CDS (II) 2022 General Knowledge · Question 33 of 120

Correct — D, Water. A hydrogen fuel cell is not an engine and nothing is burnt in it. Hydrogen from the tank is fed to one electrode, where a catalyst strips each molecule into protons and electrons; the membrane between the electrodes lets the protons across but not the electrons, so the electrons are forced round an external circuit and that flow is the electricity that drives the motor. At the other electrode the protons, the returning electrons and oxygen drawn from the air recombine. The overall reaction is 2H2 + O2 giving 2H2O, and the product is water, which leaves as vapour or as a trickle of liquid. There is nothing else for the tailpipe to emit, because there is no carbon anywhere in the fuel and no flame hot enough to force nitrogen and oxygen in the air to combine. The point of the word 'green' in the stem is upstream rather than at the tailpipe — green hydrogen is hydrogen made by splitting water with renewable electricity, so that the vehicle is clean at both ends of the chain rather than only at the exhaust.

• (a)Hydrogen peroxide — Chemically the trap for anyone half-remembering that hydrogen and oxygen are involved. Hydrogen peroxide, H2O2, is not the product of the cell reaction; it can appear as a trace intermediate inside some cells and is a nuisance because it degrades the membrane, but it is not an emission.
• (b)Nitrogen — Air is about four-fifths nitrogen, and the nitrogen that goes in with the air comes back out unchanged — but it was never an emission of the vehicle, only air passing through. What a combustion engine does emit is oxides of nitrogen, formed by the heat of the flame, and a fuel cell has no flame.
• (c)Oxygen — Oxygen is a reactant here, not a product; it is consumed at the cathode. Unreacted excess air leaves the stack, but naming oxygen as the emission reverses the chemistry.

A fuel cell electric vehicle carries hydrogen in a high-pressure tank and makes its electricity on board, so it is an electric vehicle whose battery has been replaced by a chemical plant. That gives it the refuelling time of a petrol car and the range of one, at the cost of needing a hydrogen distribution network that barely exists. Hydrogen itself is graded by how it is made — grey from natural gas by steam methane reforming, blue when the carbon dioxide from that process is captured and stored, and green when water is split by electrolysis using renewable electricity. Only the last has no carbon dioxide anywhere in its chain, which is why policy attention has moved to it.

This is a chemistry question wearing a current-affairs coat, and it should be answered from the chemistry. Whatever vehicle the stem is pointing at, the reaction in any proton-exchange-membrane fuel cell running on pure hydrogen is fixed, and its only product is water — the Toyota Mirai's own documentation puts it plainly, that water is the only substance such cars emit from their tailpipes. So the answer does not depend on identifying the launch, and this card does not certify launch details it could not source. What is worth carrying away is the distinction the stem quietly sets up between the tailpipe and the whole chain: a fuel cell car is zero-emission at the exhaust whatever hydrogen you put in it, and the adjective 'green' is doing work about where that hydrogen came from. As of the present day India's policy vehicle for this is the National Green Hydrogen Mission, approved after the 2022 exam, and the practical bottleneck remains refuelling stations rather than vehicles.

• The overall reaction in a hydrogen fuel cell is 2H2 + O2 giving 2H2O; the only tailpipe product is water.
• A fuel cell produces direct current, which an inverter and controller convert for the traction motor — nothing is burnt, so there is no flame and no oxides of nitrogen.
• Green hydrogen is made by electrolysis of water using renewable electricity; grey hydrogen comes from steam methane reforming, and blue is grey with the carbon dioxide captured.
• A fuel cell electric vehicle refuels in minutes like a petrol car, unlike a battery electric vehicle, but needs a hydrogen refuelling network.
• Fuel cells were used on the Apollo and Space Shuttle missions, where the water they produced was drinking water for the crew.

• Naming oxygen as the emission; it is what the cell consumes, not what it releases.
• Assuming a fuel cell emits oxides of nitrogen like an engine; there is no combustion and no flame temperature.
• Treating 'green hydrogen' as a statement about the exhaust. Any hydrogen fuel cell emits only water; green describes how the hydrogen was made.

As a one-line emissions question, or as a statements item separating green, blue and grey hydrogen and the uses each is put to.

• practice — not a real PYQ

  Green hydrogen is best described as hydrogen produced by

  • (a)steam reforming of natural gas
  • (b)electrolysis of water using electricity from renewable sources
  • (c)gasification of coal with carbon capture
  • (d)fractional distillation of liquid air

  Answer(b) electrolysis of water using electricity from renewable sources — the only route with no carbon dioxide anywhere in the chain.

• practice — not a real PYQ

  In a hydrogen fuel cell, oxygen from the air acts as

  • (a)the fuel
  • (b)the catalyst
  • (c)a reactant consumed at the cathode
  • (d)the electrolyte

  Answer(c) a reactant consumed at the cathode — where it joins the protons and returning electrons to form water.