Fuel Cells and the Hydrogen Economy

Hydrogen fuel cells appear in the Cambridge 0620 syllabus under chemical energetics and environmental chemistry. They are tested as an alternative to burning fossil fuels, and examiners expect candidates to evaluate both sides of the hydrogen economy — not just state that “water is the only product.”

What is a fuel cell?

A fuel cell is an electrochemical device that converts the chemical energy of a fuel directly into electrical energy, without combustion. Unlike a battery, a fuel cell does not run down — it continues to produce electricity as long as fuel (hydrogen) and oxygen are supplied.

How a hydrogen fuel cell works

The cell has two electrodes separated by an electrolyte (often a proton exchange membrane, PEM).

At the anode (negative electrode)

Hydrogen gas is fed in. Each H₂ molecule loses electrons:

H₂ → 2H⁺ + 2e⁻ (oxidation)

The electrons flow through an external circuit to the cathode, doing electrical work along the way. The H⁺ ions pass through the membrane to the cathode.

At the cathode (positive electrode)

Oxygen gas (from air) is fed in. The oxygen reacts with the H⁺ ions and the electrons arriving through the circuit:

O₂ + 4H⁺ + 4e⁻ → 2H₂O (reduction)

Overall reaction

2H₂ + O₂ → 2H₂O

The only product is water. No carbon dioxide, no carbon monoxide, no sulfur dioxide, no nitrogen oxides — none of the pollutants associated with fossil fuel combustion.

Fuel cells vs combustion engines

Feature	Hydrogen fuel cell	Petrol engine
Energy conversion	Chemical → electrical (direct)	Chemical → heat → kinetic
Efficiency	~60%	~25–30%
Products	Water only	CO₂, CO, NOₓ, H₂O
Pollutants	None at point of use	CO₂, CO, NOₓ, soot
Noise	Very quiet	Noisy
Refuelling	Hydrogen gas needed	Petrol widely available

The hydrogen economy

The “hydrogen economy” is the idea that hydrogen could replace fossil fuels as the main energy carrier. In this vision:

• Electricity from renewable sources (solar, wind, hydroelectric) is used to electrolyse water
• The hydrogen produced is stored and transported
• Fuel cells in vehicles, buildings, and power stations convert hydrogen back to electricity
• The cycle produces no net carbon emissions

Where does the hydrogen come from?

Method	Process	Carbon-free?
Electrolysis of water	2H₂O → 2H₂ + O₂ (using electricity)	Yes, if electricity is from renewables
Steam reforming of methane	CH₄ + H₂O → CO + 3H₂	No (produces CO₂)
From crude oil/natural gas	Industrial chemical processes	No

Currently, most hydrogen is produced from fossil fuels (steam reforming), so the hydrogen economy is only truly “clean” if renewable electricity is used for electrolysis.

Advantages of hydrogen fuel cells

1. Only product is water — no CO₂ or pollutant emissions at point of use
2. High efficiency — more of the fuel’s energy is converted to useful electricity
3. Quiet operation — important for urban transport
4. Hydrogen is abundant — can be made from water by electrolysis
5. Renewable — if hydrogen is produced using renewable electricity

Disadvantages and challenges

1. Hydrogen storage — H₂ is a gas at room temperature; it must be stored under high pressure or at very low temperature (-253 °C as liquid), both of which are expensive and require energy
2. Hydrogen production — electrolysis requires a lot of energy; most commercial hydrogen currently comes from fossil fuels
3. Infrastructure — hydrogen refuelling stations are rare; a new distribution network is needed
4. Cost — fuel cells use expensive catalysts (platinum) and the technology is still developing
5. Safety — hydrogen is highly flammable and forms explosive mixtures with air

Applications

Application	Example
Vehicles	Hydrogen fuel cell buses and cars (Toyota Mirai, Hyundai Nexo)
Space	NASA used hydrogen fuel cells on the Space Shuttle
Backup power	Hospitals, data centres
Portable devices	Experimental laptop and phone chargers

Worked exam question

Evaluate the use of hydrogen fuel cells as an alternative to petrol engines in cars. [4]

Hydrogen fuel cells produce only water as a product, so there are no CO₂ emissions or air pollutants at the point of use [1]. They are more efficient than petrol engines at converting chemical energy to useful energy [1]. However, hydrogen is difficult and expensive to store because it must be kept under high pressure or at very low temperature [1]. Most hydrogen is currently produced from fossil fuels (steam reforming of methane), so the overall process is not carbon-free unless renewable electricity is used for electrolysis [1].

Common exam mistakes

1. Saying fuel cells produce “no pollution” without qualification — the fuel cell itself produces only water, but producing the hydrogen may generate CO₂ if fossil fuels are used.
2. Confusing fuel cells with batteries — a battery stores a fixed amount of energy; a fuel cell generates electricity continuously as long as fuel is supplied.
3. Not mentioning storage difficulties — hydrogen’s low density means it takes up a lot of space, and compression or liquefaction adds cost.
4. Forgetting to discuss both advantages and disadvantages when the question says “evaluate” or “discuss.”