Local proton hopping mechanism in imidazolium-based plastic crystal: an ab initio molecular dynamics study

Protic organic ionic plastic crystals (POIPCs) are promising solid-state proton conductor materials in anhydrous proton exchange membrane fuel cells, due to their mechanical flexibility and high ionic conductivity in the plastic crystal phase. In typical POIPCs, the ions are orientationally disordered while the centers of mass are ordered (positional order) like the crystal phase. The local disorder provides more degrees of freedom for the translational and rotational diffusion of ions, thus enhancing proton conduction either via the vehicle mechanism or the Grotthuss mechanism. Yet the local dynamics and the interactions of the cations and anions during the proton transfer process are far from being fully understood. Here, we performed Car–Parrinello molecular dynamics (CPMD) simulation on the imidazolium methanesulfate (ImH]CH₃SO₃]) unit cell. By artificially creating one proton hole, we found that a proton can hop directly between the cations. Though the anion is not directly involved in proton hopping, the oxygen atom in the sulfonate group interacts with the proton and has a synergetic motion along with the proton hopping process. This indicates the structural disorder of imidazolium rings and the aid of an anion can facilitate Grotthuss-type proton hopping in imidazolium-based POIPCs.