Modulation Of Interface Engineering Toward Ni/Co-Based Nanomaterials And Their Studies In Electrocatalytic Water Splitting

With the rapid development of modern society,there is an urgent demand for energy.The intensive consumption of traditional fossil fuels aggravates the energy crisis and environmental pollution.It is of great significance to develop clean and efficient green energy to replace traditional fossil fuels.Hydrogen energy is one of the most promising green energy.It possesses the advantages of high energy density,no pollution,and renewable.Water electrolysis is an efficient method to produce high-purity hydrogen.It includes two parts of hydrogen evolution reaction(HER)and oxygen evolution reaction(OER),which are carried out on the cathode and anode of the electrolytic cell,respectively.During the water electrolysis,the hysteresis reaction kinetics decrease the reaction rate.Therefore,it is necessary to apply efficient electrocatalysts to reduce the reaction energy barrier and energy consumption.Noble metal-based electrocatalysts,which include Pt,IrO2,and RuO2,are found to possess excellent HER and OER electrocatalytic activities,respectively.However,high cost,limited reserves of natural resources,and poor stability impede its wide application.In the past few decades,transition metal-based electrocatalysts have been demonstrated to possess excellent electrocatalytic activities toward electrocatalytic water splitting,such as transition metal sulfides,selenides,and hydroxides.To further enhance the electrocatalytic activities of transition metalbased electrocatalysts,various strategies have been developed,such as morphology and structure control,surface engineering regulation,and interface engineering regulation.This paper focuses on the effects of the interface engineering regulation on the electronic structure and the electrocatalytic performance of Ni/Co-based nanomaterials.There exist rich heterointerfaces on the designed heterojunction nanomaterials.This is because the lattice mismatch commonly exists between the different components in the heterojunction nanomaterials,and the amounts of reactive sites expose on the heterointerfaces,which is conducive to increasing the electrochemical surface area.In addition,there exists strong electron interaction between the different components in the heterojunction nanomaterials,and electrons redistribute on the heterointerfaces until a new charge balance is reached,which is conducive to regulating the electronic structure.On the one hand,it is beneficial to improve the electrical conductivity of nanomaterials.On the other hand,it can enhance the adsorption energy toward different reaction intermediates.The reaction energy barrier in the rate-determining step is reduced,the reaction pathway is optimized,and the reaction kinetics is accelerated.In this paper,Ni/Cobased heterojunction nanomaterials were prepared by hydrothermal method,hightemperature calcination method,and other means,and the effects of the interface engineering regulation on the electronic structure and the electrocatalytic activities were studied in detail as follows:Part Ⅰ:NiS2/VS heterojunction nanosheets were synthesized on the carbon cloth by hydrothermal method and high-temperature vulcanization.Compared with single NiS2 or VS,the obtained NiS2/VS-3:1 heterojunction nanosheets electrocatalyst possessed higher electrocatalytic activities.It needed low overpotentials of only 74 and 235 mV to obtain 10 mA cm-2 toward HER and OER in 1 M KOH.The outstanding electrocatalytic performance of the NiS2/VS-3:1 electrocatalyst could be attributed to the following aspects:(1)There existed strong electron interaction between NiS2 and VS,and electrons more easily transferred from NiS2 to VS,which promoted the formation of more high valence state Ni3+and low valence state V2+in the NiS2/VS-3:1 electrocatalyst.During the OER process,high valence state Ni3+helped enhance the adsorption energy toward different reaction intermediates,reduced the reaction energy barrier of the ratedetermining step,optimized the reaction path,and speeded up the reaction kinetics.During the HER process,low valence state V2+could provide extra electrons to combine with hydrogen protons,which induced the generation of hydrogen molecules,thus helping to speed up HER kinetics.(2)Its porous nanosheet structure was beneficial to full contact with the electrolyte and accelerate the reaction kinetics.The rich heterointerfaces between NiS2 and VS also facilitated the exposure of reactive sites,which was conducive to increasing the electrochemical surface area and enhancing the electrocatalytic activities.Adding some more easily oxidized substances(such as urea,furfural,furfuryl alcohol,and 5-hydroxymethylfurfural)into 1 M KOH was expected to achieve energy-saving hydrogen production.pollutant degradation,and biomass transformation.The NiS2/VS-3:1 electrocatalyst exhibited a promising prospect.Part Ⅱ:NiSe2/Ni0.85Se heterojunction nanooctahedron electrocatalyst was synthesized by hydrothermal method and chemical etching treatment.There existed strong electron interaction between the newly-formed Ni0.85Se and NiSe2,and electrons more easily transferred from NiSe2 to Ni0.85Se,which regulated the charge-state of NiSe2 and Ni0.85Se,promoting the increase of the charge-state for Ni0.85Se and the decrease of the charge-state for NiSe2.During the HER process,Ni0.85Se with a higher charge-state could provide extra electrons to combine with hydrogen protons,which induced the generation of hydrogen molecules,thus accelerating HER kinetics.During the OER process,NiSe2 with a lower charge state was more easily oxidized into high valence state Ni3+,which was conducive to enhancing the adsorption energy toward different reaction intermediates,reducing the reaction energy barrier for the rate-determining step,optimizing the reaction pathways,and accelerating the reaction kinetics.DFT calculation manifested that interface engineering regulated the electronic structure of the heterointerfaces,and the heterointerfaces exhibited better Gibbs adsorption free energy toward the intermediate H,corresponding to the higher HER electrocatalytic activities.The electrocatalysts with the highest electrocatalytic activities could be obtained by adjusting the reaction time of chemical etching.Compared with single NiSe2 or other NiSe2/Ni0.85Se heterojunction electrocatalysts,the NiSe2/NiO.85Se-2h electrocatalyst possessed better electrocatalytic activities,which required low overpotentials of 76 and 232 mV to obtain 10 mA cm-2 toward HER and OER in 1 M KOH.When using the NiSe2/Ni0.85Se-2h electrocatalyst as both the cathode and anode,it only required a working potential of 1.54 V to obtain 10 mA cm-2.There existed rich heterointerfaces on the heterojunction electrocatalysts,and the reactive sites were completely exposed on the heterointerfaces.It could also be applied for urea oxidation reaction(UOR).It needed a low working potential of only 1.32 V to reach 10 mA cm-2 in 1 M KOH with 0.3 M urea,which indicated that the addition of urea significantly reduced the energy consumption for hydrogen production,and was beneficial to realize energy-saving hydrogen production.Part Ⅲ:CoSe2/MoSe2 heterojunction nanosheets with rich Se vacancies(CMSV)were fabricated on nickel foam by hydrothermal method,hightemperature selenization treatment,and chemical etching treatment.Compared with CoSe2/MoSe2,the CMSV electrocatalyst delivered higher electrocatalytic activities,it required low overpotentials of 74 and 242 mV to obtain 10 mA cm-2.When using the CMSV electrocatalyst as both the cathode and anode,it needed a low working potential of 1.55 V to reach 10 mA cm-2.Compared with single CoSe2,the CMSV electrocatalyst exhibited better electrocatalytic activities,it was mainly because the design of heterointerfaces regulated the electronic structure.There existed strong electron interaction between CoSe2 and MoSe2 in the CMSV electrocatalyst,and electrons more easily transferred from CoSe2 to MoSe2,which was beneficial to regulate the electronic structure at the heterointerfaces,enhance the adsorption energy toward different reaction intermediates,reduce the reaction energy barrier,optimize the reaction pathways,and accelerate the reaction kinetics.Rich heterointerfaces existed on the heterojunction electrocatalysts,which helped expose the reactive sites,increase the electrochemical surface area,and improve the electrical conductivity.In addition,the CMSV electrocatalyst exhibited better electrocatalytic activities than CoSe2/MoSe2,it was mainly because the introduction of rich Se vacancies enhanced the electrocatalytic activities of the heterojunction nanomaterials.After H2O2 etching,partial surface Se atoms in the electrocatalyst were removed,the atomic arrangement on the electrocatalyst’s surface got changed,and the electronic structure was optimized.The introduction of Se vacancies was conducive to the adsorption of different reaction intermediates,thus accelerating the reaction kinetics.Therefore,interface engineering regulation and surface engineering regulation could be simultaneously applied to enhance the electrocatalytic activities of nanomaterials.The CMSV electrocatalyst could also be applied for hydrazine oxidation reaction(HzOR),which required a low working potential of-0.058 V to reach 10 mA cm-2 in 1 M KOH with 0.5 M hydrazine.The working potential significantly decreased to 0.108 V when using the CMSV electrocatalyst as both the cathode and anode to perform hydrazine-assisted overall water splitting,which indicated that the addition of hydrazine significantly reduced the energy consumption for hydrogen production,and was conducive to realizing energy-saving hydrogen production.Part Ⅳ:Mn-doped NiCo hydroxide nanosheets intertwined with carbon nanotubes(1.5Mn-NiCo HNS/CNT)electrocatalyst was prepared by a wet chemical method.Compared with single NiCo HNS/CNT electrocatalyst,the 1.5Mn-NiCo HNS/CNT electrocatalyst delivered better OER electrocatalytic activities,an overpotential of 239 mV was needed to drive 10 mA cm-2 in 1 M KOH.The excellent OER electrocatalytic activities of the 1.5Mn-NiCo HNS/CNT electrocatalyst could be attributed to the following points:(1)Strong electron interaction existed between the Mn element and the host material,and electrons more easily transferred from Mn element to Ni or Co sites,which increased the valence state of Mn element,enhancing the adsorption energy toward different reaction intermediates,and accelerating the reaction kinetics.(2)The introduction of carbon nanotubes could effectively improve the electrical conductivity of the nanocomposites.The combination of one-dimensional carbon nanotubes and twodimensional NiCo hydroxide nanosheets helped construct a three-dimensional conductive network structure,and electrons were more easily transported to each nanosheet through carbon nanotubes,thus speeding up the transportation of electrons in the nanocomposites.Carbon nanotubes could also reduce the aggregation and the stack of nanosheets to a certain extent,which was beneficial to the full contact between the surface of nanosheets and the electrolyte,and increased the electrochemical surface area.(3)NiCo hydroxide possessed a hexagonal nanosheet structure and had a large specific surface area,which was conducive to the full contact with the electrolyte,speeding up the reaction kinetics.DFT calculation manifested that the Mn-substituted Co sites had better OER electrocatalytic activities,which reduced the reaction energy barrier for the ratedetermining step,optimized the reaction pathways,and accelerated the reaction kinetics.The 1.5Mn-NiCo HNS/CNT electrocatalyst also possessed excellent UOR and HzOR electrocatalytic activities.It required low working potentials of 1.35 V and 1 mV to obtain 10 mA cm-2 in 1 M KOH with 0.5 M urea or 1 M KOH with 0.5 M hydrazine.The addition of urea or hydrazine could help reduce the energy consumption for hydrogen production and was beneficial to realize the energysaving hydrogen production.The 1.5Mn-NiCo HNS/CNT electrocatalyst exhibited a promising prospect in the field of domestic sewage,and industrial and agricultural wastewater treatment.