Hydrogen‐Treated Rutile TiO2 Shell in Graphite‐Core Structure as a Negative Electrode for High‐Performance Vanadium Redox Flow Batteries

Hydrogen‐treated TiO2 as an electrocatalyst has shown to boost the capacity of high‐performance all‐vanadium redox flow batteries (VRFBs) as a simple and eco‐friendly strategy. The graphite felt‐based GF@TiO2:H electrode is able to inhibit the hydrogen evolution reaction (HER), which is a critical barrier for operating at high rate for long‐term cycling in VRFBs. Significant improvements in charge/discharge and electron‐transfer processes for the V3+/V2+ reaction on the surface of reduced TiO2 were achieved as a consequence of the formation of oxygen functional groups and oxygen vacancies in the lattice structure. Key performance indicators of VRFB have been improved, such as high capability rates and electrolyte‐utilization ratios (82 % at 200 mA cm−2). Additionally, high coulombic efficiencies (ca. 100 % up to the 96th cycle, afterwards >97 %) were obtained, demonstrating the feasibility of achieving long‐term stability. Hydrogen‐treated TiO2‐covered electrodes: A rutile shell is built around a graphite felt electrode to increase its selectivity for the V3+/V2+ redox reaction and to inhibit the hydrogen evolution reaction. The shell is partially reduced in hydrogen, forming oxygen vacancies in the TiO2 structure, which enhances the catalytic activity for the vanadium negative reaction. This results in an electrode that is suitable for vanadium redox flow batteries at high rates (up to 300 mA cm−2) and for long‐term cycling.