Bismuth vanadate (BiVO₄) is one of the most extensively investigated photoanodes for solar-driven water oxidation owing to its suitable bandgap, strong visible-light absorption, favourable band-edge alignment, and chemical stability. However, its photoelectrochemical (PEC) efficiency remains fundamentally limited by poor charge transport and severe electron–hole recombination. These limitations originate from intrinsically low carrier mobility and short diffusion lengths, which arise from the formation of self-trapped small polarons that strongly couple charge transport to local lattice distortions. Consequently, the photoactivity of BiVO₄ is exceptionally sensitive to atomic-scale structural perturbations, making it an ideal model system for exploring how local chemical environments influence macroscopic PEC function.

Transition-metal (TM) incorporation has emerged as a promising strategy to improve BiVO₄ performance through modification of its electronic structure, defect chemistry, and charge-transport pathways. However, despite numerous reports of enhanced photocurrents and improved charge-separation efficiencies, the microscopic origins of these improvements remain poorly understood. In particular, the relationship between TM coordination environment, electronic coupling with the host lattice, and the resulting charge-carrier dynamics has remained largely unresolved. Addressing this challenge requires correlating local structural information with electronic and photophysical behaviour across multiple length and time scales.

Here, we systematically investigate the influence of Fe, Co, and Ni incorporation on the structural, electronic, and PEC properties of BiVO₄ photoanodes. By combining synchrotron-based X-ray absorption spectroscopy (XAS), operando X-ray measurements, transient absorption spectroscopy spanning femtosecond-to-microsecond timescales, and electrochemical impedance spectroscopy, we establish direct structure–property relationships linking the local incorporation environment of transition metals to charge-separation dynamics and PEC performance.

All transition-metal-modified photoanodes exhibit enhanced water-oxidation activity compared with pristine BiVO4, following the performance trend Ni > Fe > Co > BiVO₄. However, our results reveal that these improvements originate from fundamentally different mechanisms. X-ray spectroscopic analysis demonstrates that Fe and Co are incorporated predominantly within heterogeneous oxide-like environments that remain only weakly electronically coupled to the surrounding BiVO₄ lattice. These environments introduce structural and electronic disorder, generating localized states that provide only limited improvements in charge separation and carrier extraction.

In contrast, Ni incorporation produces a markedly different local structure. Spectroscopic fingerprints reveal the formation of highly oxidized, locally octahedral Ni–O units that remain strongly integrated within the BiVO₄ framework. This incorporation induces persistent modifications of the surrounding V–O network, evidencing significant electronic communication between the dopant and host lattice. Rather than acting as isolated impurity centres, these Ni species alter the electronic landscape of BiVO₄ itself, leading to profound consequences for charge-carrier behaviour.

Ultrafast TAS measurements show that Ni incorporation substantially suppresses charge localisation and recombination from the earliest stages of photoexcitation. The resulting charge carriers exhibit significantly prolonged lifetimes across all measured temporal regimes, from femtoseconds to microseconds, indicating more efficient charge separation and enhanced carrier persistence. Operando X-ray absorption spectroscopy further demonstrates that lattice-incorporated Ni remains electronically active under working photoelectrochemical conditions, exhibiting dynamic changes in oxidation state associated with photogenerated charge accumulation. These observations directly connect the local electronic structure of the incorporated Ni species with the functional processes governing solar water oxidation.

Collectively, our findings demonstrate that the performance of transition-metal-modified BiVO₄ cannot be rationalized solely on the basis of dopant identity or nominal composition. Instead, the decisive factor is the nature of the electronic coupling established between the incorporated species and the host lattice. Strongly coupled incorporation environments, exemplified by Ni, fundamentally reshape charge-carrier dynamics and suppress recombination, whereas weakly coupled oxide-like environments yield only marginal benefits. This work provides new mechanistic insight into how local coordination chemistry governs photoelectrode function and establishes a general design principle for engineering next-generation oxide photoanodes for solar fuel production.