Station Oxygen Generation Assembly

Electrolysis Stack

Current per cell 32 A

Cells in stack 24

Cell voltage (overpotential) 1.80 V

Crew size 6

Live Readouts

O₂ output

— kg/day

H₂ byproduct

— kg/day

Stack power

— kW

Faraday rate

— mol e⁻/s

Crew O₂ balance (0.84 kg/person/day)

—

How it works

A solid-polymer-electrolyte (SPE) cell splits liquid water with a DC current across a proton-exchange membrane:

Anode:   2H2O -> O2 + 4H+ + 4e-
Cathode: 4H+ + 4e- -> 2H2
Net:     2H2O -> 2H2 + O2 (electrolysis)

Faraday's law of electrolysis ties current directly to the molar production rate — voltage only sets the power draw, not the yield:

n(O2)/dt = I * N_cells / (4F)
n(H2)/dt = I * N_cells / (2F)   (2x O2, by stoichiometry)
F = 96,485 C/mol (Faraday constant)
Power = I * N_cells * V_cell

Current per cell — sets the electron flow through the membrane; O₂ mass rate scales linearly with it.
Cells in stack — cells are wired in series/parallel banks; adding cells scales total gas output at the same current.
Cell voltage — real SPE cells run above the 1.23 V reversible potential (thermoneutral ~1.48 V) due to activation and ohmic overpotential; it changes power drawn, not O₂ yield.
Crew size — each astronaut consumes roughly 0.84 kg O₂/day; the balance readout shows whether the stack is keeping pace.

This is the same principle behind the ISS Oxygen Generation Assembly (OGA), which electrolyzes water recovered from the cabin humidity/urine-recycling loop, vents or feeds the H₂ byproduct to a Sabatier CO₂-reduction reactor, and supplies breathing O₂ to the cabin.

O₂ bubbles H₂ bubbles Membrane / cell plates

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