Scientists in the car industry are developing fuel cell vehicles, fuelled by: (i) hydrogen gas, (ii) hydrogen-rich fuels;Fuel cell A fuel cell uses the energy from the reaction of a fuel with oxygen to create a voltage Limitations of hydrogen fuel cells (i) storing and transporting hydrogen, in terms of safety, feasibility of a pressurised liquid and a limited life cycle of a solid ‘adsorber’ or ‘absorber’ (ii) limited lifetime (requiring regular replacement and disposal) and high production costs, (iii) use of toxic chemicals in their production 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 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 emf falls. A higher pressure can help counteract this 2e- + 2H+ H2 E=0V 4e- + 4H+ +O2 2H2O E=1.23V 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 Overall 2H2 + O2 2H2O E=1.23V Fuel cells will maintain a constant voltage over time as 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
/
~
~
~
/
5.2.3 Redox and electrode potentials
(k) explanation that a fuel cell uses the energy from the reaction of a fuel with oxygen to create a voltage and the changes that take place at each electrode. Recall of fuel cells and equations will not be required. Relevant electrode potentials and other data will be supplied.