Structure Modulation Of Ni-based Electrocatalysts For Enhanced Water Splitting Performance

Hydrogen energy,as a typical clean and sustainable green fuel that can replace tranditional fossil ones,has aroused considerable interest worldwide,due to its advantages of high energy density,non-polluting combustion products and wide sources.Water electrolysis technology is the most promising tactics to achieve sustainable hydrogen gas.However,the overall efficienty of water splitting is low because of the high activation energy barrier required for water electrolysis,which greatly limits the industrial-scale application.At present,noble-metal(Pt,Ir and Ru)-based materials are recognized as the best catalysts for commercial applications,but their wide application is greatly limited by high price and scarce reserves.Therefore,the design and development of efficient,stable,abundant reserves,cost-effective and environment-friendly non-precious electrocatalysts is the key to boost the efficiency of water electrolysis.Nickel-based catalysts have the superiorities of low cost,sufficient resources and good electrical conductivity,which are ideal alternative to noble-metal electrocatalysts.However,the catalytic activity and stability of most nickel-based catalysts cannot satisfy the demand of industrialization.In particular,the limited number of exposed active sites,low intrinsic activity and poor mechanical stability at high current density are the main factors to empede the improvement of its performance.In order to solve the problems,the modulation strategies including hierarchical nanoarray construction,heterostructure establishment and d-band electron density regulation of nickel-based catalysts were studied to increase the number of active site exposure,optimize its electronic structure and improve its mechanical stability under the large current density in this thesis,thereby establishing the relationship model of structure-performance-mechanism,and revealing the synergistic enhancement mechanism for catalytic activity and stability.Main conclusions are as follows:(1)A novel "all in one" NiV layer double hydroxide nanosheet array was synthesized in-situ on nickel foam(in-NiV-LDH/NF)through a facile one-step hydrothermal method.Interestingly,the valence configuration of V sites in such in-NiV-LDH are well dominated by the use of NF as the reducing regulator,achieving the in-NiV-LDH/NF with a high proportion of low-valent V ions(V3+and V4+),and then enhanced intrinsic electrocatalytic activity.Meanwhile,the integrated three-dimensional nanostructure can expose more reactive active sites,and enhance the mechanical stability during catalytic process.Therefore,the in-NiV-LDH/NF catalytic electrode requires the overpotentials of 114 mV in 1.0 mol/L KOH and 156 mV in 1.0 mol/L PBS solution to obtain the current density of 10 mA/cm2 for hydrogen evolution respectively,as well as a robust catalytic stability for at least 100 h in both conditions.(2)The vanadium-doped Ni2P/Ni12P5 nanoflower arrays in-situ grown on nickel foam(V-Ni2P/Ni12P5/NF)with 3D heterostructures were successfully prepared by one-step solid-phase phosphating method employing NiV-LDH/NF as precursor.Results indicate that V doping effectively regulates the local charge density of the heterointerface,and optimize the d-electron energy level structure and hydrogen adsorption/desorption free energy,thereby promoting the electrocatalytic HER kinetics.Meantime,the 3D nanoflower morphology possesses a large specific surface area,favoring the sufficient exposure of reactive active sites and the rapid escape of bubbles,which contributes to the synergistic improvement of the catalytic activity and stability of the electrode under high current density.Therefore,the V-Ni2P/Ni12P5/NF electrode exhibits prominent electrocatalytic HER performance,requiring the overpotentials of 215 mV,306 mV and 355 mV)in an alkaline simulated seawater medium to deliver the large current densities of 100 mA/cm2,500 mA/cm2 and 1000 mA/cm2,respectively.(3)The "all in one" vanadium-modulated Ni3Se2 nanorod@nanosheet arrays in-situ grown on nickel foam(V-Ni3Se2/NF)were successfully fabricated via solvothermal method employing NiV-LDH/NF as precursor,achieving the synergistic regulation of geometric morphology and surface/interface electronic structure.Results demonstrate that V induces the construction of superhydrophilic integrated V-Ni3Se2/NF nano-hierarchical structures,which not only fully exposes abundant catalytic active sites to accelerate the electron/ion transfer rate,but also optimizes its electronic structure to improve the intrinsic catalytic activity.Therefore,the as-prepared V-Ni3Se2/NF catalyst exhibits remarkable catalytic activity and stability for water splitting,yielding a cell voltage of 1.56 V to deliver 10 mA/cm2 together with an extraordinary long-term durability for 80 h,far outperforming the electrolytic cell composed of noble metal Pt/C/NF and IrO2/NF counterpart(1.76 V).(4)The vanadium-doped metallic NiSe/Ni3Se2 heterostructure nanoflower arrays(VNS)in-situ grown on nickel foam were successfully synthesized through a facile solvothermal method.Results show that the VNS-160 electrode prepared at 160℃ possesses the best catalytic performance,in which V atom can regulate the electronic structure of Ni/Se atom near Fermi level in VNS-160,thereby enhancing the intrinsic catalytic activity.Additionally,the construction of "all in one" VNS-160 hierarchical nanostructures is beneicial to increase the electrochemical active area and achieve a stable load on the substrate,thereby synergistically impoving the electrocatalytic activity and stability.Therefore,the VNS-160 electrode delivers the large current densities of 100 mA/cm2,500 mA/cm2 and 1000 mA/cm2 at quite low overpotentials of 175 mV,275 mV and 348 mV and maintains the cayalytic stability for at least 11 h at 100 mA/cm2 in1.0 mol/L KOH medium.(5)A novel hollow nanoarchiture assembled by N-doped carbon coated NiSe nanosheets(NiSe@NC)was synthesized by liquid-solid phase combination method using SiO2 spheres as hard template.Results indicate that the NC shell of NiSe@NC catalyst as the "armor" can effectively improve the structural stability of NiSe core in strong alkaline medium,and avoid the agglomeration of NiSe particles,thus increasing the exposure area of catalytic active sites.In addition,the N doping regulates the surface chemical state of carbon layer,and synergistically modulates the electronic structure of NiSe@NC material with NiSe,thus enhancing its intrinsic catalytic activity.Therefore,the hollow NiSe@NC catalyst merely requires an overpotential of 169 mV to deliver 10 mA/cm2 in alkaline condition for HER,and maintain stably for at least 80 h,far exceeding the catalytic activity and stability of NiSe material.