On the consequences of side chain flexibility and backbone conformation on hydration and proton dissociation in perfluorosulfonic acid membranes Stephen J. Paddison* a and James A. Elliott b Received 14th February 2006, Accepted 16th March 2006 First published as an Advance Article on the web 28th March 2006 DOI: 10.1039/b602188c The flexibility of the side chain and effects of conformational changes in the backbone on hydration and proton transfer in the short-side-chain (SSC) perfluorosulfonic acid fuel cell membrane have been investigated through first principles based molecular modelling studies. Potential energy profiles determined at the B3LYP/6-31G(d,p) level in the two pendant side chain fragments: CF 3 CF(–O(CF 2 ) 2 SO 3 H)–(CF 2 ) 7 –CF(–O(CF 2 ) 2 SO 3 H)CF 3 indicate that the largest CF 2 –CF 2 rotational barrier along the backbone is nearly 28.9 kJ mol 1 higher than the minimum energy staggered trans conformation. Furthermore, the calculations reveal that the stiffest portion of the side chain is near to its attachment site on the backbone, with CF–O and O–CF 2 barriers of 38.1 and 28.0 kJ mol 1 , respectively. The most flexible portion of the side chain is the carbon–sulfur bond, with a barrier of only 8.8 kJ mol 1 . Extensive searches for minimum energy structures (at the B3LYP/6-311G(d,p) level) of the same polymeric fragment with 4–7 explicit water molecules reveal that the perfluorocarbon backbone may adopt either an elongated geometry, with all carbons in a trans configuration, or a folded conformation as a result of the hydrogen bonding of the terminal sulfonic acids with the water. These electronic structure calculations show that the fragments displaying the latter ‘kinked’ backbone possessed stronger binding of the water to the sulfonic acid groups, and also undergo proton dissociation with fewer water molecules. The calculations point to the importance of the flexibility in both the backbone and side chains of PFSA membranes to effectively transport protons under low humidity conditions. Introduction The transfer and transport of protons feature importantly in the function and energy transformation in a number of different chemical and biological systems. 1,2 Hence, the me- chanisms of proton conduction are extensively studied both experimentally and theoretically in a variety of materials and diverse media 3 including: (most importantly) water, 4–8 mixed aqueous solutions (e.g. aqueous CH 3 OH 9 ), crystals, 10 solids 11,12 (e.g. oxides, 13 phosphates, 14 sulfates 15 ), trans-mem- brane proteins 16–18 (e.g. proton channels 19 and pumps 20 ), carbon nanotubes, 21 and polymer electrolyte membranes (PEMs). 22–29 Despite differences in the molecular structure of these systems there are common features including: the formation of a continuous network of dynamical hydrogen bonds and the mobility of the excess protonic charge with the centre of symmetry of the hydrogen bond coordination. The present work seeks to elucidate molecular features of proton transfer in minimally hydrated perfluorosulfonic acid (PFSA) polymers for the purposes of providing some direction to- wards the development of improved and highly conductive PEMs for fuel cells. 30,31 PFSA membranes remain the most commonly employed electrolyte and electrode separator in PEM fuel cells due to a considerable window of chemical stability and mechanical strength. Nafion s (DuPont) is presently considered the state-of-the-art membrane in PEM fuel cells. Although now widespread, its use has some serious limitations including a restrictive range of thermal stability, high manufacturing cost and, most importantly, the need for a significant level of hydration in order to achieve sufficient proton conductivity. 32 The properties and function of a considerable number of PEMs have recently been reviewed by several authors. 33–39 The water in these biphasic systems is dispersed in a princi- pally amorphous fluorocarbon polymeric primary phase. 40,41 The acidic groups of the polymer are solvated facilitating mobility of the protons via structural diffusion 42,43 through the hydrogen-bonded network of water molecules and con- jugate bases (i.e. sulfonates) and vehicular diffusion where there is coupling of protons and water as hydronium ions. 44 The presence of water is critical for the formation of hydrated protons (i.e. as Zundel, H 5 O 2 1 , or Eigen, H 9 O 4 1 , cations) and their mobility. The hydration requirement of conventional PEMs results in a problematically low operating temperature limited by the boiling point of water (i.e. T r 100 1C at 1 atm). Since the PEM fuel cell is deemed to possess the potential to lead to considerable energy savings and security within a ‘hydrogen economy’, along with improvements in air quality, a substantial effort is being mounted to design and synthesize a Department of Chemistry and Materials Science, University of Alabama in Huntsville, Huntsville, AL 35899, USA. E-mail: paddison@matsci.uah.edu b Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, UK CB2 3QZ. E-mail: jae1001@cam.ac.uk This journal is  c the Owner Societies 2006 Phys. Chem. Chem. Phys., 2006, 8, 2193–2203 | 2193 PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics