Deconvoluting Surface Modification Effects on Flow Battery Electrode Performance by Impedance Spectroscopy

Flow batteries have proliferated over the past decade, both in installed capacity and in the diversity of systems being developed for grid storage applications. As a long duration energy storage technology, flow batteries offer the distinct advantage of independent scaling of energy capacity through their liquid electrolytes, and of power capacity through stack design. Across flow battery systems, improving the design of reactor stacks to achieve high power densities at acceptable energy conversion efficiencies remains an important lever for lowering overall levelized costs of storage. In pursuit of this aim, understanding the reaction kinetics on the surfaces of porous carbon electrodes is key 1. However, traditional techniques such as voltammetry, impedance spectroscopy, and cell performance must be utilised with caution to avoid convoluting observations of surface kinetics with other effects. Previous studies on vanadium flow batteries have used impedance spectroscopy to circumvent this issue, utilising the shared dependence of double layer capacitance and charge transfer conductance 2. In recent years, DRT analysis of impedance spectra has also been used to further study electrode treatments for vanadium systems 3. Building on these works, we use impedance spectroscopy combined with other techniques to deconvolute the effects of electrode treatments ex-situ before demonstrating the modified electrode performance in flow cells. The accompanying abstract image illustrates how different electrode modifications can manifest comparative differences in identified faradaic features in impedance spectra. As the most well studied system, we first apply our analysis to electrodes for vanadium electrolytes, validating our method and previous studies on electrode modifications. Following this, we apply our approach to capture a range of behaviours by studying redox active species with faster kinetics, such as recently developed organic species 4. The results of this work aim to improve the evaluation of reaction kinetics for different redox active species and electrode treatments. By doing so, we seek to further understand surface reaction mechanisms and how they can be harnessed to improve stack and electrolyte performance. 1 T. V. Sawant, C. S. Yim, T. J. Henry, D. M. Miller, and J. R. McKone, ‘Harnessing Interfacial Electron Transfer in Redox Flow Batteries’, Joule , vol. 5, no. 2, pp. 360–378, Feb. 2021, doi: 10.1016/j.joule.2020.11.022. 2 H. Fink, J. Friedl, and U. Stimming, ‘Composition of the Electrode Determines Which Half-Cell’s Rate Constant is Higher in a Vanadium Flow Battery’, J. Phys. Chem. C , vol. 120, no. 29, pp. 15893–15901, Jul. 2016, doi: 10.1021/acs.jpcc.5b12098. 3 K. Köble et al. , ‘Revealing the Multifaceted Impacts of Electrode Modifications for Vanadium Redox Flow Battery Electrodes’, ACS Appl. Mater. Interfaces , vol. 15, no. 40, pp. 46775–46789, Oct. 2023, doi: 10.1021/acsami.3c07940. 4 J.-M. Fontmorin, S. Guiheneuf, T. Godet-Bar, D. Floner, and F. Geneste, ‘How anthraquinones can enable aqueous organic redox flow batteries to meet the needs of industrialization’, Current Opinion in Colloid & Interface Science , vol. 61, p. 101624, Oct. 2022, doi: 10.1016/j.cocis.2022.101624. Figure 1