Electrification of the transportation sector is critical to future energy and environmental resilience and will require high-power fuel cells (either standalone or in conjunction with batteries) to facilitate the transition to electric vehicles, from cars and trucks to boats and airplanes.
Liquid-fueled fuel cells are an attractive alternative to traditional hydrogen fuel cells because they eliminate the need to transport and store hydrogen. They can help to power unmanned underwater vehicles, drones, and, eventually, electric aircraft — all at significantly lower cost. These fuel cells could also serve as range-extenders for current battery-powered electric vehicles, thus advancing their adoption.
Engineers have developed high-power direct borohydride fuel cells (DBFCs) that operate at double the voltage of conventional hydrogen fuel cells. The research team identified an optimal range of flow rates, flow field architectures, and residence times that enable high-power operation. This approach addresses key challenges in DBFCs; namely, proper fuel and oxidant distribution and the mitigation of parasitic reactions.
The team has demonstrated a single-cell operating voltage of 1.4 or greater, double that obtained in conventional hydrogen fuel cells, with peak powers approaching 1 watt/cm2. Doubling the voltage would allow for a smaller, lighter, more efficient fuel cell design, which translates to significant gravimetric and volumetric advantages when assembling multiple cells into a stack for commercial use. Their approach is broadly applicable to other classes of liquid/liquid fuel cells.
The reactant-transport engineering approach provides a way to significantly boost the performance of these fuel cells while still using existing components; even current, commercially deployed liquid fuel cells can see gains in performance.
The key to improving any existing fuel cell technology is reducing or eliminating side reactions. The majority of efforts to achieve this goal involve developing new catalysts that face significant hurdles in terms of adoption and field deployment.
Hydrogen bubbles formed on the surface of the catalyst have long been a problem for direct sodium borohydride fuel cells and it can be minimized by the rational design of the flow field. With the development of this reactant-transport approach, scale-up and deployment are possible.
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