Synthesis Of Cobalt Sulfides And Application In Electrocatalytic Water Splitting

The development of social economy requires adequate energy supply,but the reserves of fossil fuels are limited,and its utilization will also lead to greenhouse effect and environmental pollution.Hydrogen energy shows its unique advantages,that is,high combustion calorific value,no pollution and no greenhouse gas.Splitting water to produce hydrogen is an ideal method,but there are still some technical barriers to be overcome.Its oxygen evolution reaction（OER）kinetics is slow,which hinders the whole water electrolysis process.This requires us to develop OER electrocatalyst materials that can reduce the energy barrier in the process of oxygen evolution reaction.At present,RuO₂ and IrO₂ are the most active catalysts for oxygen evolution reaction,but their high cost limits their large-scale application.Cobalt based sulfide is a good electrocatalytic water splitting catalyst because of its environmental friendliness,low price and high conductivity.However,there are many kinds of cobalt sulfides,such as Co₉S₈,CoS,Co₃S₄,CoS₂ and sulfur rich CoS_x.Due to the variable valence state,there are great differences in the surface properties of cobalt sulfur compounds.For example,as cobalt sulfur compounds change from cobalt rich to sulfur rich,the valence state of sulfur changes,and its physical and chemical properties also change.Whether the enrichment of sulfur or cobalt is conducive to electrochemical activity needs to be further verified by theory and experiment.Therefore,the preparation and research of cobalt sulfide,especially cobalt sulfide enriched in cobalt or sulfur,reflects its significance.This paper takes cobalt sulfide as the research object,and the specific contents are as follows:（1）CoS₂,Co₉S₈ and Co₃S₄ with uniform hollow nanostructures were synthesized by hydrothermal method and soft template method.The electrochemical properties of the prepared cobalt sulfide were tested.Among the three cobalt based sulfides,the order of electrochemical activity is CoS₂>Co₉S₈>Co₃S₄.When the current density of CoS₂ is 10m A·cm^-2,the overpotential can reach 310 m V It is inferred that the excellent electrochemical performance of CoS₂ is due to its sulfur rich nature,the existence of bridged S-S and more active CoS₆ octahedral coordination.The phenomenon that Co₉S₈ has better OER performance than Co₃S₄ confirms the role of Co-Co bond in Co₉S₈,which affects the electronic structure of adjacent sulfur atoms,increases conductivity and improves its electrochemical activity.In addition,we studied the performance of supercapacitor,another electrochemical application of these three cobalt sulfides,which also confirmed the above conclusion.Therefore,the work of this chapter can provide a reference for understanding and designing electrode materials for efficient application in the field of energy storage and water decomposition.（2）Using sodium polysulfide synthesized in the laboratory as the sulfur source and cobalt acetate as the cobalt source,a series of CoS_x sulfur rich cobalt sulfides with different cobalt sulfur feed ratios were synthesized by hydrothermal method without adding additional ligands and complexing agents,and the other group used hexamethylene tetramine as the ligands and complexing agents.In the process of testing and analysis,it is revealed that in the case of surplus polysulfide ions,cobalt polysulfide tends to form amorphous structure,hexamethylene tetramine plays a positive role in the formation of CoS_x with high catalytic performance,and bridged S-S and S-S-S have a positive effect on electrochemical activity.In1 m KOH solution,with the current density of 10 m A·cm^-2,the over potential can reach 308m V.（3）Using ammonium polysulfide with polysulfide ion as sulfur source and cobalt acetate as cobalt source,two groups of cobalt sulfides（one group of hexamethylene tetramine participated in the synthesis and the other group did not participate）were prepared at room temperature,exploring a more convenient preparation method of sulfur rich CoS_x.Through the study of its OER performance and mechanism,we found that the OER mechanism of them also conforms to the law revealed when sodium polysulfide is used as sulfur source,which confirms its universality.In 1 M KOH solution,with the current density of 10m A·cm^-2,the overpotential of sulfur rich cobalt sulfide（synthesized with hexamethylene tetramine）can be as low as 204 m V.Its excellent electrocatalytic oxygen evolution activity reflects the research prospect of sulfur rich cobalt sulfide in this field and anchors the direction for further research.