16B1.Redox equlibria

• Fuel cell


  Produces a voltage from the chemical reaction of a fuel with oxygen
• Types of fuel cells


  • Hydrogen-oxygen fuel cell
  • Methanol/ethanol fuel cell
  • Hydrogen-rich methanol fuel cell
• Hydrogen-oxygen fuel cell


  1. Hydrogen supplied externally as a gas

  2. Cell can operate as long as fuel supply is maintained
• Hydrogen-oxygen fuel cell


  • Both metal electrodes coated with platinum, which catalyses the reactions
  • Reactions take place in the presence of an acidic electrolyte
• Reactions in hydrogen-oxygen fuel cell

  1. At negative electrode: H2(g) → 2H+(aq) + 2e¯

  2. At positive electrode: O2(g) + 2H+(aq) + 2e¯ → H2O(l)

  3. Hydrogen ions pass through proton exchange membrane to react with oxygen
• Overall cell reaction


  H2(g) + O2(g) → H2O(l)
• Reactions in hydrogen-oxygen fuel cell with alkaline electrolyte

  1. At negative electrode: H2(g) + 2OH(aq) → 2H2O(l) + 2e¯

  2. At positive electrode: O2(g) + H2O(l) + 2e¯ → 2OH(aq)

  3. Overall cell reaction: H2(g) + O2(g) → H2O(l)
• Advantages of hydrogen-oxygen fuel cells

  • Alternative to direct use of fossil fuels
  • Products do not include CO2 or pollutants
  • Lighter and more efficient than engines using fossil fuels
• Problems with hydrogen-oxygen fuel cells

  • Hydrogen explodes easily when ignited, so care needed when transporting
  • Compressing gas has safety disadvantages
  • Adsorbing onto solids or absorbing into materials have challenges
  • Hydrogen supply is limited, expensive to produce from renewable sources
• Vehicles using fuel cell technology

  • Forklifts
  • Space applications
  • Cars (Toyota Mirai, Hyundai, Honda)
  • Trucks, buses, boats, motorcycles, bicycles
• Nickel-cadmium (NiCd) cells are used in many electronic devices because they are very reliable, economical and easy to use
• Standard electrode potentials for NiCd cells

  • Cd(OH)2(s) + 2e¯ = Cd(s) + 2OH-(aq) E = -0.88 V
  • NiO(OH)(s) + H2O(l) + e¯ = Ni(OH)2(s) + OH(aq) E = +0.52 V
• Reactions in NiCd cell

  Oxidation and reduction during discharge
• Fuel cells are being developed as an alternative to petrol and diesel in cars
• Methanol fuel cell


  Methanol supplied at negative electrode, oxygen at positive electrode
• Reaction at positive electrode in methanol fuel cell


  O2(g) + 2H+(aq) + 2e¯ → H2O(l)
• Reaction at negative electrode in methanol fuel cell


  Deduced from complete combustion of methanol in oxygen
• Advantages of fuel cells compared to combustion of petrol/diesel

  • No carbon dioxide or pollutants produced
  • More efficient
• Advantage of using methanol over hydrogen in fuel cells for cars

  • Easier to store and transport than hydrogen
• Difference between fuel cell and conventional electrochemical cell

  Fuel cell produces voltage from chemical reaction of fuel with oxygen, conventional cell uses stored chemical energy
• Ways to store hydrogen as fuel for cars

  • Compressing the gas
  • Adsorbing onto solid materials
  • Absorbing into materials
• Some scientists consider using hydrogen as a fuel for cars uses up more energy than using petrol or diesel