Cobalt-Based Molecular Catalysts Designed for Significantly Enhanced Low-Bias Photoelectrochemical Water Oxidation over BiVO₄

The development of molecular oxygen evolution catalysts (OECs) that function efficiently at a low applied potential and can be readily anchored to photoelectrodes is crucial for building bias-free photoelectrochemical (PEC) water-splitting devices. In this work, we designed three robust cobalt-based molecular OECs using distinct organic bifunctional linkers: dipicolyl alanine acid (DPAA), dipicolyl glycine acid (DPGA), and dipicolyl-4-aminobenzoic acid (DABA). These linkers were immobilized onto BiVO₄ photoanodes to coordinate Co²⁺ ions. All modified photoanodes exhibited enhanced PEC activity, with Co-DABA/BiVO₄ achieving the highest photocurrent density of 3.45 mA cm^–2 at 0.6 V_RHE─representing enhancements of 1.66-, 1.63-, and 8.92-fold over Co-DPAA/BiVO₄, Co-DPGA/BiVO₄, and pristine BiVO₄, respectively. At this potential, Co-DABA/BiVO₄ also attained a Faradaic efficiency of 91% and a turnover frequency (TOF) of 4.54 s^–1. Spectroscopic and photoelectrochemical analyses revealed prolonged hole lifetimes, improved charge separation, and accelerated charge transfer kinetics. The superior performance is attributed to the phenylene group in DABA, which promotes hole transfer and increases the oxidation capability, as supported by DFT calculations. Solid-state simulations confirmed hole localization at Co-DABA, with partial delocalization extending to the BiVO₄ surface, indicating strong electronic coupling that promotes hole transfer from BiVO₄ to the electrolyte across the Co-DABA molecular OEC. The Co-DABA/BiVO₄ demonstrated significant operational stability and a TOF surpassing many of the best-performing Ru- and Co-based molecular OECs, highlighting its strong potential for efficient, scalable solar fuel generation.