Xi, L.; Schellenberger, M.; Praeg, R.F.; Gao, D.; Drevon, D.; Plate, P.; Bogdanoff, P.; van de Krol, R.; Lange, K.: Structural Monitoring of NiBi Modified BiVO4 photoanodes Using in Situ Soft and Hard X-ray Absorption Spectroscopies. ACS Applied Energy Materials 2 (2019), p. 4126-4134
10.1021/acsaem.9b00304
Open Accesn Version

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 H2 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 BiVO4 photoanodes can be enhanced by the deposition of a nickel−borate co-catalyst layer (NiBi). We investigate the electronic structure of the NiBi 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 3dt2g 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 NiBi film is readily oxidized to higher oxidation states. This in situ soft XAS study offers the first direct observation of Ni4+ formation during solar water oxidation. Cyclic voltammetry−XAS (CV-XAS) results support that the formation of Ni4+ 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.