In Situ Activation of Amorphous NiFeMo Oxide Cocatalyst To Improve the Photoelectrochemical Water Splitting Performance of BiVO₄

Abstract

Amorphous NiFeMo oxides (a-NiFeMo) synthesized via a simple supersaturated coprecipitation method are explored as a cocatalyst to improve the photoelectrochemical water splitting performance of BiVO 4 . The performance of the composite photoelectrode is found to be further enhanced through a cyclic-voltammetry-related in situ activation process, and a photocurrent of 5.0 mA/cm 2 is achieved at 1.23 V RHE with improved photoelectrochemical stability. X-ray photoelectron spectroscopy (XPS), Raman, and electron spin resonance (ESR) characterization results indicate that the surface-loaded a-NiFeMo cocatalyst undergoes a rapid surface reconstruction during the in situ activation process, which results in an incorporation of phosphate ions in the surface-loaded a-NiFeMo, along with an increased oxidation state of Ni ions and enriched oxygen vacancies. These combined effects lead to an improved oxygen evolution reaction performance and finally result in a reduced charge-transfer resistance at the solid/liquid interface, causing the interfacial charge-injection efficiency to be enormously enhanced from the original 25.1% in pure BiVO 4 to 83.3% in BiVO 4 /a-NiFeMo after being in situ activated. Our result indicates that the rapid surface-reconstruction phenomenon shown in the amorphous materials may provide a promising strategy for designing a highly efficient cocatalyst for photoelectrodes.

Enhanced Charge Carrier Separation in WO3/BiVO4 Photoanodes Achieved via Light Absorption in the BiVO4 Layer

“…Bismuth vanadate, BiVO 4 , is a promising semiconductor oxide employed in photoanodes for the oxygen evolution reaction in water-splitting devices. , Its stability in contact with aqueous electrolytes, , its good visible light-harvesting capability, and its simple preparation through cheap wet techniques point to this material as a possible component of future commercial photoelectrochemical (PEC) cells. Furthermore, in the last 15 years the efficiency of BiVO 4 -based photoanodes (in terms of current density) rapidly grew from a few microamps per square centimeter in early reports to 4–5 mA cm –2 , with prolonged continuous operation of photoelectrodes modified with oxygen evolution cocatalysts. ,,− However, the fast charge recombination of BiVO 4 -based electrodes still hampers the efficiency of this material. , …”

Advanced oxygen evolution reaction catalysts for solar-driven photoelectrochemical water splitting

“…The photocurrent density of pristine BiVO 4 could be improved by up to 5.0 mA cm −2 at 1.23 V vs. RHE with an excellent long-term PEC stability. 80 In addition, Wang and co-workers demonstrated that the self-transformation of a precursor metal–organic framework could produce an oxygen defect enriched ultrathin Co(O)OH catalyst for improving the PEC performances of BiVO 4 photoanodes. As shown in Fig.…”

Cobalt-Modified Amorphous Nickel–Iron–Molybdate Cocatalysts to Enhance the Photoelectrochemical Water Splitting Performance of BiVO4 Photoanodes

“…The two vibrational peaks positioned at 344 and 937 cm −1 are corresponding to the bending mode and stretching mode of Mo−O in molybdate oxides, respectively, and a vibrational peak associated with NiFe−O was observed at 559 cm −1 . 27,29 Compared with the Raman characterization result of a-NiFeMo, 19,27 a-NiCoFeMo-0.03 had two additional vibrational peaks at 882.2 and 812.3 cm −1 , which may be attributed to the change in the coordination environment of Mo−O induced by the incorporation of Co. 30,31 Energy dispersive X-ray spectroscopy (EDX) revealed that Ni, Co, Fe, Mo, and O elements were uniformly distributed in a-NiCoFeMo-0.03, and no clear phase separation or element segregation can be observed (Figures 2c and S6). The results of XRD, Raman, and EDX indicate that amorphous nickel−cobalt−iron-blended molybdate oxides (a-NiCoFeMo) were successfully prepared by the supersaturated coprecipitation method when a low content of Co is incorporated.…”

“…On the other hand, surface modification with transition metal (Ni, Co, Fe)-based oxygen evolution cocatalysts can reduce the water oxidation barrier of BiVO 4 , , thereby improving the interface-related water oxidation kinetics. Actually, not only limited to BiVO 4 , a similar cocatalyst modification strategy was also demonstrated to effectively improve the PEC performance of other kinds of photoelectrodes, such as α-Fe 2 O 3 and CuInS 2 , − which further suggests that rational designing of a cocatalyst with functional properties is critical for achieving highly efficient photoelectrodes.…”

“…However, most of the oxygen evolution cocatalysts are studied in crystalline forms and little attention has been paid to their amorphous states. In a previous study, 19 we synthesized amorphous nickel−iron molybdenum oxide (a-NiFeMo) and adopted it as a cocatalyst for BiVO 4 . In contrast to the cocatalysts in crystalline forms, a rapid surface reconstruction process can be triggered in a-NiFeMo via a cyclic voltammetryrelated in situ activation process, which leads to an activated catalytic activity and results in an enhanced PEC performance in the composite BiVO 4 photoelectrode.…”