Fuel Cells

FUELFUELCELLSCELLS

KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING2008 2008

SPECIFICATIONSSPECIFICATIONS

INTRODUCTION

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KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING

FUEL CELLSFUEL CELLS

CONTENTS• Introduction

• The Proton Exchange Membrane Fuel Cell (PEMFC)

• PEMFC Cell – theory and electrode reactions

• Why use fuel cells?

• Sources of hydrogen

• Fuel cell vehicles

• Fuel cells - Advantages

• Fuel cells – Disdavantages

• The future of fuel cells

• Other types of fuel cell



INTRODUCTIONFuel cells generate electricity from an electrochemical reaction in which oxygen (from air) and a fuel (e.g. hydrogen) combine to form water.

The electricity produced can be used to power cars, buses, laptops and mobile phones. The by-product, heat, can also be used.


INTRODUCTIONFuel cells generate electricity from an electrochemical reaction in which oxygen (from air) and a fuel (e.g. hydrogen) combine to form water.

The electricity produced can be used to power cars, buses, laptops and mobile phones. The by-product, heat, can also be used.

STRUCTURE• cells consist of two electrodes, a negative anode and a positive cathode• electrodes are separated by a solid or liquid electrolyte• electrically charged particles move between the two electrodes• catalysts (e.g. Pt) are often used to speed up reactions at the electrodes• electricity is generated when oxygen and hydrogen combine

A TYPICAL FUEL CELLA TYPICAL FUEL CELLThe Proton Exchange Membrane Fuel Cell - PEMFC

Operation• hydrogen (the fuel) is oxidised to H+ ions (protons) at the anode• protons move through the electrolyte• electrons pass through the external circuit• oxygen is reduced at the cathode• water is produced• a platinum catalyst accelerates the reactions at the electrodes

PROTON EXCHANGE MEMBRANE FUEL CELLPROTON EXCHANGE MEMBRANE FUEL CELL

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HYDROGEN GAS ENTERS THE CELL



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HYDROGEN IS OXIDISED TO H+ AT THE ANODE

2H2(g) —> 4H+(aq) + 4e¯


ELECTRONS TRAVEL VIA THE EXTERNAL CIRCUIT


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H+ IONS TRAVEL THROUGH THE ELECTROLYTE AND MEMBRANE


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OXYGEN GAS ENTERS THE CELL


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OXYGEN IS REDUCED AT THE CATHODE


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O2(g) + 4H+(aq) + 4e¯ —> 2H2O(l)


A CATALYST SPEEDS UP THE REACTION BETWEEN OXYGEN AND H+


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WATER IS PRODUCED


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2H2(g) + O2(g) —> 2H2O(l)


THEORYTHEORYThe relevant half reactions are…

O2(g) + 4H+(aq) + 4e¯ 2H2O(l) E° = +1.23V 2H+(aq) + 2e¯ H2(g) E° = 0.00V



The equation with the MORE POSITIVE E° value reverses the one with the less positive E° value.

O2(g) + 4H+(aq) + 4e¯ 2H2O(l)

H2(g) 2H+(aq) + 2e¯




O2(g) + 4H+(aq) + 4e¯ 2H2O(l)

H2(g) 2H+(aq) + 2e¯

Double the second equation to balance the electrons then combine both to give the overall reaction…




O2(g) + 4H+(aq) + 4e¯ 2H2O(l)

H2(g) 2H+(aq) + 2e¯


2H2(g) + O2(g) —> 2H2O(l) E° = +1.23V




ELECTRODE REACTIONSAnode (-) 2H2(g) —> 4H+(aq) + 4e¯ OXIDATION

Cathode (+) O2(g) + 4H+(aq) + 4e¯ —> 2H2O(l) REDUCTION

O2(g) + 4H+(aq) + 4e¯ 2H2O(l)

H2(g) 2H+(aq) + 2e¯


2H2(g) + O2(g) —> 2H2O(l) E° = +1.23V

CELL REACTIONS - SUMMARYCELL REACTIONS - SUMMARY

Anode (-) 2H2(g) —> 4H+(aq) + 4e¯ E° = 0.00V OXIDATION

Cathode (+) O2(g) + 4H+(aq) + 4e¯ —> 2H2O(l) E° = +1.23V REDUCTION

overall reaction 2H2(g) + O2(g) —> 2H2O(l) E° = +1.23V

CELL REACTIONS - SUMMARYCELL REACTIONS - SUMMARY

Anode (-) 2H2(g) —> 4H+(aq) + 4e¯ E° = 0.00V OXIDATION

Cathode (+) O2(g) + 4H+(aq) + 4e¯ —> 2H2O(l) E° = +1.23V REDUCTION

overall reaction 2H2(g) + O2(g) —> 2H2O(l) E° = +1.23V

Electrolyte • Carries charged particles from one electrode to the other

• It must allow only the appropriate ions to pass between the electrodes

• If other substances travel through the electrolyte, they can disrupt the chemical reaction.

FUEL CELLS – Why use them?FUEL CELLS – Why use them?

• our society is dependent upon fossil fuels such as coal, oil and gas• fossil fuels are a non-renewable energy resource• fuel prices are rising and resources dwindling• food, transport and electricity costs are affected by fuel prices• the atmosphere is becoming more and more polluted• carbon dioxide contributes to climate change and the greenhouse effect

FUEL CELLS – Why use them?FUEL CELLS – Why use them?

• our society is dependent upon fossil fuels such as coal, oil and gas• fossil fuels are a non-renewable energy resource• fuel prices are rising and resources dwindling• food, transport and electricity costs are affected by fuel prices• the atmosphere is becoming more and more polluted• carbon dioxide contributes to climate change and the greenhouse effect

Limitations• storage of hydrogen - safety considerations• transportation of hydrogen - low density so expensive to deliver• feasibility of liquefied hydrogen under pressure - safety considerations• limited life of adsorber / absorber - economic considerations• limited life cycle of cell - economic considerations• high production costs - economic considerations• use toxic chemicals in cell production - environmental considerations

FUEL CELLS – Manufacture of hydrogenFUEL CELLS – Manufacture of hydrogen

METHODS• ideally from non-polluting and renewable resources; (solar, wind, hydro)• from hydrocarbon fuels by reforming• from natural gas (methane) or ethanol CH4 + H2O —> CO + 3H2

• electrolysis of water




REFORMINGMost of today’s hydrogen is generated by steam reforming. Unfortunately it uses non-sustainable, natural resources.Fuel is mixed with steam in the presence of a metal catalyst to produce hydrogen and carbon monoxide.This method is cost effective and efficient with conversion rates of 70-80%.




REFORMINGMost of today’s hydrogen is generated by steam reforming. Unfortunately it uses non-sustainable, natural resources.Fuel is mixed with steam in the presence of a metal catalyst to produce hydrogen and carbon monoxide.This method is cost effective and efficient with conversion rates of 70-80%.

STORAGE OF HYDROGEN• liquid stored under pressure or• adsorbed on the surface of a solid or• absorbed within a solid

FUEL CELL VEHICLES (FCV’S) - ADVANTAGES FUEL CELL VEHICLES (FCV’S) - ADVANTAGES

• produce less pollution from exhaust gases (no NOx, CO, unburnt hydrocarbons)

• produce less CO2

• are more efficient

FUEL CELLS - Advantages FUEL CELLS - Advantages

• eliminates pollution caused by burning fossil fuels; the only by-product is water

• eliminates greenhouse gases if the hydrogen used comes from electrolysis of water

• eliminates economic dependence on politically unstable countries for fossil fuel

• have a higher efficiency than diesel or gas engines

• most operate silently compared to internal combustion engines

• some have low heat transmission - ideal for military applications

• operating times are much longer than with batteries

• maintenance is simple since there are few moving parts in the system

FUEL CELLS - Disadvantages FUEL CELLS - Disadvantages

• production, transportation, distribution and storage of hydrogen is difficult

• reforming is technically challenging and not environmentally friendly

• refuelling and starting times of fuel cell vehicles (FCV’s) are longer

• driving range of cars is shorter than in a traditional vehicles

• fuel cells are generally slightly bigger than comparable batteries or engines

• currently expensive to produce, since most units are hand-made

• some use expensive materials

• the technology is not yet fully developed and few products are available

FUEL CELLS - The future FUEL CELLS - The future

Limited supplies of fossil fuels may causeus to move to a ‘hydrogen economy’.

However

• greater acceptance by the public and politicians is necessary

• handling and maintenance of hydrogen systems must be safe

• improvements to hydrogen manufacturing must be made

FUEL CELLS – Other typesFUEL CELLS – Other types

Fuel cells are classified according to the nature of the electrolyte.

Alkaline Fuel Cells (AFC) • uses an alkaline electrolyte such as potassium hydroxide• used by NASA in space shuttles

Direct Methanol Fuel Cells (DMFC) • uses a polymer membrane as an electrolyte• a catalyst on the anode draws hydrogen from liquid methanol• eliminates need for a fuel reformer, so pure methanol can be used as fuel

Molten Carbonate Fuel Cells (MCFC) • uses a molten carbonate salt as the electrolyte.• has the potential to be fuelled with coal-derived fuel gases, methane / natural gas

Phosphoric Acid Fuel Cells (PAFC) • anode and a cathode made of a finely dispersed platinum catalyst on carbon• has a silicon carbide structure that holds the phosphoric acid electrolyte• used to power many commercial premises and large vehicles, such as buses

FUEL CELLS – Other typesFUEL CELLS – Other types

Fuel cells are classified according to the nature of the electrolyte.

Proton Exchange Membrane Fuel Cells (PEMFC) • uses a polymeric membrane as the electrolyte, with platinum electrodes• operate at relatively low temperatures.• can vary their output to meet shifting power demands.• best for cars, for buildings and smaller applications

Solid Oxide Fuel Cells (SOFC) • use solid ceramic electrolytes; eg zirconium oxide stabilised with yttrium oxide• work at high temperatures• can reach efficiencies of around 60 per cent• are expected to be used for generating electricity and heat in industry• have potential for providing auxiliary power in vehicles

Regenerative Fuel Cells (RFC) • produce electricity from hydrogen and oxygen• can be reversed to produce hydrogen and oxygen; effectively storing energy

©2010 JONATHAN HOPTON & KNOCKHARDY PUBLISHING©2010 JONATHAN HOPTON & KNOCKHARDY PUBLISHING

THE ENDTHE END

FUELFUELCELLSCELLS

Fuel Cells

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