Atomically Dispersed Cu-N₃ Sites in N‑Doped Porous Carbon for Enhanced Vanadium Flow Battery Performance

Vanadium flow batteries (VFBs), although promising for large-scale energy storage, are limited by sluggish redox kinetics and parasitic side reactions, resulting in substantial efficiency loss and poor cycling stability. To overcome these intrinsic limitations, an ideal electrocatalyst should simultaneously accelerate vanadium ion conversion and suppress undesirable side reactions, yet such catalysts have rarely been realized in VFB systems. Herein, we report the development of a copper-based single-atom catalyst featuring atomically dispersed Cu sites embedded in a nitrogen-doped porous carbon matrix (Cu SAs/NC) grown in situ on graphite felt (GF) as a highly active and cost-effective electrode for VFBs. The obtained Cu SAs/NC provides abundant exposed active sites, improved electrolyte wettability, and accelerated interfacial charge transfer. Notably, the single-atom Cu sites with a Cu-N₃ coordination structure significantly enhance the redox kinetics of both VO²⁺/VO₂ ⁺ and V²⁺/V³⁺ couples by lowering the Gibbs free energy barriers of the rate-determining steps, achieving a high energy efficiency of 75.2% at 200 mA cm^-2 and maintaining 73.1% after 900 cycles, markedly surpassing conventional graphite felt. This work presents the demonstration of Cu-based single-atom catalysis in VFBs, offering a novel design strategy for high-performance flow battery electrodes.