NMR Investigation of Transport in Polybenzimidazole/Polyphosphoric Acid Membranes Prepared Via Novel Synthesis Route

Abstract

Previous work by the some of us has demonstrated enhanced electrochemical performance of polybenzimidazole (PBI) membranes with high phosphoric acid content prepared by the so-called PPA process.1 Methods of PBI synthesis that involve casting from an organic solvent cannot incorporate the more rigid PBIs characteristic of the PPA process due to their limited solubility. Recently, a carefully controlled drying method has been discovered in which a gel PBI membrane, made in the PPA process, is transformed into a dense PBI film. This method provides a synthetic route to PBI films without the use of any organic solvents. The dense PBI films with repeat unit para-PBI (Fig.1), were originally developed for flow battery applications. However they also can be re-doped in phosphoric acid to be used as a membrane in high-temperature fuel cells.The re-doped PBI films have displayed unprecedented ionic conductivity at elevated temperatures, similar to the starting PBI gel membranes prepared by the PPA process1, while also exhibiting enhanced mechanical properties. Temperature-dependent conductivity data for two of these films are displayed below. Sample A is the dried, densified, and re-acidified version of sample B, with a much lower acid content, ~18 mol PA/PBI repeat unit compared to 30 mol PA/PBI RU, respectively, but exhibits comparable conductivity (Fig. 2). Thus, in high-temperature fuel cell operation the electrochemical performance is similar, but performance degradation due to mechanical creep is reduced, leading to greater durability of the Version A PBI.In attempting to understand the enhanced proton conductivity of the modified PBI membrane despite its lower acid content, we performed both 1H and 31P nuclear magnetic resonance (NMR) measurements. Proton self-diffusion coefficients for four membranes two (A,C) prepared by the modified process and two (B,D) by the original PPA process, were measured by pulsed gradient NMR and plotted in Fig. 3.The results show significantly enhanced proton diffusivity, both in magnitude and in lower activation energy for sample A, which is consistent with its high conductivity despite lower acid content compared to the original PPA-processed membranes (B, D). We have also performed solid state 1D 31P and 2D 31P{1H} HETCOR NMR measurements, which yield additional information on structural differences between the membranes and how these differences may affect transport properties. Multinuclear NMR Study of the Effect of Acid Concentration on Ion Transport in Phosphoric Acid Doped Polybenzimidazole Membranes, N. Suarez, N.K.A.C. Kodiweera, P. Stallworth, S.G. Greenbaun, S. Yu, and B. Benicewicz, Journal of Physical Chemistry B, 116, 12545-51(2012). Figure 1