Facile Proton Transport in Ammonium Borosulfate—An Unhumidified Solid Acid Polyelectrolyte for Intermediate Temperatures

High proton conductivity is reported for unhumidified ammonium borosulfate, NH4 [B(SO4 )2 ], a solid acid coordination polymer that contains 1D, hydrogen-bonded NH4+ ···1∞ [B(SO4 )4/2 ]- chains. NH4 [B(SO4 )2 ] is thermally stable to 320 °C and is amenable to sintering into monolithic, polycrystalline discs at 200 °C and about 300 MPa of uniaxial pressure. Impedance spectroscopy measurements reveal ionic conductivities for sintered ammonium borosulfate of 0.1 mS cm-1 at 25 °C and up to 10 mS cm-1 at 180 °C in ambient air. No superprotonic transition is observed in the temperature range of 25-180 °C. Ab initio molecular dynamics simulations show these high conductivities are aided by free rotation of the NH4+ units and significant gyrational mobility of the SO4 tetrahedra, which, in turn, provide facile pathways for proton locomotion. High conductivities, a wide operational temperature window, and tolerance to both ambient and anhydrous conditions make NH4 [B(SO4 )]2 an attractive candidate electrolyte for intermediate-temperature hydrogen fuel cells that may enable operation at temperatures as high as 300 °C without active humidification.