Oxygen, water, and fuel are of paramount importance to human life. As a leading concept, the solid-oxide electrolysis cell (SOEC) is a very powerful technology, especially in aiding NASA's endeavors to pursue extraterrestrial exploration missions. This work focused on developing a robust, long-life SOEC technology that efficiently cogenerates oxygen and CO fuel directly from CO2, and is superior to the state-of-the-art Oxygen-Generation System (OGS) technologies. The principal objective of the project was to develop the system to support Mars exploration missions as part of In-Situ Resource Utilization. The key problem characteristics were the SOEC performance and longevity under various operating conditions. The prior art was built on a thick electrolyte-supported SOEC using precious metals as electrodes. Due to the nature of SOEC operating mechanisms, high pressures may build up at the interfaces of the positive electrode and the electrolyte, resulting in electrode delamination and long-term stability issues. The state-of-the-art SOEC technology also faced the scaling up and stack sealing issues.
To overcome the limitations of the state-of-the-art SOEC technologies for oxygen production directly from carbon dioxide (i.e. electrode delamination, low performance, long-term durability, and high cost issues), an advanced concept of a robust and highly efficient O2-CO cogeneration system was developed that was built upon a Tubular, Negative Electrode-supported Solid-Oxide Electrolysis Cell (Tune-SOEC) technology.
The nature of tubular design enables the assembly of tubes in modules of a convenient size in a lightweight fashion with minimum concerns over sealability. A further increase in capacity could be achieved through electrical connection of multiple modules. In this way, systems for a wide range of applications and/or scales could be assembled, and the reliability of large installments could be quite high since single modules could be serviced or replaced as needed. Thus, the design of the prototype O2-CO cogeneration system was refined, and key elements were identified for the construction of cogeneration bundles.
Tune-SOECs were constructed using proprietary electrode current collectors, which were proven for conducting current at low voltage losses. The tubes were evaluated on a standard testing fixture at 800 ºC and cogenerating O2-CO directly from a bottled CO2 gas. A four-probe configuration with two voltage and two current leads was used. As a baseline, cells were tested initially in SOFC mode to characterize the power generation using a fuel including H2 and diluted CO by CO2, after which the cell underwent CO2 electrolysis characterization. Effects of CO2 concentrations on the cell performance were evaluated. During tests, the total cell potential was measured and recorded as a function of current (density). AC impedance spectroscopy was used to measure selected cell polarizations, with the Solartron frequencies varying from 0.01 Hz to 1 MHz.
The technology development integrated the most promising degradation-resistant ceramic materials with the unique cell/bundle design. Performance improvement was realized through the refinement of the key cell materials, and the development and implementation of an electrocatalyst infiltration process. With a unique bundle design, the Tune-SOEC technology was successfully scaled up from individual cells to a bundle comprising multiple tubes, which was proven experimentally for producing 0.411 kilograms of oxygen daily.
The Tune-SOEC system did not require any in-plane sealing, and can withstand high pressure, making it potentially suitable for pressurized operation. A tubular SOEC was more rigid than the conventionally adopted planar cell that required a mechanical compression load (likely generated stresses, in turn causing weakening of ceramic cells); thus the Tune-SOEC would have high tolerance to vibration during payload launching/landing.
No metallic interconnects/gas separators were needed for the proposed Tune-SOEC system, resulting in less weight and high gravimetric power densities, and high thermal shock resistances. The Tune-SOEC platform offered much more resistance to long-term degradation than a planar architecture due to the “cell imbalance” failure mechanism.
This work was done by Greg Gege Tao and Devin McGlochlin of Materials and Systems Research Inc. for Glenn Research Center. NASA is seeking partners to further develop this technology through joint cooperative research and development. For more information about this technology and to explore opportunities, please contact here. LEW-19344-1