PEM vs Alkaline Electrolysis for Engine Retrofits
Both technologies split water into hydrogen and oxygen. They differ in what lives on the vehicle, what the workshop has to handle, and how the unit behaves under vibration, cold start and load change.
Side by side
| PEM (HydroHub™) | Alkaline (legacy HHO) | |
|---|---|---|
| Electrolyte | Solid polymer membrane, purified water only | Liquid potassium hydroxide solution |
| Routine service | Top up water; resin filter every 6–9 months | Mix, check, top up and replace caustic electrolyte |
| Operator handling | No caustic chemical onboard | PPE required; caustic stored on the vehicle |
| Gas purity | 99.99%+, no electrolyte carry-over | Electrolyte mist carry-over requires scrubbing |
| Dynamic response | Output tracks demand from cold | Output depends on electrolyte temperature |
| Vibration tolerance | No free liquid electrolyte to slosh | Liquid volume must be managed against spill |
| Purchase cost | Higher per unit of output | Lower per unit of output |
| Running cost | Water and a periodic filter | Chemical, PPE, disposal, more labor |
Why purity matters on a retrofit
Whatever leaves the cell travels into the intake path. A liquid caustic electrolyte can carry mist with the gas, which is why alkaline designs need scrubbing and periodic attention at that point. A membrane cell has no electrolyte to carry over, so water quality management moves upstream, to a resin filter on the feed water.
Where alkaline still makes sense
At industrial scale — large boilers and fixed thermal plant with staffed maintenance — alkaline electrolysis is a mature, effective and lower-capex technology. The case for PEM is specific to mobile and unstaffed applications where handling caustic chemicals and managing liquid electrolyte on a moving platform is the dominant cost.
Questions.
Is alkaline cheaper?
- Cheaper to buy, generally more expensive to run. The difference appears in labor, PPE, electrolyte replacement and disposal, which scale with fleet size.
Can alkaline benchmarks be used to size a PEM unit?
- No. A volume figure established on a contaminated alkaline stream is not a reliable guide for a high-purity PEM stream. HydroHub systems are specified against engine displacement instead.
- How PEM Hydrogen Injection Works →
- Does Hydrogen Injection Actually Work? →
- Hydrogen Injection for Fleets →
- Types of Hydrogen Technology for Engines →
- Hydrogen Injection Supplier Comparison →
- Engine Performance and Hydrogen Enrichment →
- Emissions and Hydrogen Enrichment →
- Fuel Efficiency and Hydrogen Enrichment →
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