6 Fuel cells

Fuel cells A fuel cell uses the energy from the reaction of a fuel with oxygen to create a voltage. Hydrogen Fuel cell (potassium hydroxide electrolyte) 4e- + 4H2O  2H2 +4OH- E=-0.83V 4e- + 2H2O +O2  4OH- E=+0.4V Overall reaction 2H2 + O2  2H2O E=1.23V Using standard conditions: The rate is too slow to produce an appreciable current. Higher temperatures are therefore used to increase rate but the reaction is exothermic so by applying le Chatelier would mean the E cell falls. A higher pressure can help counteract this. In acidic conditions these are the electrode potentials. The Ecell is the same as alkaline conditions as the overall equation is the same O2 from air H2O +heat H2 from fuel Fuel cells will maintain a constant voltage over time because they are continuously fed with fresh O2 and H2 so maintaining constant concentration of reactants. This differs from ordinary cells where the voltage drops over time as the reactant concentrations drop. Alkaline Conditions You should learn the details of the lithium cell and the hydrogen fuel cell in alkaline conditions. Advantages of Fuel cells over conventional petrol or diesel-powered vehicles (i) less pollution and less CO2 . (Pure hydrogen emits only water whilst hydrogen-rich fuels produce only small amounts of air pollutants and CO2 ). (ii) greater efficiency Hydrogen is readily available by the electrolysis of water, but this is expensive. To be a green fuel the electricity needed would need to be produced from renewable resources Ethanol fuel cells have also been developed. Compared to hydrogen fuel cells they have certain advantages including. Ethanol can be made from renewable sources in a carbon neutral way. Raw materials to produce ethanol by fermentation are abundant. Ethanol is less explosive and easier to store than hydrogen. New petrol stations would not be required as ethanol is a liquid fuel. Equation that occurs at oxygen electrode 4e- + 4H+ +O2  2H2O E=1.23V C2H5OH + 3O2 → 2CO2 + 3H2O Equation that occurs at ethanol electrode C2H5OH + 3H2O → 2CO2 + 12H+ + 12e- Overall equation. Limitations of hydrogen fuel cells (i) expensive (ii) storing and transporting hydrogen, in terms of safety, feasibility of a pressurised liquid and a limited life cycle of a solid ‘adsorber’ or ‘absorber’ (iii) limited lifetime (requiring regular replacement and disposal) and high production costs, (iv) use of toxic chemicals in their production. Hydrogen can be stored in fuel cells (i) as a liquid under pressure, (ii) adsorbed on the surface of a solid material, (iii) absorbed within a solid material;