Charge Transfer Regulation In Ni,Co-based Materials For Optimizing The Energy Storage/electrocatalytic Performance

Efficient energy storage and conversion technologies are highly desired because the global environmental pollution and energy issues become increasingly prominent.In terms of energy storage,to meet the development needs of electronic equipments,hybrid electric vehicles and smart grid for large power supply,it is desired to develop novel electrode materials for supercapacitors with high power density,high energy density,good high-rate capability and long cycling life.In terms of energy conversion,electrochemical water splitting,including anodic oxygen evolution(OER)and cathodic hydrogen evolution,is an easy and green process to produce hydrogen without carbon emission.However,the sluggish kinetics of the four-electron OER limit the development of electrochemical water splitting,and developing efficient and low-cost electrocatalysts is one of the solutions.In this thesis,by taking a series of Ni,Co-based materials as a platform,advaned functional materials for supercapacitor and OER have been designed via regulation of internal electric field,bonding way,doping and ion release.The main content and conclusion are as follows:1.Based on the regulation of internal electric field,to improve the capacitive performance of traditional capacitive material Co3O4,simple hydrothermal method,reflux method and calcination method are applied to synthesize ZnO/Co3O4 composites.The specific capacity,high-rate capability and cycling stability of ZnO/Co3O4 are respectively 1.4,4.7 and 1.1 times higher than these of Co3O4.The enhanced capacitive performance of ZnO/Co3O4 is ascribed to the formation of heterojunction and internal electric field,which could facilitate the transmission of OH-.Moreover,one-dimensional porous structure increases the specific surface area of the materials and shortens the ion diffusion distance.This work innovatively introduces heterojunction into electrode materials of supercapacitor,and the capacitive performance has been significantly improved.However,the high-rate capability of ZnO/Co3O4 still can not satisfy the practical application for supercapacitor.2.Based on the regulation of bonding way,to further improve the high-rate capability of Ni,Co-based materials,hydrothermal method followd by phosphorization is applied to synthesize NiCoP.The NiCoP with good conductivity exhibits urchin-like hollow structure.The phase and morphology of the products are regulated by adjusting the feed ratio of precursor and phosphorus source.The optimized material shows excellent high-rate performance,and the capacity retained 91.1%from 1 to 20 A g-1.The NiCoP is promising for high-performance supercapacitor.3.Doping plays a vital role in capacitive performance,and a novel strategy for the preparation of high-performance Ni-based electrode materials by in-situ growth is proposed.Sulfur-doped Ni(OH)2 nanoplates are in situ grown on nickel foam via one-step hydrothermal method without additional nickel source and surfactant.The prepared material exhibits excellent capacitive performance,and the maximum areal capacity,energy density and power density are 1.72 C cm-2,3.59 W h m-2 and 44.62 W m-2,respectively.The results of density functional theory calculation show that sulfate modification improves the surface wettability and conductivity of the material.4.Ion release affect the electronic structure and active site of materials,and sulfate-functionalized Ni(OH)2(S-Ni(OH)2)as an OER electrocatalyst are further investigated.Compared with traditional Ni(OH)2 materials with poor stability,S-Ni(OH)2 exhibits sustainedly self-enhanced OER activity.After 110-h reaction at 100 mA cm-2,the overpotential decreases by 70 mV instead of increasing.The self-enhanced activity is found to be related to the release of sulfate during OER by analyzing the changes in morphology,phase,crystalline and composition after anodic oxidation.New active NiOOH/Ni(SO4)0.3(OH)1.4 sites with higher catalytic activity are formed during OER.This work provides new ideas for designing highly stable OER electrocatalysts.This thesis verifies that the capacitive performance of Ni,Co-based materials can be imprved by constructing heterojunction,phosphorization and doping.The sulfate release from Ni(OH)2 is firstly demonstrated to enhance the OER activity of Ni-based materials here.These results have important scientific value and practical significance in terms of material optimization design and control for energy storage and coversion.