Density functional molecular dynamics simulations investigation of proton transfer and inter-molecular reorientation under external electrostatic field perturbation: Case studies for water and imidazole systems

Abstract

Proton transfer is a governing factor in the proton exchange efficiency in membrane fuel cells (PEMFCs), which are an alternative environmentally friendly resource. To develop the capacity of the PEMFC system, anhydrous membranes containing imidazole groups have garnered much interest. In this research, the relationship between the hydrogen bond networks, including the consequent packing structure, and the proton conductivity of water and imidazole (Im) systems have been systematically studied. The effect of external electrostatic perturbation was investigated in (H2O)H+⋯H2O, (Im)H+⋯Im, bulk water, and imidazole superlattice systems. In all of these cases, the application of an electric field in a direction opposite to that of the overall system dipole significantly reduces the activation barrier for proton transport. In isolated systems, (H2O)H+⋯H2O and (Im)H+⋯Im, the preferred orientation angle between the neighbouring molecules was 90°. From density functional molecular dynamics simulations of the bulk system, the proton diffusion coefficient was found to increase under the perturbation by the applied electric fields in range of 1.29 × 107 to 3.86 × 107 V cm−1 (0.0025–0.0075 a.u.) for both water and imidazole. To trace the efficient proton transfer, the proton movement trajectory was explicitly analysed in detail. Interestingly, a tilted proton hopping direction was found for imidazole crystal.