Microscopic visualization of NiFeO_x and NiO_x cocatalyst effects on charge carrier dynamics of BiVO₄ photoanodes

Improving the interfacial charge carrier dynamics of BiVO₄ photoanodes is essential for achieving higher efficiency in photoelectrochemical water oxidation. In this study, we investigated the effects of NiFeO_x and NiO_x cocatalysts on BiVO₄ by combining patterned-illumination time-resolved phase microscopy (PI-PM) and photoelectrochemical measurements, including impedance spectroscopy. Both cocatalysts substantially enhance the photocurrent and charge-injection efficiency (η_inj), whereas the bulk charge-separation efficiency (η_sep) remains less changed (within ±6%), indicating that the improvements originate from the interfacial hole transfer process. PI-PM visualizes the local trapped-carrier dynamics and reveals the differences in trapped-carrier activity between the cocatalysts. NiFeO_x introduces spatially localized slow-decay hole and electron components that correspond to reactive carrier populations, while NiO_x generates a more uniform distribution of reactive domains with increased spatial coverage and prolonged carrier lifetimes. These microscopic differences correlate with macroscopic performance, with NiO_x achieving the highest η_inj and photocurrent (1.98 mA cm^-2). The combined results demonstrate that NiFeO_x locally enhances the catalytic activity at selective surface regions, whereas NiO_x promotes more homogeneous interfacial charge extraction across the BiVO₄ surface.