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batteries Article Electrochemical Evaluation of Different Graphite Felt Electrode Treatments in Full Vanadium Redox Flow Batteries Itziar Azpitarte 1,*, Unai Eletxigerra 1,*, Angela Barros 1,* , Estibaliz Aranzabe 1 and Rosalía Cid 2 1 2 * Correspondence: itziar.azpitarte@tekniker.es (I.A.); unai.eletxiguerra@tekniker.es (U.E.); angela.barros@tekniker.es (A.B.) Abstract: The use of flow batteries for energy storage has attracted considerable attention with the increased use of renewable resources. It is well known that the performance of a flow battery depends, among other factors, on the properties of the electrodes, which are generally composed of graphite felt (GF). In this work, thermal, chemical and plasma treatments have been employed to modify the surface of the graphite felt to improve the electrochemical activity of the redox flow cell. The influence of the variables of each of these processes on the generation of surface functional groups and on changes in the obtained surface area have been examined. In this work, the kinetics of redox reactions relevant to the VO2+/VO2+ reaction have been studied with these treated electrodes and the relationship between the nature of the surface and electrochemical activity of the GF is discussed. As a result, an enhanced electrochemical performance (reduction over 200 mV of the separation between anodic and cathodic peaks and 110 mV of the onset potential) in comparison to the untreated GF is obtained for those GF treatments with low oxygenated groups concentration. Keywords: graphite felt; electrode; surface treatments; electrochemical performance; redox flow batteries 1. Introduction The continuous increase in electricity consumption in a world of finite fuel sources and a changing climate, urgently requires the impulse of renewable energy sources [1]. The fluctuating nature of renewable energy generation [2] entails the development of reliable large-scale energy storage systems (ESSs) that improve the stability, efficiency and sustainability of the power grid [3]. Among the different electrochemical ESSs, redox flow batteries (RFBs) are considered an excellent technology for cost-effective large-scale stationary applications [4] due to their high energy density, good cyclability, flexible architecture and distinctive decoupling between power and energy [5]. The core of RFBs lies in the electroactive species in which the energy is stored [6], and several metal-based redox couples have been investigated over the past few decades [7]. Among them, “all-vanadium” RFBs (VRFBs) are the most studied and are among the few RFBs that have been tested and demonstrated at utility scale [8,9]. The VRFB technology was developed in 1985 by Skyllas-Kazakos [10] and consists of an electrochemical cell where the energy conversions take place and two external tanks where the positive (catholyte, containing VO2+/VO2+ ions) and negative (anolyte, con- taining V2+/V3+ ions) electrolytes are stored. The electrolyte flows through each of the two half-cells, which are separated by a membrane that allows selective ion exchange to maintain charge balance, to return to the storage tank. Considering that they provide the reaction sites, electrodes are an essential compo- nent of RFBs that directly affect battery performance [11]. The main requirements of the electrodes are (1) good electrocatalytic activity, (2) chemical stability, (3) electrochemical Surface Chemistry & Nanotechnologies Unit, Tekniker, Basque Research and Technology Alliance (BRTA), C/Iñaki Goenaga, 5, 20600 Eibar, Spain Centre for Cooperative Research on Alternative Energies (CIC energiGUNE), Basque Research and Technology Alliance (BRTA), Alava Technology Park, Albert Einstein 48, 01510 Vitoria-Gasteiz, Spain Citation: Azpitarte, I.; Eletxigerra, U.; Barros, A.; Aranzabe, E.; Cid, R. Electrochemical Evaluation of Different Graphite Felt Electrode Treatments in Full Vanadium Redox Flow Batteries. Batteries 2023, 9, 39. https://doi.org/10.3390/ batteries9010039 Academic Editors: Quanqing Yu and Guanjie He Received: 30 September 2022 Revised: 29 November 2022 Accepted: 31 December 2022 Published: 5 January 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Batteries 2023, 9, 39. https://doi.org/10.3390/batteries9010039 https://www.mdpi.com/journal/batteriesPDF Image | Graphite Felt Electrode Treatments Vanadium Redox Flow
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