Influence of solvent on ion conductivity of polybenzimidazole proton exchange membranes for vanadium redox flow batteries

Aqueous Redox Flow Cells Utilizing Verdazyl Cations enabled by Polybenzimidazole Membranes

Non-aqueous organic redox flow batteries (RFB) utilizing verdazyl radicals are increasingly explored as energy storage technology. Verdazyl cations in RFBs with acidic aqueous electrolytes, however, have not been investigated yet. To advance the application in aqueous RFBs it is crucial to examine the interaction with the utilized membranes. Herein, the interactions between the 1,3,5-triphenylverdazyl cation and commercial Nafion 211 and self-casted polybenzimidazole (PBI) membranes are systematically investigated to improve the performance in RFBs. The impact of polymer backbones is studied by using mPBI and OPBI as well as different pre-treatments with KOH and H3PO4. Nafion 211 shows substantial absorption of the 1,3,5-triphenylverdazylium cation resulting in loss of conductivity. In contrast, mPBI and OPBI are chemically stable against the verdazylium cation without noticeable absorption. Pre-treatment with KOH leads to a significant increase in ionic conductivity as well as low absorption and permeation of the verdazylium cation. Symmetrical RFB cell tests on lab-scale highlight the beneficial impact of PBI membranes in terms of capacity retention and I-V curves over Nafion 211. With only 2 % d-1 capacity fading 1,3,5-triphenylverdazyl cations in acidic electrolytes with low-cost PBI based membranes exhibit a higher cycling stability compared to state-of-the-art batteries using verdazyl derivatives in non-aqueous electrolytes.

Study of Innovative GO/PBI Composites as Possible Proton Conducting Membranes for Electrochemical Devices

The appeal of combining polybenzimidazole (PBI) and graphene oxide (GO) for the manufacturing of membranes is increasingly growing, due to their versatility. Nevertheless, GO has always been used only as a filler in the PBI matrix. In such context, this work proposes the design of a simple, safe, and reproducible procedure to prepare self-assembling GO/PBI composite membranes characterized by GO-to-PBI (X:Y) mass ratios of 1:3, 1:2, 1:1, 2:1, and 3:1. SEM and XRD suggested a homogenous reciprocal dispersion of GO and PBI, which established an alternated stacked structure by mutual π-π interactions among the benzimidazole rings of PBI and the aromatic domains of GO. TGA indicated a remarkable thermal stability of the composites. From mechanical tests, improved tensile strengths but worsened maximum strains were observed with respect to pure PBI. The preliminary evaluation of the suitability of the GO/PBI X:Y composites as proton exchange membranes was executed via IEC determination and EIS. GO/PBI 2:1 (IEC: 0.42 meq g^-1; proton conductivity at 100 °C: 0.0464 S cm^-1) and GO/PBI 3:1 (IEC: 0.80 meq g^-1; proton conductivity at 100 °C: 0.0451 S cm^-1) provided equivalent or superior performances with respect to similar PBI-based state-of-the-art materials.