Fuel cells

What is a fuel cell?

In plain words: a steam power station burns fuel, makes steam and drives a turbine-generator, rejecting some energy as heat. Gas-turbine and combined-cycle plants use different arrangements. A fuel cell skips all of that. It lets hydrogen and oxygen react on two separate electrodes, so the electrons they exchange go through a wire and do work directly.

Why not just burn the fuel?

• In a thermal power plant the heat of combustion of coal, gas or oil first turns water into high-pressure steam, which runs a turbine to produce electricity. NCERT calls this not a very efficient method and a major source of pollution (page 56). Strictly, a heat engine must reject some of its heat, which limits its efficiency (reasoned).
• A fuel cell is a galvanic cell built to run on a continuous supply of fuel (NCERT page 56).

Hydrogen–oxygen fuel cell

• One of the most successful fuel cells uses the reaction of hydrogen with oxygen to form water. It provided electrical power in the Apollo space programme, and the water vapour it produced was condensed and added to the astronauts’ drinking water (NCERT page 56).
• Hydrogen and oxygen are bubbled through porous carbon electrodes into concentrated aqueous sodium hydroxide. Finely divided platinum or palladium in the electrodes acts as a catalyst, speeding up the electrode reactions (NCERT page 56).
• Beyond the NCERT text: alkaline fuel cells, the type used in the US space programme, normally use potassium hydroxide (US Department of Energy, “Fuel Cells” and “Types of Fuel Cells”). The reactions are the same; write NCERT’s sodium hydroxide unless a question offers potassium hydroxide.
• The electrode reactions (NCERT page 56):
  • cathode: O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)
  • anode: 2H₂(g) + 4OH⁻(aq) → 4H₂O(l) + 4e⁻
  • overall: 2H₂(g) + O₂(g) → 2H₂O(l)
• The cell runs continuously as long as the reactants are supplied (NCERT page 56). The hydroxide ions made at the cathode travel through the electrolyte to the anode, so the cell reaction uses up no NaOH; the water it makes would dilute the electrolyte, so it is removed continuously (reasoned from the electrode reactions; NCERT page 56).
• Its voltage. The overall reaction is the same as in acid, so the standard emf is the difference between O₂/H₂O and H⁺/H₂ in NCERT Table 2.1: 1.23 − 0.00 = 1.23 V (reasoned from NCERT Table 2.1, page 37). In the alkaline cell the hydrogen couple, 2H₂O + 2e⁻ → H₂ + 2OH⁻, is −0.83 V in NCERT Table 2.1, so the same 1.23 V puts O₂/OH⁻ at +0.40 V (reasoned).

Efficiency of fuel cells

• Fuel cells produce electricity with an efficiency of about 70%, against about 40% for thermal plants (NCERT pages 56–57).
• Beyond the NCERT text: the US Department of Energy says fuel cells can exceed 60% efficiency, and combustion power plants typically run at about 33% (US Department of Energy, “Fuel Cells” and “Types of Fuel Cells”). In an exam, use NCERT’s 70% and 40%; in a problem, use the values it gives. Real figures depend on the cell and the plant: state whether efficiency counts electricity alone or electricity plus recovered heat, and whether fuel energy uses the higher or lower heating value (reasoned).
• There has been tremendous progress in new electrode materials, better catalysts and electrolytes for increasing their efficiency; fuel cells have been used in automobiles on an experimental basis, and in view of their future importance, a variety of fuel cells have been fabricated and tried (NCERT page 57). Beyond the NCERT text: several vehicle manufacturers now have hydrogen fuel-cell vehicles (US Energy Information Administration, “Use of hydrogen”).
• NCERT slip (page 57): NCERT says fuel cells “are pollution free”. Only the hydrogen–oxygen cell earns that where it runs: its one product is water (US Department of Energy, “Fuel Cells” and “Types of Fuel Cells”). Methane and methanol cells give carbon dioxide (reasoned). Beyond the NCERT text: making the hydrogen can itself release carbon dioxide, since nearly all commercial hydrogen in the United States comes from steam reforming of methane (US Energy Information Administration, “Production of hydrogen”). In an exam, write NCERT’s “pollution free”; if a question asks about methane or methanol cells, or about where the hydrogen comes from, add the qualification.
• The standard-state ceiling at 298 K (reasoned; liquid-water product, higher-heating-value basis). The most electrical work a reaction can give is −Δ_rG, the fall in its Gibbs energy, while burning it releases −Δ_rH as heat. For H₂ + ½O₂ → H₂O(l), with n = 2:
  • Δ_rG^⊖ = −nFE^⊖ = −2 × 96 487 × 1.23 = −237 kJ mol⁻¹ (NCERT Eq. (2.16), page 40);
  • Δ_rH^⊖ = −285.8 kJ mol⁻¹, the enthalpy of formation of liquid water (NCERT Class 11 Chemistry, Unit 5, page 150);
  • so in this reversible 298 K standard-state comparison, 237/285.8 = 83% of the reaction enthalpy can come out as electricity. NCERT puts practical cells at about 70% (NCERT page 56).

The hydrogen economy

• Burning fossil fuels gives carbon dioxide, which is strengthening the greenhouse effect: the Earth’s surface warms, polar ice melts and sea levels rise, threatening low-lying coasts and island nations such as the Maldives (NCERT page 58).
• Hydrogen is an ideal alternative, because its combustion gives water only. Made by splitting water using solar energy, it can be used as a renewable and non-polluting source of energy (NCERT page 58).
• Beyond the NCERT text: hydrogen is an energy carrier, not a primary source, and it can be made from water, fossil fuels or biomass (US Energy Information Administration, “Hydrogen explained”), so its emissions depend on how it is made. Wind as well as solar power can drive the electrolysis (US Department of Energy, “Hydrogen Fuel Basics”).
• Both steps of this hydrogen economy, making hydrogen by electrolysis of water and using it in a fuel cell, rest on electrochemical principles (NCERT page 58).

Solved examples

Common mistakes to avoid

Quick revision summary

• Fuel cell: a galvanic cell fed continuously with a fuel (hydrogen, methane, methanol) and oxygen; products removed continuously.
• NCERT alkaline H_₂–O_₂ model: porous carbon electrodes with Pt or Pd and concentrated NaOH. Apollo used alkaline fuel cells; KOH is the typical space-cell electrolyte.
• Anode: 2H_₂ + 4OH^⁻ → 4H_₂O + 4e^⁻; cathode: O_₂ + 2H_₂O + 4e^⁻ → 4OH^⁻; overall 2H_₂ + O_₂ → 2H_₂O, about 1.23 V.
• NCERT comparison: about 70% versus 40%; actual system efficiencies vary. Reversible 298 K standard-state limit for liquid-water product: ΔG/ΔH ≈ 83% (HHV).
• Hydrogen economy: hydrogen from water by solar-powered electrolysis, used in fuel cells.

Questions students ask

Is a fuel cell a primary or a secondary cell?

A fuel cell is neither a primary nor a secondary cell in NCERT’s grouping: NCERT divides batteries into primary and secondary (page 54) but gives fuel cells their own Section 2.7, and its Summary lists batteries and fuel cells as two useful forms of galvanic cell (page 59). A fuel cell is refuelled, not recharged: its electrodes are not used up but only carry out the reactions of the fuel and oxygen fed to them, while in a battery the electrode materials themselves react, such as the zinc can of a dry cell (NCERT pages 54–56; reasoned). Beyond the NCERT text, the US Department of Energy also says that fuel cells do not run down or need recharging (US DOE, “Fuel Cells”).

What is the difference between a fuel cell and an electrolyser?

A hydrogen–oxygen fuel cell and a water electrolyser run one reaction in opposite directions: the fuel cell, a galvanic cell, makes electricity from 2H₂ + O₂ → 2H₂O (NCERT page 56), and the electrolyser, an electrolytic cell, uses electricity to split water (NCERT page 58). Beyond the NCERT text, both consist of an anode and a cathode separated by an electrolyte, KOH or NaOH in alkaline ones (US DOE, “Hydrogen Production: Electrolysis”). At 298 K with liquid water and standard gas conditions, the reversible fuel-cell emf is about 1.23 V; the electrolyser needs more, to overcome overpotential (reasoned; NCERT pages 37, 53).

Can a hydrogen–oxygen fuel cell use an acidic electrolyte instead of NaOH?

Yes, a hydrogen–oxygen fuel cell can work in acid, with the same reaction and 1.23 V standard emf: H₂ → 2H⁺ + 2e⁻ at the anode and O₂ + 4H⁺ + 4e⁻ → 2H₂O at the cathode (reasoned from NCERT Table 2.1, page 37). Beyond the NCERT text, the proton exchange membrane cells in cars do this, with a solid acidic polymer electrolyte and a costly platinum catalyst (US DOE, “Types of Fuel Cells”). Alkaline cells like NCERT’s react fast enough for cheaper catalysts, but even the CO₂ in air turns their hydroxide into carbonate, so conventional aqueous alkaline cells need CO₂-free oxidant, such as pure oxygen or appropriately scrubbed air (US DOE, “Types of Fuel Cells”; Wikipedia, “Alkaline fuel cell”).

Why are fuel cells not widely used yet?

Beyond the NCERT text, fuel cells are held back mainly by cost and hydrogen supply: the proton exchange membrane cells used in vehicles need a costly platinum catalyst, which carbon monoxide in impure hydrogen poisons (US DOE, “Types of Fuel Cells”). Hydrogen is usually stored as a gas in tanks at 350–700 bar or as a liquid at −252.8 °C, its boiling point (US DOE, “Hydrogen Storage”), and 95% of the hydrogen made in the United States comes from natural gas, which gives carbon dioxide (US DOE, “Hydrogen Production: Natural Gas Reforming”). NCERT notes that fuel cells have been used in automobiles on an experimental basis (page 57).

Are fuel cells in the JEE Main and NEET syllabus?

Yes, fuel cells are in both syllabuses: Unit 7, Redox Reactions and Electrochemistry, of the JEE Main syllabus ends with “dry cell and lead accumulator, fuel cells”, and Unit 7 of the NEET (UG) 2026 syllabus ends with “Dry cell and lead accumulator; Fuel cells”. NCERT Class 12 Chemistry Part I covers them in Section 2.7 (pages 56–57), with Intext Question 2.14 and the Hydrogen Economy box on page 58. Neither syllabus names the hydrogen economy, so the hydrogen–oxygen cell, its electrode reactions and its efficiency are the core to learn (reasoned).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. What are the electrodes and the electrolyte of the hydrogen–oxygen fuel cell, and why are platinum or palladium added?

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   Porous carbon electrodes and concentrated aqueous sodium hydroxide; finely divided platinum or palladium is a catalyst that speeds up the electrode reactions (NCERT page 56).

2. Q2. How was the water from the Apollo fuel cells used?

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   The water vapour was condensed and added to the astronauts’ drinking water supply (NCERT page 56).

3. Q3. Give two advantages of fuel cells over thermal power plants.

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   They are more efficient, about 70% against about 40%, and NCERT calls them pollution free (NCERT pages 56–57); the hydrogen–oxygen cell’s only product is water. Strictly, real efficiencies vary with the system, and making the hydrogen can release carbon dioxide (beyond the NCERT text; see the slip above).