Molten Salt Electrolysis

Beth Lomax/University of Glasgow

This is one possible way to get oxygen for Lunar mare basalt.

(CaCl2/Oxide Solid)


  • Chemical reaction

European Space Research and Technology Centre (ESTEC)

ESTEC has built a prototype oxygen plant in Noordwijk, Netherlands [1]. This process was developed by Metalysis; however, in their process the oxygen is an unwanted by-product [1]. The process is based on the FCC process [3].

Their process:

  • Use molten molten calcium chloride salt as an electrolyte [1].
  • Crush regolith (or basalt) into powder [1].
  • Heat to 950°C [1].
    • The regolith will remain solid at this temperature [1].
  • Place regolith, contained in a metal basket, into the molten electrolyte at the cathode (carbon composite catalyst) [1][2].
  • Pass a current through the electrolyte [1].
  • Oxygen is extracted from the regolith, migrates across the electrolyte, and is collected at the anode (made of graphite) [1][2].
    • 15 hours to extract 75% of the total oxygen [3].
    • 50 hours to extract 96% of the total oxygen [3].
  • The oxygen is then vented into the atmosphere in their current setup but eventually they will have it go into a storage system [1].
  • A by-product of this process is a “tangle of different metals” [1].


  • Lower temperature and energy consumption because the regolith remains in a solid state instead of melting [2].
  • Much of the oxygen can be extracted [3].
  • No toxic by-products [2].
  • Minimal processing. The feedstock is a powder and remains a powder at the end of the process [2].
  • Silent [1].
  • A by-product is a metal sponge that can be crushed, milled, washed, and dried into a variety of metal powders [2].
  • Calcium chloride is considered pretty safe.


  • No cast basalt as a by-product (because the regolith isn’t melted).
  • Calcium is pretty common but chlorine isn’t. That will have to be imported from Earth.
  • I haven’t found data on if the CaCl2 is consumed in the process.

Parameter priorities

  1. Able to be constructed/maintained locally
    1. UNKNOWN
  2. Minimal reliance on Earth for reagents
    1. UNKNOWN
  3. Operable in 1 SLHA
    1. UNKNOWN
  4. Ease of maintenance
    1. UNKNOWN
  5. Simple
    1. UNKNOWN
  6. Durable
    1. UNKNOWN
  7. Energy input
    1. Electrolyte heated to 950°C [1].
  8. Heat output
    1. UNKNOWN
  9. No hazardous materials are produced
    1. UNKNOWN
  10. Efficiency
    1. UNKNOWN
  11. Processes that extract more than just oxygen are preferred
    1. Process leaves behind a metal sponge from which a variety of metals can be obtained [2].
  12. Weight/mass/volume
    1. UNKNOWN
  13. Other uses for the reagents
    1. UNKNOWN


  1. European Space Agency (ESA) – ESA opens oxygen plant – making air out of moondust. (
  2. Metalysis (
  3. – Oxygen and metal from lunar regolith. (

Need to read

  • Lomax 2019. Planetary and Space Science Vol 180
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