(Invited, Digital Presentation) Polybenzimidazole and Its Use in Energy Storage and Conversion

Abstract

Polybenzimidazole (PBI) is a unique polymer. It has high mechanical strength and stability, and is not conductive. However, in contact with acids, the imidazole rings are protonated, and PBI becomes proton conductive. In contact with alkaline solutions, the amine groups are deprotonated, and PBI becomes hydroxide conductive. Control of the doping process allows to adjust the uptake and hence allows to control conductivity and permeability of PBI membranes. Therefore, PBI can be used in various ways for several applications. Phosphoric acid doped PBI blends were tested in the HT PEMFC, and showed more than >2000 hours stable, high performance operation at very challenging high current density of 800 mA/cm2 (one of world-best results).[1] Sulfuric acid doped PBI membranes were developed for use in the vanadium redox flow battery. We show that stable performances with an energy efficiency >91% and coulomb efficiency >99% @ 80 mA/cm2 can be achieved, which is one of the highest reported performances.[2, 3] By using a newly developed membrane fabrication process, hydroxide conducting membranes with a conductivity of > 310 mS/cm2 were be obtained. Porous supports enhanced the mechanical stability of these membranes, and PEM-water electrolyser-like performance was achieved in an alkaline water electrolyser, which was successfully operated for 1000 hours without failure.[4] A common way to increase the mechanical stability of membranes is reinforcement with a porous support. The drawback is that different swelling ratios of matrix and support can form voids, which results in unacceptably high gas crossover in electrolyzers and fuel cells. To tackle this issue, we pore filled an electrospun PBI fiber mat with a bomoalkylated polymer and reacted it with the imidazole groups available on the PBI fiber surface, thus forming a covalently bonded interface. In a final step, the remaining bromoalkyl groups were quaternized. The membrane was tested for 200 hours in an electrolyzer, and showed no signs of degradation. [5] [1] N. Nambi Krishnan, N.M.H. Duong, A. Konovalova, J.H. Jang, H.S. Park, H.J. Kim, A. Roznowska, A. Michalak, D. Henkensmeier, Polybenzimidazole / tetrazole-modified poly(arylene ether) blend membranes for high temperature proton exchange membrane fuel cells, J. Membr. Sci., 2020, 614, 118494. https://doi.org/10.1016/j.memsci.2020.118494[2] C. Noh, D. Serhiichuk, M. Najibah, Y. Kwon, D. Henkensmeier, Optimizing the performance of meta-polybenzimidazole membranes in vanadium redox flow batteries by adding an alkaline pre-swelling step, Chem. Eng, J., 405 (2021) 126574. https://doi.org/10.1016/j.cej.2020.126574[3] unpublished results[4] under review[5] Malikah Najibah, E. Tsoy, H. Khalid, Y. Chen, Q. Li, C. Bae, J. Hnát, M. Plevová, K. Bouzek, J.H. Jang, H.S. Park, Dirk Henkensmeier, PBI nanofiber mat-reinforced anion exchange membranes with covalently linked interfaces for use in water electrolysers, J. Membr. Sci., 2021, 640, 119832. https://doi.org/10.1016/j.memsci.2021.119832