title: Multinuclear NMR study of the effect of acid concentration on ion transport in phosphoric acid doped poly(benzimidazole) membranes.
abstract: (1)H and (31)P NMR spectra, line widths, spin-lattice relaxation times (T(1)), and (1)H self-diffusion coefficients (D) were determined for two distinct poly(benzimidazole) (PBI) proton exchange membranes (PEM), para-PBI and dihydroxy-PBI (2OH-PBI), both incorporating varying concentrations of phosphoric acid. The study was performed over the temperature range of 20-180 °C, for phosphoric acid concentrations of 30, 50, and 70 wt %. Of the two samples, less mobility was indicated for the 2OH-PBI compared with the para-PBI at all acid concentrations. It was also observed that increasing the acid content resulted in an increase in the temperature at which the T(1) minimum or plateau occurred. (31)P spectra reveal the presence of pyrophosphates and in the case of the 50 and 70 wt % para-PBI samples higher oligomers such as tripolyphosphates. (1)H D data showed the 30 wt % para-PBI having almost identical values as the 70 wt % 2OH-PBI over the entire temperature range. In general, stronger short- and long-range interactions were observed in the 2OH-PBI matrix, yielding reduced translational proton transport compared to that of para-PBI. While these stronger interactions hinder translational proton diffusion, they could enhance proton transport by the Grotthuss or structure diffusion mechanism, the more favorable transport mechanism. Activation energies obtained from the (1)H D data supports a proton-hopping mechanism, with possible assistance from fast exchange between phosphate groups.
authors.affiliation: Department of Physics, Brooklyn College/CUNY, Brooklyn, New York 11210, United States. snsuarez@brooklyn.cuny.edu
authors.forename: Sophia
authors.initials: S
authors.lastname: Suarez
authors.forename: N K A C
authors.initials: NK
authors.lastname: Kodiweera
authors.forename: P
authors.initials: P
authors.lastname: Stallworth
authors.forename: Seonghan
authors.initials: S
authors.lastname: Yu
authors.forename: S G
authors.initials: SG
authors.lastname: Greenbaum
authors.forename: B C
authors.initials: BC
authors.lastname: Benicewicz
mesh_terms: D001562:Benzimidazoles / Q000737:chemistry*; D017136:Ion Transport; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D010756:Phosphoric Acids / Q000737:chemistry*; D011108:Polymers / Q000737:chemistry*
chemical_list: D001562:Benzimidazoles; D010756:Phosphoric Acids; D011108:Polymers; C549461:poly(benzimidazole); C030242:phosphoric acid