Synthesis Of Nano-sulfures/Oxides And Its Research And Application In Electrocatalytic Water Splitting

Electrocatalytic water splitting is an important branch of water splitting.In view of the increasingly serious environmental pollution and the advocacy of sustainable development,as the main product of water splitting are H₂ and O₂,that the H₂ can be used for energy storage and the O₂ is what we depend on.Therefore,the development and application of water splitting,together with renewable energy such as wind and solar energy,has received a lot of attention.As far as the research of electrocatalytic was concerned,it has a history of more than 200 years.However,it was not until 1970’s that electrocatalytic water splitting began to be concerned,thus its development space is quite expected.At present,catalysts for electrocatalytic water splitting can be roughly divided into two categories.One is the materials that need to be physical loading on the electrode surface,and the other is the catalytic materials which are in situ loaded on the electrode surface and formed a self-supporting system.Considering the stability and uncertainty of the physical loading,we hope to synthesize the in situ supported electrocatalytic materials with good electrolytic performance and well stability.As we know that the noble metals and their alloys have good performance in water splitting.However,the small reserves of these metals restrict their widely application.Therefore,the development of low-cost and excellent non-noble catalysts is still a constant pursuit for our researcher.Nowadays,the main non-noble metal used for electrolytic water splitting are transition metal-sulfide,-oxide,-carbide,-nitride,-phosphide,-selenide,and the composite materials formed by these materials loaded on conductive materials,such as various conductive carbon materials and metal materials.Among them,sulfides and composite materials have been largely studied.These materials have good stability and electrical conductivity.Apart from these,if the material also with porous or lamellar morphology,which will expose more active sites and will finally improve the performance of the catalysts.In this dissertation,based on the reported literature,we used the method of hydrothermal synthesis and high temperature solid-state reaction,a series of bifunctional sulfur/oxide electrocatalysts with high efficiency water splitting property were designed and synthesized.After that,we regulated the synthesis conditions,characterized the structure and morphology of the product.Finally,we deeply studied the property and the possible reaction mechanism of these materials.The main research results are listed as follows:1.Ultrathin nanosheets Cr_0.06-MoS₂ was synthesized by high temperature solid-state reaction.We found that its performance in electrocatalytic hydrogen evolution was much better than that of the undoped MoS₂.Thus,we carefully studied its structure and morphology.The results showed that the Cr_0.06-MoS₂ with more uniform lamellar structure and increased specific surface area than the undoped MoS₂.The results of HRTEM,XPS and Raman showed that the Cr_0.06-MoS₂ is a mixture of 1T and 2H phases.Therefore,Cr_0.06-MoS₂ is semi-metallic material and with good conductive,this is conducive to the rapid transfer of electrons.Moreover,the exposed active sites are not only at the Mo-S edge,but also on the plane of MoS₂,which will in turn increase material’s intrinsic activity for water splitting.2.Co-Ni₃S₂ nanosheets were in situ synthesized by one-step vulcanization process through the hydrothermal method,and the as synthesized Co-Ni₃S₂/NF showed excellent property for water splitting.In the analysis of structure and morphology,we found that the XRD result of Co-Ni₃S₂ was basically the same as that of undoped Ni₃S₂.However,the SEM results are very different.The SEM of Co-Ni₃S₂ showed a film with the miscellaneous of both large ultrathin nanosheets and small nanosheets,which will enlarge the active area of material,while undoped Ni₃S₂ was only a film of single-layer nanosheets.What’s more,we find that the conductivity of Co-Ni₃S₂ was much higher than that of the undoped Ni₃S₂.All of these lead to the improved intrinsic catalytic activity of Co-Ni₃S₂.Besides,Co-Ni₃S₂/NF could achieve higher current output（>1A/cm₂）at lower voltage in the process of electrolytic water splitting.3.The Tremella-like Sn-Ni₃S₂/NF was synthesized through the hydrothermal process and showed outstanding performance for water splitting.For that,it can achieve larger current output（>1 A/cm₂）at lower voltage.Besides,the required voltage is only 1.46 V when the current density reaches to 10 m A/cm₂.At this current density,the voltage of 1.46 V is almost the lowest value in Ni₃S₂-based electrocatalysts.When we analyze the reasons of the increased activity for water splitting,we suppose that the changed morphology of the doped catalyst increased the exposed active sites.What’s more,the increased conductivity of the Sn-Ni₃S₂/NF shortened the passway of electron transfer during the water splitting process.All of these lead to the improved water splitting activity.In addition,we suppose that the introduced Sn may also be the active site of the catalyst.4.We controlled the synthesis condition of bulk K₂Fe₄O₇,adding different volume of nickel foam（NF）in the reaction system,and finally successfully synthesized the composite of nano K₂Fe₄O₇ on the NF.We analyzed the morphology of K₂Fe₄O₇/NF,and found that the nano-K₂Fe₄O₇ which was in situ loaded on the NF with a diameter of 50 nm.We used it for the application of electrocatalytic water splitting.The results showed that nano-K₂Fe₄O₇ has a good electrocatalytic water splitting performance and is a better bifunctional electrocatalyst.However,the bulk K₂Fe₄O₇ has almost no such performance.The most likely reason for this phenomenon is that the nanomaterials will expose more active sites.Besides,a bimetialic interface was formed between the nano-K₂Fe₄O₇ and basal NF,which increased the active phase for the electrocatalytic process.Other reasons for the improved electrocatalytic water splitting activity of nanoscale K₂Fe₄O₇/NF may be that the free K+ in K₂Fe₄O₇ pore channel increased the concentration of OH-near the electrode-material in the electrolyte（1.0 M KOH）or some oter possible reasons.No doublt,the increased of reactants in turn will promote to the process of electrocatalytic water splitting.