Water electrolysis splits H₂O into hydrogen and oxygen by forcing current through a cell: 2H₂O → 2H₂ + O₂. Twice as much hydrogen forms at the cathode as oxygen at the anode — watch the bubble ratio in the tank. The four technologies differ mainly in electrolyte, operating temperature and how much of that reaction's energy has to come in as electricity versus heat.
V_cell = V_rev + R·J + a·ln(1 + J/j0)
efficiency (HHV) = V_tn / V_cell × 100%
H2 rate ∝ stack_size × J / (2F)
- Current density (J) — how hard the stack is driven, in A/cm². Higher J means faster gas output but a higher cell voltage from ohmic and activation losses, so efficiency falls.
- Stack size — how many cells are wired in the stack; it scales total gas output and power linearly without changing per-cell voltage or efficiency.
- PEM — solid polymer membrane, compact, good catalysts (low activation loss), moderate temperature (~70°C), tolerates fast ramping — but needs precious-metal catalysts.
- Alkaline (ALK) — the oldest, cheapest technology (liquid KOH electrolyte), but higher ohmic resistance and slower catalysis mean a higher cell voltage at the same current density.
- AEM — anion-exchange membrane; aims for PEM-like compactness with cheaper, non-precious-metal catalysts, at a still-maturing efficiency in between PEM and ALK.
- SOEC — solid oxide cell run at 700–850°C; part of the splitting energy enters as heat rather than electricity, so its thermoneutral voltage is lower and electrical efficiency can be highest — if that heat is available for free (e.g. from waste heat).
Real-world relevance: this is exactly the trade-off green-hydrogen projects weigh — PEM and AEM for flexible, renewable-following operation, alkaline for lowest capital cost at steady load, and SOEC where cheap industrial heat is already on site.