1D Borophosphates for Use as Electrolyte Membranes in Solid Acid Fuel Cells

Although they are relatively new players in the field of hydrogen energy conversion, solid acid fuel cells (SAFCs) demonstrate several advantages compared to their more well-developed counterparts: polymer electrolyte membrane fuel cells (PEMFCs) and solid oxide fuel cells (SOFCs). The elimination of water as the proton carrier in solid acid electrolytes allows operation at intermediate temperatures (e.g., 200–350 °C), simultaneously improving catalytic activity and removing the need to manage liquid water while avoiding the high-cost, refractory materials required for operation under SOFC conditions. However, higher operational temperatures necessitate the discovery, design, and manufacture of electrolyte materials with improved thermal stability compared to PEMs.<br/><br/>Cesium hydrogen sulfate (CsHSO₄) and cesium dihydrogen phosphate (CsH₂PO₄, CDP) are two well-studied examples of solid acids: protic materials which remain solid at operational temperature while exhibiting proton conduction. CDP demonstrates particular promise as a SAFC electrolyte, as a superprotonic transition from a low-temperature monoclinic phase to a high-temperature cubic phase dramatically improves its proton conductivity above 225 °C. However, active humidification of the CDP electrolyte is required to prevent thermal decomposition (i.e., dehydration) below the superprotonic transition temperature, limiting the practical operational window of CDP-based SAFCs to 225–260 °C.<br/><br/>We synthesize and explore an isostructural family of one-dimensional inorganic polyelectrolytes, the “BOB” borophosphates – empirical formula M_5-xH_x[BOB(PO₄)₃] where M is a monovalent cation – as alternative proton conducting solid acids to commercially available membrane materials including PEMs and CDP. Comparing the previously described Rb₃H₂[BOB(PO₄)₃], Na₅[BOB(PO₄)₃], and the novel (NH₄)₃H₂[BOB(PO₄)₃], we find that the rubidium borophosphate (RbBOB) strikes an attractive balance between temperature- and humidity-dependent ionic conductivity and thermal stability. Exhibiting stability under air, inert atmosphere, and hydrogen up to 400 °C and exhibiting ionic conductivity up to 10^-4 S cm^-1 under active humidification at elevated temperature, RbBOB is an exciting new solid acid for SAFC electrolyte membranes.