Electrochemical cells in which molten carbonates would serve as electrolytes have been proposed for use in electrolyzing CO2. The proposal was made in an effort to implement a concept of in situ resource utilization (ISRU) for the exploration of Mars; the basic idea is to generate CO (if needed as a fuel) and O2 (for oxidizing fuel and/or for breathing) by electrolysis of CO2 from the Martian atmosphere. On Earth, molten-carbonate electrolyzers could be used to make breathable O2 for medical use, pure O2 for oxidizing surfaces of semiconductor chips, and CO as a feedstock for synthesis of alcohols and hydrocarbons. In both terrestrial and spacecraft life-support systems, the electrolyzers could be used to regenerate breathable O2 from CO2.

The proposed electrolyzers would amount in effect to molten-carbonate fuel cells optimized for operation in reverse. Solid-oxide electrolyzers (usually containing zirconia solid electrolytes) would be the closest competitors, were it not for their relative fragility, susceptibility to leakage, and necessity of very high operating temperatures. Sabatier systems would be the next closest competitors, except that hydrogen must be supplied for operation, making them impractical in the intended applications. Other would-be competitors include glow-discharge reactors, which must be powered with high voltages and currents; and reverse water-gas reactors, for which catalysts must be regenerated.

Molten-carbonate fuel cells have been investigated extensively, but not with respect to reverse operation for electrolysis. It should be a simple matter to adapt a commercial off-the-shelf molten-carbonate fuel cell for initial experiments in electrolysis. Topics that must be addressed in efforts to develop molten-carbonate electrolyzers include stability of the electrolytes and extension of operating lifetimes, which are short. The use of modern materials, including stabilizers for the electrolytes, is expected to be an important part of the proposed development efforts.

This work was done by Kumar Ramohalli and Gerald Voecks of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Physical Sciences category.


This Brief includes a Technical Support Package (TSP).
Molten-Carbonate Electrolyzers for Making CO and Oxygen

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This article first appeared in the November, 2001 issue of NASA Tech Briefs Magazine.

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