A hydrogen fuel cell generates electricity by reacting hydrogen and oxygen, producing only water as a by-product. Unlike batteries, fuel cells do not run flat — they operate continuously while hydrogen is supplied. The overall reaction is 2H₂ + O₂ → 2H₂O, making them an appealing low-emission power source for vehicles and buildings.
What is a hydrogen fuel cell?
A fuel cell is an electrochemical cell that converts chemical energy directly into electrical energy through a controlled reaction — like a battery that never runs flat as long as fuel is supplied.
In a hydrogen fuel cell:
- Hydrogen is supplied continuously as the fuel
- Oxygen (from air) is supplied as the oxidant
- Electricity is produced continuously
- The only product is water (as steam or liquid)
The cell consists of two electrodes separated by an electrolyte — in the most common design, a polymer electrolyte membrane (PEM) that allows hydrogen ions (H⁺) to pass through but not electrons. This forces electrons to flow through an external circuit, producing an electric current.
What are the electrode reactions in a hydrogen fuel cell?
The reactions occur at the two electrodes:
At the anode (negative electrode):
Hydrogen molecules are oxidised — each molecule loses electrons:
2H₂ → 4H⁺ + 4e⁻
The hydrogen ions (H⁺) travel through the electrolyte membrane to the cathode. The electrons travel through the external circuit, producing a current.
At the cathode (positive electrode):
Oxygen molecules from the air are reduced — they gain the electrons from the circuit and combine with the H⁺ ions to form water:
O₂ + 4H⁺ + 4e⁻ → 2H₂O
Overall reaction:
2H₂ + O₂ → 2H₂O
The only product is water — there are no carbon dioxide emissions, no particulate pollution, and no nitrogen oxides from the electrochemical reaction itself.
What are the advantages of hydrogen fuel cells?
| Advantage | Explanation |
|---|---|
| Zero direct emissions | Only product is water — no CO₂, no particulates at point of use |
| High efficiency | ~60% efficient at converting hydrogen's chemical energy to electricity; a petrol engine is ~25% efficient |
| Continuous operation | Unlike a battery, does not need recharging — hydrogen is replenished like filling a fuel tank |
| Quiet | No combustion, no moving parts in the cell itself |
| Scalable | From small portable devices to large stationary power plants |
| Fast refuelling | A hydrogen car can be refuelled in ~3–5 minutes (compared to 30+ minutes for rapid electric charging) |
What are the disadvantages and challenges of hydrogen fuel cells?
Hydrogen production:
- Most hydrogen is currently produced by the steam reforming of methane (a fossil fuel process), which releases CO₂. This means the overall carbon footprint depends on how the hydrogen is made.
- Green hydrogen (produced by electrolysis using renewable electricity) is genuinely low-carbon, but currently expensive.
Storage and transport:
- Hydrogen gas is extremely flammable and must be stored under high pressure (~700 bar in cars) or as cryogenic liquid (−253 °C).
- The energy density by volume is much lower than petrol, even compressed.
Infrastructure:
- There are very few hydrogen refuelling stations (around 10 in the UK as of 2026, compared to tens of thousands of petrol stations).
Cost:
- Fuel cells use platinum as a catalyst at the cathode, making them expensive to manufacture.
How do hydrogen fuel cells compare with rechargeable batteries?
| Feature | Hydrogen fuel cell | Rechargeable battery |
|---|---|---|
| Energy source | Hydrogen (chemical fuel) | Electrical energy stored in electrodes |
| By-product | Water only | None at point of use |
| Refuel/recharge | 3–5 minutes (refuel) | 30–60 minutes+ (rapid charge) |
| Range anxiety | Lower — tank-based range | Higher — limited by battery size |
| Mass | Lighter for long range | Heavier for equivalent range |
| Infrastructure | Very limited | Growing (EV chargers) |
| Cost (2026) | Higher | Falling rapidly |
| Mature technology? | Emerging | Well established |
Both technologies are viable depending on the application. Fuel cells suit heavy transport (lorries, buses, shipping) where battery weight and charge time are problematic; batteries suit passenger cars and short-range urban use.
Frequently asked questions
What is the overall word equation and symbol equation for the hydrogen fuel cell reaction?
The word equation is: hydrogen + oxygen → water. The balanced symbol equation is: 2H₂ + O₂ → 2H₂O. The only product is water. No carbon-containing fuels are used, so there is no carbon dioxide produced within the cell itself. Whether the overall process is truly "clean" depends on how the hydrogen was produced in the first place — green hydrogen from renewable electrolysis gives a genuinely low-carbon chain, while grey hydrogen from methane reforming does not.
Why is platinum used as a catalyst in hydrogen fuel cells?
Platinum is an excellent catalyst for the oxygen reduction reaction at the cathode (O₂ + 4H⁺ + 4e⁻ → 2H₂O). It lowers the activation energy of this reaction sufficiently for it to proceed at room temperature at useful rates. Without a catalyst, the oxygen reduction reaction is very slow. The problem is that platinum is very expensive (one of the rarest metals on Earth), which is the main reason hydrogen fuel cells remain costly. Researchers are actively looking for cheaper catalysts, including nickel and iron-nitrogen compounds.
How is hydrogen produced and why does this affect its environmental impact?
Currently, about 95% of the world's hydrogen is produced by steam reforming: CH₄ + H₂O → CO + 3H₂ (then CO + H₂O → CO₂ + H₂). This process uses natural gas and releases significant CO₂ — this is "grey hydrogen". If the CO₂ is captured and stored underground, it becomes "blue hydrogen". If hydrogen is produced by electrolysis of water using renewable electricity (wind or solar), no greenhouse gases are emitted and it is "green hydrogen". At the moment green hydrogen is more expensive, but as renewable electricity becomes cheaper and electrolyser technology improves, this gap is closing.
Why do fuel cells use a membrane instead of a liquid electrolyte?
In a polymer electrolyte membrane (PEM) fuel cell, the membrane allows hydrogen ions (H⁺) to pass from the anode side to the cathode side, but blocks electrons (which must travel through the external circuit to do useful electrical work) and prevents the hydrogen and oxygen gases from mixing directly (which would cause them to react explosively). Liquid electrolytes can spill, evaporate, and react with materials, making them unsuitable for vehicle use. The solid membrane is compact, robust, and can operate at relatively low temperatures (~80 °C), making it ideal for transport applications.
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