Redox Tolerant Solid Oxide Electrolysis Cathode for CO2 and Steam

Abstract

The production of oxygen for life support and ascent vehicle propellant oxidant is essential for human expeditions to Mars. OxEon team led the development of solid oxide electrolysis cell (SOEC) stacks for the Mars 2020 mission in collaboration with the Jet Propulsion Laboratory (JPL) and Massachusetts Institute of Technology (MIT). A stack that was installed in the Perseverance Rover has been operated twelve times so far to demonstrate the production of high purity oxygen by electrolyzing Mars atmosphere CO2.Traditionally, SOEC stacks use nickel–zirconia or nickel–ceria composite cathode to reduce the oxidized species. Nickel based electrodes are susceptible to oxidation by the feed gas (CO2 or steam) at the inlet conditions and are often irreversibly damaged during operation and start-up unless reduced species (carbon monoxide or hydrogen) are also present. Oxidation of Ni to NiO causes ~24% volume expansion, and a redox cycle with associated expansion and contraction can result in significant changes to the microstructure and loss of connectivity in the cathode and current collection layers. This challenge was encountered during early stack and testing development for the Mars mission where even short-term exposure to oxidizing dry CO2 feed caused massive performance degradation (12% of initial performance after 15 operational cycles). This necessitated a recycle loop for the stack on the Perseverance Rover that introduces a fraction of the CO-containing tail gas to the inlet to protect the cathode layers from oxidation.A simpler solution than a system recycle loop was desired for future applications. Under a NASA SBIR program OxEon investigated a combination of materials and engineering solutions to improve redox tolerance of the nickel-based cathode so that 100% dry CO2 could be fed directly into a stack without harming the electrode. A modified nickel-based cathode composition with a unique backbone and infiltrated cathode structure was developed and tested in both button cell and stack configurations. The new cathode has been shown to completely tolerate partial and full (i.e., complete oxidation of Ni to NiO before re-reduction) redox cycling, with complete performance recovery occurring in a matter of minutes even after total oxidation. It was also demonstrated that feeding a reducing gas after complete oxidation is not required since the CO generated by the applied voltage during initial CO2 electrolysis reaction is sufficient for self-reduction recovery. This self-recovery feature is particularly attractive for applications where an oxidizing gas feed cannot be easily substituted with a reducing gas feed, such as with Mars O2 generation.In-situ resource utilization (ISRU) of lunar ice and potential Martian ice for O2 and H2 generation is also of significant interest for future space missions. The redox tolerant cathode has been shown to completely tolerate steam oxidation as well, with self-generated H2 resulting in rapid reduction and recovery. Steam redox tolerance was demonstrated in both button cells and stacks and was independently verified by testing at PNNL. The redox tolerant cathode is also capable of rapid thermal cycling with ramp rates as high as 15 °C/min tested with no performance degradation. Improved coking tolerance over the traditional cathode material is an additional robustness feature that allows for higher conversion of CO2, enabling increased O2 production.This work was done under a NASA Small Business Innovation Research Contract No. 80NSSC19C0114. Validation testing at PNNL was performed under a DOE Award No. DE-FE0032105.