Solid-state NMR investigation of phosphonic acid based proton conducting materials

In this work, new promising proton conducting fuel cell membrane materials were characterized in terms of their structure and dynamic properties using solid-state nuclear magnetic resonance (NMR) spectroscopy and X-ray diffraction. Structurally different, phosphonic acid (PA) containing materials were systematically evaluated for possible high-temperature operation (e.g. at T>100°C). Notably, 1H, 2H and 31P magic angle spinning (MAS) NMR provided insight into local connectivities and dynamics of the hydrogen bonded network, while packing arrangements were identified by means of heteronuclear dipolar recoupling techniques.rnThe first part of this work introduced rather crystalline, low molecular weight ionomers for proton conducting membranes, where six different geometries such as line, triangle, screw, tetrahedron, square and hexagon, were investigated. The hexagon was identified as the most promising geometry with high-temperature bulk proton conductivities in the range of 10-3 Scm-1 at a relative humidity of 50%. However, 2H NMR and TGA-MS data suggest that the bulk proton transport is mainly due to the presence of crystal water. Single crystal X-ray data revealed that in the tetrahedron phosphonic acids form tetrameric clusters isolating the mobile protons while the phosphonic acids in the hexagon form zigzag-type pathways through the sample.rnThe second part of this work demonstrates how acid-base pairing and the choice of appropriate spacers may influence proton conduction. Different ratios of statistical copolymers of poly (vinylphosphonic acid) and poly (4-vinylpyridine) were measured to derive information about the local structure and chemical changes. Though anhydrous proton conductivities of all statistical copolymers are rather poor, the conductivity increases to 10-2 S cm-1 when exposing the sample to relative humidity of 80%. In contrast to PVPA, anhydride formation of phosphonic acids in the copolymer is not reversible even when exposing the sample to a relative humidity of 100%.rnIn addition, the influence of both spacers and degree of backbone crystallinity on bulk proton conductivity was investigated. Unlike in systems such as poly benzimidazole (PBI), spacers were inserted between the protogenic groups along the backbone. It was found that dilution of the protogenic groups decreases the conductivity, but compared to PVPA, similar apparent activation energies for local motions were obtained from both variable temperature 1H NMR and impedance spectroscopy data. These observations suggest the formation of phosphonic acid clusters with high degrees of local proton motion, where only a fraction of motions contribute to the observable bulk proton conductivity. Additionally, it was shown that gradual changes of the spacer length lead to different morphologies.rnIn summary, applying advanced solid-state NMR and X-ray analysis, structural and dynamic phenomena in proton conducting materials were identified on a molecular level. The results were discussed with respect to different proton conduction mechanisms and may contribute to a more rational design or improvement of proton conducting membranes.rn