The Technology Transfer and Partnerships Office
Molten Regolith Electrolysis
Molten Regolith Electrolysis reactor (externally heated) in laboratory operation. (Image credit: MIT/Kennedy Space Center)

Molten Regolith Electrolysis reactor (externally heated) in laboratory operation. (Image credit: MIT/Kennedy Space Center).

The co-production of oxygen and metals on the lunar surface and on other planetary bodies is potentially a paradigm-shifting step in the advancement of space exploration. If sustainable, such capability would greatly enhance the survivability of a permanent human presence on any planetary surface throughout the solar system. The efficient production of in-situ oxygen and metals can be accomplish in a single step by Molten Regolith Electrolysis (MRE) in which metal oxides are directly reduced by electrolysis. Building on pioneering work by Haskin and Keller, the ISRU Technology Development Project has supported a project led by Kennedy Space Center and the Massachusetts Institute of Technology in collaboration with the Ohio State University and Marshall Space Flight Center to advance electrochemical measurements, materials and reactor component technology for MRE.


Goal

Extract oxygen and metals from regolith by a single-step electrolysis of the minerals. The metals must be produced in their molten form that is most useful for subsequent uses. Establish the level of feasibility of MRE for space missions.


Principle of Operation

Molten oxide electrolysis is an extreme form of molten salt electrolysis, a technology that has been producing tonnage metal for over 100 years; aluminum, magnesium, lithium, sodium, and the rare-earth metals are all produced in this manner. What sets molten oxide electrolysis apart is its ability to directly electrolyze the regolith without supporting electrolyte to produce pure oxygen gas at the anode and metallic elements at the cathode.

Figure 1 shows how oxygen might be produced by the proposed technology. The reactor depicted is an electrolytic cell, a device that causes electrical energy to do chemical work. This occurs by the transfer of electrical charge between two electrodes across an ionically conducting liquid (electrolyte). The electrolyte in this case is molten regolith. Oxygen is produced at the top of the cell on the surface of the anode, which acts as the current feeder. The anode must be chemically inert with respect to both oxygen gas and the molten oxide electrolyte. Evolution of oxygen occurs according to the following reaction:


O2- (electrolyte) ⇒ 2 e- (anode) + 1/2 O2 (gas)

The cathode can be either a solid immersed in the electrolyte or a pool of molten metal at the bottom of the cell as depicted in the figure. At the interface of liquid metal and electrolyte, the electrochemical reduction of metal ions occurs according to the following reactions:


Fe2+(electrolyte) + 2 e- (cathode) ⇒ Fe (liquid)
Si4+ (electrolyte) + 4 e- (cathode) ⇒ Si (liquid)


Joule-heated Molten Regolith

Figure 1. Principle of operation of a Joule–heated Molten Regolith Electrolysis reactor. (Source: D. R. Sadoway/L. Sibille)


Technology Features

Among other electrolytic techniques, Molten Regolith Electrolysis (MRE) — a.k.a. molten oxide electrolysis and magma electrolysis — offers the only one-step process to separate oxygen from metals by directly electrowinning the molten oxides. No salts, or fluxing agents need to be imported from Earth or manufactured as an added step resulting in greater simplicity in engineering, lower overall landed mass, and lower contamination of the produced oxygen. These attributes and its higher yields of oxygen and metal per unit mass of soil gives MRE the edge over techniques requiring chemical compounds to react with mineral constituents of the soil. MRE is also the only existing technology to deliver metals in their molten form, suited for easy retrieval and casting for future use.

The melting temperature of the minerals and that of the metals to be extracted dictate the operating temperatures; that temperature is currently approximately 1600°C or 2900°F. It is the only technology designed to produce metals in molten form that enable in situ parts fabrication from regolith.

The development of such technology is multi-faceted and interdisciplinary, calling for advances in materials design, electrochemical measurements techniques at high temperatures, and designs for high temperature oxidizing environments. At its core, the effort relies on experimental research aimed at understanding the electrochemistry involved in these unique environments and evolves through the inventions of new devices to remove molten materials from the reactor, to fabricate large surface electrodes capable of sustaining long immersions in liquid metals and oxides, and to provide long-lasting containment of the molten media.


Regolith and Environment Science and Oxygen and Lunar Volatile Extraction Thermal model of Joule heating in a Molten Regolith Electrolysis reactor in which the regolith itself contains a centered pool of electrolyte heated by the electrolytic current. (Image credit: NASA Kennedy Space Center)
Small casting of molten ferrosilicon Small casting of molten ferrosilicon (lower layer) and molten oxide of lunar composition (top layer) withdrawn by counter-gravity suction at 1600C from reactor furnace (Image credit: Ohio State U./Kennedy Space Center).



























References

Joule-heated Molten Regolith Electrolysis Reactor Concepts for Oxygen and Metals Production on the Moon and Mars, L. Sibille and J. A. Dominguez, AIAA 2012-0639, 50th AIAA Aerospace Sciences Meeting. 9 - 12 January 2012, Nashville, TN.


Recent Advances in Scale-up Development of Molten Regolith Electrolysis for Oxygen Production in support of a Lunar Base, L. Sibille, D. R. Sadoway, A. Sirk, P. Tripathy, O. Melendez, E. Standish, J. A. Dominguez, D. M. Stefanescu, P. A. Curreri, S. Poizeau, AIAA 2009-659, 47th AIAA Aerospace Sciences Meeting, 5 - 8 January 2009, Orlando, FL.