Design And Synthesis Of Metallic Nickel-Based Oxide Nanomaterials For Electrocatalytic Oxygen Evolution

Developing clean and sustainable energies as alternatives to fossil fuels is in strong demand within modern society.The oxygen evolution reaction（OER）is the efficiency-limiting process in plenty of key renewable energy systems,such as electrochemical water splitting and rechargeable metal–air batteries.The development of low-cost,high-efficiency,and robust electrocatalysts for the oxygen evolution reaction is urgently needed to address the energy crisis.Ir O₂and Ru O₂,known as the state-of-the-art OER electrocatalysts,are severely limited by the high cost and low earth abundance of these noble metals.Developing noble-metal-free OER electrocatalysts with high performance has been in great demand.Apart from traditional precious-metal-based catalysts,nickel-based compounds are the most promising earth-abundant OER catalysts,attracting ever-increasing interestdue to high activity and stability.Therefore,in this work,we reported a new method to enhance the oxygen evolution performance of nickel oxide by sulfurization and desulfurization（selenization and deselenization,phosphorization and dephosphorization）.We used nickel sulfate as the source of metallic nickel,electrodeposited them on three-dimensional（3D）carbon paper.Then,the Ni S₂,Ni Se₂ and Ni₅P₄ are generated by sulfurization,selenization and phosphation at high temperature respectively,and then they are respectively subjected to in-situ electrochemical oxidation.It is used to construct a new single metal nickel oxide composite.Modified Ni O catalyst electrode was obtained by in situ electrochemical oxidation.The oxygen evolution reactions activities of these electrodes exhibited enhanced performance in comparison with the pristine-Ni O.The OER activity of the sulfide oxidation derived Ni O（SD-Ni O）,the selenide oxidation derived Ni O（Se D-Ni O）,and the phosphide oxidation derived Ni O（PD-Ni O）electrodes was increased by about 4 times（atη=360 m V）.The OER overpotential of the XD-Ni O（X=S,Se,P）catalytic electrode is also lower than that of the pristine-Ni O catalytic electrode(At the current density of 10 m A cm^-2,η_{pristine-Ni O}=440 m V,η_{SD-Ni O}=360 m V,η_{Se D-Ni O}=380 m V,η_{PD-Ni O}=365 m V).To research the reasons for OER performance enhancement,we preliminarily characterized the changes in the material preparation process.Under the electrochemical oxidation condition,the sulfur,selenium and phosphorus in Ni S₂,Ni Se₂ and Ni₅P₄ can exude,thus causing favorable changes in physical and chemical structure of materials.Moreover,the surface nanostructure of modified Ni O electrode（XD-Ni O,X=S,Se,P）formed in the electrochemical oxidation process presents amorphous state,and its change process is from the initial amorphous state to the crystalline state and then back to the amorphous state,with significantly enhanced material conductivity.We speculate that during the introduction and exudation of these nonmetals,a large number of defect sites are enriched on the material surface.Moreover,we found by XPS that S content decreased from 46%to 4%before and after electrochemical oxidation.Similarly,we introduced two other nonmetals,P and Se,and the results were strikingly similar,with Se content dropping from 17%to 1%before and after electrochemical oxidation,and P content dropping from 25%to 3%before and after electrochemical oxidation.These results indicate that the electrochemical oxidation process has almost completely converted the precursor to nickel oxide.We speculate that in the process of the introduction and exudation of these nonmetals,a large number of defect sites are enriched on the material surface,thus many active sites are exposed,which is conducive to the enhancement of OER catalytic activity.The OER catalysts described in this paper may be particularly promising,to illustrate that many materials themselves generate new substances due to instability during OER processes,which are the real active substances.Moreover,the synthesis of XD-Ni O in this paper has universality,which provides a promising guidance for improving the catalytic performance of Ni-based oxides.