Study On Geometrical And Electronic Structure Regulation Of Non-noble Electrocatalysts And Their Electrocatalytic Performances For Water Electrolysis

With the rapid development of modern industry,the exhaustion of traditional fossil energy and the environmental problems caused by fossil energy combustion have forced us to develop an efficient and clean energy source.In recent years,hydrogen energy has attracted great attention as an efficient and clean energy source.Hydrogen production form water electrolysis has become the most attractive method due to its advantages of simplicity,low pollution,high efficiency and high purity of hydrogen production.However,the technology of water electrolysis for hydrogen production suffers from the problems of high reaction overpotential and high energy consumption.How to reduce the required electrolyzer voltage of water electrolysis is the core issue of hydrogen production by water electrolysis with high efficiency and energy saving.At present,the main electrocatalysts for the anode and cathode of water electrolysis are noble metal based materials,such as Ru,Ir and Pt,which can effectively reduce the overpotential required for hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）and achieve a decrease in electrolyzer voltage.However,the high price of these noble metals greatly increases the cost of hydrogen production from water electrolysis,thus limiting the industrial development of water electrolysis.Therefore,the development of highly efficient and stable non-noble metal based electrocatalysts is the key to water electrolysis.In view of the above problems,this paper focuses on the regulation of geometry and electronic structure of electrocatalysts.From the aspects of increasing the number of active sites of catalysts and improving the intrinsic activity of catalytic sites,the following six aspects of work have been carried out:（1）Aiming at the problems of carbon layer stacking,excessive growth and agglomeration of carbide crystals during high-temperature synthesis of carbide,a"self-assembly and preshaping"strategy was developed to prepare ultra-thin nitrogen-doped graphitic carbon lamellas supported Mo₂C nanoparticles（2D Mo₂C/C-lamellas）.By self-assembling the MoO₄^2-and sodium alginate in aqueous solution and then lyophilizing,a 3D aerogel precursor constructed of 2D molybdate-organic lamellas was formed.Benefiting from the supporting effect of the preshaped 3D structure,the initial lamellar structure of each 2D unit was maintained and serious overlapping of carbon were avoided.The strong absorption effect of sodium alginates made the Mo anchored on long-chain organic matter,featuring the obtained product with highly-dispersed ultrafine Mo₂C nanoparticles.Electron microscope images showed Mo₂C particles of 3-4 nm were uniformly dispersed on graphitic carbon lamellas with a thickness of¹0 nm.Electrochemical results showed that 2D Mo₂C/C-lamellas exhibited better HER activity and stability compared with Mo₂C/C-bulk synthesized by traditional methods.（2）In order to reduce the high temperature required for carbide synthesis and the uncontrollable morphology,a controllable"metal-induced crystallization"method was developed to synthesize the 3D integrated electrode（Ni-VC@C/Ti）composed of carbon-coated nickel-modified vanadium carbide in-situ on Ti plate at a lower temperature.The Ni-VC@C/Ti achieved a high HER efficiency in both acid(η_@10mA/cm2=138 mV)and alkaline(η_@10mA/cm2=146 mV)environment with excellent stability.The incorporated Ni in vanadate precursor can selectively induce and stabilize the generation of active VC phase at a lower temperature,which avoids the damage of the 3D morphology of precursor.In addition,the pre-reduced metal Ni catalyzed the formation of graphitic carbon layer on the catalyst surface,which optimized the electron transfer and surface hydrogen adsorption energy for HER process and enhanced its corrosion resistance in strong acid and alkali.（3）Aiming at the underlying mechanism of synergistic effect of metal/metal oxide catalysts on HER,the essence of efficient HER activity of metal/metal oxide-"chimney effect"was discovered by combining DFT calculation and experiments.This special chemical environment around the interface leaded the neighboring sites to be immune to the H₂O*and OH*adsorption and to only selectively adsorbed H*properly.Meanwhile,it was also beneficial for the smooth adsorption of the reactant（H*）on the interface and the easy desorption of the product（H₂）from the catalyst surface(ΔG_H*close to zero).This phenomenon appears similar to a chimney of hydrogen evolution around the metal oxide/metal interface.Such“chimney effect”was a result of the interfacial charge transfer between the metal and metal oxide,and should be the nature of the interface-induced synergistic effect in metal oxide/metal composite catalysts for HER.Experiments further changed the content of NiO/Ni composite catalyst to adjust the content of interface active sites,and studied its relationship with HER activity.XPS and electrochemical results showed that the change trend of the interface content in NiO/Ni catalysts was consistent with its catalytic activity,indicating that the atoms at the interface were the main active sites for hydrogen evolution,which verified the correctness of the"chimney effect"of metal/metal oxide for HER.（4）Based on the guidance of above"chimney effect"of metal/metal oxide catalysts for HER,an in-situ metal precipitation method was developed for the synthesis of amorphous oxide（MoO_x）dispersed crystalline metal nanoparticles（Ni）catalyst,which maximized the amount of active metal/metal oxide interfaces.High resolution TEM images found that Ni nano particles with a diameter of⁸ nm were uniformly dispersed in MoO_x nanowires,forming a solid solution structure with abundant Ni/MoO_x interface.In addition,the critical amorphous structure of the highly oxidized MoO_x enhanced the charge transfer between Ni and MoO_x and thus effectively improved the intrinsic activity of the metal oxide/metal interface sites.Electrochemical results showed that the Ni/MoO_x catalyst owned excellent HER catalytic activity and stability exceeded Pt/C in the alkaline solution,as a small overpotential of 33 mV was required to deliver a current density of 10 mA cm^-2 and no decay was observed at 10 and 100 mA cm^-2 for over 100h.The relationship between the Ni content and the reduction temperature and the degree of crystallization of MoO_x was also investigated,as well as its influence on the amount of active interfaces and the charge transfer strength of the interface.The consistency of the Ni/MoO_x interface site content with the trend of HER activity indicated that the Ni/MoO_x interface was the main HER active site and was the determinant of HER activity of Ni/MoO_x catalysts.（5）Aiming at the low OER performance of non-noble metal catalysts and the difficulty in balancing activity and stability,a“dual ligand synergistic modulation”strategy was proposed to precisely adjust the electronic structure of transition metals in atomic level,achieving the perfect unification of high activity and stability in catalysts.The optimized NiCo₂（SOH）_x performed a small overpotential of 0.29 V at 10 mA cm^-2,and showed no decay after 30h accelerated ageing at 100 mA cm^-2,both of which exceeded the commercial RuO₂ and Ir/C benchmarks.Theoretical calculations show that the synergy between S ligands and OH groups appropriately tuned the electronic structure of NiCo₂（SOH）_x and rendered it with optimal binding energies for OER intermediates（OH*,O*,and OOH*）,which is benefit for facilitate the O₂ evolution proceeding.In addition,the OH ligands on the surface of NiCo₂（SOH）_x could attracted electrons from the antibonding orbital of M-S bonds to M-O bonds,resulting in a strengthened binding energy between metal and S and thus enhancing the stability.Besides,the varied magnetism induced by the modulation of catalysts’electronic structure significantly influenced the desorption action of paramagnetic O₂ from the catalyst surface.The special non-magnetic NiCo₂（SOH）_x suffered the lowest O₂desorption resistance and proceeded the smoothest reaction kinetics pathway among NiCo₂(S_xOH_2-x)_y catalysts,thus owning the topmost OER activity.（6）In view of the limited types of heterojunction electrocatalysts for performance regulation,a monometallic heteroligand NiO-Ni₃S₂ heteronanosheet supported on Ni foam（NiO-Ni₃S₂/NF）was developed as a bifunctional electrocatalyst for overall water splitting.The interaction between the tightly combined metallic Ni₃S₂ and NiO domains created an especial active interface zone for HER and OER.Besides,the 3D self-supported architecture of NiO-Ni₃S₂ heteronanosheet provided abundant accessible active sites and fast charge transfer paths for the catalytic reactions.As expected,such NiO-Ni₃S₂ heteronanosheets achieved synergistically enhanced activity for both HER and OER in 1M KOH solution.In particular,the NiO-Ni₃S₂/NF heteronanosheet catalyzed a HER current density of 10 mA cm^-2 with a low overpotential of 71 mV,and further surpassed the performance of Pt/C when the overpotential exceeds 150 mV.When functioning NiO-Ni₃S₂/NF as both cathode and anode,an electrolyzer voltage of1.57 V was required to deliver a current density of 10 mA cm^-2 as well as excellent durability at 100 mA cm^-2 for over 50 h.DFT calculation revealed that the established interfaces between Ni₃S₂ and NiO facilitated the synchronous chemisorption of hydrogen and oxygen-containing intermediates,consequently improving the overall electrochemical water-splitting activity.