Theoretical investigation of proton-transfer in different membranes for PEMFC applications in low humidity conditions

Abstract

A comparative theoretical study was performed to explore the effect of polymer backbone on the proton exchange in different sulfonated membranes such as poly(phenylene oxide), poly(benzimidazole), polyimide, poly(oxadiazole) and Nafion 117 at low hydration state. Geometry optimization of the membranes in the presence of different numbers of water molecules (1–3 water molecules) revealed that proton dissociation energies and hydrogen bond interactions (which are of critical importance in proton transfer) depend strongly on the polymer structure’s functional groups. When the polymer dose not stabilize hydronium ion or sulfonate anion, water molecules are arranged around sulfonate group and form a two-ring hydrogen bonded structure; in addition, this polymer showed high proton transfer energy (25.0, 27.2 and 19.4kcal/mol for sulfonated poly(phenylene oxide), sulfonated meta-poly(benzimidazole) and Nafion 117). Theoretical studies also demonstrated that the position of sulfonate group and functional groups in the polymer structure was effective in proton dissociation. In this regard, proton transfer energy for para-sulfonated poly(benzimidazole) was about 17.7kcal/mol less than sulfonated meta-poly(benzimidazole); this energy difference could be attributed to the imidazolium cation formation and its stabilizing effect on sulfonate anion. The molecular graphs for sulfonated poly(oxadiazole) indicated that, although this polymer had the highest proton transfer energy among the polymers, it formed a continuous and strong hydrogen bond chain that facilitated proton transfer by hopping mechanism.