Preparation And Electrocatalytic Properties Of Heteroatom-Doped Carbon-Based Composites

Due to increasing industrialization,excessive exploitation and combustion of non-renewable resources have caused serious environmental pollution and energy crisis,motivating people to develop new energy conversion systems.Fuel cells,zinc-air batteries and electrolytic water splitting have emerged as unique representatives of clean and efficient renewable energy technologies,and attracted extensive attention.There are three important half-reactions involved in these conversion processes,including oxygen reduction reaction(ORR),oxygen evolution reaction(OER)and hydrogen evolution reaction(HER).However,the sluggish electron-transfer kinetics,high overpotential and low energy conversion efficiency of these half-reactions renders it necessary to employ a large amount of precious metals-based electrocatalysts such as Pt and RuO₂.Unfortunately,the high cost,limited natural reserves and poor stability of these precious metals severly limit their wide-spread commercialization.Therefore,the development of low-cost,efficient and highly stable electrocatalysts is of great significance to promote the large-scale application of the electrochemical conversion systems.This dissertation is mainly focused on the enhancement of the catalytic performance by design and engineering of the microstructures of carbon-based nanocomposites.Herein,a series of nanocomposites based on nonprecious metals and doped carbon are prepared,which exhibit high efficiency and stability towards the various electrochemical reactions,and can be efficient electrocatalysts for znic-air battery and electrolytic water splitting.The main content includes the following three parts:(1)On the basis of an unconventional sulfurization strategy,carbon-based microflowers doped with ultrafine Co₃S₄ nanoparticles are fabricated by a simple hydrothermal treatment with Co(NO₃)₂·6H₂O as cobalt source and thiourea-ethylenediamine-formaldehyde as carbon and sulfur sources through Schiff-base coupling polymerization.Scanning electron microscopy(SEM)and X-ray diffraction(XRD)measurements show that ethylenediamine and formaldehyde act as strong coordinating ligands and structure-directed reagents to affect the morphology of the products and stoichiometric ratio of metal sulfides,jointly forming a hierarchical flower-like structure and Co₃S₄ nanoparticles with high intrinsic performance.Transmission electron microscopy(TEM)and electron spin resonance(ESR)measurements show that the Co₃S₄ nanoparticles entail abundant sulfur vacancies.At the mole ratio of thiourea/Co²⁺=1.5:1,the prepared Co₃S₄@FNC-Co3 exhibits the optimized electrocatalytic performance for HER and OER.The excellent bifunctional catalytic activity is attributed to the highly conductive defect-rich Co₃S₄ nanoparticles and the porous flower-like structure with fully accessible active sites.Additionally,in alkaline media,total water splitting measurements confirm that Co₃S₄@FNC-Co3 possess a high electrocatalytic performance and remarkable stability.This strategy may be exploited for the rational design and engineering of advanced multifunctional composites based on nanostructured transition metal sulfides and hierarchical carbons for a wide range of electrochemical energy devices.(2)As the aforementioned carbon substrates exhibit a low degree of graphitization,poor thermal stability and limited contributions to the electrocatalytic activity,dopamine is introduced as a nitrogen-containing compound to prepare functional polymer precursors with excellent thermal stability.Infrared and X-ray photoelectron spectroscopy(XPS)measurements show that assembly process of the thiourea-ethylenediamine-formaldehyde resin(TEFR)and polydopamine(PDA)is induced by the strong chelating interactions between the Co(II)ions(which act as mediators)and the functional groups with lone-pair electrons(such as HS-,-NH₂,and-OH)on TEFR and PDA to form a Co-TEFR@PDA complex.Pyrolysis of the precursor in an Ar/H₂ atmosphere produces the final electrocatalyst,cobalt-based nanostructured transition metal compounds(Co-NTMCs)embedded in nitrogen and sulfur codoped hierarchical porous carbon submicrospheres(Co-NTMCs@NSC).It is found that the Co-NTMCs@NSC nanocomposite comprises abundant hierarchical porous textures,a high content of active cobalt species,and a considerable amount of defective structures,endowing it with remarkable oxygen electrocatalytic activities(?E=0.76 V)and excellent stability.When used as a breathing air electrode,Co-NTMCs@NSC assembled Zn-air battery exhibits a high discharge power density and good charge-discharge reversibility.The results suggest that the controllable assembly of functional polymers may be exploited for the preparation of doped carbon/metal nanoparticle nanocomposites as high-performance catalysts for electrochemical energy technologies.(3)The morphology of the functional complex precursor is further changed into a circular sheet-like structure and tetraethyl orthosilicate is added to inhibit the agglomeration of metal particles,which increases the number of effective active sites and promots the charge transfer and mass transportation,thereby optimizing the electrocatalytic activity of the nanocomposites.The functional sheet-like composite precursor is produced via the assembly process,and carbonized to obtain silica-containing,Co/Co₉S₈ nanoparticles-decorated porous nitrogen,sulfur co-doped carbon(Silica-Co/Co₉S₈@PNSC).Different etching reagents,including HF and NH₄HF₂,are then used for post-synthesis treatment.It is found that NH₄HF₂can ecth off silica,and selectively leach metallic Co,while Co₉S₈ nanoparticles are retained to achieve the final product Co₉S₈@PNSC.The electrocatalyst possesses defective graphite carbon,high specific surface area,hierarchical pore structure and active metal nanoparticles Co₉S₈.In alkaline electrolytes,the electrocatalyst Co₉S₈@PNSC displays excellent trifunctional catalytic activities and stability towards ORR,OER and HER.The excellent electrocatalytic performance mainly originates from the synergistic effect between the defective heteroatom-doped carbon and metal nanoparticles Co₉S₈.When applied in the assembly of rechargeable Zn-air battery,Co₉S₈@PNC catalyst exhibits a higher discharge power density and better charge-discharge reversibility than commercial Pt/C.The results suggest that the selective etching strategy may provide an alternative way to the rational design of high-performance electrocatalysts for diverse applications.