In-Situ Crossover Measurements for Vanadium-Based Non Aqueous Redox Flow Battery

Abstract

The efficient operation of the non-aqueous redox flow battery (NAqRFB) requires a selective membrane that prevents the crossover of the charged storage molecules (redox species) to minimize self-discharge or mixed potentials; actually, the crossover-induced capacity loss remains one of the most challenging problems for NAqRFB. At the same time, the membrane must enable high ionic conductivity (ions from supporting electrolyte) to lower ohmic losses. The identification of a highly conductive, yet selective membrane is of paramount importance for market penetration of RFBs. Although the membrane should be extremely selective, for allowing years of continuous operation, the truth is that all commercially used membranes allow to some extent redox species permeation, especially when a thinner membrane is pursued to ensure high ionic conductivity. In terms of capacity retention, the reactant that crosses directly through the membrane is oxidized or reduced without providing electrical work, leading to lower efficiency. Therefore, crossover in a NAqRFB and its impact on cell operation is a critical phenomenon that needs to be quantified and mitigated in the development of redox flow-cells.The crossover through the membrane is problematic since it leads to loss of capacity and self-discharge. An in-situ electrochemical method based on linear sweep voltammetry was introduced to determine crossover through different membranes. The results were further confirmed through the permeance of redox ion using an ex-situ method, performance characterization, and by measuring the overpotentials. Ferrocene is reported as a model probing species for in-situ crossover measurement. The cation exchange membrane having a higher thickness (Nafion 117) and anion exchange membrane having reinforcement (FAPQ-375-PP) performed better. The model redox species (vanadium acetylacetonate) were used for the performance assessment of NARFB. The columbic efficiency was 91.47 % and voltaic efficiency of 72.9 %. The ohmic and mass transport overpotentials affecting the performance of NARFB were 0.46 V and 1.44 V. This in-situ technique will allow researchers to account for the contribution of migration-driven crossover influenced by the applied electrical field and not only the diffusion-based crossover as reported in the previous studies. Since there is no need for disassembling of the cell, so it can be used as a diagnostic tool to assess the realistic membrane situation under actual working conditions. The quantification of the permeance directly from the flow-cells and overpotentials complements the improvement in the designing and toward further optimization of the operational parameters. Due to its convenience and practicability, this technique has the potential to become the basic testing method to determine crossover and aid in membrane selection for many electrochemical cells/systems. This work is still in progress for in-depth study covering other aspects as well. Figure 1