Backbone effects on the charge transport in poly-imidazole membranes: a theoretical study

In this paper we present a theoretical investigation of proton conduction in a 2-tethered poly-vinyl-imidazole system, an N-heterocyclic-based membrane used as electrolyte in proton exchange membrane fuel cells (PEMFCs). In particular a detailed analysis, combining Density Functional Theory (DFT) and Molecular Dynamics (MD) simulations, shows the underpinning role of the backbone in determining the proton transport mechanisms. DFT calculations, carried out on a neutral trimer, suggest that proton conduction is based on a Grotthuss-like mechanism, which requires a series of proton transfers along the chain followed by a rotation of all the imidazoles. This latter represents the limiting step and it is clearly observed during the MD simulations on a larger model (oligomer of 15 imidazoles) at a typical operative temperature. From the computational point of view, this is the first time that such a mechanism is clearly evidenced in N-heterocycle-based membranes and the results give a clear explanation for the difference in proton mobility conductivities observed between P2VI and the parent poly(4-vinyl-imidazole) (P4VI). These results underline the relevant role of imidazole connectivity to the polymer backbone in determining proton conduction and could give valuable insights for the design of new and better performing protogenic groups with minimum reorientation barriers.