PEM vs alkaline electrolysis: technical differences

The two electrolysis architectures

In a PEM/SPE (proton-exchange membrane / solid-polymer electrolyte) cell, the electrolyte is the membrane itself. Deionised water is fed to the stack and split at catalyst-coated electrodes bonded to that membrane. No dissolved chemical is required, so the liquid circuit contains water only. In an alkaline cell, conductivity comes from potassium hydroxide (KOH) — or sodium hydroxide — dissolved in the water, with plain stainless plates acting as electrodes. Both routes split water into hydrogen and oxygen; the difference lies in what else is present in the cell and in how tightly the output can be regulated.

Why gas composition differs

A caustic liquid electrolyte can be entrained in the gas stream as fine aerosol or vapour, and alkaline cells typically operate warmer and wetter, so the delivered stream carries more moisture. A solid-polymer cell has no dissolved electrolyte to carry over, and membrane separation keeps the hydrogen and oxygen streams stoichiometric and consistent. This matters for engine work because the gas actually delivered to the intake — not the gas nominally generated — is what participates in combustion. Many published engine tests do not report an independent gas-composition analysis at all; see why oxyhydrogen gas composition matters.

Why dosing differs

Dosing is the hydrogen flow delivered relative to engine air and fuel flow. A membrane stack responds predictably to applied current, which makes flow repeatable across a duty cycle. Alkaline output drifts with electrolyte concentration and temperature, so a single nominal flow figure describes the operating range less precisely. Because reported engine effects are sensitive to the hydrogen fraction, a device tested at one unoptimised dose point says little about behaviour at other dose points — how oxyhydrogen dosing affects combustion.

Why engine integration differs

A pure-water circuit has no corrosive liquid to manage, contain or dispose of, which is the practical reason solid-polymer stacks are used on vehicles and mobile plant, while alkaline cells are more common in fixed installations and low-cost retrofit kits. Neither architecture guarantees a combustion outcome: results in the published literature are conditional on engine, load and hydrogen fraction. For a side-by-side reading of what the published tests used, see alkaline HHO retrofits versus PEM/SPE oxyhydrogen and the 2026 Scientific Reports HHO study review.

Related reading

• How PEM/SPE electrolysis works
• Hydrogen combustion enhancement principles
• Engine applications for PEM/SPE oxyhydrogen
• Browse all oxyhydrogen evidence reviews
• Industrial applications by duty cycle