Research On The Construction Of Polyoxometalate Composite Films And Photoelectrocatalytic Water Oxidation Performance

Fossil energy has been widely used in the world since the industrial revolution,providing convenient and continuous power for people’s daily life and travel.While fossil fuels provide convenience for our life,due to excessive carbon emissions,it has caused major climate change,which has to attract the attention of the world.The energy released within one hour from the solar radiation of the earth is even higher than the total energy consumed by the earth in one year,and no other renewable energy can match it.Therefore,it is a very promising way to use solar energy to provide energy for human daily life.For example,by photocatalytic decomposition of water into hydrogen and oxygen,in 1972,Fujishima and Honda,under the condition of Ti O2 as the electrode,first photoelectrocatalytic oxidation of water to hydrogen evolution.Subsequently,a large number of researches on photocatalytic water oxidation were carried out and made some progress.Simulated Photosynthesis System II（PSII）to help water oxidation is considered to be the most promising method at present.Polyoxometalates（POMs）are a class of polyoxometalates formed by the oxygen connection of Mo,W,V,Nb and other anions.Because of their strong electronic reception ability,they can capture photogenerated electrons in the conduction band and promote the rate of photogeneration and migration of photogenerated carriers.Many studies have shown that POMs have excellent photocatalytic performance,stability and selectivity It is considered as an excellent photocatalyst for water oxidation due to its low cost and other excellent characteristics.In this paper,three kinds of polyoxometalates containing different metals are selected as the media of charge collection and transmission to build a single component and composite photoanode to improve its photocatalytic water oxidation performance.The specific work is as follows:Firstly,a nickel-containing polyoxometalate[Ni25（H2O）2OH)18（CO3）2（PO4）6（Si W9O34）6]^50-（Ni25 POM）was synthesized.The polyoxometalate has good stability and e xhibits good catalytic effect and photoelectric response in the visible-light-driven wate r oxidation process.The modified electrode[PDDA/Ni25 POM]n/ITO was constructe d by layer-by-layer self-assembly method,and then the composite film modified electr ode PDDA/[Ni25 POM/（PDDA-r GO）]n/ITO was constructed with black inorganic se miconductor material PDDA-r GO.The composition and morphology of the film were characterized by IR,XPS and SEM.The photoelectrocatalytic water oxidation activity of the modified electrode was studied,and the water oxidation current of the two mod ified electrodes before and after illumination was compared.The photocurrent density of PDDA/[Ni25 POM/（PDDA-r GO）]6/ITO at 1.5 V potential is 2.1 m A cm^-2,which is3.5 times higher than that of[PDDA/Ni25 POM]6/ITO(0.6 m A cm^-2).Transient phot ocurrent response and electrochemical impedance test results show that the composite film modified electrode constructed by Ni25 POM and PDDA-r GO can promote charg e separation,effectively inhibit hole-electron recombination,and significantly improv e the water oxidation kinetics on the surface of the composite film.Secondly,Cobalt-containing polyoxometalate[{Co₄（OH）₃（PO₄）}₄(Si W₉O₃₄)₄]^32-（Co-Si POM）was synthesized.Bi VO4 photoanode was prepared by electrodeposition-metal organic degradation method.Then,Co-Si POM was co-constructed on the surface of Bi VO4 photoanode by layer-by-layer self-assembly method to prepare composite modified electrode[PDDA/Co-Si POM]n/Bi VO4/ITO.The successful assembly was proved by IR,XPS,UV-Vis spectroscopy and other characterization methods.The composite modified electrode exhibits excellent photoelectric catalytic activity,which can effectively reduce the surface recombination of carriers.Under the illumination of a solar energy-saving lamp and 1.5 V potential,the water oxidation current density reaches 3.37 m A cm^-2,and the incident photoelectric conversion efficiency reached76.31%.Compared with Bi VO4/ITO and[PDDA/Co-Si POM]5/ITO,the composite photoanode has higher photogenerated current per mole,smaller impedance and stronger transient photocurrent response,thus accelerating the water oxidation process.Thirdly,Fe-containing polyoxometalate[Fe₄^III（H₂O）₂(P₂W₁₅O₅₆)₂]^12-（Fe4 POM）was synthesized,and CuO materials were synthesized by hydrothermal method.Three composite films of[PDDA/Fe4 POM]n,[PDDA/（PSS/CuO）n]and[PDDA/（Fe4POM/CuO）n]were constructed on ITO by layer-by-layer self-assembly method.The composition of the films was characterized by XPS,and the growth of the films was monitored by UV-visible spectroscopy.By loading CuO material,the photocurrent density of[PDDA/（Fe4 POM/CuO）5]/ITO reached 2.57 m A cm^-2 at a potential of 1.5V,which was 6.95 times that of the modified electrode without CuO material(0.37m A cm^-2).The overpotential of catalytic water oxidation was significantly lower than that of the other two modified electrodes while the photocurrent increased,indicating that the composite significantly improved the current density and photoelectrocatalytic water oxidation performance of the photoanode material.