A fuel cell is a continuous power-generation device that produces water and electricity from hydrogen and oxygen. It is a key technology for a decarbonized society.
Basic Knowledge
Basic reactions of a fuel cell:Negative electrode: 2H2 → 4H+ + 4e-Positive electrode: O2 + 4H+ + 4e- → 2H2O
Overall reaction: 2H2 + O2 → 2H2O.
This is the combustion reaction of hydrogen itself, with ΔH=-572 kJ/(2 mol H2). The theoretical electromotive force is 1.23 V.
Types: ① PEFC(polymer electrolyte fuel cell, Nafion membrane, 80℃, used in automobiles), ② SOFC(solid oxide fuel cell, ZrO2, 800℃, used for stationary power supplies), ③ PAFC(phosphoric acid fuel cell), ④ MCFC(molten carbonate fuel cell).
Fuel cell（A device that generates electricity through continuous fuel supply）
PEFC（Polymer electrolyte type; used in automobiles）
SOFC（Solid oxide type; high temperature; stationary use）
Theoretical electromotive force（1.23 V）
Energy efficiency（Over 90% with power generation efficiency and heat recovery）
All-solid-state battery（Liquid electrolyte replaced with a solid; for next-generation EVs）
Further Exploration (Principles and Applications)
Fuel cells are not subject to the Carnot-efficiency limitation of heat engines, so they are theoretically highly efficient. Household Ene-Farm cogeneration systems achieve an overall efficiency of over 90% through power generation and waste-heat utilization.
Next-generation batteries include:
① All-solid-state batteries: replace the liquid electrolyte with a solid to reduce the risk of ignition and increase energy density
② Li-S batteries: use a sulfur positive electrode to achieve high capacity(theoretical 2,500 Wh/kg)
③ Na batteries: reduce resource risks by replacing Li
④ Redox-flow batteries: large-scale storage batteries
To realize a hydrogen society, infrastructure for production(green hydrogen=water electrolysis using renewable energy), storage, and transportation must be developed.
- Fuel cell=hydrogen+oxygen⇒water+electricity
- Theoretical electromotive force 1.23 V
- PEFC=automobiles(MIRAI, etc.)
- SOFC=stationary power generation and high temperature
- No Carnot-efficiency limitation
- All-solid-state battery=next-generation EV
- Green hydrogen=derived from renewable energy
Points to Note (Common Confusions and Frequent Mistakes)
① A fuel cell is not a storage battery(it is a power-generation device supplied continuously with fuel from outside). ② Fuel-cell vehicles(FCVs) and EVs(electric vehicles) are different. ③ Hydrogen is a secondary energy carrier(electricity is needed for its production), not an energy source. ④ Hydrogen produced from fossil fuels(gray hydrogen) emits CO2 and is not decarbonized.
Practice
- Write the negative-electrode reaction, positive-electrode reaction, and overall reaction of a hydrogen–oxygen fuel cell with an acidic electrolyte.
- Explain why the overall efficiency of fuel cells is considered higher than that of gasoline engines.
- When hydrogen is called “decarbonized energy,” what point about its production method must be considered?