Fabrication And Electrocatalytic Performance Of Highly Efficient Cobalt-based Oxygen Electrode Materials

The tremendous consumption of traditional fossil fuels and the sharp increase of greenhouse gases have severely impeded the sustainable development of society.Therefore,it is urgent to develop and popularize economic,efficient,and clean new renewable energy.Among many new renewable energy conversion and storage devices,fuel cell(FC)and unitized regenerative fuel cell(URFC)have been widely used in space,military,mobile power fields,and so on,because of the high energy-conversion efficiency,high energy density,and low emission.However,the intrinsic sluggish kinetics of the oxygen electrode reactions relied on Pt,Ir and Ru-based precious metals catalysts with scarcity,high-cost,and poor-stability,which greatly hinders the large-scale application and commercialization of FC and RFC.Besides,the dispersion degree and stability of active centers are strongly associated with the support,which plays a key role in the electrochemical performance.Therefore,the design and development of highly efficient and stable of non-precious metal(NPM)catalysts have important theoretical significance and practical application value.In this work,with the aiming of designing and fabricating efficient and stable cobalt-based electrocatalytic materials for oxygen reduction reaction(ORR)and oxygen evolution reaction(OER),various kinds of cobalt-based electrocatalytic materials were prepared by controlling and constructing defect structure,oxygen vacancy,chemical composition,morphology,support and so on.Besides,the effects of crystal structure,morphology,pore structure,active centers and surface-interface structure of cobalt-based materials on ORR/OER electrocatalytic performance and the structure-performance relationship were systematically investigated.Moreover,the electrocatalytic performance of cobalt-based oxygen electrode materials was enhanced.The main contents and results are shown as follows:(1)aiming at the regulation of morphology and pore structure of the catalysts,a series of carbon black supported hollow cobalt oxides electrocatalytic materials(CoOx/C)were prepared by a facile "surfactant template" method,and the effects of surfactant dosage and calcined temperature were systematically investigated on the morphology and pore structure of CoOx/C catalysts as well as the corresponding ORR performance.The results showed that the different dosages of sodium laurylsulfonate(SDS)played a key role in crystal structure and morphology of the catalysts.When n(SDS)=1.250 mmol,CoO nanoparticles with high dispersion,high crystallinity and hollow structure were obtained.In addition,due to the different calcined temperatures,the chemical composition,morphology and pore structure of the catalysts were different.The ORR electrocatalytic activities of CoOx/C catalysts were also affected by the calcined temperature.The CoOx/C-600 hollow cobalt oxides obtained at 600℃ exhibited an excellent ORR electrocatalytic activity and stability,which could be ascribed to the exposure of abundant active sites and high relative content of Co2+ specie,indicating the appropriate SDS dosage and calcined temperature favored to enhance the ORR performance.(2)based on the high-active cobalt oxide and oxygen-buffering cerium oxide,Co/Ce bimetal composite supported on reduced graphene oxide with high activity and high dispersion was fabricated,and carefully studied the effects of CeO2 co-catalyst on the relative content of Co2+/Co3+,number of oxygen vacancies,pore structure,as well as the ORR/OER bifunctional electrocatalytic performance.The physical characterizaon results revealed that the introduction of Ce02 co-catalyst resulted in the change/modification of crystal structure and surface property of CoOx/CeO2/RGO composite.Besides,the increasesd relative content of Co2+/Co3+,the abundant oxygen vacancies and the strong interaction between CeO2 and cobalt oxides(CoO and C03O4)aided to increase the number of acid-base sites on the surface of CoOx/CeO2/RGO catalyst,which promoted the chemical adsorption of O2 and the activation of H2O molecules,and thus improved the ORR/OER bifunctional electrocatalytic activity and stability.(3)aiming at the improvement of intrinsic conductivity in cobalt-based electrocatalysts,one step in-situ pyrolysis method was developed to prepare a series of N-doped carbon supported highly active cobalt-based multi-components electrocatalytic materials,and systematically investigated the effects of pyrolysis temperature on the crystal structure,cobalt-based components,morphology,and nitrogen species,and revealed the structure-performance relationship of cobalt-based multicomponent electrocatalytic materials.The results showed that the melamine decomposed into graphitic carbon nitride(g-C3N4)at 550℃,while the cobalt ions were introduced into g-C3N4 matrix to form many active and stable Co-Nx sites.In addition,due to the coordination between cobalt and g-C3N4,the Co-Cat-T550 catalyst appeared an encapsulated tubular structure(Co-Nx@g-C3N4),which is benefical for accelerating the electron/charge transfer,improving the conductivity,and thus achieving the enhancement of ORR electrocatalytic performance.(4)based on the host-sheet lattice orientation and interlayer-region confined effect of the layered double hydroxides(LDHs)materials,using cobalt phthalocyanine tetrasulfonate(CoPcTs)and Ni2Fe-LDH as structural units,a highly disperse bifunctional material(NiFeOx/CoNv/C)with OER species(NiFeOx)and ORR species(Co-Ny)was prepared by simple calcination of Ni2Fe-CoPcTs-LDHs precursor based on the intercalation of cobalt phthalocyanine tetrasulfonate(CoPcTs)into nickel-iron LDHs(Ni2Fe-LDHs).The results showed that the unique structure properties of intercalated LDHs precursor greatly improved the disordered degree,the dispersion degree and relative content of active centers in NiFeOx/CoNy/C catalyst,and thus enhancing the ORR/OER bifunctional electrocatalytic performance.Furthermore,it was of paramount importance to evaluate the performance of the self-assembly H2-O2 membraneless micro regenerative fuel cell with NiFeOx/CoNy/NCNH ‖ NiO-Ni/NCNH non-noble metals HER/HOR and ORR/OER bifunctional electrodes.