Surface Modification of CoO_x Loaded BiVO₄ Photoanodes with Ultrathin p-Type NiO Layers for Improved Solar Water Oxidation

Abstract

Photoelectrochemical (PEC) devices that use semiconductors to absorb solar light for water splitting offer a promising way toward the future scalable production of renewable hydrogen fuels. However, the charge recombination in the photoanode/electrolyte (solid/liquid) junction is a major energy loss and hampers the PEC performance from being efficient. Here, we show that this problem is addressed by the conformal deposition of an ultrathin p-type NiO layer on the photoanode to create a buried p/n junction as well as to reduce the charge recombination at the surface trapping states for the enlarged surface band bending. Further, the in situ formed hydroxyl-rich and hydroxyl-ion-permeable NiOOH enables the dual catalysts of CoO(x) and NiOOH for the improved water oxidation activity. Compared to the CoO(x) loaded BiVO4 (CoO(x)/BiVO4) photoanode, the ∼6 nm NiO deposited NiO/CoO(x)/BiVO4 photoanode triples the photocurrent density at 0.6 V(RHE) under AM 1.5G illumination and enables a 1.5% half-cell solar-to-hydrogen efficiency. Stoichiometric oxygen and hydrogen are generated with Faraday efficiency of unity over 12 h. This strategy could be applied to other narrow band gap semiconducting photoanodes toward the low-cost solar fuel generation devices.

Recent Advances in the BiVO4 Photocatalyst for Sun-Driven Water Oxidation: Top-Performing Photoanodes and Scale-Up Challenges

“…Zhong et al [52], in a different work, introduced CoO x as an OEC; it was found to be responsible for an increase in the photocurrent density of bare BiVO 4 from about 1 mA/cm 2 to about 1.5 mA/cm 2 at 1.2 V vs. RHE, when operating at neutral pH in a phosphate electrolyte. The same authors then used the dual OECs NiOOH (deposited in situ)/CoO x with an ultrathin ALD-deposited NiO layer on top and added them to a BiVO 4 /Ti layer.…”

Photoelectrode for water splitting: Materials, fabrication and characterization

“…7b). This method is easy to control and repeat, and many exciting results have been reported based on it [36,68].…”

Scale‐Up of BiVO4 Photoanode for Water Splitting in a Photoelectrochemical Cell: Issues and Challenges

“…Recently,K im and Choi investigatedaZnFe 2 O 4 layer to enhance the stability as well as photoelectrochemical performance of BiVO 4 in basic electrolyte. [47] Thei nitial photocurrent density at 1.23 Vv ersus RHEi ncreased to 2.74 mA cm À2 in KPi (PH 13) electrolyte, but it decreased to less than 2.0 mA cm À2 in less than 1h.Z hong et al reported an impressive long-term stability of their CoO x -loaded BiVO 4 photoanode with ultrathin NiO layer, [48] which showedo nly al ittle decay in performanceo ver 16 hb ut at relatively low applied bias of only 0.8 Vv ersus RHE. Compared with other semiconductors such as TiO 2 ,B iVO 4 is a more efficient photon absorber but is more vulnerable for photocorrosion.…”