In-situ X-ray absorption spectroscopy (XAS) at the oxygen K-edge was used to investigate the role of oxygen during the oxygen evolution reaction (OER) in an electrodeposited Ni-Fe(OxHy) electrocatalyst in alkaline pH. We show the rise of a pre-peak feature at 529 eV in the O K-edge spectra, correlated to the appearance of a shoulder at the Ni L3-edge and formation of oxidized Ni3+/4+-O. Then, for the first time, we track the spectral changes in a dynamic fashion in both the soft and hard X-ray regimes during cyclic voltammetry (in situ CV-XAS) to obtain a fine-tuned resolution of the potential-related changes. The pre-peak feature at the O K-edge likely signifies formation of an electron deficient oxygen site. The electrophilic oxygen species appears and disappears reversibly in correlation with the Ni2+ ↔ Ni3+/4+ process, and persists during OER catalysis as long the metal is oxidized. Our study provides new insight into OER electrocatalysis: Before onset of the O-O bond formation step, the catalytic oxyhydroxide has accumulated electron deficiencies by both, oxidation of transition metal ions and formation of partially oxidized oxygen sites.

Solar water splitting is a potentially scalable method to store solar energy in the form of renewable hydrogen gas. In this study, we demonstrate that the photoelectrochemical (PEC) performance of hematite photoanodes can be improved by modification with the oxygen evolution catalyst CoB. The current density at 1.23 V of the pristine hematite under one sun is 0.88 mA cm and it increases to 1.12 mA cm after CoB modification (∼27% improvement). The presence of a CoB cocatalayst layer is proposed to improve the oxygen evolution reaction (OER) kinetics and also to prevent electron-hole recombination at the surface via passivating surface defects as well as suppressing the tunneling of electrons from the hematite core, thus improving the photocurrents and resulting in a negative shift of photocurrent onset potentials. These effects of CoB modification are supported by experimental data obtained by performing electrochemical impedance spectroscopy (EIS), PEC and incident photon-to-current efficiency (IPCE) measurements. To investigate the electronic structure of the CoB cocatalyst deposited on hematite, XPS and in situ X-ray absorption spectroscopy (XAS) are employed. Co K-edge spectra at different potentials and light conditions are recorded. This makes the present work different from most of the previous studies. Using a quantitative analysis method, information on the mean oxidation state of Co in the CoB film under applied potential and illumination is revealed. We also compare different methods for determining the oxidation state from the edge position and find that the integral method and half height methods are most suitable. In summary, the present work underlines the improvement of the semiconductor/cocatalyst interface of oxygen evolving photoanodes and strengthens the importance of in situ XAS spectroscopy when studying catalysts. This study is the first report so far combining the studies of the PEC performance of a CoB modified hematite nanorod array photoanode and in situ XAS at the Co K-edge.

Photoelectrochemical (PEC) water splitting, a process using solar light and a semiconductor to split water, is proposed as a potentially scalable method to store solar energy through renewable H 2 fuels. Obtaining the electronic structure information on co-catalyst is a crucial step toward gaining a mechanistic understanding of the water oxidation reaction of this catalyst. In the present work, we show that the PEC performance of BiVO 4 photoanodes can be enhanced by the deposition of a nickel−borate co-catalyst layer (NiB i ). We investigate the electronic structure of the NiB i by in situ soft and hard X-ray absorption spectroscopies (XAS) at the Ni L-and K-edges as well as at the O Kedge under different potential and illumination conditions. We discuss the involvement of the active oxygen species related to the hybridized O 2p Ni 3dt 2g orbitals in the oxygen evolution reaction (OER) and further correlate the changes at the O K-edge with that of at the Ni L-edge. In situ soft XAS measurements show that Ni in the electrodeposited amorphous NiB i film is readily oxidized to higher oxidation states. This in situ soft XAS study offers the first direct observation of Ni 4+ formation during solar water oxidation. Cyclic voltammetry−XAS (CV-XAS) results support that the formation of Ni 4+ is prior to the formation of partly electron deficient oxygen sites. This study also provides understanding about the physical and chemical changes under potential and light illumination and represents a significant step toward obtaining a mechanistic understanding of the co-catalyst/semiconductor system. KEYWORDS: soft XAS, hard XAS, in situ, BiVO 4 , nickel borate (NiB i ), photoanode