Investigations On The Synthesis Of Cobalt-based Complexes And Their Photocatalytic Water Oxidation Properties

Hydrogen energy,as a clean and efficient renewable energy source,is expected to become an alternative to traditional fossil fuels and is therefore attracting attention.Solar energy has the advantages of clean,safe and large reserves,is the most important renewable energy source,and is also the ultimate source of energy for the earth.The development of hydrogen energy from renewable solar energy is one of the ideal ways of energy utilization.Using the most abundant water resources on the earth as raw materials,the use of solar energy to achieve water decomposition to produce hydrogen and oxygen through artificial photosynthesis process has attracted widespread attention around the world.The water decomposition reaction consists of water oxidation and proton reduction.Among them,the water oxidation reaction is a process involving four electrons transfer,and requires high activation energy,so it is also considered to be the bottleneck of the water decomposition reaction.Previous studies focused more on the development of photocatalysts for water oxidation,while less on charge transfer carriers for water oxidation reactions.This paper focuses on cobalt-based organoic complexes with structural-tunable,systematically studies the relationship between complex structure modulation and photocatalytic water oxidation performance,and obtains the following research results:（1）We synthesized a variety of cobalt-based complexes by direct synthesis and indirect synthesis,including[Co（bpy₂Cl₂）]Cl,Co（bpy₃）Cl₃,Co（bpy₃）（Cl O₄）₃,[Co（phen₂Cl₂）Cl,Co（phen₃）Cl₃ and Co（phen₃）（Cl O₄）₃,etc.,verified the purity of their synthesis by mass spectrometry,and explored their electrochemical properties through cyclic voltammetry tests.The results show that the indirect synthesis method yields a purer product than the direct synthesis method,and the six cobalt fund complexes synthesized could be stable in aqueous solution.The results of cyclic voltammetry test showed that the electrochemical redox potential of the six cobalt-based complexes was between 0.320 and 0.390 V vs.NHE,and the redox potential changed according to the different configurations and ligand types,while the redox potential of the anion did not change significantly when it was converted from Cl^-to Cl O₄^-.（2）Taking the synthetic cobalt-based complex as the charge transfer carrier and typical water oxidation photocatalysts such as Bi VO₄ and WO₃ as the research objects,we systematically studied the photocatalytic water oxidation performance under the condition of the presence of cobalt-based complexes.It is found that the cobalt-based complex exhibits good water oxidation performance in multiple photocatalyst systems,which proves its feasibility as a charge transfer carrier.Under the same photocatalyst conditions,the performance of the[Co（bpy₂Cl₂）Cl in photocatalytic water oxidation under the condition of the six synthesized cobalt-based organic complexes as charge transfer carriers was the best,and the photocatalytic water oxidation performance of the Bi VO₄ catalyst was the best when the same cobalt-based organic complex was used as the charge transfer carrier.Further research shows that a certain amount of Pt-modified Bi VO₄ catalyst can improve the performance of cobalt organic complexes in photocatalytic water oxidation,and can effectively inhibit the reversible reaction between cobalt-based complex ion pairs.In addition,with[Co（bpy₂Cl₂）]Cl as the charge transfer carrier,Pt/Bi VO₄ as the oxygen-producing photocatalyst,and Ru/Sr Ti O₃:Rh as the hydrogen-producing photocatalyst,the Z-mechanism water splitting system,and the photocatalytic complete decomposition of aquatic hydrogen and oxygen production is realized,and the hydrogen-oxygen ratio is close to the stoichiometric ratio.The series of cobalt-based complexes developed in this paper are expected to be more widely used in photocatalytic water oxidation and Z-mechanism water splitting system.In summary,from the perspective of the synthesis of cobalt-based metal organic complexes and their use as charge transport carriers in the photocatalytic water oxidation energy storage reaction,six different cobalt-based metal complexes were prepared by controllable modulation.The influence of the ligands of the compound on its redox potential and its application in the photocatalytic water oxidation energy storage system.These explorations will provide references for the further development of novel metal complexes,photocatalytic water oxidation reaction mechanisms,and charge transport carriers for efficient Z-mechanism splitting water systems.