Demonstration of Proton Conduction in Electrolyte-Supported Cesium Borophosphate Membrane Electrode Assemblies

Electrolyte-supported membrane electrode assemblies (MEAs) were fabricated using Cs₃H₂[BOB(PO₄)₃] (affectionately dubbed CsBOB) as the electrolyte and membrane separator and tested for proton conduction and power generation at temperatures up to 250 °C. The devices reached steady state ionic conductivities of 0.8-1.8 × 10^-5 S cm^-1 and produced commensurate power at nominal steady-state power densities of 0.6-1.2 × 10^-2 mW cm^-2 over the course of 160-h tests, thereby experimentally demonstrating that the CsBOB electrolyte does, in fact, conduct protons and is chemically stable under fuel cell operating conditions on the order of days. Machine-learned interatomic potentials (MLIP) trained on density functional theory calculations were used for molecular dynamics (MD) simulations, providing computational evidence confirming proton motion through bulk CsBOB. Importantly, the MLIP MD, bolstered by solid-state nuclear magnetic resonance (SS-NMR), also revealed that interstitial water plays a significant role in the CsBOB proton conduction mechanism under the operating conditions that were tested, helping provide high proton mobility. However, at these conductivities, electrochemistry is electrolyte limited; a 2-3 order-of-magnitude increase in proton conductivity is necessary for MEAs utilizing CsBOB electrolyte to achieve sufficiently high power densities for use in commercially competitive fuel cells. Future work will focus on improving CsBOB proton conductivity by increasing carrier density.