The Study On Electrocatalytic Oxygen Evolution Reaction Properties Of Heteroatom-doped Co₉S₈

With the rapid development of society and economy and the increasing demand for fossil fuels,energy crisis has attracted widespread attention from people all over the world.However,traditional fossil fuels are non-renewable primary resources and their content is limited.Therefore,the development and utilization of renewable,cleaning and non-polluting new energy sources to replace fossil energy are crucial to the environment sustainable development.Hydrogen energy is a secondary pollution-free clean energy which can be recycled.It has become the appropriate substitute for traditional fossil fuels due to its high energy density,cleanliness,pollution-free and abundant reserves.Hydrogen production by electrolysis of water is one of the most promising hydrogen production technologies due to the advantages of readily available raw materials,stable outcome and large-scale production.However,the large overpotential of the two half-reactions（the hydrogen evolution reaction,HER and the oxygen evolution reaction,OER）in the water electrolysis system largely limits its practical application,especially the OER reaction involving complex four-electron transfer process.Noble metal catalysts（such as Ru O₂and Ir O₂）are currently recognized as the most excellent OER catalysts,but their low reserves and high cost limit the large-scale commercial application.Therefore,the development of efficient and cost-effective non-noble metal based electrocatalysts is crucial for the development of the hydrogen economy.Numerous studies have shown that transition metal sulfides（especially Co₉S₈）with unique crystal structures and energy band characteristics exhibit catalytic activities comparae with noble metal catalysts.In addition,after regulating the composition and nanostructure of the electrode material,the electronic structure、electron transport rate and specific surface area can be can be also effectively improving.In this work,aiming at improving the electrocatalytic activity and stability of the catalyst,by designing the composition and microstructure of the material,a series of efficient and stable cobalt-based sulfide electrode materials were developed and constructed,which were applied to the oxygen evolution reaction.The main research contents of this work are as follows:（1）In this paper,the composition of Co₉S₈ was optimized by adjusting the doping amount of metallic element Fe,and a(Co_1-xFe_x)₉S₈ catalyst with a unique hollow spherical structure was prepared.DFT theoretical calculations and experimental results show that the Fe heteroatom pair effectively modulates the crystal structure and electronic structure of the intrinsic material Co₉S₈,and at the same time effectively alleviates the structural change of Co₉S₈ during the electrochemical process.In addition,the unique hollow sphere structure can greatly increase the contact area between the electrode material and the electrolyte.The small particle size and the rough outer surface of catalyst,which is beneficial to expose the catalytic active sites and promote the transfer charges to achieve high-efficiency electrocatalytic activity.By adjusting the doping amount of Fe,the obtained optimal catalyst CFS-0.08 can reach a current density of 10 m A cm^-2 with only an overpotential of 268 m V in an alkaline solution of1.0 M KOH,which can be comparable to Ru O₂（262 m V）.The CFS-0.08 sample also exhibits a Tafel slope close to that of commercial Ru O₂(CFS-0.08 has a Tafel of 63.9m V dec^-1,the Tafel value of Ru O₂ is 60.8 m V dec^-1).More importantly,after introducing an appropriate amount of Fe element,the stability of Co₉S₈ is greatly enhanced and can be maintained for up to 60 h.The synergistic strategy of heteroatom doping and morphology regulation in this work provides a broad prospect for the development of novel cobalt-based sulfides for efficient energy electrocatalysis fields.（2）Based on above workd of metal elements doping on Co₉S₈-based catalysts,we also investigated the effects of non-metallic elements doping on it.Among many non-metallic elements,Se is rich in content,has strong electronegativity Se element locates at the same main group with S element in the periodic table of elements.Relevant studies have shown that Se can greatly enhance the catalytic activity and stability of electrode materials in catalytic reactions.In this part,we mainly explored the control of the electronic structure of Co₉S₈ by doping the non-metallic element Se.At the same time,carbon cloth was used as the substrate material to synthesize the carbon cloth-supported Co₉S_8-xSe_x catalyst with good dispersibility.The XPS results show that the doping of Se regulates the electronic structure of Co and S.More importantly,the ratio of Co²⁺/Co³⁺in Co₉S_8-xSe_x rises with the increase of the doping amount of Se.According to the previous literatures,a higher amount of Co²⁺is beneficial to the catalytic activity of electrode materials.By adjusting the doping amount of Se,the representative sample CSSe-1.09 exhibits excellent OER performance with an overpotential as low as 271 m V at the current density of 10 m A cm^-2.Moreover,after120 h long-term galvanostatic stability test,the electrochemical performance did not decrease significantly,indicating the persistence application potential in actual industrial production of our catalysts.Density functional theory（DFT）calculations indicate that the high catalytic activity of CSSe-1.09 electrode is mainly attributable to the greatly reduced energy barrier for the rate-determining step（the process of generating*OOH from*O）of the oxygen evolution reaction during the doping of Se,while optimizing the binding energy of intermediate species to the active sites.