V doping optimizes the adsorption of H and H2O to promote the electrocatalytic hydrogen evolution of NiO nanosheets
The use of fossil fuels has resulted in serious environmental and energy crises,necessitating urgent exploration of a sustainable and environment-friendly alternative energy source to achieve the strategic goals of carbon peaking and carbon neutrality.In this regard,hydrogen energy offers the advantages of high density,pollution-free production,and abundant raw materials,and it presents a solution to the energy crisis.Electrochemical water splitting is an effective method for producing hydrogen energy,the hydrogen evolution reaction(HER)occurring at the involved cathode exhibits high Faraday efficiency,progresses under mild conditions and yields hydrogen with high purity,rendering the method suitable for use in large-scale hydrogen production.The key to achieving an efficient HER is finding a suitable catalyst.Presently,platinum-based precious-metal catalysts deliver the best electrocatalytic HER performance.However,their high cost due to scarcity restricts their large-scale application.Consequently,nonprecious-metal catalysts with abundant reserves and excellent electrolytic HER performance are urgently desired.Compared with other nonprecious metals,Ni,as a transition metal,offers high abundance,low toxicity and excellent electronic properties.Further,the outer layer of a Ni atom has unpaired 3d electrons,which easily pair with the electron in the Is orbital of a hydrogen atom to form Ni-H bonds during the HER.Ni exhibits considerable potential for catalytic application in the HER under alkaline conditions.Notably,Ni-based oxides,phosphates,sulfides,etc.show excellent electrocatalytic HER properties.Among them,NiO has recently garnered widespread research interest because of its simple preparation and low cost.For instance,researcher successfully synthesized a NiO/C nanocomposite electrocatalyst using eggshells as the carbon source.The synergistic effect between NiO and C effectively resulted in high electrocatalytic activity of the involved sample under alkaline conditions.Moreover,through oxygen vacancy regulation and cation exchange,researcher successfully prepared a NiO nanorod array electrocatalyst with abundant oxygen vacancies on a carbon fiber paper,the oxygen vacancies resulted in a substantially enhanced charge-transport performance of the involved NiO sample.Further,researcher prepared a NiO/Ni heterojunction electrocatalyst on carbon cloth through in situ surface reconstruction.The involved sample exhibited good hydrophilicity and abundant oxygen vacancies as well as excellent electrocatalytic HER activity and stability under alkaline conditions.However,because of the low conductivities and inappropriate adsorption strengths of H and H2O on the sample surface,NiO exhibited slow HER kinetics in alkaline media,considerably hindering its large-scale application as an HER catalyst.To address this issue,in this research,three-dimensional self-supported V-doped NiO nanosheet arrays were designed and successfully fabricated for catalytic application in the HER under alkaline conditions.Experimental results show that V doping can effectively improve the electrocatalytic HER activity of NiO,with the catalyst exhibiting good stability.Moreover,a V:Ni molar ratio of 10％is found to result in the best electrocatalytic HER performance.Experimental tests and density functional theory calculations show that V doping optimizes the electronic structure of NiO,effectively improves its charge transport performance,and increases its electrochemically active area.In addition,through optimization of the d-band center of the catalyst,excessive proton binding is inhibited,and through enhancement of H2O adsorption,the energy barriers involved in the rate-determining step(i.e.,water dissociation)of the HER are reduced.Overall,owing to various synergistic effects between V and NiO,the electrocatalytic HER performance of NiO is notably promoted through V doping.
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