Dynamics of benzimidazole ethylphosphonate: a solid-state NMR study of anhydrous composite proton-conducting electrolytes

Abstract

Imidazole phosphate and phosphonate solid acids model the hydrogen-bonding networks and dynamics of the anhydrous electrolyte candidate for proton exchange membrane fuel cells. Solid-state NMR reveals that phosphate and phosphonate anion dynamics dominate the rate of long-range proton transport, and that the presence of a membrane host facilitates proton mobility, as evidenced by a decreased correlation time of the composites (80 ± 15 ms) relative to the pristine salt (101 ± 5 ms). Benzimidazole ethylphosphonate (Bi-ePA) is chosen as a model salt to investigate the membrane system. The hydrogen-bonding structure of Bi-ePA is established using X-ray diffraction coupled with solid-state (1)H-(1)H DQC NMR. The anion dynamics has been determined using solid-state (31)P CODEX NMR. By comparing the dynamics of ethylphosphonate groups in pristine salt and membrane-salt composites, it is clear that the rotation process involves three-site exchange. Through data interpretation, a stretched exponential function is introduced with the stretching exponent, β, ranging 0 < β ≤ 1. The (31)P CODEX data for pristine salt are fitted with single exponential decay where β = 1; however, the data for the membrane-salt composites are fitted with stretched exponential functions, where β has a constant value of 0.5. This β value suggests a non-Gaussian distribution of the dynamic systems in the composite sample, which is introduced by the membrane host.

19F Double Quantum NMR Spectroscopy: A Tool for Probing Dynamics in Proton-Conducting Fluorinated Polymer Materials

“…14,28,34,35 For example, proton local mobility has been analyzed by monitoring the 1 H− 1 H dipolar coupling interactions using the 1 H MAS DQ recoupling technique, which has helped to establish proton conducting mechanism in solid acid materials by observing changes in H-bonding network. 34,35 This DQ NMR technique has also been applied to comparing proton conductivity in Nafion and sulfonated poly(ether ether ketone) by probing the change in H-bonding interactions. 28 In the S-PEEK materials, the investigation focused on the proton dynamics and resulting proton conduction mechanism in membrane materials.…”

“…Homonuclear recoupling ssNMR pulse sequences have been applied to understand the structure ,, and local dynamics of small molecules in the solid state. ,,, For example, proton local mobility has been analyzed by monitoring the 1 H– 1 H dipolar coupling interactions using the 1 H MAS DQ recoupling technique, which has helped to establish proton conducting mechanism in solid acid materials by observing changes in H-bonding network. , This DQ NMR technique has also been applied to comparing proton conductivity in Nafion and sulfonated poly­(ether ether ketone) by probing the change in H-bonding interactions . In the S-PEEK materials, the investigation focused on the proton dynamics and resulting proton conduction mechanism in membrane materials.…”

Advances in nuclear magnetic resonance spectroscopy: case of proton conductive materials

“…11,55 The slow (from 0.1 s −1 to 3000 s −1 ) molecular motions in solid-state electrolytes as well as their structural characterization were investigated by using the center band-only detection of exchange (CODEX) NMR technique. 56 Briey, CODEX is a stimulated-echo experiment under MAS conditions with chemical-shi-anisotropy recoupling and intensity referencing as for rotational-echo, double-resonance (REDOR) NMR. 57 The last one is a high-resolution ssNMR experiment for measuring the dipolar coupling between heteronuclear spin pairs.…”

1H–1H Double Quantum NMR Investigation of Proton Dynamics in Solid Acids

“…Promising alternatives to Nafion include inorganic and organic solid acids due to their temperature and electrochemical stability. − Inorganic solid acids were first introduced by Haile et al using CsHSO 4 , which was shown to provide high proton conductivity for up to 80 h at 250 °C . Solid-acid systems utilize the Grotthuss mechanism for proton conduction instead of the vehicle mechanism, which may enable anhydrous high temperature operating conditions.…”