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+center dot center dot center dot(1)(infinity)[B(SO4)(4/2)](-)chains. NH4[B(SO4)(2)] is thermally stable to 320 degrees C and is amenable to sintering into monolithic, polycrystalline discs at 200 degrees 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 degrees C and up to 10 mS cm(-1)at 180 degrees C in ambient air. No superprotonic transition is observed in the temperature range of 25-180 degrees C. Ab initio molecular dynamics simulations show these high conductivities are aided by free rotation of the NH(4)(+)units and significant gyrational mobility of the SO(4)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 degrees C without active humidification.