Perfluoroalkyl Phosphonic and Phosphinic Acids as Proton Conductors for Anhydrous Proton‐Exchange Membranes

Abstract

A study of proton-transport rates and mechanisms under anhydrous conditions using a series of acid model compounds, analogous to comb-branch perfluorinated ionomers functionalized with phosphonic, phosphinic, sulfonic, and carboxylic acid protogenic groups, is reported. Model compounds are characterized with respect to proton conductivity, viscosity, proton, and anion (conjugate base) self-diffusion coefficients, and Hammett acidity. The highest conductivities, and also the highest viscosities, are observed for the phosphonic and phosphinic acid model compounds. Arrhenius analysis of conductivity and viscosity for these two acids reveals much lower activation energies for ion transport than for viscous flow. Additionally, the proton self-diffusion coefficients are much higher than the conjugate-base self-diffusion coefficients for these two acids. Taken together, these data suggest that anhydrous proton transport in the phosphonic and phosphinic acid model compounds occurs primarily by a structure-diffusion, hopping-based mechanism rather than a vehicle mechanism. Further analysis of ionic conductivity and ion self-diffusion rates by using the Nernst-Einstein equation reveals that the phosphonic and phosphinic acid model compounds are relatively highly dissociated even under anhydrous conditions. In contrast, sulfonic and carboxylic acid-based systems exhibit relatively low degrees of dissociation under anhydrous conditions. These findings suggest that fluoroalkyl phosphonic and phosphinic acids are good candidates for further development as anhydrous, high-temperature proton conductors.

A new fluorinated anion for room-temperature ionic liquids

“…Data were collected using a Solatron model SI 1286 electrochemical interface coupled to a Solatron model 1254 frequency response analyzer. The conductivity experiments were carried out inside of a stainless steel apparatus using a conductivity cell constructed from PEEK (polyether-ether-ketone) material and under a nitrogen atmosphere as previously described in reference [5]. The conductivity was calculated by dividing the cell constant value by the impedance value extracted from the high-frequency intercept on the real axis of a Nyquist plot.…”

“…Imidazolium bromide [4] and bis(trifluoromethyl)phosphonic acid [5] were prepared by procedures described in literature. [BMIM][TFSI] ionic liquid was purchased from Sigma-Aldrich.…”

Influence of the Water Content on the Diffusion Coefficients of Li+ and Water across Naphthalenic Based Copolyimide Cation-Exchange Membranes

“…Haven ratios much higher than unit have also been reported for proton transport in imidazole based anhydrous membranes 45 and anhydrous ionomers. 46,47 CONCLUSIONS WAXS diffractograms of the hydrated membranes described in this work exhibit at low angles an ionomer peak located at q = 2.66 nm…”

Evaluation of carboxylic, phosphonic, and sulfonic acid protogenic moieties on tunable poly(meta‐phenylene oxide) ionomer scaffolds

“…Herein, we report the synthesis and analysis of proton conductivity of poly( m PO)s containing sulfonic, phosphonic, or carboxylic acid groups on otherwise identical polymer backbones. While some highly conductive sulfonic, carboxylic, and phosphonic acid‐functionalized polymers, MOFs, COFs, and hydrocarbon liquids have been synthesized, the lack of a versatile polymer platform has limited the direct comparison of these protogenic moieties toward rational PEM design. Herein, we explore how the identity of the acid moiety affects proton conduction.…”