Tuning Nanocomposite Structure Of Transition Metal Phosphide And Its Electrocatalytic Perfomances For Water Splitting

Nowadays,the development and exploration of renewable energy to solve a series of problems,such as the environmental pollution caused by the depletion of traditional fossil energy,has absorbed more and more attention.As the advantages of high energy density,renewable,clean and pollution-free,hydrogen is regarded as the most ideal alternative to traditional fossil fuels.Among the approaches for generating H₂,electrochemical water electrolysis is considered as the most promising method for hydrogen production because of the characteristics of clean,environmental protection and feasibility.However,due to the sluggish process in the OER process which includes four electrons transfer steps,a certain overpotential???is produced in the process of electrolysis water,reducing the efficiency of water decomposition.Therefore,it is necessary to develop the electrocatalysts with high HER and OER performances.Currently,the noble metal Pt and Ir/Ru oxided are still the most efficient catalysts for the HER and OER processes,respectively.Whereas,the practical and large-scale applications of these catalysts are limited by their high costs and scarcity.Transition metal phosphates exhibit high electrocatalytic activity due to their good thermal conductivity,electrical conductivity,thermal stability,chemical stability and unique electronic structure.In this paper,nickel cobalt based phosphates are in-situ engineering,and the effect of physio-chemical structure of electrocatalyst on its electrocatalytic properties for bifunctional electrolysis water is investigated.The main contents and conclusions are listed as follows:?1?A petal-like NiCoP/PNCF nanocomposite material was prepared by the hydrothermal method and plasma phosphating technique.The composite structure is composed by many petal-like nanosheets which are vertically and uniformly distributed on the surface of the 3D skeleton of Ni-Co alloy foam,which greatly increases the specific surface area and the number of active sites,demonstrating remarkable bifunctional electrocatalytic activity for overall water splitting.The results show that the overpotential of NiCoP/PNCF for delivering a current density of 10 mA cm^-2 in 1.0 M KOH electrolyte is 52.58 mV for the HER process.Although the value is still higher than that of Pt/C,it is still very competitive with other reported electrocatalysts.The calculated tafel slope is only 53.39 mV dec^-1,indicating a fast HER kinetic reaction.Meanwhile,the overpotentials delivering a current density of 100 mA cm^-22 is 386 mV for OER,and the tafel slope is only 53.39 mV dec^-1.Specially,the Faraday efficiency is nearly 100%because the low impedance value?0.8??.In addition,the current density is unchanged greatly after a 24 h HER and OER processes in a harsh alkaline medium,indicating the ultra stable electrocatalytic stability.?2?Using NCF as the substrate and the atom source of Ni-Co,the N-doped nickel-cobalt phosphates?N-NiCoP/NCF?was in-situ fabricated with the hydrothermal and N₂-PH₃ plasma phosphating processes.Many cracks are distributed on the surface of polyhedron N-NiCoP,which greatly increases the specific surface area and expose more active sites,demonstrating remarkable bifunctional electrocatalytic activity for overall water splitting.And the introduction of N atom changes the valence bond binding mode of the original structure of metal lattice,which further increases the electron transfer rate.At the same time,N-NiCoP/NCF with polyhedron structure are formed from the NCF frame,which increase the binding intensity between N-NiCoP and NCF,exhibiting a super electrocatalytic stability.The results show that,the overpotential is 78.2 mV for HER at a current density of 10 mA cm^-2 in 1.0 M KOH electrolyte,and the tafel slope is only 98.09 mV dec^-1,which is very competitive with other reported electrocatalyts.The overpotentials for delivering a current density of 10 mA cm^-2 in 1.0M KOH electrolyte is 225 mV for OER process,and the tafel slope is only 66.94 mV dec^-1.It is proved that N-NiCoP/NCF electrode has excellent bifunctional catalytic activity.Meanwhile,the electrochemical impedance of N-NiCoP/NCF is only 2.0?,and the Faraday efficiency is nearly100%,indicating that the electrode material has good conductivity and high efficiency.In addition,after the test for 100 h,the current density of HER and OER is not deviated greatly,which indicates that the N-NiCoP/NCF electrode material has super electrocatalytic stability.