electrochemistry lecture 5_notes

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RMIT University©yyyy School/Department/Area 1

Noble metal electrochemistry, electrocatalysis and fuel cell applications


Electrochemical characterisation of metal surfaces

• Generally for noble metals we run a CV of the metal in either an acidic or basic electrolyte

• Noble metals can oxidise under potential control

• Essentially we will do chemistry on the electrode surface

• Gold and platinum are very important materials

• They have unique electrochemical signatures

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Electrochemistry of gold• Regarded as a model system

-2 adsAu H O Au OH H e

place exchange reaction - +adsAu OH OH Au

- + 2- 2+ + -OH Au O Au H e

+ -2 2 32AuO H O Au O 2H 2e

Double layer region Oxide formation

Oxide reduction


Electrochemistry of platinum

-0.4-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4-15

-10

-5

0

5

10

I [

A]

E [V] vs Ag/AgCl

Oxide region

Double layer region

H ads/des region

Hydrogen gas evolution

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Pt electrochemistry

Hydrogen adsorption/desorption

H(aq)+ e- H(ads)

Oxide region

Pt + H2O PtOH + H+ + e-

place exchangePtOH PtO + H+ + e-

PtO + H2O PtO2 + 2H+ + 2e-


Fuel cells

Pt is extremely important as a catalyst in fuel cells

We will discuss 2 main types - proton exchange membrane (PEM) fuel cell

- direct methanol fuel cell

The anode and the cathode are generally made of high surface area carbon decorated with nanostructured platinum

Anode Reaction: 2H2 => 4H+ + 4e-

Cathode Reaction: O2 + 4H+ + 4e- => 2 H2O

Overall Cell Reaction: 2H2 + O2 => 2 H2O

The electrolyte is a solid membrane (Nafion) which when hydrated allows the movement of protons but does not conduct electrons

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What is Nafion

• Nafion is a polymeric material which can conduct ions and is termed an ionomer

• It is sulfonated tetrafluoroethylene based fluoropolymer-copolymer

• It is extremely efficient at allowing protons to diffuse through it

• Protons on the SO3H groups hop from one acidic site to the next

• Has excellent mechanical and thermal stability


PEM fuel cell

• PEM fuel cells have high-power density characteristics which makes them compact and lightweight.

• The operating temperature is less than 100ºC, which allows rapid start-up. These traits and the ability to rapidly change power output are some of the characteristics that make the PEMFC the top candidate for automotive power applications.

• As the electrolyte is a solid material, compared to a liquid, the sealing of the anode and cathode gases is simpler with a solid electrolyte, and therefore, less expensive to manufacture.

• The solid electrolyte has less problems with corrosion, compared to many of the other electrolytes, thus leading to a longer cell and stack life.

• However, since the electrolyte is required to be saturated with water to operate optimally, careful control of the moisture of the anode and cathode streams is important.

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Direct methanol fuel cell• Direct methanol fuel cells (DMFC) also employ a polymer membrane as an

electrolyte.

• The system is a variant of the polymer electrolyte membrane (PEM) cell however, the catalyst on the DMFC anode utilises hydrogen from liquid methanol.

The methanol is mixed with steam and fed to the anode

Water is consumed at the anode

Water is generated at the cathode

Overall


DMFC

• The operating temperature for DMFCs is typically around 1200C, producing an efficiency of about 40%.

• DMFC units are best suited to portable applications and are being developed for a wide variety of portable electronic products such as mobile phones and laptop computers.

• During the methanol oxidation reaction carbon monoxide (CO) is formed, which strongly adsorbs onto the platinum catalyst, reducing the surface area and thus reduces the performance of the cell. Therefore new catalysts are constantly being developed to alleviate this problem (more later).

• Methanol crossing over to the cathode through the Nafion membrane can be a problem

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Fuel cells can also be miniaturised


The fuel cell is based on how effective the catalysts are in the anode and cathode

Many redox processes are rate limited by the kinetics of the electron-transfer rates and not the rates of diffusion. These heterogeneous electron-transfer rates can be dramatically controlled by the composition of the electrode double layer with respect to specific adsorption of electroinactive inorganic ions and surface active organic species. The control of these species can be employed for a variety of applications.

• Electrocatalysis is defined in its widest sense as the study of how reactions may be accelerated at electrodes. This often requires the surface of the electrode to be modified in some way or for there to be a mediating molecule close to the electrode or in solution. These are called electrocatalysts or mediators.

• The fuel cell reactions involve multi-electron reactions which are usually sluggish at metal surfaces. However, they may all be electrocatalysed with nanostructured forms of the metal or by using alloys.

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Inner sphere V outer sphere electron transfer reactions

It is often thought that the first act that occurs in an electrochemical reaction is the transfer of an electron, but this is frequently not the case for inner-sphere heterogeneous electron transfers.

The concept of inner-sphere and outer-sphere electron transfer reactions was introduced by Taube to describe solution based redox reactions.

In outer-sphere reactions, the electron transfer occurs between two species with no bonding between them, with the electron tunneling from one to the other, probably across a solvation layer.

In inner-sphere reactions, the electron transfer occurs in an activated complex where a ligand is shared between the donor and acceptor molecules (and where the bridging ligand may or may not be transferred during the reaction)


What does this mean for electrochemical reactions?

This concept can also be extended to heterogeneous electrode reactions, where in an outer-sphere reaction the reactants, products, and intermediates do not interact strongly with the electrode material and electron transfer occurs by tunneling across at least a monolayer of solvent, while in an inner-sphere reaction there is a strong interaction of reactant or product with the electrode surface

Thus, in a heterogeneous inner-sphere reactions, the reactants, intermediates, or products are often specifically adsorbed on the electrode surface. Heterogeneous inner-sphere reactions are often called electrocatalytic reactions

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How to distinguish them

• One can typically distinguish experimentally between inner- and outersphere electrode reactions because outer-sphere reactions are generally rather insensitive to the nature of the electrode material (except for small double-layer and metal electronic effects or the presence of films that block tunneling), whereas inner sphere reactions depend very strongly on the electrode material.

Inner sphere Outer sphere

The oxidation of ferrocene methanol or the reduction of Ru(NH3)6

3+, show very similar electrochemical behavior with Pt, Au, or C electrodes. However, the reduction of oxygen or of protons is very different withthese same materials.


Challenges for real electrocatalysts• In a PEMFC, the oxygen reduction reaction (ORR) at the cathode is the

kinetically slow process dominating the entire performance. Consequently, a major goal is developing active catalysts for the ORR and is the focus of PEMFC electrocatalysis R&D.

• In general, two major approaches are underway to address the issues of slow ORR activity, high cost, and insufficient stability. One is to reduce Pt loading in PEMFC catalyst layers while maintaining high performance, and the other is to explore non-noble metal catalysts that cost much less and still display the necessary performance level under PEMFC conditions.

• Research efforts for the first approach include using alloying strategies to develop low-Pt content catalysts that do not display compromised performance, and optimizing the catalyst layers to increase Pt utilization. Since the 1990s, much work has been devoted to reducing Pt usage in PEMFCs. To date, the Pt loading in a PEMFC catalyst layer can be reduced from 2.0 mg/cm2 to 0.4 mg/cm2 without any performance loss.

• Further research effort is expected to reduce the Pt loading to 0.1 mg/cm2, although it is a challenge to sustain performance and durability at this level. The most reliable approach to reach this near- or mid-term target still relies on developing higher-performance Pt-based alloy catalysts.

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ORR at Pt electrodes

We want to promote this reaction pathway

4 electron reduction – maximises output

We want to avoid this reaction pathway – why?


Change the shape of the material to promote activity

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Change the shape of the material

Oxygen reduction reaction

Better electrocatalyst related to dominant (100) crystal face

Rotating disk electrode voltammetry

Why does the exposed crystallography affect the performance?Surface energy plays a key role – influences the adsorption of reactants and/or spectator electrolyte ions


Low index planesMetal (100)

Metal (110)

Metal (111)

AFM image of Au (111)

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However defects play a significant role also


The atoms at step and kink sites have low lattice co-ordination numbers which makes them highly active

Bulk atoms have greater stability due to more nearest neighbours


Also have adatoms on the surface – created by physical vapour deposition methods or indeed present on all surfaces

Surface adatoms will have very high energy due to even lower lattice co-ordination numbers

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Scanning Tunnelling Microscopy (STM) image of copper adatoms on Cu (111) at 21 K.


Gold Spikes Smooth gold

Lots of defect sites

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Minimising the use of precious metals

Comparison of solid and hollow Pt nanoparticles of same size. The surface area is increased

CV in acid

Methanol oxidation


Technique for today – Rotating ring disk electrodes (RRDE)

• Similar to rotating disk electrodes but consists of a double working electrode

• Electrode 1 is the central disk electrode

• Electrode 2 is a ring electrode that encompasses the internal disk electrode

• The two electrodes are separated by a non-conductive barrier and connected to the potentiostat through different leads. To operate such an electrode it is necessary to use a bipotentiostat or some potentiostat capable of controlling a four electrode system.

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Rotating ring disk electrodes

The RRDE takes advantage of the form of the laminar flow created during rotation.

As the system is rotated the solution in contact with the electrode is driven to the side of the electrode the same as with a rotating disk electrode.

As the solution flows to the side it crosses the ring electrode and back into the bulk of the solution. If the flow in the solution is laminar than the solution is brought in contact with the disk quickly followed by the ring in a very controlled manner. The resulting currents are dependent on the electrodes respective potentials, areas, and spacing as well as the rotation rate and given substrate.


This design makes a variety of experiments possible, for example a complex could be oxidized at the disk and then reduced back to the starting material at the ring. It is easy to predict what the ring/disk current ratios is if this process is entirely controlled by the flow of solution. If it is not controlled by the flow of the solution the current will deviate. For example, if the first oxidation is followed by a chemical reaction, an EC mechanism, to form a product that can not be reduced at the ring then the magnitude of the ring current would be reduced. By varying the rate of rotation it is possible to determine the rate of the chemical reaction.

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The RRDE is an important tool for characterizing the fundamental properties of electrocatalysts used in fuel cells.

The proton exchange membrane (PEM) fuel cell, oxygen reduction at the cathode consisting of platinum nanoparticles can lead to an unwanted and harmful by-product, hydrogen peroxide.

Hydrogen peroxide can damage the internal components of a PEM fuel cell, so oxygen-reduction electrocatalysts are engineered in such a way as to limit the amount of peroxide formed. An RRDE "collection experiment" can be used to probe the peroxide generating tendencies of an electrocatalyst.

In this experiment, the disk is coated with a thin layer bearing the electrocatalyst, and the disk electrode is poised at a potential which reduces the oxygen. Any products generated at the disk electrode are then swept past the ring electrode. The potential of the ring electrode is poised to detect any hydrogen peroxide that may have been generated at the ring – by application of an appropriate oxidising potential.


No hydrogen peroxide detected at the ring electrode

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Gold under acidic conditions only reduces oxygen in a 2e- process to generate hydrogen peroxide

Due to a lack of OH species to promote electrocatalysis

Here we see a synergistic effect between SnO2 and Au to give an effective electrocatalysts for the 4e-reduction of O2


Poor performance of fuel cells also comes from poisoning

• Pt is susceptible to poisoning. In particular in hydrogen gas streams there can be traces of CO.

• For DMFCs during the oxidation of methanol CO can also be formed during the course of the reaction which poisons the Pt surface and de-activates the catalyst

• To alleviate this problem the addition of another components, such as ruthenium or gold, to the catalyst is found to be effective.

• According to the most well-established theory in the field, these catalysts oxidize water to yield OH radicals: H2O → OH• + H+ + e-. The OH species from the oxidized water molecule oxidizes CO to produce CO2 which can then be released as a gas: CO + OH• → CO2 + H+ + e-.

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Yoo et al,

electrochemistry lecture 5_notes

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