Design And Preparation Of WO₃ Based Photoanodes And Their Photoelectrocatalytic Performance

Utilizing the solar energy and converting it into hydrogen through photoelectrochemical（PEC）reaction has been considered as one of the efficient ways to partially replace fossil fuels and solve the energy problem.However,the development of efficient,stable and cheap photoelectrode faces many challenges,especially under the acidic electrolyte environments,where the hydrogen evolution reaction and the practical industrial production benefit.Therefore,it is the key to design and prepare a stable and efficient photocatalytic system under strong acid conditions.This is because most semiconductor materials and no-noble cocatalysts cannot stand owing to the dissolution under acidic conditions,thus the design and preparation of efficient and acid-stable photoanodes（anodic oxidation reaction is the decisive step to prepare hydrogen by water oxidation）should be crucial.The purpose of this thesis is to improve the catalytic stability and product selectivity based on WO₃ photoanodes.In the process of continuously optimizing WO₃ by constructing heterojunction and introducing Co²⁺cocatalyst,this work find that the generation and adsorption of a large number of hydroxyl radicals（^·OH）on the surface of WO₃ electrode is the main course to affect the selectivity and stability of photoanodes.Based on this discovery,the production of hydroxyl radicals（^·OH）was successfully suppressed by simply adjusting the surface structure of pure WO₃photoanode,and finally better performance was achieved,which provided the possibility for practical application.In addition,the introduction of kinetic advantages CER and corresponding cocatalyst Ag⁺,the solar energy utilization rate and catalytic stability are further improved.The catalytic process and mechanism of PEC have been revealed by means of a series of in-situ and ex-situ physical and chemical characterizations.This study can be helpful to address the instability and low selectivity of photoelectrochemical oxidation in strong acidic or neutral conditions and enhance the economic benefits and energy efficiency of the photoelectrocatalytic hydrogen production system.The photoelectrochemical reaction process and mechanism were explored through a series of physical and chemical characterization methods.The main research results are as follows:（1）By coating a thin layer of SnO₂ on WO₃ for the formation of a heterostructure,it significantly improves the separation efficiency of WO₃ carrier in the bulk,the injection efficiency at the electrode/electrolyte interface and the photocatalytic stability.The WO₃/SnO₂ leads to the rearrangement of electrons owing to different Fermi levels,forming a built-in electric field.This structure improves the separation efficiency more than twice relative to bare WO₃ and also further promotes the bending of the energy band of WO₃ to a certain extent so as to inhibit the backward transfer of electrons from conduction band to the surface for the recombination.In addition,the SnO₂ layer suppressed the generation of^·OH,which may be closely related to the stability of WO₃Finally,the PEC OER efficiency of the photoanode was improved under acidic conditions.（2）Based on the WO₃/SnO₂ photoanode,through introducing Co²⁺ions（pH=0.3）into the electrolyte,the photogenerated holes are captured by Co²⁺that has been oxidized to Co³⁺,thus inhibiting the generation of^·OH radicals.The highly active Co³⁺can oxidize water into O₂ and the Co³⁺itself can be reduced to Co²⁺.Under illumination while in the strong acid,the conversion between Co²⁺and Co³⁺is fast,mediating the surface charge transfer and inhibiting the generation of^·OH radicals.The Faraday efficiency of oxygen has been increased from 40%to 95%,and the catalytic stability has been enhanced from¹ h to 25 h.This chapter successfully developed a redox mediator as a catalyst for water oxidation.（3）By regulating the crystal orientation of bare WO₃,the exposed crystal plane can be indirectly controlled.The water oxidation on different surfaces produces different intermediates or experiences different reaction pathways,thus through controlling the surface,it allows to improve the selectivity of oxygen and the stability of photoanodes.In this chapter,relative to{200},WO₃ photoanodes with{021}crystal plane orientation synthesized through thermal treatments inhibit the adsorption of^·OH radicals.Finally,the catalytic stability of WO₃ photoanode with{021}crystal plane orientation has been improved from 1 h to 36 h,and its Faraday efficiency for O₂ has been improved from 55%to 95%.This study provides insights into the design of acid-stable PEC systems.（4）Based on the aforementioned chapters,this chapter extended the application by constructing Sn-doped Fe₂O₃/WO₃ structures to increase light absorption and introducing the kinetically advantageous CER to further improve the solar energy utilization efficiency.In 0.5 M NaCl（pH=1）electrolyte,CER is more kinetics favorbale process over OER,the photocurrent density has been significantly improved.Finally,the Sn-doped Fe₂O₃/WO₃ photoanode can be stabilized for at least 100 h in the acidic electrolyte.In addition,the Faradaic efficiency of the CER on the photoanode was increased from⁶8%to⁹3%by introducing Ag⁺as a co-catalyst.Overall,using a highly efficient composite photoanode and replacing OER with CER significantly improved the photogenerated current and simultaneously converted Cl^-into high-valued Cl₂,enhancing the photoelectrochemical conversion efficiency and economic benefits.