An introduction to fuel cells

P. A. Christensen

Winshields Crag on the Roman Wall

“For nearly 2000 years, Hadrian’s wall has brooded over the borderlands between Scotland and England. In that time, kings and queens have come and gone, empires

have rose and fell, but not a single g of coal or ml of oil has been formed”

Genesis 1839 - 1842

The fuel cell can trace its roots back to the 1800's. A Welsh born, Oxford educated barrister, Sir William Robert Grove, who practiced patent law and also studied chemistry or "natural science" as it was known at the time.

2H2O 2H2 + O2 E0 = 1.23V

Grove realized that if electrolysis, using electricity, could split water into hydrogen and oxygen then the opposite would also be true. Combining hydrogen and oxygen, with the correct method, would produce electricity.

Grove's drawing of one of his experimental "gas batteries" from an

1843 letter

To test his reasoning, Sir William Robert Grove built a device that would combine hydrogen and oxygen to produce electricity, the world's first gas battery, later renamed the fuel cell. His invention was a success, and Grove's work advanced the understanding of the idea of conservation of energy and reversibility.

Why hydrogen?

Storage methodStorage method Energy densityEnergy density

/kWh kg/kWh kg-1-1

HydrogenHydrogen 3838

GasolineGasoline 1414

Lead acid batteryLead acid battery 0.040.04

Flywheel, fused silicaFlywheel, fused silica 0.090.09

(Methanol)(Methanol) (6)(6)

An early demise……..and resurrection 1960

The world speed record in 1899, of 104 km h-1, was held byan electric vehicle, the “Jamais Contente”.

In 1900 in the USA, there were 1681 steam-driven vehicles,1575 electric vehicles and only 936 driven by petrol engines.

All electric vehicles were powered by lead-acid batteries.

A fuel tank is lighter than a lead-acid battery and can be ‘recharged’ more rapidly. A tank of fuel gives a much longerrange than a fully charged battery- current target of 300 kmStill remains elusive (battery should not exceed ca. 1/3 of totalweight of vehicle).

The advent of the self-starter (powered by a lead-acid battery!)finally clinched the relegation of electric vehicles to milk floatsand fork-lift trucks.

In the late 19th and early 20th centuries, coal was king

But all attempts to make coal fuel cells failed, and fuel cellsfell out of favour until the 1960’s, due to interest froman out of this world source!

The Gemini capsule and fuel cell

The fuel cell concept

Fuel

Products

H+

Cath

odeA

nod

e

Oxidant

Products

The fuel cell concept

e-

Solid, liquid or polymer electrolyte

The electrolyte essentially:

•Separates fuel and oxidant•Facilitates ion transport between anolyte and catholyte•Prevents electrical short circuit between anode and cathode

And can be liquid, solid or polymeric

H2

H+

Cath

ode (P

t catalyst)An

ode

(Pt

cata

lyst

)

O2

H2O

e-

2H2 4H+ + 4e- 4H+ + 4e- + O2 2H2O

The simplest realisation – the H2/O2 fuel cell

Pt Pt

Ele

ctro

l

yte

Practicalities

The structure of Gas Diffusion Electrodes (GDE’s)(Porous carbon area up to 1000 m2 g-1)

FlowField

GasDiffusionLayer

Catalyst LayerGases:WettabilityFlooding/conductivity3-phase interface

Zone of high current density - 3 phase zone

The three-phase zone in a gas diffusion electrode

In aqueous solution H2 or O2 only soluble to ca. 1mM at 1 atm.

Electrode

Electrolyte

H2 or O2

Carbon + Pt

Carbon + Pt

Reaction zone

Gas spaceElectrolyte Pore

The three-phase zone in a single pore of a gas diffusion electrode

A typical (H2/O2) fuel cell voltage vs current plot

Thermodynamic cell voltage

Ohmic loss

Activation overpotentialloss - catalysts Mass transport

loss

Sources of hydrogen

5% Electrolysis (wind, solar, wave, nuclear….) – expensive (operating cost 50p per kWh) but pure hydrogen95% Reforming of organics -operating cost for H2 production 5p per kWh, but fuel cell system more complex and more expensive to construct.

650 – 850 C/Rh:CnH2n+2 + nH2O nCO + (2n+1)H2

CO + H2O CO2 + H2

(Methanol can be reformed at 300 C)

Either high T or CO-tolerant anode catalysts. H2S can also be present in reformed fuel.

Common Fuel cells

The most general fuel and oxidant are H2 and O2 (air). The highest temperature fuel cells (SOFC & MCFC) can usea variety of organics directly as fuels, whilst methanol is used in the low temperature Direct Methanol Fuel Cell.

1. Low temperature fuel cells

Alkaline Fuel Cells (AFC)• The simplest realisation of the fuel cell concept• Operates at 70 C, PTFE-bound porous carbon electrodes

with Pt catalysts, 30% KOH electrolyte• Runs on pure H2 + pure O2

• Power generating efficiencies of up to 70%, 0.3 – 12 kW• Compact.• Small commercial units available up to 100 kW• High power/weight ratio (hence space application)• Produces pure water and heat• Low thermal signature, silent, pollution-free exhaust• Alkaline solution – do not need noble metal catalysts

(Siemens: 1 mg cm-2 Ti-doped Raney Nickel/60 mg cm-2 Ag)

KOH

Gas DiffusionElectrode

Electrode support

The Alkaline Fuel Cell (AFD)

AFC Problems• Use of KOH as electrolyte and air as oxidant leads to

fouling by precipitation of K2CO3 • High efficiencies achieved with high catalyst loadings• H2O product dilutes KOH and reduces performance –

hence needs water evaporator• CO or H2S in reformate poisons anode catalyst• £1500 - £2500 per kW; fuel cost £0.50 per kWh

Monday 13 April 1970, 9.07 pm

200,000 miles out in space

O2 cryotank 2 explodes on Apollo 13

The Apollo fuelcell power plant.

31 cells, 100 mA cm-2, in total 1.12 kW at 28V.110 kg

Solid Polymer Electrolyte (SPE) aka Polymer Electrolyte Membrane (PEM) Fuel Cells

• Most favoured for traction (cars and buses) – small family car (800 kg unladen weight, 80 km hr-1 cruising speed) needs 6 – 12 kW. Otherwise military (submarine) and space

• Runs on pure H2 + air/O2

• Operating T up to ca. 90 C, PTFE-bound porous carbon electrodes with Pt catalysts, Solid Polymer (Nafion) electrolyte

• Small commercial units up to 500 W available

SPE Problems• £2500 - £5000 per kW; fuel cost £0.50 per kWh• Needs water separator• CO in fuel must be below 100 ppm

2 At the anode, a platinum or non- platinum catalyst causes the hydrogen to split into positive hydrogen ions (protons) and negatively charged electron.

1 Hydrogen is channelled through field flow plates to the anode on one side of the fuel cell, while oxygen from the air is channelled to the cathode on the other side of the cell

4 At the cathode, the electrons and positively charged hydrogen ions combine with oxygen to form water which flows out of the cell.

3 The Polymer Electrolyte Membrane (PEM) allows only the positively charged ions to pass through it to the cathode. The negatively charged electron must travel along an external circuit to the cathode, creating an electrical current.

Hydrogen Air (Oxygen)

Anode Cathode

PEM

H2 → 2H+ + 2e-

½O2 + 2H+ + 2e- → H2O

Methanol

CH3OH + H2O → 6H+ + CO2 + 2e-

2. Intermediate temperature fuel cells

Phosphoric Acid Fuel Cells (PAFC)• The only commercially available fuel cell (> 200 fuel cell

systems have been installed all over the world)• Runs on H2, methane, natural gas + air/O2

• Generate electricity at > 40% efficiency (ca. 85% if the steam produced is used for cogeneration; cff ca. 35% for the utility power grid in the USA)

• Graphite felt+low Pt loading, concentrated phosphoric acid (polyphosphoric acid) electrolyte absorbed in SiC

• Operating temperatures 150 - 220 C• High O2 solubility • CO tolerant ca. 1 - 2 percent% due to higher operating T• Existing PAFCs have outputs up to 200 kW (11 MW units

have been tested). Combined Heat and Power operation.

PAFC Problems• £2000 per kW; fuel cost £0.50 per kWh with reformer• H2S in reformate poisons anode catalyst• Need desulfurizer, water separator, heat exchanger and

reformer- complex (especially wrt heat management) & heavy system hence mainly stationary applications, although also buses.

• Oxidation of carbon support, agglomeration of Pt particles, flooding of electrodes and loss of acid- eg. reliability, lifetime and maintenance costs

3. High temperature fuel cells

Molten Carbonate Fuel Cell (MCFC) • Molten alkali metal carbonate (Li, Na, K) electrolyte

in a cermaic tile, Ni anode and lithiated nickel oxide cathode

• Runs on H2, methane, natural gas + air/O2

• 650 C as carbonate must be molten and conductive• Higher overall system efficiencies; combined cycle

possibility for heat usage• Greater flexibility in the use of available fuels.• Envisaged for power production and load levelling

MCFC Problems• Cost per kW not yet known, but must be brought down to

< £500 - £1000 per kW to match costs of conventional power stations; fuel cost £0.50 per kWh with reformer

• Complexity- needs water evaporator, heat exchanger and reformer (but possibility of internal reforming-right T)

• Stability of electrodes and electrolyte matrix; the high operating temperature, however, imposes limitations and constraints on choosing materials suitable for long lifetime operations

Solid Oxide Fuel Cell (SOFC)

• Solid, nonporous metal oxide electrolytes (stabilised ZrO2)• 1000 C, hence internal reforming and rapid kinetics with

nonprecious materials; nickel anode, Sr-doped LaMnO3 cathode, ZrO2.15%Y2O3 solid electrolyte

• Produces high quality heat• No restriction on the cell configuration.• Power generating efficiencies of SOFCs could reach 60%,

85% with co-generation.• Experimental systems up to few kW

SOFC Problems• Cost per kW not yet known; fuel cost £0.50 per kWh with

reformer• Complexity- needs water evaporator, heat exchanger and

reformer (but possibility of internal reforming-right T)• Stability of electrodes and electrolyte matrix; the high

operating temperature imposes limitations and constraints on choosing materials suitable for long lifetime operations. Biggest problem is thermal expansion, rendering SOFC intolerant to repeated start-up-shut-down cycles.

4. Other Fuel Cells• Bio-• Micro-

5. The Direct Methanol Fuel Cell – a lowtemperature fuel cell

In the DMFC Methanol is oxidized directly at the anode (as In the DMFC Methanol is oxidized directly at the anode (as opposed to Hopposed to H22 as in the commonly known Hydrogen PEMFC as in the commonly known Hydrogen PEMFC).).

Cell Reactions Cell Reactions Anode: CHAnode: CH33OHOH(l)(l) + H + H22O -> 6eO -> 6e-- + 6H + 6H++ + CO + CO2(g) 2(g) PtRu catalystPtRu catalyst

Cathode: 1.5 OCathode: 1.5 O2(g)2(g) + 6e + 6e-- -> 3H -> 3H22OO(l)(l) Pt catalyst Pt catalyst

Overall: CHOverall: CH33OHOH(l)(l) + 1.5 O + 1.5 O2(g)2(g) -> 3H -> 3H22O(l) + COO(l) + CO2(g) 2(g) (E(E°=°=1.2 V, 90°C)1.2 V, 90°C)

Liquid CHLiquid CH33OH is preferred over vapour due to the OH is preferred over vapour due to the

simplicity of design offered; existing liquid fuel distribution simplicity of design offered; existing liquid fuel distribution network.network.

CHCH33OH is considered by some of have lower market entry OH is considered by some of have lower market entry

barriers than Hbarriers than H22 ( (egeg. less explosive). less explosive)

DMFC problems

• Low temperature- poor kinetics at anode and cathode-much lower power density than H2/O2

• Needs Ru co-catalyst- Pt poisons otherwise• Methanol cross over through membrane to

cathode, Pt active for methanol oxidation, hence mixed potential

Medium and high temperature Fuel cells have a potentially major role in ‘Distributed power systems’

Distributed generation commonly refers to on-site power generation technology, which is tailored to meeting the needs of the consumer. Combined Heat and Power (CHP) systems are on-site generation systems, which achieve high efficiency through the concurrent production of electric power and process heat (PAFC-heat houses, MCFC and SOFC – operate steam turbine). Distributed generation is an alternative or complementary approach to reliance on grid power. It provides another means of meeting the nations future energy and security needs while increasing the reliability of power supply to the owners.

Community Project: Middlehaven

The Creation of an Energy Services Company (ESCo) to Create a New Energy Approach to a Major Regeneration Project

Coordinated new energy approach to whole development including:

Energy saving design, Gas Engine Combined heat and power, District heating and Fuel Cell System balancing heat and power requirement.

The fuel cell stack

The (SPE) fuel cell stack

Heliocentris Water-cooled PEM fuel cell stack of 20 single cells.Rated output: 300 W. Electric heat output: 300 W thermal.

Open circuit voltage: 18 V. DC rated voltage: 12 V DC.

1.5 kW H2/O2 Arbin Fuel Cell stack in Newcastle

Facts and figures

For the hydrogen economy in general and hydrogen-powered vehicles in particular, the key problems remain:

• How to generate hydrogen cheaply• How to store hydrogen safely, and without a serious

weight penalty• How to distribute hydrogen• Public perception

For the low and medium temperature fuel cells, additional problems are highlighted in the following table opposite of costs, prepared by the Center for Solar Energy and Hydrogen Research in Ulm, for a 1 kW H2/O2 PEM stack.

CostCost / /€€MEAsMEAs 60006000

SystemSystem 850850

Bipolar platesBipolar plates 74007400

SealsSeals 10001000

End plates/ End plates/ current current collectorscollectors

12001200

Purchasing and Purchasing and assembly assembly

26002600

QAQA 12001200

TOTALTOTAL 2000020000

Production of 1000 units could lower this unit cost to ca. €3000€3000

• Electricity from the National Grid is sold at ca. £0.07 per kWh

• Internal combustion engine £30 - £60 per kW• PAFC ca. £2100 per kW• Lead-acid battery £200 - £300 per kW

However:• Small lithium batteries £300 per kWh

 

Environmental Benefits: Fuel cells are considered an excellent alternative energy resource from the environmental point of view. Fuel cells are quiet and produce negligible emissions of pollutants.

Efficiency: Different types of fuel cells have varied efficiencies. Depending on the type and design of fuel cells, efficiency ranges from 40 to 60%. Alkaline fuel cells can even achieve power generating efficiencies of up to 70%.

Fuel Availability: The primary fuel source for the fuel cell is hydrogen which can be obtained from natural gas, coal gas, methanol, and other fuels containing hydrocarbons.

Comparision of carbon dioxide, nitrous oxides, sulphur dioxide and noise emissions between the four main engine types.

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