Preparation And Electrocatalytic Hydrogen Evolution Performance Of Nickel-rich Phosphide Materials

Under the guidance of the goals towards "carbon peaking" and "carbon neutrality",the exploitation and utilization of hydrogen energy have become an effective tactic to solve the problem of resource shortage and environmental pollution,and thus to achieve sustainable development.Among the current hydrogen production technologies,water electrolysis is the most promising one,but its shortcomings such as high overpotential and sluggish reaction kinetics severely restrict the energy conversion efficiency of electrolytic water devices.Although traditional noble metals,especially platinum-based electrocatalytic materials for hydrogen evolution,have demonstrated outstanding electrocatalytic activity,their applications are critically hampered by limited reserves in the natural environment and exorbitant prices.Therefore,it is of momentous scientific significance and practical value to develop high-efficiency and inexpensive nonnoble metal hydrogen evolution catalysts.Transition metal phosphides(TMPs)have attracted widespread attention due to their sufficient resources,high activity,and stability.Among them,metal-rich phosphides possess metal-like properties attributed to the internal metal-metal and metal-phosphorus bonds,and their high electrical conductivity,excellent stability,and abundant reserves are recognized as a kind of promising alternative to precious metals for hydrogen evolution electrocatalysts.In particular,metal-rich nickel phosphide materials have aroused considerable interest by their unique physicochemical properties and various stoichiometric ratios.Ni3P,as the most Nirich phosphide,with plentiful Ni-Ni bridge bonds providing sufficient electron transfer channels,has good electrical conductivity and catalytic activity,demonstrating great potential for catalytic hydrogen evolution.A variety of Ni3Pbased electrocatalysts have been developed,but their intrinsic activity and electrochemical stability need to be further improved to satisfy the requirements of practical applications.Given this,Ni3P was used as the research object in this thesis to investigate the relationship between material structure,catalytic mechanism,and catalytic performance through the micro-morphology and electronic structure regulation,construction of heterojunction,conductive carbon substrate composite,and other strategies,thus improving its intrinsic catalytic activity and increasing the exposure of active sites.The main works are as follows:(1)The mesoporous nitrogen-doped carbon nanofiber-loaded heterogeneous Ni3P/Ni nanoparticle electrocatalyst materials(Ni3P/Ni@N-CNFs)were synthesized by a one-step solid phase calcination process.Benefitting from the optimized Ni active sites resulting from the synergetic electronic coupling effect at the interface of Ni3P/Ni heterostructure,and the mesoporous N-doped nanofiber with excellent electrical conductivity and large specific area as the substrate,the Ni3P/Ni@N-CNFs performed splendid HER catalytic performance under acidic,alkaline and neutral media,requiring the low overpotential of 121,145 and 187 mV to reach the current density of 10 mA cm-2,respectively,as well as maintaining at least 60 h of catalytic stability.(2)The three-dimensional hierarchical micro flower electrocatalysts assembled from nitrogen-doped carbon encapsulated Ni3P/Ni/VN heterogeneous nanoparticles(Ni3P/Ni/VN@NC)were prepared through an in-situ carbonization and phosphating strategy using NiV layered double hydroxide(NiV-LDH)as the precursor.The unique stable hierarchical structure of NiV-LDH provides multidimensional mass and charge transport channels and plentiful active sites for HER,thus enhancing the activity of hydrogen evolution.Moreover,the kinetics of the adsorption/desorption reactions of VN and Ni3P were optimized owing to the charge pumping ability of VN during the interfacial electron coupling between the three components,which facilitated the occurrence of electron transfer between the heterogeneous interfaces and thus boosted the intrinsic catalytic activity.Therefore,the resultant Ni3P/Ni/VN@NC electrocatalyst requires quite a low overpotential of 81 and 134 mV to deliver the current density of 10 mA cm-2 in 1.0 M KOH and 0.5 M H2SO4 electrolyte,respectively,and maintains long-term durability of at least 90 h.(3)The Mott-Schottky heterostructured electrocatalyst with Ni3P/Ni nanoparticles wrapped in nitrogen-doped carbon layers(Ni3P/Ni@NC)was fabricated using a hydrothermal method combined with in-situ carbonizationphosphating strategy,where the contact between the Mott-Schottky heterogeneous interface was established by the high-index faceted Ni3P(141)and the Ni(111)facet.The experimental and theoretical results indicate that the unique Ni3P(141)Ni structure benefits from the spontaneous electron transfer at the interface between the highly active Ni3P(141)surface and Ni,which has a hydrogen adsorption free energy value of 0.022 eV,even better than that of the Pt structure(0.1 eV),representing an outstanding intrinsic HER activity.Therefore,the Ni3P/Ni@NC performed superior HER catalytic activity in a 1.0 M KOH solution,driving a current density of 10 mA cm-2 with an overpotential of merely 71 mV,and achieving constant operation for at least 120 h.