(Invited) Electrolysis at a Distance: Challenges to Autonomous Operation of MOXIE on Mars

Abstract

Listen

Translate article

Currently on Mars aboard the Perseverance rover, the Mars Oxygen ISRU Experiment (MOXIE) is a prototype of a system that will someday provide many tons of oxygen as the major component of the propellant for a Mars Ascent Vehicle that will return astronauts from the Red Planet.MOXIE (Figure 1) first collects, filters, and compresses the thin Martian air, which consists of 95% CO2 and small amounts of nitrogen and argon at a pressure of <10 mbar, using a custom scroll pump developed by Air Squared, Inc. It then pre-heats the gas to ~800˚C and injects it into a stack of 10 solid oxide electrolysis cells (SOXE) developed by Ceramatec, Inc. (now OxEon Energy). CO2 is thermally and catalytically decomposed according to the reaction CO2 → CO + O2- at the cathode of the electrolysis cells, then the oxygen ions are selectively drawn through the yttriumstabilized zirconia electrolyte where they recombine at the anode into O2 molecules. The transfer of 4 electrons from anode to cathode completes the circuit and provides the motive force for the reaction. The pure oxygen product is characterized, then released through a precision aperture, while CO fuel and unused CO2 are similarly characterized and discharged through an exhaust port.MOXIE expects to produce 6-10 grams of 98% pure O2 per hour, a factor of ~200 less than will eventually be needed on a full-scale system.Planning of MOXIE operations is dominated by the need to maintain safe conditions while at the same time exploring the limits of performance and characterizing the state of the system. As shown in Figure 2, the voltage across the MOXIE cells, the flow rates, and the SOXE temperature need to be carefully balanced to avoid crossing Boudouard boundaries. MOXIE is typically operated in a constant-current mode, introducing uncertainties into the operating voltage. To compound these uncertainties, there are significant voltage drops elsewhere in the system across series resistances that are dominated by the Inconel electrical leads. Determining the variation of performance with SOXE temperature is a one method used to distinguish the resistance across the membrane, which is a strong function of temperature, from those series resistances.Subsequent runs will evaluate operation at different inlet flows and SOXE temperature, and will test alternative control configurations such as voltage-feedback control instead of current-feedback control or cathode pressure feedback control of compressor speed. Development of a “smart” control system is a high priority for future work and will be explored in a laboratory setting during the MOXIE campaign. Acknowledgement: MOXIE is supported by a collaboration between NASA’s Human Exploration and Operations Mission Directorate and the Space Technology Mission Directorate. The Mars 2020 mission is supported by the Science Mission Directorate.