Reactant-Transport Engineering Approach to High-Power Direct Borohydride Fuel Cells

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.