Theoretical Study Of Nickel Phosphide-based Electrocatalysts For Alkaline Hydrogen Evolution Reaction

Hydrogen,a clean and efficient secondary energy carrier,is expected to fundamentally solve global problems such as energy shortages and environmental pollution coupling application with primary renewable energy.The use of electricity generated by renewable energy to dissociate water provides a sustainable hydrogen production method while providing a viable solution for the effective use of renewable energy.One of the key issues in the development of highly efficient electrolyzed water technology is the development of high-performance hydrogen evolution reaction?HER?electrocatalysts.Noble metal platinum?Pt?is well known for its excellent HER catalytic activity,but its scarcity of resources and excessively high material cost severely restrict its widespread commercial application.In the search for efficient and cheap catalysts,scholars from various countries have discovered a variety of non-noble transition metals,alloys and compound materials with application potential.Among them,transition metal phosphides get highly concerned because of their high catalytic activity,good conductivity and excellent resistance corrosive.Under acidic water electrolysis conditions,transition metal phosphides exhibit superior HER activity comparable to noble metal Pt;however,in more practical alkaline water electrolysis,they lose considerable HER catalytic performance.In this aspect,the study found that the implementation of heterogeneous atom doping can effectively improve the basic HER activity of transition metal phosphides.However,due to the lack of understanding of the relevant modification mechanism,most of the current experimental research is empirical,which largely restricts the development process of high-efficiency catalysts.In view of this situation,this paper selects the representative transition metal phosphide---nickel phosphide?Ni₂P?as the main research object,focusing on the density functional theory calculation study on the mechanism of Ni₂P toward alkaline HER reaction,as well as the metal doping modification mechanism.The main progress achieved is as follows:?1?We,for the first time,proved that the Ni₂P?001?surface can provide the synergistic active site required for alkaline HER from the theoretical calculation point of view.Ni₂P?001?surface is composed of Ni₃ clusters and P atoms distributed hexagonally around the clusters.The calculation results show that the H_3Ni hollow site in the Ni₃ cluster provides active sites for the cleavage of water molecule,while the near Ni P bridge site B_{Ni P} provides the active sites for associative desorption of H intermediates.The dissolution of water molecule is the rate-controlling step of alkaline HER for Ni₂P?001?surface,and its reaction energy barrier is high?0.79 e V?,which is consistent with the experimental observation of the poor activity of Ni₂P for alkaline HER;?2?The effect of transition metal doping on the atomic structure,electronic structure and alkaline HER dynamics of Ni₂P surface was calculated.The results show that the surface transition metal doping mainly affects the kinetics of the rate-controlling step of water molecule dissolution by changing the OH adsorption energy.Based on this,for the first time,a new viewpoint was proposed using OH binding energy as a alkaline HER activity descriptor,and further leads to a volcano diagram between Volmer reaction activity(-max(?G_eb,?G_OH*))and OH adsorption energy(?E_OH*).We also used the"d-band center theory"establishes the correlation between the OH adsorption energy and the electronic structure of surface active site.This work has theoretical guiding significance for the development of high-performance and inexpensive metal phosphide catalysts suitable for alkaline HER.?3?Combined with experimental findings,the HER mechanism of Ni_2-xMo_xP/Ni Mo O_4-ycomposite catalyst under alkaline conditions was studied.The calculation results show that Ni Mo O_4-y could effectively promote the dissolution of water molecules,and Mo doped Ni₂P could further enhance its activity to catalyze the associative desorption of H intermediates.The synergistic catalytic active site composed of the two components could efficiently catalyze alkaline HER.In this way,while explaining the experimental phenomenon,we provide a theoretical basis for the in-depth analysis of the nature of the catalytic active site.