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

High proton conductivity is reported for unhumidified ammonium borosulfate, NH₄B(SO₄)₂], a solid acid coordination polymer that contains 1D, hydrogen-bonded NH₄⁺···¹_∞B(SO₄)_4/2]^? chains. NH₄B(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 NH₄B(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.