Microscale Bipolar Interfaces for High-Power Fuel Cells

Abstract

Electrochemical devices are typically designed for operation over a narrow pH range and are constrained in the choice of catalysts and operating potentials by the pH environment of the electrodes. This is the result of a heretofore lack of a viable strategy to maintain pH gradients between the electrodes over practically significant time durations with only a minimal impact on the device performance. While bipolar interfaces are well-known, they typically result in high junction potential losses that make them impractical in real-life systems. We have demonstrated a way to overcome this long-standing challenge using our tailor-made, microscale bipolar interfaces, which allows the use of acidic electrolytes at one electrode and alkaline electrolytes at the other, without mixing over time. This allows for a much broader selection of fuel and oxidant stream catalysts (moving away from platinum group metals) and electrolytes to be used. Low-temperature aqueous fuel cells have been constrained in their operating voltage to ca. 1.2–1.5 V (as a function of fuel and oxidant combination) by the water splitting side reactions (hydrogen evolution reaction (HER) and oxygen evolution reaction (OER)). This relatively low cell voltage translates to larger stack volumes for a desired power output and poor specific power output. By employing our pH-gradient enabled microscale bipolar interfaces (PMBIs) we have demonstrated aqueous direct borohydride fuel cells (DBFC) that 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. This Account traces the >10 year development of PMBIs. We detail the development of a recessed planar electrode that provided experimental evidence that the PMBI was able to maintain a sharp local pH gradient (0.82 pH units nm^–1 on average) at the electrocatalytic reaction sites. We go on to trace the evolution of a series of highly selective anion exchange ionomer (AEIs) that enabled ever higher intercell pH gradients and culminated in the demonstration of the highest power operation of a bipolar DBFC reported in the literature. PMBIs have also enabled recent developments in high power bipolar direct methanol- and direct ethanol fuel cells (DMFC and DEFC) employing hydrogen peroxide oxidants. PMBI-based DMFC and DEFC achieved open-circuit voltages near their theoretical maxima (∼1.7 V and ∼1.65 V, respectively). Emerging strategies such as molecular modeling-guided ionomer design, self-healing bipolar interfaces, and optoelectronic coupling represent promising future directions to further enhance PMBI stability, ion transport efficiency, and expand their practical applicability in high-performance electrochemical energy conversion devices.