Proton conductivity mechanism of liquid imidazole - an ab initio molecular dynamics study

Imidazole, as a fundamental organic compound, exhibits high proton conductivity comparable to water at similar temperatures relative to the melting point. Its potential application in fuel cells has motivated numerous experimental research efforts. However, details of the proton conductivity mechanism are still ambiguous. Using multiple time-step ab initio molecular dynamics simulations, we were able to accumulate trajectories totaling 1 ns in length. The predicted proton diffusion coefficient of imidazolium is 0.52 Ų/ps at 384K, and structural diffusion is the dominant mechanism. The proton transfer event is local and must go through a geometrically restricted Zundel-type transition state at a sub-picosecond time scale. Long hydrogen-bonded chains were detected in our liquid imidazole system within which the imidazolium defect undergoes frequent identity changes from individual proton transfer events. Chain diffusion is controlled by the dynamic hydrogen bond forming and breaking via rotational reorientation at a time scale of ~30ps. The decoupling of local proton transfer and chain diffusion together explains the fast proton hopping rate and relatively large diffusion coefficient of the charge defect.