Modification Of The Transition Metal Phosphides And Nitrides For Electrocatalytic Hydrogen Evolution

Hydrogen(H2)with high energy density and clean combustion products is a contemporary clean and renewable energy with great potential.With the development of science and technology,hydrogen energy can be produced in many ways.At present,water splitting driven by renewable electricity is vigorously developed in our country.The purity of hydrogen prepared by this method can reach 99%.It is a non-pollution way to produce hydrogen and the water resources are rich.However,its industrialization development is still hindered by low economic benefits,short life of the device,low energy conversion efficiency and so on.Therefore,the development of efficient and low-cost electrochemical hydrogen evolution(HER)catalysts is the key to solve the above problems.So far,the most common non-noble metal HER catalysts include transition metal compounds,non-metallic compounds,amorphous materials and so on.Among them,transition metal compounds are widely concerned by researchers for their high conductivity,corrosion resistance and similar d-orbital electron distribution with Pt.In addition,improving the electrocatalytic HER performance of these catalysts through rational surface and interface engineering is of great significance to the large-scale application of electrolytic water splitting.In this doctoral dissertation,we are aiming to design and develop a series of lowcost and efficient transition metal phosphides and nitrides for HER catalysis.The main modification methods include heteroatom doping or loading,interface engineering and so on.The surface and atomic scale fine structure characterizations reveal the changes of element valence and the local coordination structure of active sites.Combining with the first principle simulation,the adsorption and conversion kinetic energy barrier of key intermediate species during the catalytic process are studied.These provide theoretical basis for deeply understanding of the intrinsic reasons for the catalytic performance improvement.The main contents of this paper are as follows:1.The common synthesis methods of transition metal phosphides and nitrides and their research progress in the field of HER electrocatalysis.Besides,the main contents of this paper are discussed here.2.The acidic HER performance of P-rich compounds are effectively improved by doping non-metallic N atoms.Here we taking CoP2 as the model catalyst and reveal the main reason for the limited electrocatalytic HER performance of P-rich metal phosphides based on the first principle calculation.With the increase of P content,the lattice of P-rich phosphides expand,resulting in too strong adsorption of intermediate H on non-metallic phosphorus sites of the catalytic surfaces.Based on the above analysis,heterogeneous N atoms are induced by low temperature ammonia annealing experimentally.It shows N doping can cause the lattice contraction of CoP2 and improve its acidic HER catalytic performance.N-CoP2 delivers the overpotential of 38 mV at the current density of 10 mA cm-2(η10).The fine structure characterization and theoretical simulation results show that the N doping induces the N-P interaction,which increases the overlap of atomic wave functions,and finally optimizes the H adsorption on catalytic surface.3.Aiming at solving the problem of strong adsorption of H intermediate on NiP2 surface,we optimize their interaction by loading iridium on NiP2 and effectively improve its HER catalytic performance.Density function theory(DFT)calculations reveal that the Ir loading optimizes the surface electronic structure,reduces the charge density of P and regulates the adsorption of hydrogen intermediates on the catalytic surface.Here single atom Ir loaded on NiP2(Ir/NiP2)is synthesized by chemical impregnation method.It shows the HER catalytic performance of NiP2 can be effectively improved in a wide range of pH.The η10 of Ir/NiP2 measured in acid medium is 61 mV.Meanwhile,under the same current density,the overpotential is reduced by 34 and 24 mV in alkaline and neutral electrolyte,respectively.4.Aiming at solving the sluggish water dissociation kinetics of Co4N in alkaline medium,MoN nanoparticles are coated on Co4N nanowires in situ by impregnation method.The synergistic effect between MoN and Co4N is utilized to construct a high efficient hybrid HER catalyst.Compared with Co4N catalyst,MoN-Co4N exhibits significant improved catalytic performance with η10 of 45.1 mV.DFT calculations show that more empty bands near the Fermi level and vertical orbital orientation of MoN contribute to the adsorption and activation of H2O.Besides,Co4N surface provides active sites for hydrogen adsorption and desorption.The synergistic effect between MoN and Co4N contributes to the high efficient HER catalytic performance of MoNCo4N.