Hydrogen Energy and Fuel Cells

This note covers fuel cell technology and vehicle emissions. India's Green Hydrogen policy, production targets and the National Green Hydrogen Mission are covered in Renewable Energy and Climate Mitigation Tech.

A fuel cell is electrolysis, run backwards

Electrolysis splits water into hydrogen and oxygen using an electric current. A hydrogen fuel cell does the reverse: it combines hydrogen and oxygen to produce electricity, with water as the by-product, essentially running the same chemistry in the opposite direction.

The most common type, a Proton Exchange Membrane (PEM) fuel cell, does this through a specific electrochemical sequence worth knowing rather than just the headline result. Hydrogen gas reaches the anode, where a catalyst (typically platinum) splits each hydrogen molecule into protons and electrons. The membrane at the centre of the cell allows only the protons through, to the cathode, while blocking the electrons. Those blocked electrons are forced to travel around an external circuit instead, and that flow of electrons through the circuit is the electric current that powers the vehicle. At the cathode, the protons, the electrons that completed their circuit, and oxygen from the air combine to produce the cell's only by-products: heat and water vapour.

What actually comes out of the tailpipe

This mechanism is exactly why a Hydrogen Fuel Cell Electric Vehicle (FCEV), under normal operation, emits only water vapour from its tailpipe, with no carbon dioxide and no other pollutant, because the entire process described above never involves burning anything or releasing carbon at all. This is the precise fact a past question tested by asking what an FCEV emits during normal operation.

Hydrogen enriched CNG: a genuinely different, blended technology

Hydrogen Enriched CNG (H-CNG) is a distinct concept from a pure fuel cell vehicle, and worth telling apart precisely, since a past question tested specific, exaggerated claims made about it. H-CNG blends a modest proportion of hydrogen gas into ordinary compressed natural gas, burned in a conventional internal combustion engine rather than used in a fuel cell at all. Because hydrogen burns more completely and at a different rate than methane, the blend genuinely does reduce emissions of carbon monoxide and unburned hydrocarbons compared to pure CNG. What it does not do is eliminate carbon monoxide emissions entirely, since the fuel is still overwhelmingly natural gas, a carbon-based fuel, and it is still combusted rather than reacted electrochemically the way a true fuel cell operates. Claiming complete elimination of an emission, rather than a genuine reduction, is the standard exaggeration a question in this format tests for.

Where "green" hydrogen comes into the picture

The hydrogen used in a fuel cell can be produced by different methods, and only one of those methods makes the vehicle genuinely zero-emission across its entire life cycle, not just at the tailpipe. Green hydrogen, produced by electrolysis powered by renewable electricity, has no carbon emissions anywhere in its production. Hydrogen produced from natural gas, without carbon capture, carries a significant emissions footprint upstream, even though the vehicle's own tailpipe still emits only water vapour. This is why a genuinely zero-emission hydrogen vehicle depends on the source of its fuel just as much as on the fuel cell technology itself, which is the exact link to India's own Green Hydrogen policy, covered in the neighbouring Environment note.

Quick revision points

• A fuel cell runs electrolysis in reverse: hydrogen and oxygen combine to produce electricity, with heat and water vapour as the only by-products.
• PEM fuel cell mechanism: hydrogen splits into protons and electrons at the anode; the membrane passes only protons through; blocked electrons flow through an external circuit as usable current; protons, electrons and oxygen recombine at the cathode.
• An FCEV's tailpipe emits only water vapour under normal operation, no CO2, no other pollutant. This is the specific fact a past question tested.
• H-CNG: hydrogen blended into CNG, burned in a conventional engine, genuinely reduces carbon monoxide and hydrocarbon emissions versus pure CNG, but does not eliminate carbon monoxide entirely, since it is still a combusted, carbon-based fuel. "Eliminates entirely" versus "reduces" is the standard trap.
• Only green hydrogen (produced by electrolysis using renewable electricity) makes the whole life cycle emission-free; hydrogen from unabated natural gas still carries an upstream footprint even though the vehicle's own tailpipe is clean.