Engineers at the McKelvey School of Engineering at Washington University in St. Louis (WUSTL) have developed high-power direct borohydride fuel cells (DBFC) that operate at double the voltage of conventional PEM hydrogen fuel cells. Their research was published in an open-access paper in the journal Cell Reports Physical Science.

The development of high-power fuel cells could advance the electrification of the transportation sector, including marine and air transport. Liquid-fueled fuel cells are particularly attractive for such applications as they obviate the issue of fuel transportation and storage.

Here, we report a direct borohydride fuel cell (DBFC) for high-power propulsion applications that delivers ∼0.9 W cm⁻² peak power by using a pH gradient-enabled microscale bipolar interface (PMBI) to effectively meet the incongruent pH requirements for borohydride oxidation/peroxide reduction reactions.

Reactant-transport engineering of the anode flow field architecture and fuel flowrates mitigates parasitic borohydride hydrolysis and hydrogen oxidation reactions and lessens anode passivation by hydrogen bubbles. We identify an optimal flow regime range, broadly applicable to other liquid-fed fuel cells, in terms of the standard dimensionless Reynolds number (Re) and the Damkohler number (Da).

DBFCs fulfilling these criteria provide a 2.4 times higher power density at 1.5 V compared to state-of-the-art polymer electrolyte membrane fuel cells (PEMFCs) that typically operate at 0.75 V. The high peak power density of 890 mW cm⁻² at 1.1 V may offer a pathway to reduce fuel cell stack size for propulsion applications.

—Wang et al.

The figure summarizes open circuit voltages of the representative DBFC performance in green and current density at 1.5 V in orange. DBFCs with peak power density at high voltage (>1 V) are represented by blue columns and those with peak power density at low voltage (<1 V) are represented by black columns. The present article’s work is highlighted by the yellow column. (Courtesy: Ramani Lab)

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. The approach is broadly applicable to other classes of liquid/liquid fuel cells.

The reactant-transport engineering approach provides an elegant and facile way to significantly boost the performance of these fuel cells while still using existing components. By following our guidelines, even current, commercially deployed liquid fuel cells can see gains in performance.

—Vijay Ramani, the Roma B. and Raymond H. Wittcoff Distinguished University Professor and corresponding author

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, we are on the path to scale-up and deployment.Zhongyang Wang, lead author

The technology and its underpinnings are the subject of patent filing and are available for licensing.

Resources

• Zhongyang Wang, Shrihari Sankarasubramanian, Vijay Ramani (2020) “Reactant-Transport Engineering Approach to High-Power Direct Borohydride Fuel Cells,” Cell Reports Physical Science doi: 10.1016/j.xcrp.2020.100084