Hydrogen made from fossil gas ('grey' H₂) carries a large carbon footprint. Green hydrogen is produced by electrolysis of water driven by renewable electricity, emitting only oxygen. The proton-exchange-membrane (PEM) electrolyser is favoured for renewables because it responds quickly to fluctuating wind and solar power and runs at high current density in a compact stack.
Working principle
A solid polymer membrane conducts protons (H⁺) while separating the gases. At the anode, water is oxidised — the oxygen evolution reaction (OER) — releasing O₂, protons and electrons. Protons migrate across the membrane; electrons travel the external circuit. At the cathode, protons recombine with electrons in the hydrogen evolution reaction (HER) to form H₂. Catalysts are precious metals (iridium oxide for OER, platinum for HER), a major cost driver.
| Type | Electrolyte | Strength | Limitation |
|---|---|---|---|
| Alkaline | Liquid KOH | Mature, cheap catalysts | Slow load response |
| PEM | Solid polymer | Fast, compact, dynamic | Iridium / Pt cost |
| Solid oxide | Ceramic, high T | Highest efficiency | Durability, start-up |
Cost driverGreen-hydrogen cost is dominated by electricity price and iridium scarcity; reducing precious-metal loading and pairing with cheap renewables are the central research goals.
Applications
- Decarbonising ammonia, steel and refining (industrial feedstock)
- Long-duration energy storage and grid balancing
- Heavy-transport and synthetic-fuel feedstock
References & further reading
- Carmo et al., “A comprehensive review on PEM water electrolysis,” Int. J. Hydrogen Energy, 2013.
- Buttler & Spliethoff, “Current status of water electrolysis for energy storage and Power-to-X,” Renew. Sustain. Energy Rev., 2018.
- IEA, “Global Hydrogen Review,” 2023.