Influence of Nafion and PBI Membranes on the Performance of Verdazyl Species in Aqueous Redox Flow Batteries

Abstract

The concept of redox flow batteries (RFB) was originally patented in 1949 by Dr. Kangro with inorganic active materials like iron- or chromium-ions in mind.[1] Intensive research on inorganic active materials led to the proposal of vanadium based RFBs in 1985 by the Skyllas-Kazacos group, which is the most intensively studied RFB type currently.[2],[3] Organic active materials were introduced in 2009 for RFBs and offer a potential alternative to vanadium as active species, due to potentially lower costs and the replacement of rare metal ions.[4],[5] The class of verdazyl radicals has gathered interest as organic active material for the battery application since the publication of symmetrical coin cells.[6] Verdazyl radicals can be transformed into an oxidized or reduced species by a one electron reaction in organic electrolytes, which motivates the application in symmetrical RFBs.[7] Still, degradation of the verdazyl species during operation of the symmetrical battery in organic electrolytes as well as the theoretical voltage of ≈ 1 V show the need for further optimization of the material class and electrolytes.[6] While previous electrochemical investigations of verdazyl species were limited to organic electrolytes, we used a different approach and investigated the influence of acidic electrolytes on the redox chemistry of this material class.[8] Strongly acidic electrolytes lead to a disproportionation of the radical form. The resulting species differ by two electrons and two protons and show a reversible redox reaction in cyclovoltammetry measurements. This different reaction is of interest for the RFB application due to the fast rate constant as well as the utilization of two electrons.[8] Herein, we take the next step for the possible application of the redox chemistry of verdazyl species in acidic electrolytes into aqueous RFBs by analyzing the influence of the separating membranes. We use 1,3,5-triphenylverdazyl species as model compounds in 1 M sulfuric acid. To investigate the influence of the separating membranes in aqueous RFBs, we use Nafion 211 as commercial standard and compare it to self-made mPBI and O-PBI-based polybenzimidazole (PBI) membranes with a target thickness of ≈ 25 µm. When Nafion membranes are immersed in verdazyl cation solutions, they absorb ≈ 0.5 verdazyl cations per sulfonic acid group from the solution, whereas no absorption was observed for PBI membranes. UV/vis also did not show any changes in the UV/vis spectra of the solution, indicating that the verdazyl cations did not degrade. Beside the chemical stability, the permeability through the membranes is characterized, which is detrimental for the capacity retention in an RFB. To fully characterize the membranes with verdazyl species, the conductivity of the membranes is measured and the influence of the verdazyl species onto the conductivity is investigated. As a last step, symmetrical RFBs of verdazyl species in acidic electrolytes are built and the capacity retention is compared.With this study, we show the detrimental effect of using Nafion in combination with verdazyl species. Nafion 211 shows a high affinity to verdazyl cations and absorbs them from the electrolyte, leading to fast capacity fading of the RFB. PBI membranes show chemical stability as well as low absorption and permeability.Our study presents the one of the next steps towards the implementation of verdazyl species into water-based RFBs by investigating the suitability of PBI membranes for this application. PBI membranes offer overall good stability and beneficial properties for this application, while state-of-the-art Nafion is not applicable.