R1.3.4 Hydrogen Fuel Cells - IB Chemistry

📘 Key Principle

A fuel cell converts chemical energy → electrical energy directly, bypassing the inefficient thermal step of combustion engines. Much higher efficiency.

Half-Equations (Alkaline Fuel Cell)

Electrode	Half-Equation	Process
Anode (−)	H₂ + 2OH⁻ → 2H₂O + 2e⁻	Oxidation
Cathode (+)	O₂ + 2H₂O + 4e⁻ → 4OH⁻	Reduction
Overall	2H₂ + O₂ → 2H₂O	Only product = water!

Fuel Cells vs Combustion Engines

Feature	Fuel Cell	Combustion Engine
Efficiency	~60–80%	~25–30%
Emissions	H₂O only (zero carbon)	CO₂, CO, NO_x, particulates
Noise	Silent	Loud
Moving parts	Very few	Many (pistons, valves)

Challenges of Hydrogen

Storage: Very low energy density as a gas → needs high-pressure tanks (700 bar) or cryogenic liquid (−253 °C)
Production: Most H₂ currently made by steam reforming of CH₄ → still produces CO₂
"Green" H₂: Electrolysis of water using renewable electricity → truly zero-carbon, but expensive
Infrastructure: Lack of refuelling stations; Pt catalysts are costly

📐 Worked Example: Hydrogen Fuel Cell Half-Equations

In alkaline conditions:

Anode (oxidation): H₂(g) + 2OH^-(aq) → 2H₂O(l) + 2e^-

Cathode (reduction): O₂(g) + 2H₂O(l) + 4e^- → 4OH^-(aq)

In acidic conditions:

Anode (oxidation): H₂(g) → 2H⁺(aq) + 2e^-

Cathode (reduction): O₂(g) + 4H⁺(aq) + 4e^- → 2H₂O(l)

Overall: 2H₂(g) + O₂(g) → 2H₂O(l) (same for both conditions)

Challenges of Hydrogen as a Fuel

Challenge	Explanation
Storage	H₂ is a gas with very low density. Must be compressed to high pressure or liquefied at -253 °C, both requiring energy
Production	Electrolysis of water requires electricity. Unless from renewables, this just shifts the carbon emissions
Infrastructure	Existing petrol stations and pipelines cannot be used for hydrogen without major modification
Safety	Hydrogen is highly flammable and forms explosive mixtures with air

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