Molten salt carbon felt oxidation for VRFB electrode performance improvement

Abstract

<p>In this study, carbon felt (CF) electrodes were chemically treated using <a href="https://www.sciencedirect.com/topics/materials-science/alkali-metal" title="Learn more about alkali metal from ScienceDirect's AI-generated Topic Pages">alkali metal</a> salt (potassium nitrate) in the molten state as a mild oxidizing agent. Various temperatures in the range of 380–500 °C and <a href="https://www.sciencedirect.com/topics/materials-science/oxidation-reaction" title="Learn more about oxidation from ScienceDirect's AI-generated Topic Pages">oxidation</a> duration (up to 12 h) were analyzed in order to find the most appropriate oxidation conditions. As-prepared carbon felts were electrochemically investigated in a three-electrode set-up as well as in a lab-scale vanadium <a href="https://www.sciencedirect.com/topics/materials-science/redox-flow-battery" title="Learn more about redox flow battery from ScienceDirect's AI-generated Topic Pages">redox flow battery</a> (VRFB). The battery set-up included additional <em>in-situ</em> potential measurements allowing to measure the polarization of each electrode separately and compare the performance of heat- and chemically-treated electrodes. The physico-chemical properties of CFs have been analyzed using X-ray photoelectron spectroscopy (XPS) and other advanced techniques. Combination of extended oxidation time with temperature increase causes satisfactory <a href="https://www.sciencedirect.com/topics/materials-science/surface-functionalization" title="Learn more about surface functionalization from ScienceDirect's AI-generated Topic Pages">surface functionalization</a> and enhanced vanadium <a href="https://www.sciencedirect.com/topics/materials-science/redox-process" title="Learn more about redox reaction from ScienceDirect's AI-generated Topic Pages">redox reaction</a> <a href="https://www.sciencedirect.com/topics/engineering/reversibility" title="Learn more about reversibility from ScienceDirect's AI-generated Topic Pages">reversibility</a>. The chemically treated samples showed a better performance with lower overpotentials and higher efficiencies. Improved reversibility of V²⁺/V³⁺ reaction is obtained for the carbon felt treated with salt presence. What is more, a 4 times reduced CF electrode oxidation time as usually presented in literature (12 h for heat-treated electrodes <em>vs.</em> 3 h for salt- and heat-treated ones) can be achieved with nitrate salt oxidation.<br></p>