Assessment of electrode modifications for vanadium redox flow batteries

Unlike traditional battery systems, vanadium redox flow batteries (VRFBs) have been gaining attention in the past years due to their advantages of flexible, scalable design and long-life cycle for energy storage applications. In these systems, porous electrodes such as carbon paper and carbon felts are often used due to their desirable properties including good acid resistance, high surface area, and reasonable cost. However, these materials may have drawbacks of poor reversibility and kinetics, which can affect the system overall performance. These limitations are related to interfacial phenomena at the electrode-electrolyte interface, which can be mitigated by improving wettability and active surface area. Among several approaches, thermal and chemical treatments are common in literature since these treatments can improve wettability, increase electrochemical active surface area, and can provide functional groups that are believed to facilitate vanadium redox reactions. Although different activation methods can improve electrode performance, there is a limited understanding and disparity between literature reports on how to assess the relevant properties in order to elucidate improvement mechanisms. Hence, this work aimed to systematically assess different modified electrodes so that impediments can be further understood, and reliable engineering solutions can be designed to mitigate such limitations. For that, a series of modified graphite felt electrodes were fabricated using different activation methods (thermal treatment, chemical treatment, and catalyst coating) and subjected to physical, chemical, and electrochemical characterization methods. The results revealed that wettability and electrochemical active surface area are the main physical and chemical properties that correlate to electrochemical performance improvements. More efficient electrodes can be achieved through thermal treatment by tuning process variables in order to enhance electrode properties. Moreover, performance can be further increased by combining thermal treatments and catalyst materials, though the main contribution in energy efficiency comes from the thermal activation process, which improved electrode wettability and consequently enhanced kinetics.