Design of polybenzimidazolium membranes for use in vanadium redox flow batteries.

In recent years, polybenzimidazole (PBI) membranes have been proposed for vanadium redox flow batteries (VRFBs) as an alternative to perfluoroalkylsulfonic acid membranes such as Nafion™. Despite their excellent capacity retention, PBI membranes tend to suffer from a low ionic conductivity. The formation of a polybenzimidazolium through an N-alkylation of the benzimidazole core is shown to improve the ionic conductivity of the membrane, with this class of materials having found uses in alkaline fuel cell and water electrolysis systems. However, much less is known about their incorporation into a VRFB. This article describes the use of hexamethyl-p-terphenyl polybenzimidazolium (HMT-PMBI) membranes for a vanadium redox flow battery, with the membrane characteristics in acidic media being related to their performance in a single-cell VRFB setup. A change of the degree of methylation from 56 to 65, 75, and 89% leads to an increase in ionic conductivity, correlated with an increased fraction of free water in the ionomer. The corresponding increase in cell performance is, however, accompanied by a drop in capacity retention. The membrane with a degree of methylation of 65% shows balanced properties, with a 5% higher efficiency and a two times improved capacity retention compared to Nafion™ NR212 over 200 charge-discharge cycles at 200 mA cm-2.

Highly ordered polymer films of amphiphilic, regioregular polythiophene derivatives.

The fabrication and characterization of highly ordered thin films made from amphiphilic, regioregular polythiophene derivatives are described. Films of poly(3-(11-(2-tetrahydropyranyloxy)undecyl)thiophene (PTHPUDT) were prepared by the Langmuir-Blodgett technique. The amphiphilic nature of the polymer affords layer-by-layer deposition and the formation of multilayer films of head-to-head and tail-to-tail Y-type structure. X-ray diffraction studies indicate bilayer separations of approximately 30 A. Anisotropic optical absorption in the plane of the film indicates that the thiophene backbones are preferentially oriented along the dipping direction. Further, polarized light microscopy studies indicate that these films are highly birefringent and that the optical retardation is uniform over the entire film. Ellipsometry studies confirm the sizable magnitude of the birefringence. Optical second-harmonic generation studies of multilayer films provide information regarding both the thiophene orientation within the film and the anisotropic distribution of chromophores in the surface plane. Taken together, these data offer strong evidence of highly ordered films in which the hydrophobic polythiophene chains lie parallel to the substrate surface with their alkyl chains oriented normal to the surface, as dictated by the hydrophilic nature of the alkyl chain's terminal tetrahydropyran functional group. As such, these films offer the potential for elucidating the connection between polymer morphology and physical property in materials that are otherwise subject to a sufficiently complex distribution of morphologies that such a correspondence is precluded.