A Solid-State NMR Study of Hydrogen-Bonding Networks and Ion Dynamics in Benzimidazole Salts

Abstract

On the basis of our solid-state NMR characterization of dynamics in two model salts, we draw the analogy to the fuel cell membrane candidate, phosphoric acid-doped poly(benzimidazole), and conclude that phosphate anion dynamics contribute to long-range proton transport, whereas the mobility of the polymer itself is not a contributing factor. This is contrasted with emerging membrane candidates, which rely on fully covalently bonded acid donors and acceptors, and target high-temperature PEM fuel cell operation in the absence of liquid electrolyte. The hydrogen-bonding structures of benzimidazolium phosphate and benzimidazolium methane phosphonate are established using X-ray diffraction paired with solid-state 1H DQF NMR. By comparing the dynamics of the phosphate and methane phosphonate anions with the dynamics of imidazolium and benzimidazolium cations, the relative importance of these processes in proton transport is determined. The imidazolium cation is known to undergo two-site ring reorientation on the millisecond time scale. In contrast, it is shown here that the benzimidazolium rings are immobile in analogous salts, on a time scale extending into the tens of seconds. Therefore, we look to the phosphate anions and demonstrate that the time scale of tetrahedral reorientation is comparably fast (50 ms). Moreover, the 31P CODEX NMR data clearly indicate a four-site jump process. In contrast, the methane phosphonate undergoes a three-site jump on a slower time scale (75 ms). A mechanism for a zigzag pathway of proton transport through the phosphonate salt crystallites is developed based on the 31P CODEX and 1H variable-temperature MAS NMR data.

Investigation of proton dynamics and the proton transport pathway in choline dihydrogen phosphate using solid-state NMR

“…Proton double-quantum filtered ( 1 H DQF) experiments have been used in the study of a large number of systems for determination of the packing arrangement of the hydrogen-bonding motifs. [6][7][8][9][10][11] These experiments access the information inherent to through-space dipolar coupling and the relative strength of the measured dipolar coupling can be used as a probe of molecular mobility.…”

Solid-state NMR studies of hydrogen bonding networks and proton transport pathways based on anion and cation dynamics

“…Therefore, the choice of such model salts allows the systematic study of the rates, geometries, and relative efficiencies of cation and anion dynamics in promoting proton transport in the absence of vehicular transport mechanisms. 13 A compound which bridges the gap between model salts and true polymer materials is 1,10-(1-H-imidazol-5-yl)decanephosphonic acid (Imi-d-Pa).…”

“…30 The primary mechanism of ionic conduction in this family of materials is hypothesized to be the rotation of the anion and not the ring flip of the cation. This analysis is based on previous 13 C CODEX NMR comparison of imidazolium methyl sulfonate to benzimidazolium methyl phosphonate, which yielded correlation times of 29 ms for imidazolium at 273 K and 4.2 s for benzimidazolium at 298 K, and thereby ruled out the two-site ring flip for benzimidazolium cations. 13 The decreasing melting points in the series of compounds is consistent with the dynamics of the material becoming more efficient at lower temperatures for materials with a longer alkyl chain length.…”

“…This analysis is based on previous 13 C CODEX NMR comparison of imidazolium methyl sulfonate to benzimidazolium methyl phosphonate, which yielded correlation times of 29 ms for imidazolium at 273 K and 4.2 s for benzimidazolium at 298 K, and thereby ruled out the two-site ring flip for benzimidazolium cations. 13 The decreasing melting points in the series of compounds is consistent with the dynamics of the material becoming more efficient at lower temperatures for materials with a longer alkyl chain length. To investigate the compound behaviour of the compounds with temperature and the resulting coalescence of any NMR resonances, variable temperature NMR spectra of the materials were recorded over the temperature range of our MAS probe.…”

“…27 The pulse program CODEX provides site specific determination of the geometry of the reorientation, and gives a measure of the timescale of the motion which can then be compared between the different materials. 13,22,23 The CODEX pulse sequence refocuses the chemical shift anisotropy (CSA) to obtain this site specific information. Cross polarization is typically used to saturate the nuclei of interest and 180°C…”