Structural Monitoring of NiB_i Modified BiVO₄ Photoanodes Using in Situ Soft and Hard X-ray Absorption Spectroscopies

Abstract

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

Operando Photo-Electrochemical Catalysts Synchrotron Studies

“…In a recent work by Xi et al [ 83 ], it was shown that PEC performance of BiVO 4 photoanodes can be enhanced by the deposition of a NiB i co-catalyst layer. The electronic structure of NiB i has been probed by in situ soft and hard XAS spectroscopy by monitoring the signal at Ni L and K edges and the O K edge under different potential and illumination conditions.…”

“…The advantages of this cell for the study of solids, liquids and solid/liquid interfaces under operando conditions were summarized by the authors as follows: (i) the possibility to measure XAS in transmission and fluorescence mode simultaneously, (ii) fast sample/membrane replacement (less than 5 min), (iii) reduced risk of beamline contamination with electrolyte solution due to two protection membranes and (iv) manipulation of the sample with visible light. This cell has been successfully used in a sequence of soft XAS operando studies of Ni- and Mn-modified BiVO4 photoanodes [ 83 , 101 ] and MnO [ 85 ] water oxidation catalysts.…”

The PEC performance of BiVO4 was enhanced by preparing the CoFeBi/BiVO4 photoanode using an ultrafast photoassisted electrodeposition method

“…25 Through these methods, the charge separation and transfer on both the surface and within the bulk of the photoanode can be effectively improved. In recent years, researchers have found that employing cocatalysts such as cobalt oxide (CoO x ), 26 cobalt borate (CoB i ), 27 cobalt phosphate (CoP i ), 28 iron oxyhydroxide (FeOOH), 29 iron oxyhydroxide/cobalt oxyhydroxide (FeOOH/CoOOH) 30 and nickel borate (NiB i ) 31,32 is an effective method to modify the performance of photoanodes for enhanced photocatalysis. Studies by Lifei Xi et al 33 demonstrated that cobalt borate (CoB i ) films exhibited higher activity than CoB i with increasing film thickness, ultimately displaying superior performance.…”