Preparation Of Transition Metal Single Crystal Electrodes And Their Interfacial Electrochemical Studies Covered With Graphene

Hydrogen production from water electrolysis is an important technology to obtain hydrogen energy.Its reactions include the hydrogen evolution reaction（HER）at the cathode and the oxygen evolution reaction（OER）at the anode.At present,noble metals such as Pt,Ru,Ir are the best performing HER catalysts,but they are scarce in reserves and expensive.However,inexpensive transition metals such as Fe,Co,Ni and their compounds have a huge difference in activity with noble metals.Their HER catalytic performance need to be improved through complex design.However,these catalysts have complex components and various structures,and it is often difficult to clearly understand their specific mechanisms and kinetic principles in the catalytic reaction process under the interference of many complex factors.Therefore,single crystal electrodes have attracted the attention of researchers due to their atomic level flatness and simple structure,and are an excellent platform for model catalyst research.However,the quality standards of single-crystal surfaces suitable for experimental research are extremely high and stringent.In addition,there are major challenges in the preparation of single-crystal electrodes（especially non-precious metals）and their surface modification,limiting the research and development of electrocatalysts.In this paper,a single crystal preparation platform based on induction heating in a reducing atmosphere is built,and some high-quality Ni,Co,Fe non-precious metal single crystal electrodes and Ni-Pt alloy single crystal electrodes are successfully prepared.Afterwards,a single layer of graphene（Gr）material is covered on the surface of Ni（111）,Ni₄Pt（111）and Pt（111）single crystals,and then constructed the graphene/Pt（111）,graphene/Ni（111）and graphene/Ni₄Pt（111）two-dimensional confined space.Subsequently,electrochemical studies related to single-crystal electrodes and their graphene-covered single-crystal electrodes are carried out.Using in situ electrochemical scanning tunneling microscopy（EC-STM）,we deeply studied:1)the electrochemical dynamic behavior of H in the graphene/Pt（111）confined space;2)the real-time changes of surface topography of the graphene/Ni（111）during electrochemical oxidation under alkaline conditions.The work carried out in this paper is summarized as follows:（1）A batch of non-precious metals Ni（hkl）,Co（hkl）,Fe（hkl）and Ni₄Pt（hkl）alloy single crystal electrodes were prepared after a series of orientation,polishing and annealing processes using an inductively coupled heating device filled with a reducing atmosphere.Combined with ex-situ scanning tunneling microscopy（STM）and electrochemical cyclic voltammetry（CV）tests,the results show that the prepared metal single crystal surface has the characteristics of atomically flat step surface area,low step density and clear electrochemical characteristic information.It shows that the single crystal prepared by this method is of high quality and can be used for rigorous single crystal electrochemistry experimental research.（2）Graphene monolayer was successfully prepared on the surface of Pt（111）single crystal by chemical vapor deposition（CVD）,and the selective permeability of graphene to proton H was verified by electrochemical CV test.Using in situ EC-STM,the electrochemical dynamic behavior of H on graphene/Pt（111）electrodes was investigated in the overpotential deposition（UPD）potential range of H.There are two adsorption states on the electrode:1)Potential-dependent reversible adsorption of H on surface defect sites of graphene;2)In the initial stage of adsorption,a small amount of H exhibits a " six-pointed star" cluster-like adsorption structure on the graphene upper surface or in the graphene/Pt（111）two dimensional confined space.（3）The graphene monolayer was prepared on the surface of Ni（111）single crystal by the CVD method.The CV test results of the graphene/Ni（111）electrode in 0.1 M NaOH solution demonstrate that graphene can protect the Ni（111）surface under a rather large（higher positive potential）potential window.The graphene/Ni（111）electrode was subjected to electrochemical activation（oxidation）treatment in the potential range of 0.1-1.2V（vs RHE）for 200 cycles,which could crack the surface of graphene and generate small defects.It was found that after electrochemical oxidation the HER activity of the graphene/Ni（111）electrode was significantly better than that of the bare Ni（111）electrode.Subsequently,in situ EC-STM was used to research the surface morphology changes of the graphene/Ni（111）electrode during electrochemical oxidation in real time.It is found that after the graphene defects were generated,Ni is gradually oxidized and the surface is roughened to form a graphene/Ni（OH）x/Ni composite structure,thereby enhancing the HER activity of the electrode.Based on the above experimental results,we optimized the molar ratio of Ni-Pt alloy,and successfully prepared Ni₄Pt（111）alloy single crystal and graphene-covered Gr/Ni₄Pt（111）composite electrode is used for hydrogen evolution activity test.In 0.1 M NaOH solution,the HER activity of Ni₄Pt（111）was significantly higher than that of Ni（111）and Pt（111）.In summary,in this paper,a series of easily oxidizable transition metal and its alloy single crystal electrodes are prepared by an inductively heating device,and the surface is covered with a graphene monolayer to construct a two-dimensional confinement space.Further,in situ EC-STM was used to characterize the real-time structural morphology of the graphene/metal single crystal electrode surface as a function of potential.The work in this paper has laid a solid material foundation for the wide and convenient application of non-precious metal single crystals electrode and their alloy single crystals,which is beneficial to promote the development of various electrocatalytic reactions on the modeled single crystal surface platform,and to deepen the research on the catalytic mechanism of various related nanocatalysts.