Proton transfer in a single pore of a polymer electrolyte membrane

We discuss the principles determining the activation of proton hops in pore s of polymer-electrolyte membranes and calculate the electrostatic contribu tions to the activation Gibbs energy. For this purpose, we explore a model of a flat water filled pore with charged sulfonic groups on the surfaces. W e find that when the proton transport takes place along the array of sulfon ic groups, the contribution of the work terms due to the proton charge inte raction with sulfonic groups is large. In the central region of the pore, w here the proton transport proceeds in a bulk-like fashion the work terms va nish. Moreover, the activation energy of the hopping along the surface incr eases considerably with the average distance between the sulfonic groups. S queezing the pore leaves no room for bulk like proton transfer and, though the effect on the solvent reorganization energy may be small, the overall e ffective activation energy increases strongly. This is in line with the obs erved effect of decreasing water content on membrane conductivity. (C) 2001 Elsevier Science B.V. All rights reserved.