Ammonium Acetate-Induced Ligand Exchange Catalysis: Unraveling the Unidirectional Molecular Switch Mechanism for Long-Cycle-life Iron-Chromium Redox Flow Batteries

The commercialization of iron-chromium redox flow batteries (ICRFBs) is severely hindered by the kinetically inert [Cr(H2O)6]3+complex, which simultaneously restrains Cr3+/Cr2+ redox kinetics and accelerates parasitic hydrogen evolution. Herein, we demonstrate that ammonium acetate (AMA) acts as a unidirectional molecular switch to trigger irreversible ligand-exchange catalysis via stepwise regulation of the Cr3+coordination micro-environment: the NH4+ cation first initiates the substitution of coordinated H2O by Cl-to "activate" the electroactive [Cr(H2O)5Cl]2+ intermediate, and acetate anions (Ac-) subsequently displace Cl-to "stabilize" the highly diffusive [Cr(H2O)5(Ac)]2+ complex, thus completing the one-way ligand-exchange catalytic process and enhancing the Cr3+diffusion. As a result, this unidirectional molecular switch extends the cycle life by nearly 600-fold (36% capacity retention after 1000 cycles vs. 0.06% for the pristine electrolyte with 100 cycles) and increases the hydrogen evolution reaction (HER) overpotential by 60 mV, enabling a stable discharge capacity of 345 mAh at 40 mA cm-2. This work establishes a potentially generalizable ligand-exchange design principle for high-performance aqueous redox flow battery systems with long-cycle-life.