Synthesis And Photo/Electrocatalytic Hydrogen Evolution Properties Of Nickel Based Photocatalyst

Water splitting is one of the important ways to obtain hydrogen,while the main restriction on development of water splitting is the lack of efficient and cheap catalysts.Transition metal compounds(such as transition metal oxides,sulfides,nitrides,carbides,phosphides,etc.)have attracted extensive attention from researchers due to their relatively low cost and good electrocatalytic activity,showing great potential in catalytic water splitting.Nickel and Platinum belong to one family so that have similar electronic structure.Thus,some nickel compounds show good catalytic hydrogen evolution activity.This paper aims to develop transition metal nickel-based catalysts to improve the catalytic hydrogen evolution performance of materials through size limitation,construct special structure,composition control,etc.and conduct in-depth analysis of the reasons for the improved performance according to density functional theory.The NiO quantum dots(QDs)are prepared via a confinement method with carbon materials and supported on the surface of TiO₂.The influence of added NiO on the micro-morphology of the sample is studied:The addition of a small amount of NiO QDs have few effect on the morphology of the substrate.When the amount of NiO gradually increases,the TiO₂ particles gradually agglomerate.The optimal loading ratio of NiO is determined to be 2.5%.With this loading amount,the photocatalytic hydrogen evolution performance of the TiO₂ catalyst modified by NiO QDs is improved by nearly 56 times compared with pure TiO₂,which is close to the performance of Pt-loaded TiO₂.The carrier separation efficiency of the composite catalyst has been significantly improved compared with the pure TiO₂ and NiO/TiO₂.In addition,after the TiO₂ substrate is replaced with Nb₂O₅ or Ta₂O₅,the photocatalytic hydrogen evolution performance of the composite photocatalyst is also significantly improved compared to the pure oxide,which is close to the Pt-loaded samples.Nitrogen-doped carbon coated Janus Ni₂P/Ni₅P₄ electrocatalyst is synthesized via controlling phosphating.The sample is ball-flower consisted of nanosheets,and the nanosheet composed of nanoparticles embedded in nitrogen-doped carbon.The current density of 10 m A cm^-2 in acidic and alkaline electrolytes are 104 m V and 113m V,respectively.The improvement in catalytic performance may be due to the presence of a built-in electric field at the interface within the Janus-structured nanoparticles.The built-in electric field strength at different Ni₂P/Ni₅P₄ interfaces is calculated by analyzing the change of differential charge density.Among them,the intensity of built-in electric field of the(100)crystal plane of Ni₂P and the(100)crystal plane of Ni₅P₄ is the largest,reaching 2.76 m V(?)^-1.In addition,the difference charge density(DCD),change of density of state(DOS)and the change of hydrogen adsorption Gibbs free energy(?G_H*)are compared according to theoretical calculations.The change in DOS is more consistent with?G_H*,while the DCD is not completely consistent with?G_H*.Therefore,the change in DOS is more suitable for comparing the performance of electrocatalytic hydrogen evolution.The effective electric field strength of the symmetrical laminate model and the composite laminate model is the same by adjusting the applied electric field.DOS of the surface atoms near Fermi level are very close to each other.Nanoparticles self-modified Ni_xFe_1-xB nanosheet electrocatalyst is synthesized by boron thermal reduction method.Ni and Fe elements are evenly distributed in the sample.Ni B and Fe B belong to two different space groups of Pnma and Cmcm respectively.When the content of Ni element increases to a certain value,the crystal symmetry of Ni_xFe_1-xB would change from Pnma to Cmcm.Ni_xFe_1-xB prepared with N/Fe feed ratio of 1:1 require overpotentials of 63.5 m V and 282 m V for HER and OER,respectively,to reach a current of 10 m A cm^-2.The electrocatalytic hydrogen evolution and oxygen evolution performance of the catalyst does not show significantly decrease in 20 h-stability-test.Density functional theory is used to calculate the?G_H*of Ni_xFe_1-xB(x=0,0.25,0.5,0.75,1).The calculation results show an inverted volcano curve.?G_H*of Ni_0.5Fe_0.5B is closest to 0,which is most conducive to the progress of the HER reaction.The catalysts are used as cathodes and anodes material to form an electrolytic cell for overall water splitting.A voltage of1.57 V is required to reach a current density of 10 m A cm^-2,which is less than that of most reported noble-metal-free materials.