Microscopic Visualization of NiFeOₓ and NiOₓ Cocatalyst Effects on Charge Carrier Dynamics of BiVO₄ Photoanodes

Abstract

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 NiFeOₓ and NiOₓ 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 largely unchanged (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 of the trapped-carrier activity between the cocatalysts: NiFeOₓ introduces spatially localized slow-decay hole and electron components that correspond to reactive carrier populations, while NiOₓ generates a more uniform distribution of reactive domains with increased spatial coverage and prolonged carrier lifetimes. These microscopic differences correlate with macroscopic performance, with NiOₓ achieving the highest η inj and photocurrent (1.98 mA cm⁻²). The combined results demonstrate that NiFeOₓ locally enhances catalytic activity at selective surface regions, whereas NiOₓ promotes more homogeneous interfacial charge extraction across the BiVO₄ surface.