Balanced Spin-State Energy Level Splitting Boosts Photoelectrochemical Water Oxidation on Amorphous NiFeAl-LDH Engineered BiVO₄

Amorphous oxygen evolution cocatalysts are frequently employed to enhance the performance of BiVO₄-based photoelectrochemical (PEC) systems and often outperform their crystalline counterparts. However, the fundamental mechanism underlying this enhancement has remained elusive, hindering the rational design of highly active and stable cocatalysts. Here, this knowledge gap is addressed by constructing an amorphous ternary NiFeAl layered double hydroxide (LDH) on BiVO₄. The structural disorder of the amorphous LDH promotes stronger interaction with BiVO₄, induces the formation of a distorted octahedral coordination framework, and leads to a redistribution of Ni³⁺ e_g/t_2g valence electron orbitals—from a degenerate to a non-degenerate configuration—alongside a transition from high-spin to low-spin states. As a result, the BiVO₄/NiFeAl-LDH-amorphous photoanode achieves a photocurrent density of 5.78 mA cm⁻² at 1.23 V vs RHE, and an applied bias photon-to-current efficiency of 2.21% at 0.62 V vs RHE—substantially outperforming the crystalline counterpart (2.83 mA cm⁻² and 1.19%, respectively). These results establish a clear link between amorphization and spin-state engineering, and offering a new design paradigm for efficient PEC water splitting systems.