PEM Electrolyzer Stack
Proton Exchange Membrane Water Electrolysis Stack
PEM water electrolyzer that consumes DC electrical power during the lunar day to split product water (returned from the fuel cell) and ISRU-derived feed water into high-purity hydrogen and oxygen gases for storage in cryogenic/high-pressure tanks.
Purpose
Charge half of the RFC loop — convert surplus daytime solar/nuclear electricity into chemical energy (H2 + ½O2) that can be stored for the lunar night without self-discharge, achieving an effective round-trip RFC efficiency of ~50% with multi-week storage capacity.
Context
Receives DC from L1-PDM, water from L3-ESS-FC-WTR, and feeds H2/O2 to L3-ESS-FC-H2TK and L3-ESS-FC-O2TK respectively. Coordinates with L3-ESS-FC-STACK (counter-flow loop), supported by L3-ESS-FC-BOP, supervised by L3-ESS-FC-CTRL. Heat rejected to L1-TCS.
Principles
- ▸Water electrolysis: 2H2O → 2H2 + O2; theoretical minimum voltage 1.23 V, practical cell voltage 1.8–2.0 V at high current density
- ▸PEM electrolyzers (PEMWE) use the same Nafion-class membrane as the fuel cell, but with IrO2/RuO2 anode catalyst for the oxygen evolution reaction (OER)
- ▸Cell electrolytic efficiency 70–90 % LHV; system-level efficiency ~55–65 % after auxiliaries (~55–60 kWh/kg H2)
- ▸Operating at elevated pressure (up to 30 bar) reduces downstream compression energy — direct compression electrolysis
- ▸Product gases ~99.999 % purity; oxygen carries humidity that must be removed before liquefaction or cryogenic storage
- ▸Differential pressure across the membrane is the critical safety variable — controlled by paired stack/anode-cathode pressure regulation
Typical implementations
- ▸Hydrogenics (Cummins) HyLyzer PEM modules (terrestrial, MW scale)
- ▸Bosch Hydrogen Energy PEM electrolyzer (modular 1 MW)
- ▸Nel Hydrogen MC500 PEMWE family
- ▸Giner ELX PEM electrolyzers — heritage for NASA RFC testbed at Glenn
- ▸NASA RESOURCE project — ISRU electrolyzer producing pure dry O2 from icy regolith (laboratory demo 2025)
- ▸Bramble Energy PCBFC (composite PEM electrolyzer/fuel cell technology)
Lunar considerations
- ▸Feed water purity must be very high (DI grade); ISRU water from lunar ice may carry dissolved volatiles requiring multi-stage purification
- ▸Operating temperature 50–80 °C must be maintained during lunar day when ambient may reach +127 °C — TCS must reject heat efficiently
- ▸Cold-soak protection: liquid water in stack must not freeze during shutdown — drain-and-purge protocol on idle
- ▸High-pressure electrolysis (~30 bar) reduces compressor load for cryogenic liquefaction — favored over atmospheric-pressure variant
- ▸Iridium-based anode catalyst is scarce; long-life designs minimize loading and degradation
- ▸Output humidity removal mandatory before tank storage (per NASA RESOURCE assessment)
Specifications
Functional
| primary function | Electrolyze water to produce gaseous H2 and O2 from electrical input |
| inputs | DC electrical power from L1-PDM at 80–120 V (15 kW peak), Pure water feed from L3-ESS-FC-WTR (~6 g/s at peak), Coolant flow from L1-TCS, Control commands from L3-ESS-FC-CTRL |
| outputs | Hydrogen gas at 20–30 bar to L3-ESS-FC-H2TK, Oxygen gas at 20–30 bar to L3-ESS-FC-O2TK, Waste heat ~35% of input power to L1-TCS, Stack telemetry and gas-purity sensors |
| rated input power kwe | 15 |
| h2 production rate kg per day | 6 |
| o2 production rate kg per day | 48 |
| stack efficiency lhv percent | 75 |
| system efficiency lhv percent | 62 |
| operating temperature c | 50, 80 |
| output pressure bar | 30 |
| h2 purity percent | 99.999 |
| o2 purity percent | 99.99 |
| cell count | 60 |
| design lifetime hours | 50000 |
Physical
| mass kg | 60 |
| dimensions | 500 × 400 × 400 mm |
| materials | Nafion 117 PFSA membrane (thicker than fuel-cell membrane for differential pressure), Iridium oxide (IrO2) anode catalyst, 1–2 mg/cm², Platinum cathode catalyst, 0.5 mg/cm², Titanium PTL (porous transport layer) with platinum coating, Titanium bipolar plates with corrosion-resistant coating, Titanium end plates (corrosion immunity in H2O/O2 environment) |
| operating temperature c | 50, 80 |
| storage temperature c | 5, 40 |
| freeze protection | Required (drain and dry on cold shutdown) |
Operational
| power consumption w aux | 200 |
| thermal range c | 5, 80 |
| lifetime years | 15 |
| mtbf hours | 80000 |
| duty cycle | Charge during lunar day (~330 hr per 28-day cycle) |
Interfaces
Provides
- Pressurized H2 at 30 bar, 99.999% purity
- Pressurized O2 at 30 bar, 99.99% purity
- ~5 kW waste heat at 60–80 °C
- Stack telemetry, gas purity sensors, differential pressure
Requires
- 15 kWe DC input during charge
- Purified DI feed water
- Coolant, gas-water separators, dryers, pressure regulation
- Coolant loop for waste-heat removal
- Power setpoint and safety supervision
Cite this entry
Lunar Ark Codex. "PEM Electrolyzer Stack" (L3-ESS-FC-ELEC). Retrieved 10 September 2026, from https://lunarark.com/entry/L3-ESS-FC-ELEC
Licensed CC-BY-SA 4.0. You may reuse and adapt this entry with attribution, under the same licence.