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

Abstract

Proton dynamics in polymer electrolyte membranes are multifaceted processes, and the relative contributions of various mechanisms can be difficult to distinguish. Judicious choices of model systems can aid in understanding the critical steps. In this study, we characterize anion dynamics in a series of benzimidazole-alkyl phosphonate salts, and compare those dynamics to a membrane prototype, built on a decane backbone. The series of salts are characterized, using high resolution (1)H solid-state magic angle spinning (MAS) NMR, DQ MAS NMR, and (31)P centreband-only detection of exchange (CODEX) NMR spectroscopy, to determine the influence of the nature of the alkyl group on the rates and geometries of anion dynamics, and overall proton exchange processes. The alkyl group is shown to slow the correlation times for anion reorientation, when compared at ambient temperature. However, it is also apparent that the lowered lattice energy of the salt lowers the activation energy and allows good dynamics at intermediate temperatures in both the benzimidazolium ethylphosphonate and in the HBr adduct of 1,10-(1-H-imidazol-5-yl)decanephosphonic acid (Imi-d-Pa).

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

“…[1][2][3][4][5][6] The candidates for PEMFCs rely on proton transport, which is facilitated by the hydrogen bonding in the electrolyte materials. [7][8][9][10] The polymer membranes that are commonly used as electrolytes, such as Nafion s and sulfonated poly(ether ether ketone) (sPEEK), operate optimally only under hydrous conditions since water acts as a carrier for proton transport by forming continuous water channels which promote the Grotthuss 11 mechanism transport. [1][2][3][4][5][6][7][8][9]12 This limits the operation temperature range of this type of device.…”

“…[7][8][9][10] The polymer membranes that are commonly used as electrolytes, such as Nafion s and sulfonated poly(ether ether ketone) (sPEEK), operate optimally only under hydrous conditions since water acts as a carrier for proton transport by forming continuous water channels which promote the Grotthuss 11 mechanism transport. [1][2][3][4][5][6][7][8][9]12 This limits the operation temperature range of this type of device. [1][2][3][4][5][6][7][8][9]11 Another drawback of the PEMFC system is the requirement of stringent water control.…”

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.…”

“…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. Here we establish the assignment of the proton resonances and the hydrogen-bonding structure of choline DHP by correlating the X-ray diffraction (XRD) structure to the 1 H NMR spectrum using heteronuclear and DQF homonuclear correlation techniques.…”

“…Variabletemperature 1 H NMR studies are used to characterise the local proton mobility by observing the narrowing and/or coalescence of the resonances. [10][11][12][13] The centreband-only detection of exchange (CODEX) pulse sequence measures the reorientation of a molecule by studying the changes in chemical shift anisotropy (CSA) with time. 14 It enables determination of the number of sites involved in the reorientation and the timescale of the motion.…”