Hydrogen- Oxygen Fuel Cells

A hydrogen-oxygen fuel cell has:

• A NEGATIVE electrode where H₂ is supplied.
• A POSITIVE electrode where O₂ is supplied (often just from air).
• An electrolyte (e.g. potassium hydroxide solution, or a proton-exchange membrane) between them.

The reactions:

• At negative electrode: 2 H₂ → 4 H⁺ + 4 e⁻ (oxidation).
• At positive electrode: O₂ + 4 H⁺ + 4 e⁻ → 2 H₂O (reduction).

Adding both: 2 H₂ + O₂ → 2 H₂O. The hydrogen and oxygen combine to form water — the only waste product.

The 4 electrons released at the negative electrode flow through the external circuit (powering whatever you've connected) back to the positive electrode. That flow is the electric current.

Battery: contains a fixed amount of chemical reactants inside. Once they're used up, it must be RECHARGED (rechargeable batteries) or thrown away (disposable). Performance degrades over time.

Fuel cell: reactants (H₂ and O₂) are continuously supplied from outside. As long as fuel is available, it keeps producing electricity. No recharging needed. The cell components themselves don't degrade quickly.

Strengths of hydrogen fuel cells over batteries:

• High energy density — light for the amount of energy stored (good for transport).
• Quick refuelling — minutes to refill a hydrogen tank vs hours to recharge a battery.
• No emissions at point of use.

Strengths of batteries over fuel cells:

• Cheaper to mass-produce.
• Charging infrastructure (electricity) already exists everywhere.
• Higher efficiency in stop-start city driving (regenerative braking).

This is why current debate around vehicles is partly EV (batteries) vs FCEV (fuel cells) — both are 'zero emission' at the tailpipe; both have their own advantages.

Hydrogen is sometimes called the 'fuel of the future', but there's a key catch: hydrogen is not a primary energy source. There's almost no free H₂ on Earth — it has to be made.

Ways of producing hydrogen:

• Steam reformation of methane (CH₄ + H₂O → CO + 3 H₂): cheap but emits CO₂. Most current H₂ comes from this. Called 'grey' or 'blue' hydrogen.
• Electrolysis of water (powered by renewable electricity): clean, but expensive and needs a lot of electricity. Called 'green' hydrogen.

So a hydrogen fuel cell is only as clean as the H₂ it uses. Green hydrogen + fuel cell = truly zero-emission. Grey hydrogen + fuel cell = lower emissions than petrol but not zero.

Uses today:

• Spacecraft (Apollo, ISS): H₂ fuel cells provided both electricity AND drinking water for astronauts.
• City buses (London, Tokyo, San Francisco).
• Toyota Mirai and Hyundai Nexo (consumer cars).
• Backup power for hospitals and data centres.

MYP criterion D: discuss the future of hydrogen fuel cells. Arguments for: clean at use, long range, fast refill. Arguments against: infrastructure (refilling stations are rare), energy cost of making green H₂, safety perceptions (H₂ is highly flammable).