The Study Of Electronic State Regulation And Oxygen Evolution For Cobalt-based Spinel Oxide Composite Catalysts

Hydrogen is expected to beome a new energy to replace traditional fossil energy due to its high thermal efficiency and cleaning product.Up to now,electrochemical water splitting is one of the most effective methods for hydrogen production,which consists of hydrogen evolution reaction and oxygen evolution reaction.Compared with hydrogen evolution reaction with two electron transfer process,oxygen evolution reaction with four electron transfer greatly limits the reaction rate due to its slowly kinetic mechanism.Therefore,the preparation of efficient and stable oxygen evolution catalysts to promote electrochemical water splitting is a hot topic in the field of energy and environment fields.Electronic state regulation is the key to the material performance optimization.The heterogeneous catalytic reaction occurs on the surface of the catalyst.In the electrocatalytic oxygen evolution reaction,the electronic state of material determines its catalytic activity through direct interaction with the reaction intermediates.The construction of composite phase is one of the effective measures to regulate the electronic state.The composite structure can not only combine the advantages of each component,but also induce the charge rearrangement around the active center through the synergy effect,strain effect and electron interaction.Cobalt-based spinel oxide has a variety of characteristicsis with flexible structure and low cost.At the same time,cobalt ion has abundant of d electron configuration,therefore it is widely used as a model system to investigate the structure-activity relationship between its electronic state and oxygen evolution performance.Consequently in this paper,a varity of cobaltbased spinel oxide composites were prepared and constructed by changing the reaction conditionsto optimize the electronic state which effectively improved the catalytic oxygen evolution performance of the original cobalt-based spinel.Specific works as shows follows:1.The cobalt-based spinel oxide ZnCo2O4(ZCO)systhesized by hydrothermal calcination was treated with reduction of NaBH4 to form CoO/ZnCo2O4 composites along with CoO in situ growing on the surface of spinel oxide.Experimental results demenstrate that the content of CoO phase in the composites gradually increased with the increase of NaBH4 concentration.However,excessive concentration of reducting agent will lead to the complete destruction of spinel phase with ZnCo2O4.After moderate reduction,the CoO/ZnCo2O4 composite with the highest ratio of CoO exhibites an overpotential of 351 mV,it is lower than the original phase about 68 mV The improvement of the performance of the composite catalyst is mainly attributed to the optimization of the 3d electronic configuration of cobalt ions at the octahedral site,resulting in the production of Co2+ which has a better interaction with oxygen intermediates.In addition,the strong interaction caused by in situ composite also promotes more charge transfer at the heterogeneous interface which is benefit to catalytic reaction.2.The oxygen evolution performance of catalyst is highly related to the covalency between metal and oxygen.In electrocatalytic oxygen evolution materials that were designed before,different cations are usually introduced to achieve the regulation of material covalency,relatively speaking,anionic regulation has received little attention.In this work,CoFe-LDH/Co3O4 was prepared on the surface though liquid phase reduction treating Co3O4,and then(Co,Fe)S2/Co3O4 were obtained by further treatment with sulfuration.The introduction of sulfur ions in composite materials enhances the covalency between metal and oxygen due to the effect of electron redistribution.The O-K edge test of synchotron radiation manifests that the orbital hybridization between metal and oxygen ions is enhanced after sulfuration.In addition,the density functional theory is used to calculate the partial state densities of related ions,which found that the charge transfer energy of metal 3d level and O 2p level decreased after sulfuration process,And the above results all prove that the covalency of material is improved.Finally,(Co,Fe)S2/Co3O4 is found to have the best activity with the overpotential of only 296 mV at the current density of 10 mA·cm-2.This work optimizes the covalency of metal and oxygen through the introduction of sulfur anions and provides a new perspective for the preparation of efficient electrocatalysts.