Enhancing Proton Transport and Membrane Lifetimes in Perfluorosulfonic Acid Proton Exchange Membranes: A Combined Computational and Experimental Evaluation of the Structure and Morphology Changes Due to H3PW12O40 Doping
SV Sambasivarao and Y Liu and JL Horan and S Seifert and AM Herring and CM Maupin, JOURNAL OF PHYSICAL CHEMISTRY C, 118, 20193-20202 (2014).
The impact of loading the heteropoly acid, 12-phosphotungstic acid (HPW), on a perfluorosulfonic acid (PFSA) proton exchange membrane's morphology was evaluated by means of molecular dynamics (MD) simulations and small-angle X-ray scattering (SAXS) experiments. It is found that the addition of HPW significantly modifies the solvent structure and dynamics in the PFSA membrane, which favors the formation of interconnected proton conducting networks. It is hypothesized that these HPW induced solvent modifications account for the enhanced proton conducting characteristics of these doped membranes. Radial distribution functions and water cluster analysis indicate that the HPW organizes the local solvent water and attracts the nearby excess protons thereby creating localized "nodes" of ordered water and hydronium ions. The "nodes" are found to connect surrounding water wires/channels resulting in a more efficient proton conducting network. This redistribution of solvent and hydronium ions upon addition of HPW creates a shift in the hydrophilic cluster size distribution and the overall membrane morphology. Hydrophilic cluster size analysis indicates that a high percentage of small clusters (d < 15 angstrom) exist in low HPW doped systems (i.e., 1%), while larger clusters (d > 15 angstrom) exist for the high HPW doped systems (i.e., 596). At low hydration levels, the water domains are found to be spheroidal inverted micelles embedded in an ionomer matrix, while at high hydration levels the solvent morphology shifts to a parallel spheroidal elongated cylinder. It is also observed that for the high HPW doping levels the SAXS pattern changes intensity at the low q region and Bragg peaks become present, which indicates the presence of crystalline HPW. These morphological changes create a more interconnected pathway through which the hydrated excess protons may transverse thereby enhancing the PFSA membrane's conductivity
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