Proton Conductivity in Hydrogen Phosphate/Sulfates from a Coupled Molecular Dynamics/Lattice Monte Carlo (cMD/LMC) Approach

The ionic conductivity of solid acids of CsH_nXO₄ type (X = P, S; n = 2, 1) varies upon chemical substitution P ↔ S and between different crystal structures (T_c = 503 K for CsH₂PO₄). We apply a recently developed coupled molecular dynamics/lattice Monte Carlo simulation approach (cMD/LMC,) to explain both the phosphate/sulfate and temperature/phase-related variations of the proton conductivity on a molecular level. Our simulation method elucidates the relative importance of the two key components of the Grotthuss-type proton conduction mechanism, proton hopping and structural reorientation, as a function of the chemical/thermodynamical conditions. We find that the chemical substitution leads to a substantial change in the proton hopping rate, which however results only in a modest variation of the proton diffusivity. The variation of the temperature of CsH₂PO₄ results in a significant response of the anion rotation frequency, which turns out to be the rate-limiting process for proton conduction. In particular, the dramatic conductivity response to the phase transition can be explained by a large change of the rotation frequency. In contrast to this, our simulations show that for CsHSO₄, the local proton hopping rate is the decisive mechanism which controls long-range proton transport. These findings illustrate that the actually rate limiting factor of proton conduction in such solid acids is clearly system-dependent. Our simulated results for the proton conductivities agree almost quantitatively with experimental values, providing further evidence for the high predictive capabilities of our scale-bridging cMD/LMC simulation approach.