Structural Modulation, Interface Construction Of Nickel-based Compounds And Their Performance In Electrocatalytic Water Splitting

Electrocatalytic water splitting to produce hydrogen is a very promising technology for the conversion and storage of renewable energy in the future.Currently,the noble-metals Pt and IrO2/RuO2 are commonly used as electrocatalysts for hydrogen evolution reactions(HER)and oxygen evolution reactions(OER),but their large-scale applications are limited due to their low reserves and high costs.Exploring HER and OER electrocatalysts with abundant reserves,excellent electrocatalytic performance and high electrochemical stability to replace noble-metals catalysts is of great significance for the large-scale commercial application of electrocatalytic water splitting.With the goal of industrial hydrogen production,it is significant to prepare electrocatalysts with low cost,high efficiency,and use them as bifunctional HER and OER catalysts to construct an electrocatalytic overall water-splitting system.Transition metal materials,especially nickel-based compounds,are expected to be ideal substitutes for noble-metals catalysts due to their abundance,low cost,controllable preparation and diversity of components,and have the potential to serve as bifunctional electrocatalytic water-splitting catalysts.Currently,the electrocatalytic water-splitting performance of single nickelbased transition metal hydroxides or oxides is not satisfactory because of their two-dimensional(2D)structure prone to agglomeration,low electrochemical conductivity and other problems,which lead to a small electrochemically active area,slow reaction kinetics and limited catalytic activity during the electrocatalytic reaction.This paper is based on the synthesis of various nickelbased compounds and proposes reasonable design and optimization strategies for them.Through structural regulation and interface construction,such as doping,heterostructure,defects/vacancies,etc.,the composition,microstructure,and electronic structure of nickel-based materials were adjusted.The nickel-based compounds electrocatalysts with excellent HER,OER and bifunctional electrocatalytic water-splitting performance were constructed,showing superior electrocatalytic activity and stability.The detailed research work and conclusions of this thesis are as follows:(1)A novel three-dimensional(3D)Ni3S2/Cu-NiCo LDH heterostructure nanosheet array was synthesized on nickel foam(NF)by solvothermal and hydrothermal methods,which has excellent HER,OER and overall water splitting electrocatalytic activities.The chemical composition,morphological structure,electronic structure and catalytic reaction active sites of the electrocatalysts were clarified through a series of controlled tests.The doping of Cu ions and Ni3S2 in NiCo LDH resulted in the conversion of the partial crystals to an amorphous phase,resulting in the generation of interface defects and more active sites.The 3D hierarchical nanosheet arrays can accelerate the electrolyte diffusion,resulting in fast electron/mass transport rates and excellent electrochemical stability.The electrochemical test results show the overpotentials of 119 mV and 218 mV for OER and 156 mV and 304 mV for HER at 10 and 100 mA cm-2,respectively.When using the Ni3S2/Cu-NiCo LDH heterostructure nanosheet array as both the cathode and anode of the overall water electrolytic cell,the overall water splitting at 1.75 V can reach 100 mA cm-2.(2)The nickel-based transition metal oxides have properties similar to those of hydroxides.A single nickel-based transition metal oxide also has many shortcomings,and the use of doping and sulfurization methods can effectively enhance the HER,OER and overall water splitting electrocatalytic performance of the catalyst.VS2/NiSx-Zn-NiMoO4 multiphase interface nanosheet arrays were synthesized on NF by two-step hydrothermal methods as highly active electrocatalysts.The microstructure,phase structure and electronic structure of NiMoO4 were designed and optimized using specific concentrations of Zn2+doping and multiphase transition metal,resulting in a synergistic catalytic effect on the constructed multiphase interface nanosheets,as well as fast electron transfer rate,and ultimately achieving excellent electrocatalytic performance.When VS2/NiSx-Zn-NiMoO4/NF was applied for HER and OER in an alkaline solution,it demanded only 162 and 118 mV overpotential at 100 mA cm-2 current density,respectively.The two-electrode electrolytic cell was assembled with VS2/NiSxZn-NiMoO4/NF as cathode and anode,which required only 1.60 V for overall water splitting at 100 mA cm-2,and it was able to operate continuously for 50 h at 100 mA cm-2,demonstrating excellent electrochemical stability.(3)Carbon materials exhibit high application value in the field of catalysis due to their excellent stability and unique surface structure.Therefore,the introduction of 3D network nitrogen-doped carbon frameworks into nickel-based oxides can effectively improve the conductivity and catalytic activity of electrocatalysts.Multiphase interface catalyst(P-NiFe2O4/NCNTs/NiFe)of the Pdoped NiFe2O4 nanoparticles were uniformly loaded on the bamboo-like N-doped carbon nanotubes grown via NiFe alloy autocatalysis was successfully synthesized by a step-by-step process of pyrolysis-solvothermal-phosphorization.Compared with pure NiFe2O4 nanoparticles,the catalyst can significantly increase the conversion frequency(TOF)in HER and OER.Due to the synergistic effect between P doping,ultrafine NiFe2O4 nanoparticles and NCNTs/NiFe,the intrinsic active sites are increased,thereby improving the HER,OER and overall watersplitting performance.The catalyst prepared using this structural design exhibits excellent reaction kinetics and catalytic stability.The overpotentials of HER and OER at 10 mA cm-2 in 1 M KOH is only 111 mV and 266 mV,respectively.After analyzing the effect of different concentrations of urea additions on the electrocatalytic performance,the results showed that the overpotential of UOR at 10 mA cm-2 in 1 M KOH containing 0.6 M urea is 1.34 V.The two-electrode electrolytic cell of UOR assembled using P-NiFe2O4/NCNTs/NiFe as cathode and anode requires a potential of only 1.467 V at a current density of 10 mA cm-2,lower than 1.604 V required potential for overall water splitting,and the voltage loss was only 7 mV after 50 h of continuous operation on overall water splitting containing 0.6 M urea,indicating the excellent electrocatalytic stability of the catalyst for urea electrolysis.(4)Developing an efficient and controllable strategy to precisely regulate the morphology and electronic structure of nickel-based compounds is also a new direction worth exploring.An electrochemical lithiation modulation strategy is proposed to effectively convert micron-sized LiNiO.8Co0.1Mn0.1O2 particles into novel Li1+x(NiCoMn)O2 nanosheet catalysts gradually by precisely modulating the intercalation amount of lithium ions at different lithiation voltages,and to reduce transition metal oxides into transition metal nanoparticles.This method can effectively regulate the chemical states of Ni,Co,and Mn elements and generate a large number of oxygen vacancies and some amorphous regions in the structure.The electrochemical test shows that the prepared novel nanosheet catalyst exhibits excellent electrocatalytic performance for HER,OER and overall water splitting.The overpotentials are 58 mV and 147 mV for HER and 222 mV and 341 mV for OER at 10 and 100 mA cm-2,respectively.The constructed electrolytic cell for overall water splitting only requires 1.74 V to reach 100 mA cm-2.(5)The preparation of electrodes also directly affects the catalytic activity of catalysts.Some problems exist in traditional electrodes,such as the use of binders,which cause blockage of catalyst active sites and affect the intrinsic catalytic activity of electrocatalysts.The use of self-supporting substrates makes it difficult for the catalyst to peel off on the surface,which is not conducive to recovery.By constructing a novel Ni/NiFe2O4@PPy core-shell nanosphere magnetic electrode,the catalytic performance of the electrocatalysts in HER,OER and overall water splitting can be significantly enhanced,and can be conveniently and quickly recycled by demagnetizing the electrode.The Ni/NiFe2O4@PPy core-shell nanorods magnetic electrode showed excellent electrocatalytic activity with the overpotentials of 127 mV and 236 mV for HER and 265 mV and 370 mV for OER at current densities of 10 and 100 mA cm-2,respectively.The overall water electrolytic cell with Ni/NiFe2O4@PPy electrodes can achieve 10 mA cm-2 at only 1.64 V,and the catalyst has been shown to have excellent long-term stability after 14 h of stability test.