Both cell types strip electrons from hydrogen at the anode and recombine them with oxygen at the cathode, but the ion that crosses the electrolyte — and the temperature that ion needs — is completely different. A PEM cell ships protons (H⁺) through a hydrated polymer membrane by a hopping mechanism that only works while the membrane stays wet; push the temperature toward 100 °C without pressurising it and the water flashes off, conductivity collapses, and voltage falls even though nothing else changed. A solid-oxide cell ships O²⁻ ions through a rigid ceramic lattice; that hop is thermally activated and barely conducts at all below roughly 600 °C, but keeps improving as the stack gets hotter.
V_cell = E_Nernst − η_activation − I·ASR(σ) − η_concentration
σ_PEM(T,RH) ∝ hydration(T,RH) → collapses above ~100 °C if dry
σ_SOFC(T) = σ0 · exp(−Ea / kT) → negligible below ~600 °C
- Stack temperature — one shared slider sweeps both chemistries across their real operating windows, so you can see the PEM curve die exactly where the SOFC curve is only just switching on.
- Load — current density drawn from the stack; higher load means a bigger ohmic (I·R) voltage drop for whichever electrolyte is currently a poor conductor.
- Humidification — how saturated the PEM membrane is kept; below 100% the dry-out onset shifts to a lower temperature, showing why real PEM stacks need active water management.
- Fuel supply — pure H₂ is ideal for both; switching to reformed CH₄ leaves a few percent CO in the stream, which permanently poisons a low-temperature PEM catalyst but is harmlessly tolerated (or even internally reformed) by a hot SOFC anode — the fuel-flexibility trade the article calls out for mobile vs stationary use.
Real-world relevance: this temperature/conductivity mismatch is exactly why PEM dominates cars and buses (fast cold-start, near-room-temperature operation) while SOFC dominates stationary power plants (hours-long warm-up is fine, but fuel flexibility and higher efficiency pay off over a continuous duty cycle).