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www.materialstoday.com

Materials for energyPowering the future

NOVEMBER 2011 | VOLUME 14 | NUMBER 11

MT1411pCovers.indd 1 17/10/2011 14:30:25

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MT1411pCovers.indd 2 17/10/2011 14:30:45

EDITORIAL

NOVEMBER 2011 | VOLUME 14 | NUMBER 11 505

Caroline Baillie

Queens University, Canada

Zhenan Bao

Stanford University, USA

Alejandro Lopez Briseno

University of Massachusetts, USA

Chris Ewels

CNRS, France

Peter Goodhew

University of Liverpool, UK

Alan Heeger

University of California, USA

Suwan Jayasinghe

University College London, UK

Mark Johnson

Naval Research Laboratory, USA

David Kisailus

University of California

Steven Lenhert

Florida State University, USA

Tae Won Noh

Seoul National University, Korea

Aleksandr Noy

University of California, USA

Stephen Pearton

University of Florida, USA

David N. Seidman

Northwestern University

Helena Van Swygenhoven

Paul Scherrer Institute, Switzerland

George Whitesides

Harvard University, USA

Jackie Yi-Ru Ying

Institute of Bioengineering and

Nanotechnology, Singapore

Editorial Advisory PanelEditorial Advisory Panel

You cannot pick-up a popular scientific periodical these

days, such as Materials Today or the New Scientist, without

finding some news on energy related research. The

editorial team on Materials Today are seeing a significant

surge in interest and high quality research papers

appearing on energy related topics. Whether it be new

materials in the generation of energy, storage, improved

performance or policy, energy is definitely here to stay,

as the world’s energy needs continue to increase and our

conventional energy sources become even further strained.

To further facilitate research and discovery in the field

Deborah Logan our Publishing Director in Materials Science

is launching a new journal in 2012 with Prof Zhong Lin

Wang from the Georgia Institute of Technology on Nano

Energy. You can actually hear more from Professor Wang

and the journal by listening to the podcast between

Professor Wang and Dr Stewart Bland, Assistant Editor

on Materials Today. www.materialstoday.com/podcasts.

You can also find our podcasts through your iTunes

account, search for materials today. You can find

out more information about Nano Energy by visiting

www.materialstoday.com/view/21675/nano-energy/

Professor Wang will launch Nano Energy at the Fall MRS

meeting so please come along to meet him. We will host

a meet the Editor-in-Chief event at our stand on Tuesday

29th December from 3pm where you can come along

and chat with Professor Wang and enjoy some cheese

and wine. The same evening we’ll have a more formal

reception to launch the journal at the Sheraton hotel from

6.30 – 8.30pm. To attend just pick up an invitation from

our stand during the day.

Turning to this month’s issue our lead authors Jerry D.

Murphy and Thanasit Thamsiriroj look for a single solution

in the replacement of petroleum products with renewable

transport fuels. Their lively paper entitled “what will fuel

transport systems of the future” makes fascinating reading.

Our second paper by Aaron D. LaLonde et al. looks at

thermoelectric power generation. The growth in interest

in this amazing material has been phenomenal and the

authors look at its application in waste heat recovery.

Meilin Liu et al. bring us up to date with new advances

and tools in solid oxide fuel cells.

Whilst Sergei Kalinin et al. look at the optimization of

energy storage and conversion materials by understanding

better their ionic and electrochemical functionality.

Kalinin et al. use electrochemical strain microscopy in the

studies of Li-ion cathode and anode materials.

I hope you enjoy this issue of Materials Today and if you

are travelling to the Fall MRS meeting in Boston do drop

by our stand and say hello, we look forward to welcoming

you!

Published byElsevier Ltd.The Boulevard, Langford Lane,Kidlington, OX5 1GB, UK

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Power to the future

Energy generation, storage and utilization

MT1411p505.indd 505 14/11/2011 11:08:16

MT1411p506_509.indd 506 01/11/2011 14:53:19

Contents

www.materialstoday.com CONTENTS


Regulars Editorial 505

Power to the futureEnergy generation, storage and utilization

Comment | Peter Thrower 510 Awards and prizesResearch isn’t about collecting awards, but are scientists being overlooked when it comes to recognizing and rewarding service to society?

Research News 512Memristors are made of this | Hubble bubble | Copying nature | Thermoelectrics, to go | Lights up time for graphene devices | Algae, for cheaper batteries | Synthesizing chromosomes offers DNA breakthrough | Smooth operator | Is it or isn’t it? | New photovoltaic mechanism | Belting up ultraviolet visibility | Mechanical chemistry

Updates Feature Comment 560

Scientific coopertition: can it scale and work?In materials science, collaborations tend to be limited to the cooperation of several small research groups. Meanwhile, in the world of particle physics, it’s an entirely different story. Markus Nordberg and Fabiola Gianotti from the LHC’s ATLAS experiment discuss the rewards and difficulties of large scale collaboration.

Markus Nordberg and Fabiola Gianotti

Books & Media 565

Uncovered | Benjamin J. Jones 567Nano fingerprints This month’s cover shows a back scattered electron micrograph of a fingermark developed using novel techniques. Benjamin Jones discusses the science of fingerprint detection and speculates on future techniques.

Diary 568

512 Hubble bubble.

515 Smooth operator.

517 Belting up ultraviolet visibility.

MT1411p506_509.indd 507 01/11/2011 14:53:39

CONTENTS www.materialstoday.com


www.materialstoday.com

Materials for energyPowering the future


MT1411pCovers.indd 1 17/10/2011 14:30:25

Materials for energy Review 526

Lead telluride alloy thermoelectricsThe possibility of using solid-state thermoelectrics for waste heat recovery has reinvigorated the field of thermoelectrics. In this review, Snyder et al. examine the past and present successes of PbTe as a thermoelectric material, as well as identifying the issues related to maximizing performance in other thermoelectric materials.

Aaron D. LaLonde, Yanzhong Pei, Heng Wang, and G. Jeffrey Snyder

Review 534Rational SOFC material design: new advances and toolsSolid oxide fuel cells offer great prospects for the most efficient and cost-effective utilization of a wide variety of fuels. Liu et al. highlight some of the recent progress that has been made in the tools used to study electrode reactions.

Meilin Liu, Matthew E. Lynch, Kevin Blinn, Faisal Alamgir, and

YongMan Choi

Review 548Li-ion dynamics and reactivity on the nanoscaleProgress in the development and optimization of energy storage and conversion materials necessitates understanding their ionic and electrochemical functionality on the nanometer scale of single grain clusters, grains, or extended defects. Kalinin and colleagues review electrochemical strain microscopy, focussing on applications for Li-ion cathode and anode materials.

Sergei Kalinin, Nina Balke, Stephen Jesse, Alexander Tselev,

Amit Kumar, Thomas M. Arruda, Senli Guo, and Roger Proksch

Lead story 518What will fuel transport systems of the future?Can there be a “silver bullet” to solve our energy needs for transport? In this paper, Murphy and Thamsiriroj question the notion of whether there could ever be a single solution to the replacement of petroleum products with renewable transport fuels. Their paper examines available renewable transport fuels through an analytical review of a number of technologies.

Jerry D Murphy and Thanasit Thamsiriroj

Next issueMaterials Today looks at memory and more

Organic ferroelectric opto-electronic memoriesOrganic non-volatile memory

devices based on ferroelectricity

are a promising approach

towards the development of a

low-cost memory technology

based on a simple cross-bar

array. In this review article, the

latest developments in this

area are discussed, with a focus

on the most promising opto-

electronic device concepts.

Phase change memories: properties and applicationsAfter revolutionizing the

technology of optical data

storage, phase change materials

have been successfully adopted

in nonvolatile semiconductor

memories. The paper reviews the

key physical properties making

phase change materials so special,

the quantitative framework

describing cell performance and

the future perspectives of phase-

change memory devices.

Antibody-sensed protein surface conformationUsing an antibody-modified

atomic force microscope tip,

Bhushan et al. examine protein

surface conformation. Their

finding demonstrate that block

copolymer nanomorphology

can be used to regulate protein

conformation, and potentially

cellular response.

MT1411p506_509.indd 508 01/11/2011 14:53:58

COMMENT


Awards and prizes

Just over 50 years ago I reported for duty at the

Atomic Energy Research Establishment at Harwell, UK.

After signing various documents I was unexpectedly

told that I was to work in the Carbon and Graphite

Group where I had the task of trying to understand

some of the problems of radiation damage in

graphite, a major component in current UK nuclear

power reactors. I first had to read a number of reports

and then learn to prepare few-layer graphene. Of

course, we did not use the word “graphene” because

its use was not recommended by IUPAC until 35

years later. The technique of thinning natural graphite

crystals by repeated cleavage using adhesive tape was

being explored by several researchers at that time and

some perfectly transparent samples were occasionally

produced. The resulting material could be examined

in the transmission electron microscope, and at least

two studies of dislocations in graphite prepared using

this method were published in 1960.

Readers will know that 50 years later the Nobel Prize

for Physics was awarded to Geim and Novoselov for

“producing, isolating, identifying and characterizing

graphene”. As is often the case, there were arguments

over whether other scientists should have been

recognized by the award. When I raised the subject

with a senior colleague, I received a reply that

contained the comment “science is not about awards

and prizes”.

Many countries have a system of awards, and in the

UK we have a system of honors and awards that

recognizes “outstanding achievement and service

across the whole of the United Kingdom”. The usual

awards range from the title of Sir (Knight) or Dame

to MBE (Member of the Order of the British Empire).

Many people who win gold medals in the Olympic

Games, Nobel Prize winners and people retiring

from high government office are often given the

title Sir or Dame, while an MBE may be awarded for

“achievement or service in and to the community

of a responsible kind which is outstanding in its

field”. An Honours List is published twice a year. The

awards have no monetary value. While some are for

achievement (winning an Olympic gold medal or a

Nobel Prize), most are simply a public recognition of

service to a community. As illustrated by the recent

failed bid to host the 2018 Football (Soccer) World

Cup, success is not a criterion: the people involved in

the bid were recognized for their service.

When I retired from university life almost thirteen

years ago I returned to the village in England where

I was born and raised. The village is situated in the

middle of the Norfolk Broads: a picturesque area of

shallow lakes (broads) and rivers that is renowned

for its wildlife. I know many people in the village,

some of whom I went to elementary school with. A

few years ago, one of the villagers was awarded an

MBE for “services to the Broads”. He had worked as

a reed-cutter on the Broads for around 35 years and

had given outstanding local service. [Norfolk reed

is recognized as a premium thatching material for

the roofs of houses.] This prompted a good friend to

comment: “if (he) can get an award for cutting reed,

why can’t you get one for what you do?” I must say

that the friend did not really have any knowledge of

what I do, except that I had long been involved with

editing a major scientific journal.

The answer to this question is probably quite

simple. I have never been nominated. Most people

who are nominated for awards in the UK come

from government departments, from national

to local, and quangos. The Broads Authority was

undoubtedly responsible for the nomination referred

to above. The latest Honours List includes three

school crossing wardens awarded MBEs for guiding

children across the street outside a school for many

years. They were certainly nominated by the local

council. But any organization or person is allowed to

nominate, and one wonders how often it is done?

When it was suggested that I write this Comment

I was reluctant to do so because I did not want

readers to get the impression that I was “fishing” for

a nomination. I also certainly do not want people

to think that I am of the opinion that the people

mentioned above did not deserve their awards. They

did! They all devoted a major part of their lives to

a task that was not highly rewarded financially, and

they were reliable and diligent in what they did as

a service to their local community. But such is true

of many people who do not work in a government

organization. How often do such employees get

nominated for awards? Who was the last person

recognized for “services to scientific publishing”?

Is a publisher going to take time to prepare a

nomination? Do they even think about doing so?

In the last thirteen years I have nominated several

people for different awards and I am proud to say

that they have all been successful. Why? I think there

are two reasons. First, the nominee has to be truly

worthy of the award. Second, the nomination has

to be carefully prepared, presented and supported. It

takes a lot of effort and time, but it is satisfying to

see people receive the recognition they are due.

The latest issue of my Cambridge University Alumni

Magazine reports that six people from the university

received awards in the latest Honours List. Five of

these were professors, as one might expect, but

one was presented to a department administrator,

for services to “Higher Education”. Two names

further down the list one sees a person recognized

for services to “Ploughing in Wales”. The range of

activities recognized is enormous!

Winning a Nobel Prize may guarantee consideration

for a national award, but there are those who work

“behind the scenes” that are also deserving of

recognition. It is our responsibility to see that steps

are taken to ensure that this is done.

Peter A. Thrower | Editor-in-Chief, Carbon | [email protected]

Research isn’t about collecting awards, but are scientists being overlooked when it comes to recognizing and rewarding service to society?

MT1411p510_511.indd 510 01/11/2011 14:50:08

RESEARCH NEWS


Resistors, capacitors, and inductors have been the three

most fundamental passive components of electronic

circuitry for decades. But, there is a fourth two-terminal

element: the memory resistor, or memristor.

Resistance in a memristor increases as current flows

through it in one direction, but falls when the current

is flowing in the opposite direction. What makes

memristors potentially very useful in a wide range of

applications is that when the current is switched off

a memristor maintains the value of its resistance at

that point.

Although memristors were first theorized in the

early 1970s, it was not until 2008 that researchers

announced a thin film titanium dioxide memristor

device for applications in nanoelectronics, computer

logic components, and novel computer architectures

that mimic the plasticity of the human brain. Now,

researchers in Singapore have side-stepped metal

oxides, chalcogenides, amorphous silicon, or carbon,

and polymer-nanoparticle composites to address

the possibility of creating a flexible, memristor

using more life-like molecules instead, in the form

of proteins [Chen et al., Small (2011) doi:10.1002/

smll.201101494].

Other researchers have, of course, investigated biological

macromolecules, including peptides and proteins and

nucleic acids, as possible components of nano-electronic

devices. The Singapore team has embedded a bipolar

memristive structure composed of protein into a nanogap

using the chemistry based nanofabrication technique of

on-wire lithography (OWL). Using OWL they were able

to template gold nanowires in a silicon dioxide layer in

which a controllable gap can be etched. The primary

iron-storage protein, ferritin, was then used to fill the gap

to create a redox-active constituent of their memristor.

Covalent bonds between sulfur atoms in the ferritin

molecules and the gold atoms of the template electrode

surfaces ensure strong coupling, the team explains.

The team tested the protein-based memristor and

demonstrated that there are marked forward and

reverse current biases. Moreover, the signal traces for

forward and reverse cross from positive to negative

and vice versa at zero voltage, proving that the bipolar

behavior of the memristor is not simply a capacitance

effect. They suggest a mechanism taking place within

the ferritin in which iron(III) atoms within the protein

are reduced to iron(II). Iron(II) atoms move more

readily in ferritin and thus give rise to the bias in the

device, with current flow rising as more and more

iron(III) is reduced. The iron(II) atoms do not revert

to iron(III) when the current is switched off, so the

resistance level is maintained. A reverse current nudges

the process in the other direction.

“We are now working with bioengineers who can

control the number of iron ions in ferritin, which we

think will help to modulate the memristors,”

Xiaodong told Materials Today. “We are also working

with organic chemists to synthesize bio-inspired

polymers to have iron ions, which we hope will have

memristor behaviour similar to our results in the Small

paper.”

David Bradley

Memristors are made of thisELECTRONIC MATERIALS

There has recently been an explosion of interest in

developing devices based on graphene, which is not

surprising when you consider the range of remarkable

electronic, optical, and mechanical properties it

possesses. Now, the group of researchers from the

University of Manchester that won the 2010 Nobel

Prize in Physics for their groundbreaking work on

graphene has proposed a new use for the wonder

material: adaptive focal lenses [Georgiou et al., Appl

Phys Lett (2011) 99, 093103].

The lenses that Novoselov and colleagues are

proposing rely on the bubbles that can form at

the interface between graphene and a substrate.

Although the exact origin of these bubbles is still

not quite clear, the bubbles appear to arise from

small quantities of gas that are trapped below the

graphene.

The bubbles are evident using an optical microscope,

and come in a range of shapes and sizes; from tens

of nanometers to several microns, with circular,

square, and triangular shapes. The bubbles are

visible thanks to the phenomenon of Newton’s rings:

when illuminated with monochromatic light, the

interference between the light reflected from the

curved bubble and flat substrate surface causes a

series of concentric rings to appear. By measuring

these rings it is possible to extract the height and

shape of the bubbles.

The team have suggested that a lens could be formed

from either filling the bubble with a liquid with a high

refractive index, or even by surrounding the bubble

with a liquid from the outside.

But the really exciting aspect of the lenses is that their

shape can be changed by applying a small voltage. The

team estimate that if a lens was constructed from one

of the bubbles, they could produce a device with a

focal length tuning ratio of 15 %. The ability to focus

a lens that is potentially cheap and easy to produce

could find application in a wide range of devices.

But work is ongoing, and lenses only represent the tip

of the iceberg. According to Prof Kostya Novoselov,

“The importance of the paper goes far beyond

optics. We need to learn how to control the strain in

graphene, which would allow dynamic change of the

band-structure.”

Stewart Bland

Hubble bubbleCARBON

Topography AFM scan of a bubble. Reprinted with

permission from Georgiou et al., Appl Phys Lett (2011)

99, 093103. © 2011, American Institute of Physics.

Ferritin molecule. Courtesy Xiaodong Chen.

MT1411p512_517.indd 512 31/10/2011 16:40:04

RESEARCH NEWS


Researchers have developed synthetic crystals

whose structures and properties copy those

of naturally occurring biological minerals, a

breakthrough that could help in the development

of high-performance materials made under

more environmentally friendly conditions than

conventional synthetic materials.

Biominerals, as composite materials created from

inorganic minerals such as calcium carbonate

that contain a small amount of an organic

material, are commonplace in the natural world

in seashells, bones, and teeth. Synthetic versions

of these tough structures are usually made under

high temperatures and pressures, a process

that provides a lack of full control over their

properties. However, a team of scientists from

the universities of Leeds, Sheffield, Manchester

and York in the UK, as well as the Israel Institute

of Technology, have now developed artificial

biominerals with similar properties to biominerals,

but which can be designed and produced in a

much easier and more eco-friendly way.

For the study, published in Nature Materials [Kim et al. Nature Mater (2011) doi:10.1038/

nmat3103], the researchers grew calcite

crystals in the presence of synthetic polymer

nanoparticles, which acted as artificial proteins,

with these nanoparticles being integrated into

the structure of the crystal as it grew to create

a composite material, while recording the

responses with a nanoindenter.

As leader of the study Fiona Meldrum points out,

“This method of creating synthetic biominerals

gives us a unique insight into the structure

of these incredible materials and the way the

organic molecules are incorporated into the

crystal structure at a microscopic level. We can

then relate this microscopic structure to the

mechanical properties of the material.”

Initially, they examined the properties of

biominerals, with the team keen to explore the

possibilities of using this property to develop

composite materials using a straightforward

one-pot method. The artificial biomineral

produced was much harder than the pure calcite

mineral because it is a composite material, and

allowed the addition of something soft to a hard

substance to create a material even harder than

its constituent parts. This is significant because

it offers a flexible, simple way of generating

composite materials, and an understanding of

how to control crystal properties through the

inclusion of additives in a crystal.

The study also allowed them to link the

microscopic properties of the crystals to the

macroscopic to better understand how some

of the properties of biominerals can be applied

synthetically. It is hoped the study will lead to

applying this process to the manufacture of a

range of crystals with enhanced mechanical

properties, and perhaps even for generating

many different composite systems. They will try

different additives to assess how well they can

control crystal properties, which would help in

designing new materials based on this technology.

Laurie Donaldson

Copying natureBIOMATERIALS

SEM image of a calcite crystal. Reprinted by

permission from Macmillan Publishers Ltd: Nature

Mater (2011) doi: 10.1038/nmat3103, © 2011.

Just a few months ago we reported on the use of

microwaves to quickly and safely lock away radioactive

by-products from nuclear reactors inside robust

compounds [Mater Today (2011) 14, 304]. Continuing

the theme of using domestic microwaves as a short

cut to materials synthesis, a group of researchers

from Oregon State University is now using a similar

technique to shave days off the production time of a

promising thermoelectric [Biswas et al., Mater Res Bull

(2011) doi:10.1016/j.materresbull.2011.08.058].

The potential of thermoelectric materials is obvious

when we consider how much energy is wasted as

heat. From conventional light bulbs to gasoline fueled

engines, the problem is clear. Thermoelectrics provide

the opportunity for some of that wasted heat energy

to be recovered, by converting it into electricity.

One promising group of materials for thermoelectric

applications are the filled skutterudites.

The team, led by Prof Mas Subramanian, have

managed to rapidly synthesize In0.2Co4Sb12: an indium

filled skutterrudite. Skutterudites are materials that

adopt a cubic structure, with large voids located at

their centers. By filling these voids with guest atoms,

so called rattlers, it is possible to simultaneously

increase the material’s thermoelectric power factor

while reducing the lattice component of the thermal

conductivity. The result being that filled skutterudites

make excellent thermoelectric materials.

Conventional synthesis methods for skutterrudites

can take several days, as the calcination process can

take up to 48 hours. However, by using a microwave,

Subramanian and colleagues have reduced the process

to just two minutes.

To produce the thermoelectric, the team mixed the

powdered elemental components together and placed

them within a tube inside a mass of CuO: a material

that strongly absorbs microwaves. After switching

the microwave on, the CuO rapidly increases in

temperature, reaching over 650 °C in two minutes.

The resulting materials were then ball milled in

acetone, and sintered for several hours under nitrogen

and oxygen.

The overall production time can be compressed to just

a few hours. But the new process does not just save

time; it saves energy.

Subsequent testing revealed that the samples

prepared using the new method were as good as those

produced using a conventional furnace. The team is

now hoping that it will be possible to produce different

types of skutterudites using the same method.

Stewart Bland

Thermoelectrics, to goENERGY

The unit cell of InxCo4Sb12, showing the indium

rattler at the center.

MT1411p512_517.indd 513 31/10/2011 16:40:06


RESEARCH NEWS

A new study has shown that alginates taken from

fast-growing brown algae could offer extended energy

storage capabilities for a new generation of lithium ion

batteries using environmentally friendly manufacturing

technologies. Such cheap, lightweight, and improved

batteries could benefit a range of applications, such as

electrical cars, computers, and cell phones.

The researchers, from the Georgia Institute of Technology

and Clemson University in the United States, whose work

was published in Science Express [Kovalenko et al. Science

(2011) doi:10.1126/science.1209150], showed that the

algae, which are produced in stalks as long as 60 meters

in large oceanic clusters, can provide more energy storage

and output than the two standard types of commercial

electrodes; graphite-based and silicon-based.

They initially looked for a natural replacement

binder in aquatic plants that grow in salt water with

high concentration of ions. The binder is crucial in

suspending the silicon or graphite particles that

interact with the electrolyte and produce power.

Lithium ion batteries transfer lithium ions between

two electrodes through a liquid electrolyte, so the

easier the lithium ions can enter the electrodes during

charge and discharge, then the greater the capacity of

the battery.

The low-cost alginate-nanoSi-electrode can be

extracted from seaweed using a straightforward soda-

based process that produces a uniform material. From

there, the anodes can be developed using a water-

based slurry to suspend either the silicon or graphite

nanoparticles. Such electrodes have the advantage of

being compatible with existing methods of production

and can therefore be integrated into standard designs

for batteries.

The alginate needs to deal with decomposition

occurring when the lithium ion electrolyte forms

a solid electrolyte interface (SEI) on the surface of

the anode, hampering the potential of high-energy

silicon anodes. The SEI has to be stable to allow

the lithium ions to pass through it as well as to

restrict the flow of fresh electrolyte. For graphite

particles, the SEI remains stable as the volume

does not change; the alginate also manages to bind

silicon nanoparticles to each other as well as the

anode, and coat the silicon nanoparticles themselves

to offer a rigorous support for the SEI, therefore

stopping any degradation.

As researcher Gleb Yushin points out “The carboxylic

groups in alginate actively interact with ions from

water and the intracellular environment, protecting the

cell from an excess of toxic chemicals. We utilize these

uniformly distributed carboxylic groups to improve the

performance of battery electrodes.”

Algae can be produced on salt water or waste water

land and do not use valuable agricultural land, and

also need less area to produce the same amount of

biomass as regular crops. The team expects such use

of plant matter to increase and believe this should

be prioritized to help achieve greater sustainability,

especially as alginates are already extensively used in

the paper, pharmaceutical, biotechnological, dental,

and food industries.

Laurie Donaldson

Algae, for cheaper batteriesENERGY

SEM Aliginate-nanoSi-electrode. Courtesy of Gleb Yushin.

Lights up time for graphene devicesCARBON

Graphene has been touted as the natural

successor to silicon in microelectronics devices.

Indeed, experimental and theoretical results

suggest that it is has many properties that will

make it the perfect material for building a new

generation of transistors, chemical sensors,

composites, nanoeletromechanical (NEMS)

devices and optoelectronics components that

might operate at 10 gigabits per second.

However, there is an obstacle visible on the

roadmap: graphene-based photodetectors

demonstrate a poor response when compared to

conventional semiconductor devices.

Now, UK researchers have combined graphene

with plasmonic nanostructures to boost

their photodetector sensitivity twentyfold

[Novoselov et al., Nature Commun (2011)

doi:10.1038/ncomms1464] The research also

suggests that it is possible to achieve wavelength

and polarization selectivity by tweaking the

nanoscopic geometry of the materials.

Geim and Novoselov and colleagues at

Manchester and the University of Cambridge

explain that graphene-based photodetectors

ought to have excellent characteristics in terms

of quantum efficiency and reaction time and

indeed they do. But they absorb light only

inefficiently and it is difficult to extract electrons

from the critical p-n junctions in any such device.

In order to circumvent this latter obstacle to the

development of graphene-based photodetectors,

Geim and colleagues have focused on

incorporating plasmonic nanostructures close

to the junctions. The team explains that these

plasmonic structures can absorb photons,

producing plasmonic oscillations, which in

turn boosts the local electric field guiding the

electromagnetic energy to the p-n junction.

With this in mind, the team therefore prepared

graphene flakes using their Nobel-winning

micromechanical exfoliation technique to

peel off monoatomic layers of carbon from

an adhesive surface. Raman spectroscopy and

optical contrast techniques proved that the

necessary templated layouts were produced.

They then created various nanostructures at

the p-n terminals of these graphene layouts.

They tested the photo response of the devices

using several low-intensity lasers coupled to a

microscope to scan the points of illumination.

The researchers were able to measure the local

photovoltage and photocurrent response. Their

theoretical calculations agreed with the responses,

however, they were unable to make a direct

quantitative comparison between the theoretical

field enhancement and the photovoltaic signals

obtained. Nevertheless, they point out that the

qualitative correspondence between theory and

experiment is sufficient and it “proves the viability

of the concept of using field amplification by

plasmonic nanostructures for light harvesting in

graphene-based photonic devices,” they conclude.

David Bradley

MT1411p512_517.indd 514 31/10/2011 16:40:07

RESEARCH NEWS


In what could be a crucial breakthrough in synthetic

biology, researchers have synthesized one of the

largest DNA molecules ever by replacing the DNA in

the arm of a yeast chromosome with synthetically

produced DNA. This computer-designed DNA is

different from the original but can still produce

a viable yeast cell using the chromosome arm to

produce a new way of mixing up the genetic structure

of the organism.

The study, which was published in Nature

[Dymond et al., Nature (2011) doi:10.1038/nature10403],

showed that large chunks of DNA can be synthesized

and inserted into a chromosome, and also developed a

way to change the structure of the synthetic DNA (called

scrambling), a process that could be applied to a range

of other organisms. The synthesis and scrambling of the

DNA in the chromosome arm is another stage in the

synthesis of all DNA in a yeast cell, and the similarities

between yeast cells and human cells could provide

information about which structural arrangements are

possible and compatible with life.

Although there have been previous studies

that synthesized bacterial chromosomes, yeast

chromosomes are bigger and more complex, making

them harder to synthesize. The team first developed

semi-synthetic DNA using a computer-generated

blueprint for the sequence of nucleotides, the building

blocks of DNA, before the resulting semi-synthetic

DNA replaced a particular chromosome arm of a

yeast cell without impacting its health. Researcher

Jef D. Boeke, from Johns Hopkins University School

of Medicine, stated “The yeast that underwent

this process were indistinguishable in their growth

properties from the native yeast.”

The synthesized DNA in the arm was then scrambled,

with a chemical added to the yeast culture that caused

major changes to gene-sized blocks of nucleotides in

the synthesized DNA. The shuffling caused some of the

genes to be lost, while the order of other genes was

also changed. The process was repeated for a range of

yeast cultures to produce many modified arms. The

differences between the scrambled genetic codes of

the yeast cultures meant these cultures showed trait

differences.

As Boeke pointed out “We were able to track the

changes we made relative to the native yeast and

isolate scrambled derivatives from the semi-synthetic

yeast. We thereby generated a wide range of different

derivatives from the semi-synthetic strain.” With

yeast being involved in a wide variety of industrial

fermentation processes, being able to more effectively

confer desired traits on yeast DNA and make it

more flexible could lead to the development of new

medicines, vaccines, and more efficient biofuels.

It is also hoped the ability to trigger huge

combinatorial genome rearrangements will resolve

other issues, such as how much you can reduce the

genome and still grow yeast and whether yeast is

restricted to 16 chromosomes or can be made with

less or more.

Laurie Donaldson

Synthesizing chromosomes offers DNA breakthroughBIOMATERIALS

A population of synthetic or semi-synthetic yeast

(yellowish cells) is scrambled. Courtesy Jessica S.

Dymond.

There is nothing worse than a ragged ribbon, especially

if it’s a graphene nanoribbon destined for the future

world of spintronics and nanoelectronics. Thankfully,

a US team has developed an approach to nanoribbons

that smoothes the edges paving the way for the

exploitation of their quantum-confined bandgaps and

magnetic edge states.

Rough edges and defects are part and parcel of

making graphene nanoribbons using lithography. But,

a novel approach that involves opening and unrolling

single-walled carbon nanotubes can be used to create

nanoribbon quantum dots that have all the properties

desired of such structures. Specifically, the researchers

demonstrated how these materials have well defined

quantum transport phenomena [Dai et al., Nature

Nanotechnol (2011) doi:10.1038/nnano.2011.138].

The concept of graphene nanoribbons, also referred

to as nano-graphene ribbons, was first postulated in

the 1990s as a suggestion for investigating the edge

effects and effects of nanostructure on these intriguing

materials. While a graphene sheet might be thought

of as a single layer of the carbon allotrope graphite,

it is the edges that are thought to give rise to specific

functionality. Theoretically, there are two arrangements:

the zigzag and the armchair. In the former, the

carbons along the edge resemble the carbon backbone

of a simple, unbranched, and essentially unending,

hydrocarbon chain. In the armchair motif, one might

imagine the edge as resembling an alternating E, Z

carbon=carbon double bond chain. In reality, these neat

motifs are distorted by dangling groups, gaps and other

defects and deviations from the norm.

There is good reason to investigate graphene nanoribbons

as theory suggests that quantum effects will arise that

are not observed with graphene sheets and that zigzag

ribbons are “metallic” whereas armchair ribbons are either

semiconducting or metallic depending on their width.

Studying these effects requires smooth, ordered edges

otherwise the phenomena observed would not be

consistent between experiments.

Moreover, one might thus be forced to conclude

that it is the defects that are giving rise to specific

measurements. This is supported by the studies carried

out by Dai et al. from which they infer that, “the

quantum transport features of graphene nanoribbons

are highly reflective of ribbon quality.” Moreover,

their graphene nanoribbons have conductivities 700

to 800 times higher than previously reported devices,

presumably due to their smooth edges.

“The next step should be to further improve nanoribbon

quality and edge structures, exploring the signature

of the edge-induced quantum phenomena such as

magnetic edges states in transport measurements, and

harvesting these states for future spintronic and other

device applications,” Dai told Materials Today.

David Bradley

Smooth operatorCARBON

AFM of a smooth GNR device. Courtesy Hongjie Dai.

MT1411p512_517.indd 515 31/10/2011 16:40:08


RESEARCH NEWS

Is it or isn’t it?BIOMATERIALS

A team of scientists from South Korea have

revealed that a developed self-assembled

phosphoryl cholin (PC) nanostructure is capable

of mimicking the natural cell membrane bilayer,

and significantly affect the gene delivery process

through favorable interactions with the cell

membrane. The team investigated this capability

with an experiment that utilized nanostructured

film-mediated gene delivery through an

examination of the interactions between the

cell membrane and an artificial self-assembled

nanostructure.

They used the nanostructure as a cell membrane

mimicking biointerface as it could interact with

the natural cell membrane, which has a strong

affinity toward naturally occurring PC groups

in cell membranes, and is structural similarity

with the phospholipid layer. The study, which

was published in Small [Son et al., Small (2011)

doi:10.1002/smll.201100232], produced two

key breakthroughs. The first was the effective

development of brush PC polymers that form

cell membranes that mimic multi-layer film and

elucidate the structural features of the multi-layer

structure by grazing incidence wide-angle x-ray

diffraction (GIWAXD). Also, the researchers looked

at the usefulness of brush polymers as artificial

cell membranes and the potential application for

tissue engineering and drug delivery substrates.

The other significant result was a greater

knowledge of the mechanism of the interaction

between the PC polymer surface and the natural

cell membrane by evaluating the gene delivery

efficiency. The team found that the polymer

surface interacted with the cell membrane

through the fusiogenic process, offering an

advantage for the delivery of payloads.

The research group had previously developed

polymer-based nanoparticles for the delivery of

therapeutic genes in the solution phase and, to

improve delivery, attempted to synthesize the

polymeric template, which can load therapeutic

genes and also interact with cell surfaces

efficiently. One researcher, Professor Moonhor

Ree, showed cell-mimicking synthetic polymers

that consisted of a polymeric backbone and PC

side chains. This polymer had similar structural

features to natural cell membranes, such as

aliphatic alkyl tails, hydrophilic PC head, and a

forming self-assembly structure.

The team therefore decided to use artificial

polymer films for efficient surface gene

delivery because of its close resemblance to

cell membrane. For this study, they revealed

that cells seeded on the PC polymer surfaces

could proliferate very efficiently, while the

initial cell attachment was inhibited by the

PC group. To fully investigate how useful

the PC polymer could be, it was necessary

to examine its structural features, and

ensure the polymerization conditions were

optimized. The team also stressed the

importance of understanding and mimicking

the cell membrane to unravel these biological

phenomena and the development of a

biointerface platform.

There are many potential applications for the

research. For instance, the PC polymer has good

biocompatibility with cells and can be easily

coated onto a range of substances, including

silicon wafer, glass and metals; as the polymer

scaffold also has useful biocompatibility without

any cytotoxicity, it could also help to create the

next generation of tissue engineering materials.

Laurie Donaldson

New photovoltaic mechanismELECTRONIC MATERIALS

Last year, a study in Nature Nanotechnology reported

on the discovery of a new mechanism for the

photovoltaic (PV) effect at domain walls in bismuth

ferrite, a ferroelectric that has recently been the focus

of many studies [Yang et al., Nature Nanotech (2010)

5, 143]. The researchers found that they could produce

voltages that were larger than the band gap of the

material: a value that limits the maximum voltage in

conventional semiconductor photovoltaics.

Now, the same team of researchers have succeeded

in unraveling this mystery and explaining the science

behind the phenomenon, which occurs in ferroelectrics

with periodic domain structures [Seidel et al., Phys Rev

Lett (2011) 107, 126805].

Ferroelectrics are materials that posses a net electric

polarization, such that one side is negatively charged

and one side is positive (just as a ferromagnet has a

north and south pole). However, regions with particular

polarizations can be separated within the bulk of a

material into individual domains, divided by domain walls.

The team, predominantly from Berkeley, studied thin

films of bismuth ferrite grown on insulating DyScO3

using chemical vapor deposition. By varying the

thickness of the film, the team was able to control the

domains, producing periodic structures that spanned

hundreds of microns. The domains were formed in

stripes, with widths between 50 to 300 microns, and

where each stripe has the opposite polarization to

that of its neighbour. The result is striped film, which

retains a net polarization across the film thanks to the

zig-zag ferroelectric pattern.

Just as in a regular PV, when exposed to light, free

electrons and holes are formed. But the team has

found that thanks to the presence of the domain walls,

electrons and holes collect on opposite sides of the

walls, delaying the electron/hole recombination. A

current then results that is perpendicular to the walls,

as the opposite charges attempt to recombine. But

it’s the repetitive nature of the domain structure that

is key to the large voltage, as the effect is additive.

According to co-author of the study Joel Ager “It’s like

a bucket brigade, with each bucket of electrons passed

from domain to domain. As the charge contributions

from each domain add up, the voltage increases

dramatically.”

Bismuth ferrite is not the ideal material for solar cells,

as it’s sensitive to blue and near UV light, but the

same mechanism should work in any ferroelectric

with a periodic domain structure, and the team is now

looking into new material possibilities.

Stewart Bland

Piezoresponse force microscopy image of the aligned domain walls. Reprinted with permission from Seidel at al., Phy Rev Lett (2011) 107, 126805. © 2011 by the American Physical Society.

MT1411p512_517.indd 516 31/10/2011 16:40:10

RESEARCH NEWS


A novel and facile approach to detecting ultraviolet

radiation in the hazardous 320 – 400 nm (UV-A) part

of the spectrum has been developed by researchers

in China (Fudan University) and Japan (NIMS). Their

high performance photodetector comprises niobium(V)

oxide nanobelts that are 100 – 500 nm wide and

2 – 10 micrometres long. The team synthesized these

quasi-aligned nanobelts using hydrothermal treatment

of niobium foil in potassium hydroxide solution

and subsequent proton exchange and calcinations

[Fang et al., Adv Funct Mater (2011) doi:10.1002/

adfm.201100743]. The same detectors might be useful

in optoelectronics circuitry operating in the UV-A

band.

Photodetectors are becoming increasingly important

in many applications. Fundamentally, they convert

an optical signal into an electrical one and so can

be used, as the name would suggest, simply as a

light detector, as well as acting as binary optical

switches for imaging, optical communications, and

optoelectronic circuits. Given the growing interest in

nanotechnology it requires no great stretch of the

imagination to see that integrating photodetectors at

the nanoscale is the inevitable next step. Indeed, such

nano devices are inherently more effective than their

“bulk” semiconductor counterparts because of their

higher surface to volume ratio, even in bridging the

gap between micro and nano.

Researchers across the globe have thus focused

on creating one-dimensional nanowire based

photodetectors and efforts have been made to nudge

the sensitivity of these devices into the ultraviolet

part of the electromagnetic spectrum. Unfortunately,

most efforts have led to only poorly efficient UV-A

detectors. Fang and colleagues hoped to fill the gap by

turning to niobium(V) oxide, a material transparent to

visible light that has a bandgap of 3.4 eV, which they

suggest, makes it an ideal candidate for a “visible-

blind” UV-A photodetector. The visible transparency

means that the detection process essentially ignores

incident visible light.

Tests on their nanobelt UV-A photodetector reveal

it to live up to expectations with high sensitivity

and high external quantum-efficiency of well over

6000 %. The prototype nanobelt detector also has a

photocurrent stability of more than 40 minutes. The

team suggests that optimization of the annealing

process used in the final stage of preparation of the

nanobelts, could be further optimized to improve the

active life time of the materials. There is also a need

to eliminate defects and so improve efficiency and

sensitivity still further.

David Bradley

Belting up ultraviolet visibilityNANOTECHNOLOGY

Nanobelt arrays. Courtesy Xiaosheng Fang.

Click chemistry describes the process of quickly and

easily joining smaller molecules together to form

larger ones. However, in spite of a name which implies

a kind if chemical Lego, while moving forward is easy,

reversing the reactions can be rather difficult. Such is

the case for the highly stable 1,2,3-triazole moiety,

which strongly resists being reverted into its azide

and alkyne precursors, rebuffing attempts to reverse

the reaction using simple chemical and thermal

techniques.

Tackling the problem will instead require a different

approach, and thanks to a team from the University

of Texas at Austin, it looks as though we may have a

solution to “unclick the click” [Brantley et al., Science

(2011) 333, 1606].

Prof Christopher Bielawski and colleagues implanted

the stable triazole inside a polymer chain, and then

managed to break the chain at the triazole site using

ultrasonic sound waves. Speaking to Materials Today,

Bielawski exaplained how such a mechanochemical

approach works: “The polymer chains function as

handles that respond to the forces generated

under ultrasonication. In an acoustic field, solvent

cavitation generates small bubbles that rapidly

expand and implode. Solvated polymer chains near

these growing cavities essentially are pulled toward

the void volume. If this happens to a polymer

chain attached to one side of the triazole, but not

to the polymer chain attached to the other side

of the triazole, tensile forces are generated in the

center of the chain, right where the triazole is

located. It is believed that this mechanical force

destabilizes the molecule through bond distortion,

which ultimately lowers the energy needed for the

cycloreversion to occur.”

As the length of the polymer chain is dependent

on sonochemical reactions, as well as the position

of the mechanically sensitive molecule within

the chain, the team was also able to demonstrate

that the reversion was purely the result of the

mechanical action, rather than the effect of

any induced heating. These control experiments

suggest that the triazole must be located near

the center of the chain in order to experience the

required force.

The researchers believe that such an ability to

“selectively deconstruct tiazoles with high fidelity”

could find use in mechanoresponsive materials.

Bielawski revealed, “An interesting application of

our work could be the development of systems

or sensors that use mechanical forces to

reversibly label biomolecules (e.g., proteins) with

a variety of small molecules.”

The team is “currently undertaking a theoretical

study to understand the role that mechanical forces

play in the reactivity we have observed. We are

also exploring new areas, such as the application of

mechanical forces in a biological context.”

Stewart Bland

Mechanical chemistryTOOLS AND TECHNIQUES

Bielawski and colleagues make reversing the

reaction look easy. Courtesy Christopher Bielawski.

MT1411p512_517.indd 517 31/10/2011 16:40:11

ISSN:1369 7021 © Elsevier Ltd 2011NOVEMBER 2011 | VOLUME 14 | NUMBER 11518

When discussing energy, it is important to understand the relevant

units. Box 1 outlines the relationship between the tonne of oil

equivalent (toe), the Joule, and the GWeh. In addition, primary and

final energy must also be differentiated, particularly for electricity.

Primary energy is the energy contained in primary resources prior

to conversion or transformation into a form that is used by the

final consumer. Energy used by the final consumer is final energy.

For example the energy content of the coal used in a power plant

is primary energy while final energy describes the electricity

produced by the power plant. The ratio of primary energy to final

energy is approximately 2.5 if electricity is produced at 40 %

electrical efficiency. This relationship is less clear for renewable

sources. For example, for wind power primary energy is similar to

the final energy.

Background and perspectiveThe world’s Total Primary Energy Supply (TPES) was 514 EJ

(12 267 Mtoe) in 2008, which is double the figure of 1973 (256 PJ)1. In

2008 renewable sources accounted for 12.9 % of the TPES. On a global

scale the ratio of primary energy to final energy was 1.471.

There tends (particularly in the media) to be a preoccupation

with renewable electricity rather than renewable energy. Electricity

consumption in 2008 equated to 60.6 EJ or 17.2 % of the Total Final

Consumption (TFC) while transport equated to 95 EJ or 27 % of the

TFC1. On a global scale 18.7 % of the electricity is produced from

renewable sources; the majority of which is from hydro-electricity

(15.9 %). Renewables play a smaller role in transport. In 2008 liquid

biofuels accounted for 1.92 EJ or about 2 % of the energy used in

transport2. This relatively small proportion has been controversial,

This paper seeks to decry the notion of a single solution or “silver bullet” to replace petroleum products with renewable transport fuel. At different times, different technological developments have been in vogue as the panacea for future transport needs: for quite some time hydrogen has been perceived as a transport fuel that would be all encompassing when the technology was mature. Liquid biofuels have gone from exalted to unsustainable in the last ten years. The present flavor of the month is the electric vehicle. This paper examines renewable transport fuels through a review of the literature and attempts to place an analytical perspective on a number of technologies.

Jerry D. Murphy and Thanasit Thamsiriroj

Environmental Research Institute and Department of Civil and Environmental Engineering, University College Cork, Ireland

E-mail: [email protected]; [email protected]

What will fuel transport systems of the future?

MT1411p518_525.indd 518 31/10/2011 14:24:38

What will fuel transport systems of the future? REVIEW


leading to a food versus fuel debate3,4. In 2007 – 2008 the prices

of wheat, rice, and maize increased by 130 %, 98 %, and 38 %

respectively; this was attributed to the maize ethanol market5. This may

indicate that we need to worry about transport fuels of the future.

Land, population, food, biomass, and biofuelsIf biofuels are a solution to renewable transport fuel we must consider

the available land. The land area on Earth is 149 × 106 km2; 55.7 % of

this is forest, 16.1 % (or 24 × 106 km2) is pastureland and 9.4 % (or

14 × 106 km2) is arable land6. The population of the world is growing.

In 1804 there were 1 billion people on the planet; in 2000 this number

had increased by a factor of 6. By 2013 another billion people will

occupy the planet7. Global arable land averages only 0.2 ha per person

and this number is decreasing. Life styles are such that more people

require meat. A meat diet requires more land than a vegetarian diet.

Thus our finite agricultural land is required to produce more food for

a growing population of humans and animals as well as renewable

thermal and transport energy. Is this possible?

Ireland: a case studyThe world is variable; bioenergy systems are geography specific.

Sugar cane grows in tropical climates not in temperate ones. Even in

particular climatic regions the yield of crops varies; maize for example

provides yields in the range of 9 to 30 tonnes of dry solids (tDS) per

hectare per annum8. This paper will use Ireland as an example where

necessary. The Republic of Ireland is part of the island of Ireland and

is situated at the western extreme of Europe. It has a population of

4.5 million and a land area of 6.8 million ha. The agricultural area is

of the order of 4.4 million hectares of which 4 million are deemed

pastureland (including rough grazing) and 400 000 hectares are arable9.

Energy forecasts for IrelandIreland is a member state of the EU. The EU has set targets for Ireland

of 16 % renewable energy supply (RES) in 2020. They have further

set a specific target of 10 % renewable energy supply in transport

(RES-T)10. Policy in Ireland has a particular focus on renewable

electricity (RES-E). Ireland has independently set a target of 40 %

RES-E; this will be met predominately through wind power. Ireland’s

forecast for total final energy in transport in 2020 (allowing for the

implementation of energy efficiency and renewable energy plans) is

188 PJ11 (Table 1). In 2008 Ireland had 2.497 million vehicles of which

1.92 million were private cars. The private car density thus equated to

ca. 430 per 1000 population, with an average annual distance travelled

of 16 708 kilometers12.

Contribution of electric vehicles to renewable transportThe role of electricity in renewable transportElectric vehicles (EVs) are expected to make a significant impact on

the international transport fleet with several manufacturers rolling out

EV models. The EV (Fig. 1) is recharged from the electricity grid and is

not only beneficial to the vehicle users, but also to electricity providers.

EVs can act as an energy storage system by recharging at night when

Table 1 Forecasted final energy consumption in Ireland

in 2020. Adapted from11.

PJ % total

Electricity 124 21.5

Thermal 223 38.9

Transport (road and rail) 188 32.8

Other transport (not covered by RES-T) 39 6.8

Total 574 100

Box 1 Energy units and prefixesAt a national or global scale, energy may be described in:

PJ (1015 J) or EJ (1018 J).

Alternatively, Million tonnes of oil equivalent (Mtoe) may be used.

1Mtoe = 41.9 PJ.

Electricity may be described by the TWeh (1 TWeh = 3.6 PJ).

1 kWeh = 3.6 MJ.

k = 103: M = 106: G = 109: T = 1012 :P = 1015; E = 1018.

Box 2 The role of EVs in renewable energyRenewable energy associated with EVs300 000 EVs in 2020, each travelling 16708 km/a.

5 billion km/a at a fuel efficiency of 6 km/kWeh = 835 GWeh/a or

3 PJ/a.

Final energy consumption for transport Ireland in 2020 is

projected to be 188 PJ.

300 000 EVs equates to 1.6 % of the energy in transport (2.4 %

of the energy in electricity).

The target for green electricity in 2020 is 40 %.

300 000 EVs equates to 0.64 % green energy in transport.

The relationship between EVs and 3 MWe turbinesAllowing for 8 % losses between the source and plug in point, and

a 12 % loss from plug to battery14, a 3 MWe wind turbine at a

capacity factor of 30 % generates:

3 MWe × 8760 h/a × 0.3 × 0.92 × 0.88 × 10-3 = 6.38 GWeh/a.

300 000 EVs require 3 PJ or 835 GWeh/a.

One hundred and thirty one 3 MWe wind turbines would be

required.

One 3 MWe turbine will fuel about 2300 EVs.

MT1411p518_525.indd 519 31/10/2011 14:24:44

REVIEW What will fuel transport systems of the future?


the electricity demand is normally low and electricity from wind may

otherwise be wasted. The primary disadvantage of the EV is the battery,

which has a relatively short lifetime, a long recharging time and results

in a short driving distance per charge.

The proposed role of EVs in IrelandThe Irish Government has set an ambitious target of 10 % of all

vehicles in the transport fleet to be powered by electricity by 2020.

This will require between 250 000 – 300 000 EVs12. With reference to

Box 2 it may be noted that this amounts to only 1.6 % of the energy

in transport and as only 40 % of electricity is proposed to be green,

accounts for only 0.64 % RES-T. The EU Renewable Energy Directive10

allows a weighting of 2.5 to green electricity, thus this again equates to

1.6 % RES-T. The rationale for this weighting is to incentivize EVs, but

there is some logic to this value as it is similar to the ratio of primary

energy to final energy. The value of 1.6 % RES-T is very similar to

values obtained by Foley and co-workers13.

EVs as a variable electricity storage mechanismOne issue with producing electricity from wind is its intermittency

and the inability to store it on a large scale. Much of the potential

electricity that could be produced by wind at night is lost to the

system. Electricity demand in Ireland is expected to be 124 PJ in 2020

(Table 1) and 40 % of this (ca. 50 PJ) is targeted to be renewable; as

wind is the dominant renewable energy source, the production will

be variable. On a very simplified basis it can be assumed that a third

of this electricity (ca. 17 PJ) will be produced by night when demand

is very low. The 300 000 EVs plugged in at night will require 3 PJ/a;

averaged over the year EVs could store on the order of 18 % of the

electricity produced from wind during the night.

On the most advantageous sites in Ireland a wind turbine will

generate electricity at a 40 % capacity factor; as more turbines are

built this has dropped to ca. 30 %. With reference to Box 2 it may be

noted that a 3 MWe turbine can provide electricity for 2300 EVs; one

hundred and thirty one 3 MWe turbines are required to fuel 300 000

EVs. It is obvious that although EVs have a significant role to play in

renewable energy, other sources are required.

Liquid biofuelsLiquid biofuels are dominated by bioethanol and biodiesel.

Approximately 67 billion liters of bioethanol were produced in

20089. This equates to 1.4 EJ or (1.47 % of the energy in transport).

Approximately 12 billion liters of biodiesel were produced in 20089

(400 PJ or 0.42 % of the energy in transport).

BioethanolBioethanol may be produced from sugars (sugar cane, sugar beet)

or starches (corn/maize, wheat, barley). Ethanol production requires

fermentation of six-carbon sugars with saccharomyces cerevisiae as the

prime yeast species15. Sucrose (C12H22O11) can be easily converted to

glucose and therefore juice or molasses from sugar cane and sugar beet

do not require hydrolytic pre-treatment16. Starch however is a complex

carbohydrate (C6H10O5)n which requires hydrolytic pre-treatment prior

to fermentation17. Starch is the most utilized feedstock for ethanol

production16; its conversion is energy intensive18. For example wheat

ethanol has a by-product known as wet distiller’s grain and solubles

(WDGS) which contains 33 % of the starting solids at ca. 12 % solid

content. It is used as cattle feed after drying to 9 % water content.

This drying process can account for up to 35 % of the total parasitic

thermal demand of the ethanol process18.

BiodieselBiodiesel is produced from rapeseed and sunflower in Europe,

soybean in Southern America, and palm oil in South East Asia. It is

produced through a transesterification process whereby oil (ca. 90 %)

and methanol (ca. 10 %) together with a catalyst are converted to

fatty acid methyl ester (FAME) or biodiesel (ca. 90 %) and glycerol

Fig. 1 (a) Wind turbine and (b) plug-in electric vehicle.

(a)

(b)

MT1411p518_525.indd 520 31/10/2011 14:24:46



(ca. 10 %). Physical properties of the specific oil depend on the portion

of triglycerides and free fatty acids (FFA). For example, fresh vegetable

oils comprise 90 – 98 % triglyceride with a small portion of FFA19,20

while used cooking oil is high in FFA content. Two production methods

are currently available at commercial scales21: (1) Alkaline catalyzed

transesterification; (2) Acid and alkaline catalyzed transesterification in

a two-stage process.

The first method is used to transesterify oil with low FFA content;

the process can be established on a small scale. The second method is

for oils high in FFA. The two-stage process begins with an esterification

reaction using an acid catalyst to convert FFA into biodiesel;

subsequently a transesterification reaction (method 1) is used to

convert the remaining triglyceride into biodiesel. Animal fat obtained

from the rendering process and used cooking oil from catering are

normally high in FFA content, and require the two-stage technology.

Sustainability of first generation biofuelsIssues related to liquid biofuels may be separated into energy balances

and green house gas analyses.

Energy balanceGross energy reflects the yield of biofuel per hectare. The net energy

deducts the energy input to the crop production and to the process. For

example, 18 GJ of direct and indirect energy is required per hectare to

produce wheat. Indirect sources include the energy required to produce

the fertilizers. Direct energy includes diesel to power agricultural

machinery22.

In wheat ethanol approximately 66 GJ of ethanol are produced

per hectare in Ireland (372 L ethanol / tonne of grain × 8.4 tonnes

of grain / hectare × 21.1 MJ / L of ethanol). However in a standard

ethanol system using electricity from the grid powered by fossil fuel

and natural gas for thermal energy about two thirds of the output

energy is used in the process18. Thus wheat ethanol has a gross energy

of 66 GJ/ha/a (3125 L of ethanol per hectare) while the net energy

can be as low as 4 GJ/ha/a (Fig. 2). This highlights one difference

between modern biofuel facilities in the developed world and those

in the developing world. Sugarcane ethanol facilities use the residue

of the cane (bagasse) as a source of combined heat and power to

fuel the system and as such the net energy is not much lower than

the gross energy (Fig. 2). Systems can always be improved. Murphy

and Power18 showed that by using stillage (WDGS) to produce

biomethane, and straw as a source of thermal energy, the gross

energy of the bioenergy system could be increased by 27 % and

the net energy from 4 to 43 GJ/ha/a. For an optimum sized ethanol

facility (150 million liters / annum) the land under grain (and straw)

is 48 000 ha. As straw is a bulky, voluminous biomass the developer

may find this logistically difficult and expensive. How is the developer

persuaded to be sustainable?

Greenhouse gas balanceA greenhouse gas balance outlines the sustainability of the biofuel

system. The EU Renewable Energy Directive10 states that to be deemed

sustainable the biofuel system must affect a 60 % saving in greenhouse

gas emissions compared to the displaced fossil fuel. Table 2 highlights

data from the Directive for various biofuel systems. A lot of negative

energy balances and life cycle analysis have been attributed to biofuel

systems as non-biofuel products are neglected in the analysis. Fig. 2 only

allows for energy in fuel. If, for example, the stillage from a grain ethanol

facility is fed to cattle and displaces grass silage, no credit is given to

the ethanol system. The present authors23 investigated biodiesel for use

in Ireland through comparison of indigenous Irish rape seed and palm

oil biodiesel produced in Thailand. The paper highlighted the benefits

of the palm oil system as the by-products provide the parasitic energy

demand of the palm oil biodiesel system. The paper also highlighted a

short fall in the analysis of biofuel systems in the developed world. Of

the 4 tonnes of rape seed produced, 1.2 tonnes is converted to biodiesel

while 2.8 tonnes is converted to rape cake23. A further paper by the same

Fig. 2 Gross and net energy balance of selected biofuel systems. Adapted from9.

Table 2 Typical values for greenhouse gas savings from

the EU renewable energy directive. Adapted from10.

Biofuel system % savings in greenhouse gas emissions as compared to fossil fuel replaced

Wheat ethanol 32

Rape seed biodiesel 45

Sunflower biodiesel 58

Sugarcane ethanol 61

Palm oil biodiesel 62

Biogas from MSW 80

MT1411p518_525.indd 521 31/10/2011 14:24:50



authors24 found that allocation by energy content attributes almost half

the greenhouse gas emissions to rape cake (a co-product). Rape cake

substitutes for importation of soybean from South America and thus

saves on emissions through deforestation and/or ploughing of grass

lands. They found that the system could be sustainable when produced

glycerol is used as a source of heat, and rape straw pellets are used in lieu

of peat (an environmentally damaging indigenous fuel source in Ireland).

Bioresources of first generation biofuels in Ireland Murphy and Power25 highlighted the quantity of land required to meet

the 2010 5.75 % biofuel target for Ireland. The fuel required equated to

11.3 PJ/a or 538 million L/a of ethanol or 323 million L/a of biodiesel.

The land take is excessive. With reference to Table 3, rape seed, which

is a one in four year rotational crop, requires 280 % of the arable land

involved in a rape seed rotation to meet the 5.75 % biofuel target. This

is not possible.

Second generation biofuelsSecond generation biofuels are derived from lignocellulosic feedstocks.

These feedstocks do not (directly) compete with food production but may

compete for resources such as water and land. Thus indirectly there is

potential for conflict with food if lignocellulosic crops (such as Miscanthus)

are grown on arable land. The beneficial use of whole crop (straw and

cereal) for biofuel production has a drawback in that carbon that may

have been ploughed back in (in the form of straw) is now not available.

This can lead to carbon depletion of the soil. Care must be taken that

carbon is recycled to the soil where lignocellulosic biomass is produced. In

the long term it must also be noted that fertilizer is dependent on fossil

fuel, and as fossil fuels deplete, fertilizer will become very expensive.

Lignocellulosic biomass typically comprises 35 – 50 % cellulose,

15 – 25 % hemicellulose, 15 – 30 % lignin and small amounts of extractive

substances and ash26. Bio-refineries convert lignocellulosic biomass into

biofuels and smaller quantities of high value products (e.g., chemicals)27.

Two particular issues require caution with regard to assuming that second

generation biofuels are superior to first generation biofuels, namely: the

feedstock and the process. Second generation biofuels are not always

free or cheap. In 2006 – 2007 grain prices were of the order of �110/t in

Ireland28. Straw, for example, is a second generation feedstock with a yield

of ca. 5 t/ha/a (compared to ca. 8.5 t/ha/a of wheat grain)28. Straw requires

collection, baling, and transport and has a minimum cost of ca. �65/t29. In

Denmark straw is used in CHP facilities, which drives up the price further29.

Straw is voluminous and bulky and as such has high transport costs for the

high distances associated with a commercial ethanol facility. It produces

37 % of the ethanol produced by grain per unit mass (140 L/t versus

372 L/t)30 and as such should be at a maximum 37 % the price of grain.

The process for production of straw ethanol requires a pre-treatment step

before the first generation technology30. This is typically a steam explosion

step which drives up the capital and operating costs.

Hydrogen Hydrogen is seen as a clean, abundant energy source with water vapor

as the only emission in combustion. The merits of hydrogen are based

on the fact that its energy content per unit mass is very high. The

demerit of hydrogen is that its energy value per unit volume is low. If

we consider diesel has an energy value of ca. 37 MJ/L then 1 L of diesel

has an energy content similar to 1000 L of methane and 3000 L of

hydrogen (Table 4). Hydrogen tends to be bound in compounds such as

water or in hydrocarbons such as gas. To be used as an energy source

it has to be separated from carbon in gas or oxygen in water. Typically

hydrogen is produced using two methods.

Steam reforming of natural gasApproximately 95 % of hydrogen used in the United States is

generated from natural gas31. Steam is used to reduce methane to

hydrogen and carbon dioxide. The energy demand is of the order of

20 to 30 %32. Carbon dioxide may be removed through pressure swing

adsorption and ideally carbon should be captured and stored. Hydrogen

from steam reforming will always be more expensive than natural gas.

Water electrolysisFor renewable hydrogen, renewable electricity must be sourced. The energy

efficiency of commercial electrolyzers is ca. 74 %33. This value refers only

to the efficiency with which electrical energy is converted into the chemical

energy of hydrogen. Distribution losses of 4 – 8 % must be added 34.

Table 3 Land required to meet the 2010 biofuels target

in Ireland. Adapted from25.

Biofuel Land take (kha/a)

% of agricultural land

% of arable land (9 % of agricultural land is arable)

Biodiesel Rape seed 279.1 6.3 70

Ethanol Wheat 172.3 3.9 43

Ethanol Sugar beet 107.1 2.4 26

Table 4 Comparison of hydrogen and methane as

sources of transport fuel.

Hydrogen Methane

Energy value 142 MJ/kg 55.6 MJ/kg

Molecular weight 2.016 16.042

Density 0.085 kg/mn3 0.677 kg/mn

3

Energy value 12.1 MJ/mn3 37.6 MJ/mn

3

Compression 700 bar 220 bar

Energy per unit compressed storage 8.47 MJ/L 8.27 MJ/L

Energy to compress 13 % 3.3 %

MT1411p518_525.indd 522 31/10/2011 14:24:52



Hydrogen versus natural gasWhy convert methane (natural gas) to hydrogen to use as a transport

fuel? The natural gas system in Ireland is extensive, is interconnected

to the European gas network, and at least 40 % of the population have

access to natural gas in their homes. To construct a similar hydrogen

distribution system would entail a massive infrastructural project

over many years35. Similarly, conversion of natural gas to hydrogen

requires significant infrastructural investment and is energy intensive

and expensive. Compressed natural gas is a mature technology; there

are 12 million natural gas vehicles (NGVs) in the world. Methane

is an excellent fuel in terms of local air quality and greenhouse gas

emissions. Studies suggest a reduction of 18 – 38 % and 2 – 21 % for

petrol and diesel, respectively36-38.

Hydrogen must be compressed for transport fuel use. The

current standard is compression to 700 bar. This requires 13 % of

the energy content of the gas39. In comparison, compressed natural

gas (200 – 220 bar) requires of the order of 3.3 % of the energy of

the gas25. At 700 bar the volumetric energy content of compressed

hydrogen is of a similar order to CNG at 220 bar. Safety is a key

concern as 700 bar is a very high pressure.

Efficiency of hydrogen productionHydrogen produced at a power plant or wind farm must be compressed

and transported. Losses between production and application are in

the range of 39 to 49 % for steam reforming (20 – 30 % in steam

reforming, 6 % loss in pipelines, 13 % in compression) and 49 – 53 % for

electrolysis (26 % in electrolyzing, 4 – 8 % loss in grid transmission, 6 %

loss in pipelines, 13 % in compression)40. According to Bossel41 for each

100 kWeh of electricity, the net energy used by an EV will be 69 kWeh,

while that of a fuel cell vehicle operating on hydrogen will be 23 kWeh.

Biogas and biomethaneBiogas or biomethane can be produced from a range of feed stocks such

as organic waste materials (Fig. 3a) and crops including those not used

directly for human consumption. Marginal land and land unsuitable for

food production can be used. The feedstock is digested in a sealed vessel.

The produced biogas is scrubbed and upgraded to 97 % plus methane,

which may be discharged to the gas grid (Fig. 3b) or injected directly

into a NGV vehicle25 (Fig. 3c). Table 2 highlights the sustainability of

compressed biomethane from municipal solid waste (MSW). Singh and

co-workers42 highlighted that waste resources can typically allow for

2 % of the energy in transport through digestion of slaughter waste,

slurries, and MSW. These are all highly sustainable biofuel systems. To

achieve more than 2 % of the transport fuel market, biomethane must

Table 5 Energy production from crop digestion. Adapted

from6.

Maize Fodder beet Grass

Methane yield m3/ha 5748 6624 4303

GJ/ha 217 250 163

Process energy demand for

digestion GJ/ha

33 38 24

Energy requirement in cropping

GJ/ha

17 20 17

Total energy requirement GJ/ha 50 58 41

Net energy yield GJ/ha 167 192 122

Output (GJ/ha) Input (Total

Energy)

4.3 4.3 4.0

Fig. 3 (a) Facility producing biomethane from waste food in Austria. (b) Upgrading of biogas and injection of biomethane into the natural gas grid (yellow valve centre of picture) in Austria. (c) Injecting biomethane into a bus in Linkoping, Sweden.

(a)

(b)

(c)

MT1411p518_525.indd 523 31/10/2011 14:24:53



be produced from agricultural crops. With reference to Table 5 (and

comparison to Fig. 2) it may be noted that the energy balances are far

superior to first generation liquid biofuels. A simple calculation highlights

the potential of this technology. Allowing for an average net energy yield

of 120 GJ per hectare per year produced on 20 % of all arable and pasture

land (7.6 × 106 km2 or 7.6 × 108 ha) the potential production is 91.2 EJ;

this is almost equivalent to the world’s TFC in transport (95 EJ) in 20086.

Algae are considered to be the holy grail of biofuels. The energy

balance (and associated cost) is significantly affected by the dilute

nature of micro algae and the requirement to dry the algae to allow

esterification of the lipids. It is suggested that biomethane is preferable

to liquid biofuel generated from micro algae as the process does not

require drying43,44. Marine algae (or macro algae) may be very suited

to multi-feedstock anaerobic digesters in coastal areas. Biomethane

may be the optimal vector for energy from algae.

ConclusionsThis paper, in its brevity, can not deal with every aspect of renewable

transport energy but has the ambition of assessing the big questions.

Electrification of all transport is unlikely due to the scale of energy in

transport but should be a big part of the solution, as it allows for storage

of variable night time electricity. First generation liquid biofuels do not

optimize energy return per unit of land in the form of transport biofuel, will

struggle with sustainability issues, and will only ever account for less than

10 % of the energy in transport. Second generation liquid biofuels need a

cheap abundant source of lignocellulosic feedstock. EVs are more efficient

than hydrogen fuelled vehicles when the hydrogen is sourced from

electrolysis. Methane is always cheaper than hydrogen and biomethane

has a superior energy balance to first generation liquid biofuels. Transport

fuels of the future will require numerous sources; there is no silver bullet.

Detailed research in biofuel materials and technologies are required to

optimize the resources of all renewable transport systems.

AcknowledgementsThe ideas in this broad paper were developed based on interaction

with good colleagues and by the funders of research. Particular thanks

on this paper are due to Brian O‘ Gallachoir and Clare Dunne. Thanks

to the funders, including: the Environmental Protection Agency; The

Department of Agriculture, Food and Fisheries; The Higher Education

Authority; Science Foundation Ireland; and Bord Gais Eireann.

REFERENCES

1. International Energy Agency (IEA), Key World Energy Statistics 2010, SORE GRAPH, France, 2010.

2. ENERS Energy Concept, Biofuels Platform 2009, Lausanne, Switzerland.

3. Koh, L. P., and Ghazoul J., Biological Conservation (2008) 141, 2450.

4. Rosegrant, M. W., Biofuels and Grain Prices: Impacts and policy Responses, Testimony for the U.S. Senate Committee on Homeland Security and Governmental Affairs, USA., 2008.

5. Asian Development Bank, Soaring Food Prices: Response to the Crisis, Publication Stock No. 041908, the Philippines, 2008.

6. Murphy, J., et al., Biogas from Crop Digestion, IEA Bioenergy Task 37 – Energy from Biogas, 2011

7. World Bank, World Development Indicators 2011, USA. 2011.

8. KTBL, Faustzahlen für die Landwirtschaft, Kuratorium für Technik und Bauwesen in der Landwirtschaft e.V. (KTBL), Germany, (2005), 1095 S.

9. Smyth, B., et al., J Clean Prod (2010) 18, 1671.

10 Official Journal of the European Union. Directive 2009/28/EC of the European Parliament and of the Council of 23 April on the promotion of the use of energy from renewable sources and amending and subsequently repealing Directives 2001/77/EC and 2003/30/EC. 5 June 2009.

11. Clancy, M., et al., Energy Forecasts for Ireland to 2020, Sustainable Energy Association of Ireland, 2010.

12. Howley, M., et al., Energy in transport 2009 report, Sustainable Energy Association of Ireland, 2010.

13. Foley, A., et al., Electric vehicles and energy storage: a case study on Ireland. In IEEE International Vehicle Power and Propulsion Conference. Institute of Electrical and Electronics Engineers, Dearborn, Michigan, (2009).

14. Foley, A., et al., Electric vehicles and displaced gaseous emissions. In IEEE Vehicle

Power and Propulsion Conference pp.1-6, doi:10.1109/VPPC.2010.5729228 (2010)

15. Bai, F. W., et al., Biotechnology Advances (2008) 26, 89.

16. Cardona, C. A., and Sánchez, Ó. J., Bioresource Technol (2007) 98, 2415.

17. Verma, G., et al., Bioresource Technol (2000) 72, 261.

18. Murphy, J. D., and Power, N., Fuel (2008) 87, 1799.

19. Srivastava, A., and Prasad, R., Renew Sust Energ Rev (2000) 4, 111.

20. Sharma, Y. C., et al., Fuel (2008) 87, 2355.

21. Thamsiriroj, T., and Murphy, J. D., Fuel (2010) 89, 3579.

22. Nicholas, E. K., et al., Biofuel Bioprod Bior (2010) 4, 310.

23. Thamsiriroj, T., and Murphy J. D., Appl Energ (2009) 86, 595.

24. Thamsiriroj, T., and Murphy, J. D., Energy Fuels (2010) 24, 1720.

25. Murphy J. D., and Power, N., Biomass Bioenerg (2009) 33, 504.

26. Kamm, B., et al., Lignocellulose-Based Chemical Products and Product Family Trees, In Biorefineries - Industrial Processes and Products: Status Quo and future Directions, Kamm, B., et al., (eds.), Wiley-VCH Verlag GmbH & Co. KGaA, Germany (2006) 2, 97.

27. International Energy Agency, From 1st- to 2nd- Generation Biofuel Technologies:

An Overview of Current Industry and RD&D Activities, OECD/IEA, 2008.

28. Power, N., et al., Renew Energ (2008) 33, 1444.

29. Smyth, B. Optimal biomass technology for the production of heat and power, MEngSc Thesis, University College Cork, Ireland, 2007.

30. Ballesteros, I., et al., Appl Biochem Biotech (2006) 129-132, 496.

31. U.S. Department of Energy Hydrogen Program, Hydrogen Production, 2006.

32. Mueller-Langer F., et al., Int J Hydrogen Energ (2007) 32, 3797.

33. Hammerschlag, R., and Mazza, P., Energ Policy (2005) 33, 2039.

34. Belati, E. A., and da Costa, G. R. M., Int J Elec Power (2008) 30, 291.

35. Thamsiriroj, T., et al., Renew Sust Energ Rev (2011) doi:10.1016/j.rser.2011.07088.

36. Hekkert, M.P., et al., Energ Policy (2005) 33, 579.

37. Engerer, H., and Horn, M., Energ Policy (2010) 38, 1017.

38. EUCAR, CONCAWE, JRC. Well-to-wheels analysis of future automotive fuels and

powertrains in the European context, Well-to-Wheels Report Version 2b. May 2006.

39. Jensen, J. O., et al., J Alloy Compsd (2007) 446-447, 723.

40. Page S., and Krumdieck, S., Energ Policy (2009) 37, 3325.

41. Bossel, U., P IEEE (2006) 94, 1826.

42. Singh, A., et al., Renew Sust Energ Rev (2010) 14, 277.

43. Singh, A., et al., Bioresource Technol (2011) 102, 10.

44. Singh, A., et al., Bioresource Technol (2011) 102, 26.

MT1411p518_525.indd 524 31/10/2011 14:24:58

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The basic concept of thermoelectric power generation is rather

simple; when a temperature difference exists across a material

a proportional voltage is generated between opposing ends

of the material, which can be connected to a load to provide

electrical power (Fig. 1a1). Because the charge carriers are directly

driven by the flow of heat through the material, thermoelectric

generators have a distinct advantage over other heat engines by

operating without moving parts, thus providing a device that is

robust and requires no maintenance. While the heat in a typical

generator is provided by burning a fuel, or through radioactive

decay, the possibility of renewable sources of heat such as energy

harvesting from body heat2 or waste heat recovery from industry

or automobiles has renewed interest in thermoelectrics to target

energy sustainability3.

A thermoelectric material’s potential to convert heat into electricity

is quantified by the thermoelectric figure of merit, zT. To date, the

widely believed peak zT for single phase PbTe-based material, which

has been successfully used for several NASA space missions since the

1960s, has been ~0.8. Recent studies including precise compositional

control and modern characterization have revealed that maximum zT

The opportunity to use solid-state thermoelectrics for waste heat recovery has reinvigorated the field of thermoelectrics in tackling the challenges of energy sustainability. While thermoelectric generators have decades of proven reliability in space, from the 1960s to the present, terrestrial uses have so far been limited to niche applications on Earth because of a relatively low material efficiency. Lead telluride alloys were some of the first materials investigated and commercialized for generators but their full potential for thermoelectrics has only recently been revealed to be far greater than commonly believed. By reviewing some of the past and present successes of PbTe as a thermoelectric material we identify the issues for achieving maximum performance and successful band structure engineering strategies for further improvements that can be applied to other thermoelectric materials systems.

Aaron D. LaLonde, Yanzhong Pei, Heng Wang, and G. Jeffrey Snyder*

California Institute of Technology, Materials Science, 1200 East California Boulevard, Pasadena CA 91125, USA

*E-mail: [email protected]

Lead telluride alloy thermoelectrics

MT1411p526_533.indd 526 31/10/2011 14:27:40

Lead telluride alloy thermoelectrics REVIEW


values of ~1.4 are, in fact, intrinsic to this material for both n- and

p-type materials. Further enhancement of the figure of merit has

been achieved in alloys where zT reaches a value approaching ~1.8

for homogeneous PbTe-PbSe materials. These recent findings from

PbTe-based alloys have shed new light on this classic thermoelectric

material and provide encouragement for the further development of

thermoelectric technologies on Earth.

Many types of PbTe and related compounds, alloys, and composites

have been studied as thermoelectrics for many years. There are several

comprehensive reviews of the older results that can be found in

reference 4, while recent developments in PbTe based nanostructured

composites are described in references 5 and 6. The focus of this review

is to highlight the potential of PbTe utilizing only small concentrations

of dopants (assuming the bands remain rigid) or alloying that produces

only minor perturbations to tune the band structure.

The history of PbTeDuring the Cold War and the Space Race of the middle part of the 20th

century it was with a sense of pride that President Eisenhower of the

United States was presented with the “world’s first atomic battery”

in the oval office of the White House7,8 (Fig. 1b9). This radioisotope

thermoelectric generator (RTG) contained simple alloys of PbTe for

both the n- and p-type elements. NASA used this design for its first

RTG powered spacecraft, the Transit 4A, and modified designs and

materials based on PbTe in the Apollo missions and the 1975 launch

of the Viking 2 mission to Mars10. With the advent of the Voyager

missions, NASA switched to Si-Ge alloys which were of more interest

to the scientific community following the 1960s. Although the

excitment of the Space Race has subsided, there is new enthusiasm in

NASA’s forthcoming mission; the Mars Science Laboratory (MSL), which

will probe the possibility of life on Mars. Although it has been nearly

35 years, NASA will return to PbTe based alloys as the thermoelectric

material of choice to provide power to the most sophisticated Mars

rover to date10 ( Fig. 1c11).

The early promotion of PbTe in thermoelectric generators was

made by Soviet physicist A. F. Ioffe, reportedly as early as 192812,

and numerous thorough investigations were performed at the

Semiconductor Institute in Leningrad. While the Soviets initiated the

Fig. 1 Thermoelectric materials and electric power generators. (a) Schematic of a thermoelectric device consisting of both n- and p-type thermoelectric materials1.

(b) President Eisenhower (far left) in the Oval Office of the White House being presented the “world’s first atomic battery” by officials from the Atomic Energy

Commission. The radioisotope powered thermoelectric generator (left most object on the desk) is powering a fan next to it9. Image courtesy of the US Department

of Energy. (c) A depiction of the Mars Science Lab rover Curiosity showing the thermoelectric power source (right end of the rover) containing PbTe-based

materials11. Image courtesy of NASA.

(a)(b)

(c)

MT1411p526_533.indd 527 31/10/2011 14:27:45

REVIEW Lead telluride alloy thermoelectrics


lead telluride research, the 3M Corporation was actively pursuing

similar work in the United States. Scientific reports on these materials

started appearing in the literature in the early 1950s and 1960s from

both the United States and the Soviet Union12-15, during which time

a vast amount of experimental data was gathered on PbTe and similar

alloys for thermoelectric applications.

Evolution of high temperature thermal conductivity measurementsThe experimental data that are most frequently acquired to

characterize the performance of a thermoelectric material are

the Seebeck coefficient (S), the electrical resistivity (ρ), and the

total thermal conductivity (κ). These three properties, along with

temperature (T), constitute the dimensionless thermoelectric figure

of merit, zT = (S2T)/(ρκ). The thermal conductivity of a material is

given by κ = κE + κL, where κE is the electronic component and κL

is the lattice component. The electronic component is related to the

electrical resistivity and is calculated by the Wiedemann-Franz law,

κE = LT/ρ, where L is the Lorenz number. The lattice component can be

estimated by subtracting the electronic component, calculated using

the measured electrical resistivity, from the measured total thermal

conductivity.

At the time of the initial interest in PbTe the measurement of the

electrical resistivity and the Seebeck coefficient could be performed

accurately in research laboratories throughout the world. On the

other hand, the measurement of the thermal conductivity at high

temperatures was known to be a very difficult measurement to

perform accurately16. It is likely that as a result of this difficulty,

Fritts at 3M used a more confidently measured room temperature

thermal conductivity value, in combination with the resistivity and

Seebeck measurements, to estimate κ and zT at the temperatures of

interest for PbTe thermoelectric materials15. The room temperature

κL value was treated as a constant (Fig. 2) and combined with the

reliable electrical resistivity measurements at various temperatures

to determine κE and thus determine a value of κ without actual

measurements of the high temperature thermal conductivity.

Fritts himself knew that this approach would result in a significant

overestimate of κ and therefore lead to underestimated values

of zT. By the time an acceptable high temperature measurement

method, the flash diffusivity technique, became available in the

USA in the early 1960s16, the focus of thermoelectric research in

the United States had shifted away from PbTe. The research in the

Soviet Union continued to pursue the understanding of the physics

of PbTe, however, the work did not utilize the newly available flash

diffusivity technique17-21. These initial results for the high temperature

thermal properties of PbTe, which were more recently used for

thermal conductivity calculations22, give values for κ that are up to

~30 % higher than those measured today using the flash diffusivity

technique. Despite extensive research comparing results to PbTe, the

underestimated figure of merit values by Fritts have subsisted within

the field until it was revealed very recently that optimally doped PbTe

is, in fact, almost two times better than commonly believed23,24.

By using the now commonplace flash thermal diffusivity

measurement technique the thermal conductivity of a material can

be accurately determined if the specific heat capacity and density are

known. Because the largest source of error with this method is the

measurement of specific heat capacity, published values measured

by drop-calorimetry are likely to be the most accurate. The recent

findings shown i n Fig. 3 reveal that, in fact, the zT value of the historic

n-type PbTe material is ~1.4 for several sample compositions over

a temperature range of 150 degrees (700 – 850 K)24. The electronic

transport properties (ρ and S) were found to be in excellent agreement

with numerous previously reported studies on the same compositions,

allowing the increase in zT to be almost entirely attributed to the

difference in thermal conductivity determined for the material. These

results have revealed the inherent properties of PbTe and provide

new motivation to continue research and development of this highly

functional thermoelectric material.

The low thermal conductivity intrinsic to PbTe may be due to

many factors. In the Umklapp dominated phonon scattering regime

above 300 K, this can be traced to the low speed of sound due to

Fig. 2 Comparison of the thermal conductivity recently reported for PbTe,

determined by laser flash diffusivity measurement, and thermal conductivity

data reported by Fritts in 1960. The Fritts data is from n-type 0.055 %

PbI2 and p-type 1 % Na15. The recently reported data is from n-type

PbTe1-xIx (x = 0.0012)24 and p-type NaxPb1-xTe (x=0.01)23. Also shown is

the constant κL for both n- and p-type PbTe at high temperatures; a value that

Fritts a ssumed.

MT1411p526_533.indd 528 31/10/2011 14:27:51



the harmonic lattice vibrations and the relatively high anharmonicity,

which is quantified by the Grüneisen parameter. Low speed of sound is

generally found in materials with soft bonds from large, heavy atoms,

which also correlates with a high coordination number and Grüneisen

parameter. The large Grüneisen parameter may also be due to unusual

structural features relating to the near ferroelectric instability in PbTe

and related materials25-28.

Maximizing PbTe performance via doping optimizationA similar oversight in thermal characterization also exists for

p-type PbTe doped with sodium, significantly contributing to an

underestimated zT value23. The value found recently is nearly double

the value of Fritts that is commonly reported, showing p-type

PbTe with a maximum zT value of 0.7 , (Fig. 3). However, in this

case, the accurately measured thermal conductivity is only partially

responsible for the large discrepancy in the figure of merit. Additionally

contributing to the larger zT value is an improvement of the electronic

transport properties, which are attributed to two-band conduction

behavior in heavily doped p-type PbTe.

The two-band model has been proposed because of significant

deviations of the transport data compared to that expected from

a single band model12,29-34. As schematically shown in (Fig. 4a), at

lower carrier concentrations the transport properties are dominated

by a light mass band (with extrema at the L point of Brillouin zone)

and as the carrier density increases a heavy effective mass band

(with extrema along the Σ line of the Brillouin zone) plays a more

significant role contributing to the carrier transport. Additionally, the

position of the bands is found to be temperature dependent so that

the importance of the Σ band to the transport properties increases

with temperature12,29,35-37. As the contribution from each band

is dependent on both carrier concentration and temperature, it is

intuitive that there will be optimized values that will result in the best

thermoelectric performance. Correspondingly, it is possible to “tune”

carrier concentration (and the band structure as described below) for

higher zT at the desired temperature.

The zT for each individual band (as if the other valence band did

not exist), as well as for the coexistence of both bands as a function

of carrier concentration is show n in Fig. 4b, where it is clear that the

performance of the individual bands is optimized in different regions.

The early material development per formed by Fritts for NASA is

now known to have been too lightly doped15,23,38. It can be seen

quantitative ly in Fig. 4b that at 750 K the carrier concentration in

Fritts’ (and similar material from the USA38,39) is only about half of

the optimally required amount for maximizing zT. It can also be s een

in Fig. 4b that if the transport properties were to be the result of the

Σ band alone (no L band) that the zT would have a maximum value of

only 1.1. However, when the Σ band is supplemented by the light mass

carriers of the L band, a maximum zT of ~1.4 can be realized at 750 K

(Fig. 3b) as long as it is properly doped (~2 × 1020 cm-3).

High performance PbTe analogsEarly studies, particularly from the 1950s to the 1970s have

surveyed many IV-VI compounds (such as PbTe, PbSe, GeTe, etc.) for

thermoelectric applications4, which have similar atomic and electronic

structures. PbTe was considered the best candidate and thus is the

Fig. 3 Overview of zT values for materials reviewed here and those in reference 1, including the n- and p-type values for PbTe reported by Fritts in 1960 for (a)

p-type and (b) n-type thermoelectric materials. The n-type PbTe1-xIx (x = 0.0012) and p-type NaxPb1-xTe (x = 0.01)23,24 are shown to have significantly higher zT

values than previously believed. Additionally, it can be seen that p-type PbSe (carrier concentration 3 × 1019 by Na doping43) is a very promising alternative to PbTe

and that Na doped alloys of PbTe1-xSex (x = 0.15)50 show extraordinary perfor mance.

(a) (b)

MT1411p526_533.indd 529 31/10/2011 14:27:52



most studied of the lead salts4,12,13,41. As a result of the growing

expense of Te there is a renewed interest in materials that do not

contain Te. Among the alternate materials is PbSe, which had been

thought to have much lower performance compared with PbTe12,20

due to its lower band gap (actually, only at low temperatures) and

the general trend that isostructural compounds composed of lighter

elements have higher κL. However, recent theoretical work and

experimental results have reported that this material may in fact

outperform PbTe at high temperatures42, 43.

When PbSe is made p-type by doping with Na it is found that

the electronic transport properties can be explained by the same

type of complex band structure as that found in Na-doped PbTe43

and this leads to substan tial zT (Fig. 3). The maximum zT achieved

in samples with hole concentrations of 3 × 1019 cm-3 is near 1.

However, a significant contribution from the presence of the heavy

hole band and a resulting peak zT > 1.2 is realized in samples having

hole concentrations even greater than 1 × 1020 cm-3. In heavily doped

(1 × 1020 cm-3) PbSe samples the large band gap at high temperatures

inhibits thermally activated minority carriers that degrade charge

carrier transport, the bipolar effect, and consequently the zT of these

samples does not appear to be approaching a maximum value at

850 K.

While the increase in Seebeck accounts considerably for the zT

values observed, the thermal conductivity of PbSe is much lower than

expected as compared to PbTe, which has a larger molar mass, and is

partially responsible for the high zT observed. The low lattice thermal

conductivity can be attributed to the larger Grüneisen parameter12

corresponding to the stronger anharmonic nature of the lattice

vibrations25.

High zT is well known in p-type AgSbTe2 and TAGS (GeTe alloy

with 15 % AgSbTe2)25,40 and is likely due to the structure, which is

similar to PbTe, along with the same reasons that make PbTe and PbSe

good thermoelectric materials, although additional factors (e.g., due to

alloying) are also present.

Band structure engineering by alloyingIncorporating additional elements into PbTe opens new opportunities

for tuning the electronic transport properties through band

structure engineering as well as providing a route to κL reductions.

The reduction of lattice thermal conductivity by alloying (such as

PbTe1-xSex) due to the scattering of phonons by point defects is

well known and was promoted by A. F. Ioffe well before 195713.

The substitution of Te with Se reduces κL by ~35 % at 300 K for

x = 0.15, but greater amounts of Se present in the material result in

a detrimental net effect due to the simultaneous reduction of the

carrier mobility caused by scattering of carriers by the additional

Se atoms.

Concurrent to the rather simple effects on κL due to the addition

of Se in PbTe, a more complicated impact on the electronic band

structure is also observed, which provides an additional avenue for

Fig. 4 (a) Schematic of energy bands in PbTe and how the bands move as temperature increases. The shaded areas schematically represent the hole population of

each band. The orange line indicates the movement of the L band as temperature increases. (b) The calculated zT value as a function of carrier concentration for

p-PbTe comparing the result of transport from the L or Σ band alone (as if the other band did not exist). It is seen that when the interaction between the two bands

is modeled, "Σ + L", the result is a significant increase of peak zT, which can only be realized at carrier concentrations twice that reported by Fritts.

(a) (b)

MT1411p526_533.indd 530 31/10/2011 14:27:54



developing high performance thermoelectric materials based on band

structure engineering.

It is straightforward to show that if it can be assumed that carrier

concentration can be tuned to optimize zT, this optimized zT value

depends on the thermoelectric quality factor, B, determined by the

lattice thermal conductivity, κL, and the electronic band structure44-47.

B ∝ μ—N—v

κL—Mb—

*3/

–2

– (1)

Here, μ is the carrier mobility, κL is the lattice thermal conductivity,

and mb* is the density-of-states effective mass of a single carrier

pocket. While μ decreases rapidly with mb*, it is clear that large

valley degeneracy (NV) is good for thermoelectric materials. Valley

degeneracy arises when multiple bands have the same energy (are

degenerate) at the band extrema (orbital degeneracy), or when there

are multiple degenerate carrier pockets in the Brillouin zone due to the

symmetry of the crystal. High symmetry crystals can have very high

valley degeneracy when the band extrema are located at low symmetry

points in the Brillouin zone48. The known good thermoelectric materials

such as Si1-xGex, Bi2Te3, and CoSb3 have NV of 6 or less. In p-type

PbTe the valence band maxima occur at the L point in the Brillouin

zone where NV is 4, while the Σ band has an exceptionally large NV

of 1212,41,49,50, which certainly contributes to its good electronic

properties and high zT at high carrier concentration. One can think

of valley degeneracy as leading to multiple (NV) pathways for charge

carriers to participate in electronic transport without altering the

Seebeck coefficient (determined by the Fermi level).

One way to increase NV, and therefore zT, is to converge different

bands which are not required to be degenerate because of orbital

or Brillouin zone symmetry. Such bands are effectively converged

when their band extrema are within a few kBT of each other (kB is

the Boltzmann constant). This concept formed the basis of early

carrier pocket engineering attempts to increase zT in low dimensional

thermoelectric materials51.

Perhaps the most dramatic demonstration of band structure

engineering to increase NV, to date, has been in p-type PbTe1-xSex

alloys where the alloying allows small, controlled manipulation of the

band energies. The relative energies of the L and Σ bands in PbTe are

temperature dependent and as the temperature increases the band

energies converge to produce a combined valley degeneracy of 16. As

Se is added to PbTe the energy difference between the L and Σ bands

increases, raises the temperature at which the band convergence

occurs, and makes the convergence effect more noticeable as the peak

zT approaches ~1.8 in bulk PbTe1-xSex.

Alloying can also be used to introduce resonant electronic states,

such as Tl in PbTe52. Additional electronic states brought by the

incorporation of resonant impurities to increase NV will increase

thermoelectric properties in the same manner as discussed above.

Resonant states can also introduce resonant scattering, which is

proposed to improve thermoelectric performance at low temperature

by a different means53.

Band convergence by alloying has a distinct advantage over

nanowires or superlattices because the high symmetry (e.g., cubic in

PbTe), and therefore high inherent NV, for each band is maintained

while low dimensional structures break this symmetry. In addition,

small concentrations of alloying elements can be used to enable fine,

precise adjustments in the band energies.

Potential for future band structure engineeringThe effectiveness of alloying to induce band convergence has only

begun to be fully appreciated in PbTe and a few other systems54,55.

The quality factor can be further developed to encourage other

strategies for band structure engineering, or even the search for entirely

new thermoelectric materials. Since virtually all good thermoelectric

materials have carrier mobility limited by acoustic phonon scattering,

the deformation potential theory15,56 can be used to show that

B ∝ T m—

I*Nv—

Ξ2– Cκ—l

L— (2)

where Cl is a combination of elastic constants57 that will be correlated

with κL, mI* is the inertial effective mass along the conduction

direction, and Ξ is the deformation potential coefficient that describes

the change in energy of the electronic band with elastic deformation.

This formula suggests that low effective masses and low deformation

should be targeted in future band structure engineering58,59. In

addition, it is also desirable to engineer increases in band gap, as long

as it does not include detrimental effects to NV, mI*,or Ξ, as it enables

higher temperature operation without the influence of minority

carriers16,45.

Ultimately, however, band structure engineering will be used to

improve the average zT (or more precisely, device zT1) for higher

overall thermoelectric efficiency rather than peak zT, which will lead to

somewhat different goals60,61.

Summary and outlook Simple binary lead telluride alloy thermoelectric materials have

demonstrated exceptional thermoelectric performances, with an

optimized peak zT of ~1.4, far exceeding the values commonly reported

since 1960. Two key aspects for high performance n- and p-type PbTe

are the use of modern thermal diffusivity measurements for thermal

conductivity characterization and optimal dopant concentration.

The large Grüneisen parameter and high valley degeneracy in p-type

materials contribute to the exceptional zT in PbTe and related IV-VI

semiconductors. These trends, understandable from the perspective

of simple semiconductor physics, can help guide other thermoelectric

material systems as well as the search for new thermoelectric

materials.

MT1411p526_533.indd 531 31/10/2011 14:27:55



Additionally, the concept of band structure engineering to achieve

band convergence is demonstrated with the exceptional peak zT

of ~1.8 in PbTe1-xSex alloys. Such a small modification of the band

structure through alloying promises to be a fruitful route to tune other

band parameters as well. Alternatively, the more dramatic changes

brought about by resonant states may provide another mechanism

for increasing the number of converged bands and would expand the

possibilities for further improvements. A summary of the materials

reviewed in this paper and the corresponding zT values is shown in

Fig. 3, as compared to other thermoelectric materials.

While alloying has proven successful in reducing lattice thermal

conductivity, other methods such as nanostructuring6,62 should lead

to further improvements, particularly at low temperatures where

boundary scattering of phonons is most effective, raising the average

zT60. However, with such strategies, it will be important to consider the

effect on other parameters that determine the thermoelectric quality

factor to ensure a net increase in the performance of the material.

Small improvements in sample homogeneity, fine tuning of the doping

and alloy concentrations, microstructure and composite control, as

well as functionally grading63 could all combine to produce a truly

optimized thermoelectric material.

Even though PbTe is one of the oldest and most studied

thermoelectric materials for power generation, recent work has

demonstrated several new possibilities that can be explored to ensure

a bright future for the further development and use of PbTe-based

thermoelectric ma terials.

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16. Parker, W., et al., J Appl Phys (1961) 32, 1679.

17. Petrov, A., In: Thermoelectric properties of Semiconductors, edited by Kutasov, V. (Consultants Bureau, New York, 1964) p. 17.

18. Devyatkova, E. and Saakyan, V., Izvestiia Akademii Nauk SSSR (1967) 2, 14.

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21. Alekseeva, G., et al., Fiz Tverd Tela (1981) 23, 2888.

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23. Pei, Y., et al., Energy Environ Sci (2011) 4, 2085.

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25. Morelli, D.T., et al., Phys Rev Lett (2008) 101, 035901.

26. Bozin, E., et al., Science (2010) 330, 1660.

27. An, J., et al., Solid State Commun (2008) 148, 417.

28. Delaire, O., et al., Nat Mater (2011) 10, 614.

29. Andreev, A. and Radinov, V., Sov Phys Semicond (1967) 1, 145.

30. Chernik, I., et al., Sov Phys Semicond (1968) 2, 645.

31. Crocker, A. and Rogers, L., J Phys Colloques (1968) C4, 129.

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47. Mahan, G. Solid State Physics (Academic Press, 1998) pp. 81–157.

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MT1411p526_533.indd 532 31/10/2011 14:27:55

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MT1411p526_533.indd 533 31/10/2011 14:27:56


The demand for clean, secure, and sustainable energy sources

has stimulated great interest in fuel cells: devices that convert

a chemical fuel directly to electricity. Among all types of fuel

cell, solid oxide fuel cells (SOFCs) represent the cleanest, most

efficient, and versatile energy conversion system1, offering the

prospect of efficient and cost effective utilization of hydrocarbon

fuels, coal gas, biomass, and other renewable fuels2,3. However,

SOFCs must be economically competitive to be commercially

viable. An effective approach to cost reduction is to drastically

reduce the operating temperature to 400 – 700 ºC, thereby

allowing the use of much less expensive materials in the

components4. Unfortunately, lowering the operating temperature

also lowers the fuel cell performance, as the electrode and

electrolyte materials become less conductive and less catalytically

active. Long term performance of SOFCs also degrades due to

poisoning of the cathode by chromium from interconnect layers5,6,

deactivation of the conventional anode by carbon deposition4-7

and poisoning by contaminants (e.g., sulfur) in the fuel gas8-12.

One of the grand challenges facing the development of a new

generation of low cost SOFCs is the creation of novel materials with

unique compositions, structures, morphologies, and architectures that

promote the fast transport of ionic and electronic defects, facilitate

rapid surface electrochemical reactions, and enhance the tolerance to

contaminants at low temperatures. In this article, we will highlight some

Solid oxide fuel cells (SOFCs) offer great prospects for the most efficient and cost-effective utilization of a wide variety of fuels. However, their commercialization hinges on the rational design of low cost materials with exceptional functionalities. This article highlights some recent progress in probing and mapping surface species and incipient phases relevant to electrode reactions using in situ Raman spectroscopy, synchrotron based x-ray analysis, and multi-scale modeling of charge and mass transport. The combination of in situ characterization and multi-scale modeling is imperative to unraveling the mechanisms of chemical and energy transformation: a vital step for the rational design of next generation SOFC materials.

Meilin Liua,b*, Matthew E. Lyncha, Kevin Blinna, Faisal M. Alamgira, and YongMan Choia,c

a Center for Innovative Fuel Cell and Battery Technologies, School of Materials Science and Engineering, Georgia Institute of Technology,

771 Ferst Drive, Atlanta, GA 30332-0245, USAb World Class University (WCU), UNIST, South Koreac Current address: Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA

* E-mail: [email protected]

Rational SOFC material design: new advances and tools

MT1411p534_547.indd 534 01/11/2011 14:33:43

Rational SOFC material design: new advances and tools REVIEW


recent progress, the remaining challenges, and future perspectives in the

modeling, simulation, and in situ characterization of SOFC materials, in

order to unravel the mechanism of electrode processes and ultimately

achieve rational design of new materials and structures using a multi-

scale computational framework rigorously validated by experiments at

each scale.

Recent progress in materials developmentThe electrolyteOxygen ion (or vacancy), proton, and mixed ion conductors have been

used for SOFCs (Fig. 1)13. As is well known, the advantages of SOFCs

based on oxygen ion conductors include the formation of H2O and

CO2 on the fuel side of the cell, which facilitates the use of carbon

containing fuels through steam (H2O) and dry (CO2) reforming.

However, the reaction products dilute the fuel. Although many

candidate oxygen ion conductors have been studied14,15, the materials

that attract the most attention include doped zirconia, ceria, and

LaGaO3. For SOFCs based on proton conductors14, the H2O will form

on the cathode side, diluting the air, not the fuel. Direct utilization of

carbon-containing fuels is no longer possible with proton-conducting

electrolytes. One prominent group of proton conductors is the

BaZr0.1Ce0.7Y0.2O3-δ (BZCY) system16, representing a good compromise

between ionic conductivity and stability. It is also reported as a mixed

ion conductor, allowing transport of both proton and oxygen ions.

In particular, the BaZr0.1Ce0.7Y0.2-xYbxO3-δ (BZCYYb) system offers

the highest ionic conductivity in the intermediate temperature range

and the ionic transference number may be tailored to some degree2.

An ideal situation is to tailor the proton and oxygen ion transference

number of the mixed ion conductor, allowing CO2 to form on the

fuel side while allowing most of the H2O to form on the air side.

Proton conductors have attracted considerable attention because of

the low activation energy for proton conduction and thus high ionic

conductivity at low temperatures. This class of mixed proton and

oxygen ion conductors holds great potential for a new generation of

low temperature SOFCs2.

The air electrode (or cathode)Cathode polarization still contributes considerably to energy loss

in SOFC operation, more so at lower operating temperatures. The

search for materials and architectures that are more active for

oxygen reduction reactions (ORR) at lower temperatures has led

to many candidate cathode materials17, including doped SrFeO318,

Ba0.5Sr0.5Co0.8Fe0.2O3-δ composites19, and Sr0.5Sm0.5CoO3-δ with

a cone-shaped microstructure20. In particular, mixed ionic and

electronic conductors (MIECs) have attracted attention because

a high ambipolar conductivity may extend the active sites

beyond the triple-phase boundaries (TPBs), thus offering better

performance than a predominantly electronic conductor such as

La1-xSrxMn O3−δ (LSM). To date, however, LSM-based composites

(> 800 °C) and La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) (< 750 °C) still remain

the most widely used cathodes for SOFC development; the adoption

of alternative cathode materials is hindered by their unproven

long-term stability and limited compatibility with electrolyte/other

cell components, especially at high temperatures required for cell

fabrication.

To counter this problem, many different types of catalytically active

cathode materials have been infiltrated into the scaffolds of electrolytes

(to form composite cathodes) followed by firing at a much lower

temperature to avoid reactions between the electrolyte and electrode

materials21. While it appears possible to make use of more active

cathode materials and to show reasonable performance in small button

cells using heavy coatings of Pt paste or mesh as a current collector,

several critical issues still remain: the long-term stability of the cathodes

is yet to be demonstrated due to degradation issues21 and the poor

conductivity for current collection could be much more severe in real

cells or stacks where the use of Pt is no longer practical.

One reliable and effective approach is to modify the surface of the

state-of-the-art cathode with a thin-film coating of a catalyst with higher

stability and catalytic activity toward ORR. One example is a cathode

consisting of a porous LSCF backbone and a thin coating of LSM22. The

LSM-infiltrated LSCF allows the use of the best properties of two different

materials: the excellent ambipolar conductivity of LSCF and the high

stability and catalytic activity of LSM. The catalyst coating could be a

porous layer of discrete particles or a dense, continuous film, as shown in

Figs. 2a and 2b, respectively. Nanoparticles of ionic conductors such as

Fig. 1 (a) Schematic of an SOFC single cell based on proton and/or oxide ion conductors and (b) a cross-sectional view (SEM micrograph) of an SOFC single cell showing the typical microstructures of a dense electrolyte and porous electrodes.

(a)

(b)

MT1411p534_547.indd 535 01/11/2011 14:33:47

REVIEW Rational SOFC material design: new advances and tools


SDC have been successfully deposited on LSCF surfaces by the infiltration

of aqueous nitrate solutions23. In contrast, dense films of LSM have

been prepared using non-aqueous solutions24, as shown in Figs. 2c-e.

The challenges lie in how to achieve a rational design of the desired

architecture and microstructure for each component, and how to fabricate

a cathode with a reduced polarization and enhanced stability at low cost.

It is not always completely clear how to correlate the electrochemical

performance quantitatively with the local structure, composition, and

morphology of surfaces and interfaces of a cathode. Most approaches

to electrode materials development tend to be very empirical in nature:

a qualitative idea is developed, an electrode is fabricated, and a test is

performed. The idea is considered a success if the performance meets

or exceeds expectations. Part of the reason for the continued role of

empiricism is the inability to establish the scientific basis for the rational

design of better cathodes with enhanced stability and activity for

oxygen reduction, which is a very hard problem. Nevertheless, rational

design provides important insights into how to achieve higher cathode

efficiencies through new architectures and new materials. Recent efforts

in this direction are discussed later in this paper.

The fuel electrode (or anode)Ni-YSZ cermet anodes are known to exhibit excellent performance in

clean hydrogen or reformed fuels; however, Ni metal is susceptible to

re-oxidation, carbon buildup (coking) in carbon-containing fuels, and

deactivation by fuel contaminants (e.g., sulfur).

To overcome these critical technical barriers to fuel flexibility,

a large number of “Ni-free” alternative anode materials have been

developed; one prominent group of which is the conducting metal oxides,

including La0.75Sr0.25Cr0.5Mn0.5O3-δ, (with a Ce0.8Gd0.2O2-δ interlayer)25,

Sr2Mg1−xMnxMoO6−δ (0 ≤ x ≤ 1)26, and doped (La,Sr)(Ti)O327,28. Indeed,

many alternative anode materials have shown much-improved tolerance

to coking and contaminant poisoning; however, they have limited

Fig. 2 A highly efficient cathode consists of an LSCF backbone having high ionic and electronic conductivity and (a) a porous layer of catalyst particles or (b) a dense

thin film of catalyst having high stability and catalytic activity toward O2 reduction (e.g., LSM), making effective use of the desirable properties of two different

materials. (c) An LSM film (40 to 50 nm thick) on an LSCF substrate, (d) LSM infiltrated into a porous LSCF cathode (TEM image) after operation at 750 °C at 0.8 V

for 900 hours, and (e) a closer view of the LSM coated LSCF grain in the LSM infiltrated LSCF cathode shown in (d). Parts b-e reprinted from22 with permission of the

Royal Society of Chemistry.

(a) (b)

(c)

(d) (e)

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physical, chemical, and thermal compatibility with the YSZ electrolyte

during fabrication at high temperatures, which severely hinders their

applicability to actual fuel cell systems.

Fabrication by infiltration of alternative electrode materials into a

scaffold of electrolyte is perceived to offer advantages for improved

structural stability and better thermal expansion matching. However,

these claims are yet to be proven by experimental results.

Recently, a very different approach was adopted to achieve better

sulfur tolerance: replacing the oxygen ion conductor YSZ in a Ni-YSZ

cermet anode with a mixed-ion conductor like BZCYYb2; the -OH

groups produced on anode surfaces by dissociative adsorption of water

greatly facilitate oxidation of H2S to SO2 (thus removing sulfur from

anode surfaces) and in situ reformation of carbon-containing fuels

(thus minimizing coking) under typical SOFC operating conditions. This

Ni-BZCYYb cermet anode showed superior sulfur tolerance at 750 °C for

up to ~20 ppm H2S using a cell based on a dense BZCYYb electrolyte

and up to ~50 ppm H2S using a cell based on Sm-doped ceria (SDC)

dense electrolyte, suggesting that the critical pH2S/pH2 values are two

to three orders of magnitude higher than that for a conventional Ni-YSZ

cermet anode under similar conditions10,29. The sulfur tolerance exhibited

is also significantly better than the Ni-YSZ anodes with YSZ replaced by

other oxygen ion conductors of higher conductivity such as Gd-doped

ceria (GDC) and Sc-stabilized zirconia (ScSZ)30.

Among all anode materials ever studied, a composite anode

consisting of Ni and the electrolyte still represents the state-of-the-art

for anode-supported SOFCs because of the excellent catalytic activity

for hydrogen oxidation, electrical conductivity for current collection,

and compatibility with the electrolyte. One effective approach to

making these composite anodes contaminant-tolerant is to modify

the Ni-electrolyte surface by particles of catalysts that may promote

the removal of contaminants (e.g., carbon or sulfur) while maintaining

the unique properties of Ni required for high performance. As reported

recently3, small amounts of BaO spread over the surface of Ni grains

in a Ni-BZCYYb cermet anode during processing at high temperatures

may play a vital role in achieving the observed sulfur tolerance. In

fact, when nano-sized BaO islands were created on the surface of the

Ni grains in a Ni-YSZ cermet anode using a vapor phase deposition

(Fig. 3), the resistance to coking was dramatically enhanced3. The

nanostructured BaO/Ni interfaces seem very efficient for a water-

mediated carbon removal. They also showed good sulfur tolerance

while maintaining high performance.

Challenges in the rational design of materialsTo date the development of new materials or structures has been

guided largely by experience and chemical/physical intuition rather

than by scientific theories or models, because the mechanisms of

many charge and mass transport processes associated with fuel

cell operation are still lacking. In this section we outline the main

challenges we are facing in unraveling the mechanisms of electrode

processes and the nature of the rate-limiting step, using both

computational and experimental approaches, in order to control the

rate of electrode processes or to achieve the rational design of better

materials by changing the composition, structure, and morphology of

materials.

Fig. 3 Microanalysis of a BaO-modified Ni-YSZ anode: (a) top view (SEM image) showing the nano-islands of BaO on a Ni grain, (b) a cross-sectional view (bright-field

TEM image) of a BaO/Ni interface, (c) HRTEM image of the BaO/Ni interface (the [112–

] zone axis of the Ni under the BaO island is perpendicular to the page), and

(d) Fourier-filtered [112–

] zone axis image of the Ni under the BaO island. Adapted from3 by permission from Macmillan Publishers Ltd: Nature Communications, ©2011.

(a)

(b)

(c) (d)

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Well-designed cells for electrochemical measurementsThe rates of many chemical and energy transformation processes are

limited by the charge and mass transfer along surfaces and across

interfaces. A fundamental understanding of these processes, especially

the rate-limiting step, is vital to enhancing electrode performance.

Electrochemical measurement is an effective technique for

quantifying the performance of a cell or a cell component, offering

phenomenological parameters such as charge or mass transfer

resistance or area specific resistance (ASR), exchange current density,

and transference numbers. These parameters are helpful when

predicting performance using continuum models. However, they may

not represent the intrinsic catalytic properties of an electrode material

because they may be influenced by many factors which are difficult

to control experimentally, including the geometry, microstructure,

and transport properties of the electrode as well as its physical and

chemical compatibility with the electrolyte.

To preclude the effect of extrinsic factors (e.g., geometry and

microstructure), a test cell platform with well-controlled geometry

must be used31. One example is a dense or patterned film electrode

of the material to be examined, which functions as the working

electrode, as schematically shown in Figs. 4a and b, respectively22.

The secondary current collection layer, the MIEC layer in Fig. 4a, is

sufficiently thick to alleviate sheet resistance and therefore does not

require a densely packed metal current collector, allowing exposure of

the working electrode surface for other in situ characterization. This

cell is ideally suited for characterization of the intrinsic properties

of an electrode material (the top layer), which is also open to other

in situ characterization such as Raman spectroscopy and scanning

probe microscopy. When dense, the working electrode material must

have some degree of mixed conductivity (poor ionic conductivity

may be mitigated by reducing the thickness). To minimize the sheet

resistance effect of a thin-film electrode, an MIEC of high ambipolar

conductivity has to be used as the current collector, which can be

a homogenous (like LSCF) or a composite (like YSZ-LSM or Ni-YSZ,

Fig. 4c) MIEC. Another advantage of this cell design is that only the

top surface is active for electrochemical reactions, which is open to

other simultaneous in situ measurements such as Raman spectroscopy

or x-ray analysis due to the underlying current collection layer and,

Fig. 4 (a) Schematic of a test cell with well controlled geometry: an MIEC film (current collector, the 2nd layer) coated with a dense film of a second cathode material

(working electrode, the top layer), both deposited onto an electrolyte layer with a highly active counter electrode opposite. (b) A top and cross-sectional view of a

test cell with a patterned electrode. Reproduced with permission from33. © 2008, The Electrochemical Society. (c) A schematic and an SEM image of a composite MIEC

consisting of two phases (e.g., Ni-YSZ), which can be used as the current collector (as the 2nd layer) or be characterized (as the top layer) in the test cell.

(a)

(b)

(c)

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therefore, lack of interference by bulky metal mesh or even metal

paste.

To further isolate certain charge or mass transfer step(s) of the

electrode reactions, continuum models have been developed for

the careful design of cell geometry (e.g., the thickness of MIEC film)

to minimize the sheet-resistance effect in the thin-film working

electrodes32. Models have also been developed for the design of cells

with patterned electrodes for direct correlation between performance

and electrode geometry (e.g., thickness, TPB length) to minimize

experimental errors33. These cell designs are instrumental for the

reliable determination of the intrinsic properties of electrode materials.

The use of well-designed cells is vital for the collection of useful

information, since the interpretation of data becomes difficult for cells

with complicated porous electrodes. For example, while it is possible

to achieve satisfactory or even perfect curve fitting to impedance data

involving intricate charge and mass transfer processes with similar

relaxation time constants, there is no way of knowing if the assumed

number of processes adequately reflects what is occurring in the cell,

if the proposed equivalent circuits are meaningful, or if the assigned

values for circuit elements are valid, let alone the errors associated

with the de-convolution of the overlapping data. When complications

occur in data interpretation, the best solution is to simplify the cell

for electrochemical measurements by isolating the response from a

cell component or separating charge from mass transfer, as discussed

earlier. It is noted, however, that some difference may exist between

bulk properties of a porous material and those of the material as a

thin film34,35. Careful validation is necessary to make well designed

test cells a valuable tool for the evaluation of fundamental electrode

properties.

However, an electrochemical measurement cannot provide direct

information for identifying the chemical species involved in electrode

reactions or changes in electrode materials under the test conditions.

In situ characterization techniques can be performed alongside

electrochemical measurements, including Raman spectroscopy and

AFM (see Fig. 4a). Probing and mapping the evolution of surface

composition and structure or incipient new phase formation on

electrode surfaces relevant to electrode reactions under practical

fuel cell operating conditions may provide critical insights into the

Fig. 5 (a) Optical micrograph of a patterned Ni electrode (lighter area) on YSZ after exposure to CH4 at 625 °C for 12h. (b) Raman intensity map for a 1580 cm-1

Raman shift (carbon G-band) of the same area shown in (a). The detailed experimental arrangement for the Raman measurement is described elsewhere10.

(c) Comparison of coking behavior between bare a Ni mesh electrode and BaO-modified electrode using Raman spectromicroscopy.

(a)

(b)

(c)

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mechanisms of these reactions, which are vital to achieving rational

design of better electrodes, catalysts, and interfaces.

In situ Raman spectroscopy Since a number of chemical and electrochemical reactions limit SOFC

performance, a detailed knowledge of the surface species involved

in those processes is vital to the design of new SOFC materials and

microstructures. Among the relatively few in situ surface analysis

methods (FTIR, Raman, EXAFS, and small-angle x-ray scattering)36,

Raman spectroscopy is the most flexible in terms of having the largest

window of operating conditions and the greatest range of surface

species that can be probed and mapped, especially oxygen37,38,

sulfur10,39,40, carbon9,41,42, hydrocarbons43, and water2,3. Raman

spectroscopy can be used in situ (and ex situ) alongside electrochemical

measurements to probe and map surface species (e.g., reaction

intermediates) and chemical phases relevant to the electrode reactions

under practical fuel cell operating conditions, allowing the direct

correlation of electrochemical performance to surface chemistry and

structure in the electrochemical environment of an operating cell.

A typical arrangement for in situ Raman analysis of SOFC electrode

materials is described elsewhere10.

A multitude of research efforts in recent years have led to marked

progress in the development of Raman spectroscopy as an effective

tool for studying SOFC materials, including the investigation of sulfur

poisoning on Ni-based anodes39,40, chromium poisoning of LSM

cathodes in cells with metallic interconnects6, and the oxidation state

of GDC electrolytes under fuel cell atmospheres44. Studies of coking

on SOFC anodes operating on carbon-containing fuels have garnered

attention from researchers in this area due to the reasonable sensitivity

of Raman spectroscopy to carbon species9,41,42,45-47. Walker’s group

was the first to develop and demonstrate a Raman microscope system

for in situ studies of coking on Ni-YSZ anodes45, and this methodology

was adapted for similar investigations by others42,46,47.

More recently, efforts have been devoted towards vastly improving

spatial resolution in these types of studies41. For example, Fig. 5a

shows an optical image of a patterned Ni strip electrode deposited

by PVD on YSZ that was exposed to CH4 at 625 °C for 12 hours,

while Fig. 5b shows an intensity map of the same area for a 1580

cm-1 Raman shift, corresponding to the carbon G-band. The intensity

map very clearly resolves the Ni electrode, on which the coking

should preferentially occur. Additionally, Raman spectromicroscopy

has been used to characterize coking resistance conveyed by surface

modifications to Ni electrodes. Fig. 5c shows a comparison of coking

behavior between a plain Ni mesh electrode and one that has been

modified with BaO (see above), the latter of which shows no coking

even after 16 hours of exposure to C3H8 at 625 °C under OCV

conditions due to BaO islands on the surface, of which clusters are

visible in the micrograph. The spectra shown were collected in situ

under treatment conditions.

In the future, Raman spectroscopy will provide critical insights into

the pathway, sequence, and mechanism of reactions occurring on

SOFC electrode surfaces under electrochemically polarized conditions.

The presence of specific chemical species on electrode surfaces will be

correlated with impedance spectroscopy data under various operating

conditions to understand how different species impact fuel cell

performance. A profound understanding of the detailed electrode reaction

mechanism and knowing which step affects cell performance the most will

guide the development of new electrode materials and microstructures.

While Raman spectroscopy has been successfully used to probe

and map carbon, sulfur, Cr-containing phases, water, and oxygen

species, many challenges still remain in pin-pointing important reaction

mechanisms relevant to fuel cell operation. These obstacles include a

lack of sensitivity for probing and mapping the earliest stages of phase

formation related to coking and sulfur poisoning as well as the limited

amount of signal from oxygen species on cathode surfaces, due to the

small Raman cross-sections of those species. In particular, for common

cathode materials like LSM and LSCF, a high tendency for fluorescence

necessitates the use of a lower excitation power, diminishing the useful

Raman signal. Additionally, the high temperatures associated with

in situ testing further decrease the sensitivity and may lead to a shifting

of the characteristic Raman bands. To overcome these difficulties,

surface-enhanced Raman scattering (SERS) methods applicable to SOFC

materials are being developed to increase sensitivity48. In addition,

Raman spectroscopy integrated with scanning probe methods, which

can allow for tip-enhanced Raman spectroscopy (TERS) can potentially

increase both sensitivity and spatial resolution49.

In situ synchrotron-based XRD/XAS Synchrotron-based x-ray techniques provide yet another methodology

for in situ investigations of SOFC materials. When acting as in situ

Fig. 6 A schematic arrangement for an in situ synchrotron-based XAS study of SOFC materials under conditions similar to fuel cell operation alongside electrochemical measurements.

MT1411p534_547.indd 540 01/11/2011 14:34:03



structure probes for SOFC materials, x-ray absorption spectroscopy

(XAS) is excellent in probing the local atomistic and electronic

structure, as shown schematically in Fig. 6, while x-ray diffraction

(XRD) is a powerful tool for eliciting lattice or long-range order. Often,

the structures of SOFC materials may change due to a chemical,

electrochemical, and/or thermal effect under typical operating

conditions. The atom specificity of XAS makes the technique very

powerful for characterizing electrode reaction mechanisms. Since the

near-edge structure of XAS data (XANES) probes the chemical state

of a particular elemental component while the extended fine structure

(EXAFS) describes its local atomic environment, combining XANES and

EXAFS of multiple atomic species over an SOFC reaction can be used

to create a reaction roadmap. At the same time, XRD can identify the

crystalline phases involved in the reactions and draw out the phase

transformation windows in temperature, PO2, or other external stimuli.

XAS can be performed in multiple modes simultaneously, barring

constraints of sample geometry and sample chamber pressure and

temperature. Transmission mode XAS measurements at hard x-ray

energies are forgiving experiments to set up under in situ conditions

and provide access to the K-edges of 3d transition metals, Sr,

and Y, and the L3-edges of Ba, La, and Ta. Fluorescence yield (FY)

measurements here are particularly useful when the element being

probed is a dilute component of the material (e.g., the dopants).

Near-surface electronic structure measurements, using total or

partial electron yield (TEY/PEY), can be made simultaneously with

corresponding bulk measurements using fluorescence yield (FY) or

direct transmission, and provide contrast between surface and bulk

phenomena. As described earlier, XANES provides the chemical state

of a species, but to be more specific, it provides a spectral function

that is proportional to the unoccupied densities of state local to that

species. In the commonly found transition metals with octahedral

symmetry in SOFC materials, for example, the oxygen K-edge or the

metal L-edge will show the split t2g and eg LUMO orbitals (provided

they have empty states). Fig. 7 shows the deconvoluted oxygen

K-edge (transition between the oxygen 1s initial state and LUMO final

states with p-symmetry) in LSF with and without Ti and Ta doping50.

After deconvoluting the oxygen K-edge prepeaks into individual

t2g and eg spin up and spin down bands, Braun et al. discovered a

correlation between a ratio of peak heights, the conductivity, and

the Fe4+ /Fe3+ ratio. Also shown in Fig. 7 is the Mn L-edge data

(Mn 2p electrons excited to Mn 3d final states) in LSM, clearly showing

the contributions of three types of Mn ion51.

Fig. 7 (a-d) Deconvoluted oxygen 1s spectra with a pre-peak near EF for LSF and LSF doped with Ti (20 mol.%) and Ta (10 and 20 mol.%). Numerals indicate

peak position and spectral weight. Reproduced from50 with permission of the American Institute of Physics. (e) Mn L3-edge XAS of LSM as-processed (blue), SOFC

environment exposed (black), and activated (red) films. The Mn charge-state multiplet line shape peak energies are plotted as vertical lines. Reproduced with

permission from51. © 2011, The Electrochemical Society.

(a) (b)

(c) (d)

(e)

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Transmission in situ XAS can be carried out in a proper furnace at

reaction temperature and PO2 like the one at beamline X18B of the

National Synchrotron Light Source where multiple samples can be

placed in a carousel and exposed to the same reaction conditions.

An example of in situ XANES of SOFCs can be seen in the work of

Yildiz and co-workers who studied the activation of La0.8Sr0.2MnO3

and La0.8Ca0.2MnO3 on single crystal YSZ electrolytes using in situ

XANES (Mn K-edge and La L2-edge and x-ray reflectivity)52. For

enhanced surface sensitivity under in situ conditions, a grazing

incidence FY geometry can be used, such as the one demonstrated by

Shinoda et al.53.

There are currently a handful of examples for the use of in situ

XRD for structural characterization of SOFC materials. Liu et al.

studied phase and strain distributions associated with reactive

contaminants in SOFCs54. Using in situ measurements, Shultz et al.

characterized the dissociation and crystallization of the amorphous

precursor powders of lanthanum strontium gallates55. More recently,

Hashimoto et al. investigated the changes in the lattice parameters

of La0.6Sr0.4Co1−yFeyO3−δ (y = 0.2, 0.4, 0.6, and 0.8)56. Further, a

combination of both XAS and XRD would be a very powerful approach

to probing local and long-range atomistic structures of SOFC materials

under practical fuel cell operating conditions, offering structural

information that has never before been accessible.

Prediction of intrinsic properties of materials Proper modeling techniques and prediction tools are essential to a

comprehensive understanding of SOFC materials. For example, density

functional theory (DFT)57 is an effective computational framework for

prediction of electronic structures and other fundamental properties

of materials58-60. In particular, it has provided detailed, molecular-

level information that may not be readily obtained experimentally,

including probable electrode reaction sequence, mechanisms, and

stable intermediates, surface coverage of adsorption sites, mobility of

electro-active species along surfaces, and rate constants for adsorption/

dissociation, reduction/oxidation, and incorporation/release of species

at surface sites. For example, the significance of the MnO2-terminated

(001) surface of LaMnO3 was reported61 and, in conjunction with

molecular dynamics (MD) simulations62 and kinetic theory63, several

properties of LaMnO3-based materials were examined for SOFC

applications62-64 including reaction sequences and charge transfer

mechanism. Recently, an ab initio thermodynamic approach was used

to examine oxygen reduction on SOFC cathodes.65,66 Fig. 8 depicts a

molecular-level computational screening based on surface and bulk

properties to search for cathode materials of higher performance than

conventional ones67. DFT calculations were also used to design sulfur-

and carbon-tolerant anode materials59,68-70. The interactions of sulfur-

containing compounds (e.g., H2S) with anode materials under SOFC

operating conditions were predicted using ab initio thermodynamics12;

providing important insights into the mechanism of sulfur poisoning.

Recently, a water-mediated removal of carbon species from nano-

structured BaO/Ni interfaces was modeled successfully (Fig. 9)3.

Furthermore, the predicted vibrational frequencies of surface species and

new phases were confirmed using Raman spectroscopy71.

Fig. 9 DFT-predicted energy profile for removing chemisorbed carbon species

on BaO/Ni(111) interfaces through a water mediated process, where *

denotes an adsorbed species on the surface. Adapted from3 by permission from

Macmillan Publishers Ltd, © 2011.

Fig. 8 (a) Illustration of a bulk La0.5Sr0.5MnO2.75 (110) structure before and

after ionic conduction. V and ON (N = 1 or 2) are, respectively, an oxygen

vacancy and the nearest neighboring oxygen to V. (b) Trajectory of oxygen ion

conduction through La0.5Sr0.5MnO2.75 (110). O1 and O2 are the initial and

final states of the oxygen ion conduction. Reprinted from67 with permission

from Elsevier, © 2011.

(a)

(b)

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While DFT-based calculations have been instrumental in gaining

critical insights into intrinsic properties of many materials72, new

methodologies must be developed to bridge the gap between

theoretical predictions and experimental measurements, notably for the

materials with open shells of d or f electrons commonly encountered in

SOFC systems60,73. For example, the DFT+U theory has been proposed

to mitigate the limitations of conventional DFT for these materials.

Further, time-dependant DFT60 has also been applied to the analyses of

x-ray absorption spectra of SOFC materials.

On the other hand, DFT calculations become increasingly more

difficult for more complex material systems, such as the BZCYYb

electrolyte, the LSCF cathode, and LSTC anode materials, due

partially to the large number of atoms/ions (or potential reactive

sites) that must be considered in the calculations. It is still a grand

challenge to identify approximate descriptors for the computational

design of better SOFC materials, something similar to the d band

model applied to the characterization of the oxygen reduction reactions

(ORR) in low temperature fuel cells. Moreover, electrochemical hot

spots, such as the heterogeneous boundaries and junctures in a porous

electrode (e.g., TPB), are even more difficult to predict using DFT-based

calculations. The quantum mechanics/molecular mechanics (QM/MM)

methodology74 may help model reactions at or near the TPB region.

To link the intrinsic properties of a material with its performance

in a fuel cell, however, DFT/MD simulations must be combined with

continuum modeling that can predict the phenomenological behavior

of materials, which can be confirmed directly by experiments.

Continuum modeling Coupled continuum and phenomenological models can be used to

predict experimentally measurable parameters, such as exchange

current density and ASR of a cell, providing a means to evaluate

response over larger length scales than those available to DFT/MD

simulations. Phenomenological models provide a means to understand

the rates of charge and mass transfer processes in detail and predict

them in a variety of circumstances. The rate expressions serve as

boundary conditions for continuum models. Fig. 10 illustrates the

interdependence of models at different length and time scales (from

DFT to continuum) for the rational study of SOFC materials.

Once phenomenological parameters are known, these models in

turn can predict the behavior of materials or the performance of fuel

cells under various conditions. For example, continuum models have

been successfully applied to various aspects of electrode operation75,

from the global response of porous mixed conducting electrodes76,77,

to the performance of heterogeneous composite electrodes described

using particle/resistor networks78 and homogenized treatment79,

and to detailed reaction rates or intermediate steps of surface and

interfacial processes80.

Further, continuum models can link DFT/MD calculations indirectly

to experimental measurements, thus providing a means of verifying

their predictions. For example, continuum models can be used to

predict performance of fuel cells from materials properties derived from

DFT/MD simulations, including the molecular level reaction sequence,

rate-limiting steps, detailed defect structures, surface structures,

and phenomenological parameters (e.g., surface adsorbed oxygen

concentration73), some of which may not be readily accessible from

experiments. Phenomenological/continuum models guided by DFT/MD

can then be compared to experimental results to verify and/or refine

these calculations.

Moreover, phenomenological/continuum models conformal to

electrode geometry have been successfully used with cells consisting

of thin-film/patterned electrodes. On the simplest level, they help to

quantify the characteristic activity under the framework of linearized

parameters, such as the length-normalized resistance to oxygen

reduction at the TPB81 or the area-normalized equivalent circuit

parameters of the bulk pathway82. A model conformal to electrode

geometry is required to examine the effect of the microstructure on

the performance of a porous electrode. Conformal models have been

successfully coupled with phenomenological models to explain83 and

help mitigate32 sheet resistance observed in experimental patterned

electrode results, thus providing guidelines for the better design

of test cells and for the proper interpretation of electrochemical

measurements84. As described in detail elsewhere32,33, these models

have been used to predict potential and defect distribution in a thin-

film working electrode with current collectors of different geometries

(strips, grids, and circular pads), the critical spacing between current

collectors to minimize the effect of sheet resistance on performance,

and the relative contributions from competing bulk and TPB pathways

of a patterned electrode under cathodic polarization. Recently,

phenomenological modeling was used together with DFT simulation

predictions and TEM analysis to examine the trend of the ASR of

uncoated and LSM-coated LSCF electrodes under various cathodic

polarizations22.

The reactions on both cathode and anode are quite complex, often

involving adsorption, dissociation, and reduction/oxidation of gas

molecules, transport of adsorbed surface species, and participation of

point defects (e.g., oxygen vacancies and electrons/electron holes). An

electrochemical driving force may not only alter the concentrations

of surface species within a mixed conductor but also change their

energies85. Further, the composition and structure of an active

electrode surface may be different from those of the bulk phase due

to surface elemental segregation86. The linking of electrochemical

response with detailed surface properties and reaction mechanisms,

therefore, is quite a challenge and continues to be an important

research pursuit.

Quantification of the microstructure effectThe performance of an electrode is determined critically by the porous

3D microstructure. Important factors include the exposed catalyst

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surface area, facility of gas transport through pores, resistance to ionic

and electronic transport through solid phase, and length of TPB lines.

The complexity of mass and charge transport in just the solid phase

is illustrated in Figs. 11a,b. Many strategies have been employed to

optimize the microstructure, including the formation of composite

electrodes, functionally graded microstructures, and infiltration of active

electrode phases on electrolyte scaffolds. Optimization of the electrodes

is a very difficult task because many of the important features compete

with one another; for example, surface area may increase at the expense

of gas-phase diffusion.

Modeling on an electrode level75,87 is useful for understanding

performance. In particular, equivalent circuit models78,88 describe the

performance based upon linearized parameters. Another model type

for mixed ionic-electronic conducting electrodes is based upon porous

electrode theory76 and uses homogenized microstructural parameters

and linear irreversible thermodynamics in reaction rates. Recent results

show that it is reasonable for many MIECs, but not for those where

diffusion length is on the order of particle size89, because they neglect

the fine details of the microstructure and can lack detailed predictive

capability.

3D reconstruction by FIB/SEM90,91 and phase-sensitive x-ray computed

tomography92 is a recent and promising development, providing high-

resolution microstructural details of porous electrodes. This information

has been used in homogenized models for performance prediction.

Recently, researchers also began to use the 3D reconstructions

as the domain for electrochemical simulations using the Lattice-

Boltzmann method93,94 or the finite element method95. The former

has been applied primarily in the anode using models developed for

nickel patterned electrodes, gas diffusion, and ionic transport. The latter

has been applied to an LSCF cathode using effective linear irreversible

thermodynamic parameters (not detailed reaction rates) based on

surface exchange and tracer diffusion coefficients. We have developed

FEM models for the simulation of the electrochemical response of

3D porous electrodes reconstructed by x-ray computed tomography92;

the preliminary results are shown in Fig. 11c.96

While simulations conformal to the reconstructed electrode

microstructures constitute a powerful computational framework,

some challenges still remain. First, the 3D reconstructions may require

extensive and skilled FIB/SEM or synchrotron work. Second, simulations

deployed on the actual porous structures require sophisticated

numerical methods and, depending on the complexity of phenomena

modeled, can require complicated constitutive equations and/or

parameter determination. All are the subject of current research within

the field.

Such simulations can corroborate the accuracy of homogenized

models and, when homogenized models break down, provide the

most accurate and detailed means of simulation. The detailed

microstructure may also be able to act as a sort of well-defined

electrode in and of itself: the a priori digital representation of explicit

microstructural geometry might allow fundamental study. The

ultimate goal is to use the 3D geometry for numerical simulation

of electrode performance in engineering design, in conjunction with

Fig. 10 Interdependence of models at different length and time scale and characterization techniques for the rational study and design of SOFC materials.


MT1411p534_547.indd 544 01/11/2011 14:34:10



detailed, mechanistic, nonlinear phenomenological rate expressions

serving as boundary conditions97 and informed by parameters derived

from patterned or porous electrodes.

New directions and future perspectives One important direction is to exploit nanostructures and nano-

architectures derived from a variety of templates in order to transcend

some of the difficulties facing materials development for energy

applications98. In particular, hierarchical 3D porous architectures99

may dramatically enhance the rates of change and mass transfer

processes while improving the mechanical integrity and robustness.

These nanostructured electrodes and interfaces are known for increased

numbers of active sites, reduced length of ion diffusion to active sites,

and greater flexibility in surface modification for catalysis and electro-

catalysis98,100.

Another important new direction is to develop a predictive

multi-scale (from DFT to continuum) computational framework,

through rigorous validation at each scale by carefully designed

experiments under in situ conditions, for the rational design of

better materials and structures for a new generation of SOFCs to

be powered by readily available fuels. While significant progress has

been made in developing SOFC materials in probing and mapping

electrode surface species relevant to electrode processes, and in

unraveling some of the mechanisms of the electrode processes,

many challenges still remain to bridge the gaps between models at

different scales or between theoretical predictions and experimental

observations10. Only when the detailed mechanisms of the rate-

limiting steps are clearly understood will it be possible to rationally

design better materials.

It is vital to perform well designed experiments at each scale under

in situ conditions in order to validate and perfect the predictability of

the individual models at different scales. It is still a grand challenge

to link the global performance or functionality of a 3D porous

electrode with the local structure, composition, and morphology of

nanostructured surfaces and interfaces. Validation and integration of

information collected from different scales are critical to developing a

computational framework across multiple scales for the rational design

of materials with exceptional functionality.

AcknowledgmentsThis material is based upon work supported as part of the HeteroFoaM

Center, an Energy Frontier Research Center funded by the U.S.

Department of Energy (DOE), Office of Science, Office of Basic

Energy Sciences (BES) under Award Number DE-SC0001061. Partial

support from the World Class University (WCU) program, UNIST, South

Korea, is also acknowledged. The authors wish to thank Prof. Wilson

Chiu, Dr. George Nelson, William Harris, and Jeffrey Lombardo at

the University of Connecticut for x-ray tomography data used for the

simulation shown in Fig. 11c.

Fig. 11 Schematic diagram of charge and mass transport within and on the

surface of (a) a single-phase mixed conducting porous electrode and (b) a

composite (mixed conductor + electrolyte) porous electrode. (c) Initial 3D

FEM simulation of adsorbed oxygen species on the surface and at the TPBs of a

porous LSM electrode96.

(a)

(b)

(c)

MT1411p534_547.indd 545 01/11/2011 14:34:12



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Ionic transport and electrochemical transformations underpin a

broad variety of modern energy and information technologies.

The classical examples include primary and secondary batteries

ranging from centuries-old Volta piles1 and Leclanche elements2 to

modern Li-ion and flow batteries that hold the promise of viable

hybrid and electric vehicle technologies and grid-level storage3 ,4 .

Ionic phenomena are at the core of the operation of solid oxide

and polymer electrolyte fuel cells, which offer some of the

highest efficiencies of fuel-to-energy conversion5-7. Finally, the

development of secondary metal-air and metal-water batteries

will potentially open the pathway for energy storage at densities

comparable to fossil fuels8.

Equally important are ionic phenomena in many areas of condensed

matter physics. Recent examples include electroresistive and

memristive electronic devices as components of non-volatile storage

and neuromorphic logic9,10. Similarly, ionic effects can play a significant

and potentially definitive role in the functionality of molecular

electronic devices11, strongly affect the piezoresistance12, induce

ferroelectric-like dielectric behaviors13, contribute to ferroelectric

resistive switching14, and couple to other physical phenomena in

nanoscale oxides. Finally, ionic phenomena are an integral part of

the long-term degradation phenomena in ferroelectrics and dielectric

materials15-17, and hence are directly relevant to the optimization and

implementation of oxide electronic devices.

Progress in the development and optimization of energy storage and conversion materials necessitates understanding their ionic and electrochemical functionality on the nanometer scale of single grain clusters, grains, or extended defects. Classical electrochemical strategies based on Faradaic current detection are fundamentally limited on the nanoscale. Here, we review principles and recent applications of electrochemical strain microscopy (ESM), a scanning probe microscopy (SPM) technique utilizing intrinsic coupling between ionic phenomena and molar volumes. ESM imaging, as well as time and voltage spectroscopies, are illustrated for several Li-ion cathode and anode materials. Finally, perspectives for future ESM developments and applications to other ionic systems are discussed.

Sergei Kalinina,* Nina Balkea, Stephen Jessea, Alexander Tseleva, Amit Kumara, Thomas M. Arrudaa, Senli Guoa, and Roger Prokschb

aThe Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USAbAsylum Research Corporation, Santa Barbara, CA, USA

*E-mail: [email protected]

Li-ion dynamics and reactivity on the nanoscale

MT1411p548_559.indd 548 31/10/2011 14:40:31

Li-ion dynamics and reactivity on the nanoscale REVIEW


Progress in these applications requires probing electrochemical and

ionic phenomena on the nanometer scale, establishing the origins of

observed physical behaviors, and linking macroscopic device or material

functionality and advanced theoretical studies18. However, while

such an understanding was developed for, e.g., semiconductor and

structural materials leading to exponential technology development

(Moore’s law), an understanding of the electrochemical processes in

solids and at solid-gas/liquid interfaces remains elusive.

This dearth of information is related to two factors, namely

the complexity of ionic systems and the very small length scales

of relevant interactions. For example, operation of battery or fuel

cell devices will involve stages of simultaneous electronic and

ionic transport (possibly mediated by the presence of electrolytes,

percolating and non-percolating diffusion paths, second phase

inclusions, and conductive dopants) in cathodes and anodes, chemical

reactions at the interfaces with solid or liquid electrolytes, and

ionic transport in electrolytes3,4. The electrochemical reactions and

intercalation processes in solid components are typically associated

with significant volume change effects that can lead to large (and

poorly understood) strains and, potentially, failure of the material. Also

significant, and poorly understood, are irreversible processes associated

with the formation stages of the device (i.e., initial operational cycle),

including solid-electrolyte interphase (SEI) formation in batteries19-21

and electroforming in memristors and electroresistive materials22,23.

Complimentary to this extreme complexity are the small length

scales of relevant physical phenomena and materials morphologies.

For example, modern Li-ion battery cathodes are formed by multilevel

assemblies, with the characteristic sizes of particles of the order

of 50 – 200 nm24,25. Similarly, the active zone of the TiO2 based

memristor device is often a nanometer-scale filament formed by

conductive Magneli phases26. More generally, it is well recognized that

the functionality of solids is controlled by atomic- and nanometer scale

defects that act as nucleation centers for new phases, pinning centers

for moving transformation fronts, etc. Defects and defect-mediated

functionality thus play a universally important role in virtually all phase

and chemical transformations, including those in energy systems.

Characterization of these phenomena on the level of a single

morphological or structural element and eventually, single defects

(or defect-free segment of material), requires extending classical

electroche mi cal strategies27,28 to the nanometer scale. While clearly

a challenge, recent progress in several areas of nanoscale science

suggests that such developments are possible once the proper tools

are developed. As a comparative example, the thermodynamics and

kinetics of macromolecular reactions have become accessible on a

singl e molecule level29-31, as a result of development of molecular

unfolding spectroscopy. Similarly, the development of Piezoresponse

Force Microscopy (PFM) and associated spectroscopic techniques

have allowed the exploration and control of polarization switching in

ferroelectric and mu ltiferroic materials at a single defect level32-34.

These examples suggest that comparable progress can be achieved in

all areas related to ionic and electrochemical behavior in solids once

proper nanoscale characterization tools are developed.

However, classical electrochemical methods based on the detection

of Faradaic currents offer very stringent limitations on the minimum

amount of material that can be probed. This necessitates the

development of alternative strategies to probe local electrochemical

functionality. In this review, we summarize the principles and

applications of electrochemical strain microscopy (ESM), a novel

scanning probe microscopy (SPM) method specifically aimed at probing

electrochemical and ionic phenomena in solids on the nanometer scale.

Scanning probe microscopy in electrochemistry: current and strain detectionProbing electrochemical processes by scanning probe microscopy

brings the dual challenge of inducing electrochemical processes below

the tip and detecting the associated changes in materials, e.g., changes

in the ionic concentration, size of the nucleated second phase, etc.

Tip-induced electrochemical reactions have been reported since the

very dawn of SPM, usually in the context of (highly undesirable)

processes that interfere with SPM imaging35,36. In the 1990s, it was

recognized that local electrochemical reactions can be used as a

basis for nanofabrication in the nano-oxidation of semiconductors

and metals37,38, electromachining, or deposition of carbon39,

semiconductors40, or metals41. In all these cases, the presence and

extent of (thermo)electrochemical processes can be established from

the changes in surface topography readily accessible by SPM in situ or

post mortem, or using spatially-resolved chemical analysis (e.g., micro

Raman).

The measurements of reversible ionic and electrochemical processes

necessitate the detection of transient signals directly during SPM imaging

or spectrum acquisition. One such approach utilizes the local surface

potential with variants of kelvin probe force microscopy42 or electrostatic

force microscopy43 following the application of bias pulses to the probe.

The relaxation of induced surface charge directly coupled to the local

ion concentration yie lds information on ion dynamics44-46. Schirmeisen

and his colleagues47,48 explored the direct relaxation of the electrostatic

force microscopy (rEFM) signal, extending the detection limit to the

millisecond range. Similar time resolved spectroscopies are actively being

pursued by the Ginger group for mapping light-induced phenomena in

photovoltaic systems49. However, the primary limitation of this approach

is only an indirect link between the potential and ionic concentration (e.g.,

the uncompensated injected charges are expected to affect electrostatic

forces much stronger then weak changes in the work function induced by

changes of ionic concentration while maintaining electroneutrality).

The alternative strategy for detecting bias-induced transformations

under the tip is direct detection of the electronic current by an SPM probe.

For materials with electronic and mixed electronic-ionic conductivities

the detected current will be dominated by electronic currents and contain

MT1411p548_559.indd 549 31/10/2011 14:40:34

REVIEW Li-ion dynamics and reactivity on the nanoscale


only indirect information on the electrochemical transformations50-53.

For materials with purely ionic conductivity (transference number t = 1),

the electronic current flowing though the (metallic) tip is balanced by

the ionic current though the material, as illustrated, for example, for

proton-conducting Nafion membranes by Burrato54 and molten salts

by Haile55. However, the limitations of this approach stem directly from

the sensitivity limits of current AFM. For example, a ~10 pA current

for 1 second is equivalent to 6.2 × 107 electrons, or, e.g., complete

electrochemical transformation in a 1.43 × 105 nm3 volume of material

such as LiCoO2 (assuming a cathode with 1 g LiCoO2 and the theoretical

capacity of 275 mAh/g). This introduces stringent limits on Faradaic

current measurements in SPM. In comparison, static and dynamic strain

detection can probe volumes as small as several 10s of nm3 as discussed

below, suggesting that strain detection can be used to probe local

electrochemistry at much higher resolutions.

As an illustration, Fig. 1 shows the surface topography and I-V

curve on th surface of the Li-ion conductive electrolyte56. Currents

on the order of 10s of nA are observed for biases in the range of

8 to 13 V, corresponding to purely Faradaic currents in the reaction

of Li+ + 1 e- → Li(s). Note that the total number of metallic Li atoms

deposited (via integration of topography) on the surface is on the

order of ~1010 which scales linearly with the total transferred charge.

However, the fact that AFM can readily detect nanoparticles of the

~1 – 2 nm scale (depending on background surface roughness) suggests

that current measurement is indeed not the optimal measurement

strategy for local electrochemical probing. For example, a Li particle

with a 4 nm diameter and 4 nm height having the same shape as the

Fig. 2 Limitations of conductive SPM based on environment and material. (a) Schematic image of current detection in liquid showing that stray currents are dominant and lead to non-locality of current detection. (b) Surface topography and (c) current image at 0 V bias for a LiCoO2 surface, illustrating electrical inhomogeneity in the electrode material.

Fig. 1 Surface topography of Li-ion conductive glass ceramics (LiCGC) (a) after and (b) before application of a bias pulse, and (c) locally measured I-V curve with inset showing the linear correlation (fit: y = 0.9389x) between the total charge transferred and volume of Li deposited.

(a)

(b)

(c)

(a) (b)

(c)

MT1411p548_559.indd 550 31/10/2011 14:40:35



particles in Fig. 1a, would require < 1000 electrons (corresponding

to sub ~fA currents for 1 s), which is well below the noise levels and

detection limit of typical current measuring amplifiers used in SPM.

Even given the limitations of direct Faradaic current detection,

local conductivity can offer valuable indirect information on local phase

composition (conductive/non-conductive phases) and bias-induced

transformations (e.g., electroforming57, electrochemical reactions at

the junction58, or polarization switching in ferroelectrics59). However,

the significant limitation of the current-based techniques follows from

the non-local nature of the detection mechanism, as illustrated in

Fig. 2. In conductive electrolytes, the current will be dominated by non-

local current flow to the tip and cantilever. Even with the introduction

of insulated and shielded probes60-62, the measured current will be

determined by all resistive elements between the tip-surface junction and

the current collector. While for (homogeneous) materials the spreading

resistance of the tip-surface-junction is the dominant resistive element,

this is not necessarily the case for complex architectures forming energy

materials with multiple interfaces and grain boundaries, thus complicating

the interpretation of conductive AFM (cAFM) data. Finally, the presence

of multiple electrochemically active pairs (e.g., cathode materials and

carbon particles) results in a large inhomogeneity of current responses

and presence of “nanobatteries”, as can be deduced from non-zero

currents at zero bias (e.g., shown in Figs. 2b,c). This inhomogeneity further

complicates interpretation of cAFM data in terms of battery functionality.

We further note that some information on the local chemical changes

induced by bias can be obtained from mechanical property changes,

e.g., in atomic force acoustic microscopy and related techniques or even

AFM phase imaging. Comparative analysis of SPM methods for probing

electrochemical processes in solids is given in reference63.

Imaging by electrochemical strain microscopyRecently, the electrochemical strain microscopy approach was

suggested for probing ionic dynamics based on local dynamic strain

detection64. While scanning tunneling microscopes (STMs) measure

electronic currents and atomic force microscopies (AFM) measures

forces, ESM measures the direct coupling of ionic currents to strain

(or position) measurements, providing a new tool for mapping

electrochemical phenomena on the nanoscale. The operation of ESM

is reminiscent of the dilatometric measurements broadly used for

the characterization of oxide conductors65-67 in that the deformation

of the material in response to electrochemical stimulus (tip bias) is

determined. Note that the direct (deformation induced by bias) and

reverse (bias induced by deformation) electromechanical coupling

effects have been reported for macroscopic geometries as well68,69.

Principles of ESMESM imaging (Fig. 3) is based on detecting the strain response of a

material to an applied electric field though a blocking or electrochemically

active SPM tip (functionalized directly or placed in an ion-containing

medium). The biased SPM tip concentrates an electric field in a

nanometer-scale volume of material, inducing interfacial electrochemical

processes at the tip-surface junction and ionic currents through the

solid71. The intrinsic link between concentration of ionic species and/or

oxidation states of the host cation and the molar volume of the material

Fig. 3 The principle of electrochemical strain microscopy. (a) In ESM, a periodic bias is applied to the SPM tip in contact with the sample surface. The applied bias

induces ionic motion in the sample and the resulting surface deformation is detected by the SPM probe and electronics, generating an image that maps the ionic

motion at the nanoscale. Reprinted from63 with permission from Wiley. (b) The dependence of the c-lattice parameter (perpendicular to the layers) on lithiation in

a prototypical LixCoO2 cathode material. Reproduced from70 by permission of The Electrochemical Society.

(a) (b)

MT1411p548_559.indd 551 31/10/2011 14:40:38



results in electrochemical strain and surface displacement. This is the case

for many ionic and mixed ionic-electronic conductors such as ceria72,

cobaltites73,74, nikelates74, and manganites75. Similarly, insertion and

extraction of Li-ions in a Li-battery electrodes produce changes of molar

volume75,76. For the example of LiCoO2, the lattice parameter changes

with the degree of lithiation, x, by 40 pm (Fig. 3b) in the operation region

of LixCoO2 from x = 1 to x = 0.5. Combined with the 3 – 4 pm sensitivity

limit of modern AFMs, this suggests that a lithiation state change of just

10 % can be measured through 1 unit cell of material.

The electrochemical interactions in the tip-surface junction are

governed by the nature of the tip and surface material and the

surrounding medium, much like reversible and polarizable electrodes in

classical electrochemical strategies29,30. For the blocking tip electrode,

the electron transfer between tip and surface and the non-uniform

electrostatic field result in mobile ion redistribution within the solid but

no electrochemical process at the interface. Note that for sufficiently

high bias, “non-standard” electrochemical processes become possible,

e.g., the formation of SiC on the SiO2 surface in hexane77, formation

of carbon from CO278, or electrochemical injection of dislocations in

oxides79. In principle, ESM can be performed in a liquid Li-containing

electrolyte, even for finite electronic conductivities the ac electric field

is concentrated in the tip-surface junction, as recently demonstrated in

ferroelectric materials80,81. However, corresponding image formation

mechanisms can be expected to differ significantly from the ambient

Fig. 4 Mapping the electrochemical strain response with an amorphous Si anode. (a) Contact resonance amplitude map of a 1 × 1 μm area with the AFM deflection (topography) signal (inset), and (b) resonance frequency map showing the heterogeneous strain response and a strong correlation between the resonance frequency and topography. (c) Single-point contact resonance peak with (d) the corresponding phase from the grain and grain boundary region. (e,f) Corresponding amplitude and phase profiles illustrating variations of signal strength (color) and resonant frequency (vertical axis) along the surface (horizontal axis).

(a) (b)

(c) (d)

(e)

(f)

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situation. Under ambient conditions, the formation of a liquid

droplet at the tip-surface junction82,83 provides a Li-ion reservoir,

rendering electrodes partially reversible. A similar effect can occur for

blocking electrodes at high biases (Li-extraction and tip plating) or for

Li-electrolyte-coated electrodes. Finally, ESM can be performed on

the surface of top-electrode devices, or on a uniformly biased mixed

electron-ionic conductor, similar to piezoresponse force microscopy of

capacitor structures36,84.

Dynamic modes in ESMIn contrast to the well-polished ceramics or smooth epitaxial films

typically studied in the context of condensed matter physics and

materials science, one of the significant difficulties in imaging

electrochemical materials is their high intrinsic surface roughness.

This topographic roughness can couple into the measured ESM signal

through direct effects, where the signal depends on contact radius

and local slope (e.g., contact stiffness), and indirect effects where the

frequency dependent transfer function of the cantilever depends on

the contact radius and local slope. This cross coupling is well known

in SPM (generally referred to as “topographic cross-talk”), and hinders

quantitative and even qualitative measurements.

Crosstalk with surface topography can be avoided or minimized using

multifrequency excitation schemes including band excitation (BE)85, dual

AC resonance tracking (DART)86, and other dynamic methods87-89. In BE,

Fig. 5 (a) Correlation between OP and IP ESM response and grain orientation. (b) Topography, (c) deflection, (d) OP ESM map, (e) IP ESM map for a 2 × 2 μm area

on a LiCoO2 thin film. Note the complementary character of information in the OP and IP maps in (d) and (e).

(a)

(e)

(b) (c)

(d)

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the excitation and detection are performed using a signal with defined

amplitude and phase content over a given frequency interval. In DART,

the amplitude-based feedback is used to track cantilever resonance and

its quality factor. These methods allow effective use of cantilever resonant

amplification and decouple the intrinsic surface response from variations in

resonant frequency induced by surface topography. This approach avoids

the indirect topographic cross-talk inevitable in single frequency SPMs90.

As an illustration of BE ESM imaging, Fig. 4 shows an example of

an amorphous Si anode. Figs. 4a,b illustrate the electrochemical strain

microscopy response amplitude and resonant frequency obtained

at each spatial point from resonant frequency curves as shown in

Figs. 4c,d. Note the strong variability of resonant frequency within

the grains by ~30 kHz, as compared to the resonant peak width of

3 – 5 kHz. In the constant frequency method, this will lead to very

strong cross-talk with topography, whereas feedback-based methods

will lose stability due to a lack of a well-defined response phase

(for phase locked-loop based methods) or zero amplitude at certain

locations (for DART). The typical evolution of the amplitude and phase

response signal across the line of the surface is illustrated in Figs. 4e,f.

Vector ESM imagingThe bias-induced surface displacement in ESM is generally a vector

having three non-zero components. The full surface displacement vector

in ESM can be characterized through cantilever deflection and torsion

measurements, similar to the approach used in vector piezoresponse force

microscopy of ferroelectric and piezoelectric materials91. The measurement

of the full displacement vector is especially important for materials with an

anisotropic ionic conduction and volume change, as schematically shown in

Fig. 5. For layered LiCoO2, chosen here as an example, the ionic transport

is fastest along the (001) Li-planes, whereas the strongest volume change

occurs along the [001] direction perpendicular to the planes. The vertical

(out-of-plane, OP) and lateral (in-plane, IP) component of the surface

displacements for differently oriented LiCoO2 is shown (Fig. 5a).

Figs. 5b-e display the correlation between topography and the

measured OP and IP ESM amplitude signals for LiCoO2 thin films. Here,

OP and IP ESM signals are recorded around the deflection and torsional

contact resonance frequencies of the cantilever. The topography and

deflection signals are shown in Figs. 5b and c, respectively. The maximum

OP and IP ESM amplitudes are displayed in Figs. 5d,e. Both images show

strong variations in the ESM response across the scanned area. In addition,

the OP and IP ESM amplitude maps do not show the same features,

demonstrating no or minimum cross-talk between the cantilever deflection

and torsion. When Figs. 5d,e are compared, grains with OP and IP response

(#1), no OP but IP response (#2), and OP but no IP response (#3) can be

identified. In the future, these local variations of IP and OP ESM can be

related to crystallographic orientation and electrochemical activity.

Spatially-resolved spectroscopies in ESMThe ESM signal can be used as a basis for a broad set of voltage and

time spectroscopies. The spectroscopic techniques in ESM have been

Fig. 6 Mapping of Li-ion relaxation in an amorphous thin film Si anode. (a) Scheme of voltage pulses applied to measure relaxation maps. (b) Single-point relaxation curves from two points in the map. (c) Deflection signal of a 1 × 1 μm area showing grain boundaries. (d) Map of maximum displacement measured after a −18 V voltage pulse of 30 ms duration.

(a) (b)

(c) (d)


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developed following the protocols of classical electrochemical methods

(e.g., potentiostatic intermittent titration, galvanostatic intermittent

titration, electrochemical impedance spectroscopy), in which the local

electrochemical strain signal substitutes for macroscopic Faradaic

currents. Once properly calibrated, these techniques offer the potential

for implementation of the rich panoply of electrochemical techniques

on the nanoscale level in a spatially-resolved fashion, some of which are

illustrated below.

Fig. 7 Mapping Li-ion diffusion. (a) Deflection signal of a 1 × 1 μm area showing a triple boundary junction of an amorphous Si anode. (b) Map of displacement

loop opening for a voltage sweep of 7 Hz and ±15 V. Reprinted with permission from98. ©2010 American Chemical Society. The loop opening is a direct measure of

Li-ion diffusivity. (c) Single-point displacement loops from three different areas as indicated in (b). Similarly, (d) deflection, (e) displacement loop opening map,

and (f) single-point displacement loops for a LiCoO2 cathode film.

Fig. 8 Separation of transport and electrochemical reaction within a Si anode. (a) Scheme of the expected displacement loop opening as a function of voltage used

to measure the displacement loops. The case of linear diffusion and diffusion after activating the electrochemical reaction with an onset voltage is shown.

(b) Displacement loops as a function of maximum bias voltage. (c) Map of electrochemical reaction onset voltage in a 500 × 500 nm area around a triple boundary

junction. (d) Single-point curves extracted from the boundary and grain region as indicated in (c).

(a) (b) (c)

(d) (e) (f)

(a) (b)

(c) (d)

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ESM time spectroscopyIn ESM time spectroscopy92,93, the signal is measured after the application

of a single voltage pulse to the probe, and the response is measured

over a long time (ideally, comparable to the diffusion time of Li-ions).

In the mapping mode, the relaxation curves are measured over a grid

of locations on the sample surface giving rise to a 3D data set, and the

characteristic parameters (relaxation times, relaxation amplitude) are then

extracted from each relaxation curve and plotted as 2D maps94,95. The

diffusion length can often be determined from spatially resolved images,

thus allowing quantitative determination of the diffusion coefficient.

This spectroscopic method is somewhat reminiscent of the well-known

potentiostatic and galvanostatic intermittent titration techniques96,97, but

is performed on the nanometer scale in a spatially-resolved fashion.

An example of relaxation mapping of the amorphous Si anode

is illustrated in Fig. 6. Here, the excitation waveform at each spatial

location is formed by a sequence of positive and negative bias pulses

to minimize the device charging. Following the bias pulse application,

the relaxation of the electromechanical signal is probed by a sequence

of BE pulses. The relaxation curves from two surface locations are

shown in Fig. 6b. The corresponding surface deflection map is shown in

Fig. 6c. The maximum relaxation amplitude shown in Fig. 6d illustrates

that strong relaxation of the ESM signal is observed only in the grain

boundary like regions and a number of “hot spots” within the material.

These regions thus correspond to the locations with maximal ionic

activity and can be mapped with ~10 nm spatial resolution.

Voltage spectroscopy As an alternative to time spectroscopy, the ESM measurements can be

performed in voltage spectroscopic mode. In this ca se, voltage pulses of

increasing and decreasing amplitude are applied to the probe, and the

electrochemical strain response is tested after each pulse. The voltage

sweep provides the advantage of faster measurements (compared to

time spectroscopy, where only one voltage is tested at a time) and

yields information about voltage-activated electrochemical processes

and transport in the probed volume.

Fig. 7 shows an example of ESM voltage spectroscopy of Si anode

and LiCoO2 cathode materials. The surface topography of the Si anode

illustrates the presence of grain-boundary-like features, likely induced

by the roughness of the alumina substrate and associated with the

disruption of short range order in the amorphous Si (as evidenced by

sharpness of the feature). The ESM hysteresis loops are measured and

the areas of the loops are plotted as a 2D spatial map in Fig. 7b. The

hysteresis loops extracted from several spatial locations are shown in

Fig. 7c. The open loops are highly localized at the grain-boundary-like

features, with effective resolution well below 10 nm. This behavior is

indicative of high localization of Li activity at the grain boundaries. Note

that this behavior cannot be ascribed to topographic crosstalk, since the

latter primarily affects conservative tip-surface interactions rather than

the hysteresis of bias response. Figs. 7d-f show the same measurement

performed on a thin film LiCoO2 cathode material. As for the Si anode,

areas of different Li-ion transport properties can be identified.

Fig. 9 Evolution of loop opening maps with battery cycling at 7 Hz with ±16 V. Loop opening map after (a) 103, (b) 104, (c) 3 × 104, and (d) 105 measurement cycles. (e) Strain hysteresis of the boundary region for increasing cycle number. (f) Charging curves of fresh and high-frequency, strongly cycled batteries. Reprinted with permission from98. ©2010 American Chemical Society.

(a) (b) (c) (d)

(e) (f)

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Reaction-transport separationElectrochemical processes are typically composed of several interfacial

reactions and diffusion steps. A reaction is typically exponentially

dependent on overpotential while transport (diffusion and/or

migration) changes linearly with driving force. Consequently, for low

potentials the process is limited by reaction, while for high potentials it

is limited by diffusion. Hence, measuring the ESM signal as a function

of bias pulse magnitude allows differentiation of the reaction and

diffusion, as illustrated in Fig. 8 for the case of the Si anode.

Evolution of Li concentration in Si anode on chargeESM allows one to systematically follow in detail the changes in Li-ion

diffusion on a local scale during battery charging/discharging or battery

fading. Fig. 9 illustrates the evolution of ESM activity on a silicon anode

surface at different stages of cycling. Note the gradual disappearance

of defect-related “hot spots” and the increase of contrast at grain

boundaries. This behavior is attributed to the accumulation of Li-ions in

the grain boundaries and exclusion of the dominant parts of the sample

from the electrochemical process, providing insight into the causes of

the loss of capacity on subsequent charging.

Future challengesAs reviewed above, ESM is a technique capable of probing

electrochemical reactivity and ionic flows in solids on the sub-10

nanometer level, made possible using the intrinsic link between

electrochemical processes and strains. ESM can be extended to a

broad spectrum of time and voltage spectroscopies; however, broad

acceptance and implementation of ESM hinges on a number of technical

breakthroughs. Some of these are standard targets of SPM development,

such as higher spatial resolution, sensitivity, energy and time resolutions.

At the same time, several development directions are specific to ESM,

including development of SPM platforms operating under realistic

electrochemical conditions (electrolytes, reactive gases, temperature

ranges), theoretical understanding of ESM signals, extending ESM

towards mapping irreversible electrochemical processes, and collecting

chemical information.

For in situ ESM, imaging in conductive liquids presents an obvious

problem. While ac electric fields can be localized even in conductive

liquids91, the dc tip-surface potential difference cannot be maintained92.

While multiple prototypes of insulated and shielded probes have been

developed68-70, such probes are not yet broadly available. Similarly, ESM

studies of fuel call materials necessitates development of appropriate

environmental cells maintaining temperatures, gas compositions and

pressures similar to operational fuel cells.

Quantitative interpretation of ESM requires both understanding

aspects of SPM operation (i.e., the relationship between the surface

deformation below the tip and signal detected by SPM electronics)

and materials behavior (relationship between electromechanical

response and local electrochemical functionality) which has to be

addressed by combining experiment and theory. For a simplified case

of purely diffusional coupling, the theoretical principles of ESM have

been considered in several recent publications99,100. However, the

future development of ESM requires the development of analytical

approximations and numerical models reproducing voltage-divider effects

in the tip-surface junction and bulk, strain evolution in the material,

and associated surface deformations. Recent analyses by Garcia101 and

Ciucci102,103 illustrated possible pathways toward addressing these

problems.

The primarily limitation of ESM is the lack of chemical information,

especially limiting for multi-component materials or in studying complex

sets of electrochemical and chemical transformations. The combination of

ESM and microRaman/near field scanning optical microscopy (NSOM) will

allow us to add local chemical sensitivity and systematically explore the

local electrochemistry at the tip-surface junction. Similar opportunities

can be provided by the combination of ESM with focused synchrotron

x-ray microdiffraction and microfluorescence which can be a powerful

tool to provide the needed phase composition, crystal orientation, and

micron-scale chemical and strain changes104.

OutlookThe capability for probing electrochemical processes and ionic transport

in solids is invaluable for the study and improvement of a broad range

of energy technologies and applications, including batteries and fuel

cells. However, the progress in this field has been limited by an almost

complete lack of tools capable of probing the local electrochemical

activity on the nanoscale.

ESM offers a universal method for probing ionic and electrochemical

processes in solids on the nanoscale. To date, ESM has been demonstrated

for a variety of lithium-ion materials (including layered transition metal

oxide cathodes79, silicon anodes105, and electrolytes such as LISICON),

oxygen electrolytes (including yttria-stabilized zirconia [YSZ] and

samarium-doped ceria106), mixed electronic-ionic conductors for fuel cell

cathodes, and some proton conductors. The data, as well as the near-

universal presence of chemical expansion in electrochemical systems,

suggest an extremely promising potential for future developments and

applications.

AcknowledgementsThe effort by SVK and NB was supported as a part of the Fluid

Interface Reactions, Structures and Transport (FIRST) Center at Oak Ridge

National Laboratory, an Energy Frontier Research Center funded by the U.S.

Department of Energy, Office of Science, Office of Basic Energy Sciences

under Award Number ERKCC61. Parts of this research (SJ, TMA, AK, AT) were

performed at the Center for Nanophase Materials Science sponsored by the

Office of Science, Basic Energy Sciences Program, Division of User Facilities.

TMA was supported in part by DOE SISGR program. The authors are deeply

grateful to J. Budai for valuable advice regarding x-ray microprobe, and A.

Borisevich and R. Unocic for multiple discussion of STEM-SPM combinations.

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MT1411p548_559.indd 559 31/10/2011 14:40:54


FEATURE COMMENT

Scientific coopertition: can it scale and work?

Collaboration and competition. The two words any ambitious scientist

knows too well and has learned to live with. On the one hand, addressing

cutting-edge scientific problems requires an increasing amount of knowledge

and expertise, as well as inter-disciplinary skills, demanding some form of

collaboration with colleagues and external experts. But on the other hand,

there are only limited number of good research and teaching positions, and

research funds are limited. Time is also of the essence. Direct competition is

therefore unavoidable.

In industry, this mix of collaboration and competition is better known as

“coopertition” and has been exercised for years. For example, in the automobile

In materials science, collaborations tend to be limited to the cooperation of several small research groups. Meanwhile, in the world of particle physics, it’s an entirely different story. Markus Nordberg and Fabiola Gianotti from the LHC’s ATLAS experiment, discuss the rewards and difficulties of large scale collaboration.

Markus Nordberg and Fabiola Gianotti | ATLAS Experiment, CERN | [email protected]

The ATLAS detector. Forty five meters long and seven

thousand tons in weight. ATLAS Experiment © 2011 CERN.

MT1411p560_563.indd 560 13/10/2011 12:49:44


and avionic industries, large manufacturers use the same few subcontractors and

occasionally form specific alliances among each other. The number of competing

partners in such alliances can run in the excess of tens of companies.

But how about in scientific research, say in physics in general, where the

specific research domains are rather well established, not expanding rapidly, and

where scientists (more or less) know each other? Can research undertakings

involving a lot more than say, twenty contributing scientists, work? And above

all, does it make any sense?

Come together, right nowThis question is surely not new. A healthy suspicion about scaling up scientific

collaborations has existed ever since the mid 1930s when the epoch of “Big

Science” started. This is when physicists in the US started to team up to design

and run particle physics accelerators and later on, to construct and operate large

physics experiments. The trend was later picked up in Europe and elsewhere.

And the last 60 years or so has shown that this approach works, and that highly

ambitious, complex scientific goals are accessible with enough people, cash, and

even more patience.

As an example, we can examine the ATLAS experiment at the Large Hadron

Collider (LHC) at CERN, of which we are members. ATLAS brings together more

than 3000 scientists, engineers, and PhD students from more than 170 scientific

institutions around the world. We study the conditions of our Universe just a

fraction after the Big Bang. We look at traces of particles we believe existed

then and try to understand the underlying laws behind their interactions. To

achieve our research goals, we needed a new tool: a large particle detector.

ATLAS was built by our own community and completed in 2008. It lies in a large

FEATURE COMMENT

The Large Hadron Collider is the world’s largest particle accelerator; measuring twenty seven kilometers in circumference, and buried beneath the Franco-Swiss border. © 2011 CERN.

“Can research undertakings involving a lot more than say, twenty contributing scientists, work?

And above all, does it make any sense?”

MT1411p560_563.indd 561 13/10/2011 12:49:50


FEATURE COMMENT

cavern some 100 meters below the ground. It is about 45 meters long, 23 wide,

and weights about 7000 tons: as much as the Eiffel Tower. It has over 10 million

functional elements, perhaps ten times more in terms of pieces touched by a

human hand at least once during the production phase that took over 10 years

to complete.

In order to bring this all together, a community had to be established and

it had to learn to work together. How did this happen? First, a small group of

scientists got together in a rather informal way to discuss how a detector could

be built to best explore the expected new physics. These coffee-table discussions

then started to include more colleagues, who performed computer simulations

of both the physical processes and behavior of the detector, explored the use of

suitable materials, and considered the related signal collection and processing

technologies. These interactions in turn led into larger, common meetings,

where scientists and engineers started to work towards splitting the detector

into meaningful sub-projects or sub-systems.

Second, communities started to emerge around the proposed sub-systems

and they in turn started to organize themselves, benefitting from general

guidance from the core group that had started the process. Moreover, technical

review panels of external experts were set up to evaluate the documented

technical solutions that the sub-systems were proposing. In parallel, CERN, in

its role of the Host Lab, put in place a peer review system to follow and finally

approve the progress being made on the sub-system level.

Third, an overall collaboration structure was put in place. This meant

establishing a Collaboration Board that every participating institute could join,

all with equal voting powers. The rules for such a Collaboration Board were

worked out, again based on the principle of minimum control. The rules laid out

procedures for joining the experiment, formalized the shared obligations, both in

terms of the nature of the scientific and financial input, and stated the common

scientific policies (e.g., concerning working together on individual topics and

signing off scientific papers). In addition to the Collaboration, an Executive Board

was established for the running of day-to-day operations, spearheaded by a small

Management Team. Each sub-system in turn set up their own internal structures

including their Institute Board and Project Leaders, somewhat in a symmetrical

way to the overall ATLAS organization. This is probably the key feature that allows

the collaboration to scale. Later on, as the experiment started to collect data, the

physics groups consolidated their structures around topics of major interest.

Fourth, the funding mechanisms and resources allocation processes were

established. This meant creating a Resources Review Board so that the ATLAS

Management could present, to all participating Funding Agencies, on a periodic

basis, the progress made and the resources consumed, and make resource

requests for the future. As a point of reference, the (direct) construction costs

of ATLAS amounted to some 540 million Swiss Francs and the annual operation

costs amount to about 4 % of that.

Finally, all of the above was recorded and agreed to in a nine page

Memorandum of Understanding (MoU) which all of the Funding Agencies

signed. This MoU was light in structure and mainly based on the principle of

deliverables, where participating institutes commit to building an agreed set of

hardware or software at a fixed cost. This meant that the bulk of the spending

took place at home, facilitating book keeping and financial reporting. The MoU

tried to give maximum flexibility to all partners to facilitate what was expected

from them and in fact, the MoU was declared as legally non-binding. We were

later told by lawyers that the notion of such an agreement is very strange, but

was undoubtedly effective!

Should you try this at home?Although putting all this together may sound straightforward, even if it appears

bulky, it took almost 10 years for the ATLAS community to establish itself

and commit on paper to build the detector described above. The way forward

was determined by history, a compelling vision, a common passion for new

discoveries, pragmatism, tolerance, and common sense; rather than by glossy

diagrams adapted from business management books. We are often asked

whether our structures or management approach was influenced by large,

A proton collision, as measured by ATLAS. ATLAS Experiment © 2011 CERN.

“A lot of diplomatic skills are required, and in a scientific community like ours it can

sometimes be difficult to find!”

“Highly ambitious, complex scientific goals are accessible

with enough people, cash, and even more patience.”

MT1411p560_563.indd 562 13/10/2011 12:49:57


high-tech corporations. It was not, we are embarrassed to admit; although

some management scholars think we should not be!

And such a collaboration had many advantages. First, the collaboration

mode offered full access to bright individuals and expertise in 38 different

countries. This allowed an effective way to expose, address, and solve many

scientific and technical challenges as they emerged that otherwise may have

jeopardized the success of ATLAS in the later years. Second, the collaboration

permitted us to make the best of available industrial technologies at the best

cost: components of ATLAS could be manufactured in the countries most

suitable for producing them, benefiting from a local ATLAS presence. Third, it

offered a new and exciting learning environment for young PhD students: today,

ATLAS has more than 1000 PhD students working on physics and detector

technologies. They get immediate exposure to an international community and

get to present their work in front of a large international audience of leading

experts. They are allowed to grow and blossom in a multi-cultural environment

and learn about tolerance and interacting within the community.

But it’s obvious also that there is a downside. Working together with 3000

scientists and engineers is not the same as doing “garage experiments”. One

needs structures, procedures, and patience. It can sometimes be heavy and

slow and the project reporting lines unclear. The construction phase certainly

was not without some very difficult moments. For example, in the early days

there were several parallel and promising R&D projects on-going to solve

serious technological challenges. Due to limited funding, it became obvious

that only one solution could be followed, and that the funding from the other

competing projects should be used to implement it. So which one to choose?

How to keep everyone on board? How to avoid creating an unhelpful class

of “winners” and “losers”? A lot of diplomatic skills are required, and in a

scientific community like ours it can sometimes be difficult to find!

Working in a large collaboration like ATLAS also has implications back

at the home institutions. Our colleagues do need help when defending their

applications for tenure when being asked by colleagues in other fields about

their individual contribution to a scientific paper carrying 3000 authors (even if

there will soon be over a hundred such papers). It is certainly a valid question

and answering it requires explaining the nature of large collaborations and the

structures within.

The question of competition between ATLAS collaborators and those

in other experiments is also an important factor. Luckily, it has so far not

jeopardized the success of ATLAS. We believe this is because there is a common,

shared motivation which overrides the desire to withhold knowledge from

colleagues in fear of compromising future tenure or senior research positions.

The individuals are usually known within the overall particle physics community

and their work can be assessed and ranked rather well, despite any boundary

conditions that may be implied from their association to a given experiment.

Competition is tough but fair.

Despite the above hardship, we believe large collaborations provide the

required academic, educational, and financial stability to encounter ambitious

projects in fundamental science which extend beyond a decade or two.

Youngsters need to enter the game early and learn the tricks of the trade.

Working closely with people from so many different countries and cultures gives

a totally new meaning to the concept of problem solving and innovation. Even

if one shares a common language, in ATLAS it is English, people see the same

thing in many different ways. It’s a bit like making a raw diamond shine: it’s

hard work, not always fun, but it’s worth it.

Follow the Yellow Brick Road?Although large scale collaboration is the norm in the modern world of particle

physics, it is not intended to imply that our colleagues in other fields of science

should necessarily follow suit. Although following the Yellow Brick Road may

offer plenty of excitement and magic along the way, size alone does not always

matter. What matters is the nature of the scientific goals, the anticipated means

to achieve these goals within a meaningful time frame, and concrete, available

funding. And this, of course, varies across different scientific disciplines.

As illustrated earlier, the strive for such large, global collaboration in

experimental particle physics has been determined by a pragmatic need to

pool resources together and share the work load. This is necessary because the

increased scientific and technological complexity and the way forward has been

part of a natural evolution over decades to accommodate the many boundary

conditions imposed by academic institutions and their funding agencies around

the world. There is some indication that our colleagues in the fields of nuclear

physics, bioinformatics, and computing are testing their toes in the strange

waters of truly global collaboration. Whether other fields find themselves facing

similar challenges remains to be seen.

And for ourselves, shall we continue growing bigger? We don’t know.

Much depends on what new exciting physics comes out of the LHC program

and what the next steps should be. Our expectations are high but in the end,

we can’t force Mother Nature’s hand, either. But irrespective of what there

is for us to discover, we do know that the next generation of collaborations

will look different from the present ones. Perhaps future efforts will be even

more concentrated geographically. Perhaps the community needs to shrink

in size. In any event, the passion to dream about new worlds will remain and

a set of minimal, simple rules will guide the crowd forward. And this is what

fundamental research has always been about.

FEATURE COMMENT

“Although large scale collaboration is the norm in the modern world of particle physics, it is not

intended to imply that our colleagues in other fields of science should necessarily follow suit.”

MT1411p560_563.indd 563 13/10/2011 12:50:01

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MT1411p564_565.indd 564 31/10/2011 14:42:19

BOOKS & MEDIA


Q & AKirsten BodleySTEMNET | www.stemnet.org.uk

Kirsten Bodley is the Chief Executive

of STEMNET, an organization

dedicated to inspiring young people

in science, technology, engineering,

and mathematics. Materials Today

caught up with Kirsten to find out

what inspires her.

MT: When did your interest in science begin?

KB: I became interested in science at school, thanks to encouraging chemistry and physics teachers who let us take part in engaging and exciting experiments; having a go ourselves. Seeing scientific theories come to life through practicals made learning much more interesting.

MT: What’s the most rewarding part of your job?

KB: One of the most rewarding aspects of my job is working with our nationwide network of volunteers: STEM Ambassadors. I enjoy meeting these role models who volunteer their time to inspire young people in STEM: they always have great stories to share from the school activities they have taken part in!

MT: What did you want to be when you were younger?

KB: Believe it or not, a ballerina! However, It was not to be. I found chemistry at school and always wanted to be in an industry related to it.

MT: What’s your biggest achievement?

KB: Completing and enjoying my NQT year, before moving on to teach Year 5/6. Although challenging (the children never miss a trick!), I found these experiences richly rewarding.

MT: Which scientists have inspired you?

KB: Scientists from the past who have inspired me include Rosalind Franklin, for her pioneering work on the DNA structure, and Marie Curie. In the present day, I find Maggie Aderin-Pocock hugely inspirational, for not only encouraging the next generation of scientists and engineers, but also for increasing public engagement with science. It was an honor to have her as a presenter at The Big Bang London and South East in July; a celebration of STEM for young people, coordinated by STEMNET.

To hear more from Kirsten, as well as a

plethora of speakers from academia and

industry, visit our podcast page at

www.materialstoday.com/podcasts

or by searching for Materials Today on

iTunes.

Industrial Biofouling

Reg Bott started to investigate industrial biofouling long before it acquired the attention that it actually deserves, and he was the first to state through straightforward, elegant experiments, published in the eighties, that nutrients are a key factor for the progress of biofouling, leading to the conclusion that nutrients are a major fouling factor. This is remarkable compared to the common anti-fouling approaches which simply focus on killing bacteria instead of limiting their food.

He now presents a brand-new book on “Industrial Biofouling”. He did his major work on the biofouling of heat exchangers, and, consequently, he begins with a chapter on fluid flow, mass, and heat transfer. Here, solid text book material is presented, helping readers to understand the influence of biofilms on all of these factors. A clear definition of biofouling would have been good, but is missing.

A general chapter on biofilms follows, which unfortunately is strangely outdated. In the introduction, he elaborates briefly on bacteria, but from a time before molecular biology took over and provided a much better understanding of bacterial species, physiology, interactions, and identification methods. The entire chapter is derived from work from the eighties, as is represented by the references: out of 66 citations, 33 are older than 20 years, another 28 are older than 10 years, leaving only 5 recent publications; two of them by the author. In the section about primary adhesion, he explains the physico-chemical approaches for understanding the mechanisms. However, it has long been known that none of these approaches really fit and allow for predictions. As a graphical description of biofilms, unfortunately, he chose an image published in New Scientist that is clearly very uninspired and schematic. In the meantime, there are many other, much better biofilm schemes available; and much deeper, detailed, and process-oriented descriptions of biofilms too.

The chapter on biofouling control reflects the ample experience of the author. Much of it is presented without reference but is very trustworthy. As the obvious, most common countermeasure, he presents the use of biocides. Implicitly, he distinguishes between the killing of bacteria (which will result in leaving biofilms in place) and of the removing of biofilms. But this aspect should have deserved much more attention as it is one of the main reasons for failures of biocide applications: these failures are based on the medical paradigm that killing the problem-causing microorganisms will cure the problem.

For living systems this applies because the immune system performs the cleaning. In technical environments, there are no such systems and dead biomass represents nutrients for subsequent living organisms. It would have been particularly interesting to have provided more information on biodispersants, because they hold the potential to remove biofilms. The methods of physical control concentrate mainly on mechanical principles and are very interesting. It could easily have been amalgamated with the subsequent, very brief, chapter on cleaning.

Then he addresses a very important aspect in anti-fouling strategies; that of monitoring. As we usually don´t have

“eyes in the system”, there is a poor capacity for early warning, and problems are recognized late and not really localized. The practice of water sampling is insufficient to locate or quantify biofilms. He presents a very interesting system, but many others are not mentioned, although in heat exchanger technology, a number of interesting and promising methods have been developed, e.g., based on ultrasound, heat transfer, friction resistance, or optical/spectroscopic principles. It remains surprising and disappointing that this entire branch of industry still operates without proper monitoring as long as they can get away with just

dumping biocides, saving the investment for developing

monitors for industrial application. Here, a considerable optimization potential is wasted. And the chapter is very scarce about these aspects.

Finally, a chapter on biofilms in industry follows. It begins with a superficial section on biofilm reactors, followed by brief sections on biofilms in water treatment and in the food industry. Again, two thirds of the references are more than 10 years old, and are mostly textbook material.

In the concluding remarks, Reg Bott states “As many of the references quoted in this publication illustrate, much research work has been done or is in progress to provide a better understanding of the fundamentals of biofouling and hence the development of improved methods of control.” He is absolutely right. And quite a bit has been achieved in the meantime. Why didn’t he say more about it?

Reg Bott is a long standing member of my hall of fame, and in this text he discusses the challenges and benefits of biofilms on industrial surfacesHans-Curt Flemming | [email protected]

T. Reg. Bott

Industrial Biofouling

Elsevier • 2011 • 220 pp

ISBN: 978-0-444-53224-4

$260.00

MT1411p564_565.indd 565 31/10/2011 14:42:21

Use ScienceDirect to target your customers in the energy sector

The energy part of ScienceDirect is an invaluable tool generating over 797,000 page views per month.

There are 5 sub topics in this area to place a high impact, targeted advertising campaign.

Use ScienceDirect to target your customers in the energy sector

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MT1411p566-567.indd 566 31/10/2011 14:51:01

UNCOVERED


Nano fingerprints

The use of powders to develop latent fingerprints left after criminal activity has been established for many years. However, various types of substrate surfaces, such as rough materials, fabrics, and adhesives are not well suited to this type of technique. Other methods have been developed, including acid dyes, cyanoacrylate fuming (CA), and the evaporation of metals such as gold, zinc, and silver. Protocol tables have been established that apply broad classifications to surfaces and outline appropriate development techniques.

Fingerprints are biochemically complex, containing fatty acids, glycerides, amino acids, and metal ions in various proportions, excreted from eccrine and sebaceous glands. Print composition also varies from person to person, and is strongly affected by factors such emotional state, grooming regime, and intake of food and drugs. This inter- and intra- donor variability further complicates detection and interpretation.

A US report in February 2009 outlined the need for additional and rigorous research on forensic techniques, and this was backed up by practitioners such as the chief forensic pathologist of the New York State Police, who stated in a New Scientist interview, “so many innocent people get convicted … based on junk science”1. More recently, the UK Forensic Science Regulator has stressed the importance of ensuring the validity of methods, with peer-review and publication featuring as cornerstones of quality in forensic practice. This conclusion is reinforced by other studies and court actions that challenge the application and interpretation of fingerprint evidence. Therefore the development of a scientific understanding of forensic fingerprint evidence is both timely and critical in ensuring the continued trust in forensics and the validity of investigative methodology. Researchers across academia and the forensic provider sector have stepped up to the challenge, with recent advances in areas such as quantum dots for fingerprint development, detection of drug residues within fingerprint deposits, and statistical analysis and representation of uncertainties within the courtroom.

Research at Brunel University in London, in association with the UK Home Office, has been pioneering the use

of micro and nanotechnological analysis to improve understanding of the operation and interaction of fingerprint development techniques. A recent study investigated titanium dioxide powders in suspension for developing fingerprints on adhesive tapes, for example from drug packaging2. This work demonstrated a nanoscale variation in particulate coating in commercial formulations that is responsible for the significant differences in the effectiveness of different powder suspensions. This also highlights a problem in detecting fingerprints: no one formulation is effective across all fingerprint donors, or on every material. Research on the surface interaction of development agents can

therefore help to improve development agent selection and hence enhance the detection process3.

Multiple techniques can sometimes be utilized to aid development of fingermarks or obtain additional details, for example, when investigating fingermarks in blood4. However, the interaction of two techniques can sometimes be detrimental and obscure information from the fingerprint, therefore further elucidation of the operation of multiple techniques helps to ensure validity.

This month’s cover image shows a back scattered electron micrograph of a fingermark developed with two sequential techniques. Here, vacuum metal deposition of gold and zinc, following cyanoacrylate development of a latent print leads to zinc nanoparticulate decoration of the polycyanoacrylate deposits5. The image was captured using a field emission scanning electron microscope, the contrast

is dependent on atomic number. Operating in variable pressure mode enables imaging and analysis without the usual addition of a conducting coating.

There is more that a fingerprint could tell us. A wide consortium of research laboratories is investigating the potential to capitalize on the inter-donor variability of the biochemistry of fingerprints. Although a problem in developing prints and designing effective techniques, this variability may make it possible to gather extra intelligence about the victims or perpetrators of crime, such as age, gender, smoking, or drug habits, which could facilitate criminal investigations.

This work aims to improve the efficiency and performance of fingerprint detection, as well as aid the selection of the most appropriate development systems, and so facilitate enhanced and reliable collection of forensic information.

Gathering intelligence

Benjamin J. Jones

Experimental Techniques Centre, Brunel University, UK

E-mail: [email protected]

REFERENCES

1. Geddes, L., New Sci (2009) 201(2697), 6.

2. Jones, B. J., et al., Sci Justice (2010) 50, 150.

3. Jones, B. J., et al., Surf Interface Anal (2010) 42, 438.

4. Au, C., et al., Forensic Sci Int (2011) 204, 13.

5. Jones, B. J., et al., J Forensic Sci, doi: 10.1111/j.1556-4029.2011.01952.x.

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combined with analytical capabilities makes this high end FE-SEM suitable for a wide range of applications in materials science, life science, and semiconductor technology. The large specimen chamber for the integration of optional detectors and accessories enables the user to configure the SUPRA® for specific applications without sacrificing productivity or efficiency.

For more information about Carl Zeiss NTS and our comprehensive particle-beam product portfolio including SEM, TEM, FIB-SEM and the unique Helium-Ion microscopes, go to www.zeiss.com/nts.

MT1411p566-567.indd 567 31/10/2011 14:51:02

EVENTS DIARY


28 November – 2 December 2011

2011 MRS Fall Meeting & Exhibit

Boston – MA – USA

MRS conducts two major meetings every year, the Spring Meeting in San Francisco and the Fall Meeting in Boston. Consisting of topical symposia, the meetings have become important and not-to-be-missed events for materials researchers around the world for presenting their work, for getting information on up-to-the minute developments in their field, and for networking.

www.mrs.org/fall2011/

8 – 10 December 2011

ICANN-2011 — 2nd International Conference on Advanced Nanomaterials and Nanotechnology

Guwahati – India

The 2nd International Conference on ‘Advanced Nanomaterials and Nanotechnology (ICANN-2011)’ is being organized jointly by the Department of Physics and Centre for Nanotechnology at the Indian Institute of Technology Guwahati (IITG), India. This is going to be another major international conference being held in the North-Eastern region of India, in the area of Nanoscience and Nanotechnolgy. The international conference intends to bring eminent scientists, technologists, and young researchers from several disciplines across the globe together to provide a common platform for discussing their achievements and new directions of research.

www.iitg.ernet.in/icann2011/

11 – 15 December 2011

2nd Nanotoday Conference

Marriott Resort Waikoloa – Hawaii – USA

This second international meeting on nanostructured materials and devices, organized by the journal NanoToday, will showcase the latest research and advances in this increasingly multidisciplinary field. This conference will present the latest research achievements and commercial applications of nanostructured materials and devices.

www.nanotoday-conference.com

11 – 14 December 2011

4th International Conference on the Mechanical Biomaterials and Tissues

Marriott Resort Waikoloa – Hawaii – USA

The conference provides a forum for the discussion of the modeling and measurement of deformation and fracture behavior in biological and replacement materials, and the role mechanical properties play in physiological and disease conditions.

www.mechanicsofbiomaterials.com

8 – 14 January 2012

2012 Winter Conference on Plasma Spectrochemistry

Tucson – Arizona – USA

More than 500 scientists are expected to participate, and over 300 papers on modern plasma spectrochemistry will be presented. A three day exhibition will feature new plasma instrumentation and many supporting products. Six plenary lectures and 24 invited speakers will highlight critical topics in 12 symposia. In addition, six Heritage Lectures will feature outstanding senior researchers.

http://icpinformation.org/2010_Winter_Conference.html

9 – 11 January 2012

BTS 2012 — Biotech Showcase 2012

San Francisco – United States

Biotech Showcase™ is a forum devoted to providing biotechnology and medtech companies, investors, and pharmaceutical executives an opportunity to meet in one place during the course of one of the largest annual healthcare conferences that attracts investors and biopharmaceutical executives from around the world.

www.ebdgroup.com

18 – 20 January 2012

Electronic Materials and Applications 2012

Orlando – Florida – USA

EMA 2012 focuses on electronic materials for energy generation, conversion, and storage applications, highlighting renewable energy, innovative hybrid and all-electric transportation development, electrical ceramics, and advanced microelectronics.

http://ceramics.org/meetings/electronic-materials-and-applications-2012

21 – 26 January 2012

SPIE Photonics West

San Francisco – California – USA

See over 1150 industry-leading companies at the industry’s largest photonics and laser event. If you’re in the photonics and laser industry, kick off the new year at SPIE Photonics West, the essential photonics and laser event.

http://spie.org/x2584.xml

22 – 27 January 2012

36th International Conference and Expo on Advanced Ceramics and Composites

Daytona Beach – Florida – USA

ICACC’12 showcases cutting-edge research and product developments in advanced ceramics, armor ceramics, solid oxide fuel cells, ceramic coatings, bioceramics and more.

http://ceramics.org/meetings/36th-international-conference-and-expo-on-advanced-ceramics-and-composites

15 – 17 February 2012

BioMed 2012 – The Ninth International Conference on Biomedical Engineering

Innsbruck – Austria

In recent years, with the aid of engineering and information technology, biomedical engineering has emerged as a high-tech field, generating innovation in such areas as medical imaging, bioinformatics, MEMS and nanotechnology, new biomaterials and sensors, medical robotics, and neurobiology. Scientists and engineers in this field have recently been working towards such advances as developing artificial organs that mimic natural human organs, conducting telemedicine, performing surgeries with robots, creating a laboratory-on-a-chip, and controlling robots through natural animal brain matter.

www.iasted.org/conferences/home-764.html

26 February – 1 March 2012

Materials Challenges in Alternative and Renewable Energy

Clearwater – Florida – USA

MCARE 2012 facilitates information sharing on the latest materials developments and innovations for solar, wind, hydro, geothermal, biomass, nuclear, hydrogen, electric grid, materials availability, and battery and energy storage.

http://ceramics.org/meetings/materials-challenges-in-alternative-rewable-energy-2012

27 February – 2 March 2012

APS March Meeting 2012

Boston – Massachusetts – USA

The American Physical Society will hold its 2012 March Meeting in Boston, Massachusetts. The conference is expected to play host to over 7000 top scientists involved in physics research and applied physics from around the world.

www.aps.org/meetings/march/

6 – 7 March 2012

Nanomaterials for Biomedical Technologies

Frankfurt am Main – Germany

Nanomaterials in biomedical applications either in vitro or in vivo have raised high expectations for new and ground breaking diagnostic and therapeutic solutions in health care and are already moving from the laboratory bench to clinical application. The success of nanomaterials in these fields is founded on our advanced understanding of molecular mechanisms in biology, the progress of nanostructure sciences in physics, chemistry, and engineering, and our quickly improving ability to mimic biological signals by increasingly complex synthetic structures and interaction functionalities.

www.processnet.org/nanoBiomed2012

FORTHCOMING EVENTS

diaryIf you are organizing a future conference or workshop and would like to have it listed in Materials Today please contact Jonathan Agbenyega – [email protected].

Events Materials Today has a contra deal with and that are relevant to the current issue of the magazine are listed below.

If, as an organizer, you would like to discuss a contra deal, please contact Lucy Rodzynska – [email protected]

For further information please visit www.materialstoday.com/events

28 November – 2 December 2011

2011 MRS Fall Meeting & Exhibit

Boston – MA – USA

MRS conducts two major meetings every year, the Spring meeting in San Francisco and the Fall Meeting in Boston. Consisting of topical symposia, the Meetings have become important and not-to-be-missed events for materials researchers around the world for presenting their work, for getting information on up-to-the minute developments in their field, and for networking.

www.mrs.org/fall2011/

11 – 15 December 2011

2nd Nanotoday Conference

Waikoloa – Hawaii – USA

This second international meeting on nanostructured materials and devices, organized by the journal NanoToday, will showcase the latest research and advances in this increasingly multidisciplinary field. This conference will present the latest research achievements and commercial applications of nanostructured materials and devices.

www.nanotoday-conference.com

MT1411p568.indd 568 11/10/2011 16:03:22

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