(A) study on carbon felt electrode for vanadium redox flow battery to enhance reactivity and reduce contact resistance

Abstract

Energy-storage systems (ESSs) are indispensable for renewable-energy sources such as solar and wind power, which supply intermittent electricity. Vanadium redox flow batteries (VRFBs) are promising as next-generation ESSs owing to their weak explosive properties and independence of power and capacity. However, the technical readiness level of VRFBs is still insufficient for commercialization, and the low energy density and energy efficiency are critical issues that need to be addressed. In particular, it has been reported that the electrodes dominantly influence the performance of VRFB systems because the redox reaction occurs at their surface. Additionally, the electrodes not only directly affect the chemical reaction but also act as a medium through which the liquid electrolyte flows and as a path for electrons to flow. In this study, a method to improve the reactivity of electrode and a method to reduce the contact resistance between components were proposed. First, a glucose-based carbon coating layer was introduced on the electrode surface to improve the reactivity of the electrode. The performance of the prepared carbon coating layer was evaluated through cyclic voltammetry, electrochemical impedance spectroscopy, and charge-discharge tests. It was confirmed that it showed excellent reactivity compared to the conventional heat treatment method. The reversibility of the reaction was superior, so that the performance was maintained for a long cycle. The correlation between the chemical composition and performance of the carbon layer was investigated through surface analysis. As the other approach, an integration method was presented to reduce the contact resistance between the electrode and the bipolar plate. Before the compression molding to fabricate the bipolar plate part, glucose was pre-impregnated and cured to the electrode part to prevent a change in porosity owing to the compression of the carbon felt. In addition, the integrated structure was fabricated with a single sheet of graphite felt so that the electrode and bipolar plate could be interconnected with graphite fibers. Finally, a porous electrode with a gradient of compressive stiffness in the through-thickness direction was proposed to simultaneously improve the reactivity and contact resistance of the electrode. When assembling a cell using an electrode with a stiffness gradient, the contact resistance between the bipolar plate and the electrode was increased, which contributed to reducing the contact resistance. These results in this study are expected to improve energy efficiency of VRFBs, which contribute to the efficient and stable energy supply of a large scale ESSs.