(Invited) Solid Oxide Cell Design for High Power Density and Reliability: Applications in Power and Fuels Production
Authors: Elangovan S , Hafen Tyler , Larsen Dennis , Alvarado Esteban , Oldham Claire Decker , Abernathy Harry W. , Guan Bo , Liu Jian , Le Long , Marina Olga A , Coyle Chris , Pike Jenna 
Published: August 2024

Solid oxide fuel cells (SOFC) offer a carbon-neutral solution to power generation, fuel flexibility, and robust design suitable for stationary and air transportation applications. OxEon Energy developed an air electrode supported (AES) cell design that demonstrated 63% increase in power density compared with its heritage cell design in short stack tests.

OxEon Energy team has been advancing solid oxide technology for over three decades and the stacks operate fully reversibly in fuel cell and electrolysis cell modes. The SOEC/SOFC technology builds on the success of the SOEC stack installed on NASA’s Mars Perseverance Rover as part of the MOXIE (Mars Oxygen In-situ resource utilization Experiment) mission. The MOXIE stack produced propellant grade, high-purity O2 by electrolyzing Mars atmosphere CO2. MOXIE stack used an electrolyte supported design to meet the reliability requirements for a space mission.

An infiltrated air electrode cell design was developed to increase fuel cell stack specific power (kW/kg). Electrolyte thickness, the most resistive element of the SOFC, was reduced from ~250 microns to 70 microns. The fabrication process results in a bi-layer of dense scandia stabilized zirconia (ScSZ) electrolyte and a porous yttria stabilized zirconia (YSZ) matrix for air electrode infiltration. Fuel electrodes are screen printed. The reduced thickness of the electrolyte offers a compromise between improved performance over electrolyte supported design, while meeting robustness required for aerospace applications.

Producing infiltrated electrodes with 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 prevent interaction between electrode layers; intermediate calcination temperatures; and process development using infiltration equipment that enables volume production. OxEon partnered with National Energy Technology Laboratory (NETL) for electrode infiltration process development and PNNL for button cell and short stack validation testing.

SOFC fuel flexibility was demonstrated in button cells and stacks previously; performance was nearly identical among hydrogen, ammonia, and reformed natural gas. Durability testing shows similarly stable performance in each fuel.

A 10-cell AES stack was tested in SOFC mode at 800 °C in H2 and NH3 fuel feeds at 0.7 V/cell, and the same stack was operated in steam electrolysis to produce H2. SOFC testing demonstrated powder density of 0.35 W/ cm2 at 0.7 V/cell in the AES stack in ammonia feed; an electrolyte supported stack with equivalent materials measured 0.20 W/cm2 at 0.7 V. Long term testing in steam electrolysis showed stable performance at 800 °C and 1.2 V/ cell.

Button cell tests show the thin electrolyte cell structure is capable of an area specific resistance (ASR) of 0.25 Ω-cm2 in H2- and NH3 fuel feeds. OxEon’s continued AES cell development is focused on improving manufacturability, infiltrated electrode stability, and implementing degradation mitigation strategies developed for electrolyte supported cell structures. Improved performance with AES cell design has critical implications for future integration in OxEon’s technology suite: SOEC systems to generate H2 and syngas from renewable energy, and Fischer-Tropsch to produce synthetic fuels.

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.

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