Preparation Of Ternary Bismuth Vanadate-Based Photoanode For Photoelectrochemical Water Splitting

With the increasing pressure of environmental issues such as climate change,water pollution,air pollution and depletion and destruction of the ozone layer,it is necessary to explore more environmentally friendly and efficient energy sources to mitigate the harm to the environment and human beings.Hydrogen has great potential as an efficient renewable energy source due to its high energy density,clean and non-polluting properties.Photoelectrochemical water splitting can directly convert the inexhaustible solar energy into chemical energy in the form of hydrogen as one of the green and environmentally friendly technologies to solve the future environmental pollution and energy crisis.Various n-type semiconductor materials have been widely used as photoanodes,including Ti O₂,BiVO₄,WO₃,Zn O andα-Fe₂O₃.Among them,BiVO₄ is considered as a promising photoanode due to its excellent visible light absorption ability,suitable redox potential and good chemical stability.However,BiVO₄suffers from the problems of rapid charge carrieres recombination and sluggish interfacial water oxidation kinetics,which limit its wide application in the field of PEC water splitting.To solve these problems,the water oxidation cocatalysts loaded on the surface of BiVO₄ can effectively reduce the overpotential of water oxidation reaction and enhance the water oxidation kinetics.However,the inefficient charge transfer and separation between BiVO₄ photoanode and water oxidation cocatalyst interface has been ignored in most cases.The PEC water splitting performance is hindered by the interfacial carrier recombination rate and not only by the water oxidation reaction kinetics.Herein,this thesis introduces an intermediate layer between the interface of BiVO₄photoanode and water oxidation cocatalyst by a facile synthetic method to reduce the interfacial resistance,thus effectively promoting the effective separation of photogenerated charges.Ultimately,it provides some enlightenment into the design of new ternary high-efficiency photoanodes,which is of great significance to solve the problems of environmental pollution and energy crisis.The main research contents and conclusions of this thesis are as follows:1.MgO was loaded on the BiVO₄ surface by electrodeposition method,followed by further loading of the Co-Pi water oxidation cocatalyst layer by photoassisted electrodeposition method to construct a layer-stacked Co-Pi/MgO/BiVO₄ photoanode for PEC water splitting.The optimal Co-Pi/MgO/BiVO₄ photoanode showed that the current density reached 4.43 m A cm^-2at 1.23 V vs.RHE under simulated sunlight irradiation,which is nearly 282%stronger than the BiVO₄ film.Moreover,the hydrogen production performance of Co-Pi/MgO/BiVO₄photoanode reaches 81.20μmol cm^-2 within 1 h,which is much higher than that of Co-Pi/BiVO₄and BiVO₄ photoanodes.To verify the universality and pervasiveness of the MgO interlayer,the prepared Co-Si/MgO/BiVO₄,Co-Bi/MgO/BiVO₄ and Fe OOH/MgO/BiVO₄ photoanodes also exhibit similar PEC performance.Systematic studies show that MgO can act as an effective shuttle medium that greatly facilitates the transfer of electrons and holes between Co-Pi and BiVO₄,while Co-Pi acts as a water oxidation cocatalysts layer that accelerates the injection of holes from the Co-Pi/MgO/BiVO₄ photoanode into the electrolyte solution to participate in the water oxidation reaction.2.A novel ternary Ni Co₂O₄/MgO/BiVO₄ photoanode was developed and designed by a simple method for the PEC water splitting.It was found that the MgO interlayer plays a key role in the carrier transport.When the deposition time of MgO is 20s and the load voltage is-0.7 V vs.Ag/Ag Cl,the Ni Co₂O₄/MgO/BiVO₄ composite electrode achieves the excellent photocurrent density of 4.6 m A cm^-2 at 1.23V vs.RHE,which is 1.5 and 2.9 times higher than that of the Ni Co₂O₄/BiVO₄ and BiVO₄ photoanodes,respectively.The PEC water splitting experiments showed that the H₂ and O₂ yields of Ni Co₂O₄/MgO/BiVO₄ photoanode after 180min were 201μmol cm^-2 and 102μmol cm^-2,respectively,which significantly improved the PEC hydrogen production activity of BiVO₄.Density functional theory calculations showed that electrons were mainly transferred from the MgO interlayer to the BiVO₄ photoanode,which effectively suppressed the charge recombination between the interface of Ni Co₂O₄ cocatalyst and BiVO₄ photoanode.The experimental results and conclusions in this chapter correspond to those in the previous chapter.3.A new CoFe_1.5Cr_0.5S₃O/COFs/BiVO₄ photoanode was constructed by introducing an intermediate layer of COFs between BiVO₄ and CoFe_1.5Cr_0.5S₃O cocatalyst.The experimental results show that the CoFe_1.5Cr_0.5S₃O/COFs/BiVO₄ photoanode induces a shift of the BiVO₄light absorption edge from 500 nm to 550 nm and the light harvesting efficiency is further enhanced,thus improving the utilization of visible light.The CoFe_1.5Cr_0.5S₃O/COFs/BiVO₄photoanode obtained excellent water oxidation photocurrent of 5.1 m A cm^-2 at 1.23 V vs.RHE,which is higher than that of pristine BiVO₄,binary COFs/BiVO₄ and CFCOS/BiVO₄photoanodes.After 180 min,the H₂ and O₂ yields of the CFCOS/COFs/BiVO₄ photoanode were 234.30μmol cm^-2 and 117.15μmol cm^-2,respectively,which were approximately 19 times higher than the H₂ and O₂ yields of the pure BiVO₄ electrode.This work provides theoretical guidance for the design of BiVO₄-based photoanodes with cocatalysts and interlayers,which can simultaneously optimize the surface dynamics,improve the charge carrier utilization efficiency and achieve efficient solar-to-fuel conversion.4.A novel Ni Fe Ox/Al₂O₃/BiVO₄ photoanodes were prepared by chemical bath deposition and photoassisted electrodeposition method.It was found that the photocurrent density of Ni Fe Ox/Al₂O₃/BiVO₄ photoanode was 5.87 m A cm^-2 at 1.23 V vs.RHE,which was close to the maximum theoretical photocurrent density of 7.5 m A cm^-2 of BiVO₄ photoanode and significantly higher than that of the pristine BiVO₄,Al₂O₃/BiVO₄and Ni Fe Ox/BiVO₄ films.After 120 min,the gas chromatography showed that the H₂ and O₂ yields of Ni Fe Ox/Al₂O₃/BiVO₄ photoanode were 222.64μmol cm^-2 and 110.31μmol cm^-2.Moreover,the photocurrent density of Ni Fe Ox/Al₂O₃/BiVO₄ photoanode was higher and more stable than that of BiVO₄ photoanode after 120 min test,indicating that the PEC water splitting performance and stability of BiVO₄ photoanode were enhanced.Benefiting from the synergistic effect of Ni Fe Ox cocatalyst layer and Al₂O₃,the Ni Fe Ox/Al₂O₃/BiVO₄ photoanode has an efficient and stable PEC water splitting performance.