Molecular dynamics simulations of proton conducting media containing phosphoric acid

An anhydrous proton conducting electrolyte is a key material in realizing medium-temperature fuel cells that can drastically simplify heat radiation systems in transportation applications. However, practical applications are limited by the low proton conductivity. To clarify the rate limiting process, molecular dynamics simulations using machine-learned inter-atomic potentials were conducted on three materials: liquid phosphoric acid, solid acid CsH₂PO₄, and coordination polymer [Zn(HPO₄)(H₂PO₄)₂](ImH₂)₂. The simulations showed proton hopping between phosphoric acid anions in the 100-300 fs time scale in all the three materials. However, the calculated diffusion coefficient of protons in three materials spans over orders of magnitude as observed experimentally. The rotational rates of the anions showed a remarkable difference; in the proton conducting liquid phosphoric acid and cubic CsH₂PO₄, the anions rotate at the 100 ps time scale whereas they rarely rotate in other media. This result clearly indicates that proton conduction is limited by the reorientations of the anions.