Energy generation and storage are critical to achieve energy resiliency. Decoupling fuel storage and power generation increases availability of intermittent renewable energy by storing excess energy and providing sustained power when renewable energy output is low, or grid power is interrupted. Reversible solid oxide cell (rSOC) systems operate in both electrolysis and fuel cell modes to shift renewable power from when it is available to when it is needed, addressing the intermittency of renewable energy, a critical technology area for US Department of Air Force (DAF), and DAF Operational Imperatives to secure resilient basing and power for spacecraft. With the support of DAF Direct to Phase 2 contract FA864924P1033, OxEon is designing, building, and demonstrating a rSOC system to generate electrical power from stored hydrogen gas in fuel cell mode and produce hydrogen fuel from steam in electrolysis mode. The system includes a balance of plant (BOP) and hydrogen compression system that increases the stack-produced H 2 to 5 barg for storage. The rSOC system incorporates a 65-cell stack that generates 1 kW power in solid oxide fuel cell (SOFC) operation with stored H 2 , and produces H 2 with an input of 2 kW e , with a target H 2 production rate of 1.4 kg/ day. System modeling, based on selected BOP components, identified the capability to cycle the system between electrolysis and fuel cell operation with a 15.8 hr initial solid oxide electrolysis cell (SOEC) cycle, followed by repeating 3.4 hr SOFC and 5.3 hr SOEC cycles. The demonstration system combines recent advances in OxEon’s rSOC technology. OxEon’s ruggedized, hermetic SOEC stack can operate at 1.5 to 3 barg pressure which can eliminate the largest and most costly first stage hydrogen compressor. The modestly pressurized stack produced hydrogen can then be compressed to an appropriate storage pressure with a smaller compressor. In a previous project (DOE, contract DE-FE0032105), OxEon operated a 6-cell stack to generate hydrogen at >80% steam conversion, and oxygen above 98.5% purity during pressurized operation. Both hydrogen and oxygen were generated at 3 barg pressure without the use of a pressure vessel. In addition, the test sequence included electrolysis operation at 1 bar differential pressure across anode and cathode that demonstrated substantial robustness of the cell and seal. A redox tolerant fuel electrode allows recovery from accidental oxidation of the Ni-based fuel electrode, which further increases the robustness of the stack. Under the DOE project, a stack test with the redox tolerant fuel electrode exhibited stable performance in multiple redox and thermal cycling, and stable performance in testing for 500 hours in SOEC mode, followed by 300 hours of cycling between SOEC and SOFC tests. Degradation during SOEC operation was 1.8 %/ 1,000 hours, based on a 500 hour test. In general, degradation is non-linear and reduces over time. Investigation is underway to identify contributions to early degradation. System operation benefits from improved performance stability resulting from materials development that targeted stabilizing the air electrode and improving the air electrode barrier layer. Seal materials developed under Mars Oxygen In-Situ Resource Utilization Experiment (MOXIE) program enable pressure differential from SOEC stack and its exterior. The stack on Mars successfully operated 16 times, meeting all the operational objectives and producing propellant quality (>99.6% purity) oxygen by electrolyzing Mars atmosphere CO 2 . OxEon specializes in four related energy conversion technologies. They are (1) SOFC for high efficiency generation of electricity using hydrogen and hydrocarbon derived syngas; (2) SOEC for production of hydrogen by electrolyzing steam, and syngas by co-electrolyzing steam and CO2; (3) low energy plasma-based reformer for conversion of gaseous and liquid hydrocarbons to generate syngas; and (4) FT reactor to produce liquid hydrocarbon fuel and wax using syngas as the feed. The overall objective of the company is to develop technologies for transformative cross sector energy conversion to store renewable energy in the form of hydrogen and drop-in transportation fuel. OxEon’s Project EquinOx, supported by the Department of Energy under Award Number DE-EE0011305, will implement a 25manufacturing facility for SOC stacks, 25 MWe annual stack production capacity, scalable to a fully automated GWe facility. A strategy of manufacturing scale up and performance improvement will be implemented to lower the cost of SOE stacks on a per kW basis. Figure 1
An experiment was conducted to test the hypothesis that pigs fed a low crude protein diet supplemented on a product-to-product (wt-wt) basis with 65 parts of DL-methionine (DL-Met, 99%) or 100 parts of the calcium salt of the hydroxy analog of DL-Met (MHA-Ca, 84%) will not have different N retention. A mash Met-deficient basal diet [0.23% standardized ileal digestible (SID) Met and 0.55% SID Met + Cys], which was 19% below the requirement for 11 to 25 kg pigs was formulated. Four additional diets were prepared by supplementing the basal diet with 0.030 or 0.090% DL-Met, or 0.046 or 0.138% MHA-Ca. The greatest level of Met supplementation (0.64% SID Met + Cys) was close to the requirement. Thirty barrows (initial body weight: 16.37 ± 1.28 kg) were housed individually in metabolism crates and allotted to a randomized complete block design with 5 diets and 6 replicate pigs per diet. Feed was provided at 3.2 times the energy requirement for maintenance in 2 equal daily meals. Pigs had ad libitum access to water. After a 7-day adaptation period, fecal and urine samples were collected for 4 days to determine N balance. Data were analyzed using the MIXED procedure of SAS, and orthogonal-polynomial contrasts were used to determine linear effects of DL-Met and MHA-Ca levels and the effect of Met sources. Results indicated that pigs fed diets without supplementation of Met had reduced (P < 0.05) final BW, feed intake, and N intake compared with pigs fed diets supplemented with DL-Met or MHA-Ca, and no differences in final BW, feed intake, or N intake between pigs fed DL-Met or MHA-Ca were observed (Table 1). Fecal N increased (linear, P < 0.05) by supplementation of DL-Met to the diets, and urine N decreased (linear, P < 0.05) as MHA-Ca increased in the diet. Retention of N (g/day) increased (linear, P < 0.05) as DL-Met or MHA-Ca increased and N retention calculated as percent of intake increased (linear, P < 0.05) if MHA-Ca was added to the diet and tended to increase (linear, P < 0.10) if DL-Met was added to the diets. Retention of N calculated as percent of absorbed N also increased (linear, P ≤ 0.05) with increased Met in the diet regardless of source. However, retention of N measured as percent of intake was not different between DL-Met and MHA-Ca. In conclusion, pigs fed Met-deficient diets and supplemented with 65 parts of DL-Met or 100 parts of MHA-Ca on a product-to-product (wt/wt) basis did not have different N retention expressed as g per day, as percent of intake, or as percent of absorbed.
The OxEon team provided the Solid Oxide Electrolysis Cell (SOEC) stack for the MOXIE (Mars OXygen In-situ resource utilization Experiment) project for the Perseverance Rover. The SOEC stack on Mars successfully operated 16 times meeting all the operational objectives and producing propellant quality (>99.6% purity) oxygen by electrolyzing Mars atmosphere CO 2 and represents the first ISRU (In-Situ Resource Utilization) demonstration on another planet. [1] ISRU presents the opportunity to reduce payload and launch costs by utilizing resources already available on the moon and Mars. Since MOXIE, the SOEC stack using identical set of materials has been successfully scaled by 35x and incorporated into breadboard demonstration systems for propellant production on both the moon and Mars. [2], [3] Additionally, materials development since MOXIE has resulted in improved performance and capabilities of the nickel-based cathode material. With the support of NASA, OxEon is continuing SOEC materials and hardware development to design, build, and operate a propellant production system capable of producing methane and oxygen from H 2 O and CO 2 . The MOXIE SOEC device was a 0.5% scale demonstration that produced between 6-12 g/hour high purity O 2 (>99.6%). The stack utilized a traditional nickel-based electrode which is susceptible to oxidation by the feed gas (in this case CO 2 ) at the inlet conditions without a reducing species such as carbon monoxide being present in the feed. To avoid oxidation of the cathode, a recycle loop was implemented in the MOXIE system. Since completion of MOXIE, materials development has occurred resulting in a redox tolerant cathode shown to completely tolerate partial and full (i.e., complete oxidation of Ni to NiO before re-reduction) redox cycling, with performance recovery occurring in a matter of minutes using CO generated by the electrolysis reaction. [4] This materials advancement eliminates the need for a recycle stream in future systems. Dry-CO 2 electrolysis using an SOEC stack to produce high purity oxygen was demonstrated on Mars, but subsequent ISRU SOEC work has coupled that capability with H 2 O electrolysis for additional functionality. Stack testing has been done to demonstrate steam electrolysis, dry-CO2 electrolysis, and co-electrolysis with a range of H 2 O/ CO 2 feed ratios all with the same device. In the case of co-electrolysis, the synthesis gas produced has a H 2 / CO outlet ratio that tracks the feed composition allowing for a gas composition tailored to the production of a range of chemicals including methane, methanol, lubricants, and liquid fuels. OxEon developed, fabricated, and relevant condition tested a breadboard system containing an SOEC stack 35x the size of the MOXIE stack, integrated with a methanation reactor. The integrated system was installed and tested in the Mars chamber at the Jet Propulsion Laboratory to demonstrate production of methane and high purity oxygen from H 2 O and CO 2 at relevant Mars conditions. The system demonstrated oxygen production as high as 680 g/hour with a purity >99.996% and methane production as high as 170 g/hour. A second breadboard system was developed, fabricated, and relevant condition tested for lunar propellant production. An SOEC stack 35x the size of the MOXIE stack was thermally integrated with a Balance of Plant (BOP) designed to intake near freezing water (to simulate water extracted from a Permanently Shadowed Region (PSR) on the Lunar surface) and produce propellant H 2 and O 2 . The integrated SOEC/ BOP system was relevant condition tested in a cryo-vacuum chamber at the Colorado School of Mines to simulate operation in a Lunar PSR. The system successfully met nominal operating targets of 1.8 kg/day H 2 production and 14.4 kg/day O 2 production with a specific energy as low as 48 kWh elec /kg H2 . Additionally, testing was done to demonstrate the ability to electrochemically pressurize the produced oxygen product up to pressures as high as 3.7 bar as a benefit to downstream processing and storage. Remote operation of SOEC system for Mars and Lunar applications imposes a significant demand for performance stability and reliability. Each of the materials set used in the construction of the stack has been evaluated for potential improvement for robust operation. The redox tolerant fuel electrode previously developed under a NASA SBIR project has continued to be utilized for redox capability (Figure 1, left) but also due to its ability for higher conversion of CO 2 when compared to the heritage (MOXIE) fuel electrode. Optimization of the redox tolerant fuel electrode is ongoing but recent materials development has also focused on enhanced electrolyte, air electrode, and barrier layers for improved long-term cell stability. Electrolyte performance and stability during cell operation is a function of the crystallographic phase(s) present at the beginning (thermal history) and any phase changes during operation. Zirconia electrolyte is expected to have a combination of cubic and tetragonal phases; however, a detrimental monoclinic phase can also form that affects the robustness of the cells during stack sealing and eventual performance. An investigation determined that exposure to certain temperature ranges should be limited to avoid monoclinic formation, and changes made to thermal processing resulted in more durable cells. The air/O 2 side of the cell contains an LSCF-based perovskite electrode along with a ceria barrier layer between the electrode and electrolyte. The barrier layer prevents detrimental interactions between the electrode and electrolyte materials, namely the formation of resistive lanthanum zirconate and strontium zirconate, that can take place during cell processing and operation. The ceria barrier layer also forms a resistive solid solution with zirconia electrolyte if the sintering temperature is too high. Multiple improvements were made to the barrier layer to increase cell performance and long-term stability, including use of a sintering aid to reduce the sintering temperature, printing optimizations to prevent barrier cracks, and post-processing steps to increase layer density. Unlike lanthanum, strontium has a strong tendency to migrate from the electrode to the electrolyte boundary through any gaps/cracks or regions of high porosity in the barrier layer. Therefore, a high performance strontium-free electrode is also under investigation to avoid the strontium migration and degradation problem altogether. The combined approach of improved barrier layer and strontium elimination from the electrode layer will be tested for long-term durability in button cell testing. Developments in air electrode treatments have shown that the initial performance of the electrode can be fully recovered, even after 5,000 hours of operation. Baseline electrode treatment results in full performance recovery and improved stability. A modified treatment applied to an air electrode symmetric button cell (half-cell) has demonstrated full performance recovery again, and with significantly improved stability (Figure 1, right). The 0.4-volt hold used for the test (OCV = 0 with air on both sides) results in fuel cell mode of operation for one air electrode and electrolysis mode of operation for the opposite air electrode. Typically, an initial break-in period of about 300 hours is required before reaching stabilized performance. After three thermal cycles to room temperature, around 800 hours test time to verify cycling stability, the cell was cooled for baseline treatment. The cell was re-heated and tested for an additional 4,800 hours until the previous stable performance of 0.3 A/cm 2 was reached (not shown), at which point the cell was cooled for the modified treatment. The performance once again recovered and the cell stabilized slightly below 0.4 A/cm 2 , showing a marked improvement over untreated air electrode and baseline treatment. This improved materials set will be incorporated into full-size SOEC cells and stacks for quantitative and long-term verification testing. Testing will evaluate degradation, performance, and coking resistant properties. A 5,000-10,000 hour long-term test is planned to monitor performance stability. A series of shorter length tests will operate stacks at various operating voltages, including voltages above the point where carbon is expected to form via electrolysis of CO. These results will provide the design basis for a system capable of producing 1-3 kg/hour high purity O 2 and associated methane from H 2 O and CO 2 , at pressures of 1-4 bar. The final hardware deliverable will be compatible with lunar ISRU and Mars ISRU applications. [1] Hoffman, J.A., Hecht, M.H., Rapp, D., Hartvigsen, J.J., SooHoo, J.G., Aboobaker, A.M., McClean, J.B., Liu, A.M., Hinterman, E.D., Nasr, M., “Mars Oxygen ISRU Experiment (MOXIE)—Preparing for human Mars exploration,” Sci. Adv. , 8 , https://doi.org/10.1126/sciadv.abp8636 (2022). [2] Hartvigsen, J., Elangovan, S., Elwell, J., Hollist, M., Lasen, D., Hafen, T., Calus, D., Valdez, S., Gomez, A., Czernichowski, P., Yarosh, A., Pike, J., and Davis, N., “Scale Up and Coupling of the MOXIE Solid Oxide Electrolyzer for Propellant Production on Mars,” 51 st International Conference on Environmental Systems, Minneapolis, Minnesota, ICES-2022-280, https://hdl.handle.net/2346/89749 (2022) [3] Hollist, M., Hartvigsen, J., Elwell, J., Elangovan, S., Gomez, A., Claus, D., Wilson, M., Jackson, G., Dreyer, C., Sowers, G., Dickson, D., Schmit, J., and Emadi, N., “Solid Oxide Electrolysis Based Lunar PSR Ice Processing System for Propellant Hydrogen and Oxygen Production,” 51 st International Conference on Environmental Systems, Minneapolis, Minnesota, ICES-2022-280 (2022) [4] Hafen, T., Rane, T., Larsen, D., Pike, J., Hartvigsen, J., Elwell, J., and Elangovan, S., “Solid Oxide Electrolysis Cathode for Increased Robustness for ISRU Application,” 51 st International Conference on Environmental Systems, Minneapolis, Minnesota, (2022) Figure 1
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.
