Study On Water Oxidation Performance Of Fe₂O₃ Photoelectrode Modified By Passivation Layer And Co-catalyst

With the development of modern society,the energy crisis and environmental pollution are the two major issues that hinders the improvement of human living standards.Therefore,developing new clean energy sources to replace fossil fuels and reduce emissions of harmful gases are still the challenge.As an ideal clean energy,hydrogen（H₂）has attracted widespread attention by virtue of its high energy density and environmental friendliness.Thus,how to efficiently produce H₂ has become a hot topic.Semiconductor-based photoelectrochemical（PEC）water splitting to generate H₂,as a technology that can capture and store solar energy,has aroused extensive research interest.The PEC water splitting process includes two half-reactions that are oxygen evolution reaction（OER）at the anode and hydrogen evolution reaction（HER）at the cathode.Compared with HER,the OER involves multi-electron transfer,which requires high overpotential for water oxidation due to the sluggish kinetics.Thus,the design of photoanode is of great importance.A good photoanode material should meet several requirements,such as suitable band gap,an energy band structure matching the water oxidation reaction,good electrical/photochemical stability,and non-toxic.Hematite（α-Fe₂O₃）is an anode material for PEC water oxidation with above advantages.However,α-Fe₂O₃ suffers shortcomings including the abundance of surface defects leading to serious charge recombination,slow oxidation kinetics,etc.,which results in low PEC efficiency.In this work,the couping of passivating surface state and introducing oxygen vacancies in Fe₂O₃ was adopted to improve the surface hole injection efficiency as well as the separation of electron-hole pairs,thereby improving the PEC efficiency.In addition,loading co-catalysts can increase the reaction active sites and water oxidation kinetics.Therefore,we loaded molecular co-catalysts on Fe₂O₃ photoanode to improve its oxidation kinetics.The specific contents of this paper are as follows:（1）A facile impregnation method with an annealing process was developed to synthesize the defect-rich Mo O_3-x/Fe₂O_3-x photoanode.As expected,the optimized Mo O_3-x/Fe₂O_3-x photoanode exhibits excellent PEC performance for water oxidation.Based on a series of characterization and PEC results,it is found that Mo O₃ as a passivation layer can effectively reduce the density of surface state of Fe₂O₃,thereby reducing the recombination of surface carriers,resulting in improved holes injecting efficiency into the electrolyte.Meanwhile,the introduction of oxygen vacancies not only can be used as electron capture center to improve the separation efficiency of electron-hole pairs,but also enhance the conductivity to promote electron transfer,further boosting the PEC water oxidation efficiency.（2）A hydrothermal method was conducted to successfully load the molecular catalyst of metal-tannic acid complex（M-TA,M=Fe,Ni）on the surface of Fe₂O₃photoanode,which aims at study their roles in influencing the PEC water oxidation.Compared with Fe₂O₃ and Fe-TA,the Ni-Fe-TA modified Fe₂O₃ photoanode showed a significantly increased photocurrent and negatively shifted onset potential.Based on the results of characterizations and PEC observations,it is understandable that when only Fe-TA is modified,the oxidation kinetics of the photoanode is inhibited,resulting in an increase in the recombination of electrons and holes.It is fascinating that an apparent negative shift of photocurrent onset potential is observed when Ni²⁺is further introduced into Fe-TA,proving the accelerated water oxidation kinetics.As a result,the separation of electron-hole pairs is highly promoted and thus improving the PEC water oxidation performance.