Design Of Transition Metal Sulfide/Carbon Nanostructure Interface And Regulation Of Electrolysis Water Catalytic Characteristics

The two half reactions of water decomposition,i.e.hydrogen evolution reaction(HER)and oxygen evolution reaction(OER),with high activation barriers,which are not easy to occur in thermodynamics.As a result,the overpotential of total electrolytic water is higher,and the actual meaningful decomposition water needs more than 1.8 V.At present,noble metal catalysts such as Pt,Ru and Ir are the most efficient catalysts for water electrolysis,but their rare content on earth and high price limit their application.The use of highly efficient non-noble metal electrocatalysts can solve this problem.Among them,transition metal sulfide is one of the ideal precious metal substitute materials because of its high catalytic activity,low cost,simple and reliable synthesis method.However,due to the lack of bifunctional catalytic activity and stability of sulfides,the performances of sulfides in the practical application of total water decomposition still need to improve.In this paper,the surface,interface characteristics and electronic structure of metal sulfide(nickel sulfide as an example)catalysts were controlled by four ways:multi-component design,nano morphology design,carbon coated interface design and hetero element doping design.The Gibbs free energy of oxygen intermediate adsorption and hydrogen adsorption on the surface of metal sulfide catalysts were changed,and the bifunctional catalytic activities of HER and OER were improved.The specific design theory and core ideas are as follows:(1)Multiphase interface structure of nickel sulfide and design of single Ni₃S₂catalyst:firstly,this paper selects Ni S/Ni S₂disulfide as an example to explore the effect of multiphase interface on her and OER catalytic activity.Compared with single Ni₃S₂catalyst,the electrolytic water performance is greatly enhanced(the current density of Ni S/Ni S₂can reach 10 mA cm^-2at 1.61V in full electrolysis test),It is 180 mV lower than the 1.79 V of single Ni₃S₂).Considering the poor performance of single Ni₃S₂,this paper further takes Ni₃S₂as the research object,optimizes the multi-scale structures such as Ni₃S₂nanoparticles,Ni₃S₂nano polyhedrons and Ni₃S₂nanowires by Q-CVD technology,and further constructs a composite structure with three-dimensional(or nitrogen doped)graphene,so as to achieve the ultimate goal of greatly improving the catalytic efficiency of Ni₃S₂nano catalyst for electrolytic water.(2)Design of Ni₃S₂nanoparticle@3D graphene nanosheets catalyst:in this paper,a new type of nickel foam(NF)self-supporting three-dimensional graphene nanosheet coated with Ni₃S₂nanoparticles(Ni₃S₂NPs@3-D GNs/NF)was successfully constructed by Q-CVD technology.The high-density Ni₃S₂nanoparticles were dispersed in the porous network of three-dimensional graphene.It provides two channels for ion diffusion and electron transport of efficient catalytic components;The intrinsic activity of metal sulfide and the enhancement effect of graphene play a leading role in the excellent catalytic performance of the catalyst.The synthesized Ni₃S₂NPs@3-D GNs/NF can be used as a binder free self-supporting catalytic electrode for electrocatalytic decomposition of water,showing significant oer and her catalytic activities(OER and HER overpotentials are 296 mV and 158 mV at 10 mA cm^-2current density,and the voltage at 10 mA cm^-2current density is 1.55 V in total electrolysis test).(3)Design of Ni₃S₂nanowire@nitrogen doped graphene nanotube catalyst:in this paper,a new type of Ni₃S₂nanowire@N doped graphene carbon nanotube based on foam nickel(Ni₃S₂@NGCLs/NF)was prepared by Q-CVD technology.A unique core-shell nanowire structure,has important advantages in catalytic reactions,usually,electrons can penetrate from the inner metal to the outer carbon layer to accelerate the catalytic reaction.Through the combination of carbon nanotubes and Ni₃S₂nanowires,the high conductivity of carbon nanotubes makes it easy for electrons to shuttle to Ni₃S₂nanowires;The chemical and mechanical stability of carbon nanotubes improves the catalytic stability of Ni₃S₂nanowires;Since Ni OOH is a highly active substance of OER,the core-shell cable like nanostructure of Ni₃S₂nanowires coated by NGCLs provides a slow-release effect for the continuous formation of Ni OOH by Ni₃S₂.therefore Ni₃S₂@NGCLs/NF composite electrocatalyst showed good overall electrocatalytic water decomposition performance(OER and HER overpotentials were 271 mV and 132 mV at 10 mA cm^-2current density,and the voltage at 10 mA cm^-2current density was 1.55 V in full electrolysis test).(4)Design of nitrogen doped Ni₃S₂nanocube@nitrogen doped graphene catalyst:in this paper,a nitrogen doped Ni₃S₂nanoscale and nitrogen doped graphene complex based on foam nickel(N-Ni₃S₂@NG/NF)was prepared by Q-CVD Technology.Heterogeneous element doping of carbon materials and metal compounds has strong modulation ability for the chemical adsorption of reaction intermediates on the surface of electrocatalysts,and can effectively accelerate the cracking rate of water.Due to the in-situ formation of N-Ni₃S₂@NG/NF heterogeneous interface,the incorporation of nitrogen anion and N-doped carbon coated nano cubic structure change the crystal structure and electronic structure of Ni₃S₂,so as to significantly improve the catalytic activity of the composites.Prepared self-supporting N-Ni₃S₂@NG/NF electrode showed high electrocatalytic activity for HER and OER in alkaline medium(OER and HER overpotential were 238 mV and 100 mV at 10 mA cm^-2current density,and the voltage at 10 mA cm^-2current density was 1.53 V in full electrolysis test).