Strategic Facet Design of In ₂ O ₃ Catalysts for Enhanced Kinetics and Hydrogen Suppression in Iron–Chromium Flow Batteries

Abstract Iron‐chromium redox flow batteries (ICRFBs) show promise for large‐scale energy storage, but their performance is hindered by the hydrogen evolution reaction (HER) and sluggish anode Cr 3 ⁺/Cr 2 ⁺ redox kinetics. Here, an octahedral In 2 O 3 catalyst with exposed high‐activity (222) crystal planes is reported, synthesized via high‐temperature solution thermal decomposition and grown in situ on carbon cloth. The catalyst is grown in situ on carbon cloth to form a nanostructured indium‐based electrode (In 2 O 3 ‐TCC). Grazing incidence wide‐angle X‐ray scattering confirms In 2 O 3 phase formation, while XANES reveals abundant oxygen vacancies (Ov) serving as anode reaction active sites. In 2 O 3 ‐TCC exhibits enhanced electrochemical properties, including a tripled double‐layer capacitance (8.92 mF cm − 2 ), a reduced charge transfer resistance (1.042 Ω), and improved Cr 3 ⁺/Cr 2 ⁺ kinetics. Density functional theory (DFT) shows that anode HER suppression arises from favorable H⁺ adsorption energy and a high desorption barrier. Furthermore, an in situ differential electrochemical mass spectrometer (DEMS) confirms effective anode HER suppression. The electrode achieves an energy efficiency of 84.02% at 140 mA cm − 2 and stable performance over 500 cycles. This work offers a new pathway for designing high‐efficiency, long‐lifetime ICRFB electrodes.