Gradient oxygen-containing graphite felt electrode for improved current density distribution in vanadium redox flow batteries

As a promising candidate for large-scale energy storage systems, vanadium redox flow batteries (VRFBs) are nevertheless limited by the uneven electrolyte distribution and reaction localization in conventional homogeneous electrodes. To address this critical issue, we propose a novel paradigm of spatially decoupling the mass transport and electrochemical reaction functions within a single electrode. This is realized by fabricating a gradient oxygen-functionalized graphite felt (GOC-GF) via a rapid (<10 min) and tunable air plasma process. This oxygen gradient creates two synergistic effects. First, it establishes a gradient in catalytic activity that confines the primary electrochemical reactions near the membrane side, shortening the proton transport path. Second, it induces a corresponding hydrophilicity gradient. This wettability gradient acts as an intrinsic, capillary-driven force that guides electrolyte penetration from the flow side into the electrode depth, eliminating mass transport dead zones. Consequently, the VRFB with the optimized GOC-GF electrode achieves an 8.4% higher energy efficiency (EE) at 200 mA cm−2 than the pristine GF and sustains stable operation at 250 mA cm−2. Furthermore, the introduced gradient in wettability provides a straightforward route to directionally load a broad range of catalyst precursors from aqueous solutions, significantly broadening the applicability of this design. This work presents the functional gradient architecture as an innovative and generalizable design principle for developing advanced electrodes in high- performance VRFB systems.