Unveiling the Influence of Lower‐Valence Ni in Hydroxide Co‐Catalyst and Attaining Efficient Photoanodes with FeOOH Holes Transfer Layer for Photoelectrochemical Water Splitting

Abstract Electronic structure regulation is a prevailing approach to modifying the electrocatalysts in the oxygen evolution reaction (OER), yet its impact on the co‐catalysts modified photoanodes remains uncertain. Herein, B‐modified NiFe‐LDH (NiFeB‐hydx) co‐catalyst in situ is immobilized onto Ti‐Fe₂O₃ and emphasizes the effect of lower‐valence Ni (Ni^2‐δ) in the photoelectrochemical (PEC) process. A flexible adjustment in the oxidation state of Ni from +2 to 0 by manipulating the Ni content and the corresponding B quantity is achieved. Interestingly, the Ni⁰ significantly reduces the onset potential low to 0.53 V_RHE, albeit leading to wasted holes due to incomplete redox transition. An optimal amount of Ni^2‐δ effectively facilitates the redox transition of Ni and achieves a delicate balance between hole extraction and consumption, substantially enhancing the PEC performance. Density functional theory calculations verifies the electron enrichment of Ni and the corresponding benefits for enhancing OER. Furthermore, introducing FeOOH as a hole transfer layer on Ti‐Fe₂O₃ induces a pronounced band bending effect, remarkably promoting the meticulously engineered NiFeB‐hydx/FeOOH/Ti‐Fe₂O₃ photoanode to a photocurrent density of 3.29 mA cm⁻² at 1.23 V_RHE. NiFeB‐hydx/FeOOH can be universally applied to the BiVO₄ photoanode, resulting in doubled photocurrent density (4.8 mA cm⁻² at 1.23 V_RHE).