 Fuel cells transform chemical energy from fuels such as hydrogen and methanol into electrical energy  The fuel is oxidised by oxygen from the air

 A hydrogen-oxygen fuel cell comprises of two inert, porous electrodes made of nickel or nickel oxide  These electrodes are also coated with a catalyst such as platinum to speed up the electrode reactions  The porous electrodes allow reactant and product gases to pass through  The electrodes are immersed in a solution of aqueous potassium hydroxide

 Hydrogen gas flows to the anode (negative electrode)  Air (source of oxygen) flows to the cathode (positive electrode)  Water vapour is the reaction product and it is pushed out by the stream of air

 At the anode (negative electrode) hydrogen is oxidised H 2 (g) → 2H + (aq) + 2e -  The hydrogen ions (H + ) move towards the cathode  The electrons flow through the circuit where the components are

 At the cathode (positive electrode), oxygen is reduced to water vapour 4H + (aq) + 4e - + O 2 (g) → 2H 2 O (g)  The electrons are gained from the circuit

 Overall reaction: H 2 (g) → 2H + (aq) + 2e - 4H + (aq) + 4e - + O 2 (g) → 2H 2 O (g) 2H 2 (g) → 4H + (aq) + 4e - 4H + (aq) + 4e - + O 2 (g) → 2H 2 O (g) 2H 2 (g) + O 2 (g) → 2H 2 O (g)  The efficiency of the fuel cell is around 40%

 The fuel cell produces an e.m.f. of V under standard conditions (100 kPa and 298 K)  The rate of reaction is too slow at these conditions  Therefore the temperature is increased to 200 °C  However the cell reaction is exothermic and an increase in temperature will decrease the e.m.f. (Le Chatelier)  However an increase in pressure can compensate for the increase in temperature

 Fuel cells provide a constant e.m.f. as long as the reactants are continuously supplied  Fuel cells do not need to be recharged

 Oxygen in the air is in plentiful supply but hydrogen is not  Hydrogen is made by the steam reforming reaction on natural gas (CH 4 ) CH 4 (g) + 2H 2 O (g) → CO 2 (g) + 4H 2 (g)  This reaction needs a large input of energy and it also releases carbon dioxide  However the efficiency of fuel cells can outweigh the energy demands of hydrogen production

 Water vapour is the only waste product of hydrogen- oxygen fuel cells  Fuel cells may replace petrol and diesel engines reducing the amount of carbon dioxide produced if the hydrogen is produced without fossil fuels  Also hydrogen is highly flammable and difficult to store