Inside a PEM fuel cell, hydrogen splits at the anode catalyst into protons (crossing the polymer membrane) and electrons (routed through an external circuit to the motor). Protons and electrons recombine with oxygen at the cathode, producing water vapor and waste heat — a direct chemical-to-electrical conversion, unlike combustion's heat-to-mechanical path.
Anode: H2 -> 2H+ + 2e-
Cathode: 1/2 O2 + 2H+ + 2e- -> H2O
eff = P_electrical / (H2_flow * LHV_H2), LHV_H2 ~ 120 MJ/kg
- Load demand — power drawn by the traction motor; efficiency peaks at partial load and drops at very high or low load.
- H2 flow rate — hydrogen delivery rate to the anode; must roughly track load demand for stable operation.
- Stack temperature — operating temperature affects reaction kinetics; too cold slows the reaction, too hot dries the membrane.
- Membrane humidity — proton conductivity of the polymer membrane depends on water content; too dry raises internal resistance.
Unlike a combustion engine (capped by the Carnot limit), fuel cells aren't heat engines — but real-world efficiency still falls well short of the thermodynamic maximum due to activation losses, ohmic resistance, and water/thermal management challenges shown here.