Hydrogen Fuel Cell (PEMFC) Electrochemistry
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Generating electricity from hydrogen with only water as exhaust
Overview
Proton exchange membrane fuel cells (PEMFCs) convert hydrogen and oxygen directly into electricity and water through electrochemical reactions, achieving 50-60% electrical efficiency — significantly higher than combustion engines. PEMFCs power fuel cell electric vehicles (FCEVs), backup power systems, buses, trucks, forklifts, and marine vessels. The technology uses the same Nafion membrane as PEM electrolyzers but in reverse. Toyota Mirai and Hyundai Nexo are commercial FCEV models. Fuel cell trucks from Hyundai and Nikola are entering fleet service.
Chemical Process
Hydrogen gas is fed to the anode where it is oxidized on a Pt/C catalyst, releasing protons and electrons. Protons migrate through the Nafion membrane to the cathode. Electrons travel through an external circuit (producing electricity). At the cathode, oxygen from air reacts with protons and electrons on a Pt/C catalyst to form water. Individual cells produce ~0.7 V and are stacked in series to achieve the desired voltage.
Cathode: ½O₂ + 2H⁺ + 2e⁻ → H₂O (oxygen reduction on Pt/C, E° = +1.23 V)
Overall: H₂ + ½O₂ → H₂O (ΔG = −237 kJ/mol, theoretical cell voltage 1.23 V)
Raw Materials
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Hydrogen gas (H₂) — Electrolysis, SMR, or byproduct hydrogen (Fuel (99.97% purity for PEMFC))
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Air (O₂) — Atmosphere (compressed) (Oxidant)
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Platinum catalyst (Pt/C) — PGM mining (South Africa, Russia) (Anode and cathode electrocatalyst)
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Nafion membrane — Chemours specialty polymer (Proton conductor and gas separator)
End Products
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Electricity (DC power) — Vehicle propulsion, stationary power, portable power (50-60% electrical efficiency, 80%+ with heat recovery)
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Water (H₂O) — Only emission product — exhausted as steam/liquid (Ultra-pure water, ~0.5 L/kWh)
Environmental Impact
Fuel cells produce zero local emissions (only water vapor). Environmental impact depends on the hydrogen source — green hydrogen yields near-zero life-cycle emissions, while grey hydrogen shifts emissions to the SMR plant. Platinum mining has significant environmental impact. Membrane and MEA recycling processes are being developed.
Safety Considerations
- ⚠ Hydrogen fuel is flammable and explosive — vehicle tanks tested to withstand ballistic impact
- ⚠ High-voltage DC power from fuel cell stacks — electrical safety required
- ⚠ Membrane dehydration causes irreversible damage
- ⚠ CO poisoning of platinum catalyst from impure hydrogen
Recent Innovations
Ultra-low platinum loading catalysts (<0.1 mg/cm²) reduce cost.
PGM-free catalysts (Fe-N-C) for oxygen reduction are approaching commercially viable activity.
High-temperature PEM fuel cells (120-200 degrees C, PBI membrane) tolerate CO impurities in reformed hydrogen.
Solid oxide fuel cells (SOFCs) operating at 600-800 degrees C achieve even higher efficiency.
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