Structure Control And Mechanism Of Platinum And Silver Nobel Metal Materials For Hydrogen Evolution Reaction

The development and application of clean energy is very important to solve the global energy crisis and environmental problems.Among many clean energy production technologies,electrolytic water hydrogen production has many advantages,such as simple process,no pollution,high production efficiency and high purity of hydrogen,which has been widely concerned by people.China is rich in wind and water resources.Excess wind,ocean and tidal energy can be used as energy donors of electrolyzed water,which can be converted into storable hydrogen energy,while reducing energy waste.However,in order to produce hydrogen efficiently and economically,it is necessary to develop the hydrogen evolution reaction(HER)electrocatalytic materials with excellent performance and relatively low cost.At present,the most excellent HER catalysts are Pt-group materials,but the high price and terrestrial scarcity are obstacles which prevent Pt catalysts from being widely used.Many investigations have proved that the addition of transition metals(Fe,Co,Ni)to noble metal-based nanomaterials could reduce the cost of catalysts.In addition,by adjusting the composition,structure and morphology of the noble metal composite,the HER activity and stability can be further improved.In this paper,a series of silver and platinum noble metal composite nanocrystals were prepared by precise controlled method.The content of precious metals was reduced through adding transition metals.By changing the compositions,morphologies,atom distributions and defects of the catalysts,the relationship between structure of the catalysts and HER performance was explored.By the combination of experiments and density functional theory(DFT)calculations,the controlled mechanism of catalyst structure and the reaction mechanism of HER were proposed,which provided theoretical guidance and material basis for the design and development of HER catalysts.The main contents are as follows:1.One-dimensional Ag-Ni core-shell nanowires were successfully synthesized by a simple two-step method,in which Ni shells were epitaxially grown at the surface of silver nanowires(Ag NWs)to form the unique core-shell structure.The thickness of Ni shells can be finely controlled from 50 to 150 nm by adjusting the initial Ni to Ag atomic ratios.The electrocatalytic performance of the Ag-Ni core-shell NWs can be adjusted by tuning their compositions,and the optimized Ag to Ni atomic ratio is 1:1.The Ag-Ni core-shell nanowires exhibit superior electrocatalytic activity to pure Ni nanoparticles and Ag NWs,showing a strong synergistic effect toward alkaline HER.A morphological effect was demonstrated by comparing with Ag-Ni core-shell nanoparticles.The enhanced HER activity of Ag-Ni core-shell NWs can be attributed to the synergetic effect between Ag and Ni,the more surface active sites and the fast charge transport.This approach provides a reasonable attempt for designing and developing one-dimensional eletrocatalysts based on Ag nanowires.2.The Pt-based materials with heterostructure have been showed excellent electrocatalytic performance.However,there are few investigations about the transformation from alloy to heterostructure and the main reason of the superior activity.This paper demonstrated that Pt-Ni heterostructure could be formed by only introducing NaOH in the synthesis of Pt-Ni alloy.DFT calculations reveal the added OH-can influence the energy barriers of atomic reduction and immigration,thus driving the formation of heterostructure with atomic segregation.Specifically,Ni(OH)2 was settled immediately before the reduction of Pt2+ species on the surface of Ni atoms.Then,the Pt atoms were formed and migrated from the surface to the edge of the crystal at the same time,and the formed Pt species accelerated the reduction of Ni(OH)2 to Ni atoms.Moreover,it is proved that the enhanced HER performance is attributed to favorable H and OH energetics arising from the atomic segregation.Acid treatment emphasizes the effect of H adsorption and OH adsorption,which are the key factors in determining alkaline HER activity.This work proves that adding OH-can lead the formation of nanocatalysts with atomic segregation,and tuning atomic segregation of Pt-Ni catalysts is an effective way to optimize their H and OH adsorption abilities,and thus enhance the alkaline HER performance.3.In this work,PtNi5 catalysts with surface segregation were prepared by a one-step solvothermal synthesis method,using H2 as the structure directing agent,which is simple and friendly to the environment.Combined with DFT calculation,the mechanism of H2 regulation and the relationship between catalyst surface and alkaline HER activity were proposed.From the perspective of reduction kinetics,H2 can regulate the relative reduction and growth rates of Pt and Ni,resulting in the formation of catalysts with different atomic distributions.In addition,the thermodynamic stable surface under different gas atmospheres was studied by DFT calculation:the Pt content of the most stable surface at 0.3 MPa and 0.5 MPa of H2 was 50%and 38%,respectively.The PtNi5 catalyst with 46%Pt-surface affords HER activity with an overpotential of 12.8 mV(1 M KOH)at-10 mA cm-2.DFT results demonstrated that the optimized surface segregation(50%Pt-surface)is beneficial to reduce the energy barrier of hydrolysis and hydrogen formation,which are the keys in boosting alkaline HER activity.4.It is a challenge for developing low-Pt catalyst with high efficiency and superior stability.In this work,a facile H2-assisted defect engineering strategy is designed to fabricate Pt/MoO3-x catalysts with promoted Pt dispersion,and to achieve ultra-low Pt loading for both acidic and alkaline HER.By inducing defect engineering,the average particle size of incorporated Pt nanoparticles(NPs)was decreased from 2.4 nm in Pt/MoO3 to 1.3 nm in Pt/MoO3-x,and total Pt dispersion was improved from 24.46%to 66.19%.DFT calculations confirmed that oxygen vacancy of MoO3-x is beneficial to improve the dispersion of Pt NPs and strengthen the interaction between Pt and MoO3-x.The optimized Pt/MoO3-x exhibits excellent electrocatalytic performance with ultra-low Pt mass loading(3.1 ?g cm-2),and needs overpotentials(?)of 38.9 and 28.1 mV to attain current density of-10 mA cm-2 in alkaline and acidic conditions,respectively.HER mechanism studies found that the oxygen vacancy can not only enhance the electrical conductivity,but also reduce the energy barriers of water dissociation and hydrogen formation steps,thus accelerating HER rates in both acidic and alkaline media.Our finding provides an effectively defect engineering strategy to reduce the amount of Pt in catalysts while maintaining their high activity and stability.