Controlled Construction Of Cobalt-based Three-dimensional Self-supported Integrated Electrodes And Their Application In Electrochemical Energy Conversion And Storage

Currently,exploring and developing nanoscale catalysts with excellent performance and stability,and energy storage devices with high specific capacitance is the research hotpots in the field of electrochemical energy conversion and storage.Due to the unique electronic structure and efficient catalytic activity,cobalt-based nano materials have attracted the attention.However,their catalytic performances are still poor when compared with precious metal-based catalysts,and the cobalt-based catalysts also suffer from their instability when working in the acidic or alkaline medium for long time.Therefore,enhancing the activity and durability is significant in the development of cobalt-based materials.Another problem is that the cobalt-based materials reported in previous work are often synthesized in the form of powder.Usually,polymer binders are applied but this pathway can result in an addition of the charge transfer resistance,block the active sites,and hinder the diffusion of the electrolyte,leading to decreased electrocatalytic performance.Thus,developing cobalt-based nanomaterials in situ on current-collecting substrates with well-designed strategy directly is attractive to guarantee industrial application prospect.A series of cobalt-based three-dimensional(3D)self-supported composite electrodes were constructed based of a novel precursor lamellar nitrate-hydroxide cobalt,which is developed in this paper.The composition,structure,morphology,element valence state and surface electronic state of the series electrodes are effectively controlled.And systematically researches on the electrodes for the applications of overall water splitting,oxygen reduction reaction(ORR)and supercapacitor are studied.The relationship among the composition,structure,and catalytic performance is revealed.It provides theoretical guidance and technical support for the large-scale production and application of cobalt-based 3D self-supported integrated electrodes.In the first chapter,the research status of cobalt compounds-based catalysts and energy storage devices is summarized.The commonly modification approaches of cobalt-based electrodes and their applications in different electrochemical energy conversion and storage fields are reviewed.The topic basis and research contents of this paper are proposed.In the second chapter,a novel approach a novel approach to prepare 3D porous nitrogen-doped carbon(NC)nanosheet arrays assembled into a honeycomb layer with Co P/Co₂P single-crystal nanorods encapsulated on both sides of an inch-size Ti foil(Co P/Co₂P@NC/Ti)as an efficient self-supported electrode for HER is developed.It is constructed through a facile low-temperature-solution approach by growing a large-scale honeycomb lamellar cobalt nitrate-hydroxide in situ on Ti foil.The type and size of honeycomb precursor deposited on two sides of a Ti foil can be easily tuned by adjusting the placement of titanium plates in growing process and this unusual precursor can effectively regulate the morphology,structure,and crystalline phase of phosphatized product during phosphidation.In Co P/Co₂P@NC/Ti-10-350 electrode,the large honeycomb pores as well as the mesopores within the walls can act as diffusion channels for electrolyte to increase the active sites.The synergistic effect between Co P(200)crystal plane,Co₂P(121)crystal plane and the NC shell in the electrode can act as highly efficient active sites for hydrogen evolution reaction(HER).In the third chapter,the construction of novel 3D self-supported electrodes consituting nitrogen-doped carbon(NC)layers packaged nickel cobalt phosphides that assemble into the form of nanowall arrays(NA)on Ni foam(Ni_xCo_2-xP@NC NA/NF)as efficient bifunctional electrocatalysts for overall water splitting.These electrodes are composed of highly dispersed Ni_xCo_2-xP nanoparticles,nanorods,nanocapsules and nanodendrites dependent on the stoichiometry of Ni/Co of Ni_xCo_2-xP embedded in NC matrix on NF.Among these electrodes,Ni Co P@NC NA/NF exhibits the most robust electrocatalytic performance for HER.Density functional theory(DFT)calculation indicates that the synergistic effect between Ni Co P and NC shell leads to its high activity towards HER.Moreover,during the oxygen evolution reaction(OER)process,the?-Ni_0.5Co_0.5(OOH)₂ converted from Ni Co P can be used as efficient OER electrocatalyst.Therefore,the Ni Co P@NC NA/NF electrode shows excellent performance when employed as both anode and cathode for overall water splitting.In the fourth chapter,a 3D self-supported integrated electrode,consisting of heteroatomic nitrogen-doped carbon nanotube(N-CNT)arrays on carbon cloth(CC)with confined ultrafine Co₄N nanoparticles and a distribution of anchored single-atom Co(Co₄N@Co SA/N-CNT/CC),is fabricated via a cobalt-catalyzed growth strategy using dicyandiamide as the nitrogen and carbon source and a layered cobalt hydroxide-nitrate salt as the precursor.The abundance of exposed active sites,namely,the Co₄N nanoparticles,single-atom Co,and heteroatomic N-doped carbon nanotubes,and the multiple synergistic effects among these components provide suitable tailoring of the d-band center for facilitating vectorial electron transfer and efficient electrocatalysis.Due to the confinement effect of N-CNT on the encapsulated Co₄N nanoparticles,the diameters of Co₄N nanoparticles are only about 10 nm.Moreover,the calcination process under the atmosphere of ammonia not only creates a large number of defects on the surface of N-CNT but also increases the number of doped N atoms,which further promotes the increase of Co SA.Benefiting from the merits of its structural features and electronic configuration,the prepared electrode exhibits robust performance toward the hydrogen evolution reaction in acidic and basic electrolytes.DFT calculations reveal that the occurrence of the d-band center downshift,due to the effective regulation of the confined Co₄N nanoparticles,endows the Co₄N@Co SA/N-CNT/CC electrode for efficiently catalyzing the HER.The HER performance and durability test results of Co₄N@Co SA/N-CNT/CC in both acidic and alkaline medium are superior to that of other reported transition metal-based and CNT-based electrocatalysts.In the fifth chapter,Co S_x@Co SA/NS-CNT/CC electrodes are successfully constructed by employing S as dopant in the Co@Co SA/N-CNT/CC,and their ORR performance is studied.The ratio between Co-S covalent and Co-S ionic bond can be controlled by adjusting the crystal phases of cobalt sulfide,which can further contribute to regulate the reaction mechanism and pathway of ORR.Co S₂@Co SA/NS-CNT/CC electrode obeys a four-electron process while Co S@Co SA/NS-CNT/CC follows a two-electron process when catalyze ORR process.The Co S₂@Co SA/NS-CNT/CC electrode exhibits a high onset potential and half-wave potential,which close to commercial Pt/C,and is also better than that most of cobalt-based ORR catalysts reported in the previous works.In the sixth chapter,cobalt nitride nanoparticles(Co₄N NPs)encapsulated in lanthanum oxide cyanamide film(LOCF),which synthesized on carbon colth(CC)as a novel 3D self-supported integrated composite electrode(Co₄N@LOCF/CC)is developed.In this electrode,the size of Co₄N nanoparticles are limited by the LOCF,resulting in a ultrafine nanoparticles with a diameter of only 10?20 nm.Moreover,La-N species exist on the surface of LOCF,which can provide an active subsurface layer with effective charge accumulation to improve the capacitance of the electrode.Benefiting from the unique structures,the Co₄N@LOCF/CC exhibits a high specific capacitance when act as supercapacitor positive electrode material,which superior to most of the nitride-based supercapacitor electrodes.