Facile Proton Transport in Ammonium Borosulfate-An Unhumidified Solid Acid Polyelectrolyte for Intermediate Temperatures.
basic_science · Level V
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- Record sourced from PubMed, PMID 32924200.
- Also identified by DOI 10.1002/adma.202003667.
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Abstract
High proton conductivity is reported for unhumidified ammonium borosulfate, NH<sub>4</sub> [B(SO<sub>4</sub> )<sub>2</sub> ], a solid acid coordination polymer that contains 1D, hydrogen-bonded NH<sub>4</sub> <sup>+</sup> ···<sup>1</sup> <sub>∞</sub> [B(SO<sub>4</sub> )<sub>4/2</sub> ]<sup>-</sup> chains. NH<sub>4</sub> [B(SO<sub>4</sub> )<sub>2</sub> ] 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<sup>-1</sup> at 25 °C and up to 10 mS cm<sup>-1</sup> 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 NH<sub>4</sub> <sup>+</sup> units and significant gyrational mobility of the SO<sub>4</sub> 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 NH<sub>4</sub> [B(SO<sub>4</sub> )]<sub>2</sub> an attractive candidate electrolyte for intermediate-temperature hydrogen fuel cells that may enable operation at temperatures as high as 300 °C without active humidification.