Ceramatec is developing the solid oxide electrolysis cell (SOEC, aka SOXE) stack for MOXIE. The rover host platform for the MOXIE project imposes severe constraints on mass, volume, peak power and total cycle energy, but it offers an early opportunity to demonstrate non-terrestrial ISRU with only the incremental cost of developing and delivering the 15kg MOXIE system on a 1050kg rover. Additional challenges arise in an unmanned operational environment, with once daily uplink and downlink schedules making man in the loop operation infeasible. Therefore, care must be taken to define a safe operating envelope in such a way that the system can be reliably operated without damaging itself as there is no option for a service call as was done for Hubble.
Mars atmosphere, which is about 96% CO2, is supplied to the solid oxide electrolysis stack by means of a scroll compressor. The scroll pump is a fixed displacement device, so the delivered flow rate of CO2 will depend on pump rpm and the atmospheric density, which varies as a function of landing site elevation as well as seasonal and diurnal atmosphere cycles. The landing site has not yet been selected, but even after determining the landing site, the atmospheric density still varies over a wide range. Therefore, MOXIE must be able to operate over a wide range of feeds rates.
Other than electric power, limited thermal stability and telemetry, the rover provides no process utilities such as air, hydrogen, nitrogen, or water to MOXIE. MOXIE can only take in filtered Mars atmosphere, and in an unattended operation, produce oxygen from the CO2 in the atmosphere.
A nickel cermet cathode is oxidized by pure CO2 at 800°C which results in a loss of conductivity and a disruption of the microstructure due to volume expansion on oxidation. A combination of high cell operating voltage and CO2 utilization will lead to carbon deposition which can irreversibly destroy the cathode and plug the cathode flow channels. These thermodynamic and resulting electrochemical boundaries have been mapped analytically and have been used to develop system configurations and operating strategies to protect against cathode oxidation and CO reduction to solid carbon. These approaches have been and continue to be validated experimentally with increasing degrees of system integration. The MOXIE stack process integration and operational development path and results will be presented.
Acknowledgment: This material is based upon work supported by NASA through JPL’s prime contract under JPL subcontract number 1515459.