UPSC Prelims 2024 Q37: Science & Technology – Energy

UPSC Prelims 2024, General Studies Paper I · Science & Technology

Which one of the following is the exhaust pipe emission from Fuel Cell Electric Vehicles, powered by hydrogen ?

Answer: (d) Water vapour

Explanation

Fuel Cell Electric Vehicles (FCEVs) — what comes out of the exhaust?

Part of the reaction	What happens
Anode	Hydrogen (H₂) splits into protons (H⁺) and electrons
Electrolyte	Only protons pass through (in PEM fuel cells)
External circuit	Electrons flow and power the motor
Cathode	Protons, electrons and oxygen from air combine to form water

Overall reaction: 2H₂ + O₂ → 2H₂O + electricity + heat

The only tailpipe output is water vapour, so FCEVs have zero tailpipe emissions.

FCEV vs battery EV:

Answer: (d) Water vapour.

(a) Incorrect. Hydrogen peroxide is not formed in a fuel cell's normal reaction.
(b) Incorrect. Hydronium (H₃O⁺) ions exist in solution; they are not released as exhaust.
(c) Incorrect. Oxygen is taken in from air at the cathode, not given out.
(d) CORRECT. Hydrogen combines with oxygen to form water vapour, the only exhaust.

Hydrogen fuel cell: 2H₂ + O₂ → 2H₂O + electricity. The only exhaust is water vapour.

A hydrogen fuel cell car emits only water vapour.

Thinking the vehicle releases oxygen; it actually consumes oxygen from the air.
Assuming FCEVs burn hydrogen like petrol; it is an electrochemical reaction, not combustion.
Forgetting that 'green' depends on how the hydrogen was made — green hydrogen uses renewable electricity.

Hydrogen + oxygen always give water. Any option other than water is wrong for an FCEV exhaust.