Unlocking Fast Fe–Cr Flow Battery Kinetics and Suppressing Hydrogen Evolution via Sn Nanoparticle‐Mediated Chloride‐Bridged Catalysis

Iron-chromium flow batteries (ICFBs) offer substantial promise for integrating intermittent renewable energy into electrical grids. However, their practical deployment remains fundamentally constrained by severe bottlenecks on sluggish Cr²⁺/Cr³⁺ redox kinetics and detrimental hydrogen evolution reactions (HERs). To overcome the critical limitations, a simple yet scalable electrodeposition method is introduced to uniformly decorate graphite felt with Sn nanoparticles, achieving enhanced catalytic activity and selective suppression of HER. Detailed mechanistic analyses indicate that Sn nanoparticles mediate chloride-bridging interactions, stabilizing reaction intermediates and notably reducing the activation energy barriers of inner-sphere electron transfers. Additionally, Sn increases the hydrogen adsorption free energy, significantly lowering HER propensity. Spatially segregating Cr reaction sites from proton-rich zones via selective chloride-bridging further suppresses HER pathways. Leveraging these insights, the ICFB equipped with Sn-modified graphite felt demonstrates superior performance, including an energy efficiency of 79.39 ± 0.18% and coulombic efficiency of 98.36 ± 0.18% at a current density of 200 mA cm^-2. Moreover, an 1800 W-scale battery stack exhibits stable efficiency over 100 cycles, underscoring the practical applicability and superiority of the strategy. This work establishes Sn nanoparticle catalysts as pivotal in resolving fundamental bottlenecks, thereby advancing Fe-Cr flow batteries toward practical applications.