Effect of anion reorientation on proton mobility in the solid acids family CsH_yXO₄(X = S, P, Se,y= 1, 2) fromab initiomolecular dynamics simulations

Abstract

The high temperature phases of the solid acids CsHSeO4, CsHSO4 and CsH2PO4 show extraordinary high proton conductivities, which are enabled by the interplay of high proton transfer rates and frequent anion reorientation.

Molecular dynamics simulations of proton conducting media containing phosphoric acid

“…In addition, two properties were examined in this study: the lifetime of the ionic state of H n PO 4 (3Àn)À (n = 0, 1, 2, 3, or 4) and the reorientation time averaged over all H n PO 4 (3Àn)À . Previous FPMD studies 14,19,22 have indicated a significant contribution of the Grotthuss mechanism in the liquid PA. In this mechanism, proton hopping between PA anions and their reorientations strongly affect the proton diffusivity.…”

“…Similar to a previous study, 22 the reorientation time averaged over all H n PO 4 (3Àn)À was calculated from the autocorrelation function:…”

“…where N PO is the number of P-O bonds, and p ˆi is the P-O unit vector. The reorientation time t rot was calculated by fitting the following biexponential function 16,21,22 to the autocorrelation function,…”

“…In reality, progressive proton transfer requires reorientations of H n PO 4 (3Àn)À ions. 16,19,22 Therefore, proton conduction can be limited by slow reorientations of H n PO 4 (3Àn)À ions. The slow reorientation is demonstrated by the long relaxation time calculated from the autocorrelation functions of the P-O vectors defined as eqn (6).…”

“…The first principles molecular dynamics (FPMD) simulation is a powerful tool to provide atomic-scale structures and dynamics of proton-conducting electrolytes. [13][14][15][16][17][18][19][20][21][22] Many of these simulations have produced important information on detailed mixed vehicle and Grotthuss transport dynamics composed of proton hopping and molecular reorientation in a wide variety of proton conducting materials. Its high computational cost, however, often limits systematic comparisons between different materials, not to mention the statistical samplings required to examine phase stability and molecular reorientation phenomena.…”