Study On Controllable Synthesis, Assembly And Electrocatalytic Performance Of Nickel Phosphide Nanocrystals For Hydrogen Evolution

With the increase of global energy demands and the aggravation of environmental problems,more and more attention is being paid to the production of hydrogen,which can be considered as a new energy source.Electrolysis of water is the most promising technology to produce hydrogen,but this reaction requires efficient catalyst.Pt-based noble metal materials are the most effective electrocatalysts for the hydrogen evolution reaction?HER?.However,it is difficult to achieve industrialization because of the high price and resource scarcity.Therefore,it is important to develop non-noble metal catalysts with low-cost and high abundance.As the research object to non-noble metal nickel phosphide in this paper,we studied the controllable synthesis,assembly and electrocatalytic performance of nickel phosphide nanocrystals?NCs?for the HER.We designed a controllable synthesis scheme,which can realize the crystal phase-and size-controlled synthesis of nickel phosphide NCs.Furthermore,the structure-activity relationship between nickel phosphide NCs with different microstructures and the electrocatalytic performance was revealed.Based on the above research,we designed a series of nickel phosphide-based HER catalysts with high efficiency,low cost and novel structure by material composite,component regulation and element doping strategies to further enhance the electrocatalytic activity,stability.These results have certain theory directive significance to design and synthesize other non-noble metal catalysts.It is greatly meaningful to push the practical application of nickel phosphide-based catalysts in large-scale water splitting.Firstly,the synthesis process of nickel phosphide NCs was studied by metal organic phosphating method and metal phosphide precursor method.A series of monodispersed nickel phosphide NCs were synthesized using nickel acetylacetonate as the nickel source,trioctylphosphine as the phosphorus source and oleylamine?OAm?as the reductant.The effect of synthetic conditions such as P:Ni precursor ratio,reaction temperature,reaction time,OAm quantity and the species of additives on the phase and size of the nickel phosphide NCs was discussed in detail.The controllable synthesis of nickel phosphide NCs with different microstructures was realized using bis?triphenylphosphine?nickel dichloride?BTND?as single precursor in the presence of OAm.The metal organic compound was introduced into the preparation process of nickel phosphide NCs for the first time.The growth mechanism of nickel phosphide NCs was proposed:?nucleation-phosphating-diffusion aggregation growth?.The experimental procedure was simplified and the yield was improved.Secondly,On the basis of the above controllable synthesis rule,we synthesized three nickel phosphide NCs with different phases(Ni₁₂P₅,Ni₂P and Ni₅P₄).The influence of the phase of the nickel phosphide NCs on the electrocatalytic properties for the HER was further investigated.All phases showed good electrocatalytic performance and followed the order Ni₅P₄>Ni₂P>Ni₁₂P₅.The superior catalytic activity of Ni₅P₄ is attributed to the higher positive charge of Ni and the stronger ensemble effect of P in Ni₅P₄ NCs.In order to further improve the electrocatalytic performance of nickel phosphide catalysts,a series of nanocomposites composed of nickel phosphide NCs with different nanostructures and carbon-based materials with different types?carbon nanotubes,carbon nanospheres and nitrogen doped reduced graphene oxide?were synthesized successfully.The structure of nanocomposites was characterized by various techniques and the electrocatalytic performances were evaluated.The effect of nickel phosphide NCs with different phases,the content of carbon materials and the doping of nitrogen on the electrocatalytic performance for the HER was further investigated and the action mechanism was discussed in detail.The results show that,the electrocatalytic performance of nickel phosphide catalysts can be enhanced remarkably by introducing the carbon materials due to their large specific surface area and high conductivity as well as the interaction between nickel phosphide nanoparticles and carbon materials,which enhance the dispersion of nickel phosphide nanoparticles effectively and expose more active sites.In addition,we also found that,after nitrogen doping into reduced graphene oxide,the electrocatalytic activity also can be enhanced.The enhanced catalytic activity can be attributed to the following reasons.On the one hand,after nitrogen doping,the formation of pyridinic and pyrrolic C-N species,which are another active center for HER.On the other hand,N atoms are inherently advantageous in interacting with H⁺than carbon atoms for HER,which further provide the basis for the excellent catalytic activity.Finally,on the basis of above experiments,we further improved the electrocatalytic performance of nickel phosphide-based catalysts by regulating the active component.We designed and synthesized a series of Co_2-xNi_xP/CNTs catalysts with different Co contents by in situ liquid phase assembly approach.The structure,morphology and composition of nanocomposites were characterized by various techniques and the electrocatalytic performances were compared.The effect of the introduction of Co with different contents on the structure and performance was further investigated.The results show that,the crystal phase structure of Ni₂P was changed gradually with the increase of Co contents.The Co atoms only replaced the position of Ni atoms of Ni₂P at low Co introduction content,the isomorphous substitution effect can be observed.However,with the further increase of Co contents,the crystal structure of Ni₂P was destroyed,forming compound crystal phase composed of CoP and Ni₂P,thus promoting the HER.The introduction of Co atoms as well as the strong synergistic effects between CNTs and Co_2-xNi_xP contributed to the superior HER catalytic activity.Additionally,the detailed reaction kinetic pathway and the energy barrier of hydrogen atom adsorption for HER were also investigated using the density functional theory calculation.The results show that the higher HER catalytic activity of the Co_2-xNi_xP/CNTs can be attributed to the splendid migratory aptitude of adsorbed single H atoms and the lower energy barrier for H₂ formation.The results of calculation verified the experimental results theoretically.