Efficient pH-gradient-enabled microscale bipolar interfaces in direct borohydride fuel cells

The disparate pH requirements for borohydride oxidation and peroxide reduction in direct borohydride fuel cells (DBFCs) currently hinder their performance and efficiency. Here we develop a pH-gradient-enabled microscale bipolar interface (PMBI) that facilitates sharply different local pH environments at the anode and cathode of a DBFC. Using a recessed planar electrode in conjunction with transmission electron microscopy, we show that the PMBI maintained a sharp local pH gradient (0.82 pH units nm–1 on average) at the electrocatalytic reaction site. The PMBI configuration enabled enhanced performance in a DBFC compared with either all-anion- or all-cation-exchange configurations (330 mA cm–2 at 1.5 V and a peak power density of 630 mW cm–2 at 1.0 V, respectively). The high power densities obtained at voltages well above 1.0 V—achieved by virtue of the effective separation of anolyte and catholyte locally at the electrocatalytically active sites by the PMBI—provide a pathway to reduce fuel cell stack size for autonomous propulsion applications.