FUEL CELLS KNOCKHARDY PUBLISHING 2008 SPECIFICATIONS KNOCKHARDY PUBLISHING

FUEL CELLS KNOCKHARDY PUBLISHING 2008 SPECIFICATIONS

KNOCKHARDY PUBLISHING FUEL CELLS INTRODUCTION This Powerpoint show is one of several produced to help students understand selected topics at AS and A 2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards. Individual students may use the material at home for revision purposes or it may be used for classroom teaching with an interactive white board. Accompanying notes on this, and the full range of AS and A 2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at. . . www. knockhardy. org. uk/sci. htm Navigation is achieved by. . . either clicking on the grey arrows at the foot of each page or using the left and right arrow keys on the keyboard

FUEL 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

FUEL CELLS INTRODUCTION Fuel 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.

FUEL CELLS INTRODUCTION Fuel 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 CELL The 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 CELL M E M B R A N E A N O D E C A T H T O D E CLICK ON A NUMBER FOR AN EXPLANATION OF WHAT IS TAKING PLACE

PROTON EXCHANGE MEMBRANE FUEL CELL M E M B R A N E A N O D E C A T H T O D E HYDROGEN GAS ENTERS THE CELL CLICK ON A NUMBER FOR AN EXPLANATION OF WHAT IS TAKING PLACE

PROTON EXCHANGE MEMBRANE FUEL CELL M E M B R A N E A N O D E C A T H T O D E HYDROGEN IS OXIDISED TO H+ AT THE ANODE 2 H 2(g) —> 4 H+(aq) + 4 e¯ CLICK ON A NUMBER FOR AN EXPLANATION OF WHAT IS TAKING PLACE

PROTON EXCHANGE MEMBRANE FUEL CELL M E M B R A N E A N O D E C A T H T O D E ELECTRONS TRAVEL VIA THE EXTERNAL CIRCUIT CLICK ON A NUMBER FOR AN EXPLANATION OF WHAT IS TAKING PLACE

PROTON EXCHANGE MEMBRANE FUEL CELL M E M B R A N E A N O D E C A T H T O D E H+ IONS TRAVEL THROUGH THE ELECTROLYTE AND MEMBRANE CLICK ON A NUMBER FOR AN EXPLANATION OF WHAT IS TAKING PLACE

PROTON EXCHANGE MEMBRANE FUEL CELL M E M B R A N E A N O D E C A T H T O D E OXYGEN GAS ENTERS THE CELL CLICK ON A NUMBER FOR AN EXPLANATION OF WHAT IS TAKING PLACE

PROTON EXCHANGE MEMBRANE FUEL CELL M E M B R A N E A N O D E C A T H T O D E OXYGEN IS REDUCED AT THE CATHODE O 2(g) + 4 H+(aq) + 4 e¯ —> 2 H 2 O(l) CLICK ON A NUMBER FOR AN EXPLANATION OF WHAT IS TAKING PLACE

PROTON EXCHANGE MEMBRANE FUEL CELL M E M B R A N E A N O D E C A T H T O D E A CATALYST SPEEDS UP THE REACTION BETWEEN OXYGEN AND H + CLICK ON A NUMBER FOR AN EXPLANATION OF WHAT IS TAKING PLACE

PROTON EXCHANGE MEMBRANE FUEL CELL M E M B R A N E A N O D E WATER IS PRODUCED C A T H T O D E 2 H 2(g) + O 2(g) —> 2 H 2 O(l) CLICK ON A NUMBER FOR AN EXPLANATION OF WHAT IS TAKING PLACE

THEORY The relevant half reactions are… O 2(g) + 4 H+(aq) + 4 e¯ 2 H+(aq) + 2 e¯ 2 H 2 O(l) E° = +1. 23 V H 2(g) E° = 0. 00 V

THEORY The relevant half reactions are… O 2(g) + 4 H+(aq) + 4 e¯ 2 H+(aq) + 2 e¯ 2 H 2 O(l) E° = +1. 23 V H 2(g) E° = 0. 00 V The equation with the MORE POSITIVE E° value reverses the one with the less positive E° value. O 2(g) + 4 H+(aq) + 4 e¯ H 2(g) 2 H 2 O(l) 2 H+(aq) + 2 e¯

THEORY The relevant half reactions are… O 2(g) + 4 H+(aq) + 4 e¯ 2 H+(aq) + 2 e¯ 2 H 2 O(l) E° = +1. 23 V H 2(g) E° = 0. 00 V The equation with the MORE POSITIVE E° value reverses the one with the less positive E° value. O 2(g) + 4 H+(aq) + 4 e¯ H 2(g) 2 H 2 O(l) 2 H+(aq) + 2 e¯ Double the second equation to balance the electrons then combine both to give the overall reaction…

THEORY The relevant half reactions are… O 2(g) + 4 H+(aq) + 4 e¯ 2 H+(aq) + 2 e¯ 2 H 2 O(l) E° = +1. 23 V H 2(g) E° = 0. 00 V The equation with the MORE POSITIVE E° value reverses the one with the less positive E° value. O 2(g) + 4 H+(aq) + 4 e¯ H 2(g) 2 H 2 O(l) 2 H+(aq) + 2 e¯ Double the second equation to balance the electrons then combine both to give the overall reaction… 2 H 2(g) + O 2(g) —> 2 H 2 O(l) E° = +1. 23 V

THEORY The relevant half reactions are… O 2(g) + 4 H+(aq) + 4 e¯ 2 H+(aq) + 2 e¯ 2 H 2 O(l) E° = +1. 23 V H 2(g) E° = 0. 00 V The equation with the MORE POSITIVE E° value reverses the one with the less positive E° value. O 2(g) + 4 H+(aq) + 4 e¯ H 2(g) 2 H 2 O(l) 2 H+(aq) + 2 e¯ Double the second equation to balance the electrons then combine both to give the overall reaction… 2 H 2(g) + O 2(g) —> 2 H 2 O(l) E° = +1. 23 V ELECTRODE REACTIONS Anode (-) 2 H 2(g) —> 4 H+(aq) + 4 e¯ OXIDATION Cathode (+) O 2(g) + 4 H+(aq) + 4 e¯ —> 2 H 2 O(l) REDUCTION

CELL REACTIONS - SUMMARY Anode (-) OXIDATION 2 H 2(g) —> 4 H+(aq) + 4 e¯ E° = 0. 00 V Cathode (+) REDUCTION O 2(g) + 4 H+(aq) + 4 e¯ —> 2 H 2 O(l) E° = +1. 23 V overall reaction 2 H 2(g) + O 2(g) —> 2 H 2 O(l) E° = +1. 23 V

CELL REACTIONS - SUMMARY Anode (-) OXIDATION 2 H 2(g) —> 4 H+(aq) + 4 e¯ E° = 0. 00 V Cathode (+) REDUCTION O 2(g) + 4 H+(aq) + 4 e¯ —> 2 H 2 O(l) E° = +1. 23 V overall reaction Electrolyte 2 H 2(g) + O 2(g) —> 2 H 2 O(l) E° = +1. 23 V • 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? • • • 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? • • • 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 hydrogen METHODS • ideally from non-polluting and renewable resources; (solar, wind, hydro) • from hydrocarbon fuels by reforming • from natural gas (methane) or ethanol CH 4 + H 2 O —> CO + 3 H 2 • electrolysis of water

FUEL 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 CH 4 + H 2 O —> CO + 3 H 2 • electrolysis of water REFORMING Most 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%.

FUEL 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 CH 4 + H 2 O —> CO + 3 H 2 • electrolysis of water REFORMING Most 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 • adsorbed on the surface of a solid • absorbed within a solid or or

FUEL CELL VEHICLES (FCV’S) - ADVANTAGES • produce less pollution from exhaust gases (no NOx, CO, unburnt hydrocarbons) • produce less CO 2 • are more efficient

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 • 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 Limited supplies of fossil fuels may cause us 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 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 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