Electrode Investigation for the Solid-Oxide Electrolysis of Dry CO2 for O2 Production

One of the greatest challenges for any manned space mission is the amount of oxygen that is required, both for life support and as a chemical oxidant.  It is not feasible to carry all the oxygen needed for an extended mission to any planet. In the case of Mars, an oxygen generator may be used to separate and collect oxygen from a planet’s atmosphere.  The Martian atmosphere is composed of ~95% CO₂, which may be electrolyzed to produce oxygen (O₂) and carbon monoxide (CO) (and possibly water via reaction with H₂) for human consumption or use as a chemical oxidant for power generation systems.  There are a few established methods for generating oxygen from oxygen-containing gases, but these methods have significant power requirements, and the systems produce lower purity oxygen (<97%).  A few authors have reported work on the fabrication and testing of high purity oxygen generators using various selective solid-oxide electrolysis (SOE) membranes.  Solid-oxide electrolysis membranes function by the movement of oxygen ions through a solid electrolyte membrane due to an applied field or pressure driving force.  Many recent works have centered on the co-electrolysis of CO₂/H₂O, but the study of direct electrolysis of pure CO₂ to O₂ and CO is still limited.

The objective of this work was to investigate cermet electrodes for use in solid-oxide electrolysis cells (SOECs) for the efficient reduction of CO₂.  This work includes the investigation of the optimal electrode microstructure that is required for performance stability.  In this work, palladium (Pd)- and platinum (Pt)-based compositions were mixed with yttrium-stabilized zirconia (YSZ) and gadolinium-doped ceria (GDC) to form cermet composite compositions.  The compositions were synthesized by a mixed-oxide synthesis process in which the precious metal component is mixed at various ratios with the electrolyte powders.  The particle size distribution of the electrolyte powders were varied (by combining different powder lots with different particle size distributions) in order to alter the final pore size and electrode continuity throughout the final sintered film. In addition to the particle size distribution effects on the microstructure, various levels of Pd levels within the electrolytes were investigated.  Screen-printing inks were prepared for the powders produced.  The electrode compositions were screen-printed and sintered on YSZ electrolyte substrates (~150 mm thick) to produce symmetrical cells for electrolysis testing.  Platinum electrode mesh were attached to each side of the sample with small amount of the similar ink.  Electrode interfacial polarization measurements were completed on the compositions by impedance spectroscopy (at temperatures 650-850°C) using a Solartron 1260 in air and CO₂.   Four probe current-voltage (I-V) measurements were completed in dry CO₂ on the symmetrically printed cells.  The current density versus the applied voltage were related to the calculated oxygen separation efficiency. The optimal Pd-based electrodes demonstrated O₂ production from dry CO₂ at higher level than previous reported SOE cells, with production rates as high as 9.8´10^-5 mol×cm^-2×min^-1 (for 1.5 DCV) at 800°C.

 Acknowledgements:

A portion of this work was sponsored by the NASA West Virginia Space Grant Consortium (WVSGC) and by the WVU Statler College of Engineering and Mineral Resources. The authors would also like to acknowledge West Virginia University Shared Research Facilities for support through materials characterization.