Lightweight Solid Oxide Cell Design for Ammonia-Fed SOFC Systems

Abstract

OxEon Energy team has been advancing solid oxide fuel cell (SOFC) technology for over three decades and solid oxide electrolysis cell (SOEC) technology using identical stacks for 15 years.OxEon’s SOEC/SOFC technology builds on the success of the SOEC stack installed on NASA’s Mars Perseverance Rover that produces high-purity O2 by electrolyzing Mars atmosphere CO2. The OxEon team worked with Jet Propulsion Laboratory (JPL) and Massachusetts Institute of Technology (MIT) to validate the MOXIE (Mars Oxygen In-situ resource utilization Experiment) stack.MOXIE stack demonstrated that the operational performance on Mars matched results from pre-launch earth testing. MOXIE stack design forms the basis for developing lightweight fuel cell design suitable for an electric Vertical Take-off and Landing (eVTOL). Demonstration of SOFC performance using carbon-free fuels such as ammonia is another critical objective.A lightweight cell design was developed to increase specific power (kW/kg) of the fuel cell stack. The electrolyte thickness, the most resistive element of the SOFC, was reduced from ~250 microns to 70 microns (72% decrease). The fabrication process results in a dense scandia stabilized zirconia ScSZ electrolyte and a porous yttria stabilized zirconia (YSZ) matrix for cathode (air electrode) infiltration. For the fuel electrode, both an infiltrated and screen printed options are evaluated.Producing an infiltrated electrode with a well-adhered and homogeneous distribution of active material is a challenge that requires multiple infiltration-calcination cycles to deposit sufficient material. Key variables are precursor chemistry to reduce the number of infiltrations and to prevent interaction between electrode layers; intermediate calcination temperatures, and process development using an infiltration equipment that enables volume production. OxEon partnered with National Energy Technology Laboratory (NETL) for electrode infiltration process development and PNNL for validation testing of cells. Full size cells are in production for stack testing.Fuel flexibility was demonstrated in button cells and stacks previously. Button cell and stack performance was nearly identical among hydrogen, ammonia, and reformed natural gas. Durability testing shows similarly stable performance in each fuel.A button cell test in ammonia at 800 °C using the thin electrolyte cell structure measured a button cell area specific resistance (ASR) of 0.25 Ω-cm2 in H2/N2 and NH3 fuel feeds and showed a peak power density of 0.98 W/ cm2 in NH3 fuel feed. Short term hold at 0.7 V showed stable performance.Analysis of additional button cell tests and stack tests using thin electrolyte cell structure will inform test stand design modifications that address balance of plant size requirements.This material is based on research sponsored by Air Force Research Laboratory under agreement number FA8649-22-P-0792. The views expressed are those of the authors and do not necessarily reflect the official policy or position of the Department of the Air Force, the Department of Defense, or the U.S. government. Approved for public release; distribution is unlimited. Public Affairs release approval #_______________. Figure 1