How do hydrogen fuel cells work in cars?

In one line

The rule

A hydrogen fuel cell is a device that converts chemical energy from hydrogen and oxygen into electricity, water, and heat through an electrochemical reaction.

The main reaction is: 2H₂ + O₂ → 2H₂O + energy (electricity + heat)

• H₂ = hydrogen gas
• O₂ = oxygen gas
• H₂O = water

Let's understand it simply

Imagine you have a pressure cooker. You put water inside and heat it. The water turns to steam and pushes the whistle up. Now, think of hydrogen as the fuel, like the water, and oxygen as the heat. But instead of making steam, a fuel cell makes electricity.

In a hydrogen fuel cell, hydrogen gas goes in from one side, and oxygen (from the air) goes in from the other. Inside, they meet in a special way that makes electricity, just like a battery. The only thing that comes out is water, like the steam from your cooker, but much cleaner!

How it works

1. Hydrogen gas (H₂) enters the fuel cell at the anode (one side).
2. Oxygen gas (O₂) from the air enters at the cathode (the other side).
3. At the anode, a catalyst (usually platinum) splits each hydrogen molecule into two protons (H⁺) and two electrons (e⁻).
4. The protons pass through a special membrane (PEM: Proton Exchange Membrane) to the cathode.
5. The electrons cannot pass through the membrane, so they travel through a wire—this flow is electricity, which powers the car's motor.
6. At the cathode, the protons, electrons, and oxygen combine to form water (H₂O), which leaves as harmless water vapor.

The story behind it

The idea of the fuel cell was first shown by Sir William Grove in 1839 in England. He wanted to find a way to make electricity directly from chemical reactions, not by burning things. But it took more than 100 years for fuel cells to be used in real machines. NASA used them in the Apollo missions to power spacecraft and give astronauts clean water. Today, scientists and engineers are working to make fuel cells cheap and strong enough for cars and buses.

Where you see it in real life

• Toyota Mirai car → Uses hydrogen fuel cells to run quietly and emits only water.
• Buses in Delhi → Some experimental buses use fuel cells for clean city transport.
• Apollo spacecraft → Used fuel cells for electricity and water in space.
• Trains in Germany → Some trains run on hydrogen fuel cells instead of diesel.
• Mobile phone chargers → Small fuel cells can charge phones where there is no electricity.
• Backup power for hospitals → Fuel cells keep important machines running during power cuts.

Why it matters

• No pollution: The only exhaust is water, so cities stay cleaner.
• Renewable energy: Hydrogen can be made from water using solar or wind power.
• Energy security: Less need for petrol or diesel from other countries.
• Future jobs: Engineers, scientists, and workers will be needed to build and run these systems.

Worked example

Let’s see how much water is made if a car uses 1 kg of hydrogen:

Given: 1 kg hydrogen (H₂)
Step 1: Molar mass of H₂ = 2 g/mol, so 1 kg = 1000 g / 2 = 500 mol H₂
Step 2: Reaction: 2H₂ + O₂ → 2H₂O
Step 3: 2 mol H₂ makes 2 mol H₂O, so 500 mol H₂ makes 500 mol H₂O
Step 4: Molar mass of H₂O = 18 g/mol
Step 5: 500 mol × 18 g/mol = 9000 g = 9 kg water
Answer: Burning 1 kg hydrogen in a fuel cell makes about 9 kg water.

Common mistakes

• Thinking it burns hydrogen: Fuel cells do not burn hydrogen; they react it slowly to make electricity.
• Believing it releases harmful gases: The only exhaust is water vapor, not smoke or CO₂.
• Confusing with batteries: Batteries store electricity; fuel cells make it as long as hydrogen is supplied.
• Assuming hydrogen is always green: If hydrogen is made from coal, it is not clean. It must be made from water using clean energy.

Remember it

Hydrogen fuel cells in cars = hydrogen + oxygen → electricity + water. Clean power, no smoke!