Today, the utilization of renewable energy resources, particularly solar and wind energy, has received widespread attention. In such systems, due to seasonal and daily fluctuations in energy generation, the need for an energy storage source is consistently recognized.

Among available technologies, redox flow batteries have emerged over the past decade as one of the most efficient energy storage systems worldwide, owing to their high efficiency, long service life, and environmentally benign operation [1], [2].

A schematic representation of a vanadium redox flow battery is presented in Fig. 1. Two electrolyte reservoirs, one connected to the positive electrode and the other to the negative electrode, enable the circulation of large volumes of electrolyte through the cell. An ion-exchange membrane prevents the mixing of the two electrolytes. During battery charging, V4+ at the positive electrode loses one electron through an external power supply and is oxidized to V5+. The electron is transferred from the positive electrolyte to the negative terminal of the battery, where it reduces V3+ to V2+. During discharge, the reverse process occurs

As in other battery systems, electrodes play a critical role in Vanadium Redox Flow Batteries (VRFBs). Conventional materials used for vanadium battery electrodes include metals and carbon-based materials. Metallic materials, such as noble metals and their oxides, are used less frequently in vanadium batteries due to their high cost and low specific surface area. In contrast, carbon-based materials, including Carbon felt, carbon paper, and graphite powder, are widely employed in vanadium flow batteries because they offer significant advantages such as high specific surface area, a wide operational potential window, stability, and cost-effectiveness [4], [5].

However, the inherently sluggish kinetics and poor reversibility of carbon-based materials limit their application in VRFBs. Although extensive efforts have been devoted to modifying and enhancing their electrochemical properties [6], such modified electrodes have not yet been widely implemented in practical vanadium batteries or, in many cases, do not deliver satisfactory performance within full-cell configurations. Due to their high specific surface area and excellent electrical conductivity, Carbon Nanotubes (CNTs) can be employed as electrocatalysts in a wide range of electrochemical processes. In this study, the surface activation of a carbon felt electrode using CNT is investigated with the aim of enhancing the performance of this electrode for application in a vanadium redox flow battery.

As shown in Fig. 2, which presents the cyclic voltammograms of the pristine graphite electrode and the Carbon nanotube-modified graphite electrode (C/CNT), it can be observed that the modified electrode exhibits an increased current response and a voltammogram that approaches more reversible behavior. Accordingly, the suitability of CNT for the modification of carbon-based electrodes for application in VRFBs Fig. 3 presents the CV response of bare Carbon felt and CNT-modified Carbon felt. The incorporation of CNT was found to reduce the peak-to-peak separation (ΔEp), indicating an improvement in the electrochemical reversibility of the system. Consequently, the CNT-modified electrode exhibited enhanced redox reversibility compared to the pristine Carbon felt.

Thermal treatment is a widely used method for activating carbon electrodes employed in VRFBs. Accordingly, a thermal–acid treatment was applied to complete the electrode modification process.

In the following, the CV responses of electrodes modified by thermal–acid treatment alone, as well as those modified by the combined use of CNT and thermal–acid treatment, are presented.

In Fig. 4, which corresponds to the thermal–acid treatment of the Carbon felt electrode, a decrease in ΔEp along with an increase in current is observed. This behavior indicates enhanced charge-transfer kinetics and faster electron transfer in the redox reactions occurring at the surface of the modified electrode [7].

When CNT was combined with the thermal–acid treatment for Carbon felt modification, a further increase in current was observed (Fig. 5). This result confirms the electrocatalytic effect of CNT on the electrochemical processes of VRFBs.

In the present study, CNTs were employed as electrocatalysts to modify the surface of a Carbon felt electrode. A combination of thermal and chemical treatments was applied in order to enhance the quality and electrochemical performance of the Carbon felt electrode for application in VRFBs.

The results indicate that the performance of the electrode can be significantly improved through the deposition of CNT onto the Carbon felt surface, followed by thermal–acid treatment. This enhancement is attributed to the presence of oxygen-containing functional groups on the surface of the activated CNT, which are capable of facilitating the adsorption of vanadium species. Consequently, electrochemical reactions proceed more efficiently on the surface of the modified electrode.

The authors declare that there is no conflict of interest regarding the publication of this article.

All data generated or analyzed during this study are included in this published article. No additional data are available.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.