UPSC Prelims 2024 Q37: Hydrogen Fuel Cell EV Exhaust — Explained

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

  1. Hydrogen peroxide
  2. Hydronium
  3. Oxygen
  4. Water vapour

Answer: (d) Water vapour

Statement-wise explanation

  • a: incorrect — Hydrogen peroxide (H2O2) is not what a fuel cell vehicle releases. In a proton-exchange-membrane (PEM) fuel cell, small amounts of peroxide can form as an unwanted side-reaction at the cathode. When that happens it damages the membrane from inside the cell. It is never released as an exhaust product. The designed reaction reduces oxygen all the way to water.
  • b: incorrect — Hydronium (H3O+) is an ion that exists only inside a liquid or a wet membrane, such as an acid solution. Inside the fuel cell, protons do cross the wet membrane from the anode to the cathode. At the cathode they combine with oxygen and electrons to form neutral water. A charged ion cannot come out of a tailpipe as an exhaust gas.
  • c: incorrect — Oxygen is a reactant, not a product. The fuel cell takes oxygen from the air and uses it up at the cathode. Air that has lost some of its oxygen does leave the stack, but that is unused intake air, not an emission produced by the vehicle. PIB describes the fuel cell as using 'hydrogen and air to generate electricity' with water as the only by-product.
  • d: correct — The overall reaction in a hydrogen fuel cell is 2H2 + O2 → 2H2O, plus electricity and heat. The stack runs warm, so the water mostly leaves as vapour. PIB's release on the first green hydrogen fuel cell bus, flagged off from Kartavya Path on 25 September 2023, says the only by-product is water. PIB's release on the Indian Army's hydrogen bus (2024) says the process leaves 'water vapour as the only by product, thus ensuring zero emission'.

How to eliminate

This can be solved with Class 10 chemistry, without knowing anything about vehicles. A fuel cell runs on hydrogen and oxygen from the air. The only compound you can make from those two elements in a clean, complete reaction is water (2H2 + O2 → 2H2O). Oxygen goes in as a reactant, so it cannot be the main product: eliminate it. Hydronium is a charged ion that exists only in solution. Ions do not leave a tailpipe as gas: eliminate it. Hydrogen peroxide is a reactive, unstable compound. No vehicle sold as 'zero emission' would release it: eliminate it. That leaves water vapour. Fact to remember: hydrogen FCEVs (fuel cell electric vehicles) are 'zero tailpipe emission' because they emit only water vapour and warm air. Any CO2 linked to them comes from how the hydrogen was made, not from the vehicle.

Know more

How a hydrogen fuel cell works

A fuel cell works like a battery that never runs down as long as hydrogen is supplied. It produces electricity through a chemical reaction, without burning anything.

  • Anode: H2 → 2H⁺ + 2e⁻. The electrons flow through an outside circuit and drive the electric motor.
  • Membrane (PEM): only the protons (H⁺) pass through it.
  • Cathode: O2 (from air) + 4H⁺ + 4e⁻ → 2H2O.

The result is electricity, heat and water. The stack runs at about 60–80 °C, so the water leaves mostly as vapour. A fuel cell vehicle is still an electric vehicle: the motor runs on electricity, but the electricity is made on board instead of being stored in a large battery.

Feature Petrol/diesel car Battery EV Hydrogen fuel cell EV
Energy stored as Liquid fuel Electricity in battery Compressed H2 gas (350–700 bar)
Tailpipe output CO2, CO, NOx, particulates None (no tailpipe) Water vapour and warm air
Refuelling Minutes Hours to charge Minutes
Example in India — — Toyota Mirai pilot (2022); IOCL buses; Jind–Sonipat train (2026)

Zero tailpipe does not mean zero carbon. The climate benefit depends on how the hydrogen was made:

  • Grey hydrogen comes from natural gas through steam methane reforming, and the CO2 is released.
  • Blue hydrogen is made the same way, but the CO2 is captured and stored.
  • Green hydrogen is made by electrolysing water with renewable electricity.

India's National Green Hydrogen Mission was approved in January 2023 with an outlay of ₹19,744 crore. It aims to make green hydrogen the fuel source for this kind of mobility.

In the news

In September 2023, Union Minister Hardeep Puri flagged off India's first green hydrogen fuel cell buses from Kartavya Path, New Delhi. These IndianOil buses refuel at 350 bar. The National Green Hydrogen Mission had been approved in January 2023. Both put hydrogen fuel cell vehicles and their 'water-only' emissions in the news in the run-up to Prelims 2024.

NCERT Concepts to Revise

  • Green hydrogen: Explains why an FCEV's 'zero emission' claim depends on the hydrogen being made by electrolysis using renewable power.
  • National Green Hydrogen Mission: The 2023 mission behind India's push for hydrogen buses, cars and trains, the vehicles this question is about.

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