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

Abstract

Choline dihydrogen phosphate has previously been shown to be a good ionic conductor as well as an excellent host for acid doping, leading to high proton conductivities required for e.g., electrochemical devices including proton membrane fuel cells and sensors. A combination of variable-temperature 1 H solid-state NMR and 2D NMR pulse sequences, including 31 P and 13 C CODEX and 1 H BaBa, show that the proton conduction mechanism primarily involves assisted transport via a restricted three-site motion of the phosphate unit around the P-O bond that is hydrogen bonded to the choline and exchange of protons between these anions. In other words, proton transport at ambient temperatures appears to occur most favorably along the crystallographic b axis, from phosphate dimer to dimer. At elevated temperatures exchange between the protons of the hydroxyl group on the choline cation and the hydrogen-bonded dihydrogen phosphate groups also contributes to the structural diffusion of the protons in this solid state conductor.

Protic plastic crystal/PVDF composite membranes for Proton Exchange Membrane Fuel Cells under non-humidified conditions

“…well studied, initially prompted by the excellent proton conductivities of phosphoric-acid-based materials. The proton transport of this material is proposed to be facilitated by a three-fold rotation of the dihydrogenphosphate anion [15,16]. High proton diffusivities can be obtained after doping with acid [17].…”

Fluorescence and Circular Dichroism Spectroscopy of Cytochrome c in Alkylammonium Formate Ionic Liquids

“…Strongly kosmotropic anions such as dihydrogen phosphate (dhp) and a chaotropic cation such as choline are expected to stabilize proteins (20). However, choline dhp is actually a white solid and not a room temperature ionic liquid (22). Patterns of protein unfolding and protein aggregation have been compared for ribonuclease A in choline dihydrogen phosphate and two imidazolium ionic liquid solutions from 0 to 4 M. The choline dihydrogen phosphate enhanced thermostability while the 1-ethyl-3-methylimidazolium dicyanide was a strong denaturant (23).…”

Utilizing Water Activity as a Simple Measure to Understand Hydrophobicity in Ionic Liquids

“…ChoDHP is the most hydrophilic IL in this study, although it is formed by hydrophilic cation and hydrophilic anion based on their Log P (Table 2). Previous studies have shown that ChoDHP has an extended network of hydrogen bonding particularly though the DHP anions containing two protons (Fujita et al, 2009; Cahill et al, 2010). The proton of DHP anion transports via the reorganization of hydrogen bonds (Rana et al, 2010), and it is possible that this infinite and complicated network of hydrogen bonds hinder the interaction with water, compared with other ILs in this study.…”