Human-occupied vehicles and autonomous vehicles such as rovers and landers may benefit from the fuel flexibility and high energy density of solid oxide fuel cells (SOFCs), compared to batteries and polymer electrolyte membrane (PEM) systems. Fuel systems greater than 1 kW are traditionally planar and exhibit high volumetric power density; however, due to large sealing areas, they have poor cycling characteristics. Recently, 250 cycles on a Tubular SOFC (T-SOFC) system (Protonex Technology Corp.) was demonstrated. Hot zones designed around T-SOFCs have a lower packing density, but significantly better cycle life and start times, making them an ideal solution. By increasing the power density of T-SOFCs, overall hot zone and system volumetric power densities can be greatly improved. Extending the methodology of freeze-casting to T-SOFCs will provide a system with the micro-structural advantages of their planar counterpart, but with the rapid thermal cycling capacity of traditional extruded SOFCs.

The first casting mold, fabricated from aluminum, was a single-part mold with a straight bore.

Another key advantage to the use of the SOFC generators as a power system for space applications is the use of logistical fuels and oxidants. This is because a SOFC can directly take syngas (a mixture of carbon monoxide and hydrogen), a product from catalytically reformed hydrocarbon fuels, in comparison to PEM fuel cells that have to run on pure hydrogen.

A new method for the fabrication of freeze-cast T-SOFCs was developed. The use of ice as a sublime able mold barrier allows for easy casting and removal of tubes from the mold. The uniform thermal environment created radial freezing, and the use of a zirconia (8YSZ), ethanol xylene-based electrolyte coating showed good sintering to the support. This process generated the desired unique finger-like microstructure that allows better fuel gas diffusion and enhances the anode triple-phase boundaries. An electrolyte layer was fully sintered. The electrolyte and the outside of the freeze-cast anode had strong binding interface. Previous experience on T-SOFCs (at both single cell level and stack level) reveals that the tubular geometry does have great capacity for fast start-up and cool-down.

This work was done by Yanhai Du and Joshua Persky of Yanhai Power, LLC 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 https://technology.grc.nasa.gov/. LEW-19308-1