Structure Oriented Design Of Nickel And Cobalt Based Catalysts For Zn-air Batteries

Facing the energy crisis and environmental problems,the importance of new energy storage and conversion technology are increasingly realized for sustainable development.Zn-air batteries is one of the new energy technologies with great development potential.However,the air electrode still needs catalysts to accelerate the chemical reactions involved in charge and discharge process of battery.It is a key and challenge to develop efficient,low-cost and stable non-noble metal based catalysts.Based on the structure-activity relationship between structure and performance,nickel and cobalt based catalytic materials with good intrinsic catalytic activity were selected.Here,a series of catalysts with higher catalytic activity were designed and synthesized through the improvement strategies,including partial substitution of metal atoms,doping,morphology regulation and heterointerface construction,and their performance in Zn-air battery was explored.The mainly work in this dissertation is as follows:1.Single phase bimetallic nickel cobalt sulfide（Ni,Co）S₂ was prepared by hydrothermal method and subsequent low-temperature vulcanization,which was used as an efficient bifunctional electrocatalyst for oxygen evolution reaction（OER）and oxygen reduction reaction（ORR）.The synergistic effect of nickel and cobalt improves the electron transport performance and reduces the reaction energy barrier.Therefore,（Ni,Co）S2 shows excellent electrocatalytic activity for OER/ORR in alkaline electrolyte.The first principle calculation results show that the overpotential required for the potential limiting step of（Ni,Co）S₂ is much lower than that of the single metal sulfide,endowing the better catalytic activity.（Ni,Co）S₂ based Zn-air battery has excellent performance and can effectively provide power for the electrochemical water splitting device with（Ni,Co）S2 as two electrodes,indicating that it has broad application prospects in energy conversion and energy storage devices in the future.2.Nitrogen doped NiS_1.03 hollow sphere(N-NiS_1.03 HS)was designed and synthesized,which was used as an efficient bifunctional ORR/OER electrocatalyst.Through the electronic regulation of nitrogen doping and the structural design of hollow spheres,the catalytic activity of N-NiS_1.03 HS has been greatly improved.The required overpotential is 250 mV to reach the current density of 10 mA·cm-2 for OER,and the half wave potential for ORR is 0.72 V.The results of density functional theory show that nitrogen doping can downshift the d-band center of the catalyst and reduce the reaction energy barrier simultaneously,which is conducive to the adsorption/desorption of oxygen intermediates on the active sites.When it is used as the air electrode of Zn-air battery,it displays the high open circuit potential（1.41 V）,large specific capacity（821 mAh·g-1Zn at the current density of 10 mA·cm-2）,and excellent charge discharge cycle stability（working for 60 hours at 5 mA·cm-2）.It shows great potential in the application of energy-saving zinc air battery in the future.3.A self-supporting NiO/Co₃O₄ heterojunction was constructed,which showed efficient catalytic activity for oxygen evolution.Thanks to the abundant heterogeneous interface and oxygen vacancies at the interface,numerous low coordination atoms emerge in NiO/Co₃O₄.These low coordination atoms effectively regulate the electronic structure of the surrounding atoms and act as active centers.Therefore,NiO/Co₃O₄ heterojunction exhibits enhanced electrocatalytic activity for OER/ORR process.Density functional theory（DFT）calculation confirmed that the d electronic structure in the material was effectively adjusted.In addition,the d-band center of Co near the heterointerface shifts away from the Fermi level,which weakens the adsorption of oxygen intermediates by active sites in the catalytic process,which is conducive to the rapid desorption,thus accelerating the overall reaction efficiency.Finally,NiO/Co₃O₄ based rechargeable Zn-air battery showed high open circuit potential（1.42 V）,large power density（maximum 103.8 mW·cm-2）and excellent charge discharge cycle stability（510 charge discharge cycles at 3 mA·cm-2）.This study provides an effective way for the rational construction of heterostructure interface.4.The electronic structure of perovskite LaCoO₃ was effectively regulated by double cation gradient etching technology.The excessive desolvation of cations leads to the collapse of perovskite structure and surface reconstruction,and finally the perovskite/spinel heterostructure V-LCO/Co₃O₄ with rich cation vacancies is obtained.Due to the heterogeneous interface effect and abundant vacancies,the surface electronic structure of the catalyst is effectively adjusted,which significantly improves the catalytic activity.Compared with initial perovskite（3.94×1019 cm-3）,V-LCO/Co₃O₄ has lower electrochemical activation energy and higher carrier concentration（1.36×1021 cm-3）.DFT calculation shows that the directional reconstruction of surface electronic structure makes the d-band center of V-LCO/Co₃O₄ in a medium position,endowing it moderate oxygen adsorption strength and optimize the adsorption/desorption kinetics.V-LCO/Co₃O₄ displays good bifunctional activity in Zn-air battery and is a promising catalyst.