Internal Ligand Substitution Improves External Oxygen Evolution Reaction in Metal Hydroxide-Organic Frameworks

Precisely engineering coordination defects to boost catalytic activity without sacrificing structural stability is a pivotal yet challenging goal. Herein, we report a novel internal ligand substitution strategy for metal hydroxide-organic frameworks (MHOFs) that achieves this dual objective. By selectively replacing 25% of dicarboxylic acid linkers (ABDA) with monocarboxylic benzoic acid (BA), we construct Ni-(ABDA)_0.75(BA)_0.25, which preserves long-range order via robust π-π stacking while generating undercoordinated Ni sites. The optimized catalyst exhibits a significantly reduced overpotential of 326 mV at 1 mA cm^-2 in 0.1 M KOH, surpassing the pristine Ni-ABDA by 122 mV and outperforming the commercial IrO₂ benchmark. Combined experimental and theoretical analyses reveal that the ligand defects induce electron density redistribution around the Ni centers, thereby lowering the thermodynamic barrier for the rate-determining step. More importantly, the tailored electronic structure at the undercoordinated Ni sites breaks the scaling relationship between *O and *OOH intermediates by balancing their adsorption strengths. This study demonstrates that defect engineering in MHOFs through internal coordination structure modulation drives catalytic performance, establishing a theoretical framework for designing high-efficiency, durable electrocatalysts to enable sustainable water electrolysis.