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.
NPO-20468
This Brief includes a Technical Support Package (TSP).
Molten-Carbonate Electrolyzers for Making CO and Oxygen
(reference NPO-20468) is currently available for download from the TSP library.
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Overview
The document discusses the development and potential applications of molten-carbonate electrolyzers, particularly in the context of space exploration and in situ resource utilization (ISRU) on Mars. Authored by Kumar Ramohalli and Gerald Voecks from NASA's Jet Propulsion Laboratory, it outlines how these electrolyzers can efficiently convert carbon dioxide (CO2) from the Martian atmosphere into carbon monoxide (CO) and oxygen (O2).
The primary goal of using molten-carbonate electrolyzers is to generate breathable oxygen for life-support systems and CO as a feedstock for synthesizing fuels and hydrocarbons. This technology is particularly relevant for long-duration missions to Mars, where resources must be utilized effectively to support human life and operations. The electrolyzers operate in reverse compared to molten-carbonate fuel cells, which have been extensively studied but not specifically for electrolysis applications.
The document highlights the advantages of molten-carbonate electrolyzers over competing technologies, such as solid-oxide electrolyzers and Sabatier systems. Solid-oxide electrolyzers, while effective, are fragile and require very high operating temperatures, making them less practical. Sabatier systems, on the other hand, necessitate an external supply of hydrogen, which complicates their use in isolated environments like Mars. Other alternatives, such as glow-discharge reactors and reverse water-gas reactors, also face significant operational challenges.
Key challenges in developing molten-carbonate electrolyzers include improving the stability of the electrolytes and extending their operational lifetimes, which are currently limited. The document suggests that advancements in modern materials, including stabilizers for the electrolytes, will be crucial for overcoming these hurdles.
The research and development efforts described in the document are part of a broader initiative to enhance the feasibility of human exploration of Mars by ensuring that essential resources can be generated on-site. The findings and proposed technologies could significantly impact future missions, enabling astronauts to produce the necessary gases for breathing and fuel synthesis directly from the Martian environment.
In summary, the document presents a promising approach to resource utilization in space, emphasizing the potential of molten-carbonate electrolyzers to support sustainable human presence on Mars and contribute to various applications on Earth.
