Molecular Modeling of the Short-Side-Chain Perfluorosulfonic Acid Membrane Stephen J. Paddison* Department of Chemistry and Materials Science, UniVersity of Alabama in HuntsVille, HuntsVille, Alabama 35899 James A. Elliott* Department of Materials Science and Metallurgy, UniVersity of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, UK ReceiVed: May 11, 2005; In Final Form: June 30, 2005 Presented here is a first principles based molecular modeling investigation of the possible role of the side chain in effecting proton transfer in the short-side-chain perfluorosulfonic acid fuel cell membrane under minimal hydration conditions. Extensive searches for the global minimum energy structures of fragments of the polymer having two pendant side chains of distinct separation (with chemical formula: CF 3 CF(O(CF 2 ) 2 - SO 3 H)(CF 2 ) n CF(O(CF 2 ) 2 SO 3 H)CF 3 , where n ) 5, 7, and 9) with and without explicit water molecules have shown that the side chain separation influences both the extent and nature of the hydrogen bonding between the terminal sulfonic acid groups and the number of water molecules required to transfer the proton to the water molecules of the first hydration shell. Specifically, we have found that fully optimized structures at the B3LYP/6-311G** level revealed that the number of water molecules needed to connect the sulfonic acid groups scaled as a function of the number of fluoromethylene groups in the backbone, with one, two, and three water molecules required to connect the sulfonic acid groups in fragments with n ) 5, 7, and 9, respectively. With the addition of explicit water molecules to each of the polymeric fragments, we found that the minimum number of water molecules required to effect proton transfer also increases as the number of separating tetrafluoroethylene units in the backbone is increased. Furthermore, calculation of water binding energies on CP-corrected potential energy surfaces showed that the water molecules bound more strongly after proton dissociation had occurred from the terminal sulfonic acid groups independent of the degree of separation of the side chains. Our calculations provide a baseline for molecular results that can be used to assess the impact of changes of polymer chemistry on proton conduction, including the side chain length and acidic functional group. Introduction Polymeric materials that function as the critical electrolyte and electrode separator in proton exchange membrane (PEM) fuel cells exhibit a nano-phase-separated morphology when hydrated. However, only at high degrees of hydration are the currently available state-of-the-art PEMs able to conduct protons through the membrane at sufficiently high rates for successful operation of direct hydrogen fuel cells. 1 These power sources are deemed to possess the potential to lead to considerable energy savings, energy security, and air quality improvements through a wide range of modular power applications subject to the advent of improved materials (membranes, catalysts, etc). 2 Hence, the development of PEMs which operate at high temperature (i.e. > 100 °C) and low humidity conditions (thus without requiring pressurization of the system) and exclusively transport protons is widely regarded as an important research and development requirement for PEM fuel cell technology. The design and synthesis of new membranes possessing improved performance characteristics (along with decreased manufacturing costs) will require a fundamental, molecular- based understanding of the mechanisms of proton and water transport as a function of membrane hydration, morphology, and polymer chemistry. 3 This information cannot come from experimental investigations alone, but will require knowledge of how membrane morphology and chemical composition affect the transport of both protons and water in the PEM through multiscale modeling that bridges many distinct time and length scales, connecting the equilibrium conformational structure of a membrane and its general composition to molecular processes including proton dissociation, transfer, and diffusion, and hydrogen bonding, distribution, and diffusion of water. This work seeks to contribute to this goal through a first principles based investigation into the ingredients of proton conduction in the short-side-chain perfluorosulfonic acid (PFSA) membrane at minimal hydration. Interest in the mechanisms of proton conduction is, of course, not restricted to polymeric ionomers, as proton transfer and transport feature importantly in the function of many different chemical and biological systems. 4,5 Proton transport continues to be extensively studied both experimentally and theoretically in a variety of materials and diverse media 6 including (most importantly) water, 7-11 mixed aqueous solutions (i.e. aqueous CH 3 OH 12 ), acids, 13 solids 14,15 (e.g. oxides, 16 phosphates, 17 sulfates 18 ) transmembrane proteins 19-21 (i.e. proton channels 22 and pumps 23 ), carbon nanotubes, 24 and PEMs. 25-32 Clearly, an * Correspondence may be addressed to either author. E-mail: paddison@ matsci.uah.edu or jae1001@cam.ac.uk. 7583 J. Phys. Chem. A 2005, 109, 7583-7593 10.1021/jp0524734 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/29/2005