PEM Fuel Cell Stack
Proton Exchange Membrane H2/O2 Fuel Cell Stack
Stack of proton exchange membrane (PEM) fuel cells electrochemically combining stored gaseous hydrogen and oxygen to produce DC electrical power and pure liquid water during the 336-hour lunar night, with a target stack-level efficiency of 55–60% LHV.
Purpose
Provide the discharge half of the Ark's regenerative fuel cell (RFC) energy storage loop, releasing energy that was stored as H2 and O2 by the electrolyzer (L3-ESS-FC-ELEC) during lunar daytime; chosen over batteries for multi-week storage because reactant tanks decouple energy capacity from stack mass.
Context
Discharges from H2 storage (L3-ESS-FC-H2TK) and O2 storage (L3-ESS-FC-O2TK), produces water captured by L3-ESS-FC-WTR, supported by balance-of-plant pumps/valves (L3-ESS-FC-BOP), and supervised by L3-ESS-FC-CTRL. Output feeds L1-PDM at ~120–150 VDC stack voltage. Heat rejected to L1-TCS.
Principles
- ▸PEM fuel cell electrochemistry: H2 → 2H+ + 2e- at anode; ½O2 + 2H+ + 2e- → H2O at cathode; net cell EMF ~1.23 V theoretical, ~0.7 V typical at high current density
- ▸Nafion-class perfluorinated sulfonic-acid membranes conduct protons while blocking gas crossover; require hydration to function
- ▸Stack voltage scales with cell count; typical 100–300 cells in series yield 70–200 V DC
- ▸Operating window 50–80 °C balances kinetics, membrane hydration, and waste-heat rejection
- ▸Pt or Pt-alloy catalysts on carbon support; areal loadings 0.2–0.4 mg/cm² define stack cost and durability
- ▸Bipolar plates (graphite composite or coated stainless) provide flow fields and electrical series connection
Typical implementations
- ▸Apollo Service Module alkaline fuel cells (Bacon-type, 28 V, 1.5 kW each) — first crewed-mission fuel cells
- ▸Space Shuttle Orbiter alkaline fuel cells by UTC Power — 7 kW each, three units per Orbiter
- ▸Boeing/UTC RFC for ISS demonstration; later NASA Glenn lunar RFC testbed (operational 2026)
- ▸ESA Astrobotic-class lunar lander PEM fuel cell demonstrators
- ▸Toyota Mirai / Hyundai NEXO automotive PEM stacks (~1.5–2 kW/kg) — heritage for cell-level engineering
Lunar considerations
- ▸Lunar night length of 336 hr at the equator and indefinite at PSRs makes RFC strongly preferred over batteries at large energy capacity (battery mass scales with Wh, RFC stack mass scales with peak W)
- ▸Vacuum and dust environment requires hermetic stack enclosure with managed reactant feed lines
- ▸Sub-zero start-up: water inside the stack freezes when idle; freeze-tolerant designs (e.g., heated end-plates, draining cycle) are mandatory
- ▸Membrane drying at low load / high temperature degrades performance; lunar thermal swings demand careful BOP humidity control
- ▸Catalyst sintering and Pt dissolution under load cycling sets durability; mission-life >50,000 hr requires alloy catalysts (PtCo, PtNi)
- ▸Radiation effects on Nafion membranes are mild compared to electronics (membrane mostly fluorinated polymer) — minor concern over 100 yr
Specifications
Functional
| primary function | Generate DC electrical power from stored H2 and O2 reactants |
| inputs | Hydrogen gas from L3-ESS-FC-H2TK at 5–10 bar, Oxygen gas from L3-ESS-FC-O2TK at 5–10 bar, Coolant flow from L1-TCS, Control signals from L3-ESS-FC-CTRL |
| outputs | DC electrical power at ~120–150 V to L1-PDM, Product water (liquid) to L3-ESS-FC-WTR for recycling, Waste heat ~40% of input chemical energy to L1-TCS, Stack voltage, current, individual cell voltages, temperature telemetry |
| rated power kwe | 10 |
| peak power kwe | 15 |
| stack voltage v | 120, 200 |
| efficiency lhv percent | 58 |
| operating temperature c | 50, 80 |
| operating pressure bar | 3, 10 |
| cell count | 180 |
| active area cm2 per cell | 400 |
| design lifetime hours | 50000 |
| freeze tolerance temperature c | -40 |
Physical
| mass kg | 80 |
| dimensions | 600 × 400 × 400 mm (single stack) |
| materials | Nafion 211/212 PFSA membrane (25–50 µm), Pt-Co alloy catalyst on Vulcan carbon (0.3 mg/cm² cathode, 0.05 mg/cm² anode), Carbon paper gas diffusion layer (Toray TGP-H-060) with PTFE wet-proofing, Coated 316L stainless or graphite composite bipolar plates, Fluoroelastomer (Viton) seals, End-plate alloy: 7075-T6 aluminum or titanium 6Al-4V |
| operating temperature c | 50, 80 |
| storage temperature c | -40, 60 |
| vacuum compatible | True |
| vibration qual grms | 14 |
Operational
| power consumption w aux | 50 |
| thermal range c | -40, 80 |
| lifetime years | 15 |
| mtbf hours | 80000 |
| duty cycle | Discharge during lunar night (~330 hr per 28-day cycle) |
Interfaces
Provides
- 10 kWe DC output at 120–200 V during discharge
- Product water for recycling to electrolyzer
- ~7 kW waste heat at 60–80 °C
- Stack telemetry: voltage, current, temperature, individual cell voltages
Requires
- Hydrogen feed at 5–10 bar regulated
- Oxygen feed at 5–10 bar regulated
- Coolant circulation, humidification, purge gas
- Coolant loop interface for heat removal
- Load setpoints, start-up sequence, shutdown commands
Cite this entry
Lunar Ark Codex. "PEM Fuel Cell Stack" (L3-ESS-FC-STACK). Retrieved 10 September 2026, from https://lunarark.com/entry/L3-ESS-FC-STACK
Licensed CC-BY-SA 4.0. You may reuse and adapt this entry with attribution, under the same licence.