Three series of polybenzimidazole (PBI) copolymers (3,5‐pyridine‐r‐2OH‐PBI, 3,5‐pyridine‐r‐para‐PBI, and 3,5‐pyridine‐r‐meta‐PBI) were polymerized and cast into membranes by the polyphosphoric acid (PPA) process. Monomer pairs with high and low solubility characteristics were used to define phase stability‐processing windows for preparing membranes with high temperature membrane gel stability. Creep compliance of these membranes (measured in compression at 180 °C) generally decreased with increasing polymer content. Membrane proton conductivities decreased linearly with increasing membrane polymer content. Fuel cell performances of some high‐solids 3,5‐pyridine‐based copolymer membranes (up to 0.66 V at 0.2 A cm–2 following break‐in) were comparable to para‐PBI (0.68 V at 0.2 A cm–2) despite lower phosphoric acid (PA) loadings in the high solids membranes. Long‐term steady‐state fuel cell studies showed 3,5‐pyridine‐r‐para‐PBI copolymers maintained a consistent fuel cell voltage of >0.6 V at 0.2 A cm–2 for over 2,300 h. Phosphoric acid that was continuously collected from the long‐term study demonstrated that acid loss is not a significant mode of degradation for these membranes. The PBI copolymer membranes' reduced high‐temperature creep and long‐term operational stability suggests that they are excellent candidates for use in extended lifetime electrochemical applications.

Polybenzimidazoles (PBI) are an important class of heterocyclic polymers that exhibit high thermal and oxidative stabilities. The two dominant polymerization methods used for the synthesis of PBI are the melt/solid polymerization route and solution polymerization using polyphosphoric acid as the solvent. Both methods have been widely used to produce high-molecular weight PBI, but also highlight the obvious absence of a practical organic solution-based method of polymerization. This current work explores the synthesis of highmolecular weight meta-PBI in N,N-dimethyl acetamide (DMAc). Initially, model compound studies examined the reactivity of small molecules with various chemical functionalities that could be used to produce 2-phenyl-benzimidazole in high yield with minimal side reactions. 1 H NMR and FTIR studies indicated that benzimidazoles could be efficiently synthesized in DMAc by reaction of an o-diamine and the bisulfite adduct of an aromatic aldehyde. Polymerizations were conducted at various polymer concentrations (2-26 wt % polymer) using difunctional monomers to optimize reaction conditions in DMAc which resulted in the preparation of high-molecular weight m-PBI (inherent viscosities up to 1.3 dL g 21 ). TGA and DSC confirmed that m-PBI produced via this route has comparable properties to that of commercial m-PBI. This method is advantageous in that it not only allows for high-polymer concentrations of m-PBI to be synthesized directly and efficiently, but can be applied to the synthesis of many PBI derivatives.

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Phosphoric acid (PA) doped polybenzimidazole membranes made by the PPA process have been the main focus of our research efforts for more than a decade.1 These membranes exhibit an excellent combination of properties including high proton conductivities, excellent fuel cell performance, and long-term durability. For both fuel cell and hydrogen pump devices, the improvement in membrane mechanical properties at high temperatures could lead to further increases in durability and lifetimes of the membranes and resulting devices. We have been particularly interested in membrane compressive creep properties and have developed test protocols which are being used to understand the compressive creep properties and develop structure-property relationships for PPA processed membranes. In this work, we have investigated PBI copolymer membranes that showed higher mechanical properties (in particular, creep resistance) than homo-para-PBI membranes, and still possessed good proton conductivities at 180˚C under anhydrous conditions. Membrane electrode assemblies were prepared and tested in single cells to demonstrate long-term durability at different current densities and low phosphoric acid leaching rates. We will also discuss recent investigations in terms of hydrogen pumping applications, which may have broad applications in both emerging energy applications and current industrial gas markets. 1. L. Xiao; H. Zhang; E. Scanlon; L.S.Ramanathan; E.-W. Choe; D. Rogers; T. Apple; B.C. Benicewicz. Chem. Materials, 2005, 17, 5328-5333.