Oxygen vacancy-engineered BiOX catalysts: Synergistically boosting redox kinetics and suppressing hydrogen evolution in Iron chromium flow batteries

Iron‑chromium redox flow battery (ICRFB) offers an attractive technological pathway for low-cost, grid-scale energy storage integrated with renewable energy sources. However, the slow Cr 3+ /Cr 2+ redox kinetics at the anode and the coupled parasitic hydrogen evolution reaction (HER), particularly at high current densities, remain major obstacles to their practical application. This study reports a layered structure coupled with oxygen vacancy (OV) engineering, which decouples the Cr redox activation and HER suppression processes to fully unlock the catalytic potential of halogenated bismuth (BiOX, X = Cl, Br, I). The designed BiOX catalyst leverages lattice distortion induced by halogen elements to create abundant OVs, while the asymmetric Bi-O-Cr bridging structure accelerates electron transfer during the Cr 3+ /Cr 2+ conversion. The mesoporous layered framework ensures rapid accessibility of redox-active species. By in situ anchoring this OV-rich BiOX catalyst onto porous carbon fiber, a high-performance electrode is successfully fabricated. Consequently, the ICRFB with OVs-rich BiOX-modified electrodes achieves an energy efficiency of 84.5% at 140 mA cm −2 , with exceptional stability over 400 cycles. This work elucidates the multifunctional role of OVs in BiOX for synergistically boosting Cr redox activity and mitigating parasitic HER, providing insights for the rational design of high-efficiency catalysts for ICRFB.