Structure Regulation Research Of Cobalt Based Nanomaterials And Their Oxygen Electrocatalytic Application

Energy storage and conversion equipments such as Zn-air battery,water splitting and hydrogen fuel cell become the key technologies to solve the bottleneck of large-scale application of renewable energy.However,the lack of cost-effective oxygen evolution,oxygen reduction and bifunctional catalysts has restricted the commercialization of these new energy devices.Cobalt-based spinel and carbon supported cobalt-based single atom catalysts are considered as potential substitutes for noble metal-based oxygen electrocatalysts due to their adjustable active sites,low cost,good activity and stability,but their catalytic performance still needs to be further improved.This paper is aimed at improving catalytic performance through regulating microtopography,electronic structure of active sites and coordination environment for Co-based catalyst,which mainly involves the controlled preparation of catalyst and in-depth study of electronic regulation mechanism.Besides,the optimized catalyst was used in liquid and flexible Zn-air battery test.The main research contents are as follows:（1）Spinel Cu_1-xCo_2+xS₄ sulfide and Cu_1-xCo_2+xO₄ oxide were synthesized under hydrothermal conditions,respectively.Cu_1-xCo_2+xO₄ formed porous flake morphology with the action of ammonia water,and the crystallinity of Cu_1-xCo_2+xO₄ was adjusted by controlling precursor amount.Based on the two materials,the effect of O and S nonmetallic elements on the catalytic activity of octahedron Co sites in spinel and the regulation mechanism of electronic structure were studied.The XPS characterization results show that the more electronegative O leads to a higher valence state at the Co site.DFT analysis manifests that O not only increases the charge loss of Co,but also enriches the number of empty orbitals near the Fermi level.Electrochemical tests prove that Cu_1-xCo_2+xO₄ has better OER activity and stability than Cu_1-xCo_2+xS₄ as the overpotential of 267 m V at current density of 10 m A cm^-2,which is much lower than that of commercial Ru O₂.（2）Co based spinel oxides with same morphology were controbally obtained through orientated self-assembly in hydrothermal reaction and subsequent low-temperature oxidation.The influence of Ni,Cu,Zn doping,especially the regulation mechanism of different metal cations on the catalytic activity of octahedral Co site,and the influence of Cu/Co precursor molar ratio and oxidation temperature on the activity of oxides were studied.The XPS and XANES results demonstrate that cationic doping increases the valence state of Co site,and Cu⁺species promote the highest Co oxidation state.DFT calculation reveals that the presence of Cu⁺makes O tend to acquire more electrons from the octahedral Co site,and also increases the empty orbitals of Co atoms,which facilitates electron transfer in catalytic process and promote catalytic conversion process.OER performance test shows that Cu-doped sample（CCO）has significantly better catalytic activity than Ni/Zn doped one as the overpotential decreasesing to207 m V at 10 m A cm^-2 current density,and also possesses better water splitting performance than commercial Ru O₂.（3）Co₃O₄ spinel can be a potential bifunctional catalytic platform due to its polyvalent characteristics of Co site,but its catalytic activity still needs to be improved.The nitrogen doped Co₃O₄ with surface sulfate ions mmodification（N-Co₃O₄-SO₄）was produced by solvothermal synthesis and subsequent low temperature oxidation process,and the regulation mechanism of nitrogen and sulfate on octahedral Co active site was studied.The existence of sulfate on the surface of Co₃O₄ and coordination of Co-N were demonstrated by XPS analysis,and the change of Co coordination environment was further confirmed by XAFS characterization.Moreover,DFT simulations show that N and sulfate ions synergically regulate the charge distribution of Co site,optimize the adsorption of oxygen-containing intermediates,and improve the charge transfer rate and efficiency by adjusting the Co occupied state and empty orbital distribution,which is beneficial to electrochemical reaction.The electrochemical results demonstrate that N and sulfate significantly enhanced the alkaline ORR and OER catalytic activities of Co₃O₄,and the peak power densities of liquid and flexible solid Zn-air batteries assembled with N-Co₃O₄-SO₄ catalyst reached 144.5 m W cm^-2 and 82.9 m W cm^-2,respectively,which is superior to that of Pt/C+Ru O₂ catalyst.（4）In order to further improve the performance of rechargeable Zn-air battery,catalyst of Co single atoms and orderly carbon shell coated metal Co on the porous carbon was synthesized through dopamine coating and subsequent high temperature pyrolysis.The effect of heat treatment temperature on the microstructure and catalytic activity of the catalyst were studied,as well as the electronic regulation mechanism of Co particles on the single atom site.Co single atom coordinated by four pyrrole N and coexisted with Co metal particles were proved by XPS and XAFS measurements.DFT simulations demonstrated that Co single atom and Co particles are the main active sites of ORR and OER,respectively.Meanwhile,Co particles could increase the occupied state electrons near the Fermi level of Co single atom site,and improve the charge transfer rate in ORR process.The difference between alkaline ORR half-wave potential and OER potential at 10 m A cm^-2 current density for Cop@Co NC catalyst is only0.68 V.The peak power density of liquid and flexible solid Zn-air battery assembled with Cop@Co NC is 188.8 m W cm^-2 and 121.8 m W cm^-2,respectively,which is superior to that of Pt/C+Ru O₂ catalyst.（5）To enhance the acid ORR performance of Co-based single atom catalyst,Fe-Co dual atom catalyst was constructed based on ZIF carrier containing Co.The adjacent Fe-Co atomic distance in catalyst can be adjusted by controlling the addition amount of Fe precursor.The effect of atomic distance on microstructure,coordination environment and ORR activity of catalyst was discussed,and the electronic regulation mechanism of atomic distance on single atom sites was in-depth studied.The AC-STEM images of the catalysts showed that the atomic distance were about 0.23 nm,0.39 nm and 0.5 nm for（Co,Fe）_S-N-C,（Co,Fe）_M-N-C and（Co,Fe）_L-N-C,respectively.The influence of Fe-Co atomic distance on the electronic structure and coordination environment of Fe single atom sites was confirmed by XPS and XAFS measurements.DFT simulation manifests that the Fe site for（Co,Fe）_M-N-C has the optimal adsorption and desorption strength for the intermediate.Besides,the d orbital of Fe and the p orbital of O have the strongest hybridization near the Fermi level,which is beneficial to accelerate the charge transfer in ORR.The electrochemical test demonstrates that（Co,Fe）_M-N-C sample possesses the best catalytic activity.The alkaline and acidic half-wave potentials of（Co,Fe）_M-N-C can reach 0.94 V and 0.85 V,respectively,superior to the commercial Pt/C catalyst.Furthermore,the assembled liquid Zn-air batteries also showed high peak power density of 250 m W cm^-2.