Comparing Proton Conduction in Potassium and Ammonium Borosulfate—Isostructural Inorganic Polyelectrolytes Exhibiting High Proton Mobility

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Abstract
A scalable new method for the synthesis of borosulfate compounds in sulfuric acid providing control over product crystallite size is reported as an alternative to traditional methods requiring slow growth from oleum. This new synthetic approach is used to prepare three isostructural, 1D borosulfates: one containing only ammonium cations, another containing only potassium cations, and the third sample with a solid solution of 1:1 ammonium–potassium. Proton conduction in polycrystalline pellets of these borosulfate electrolytes is compared by electrochemical impedance spectroscopy (EIS) and ab initio molecular dynamics (AIMD) simulations. For a given cation (e.g., NH4 +), conductivity decreases by three orders of magnitude with decreasing particle size while maintaining constant activation energy, indicating that proton conduction is not primarily a grain‐boundary process. AIMD simulations show that excess proton mobility in K[B(SO4)2] is in line with that of NH4[B(SO4)2], being a backbone (not cation) mediated process. Although K[B(SO4)2] exhibits higher activation energy (60.6 ± 2.0 kJ mol−1) than NH4[B(SO4)2] (33.8 ± 1.0 kJ mol−1), at 200 °C it achieves comparable conductivity to NH4[B(SO4)2] samples, which is attributable to hydrolytic B–O–H defects being the common source of mobile protons in these materials.

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