Constructions And Performances Of The Mesostructured Materials Based On Carbon Nanocages

Electrode material is one of the key parts of energy conversion and storage devices.The design and development of novel carbon-based nanomaterials is the frontier of energy material science.Based on the progresses and problems in the oxygen electrocatalysts for fuel cells and metal-air batteries as well as the anode materials for Li-ion batteries,this dissertation focuses on the controllable preparation and energy-related applications of 3D hierarchical carbon nanocages-based materials.The main progresses are summarized as followed.1.Tailoring the nanoscaled heterointerface of α-Fe2O3/Fe3O4 nanocrystals on hierarchical nitrogen-doped carbon towards superb oxygen reduction.The main challenge for large-scale applications of fuel cells is the design of cheap and stable non-precious metal(NPM)electrocatalysts for oxygen reduction reaction(ORR).Herein,we report a convenient approach to highly active NPM ORR electrocatalyst by tailoring the nano heterointerface ofα-Fe2O3/Fe3O4 on hierarchical nitrogen-doped carbon nanocages(hNCNC)via the partial carbothermal reduction of α-Fe2O3 nanocrystals to Fe3O4.The so-constructed heterostructuralα-Fe2O3/Fe3O4/hNCNC exhibits excellent ORR performance surpassing Pt/C,with a high onset potential of 1.030 V and half-wave potential of 0.838 V(versus reversible hydrogen electrode,vs.RHE),large limiting current density of 6.02 mA cm2 at 0.50 V,robust durability and methanol tolerance.The heterostructure of α-Fe2O3/Fe3O4/hNCNC is identified by the characterization of high-angle annular dark field-scanning transmission electron microscopy coupled with electron energy-loss spectroscopy.The acid-leaching experiment and theoretical calculation results indicate the heterostructural α-Fe2O3/Fe3O4 species with variable Fe valences and modified electronic state are responsible for the high ORR performance.In principle,regulating the nano heterointerface based on the multivalent metal species in nanocrystals matrix could become a new convenient approach to develop high-performance NPM electrocatalysts or even various functional materials.2.Boosting oxygen reduction activity of spinel CoFe2O4 by strong interaction with hNCNC.The unique hNCNC is used as a new support to homogeneously immobilize spinel CoFe2O4 nanoparticles by a facile solvothermal method.The so-constructed 3D hierarchical CoFe2O4/hNCNC shows a high oxygen reduction activity with an onset potential of 0.966 V and half-wave potential of 0.819 V(vs.RHE),far superior to the corresponding 0.846 and 0.742 V for its counterpart of CoFe2O4/hCNC with undoped hierarchical carbon nanocages(hCNC)as the support,which locates at the top level for spinel-based catalysts to date.Consequently,the CoFe2O4/hNCNC displays the superior high open-circuit potential(OCP,1.55 V vs.Al),discharge capacity(347 mAh g-1)and peak power density(51 mW cm-2)to CoFe2O4/hCNC,when used as the air-cathode catalysts in the primary Al-air batteries.X-ray photoelectron spectroscopy characterizations reveal the more charge transfer from CoFe2O4 to hNCNC than to hCNC,indicating the stronger interaction between CoFe2O4 and hNCNC due to the N participation.The enhanced interaction and hierarchical structure favor the modification of electronic states and high dispersion for the active species as well as the mass transport during the ORR process,which plays a key role in boosting the electrocatalytic performances.In addition,we noticed the high sensitivity of O 1s spectrum to the particle size and chemical environment for spinel oxides,which is used as an indicator to understand the evolution of ORR activities for all the CoFe2O4-related contrast catalysts.Accordingly,the well-defined structure-performance relationship is demonstrated by the combination of experimental characterizations with theoretical calculations.3.Constructing 3D hierarchical sulfur and nitrogen co-doped carbon nanocages as efficient metal-free bifunctional electrocatalysts for oxygen reduction and oxygen evolution.Exploring highly efficient and inexpensive NPM-based bifunctional oxygen electrocatalysts to replace noble metal-based ones is critical for the practical applications of rechargeable metal-air batteries.Here we expand the in-situ MgO template method for the synthesis of 3D hierarchical sulfur and nitrogen co-doped carbon nanocages(hSNCNC),which presents an excellent bifunctional electrocatalytic activity and stability for oxygen reduction and evolution superior to commercial Pt/C and IrO2.As the air-cathode catalyst of Zn-air batteries,hSNCNC demonstrates the comparable primary battery performance to Pt/C,e.g.,OCP of 1.48 V(vs.Zn),specific capacity of 695 mAh g-1,and peak power density of 56 mW cm-2.The rechargeable battery in an open-configuration displays the higher reversibility and stability than Pt/C+IrO2 over long charge/discharge cycles.The remarkable properties are mainly attributed to the uniform distribution and synergistic effect of ORR-and OER-active S and N sites as well as 3D hierarchical structures.4.Confining tin dioxide in nitrogen-doped carbon nanocages for superior lithium storage.To develop new anode materials is still the main challenge for high-performance Li-ion batteries(LIBs).SnO2 is a promising candidate but suffers from the problems of intrinsic low conductivity,easy agglomeration and large volume variation during the charge/discharge processes.Noticeably,encapsulating active species in sp2 carbons is an effective strategy to enhance the lithium storage performance.Herein,taking the advantages of hNCNC with interconnected hollow nanocages,we develop a facile vacuum-filling method to prepare SnO2 encapsulated in hNCNC(SnO2@hNCNC).The so-constructed SnO2@hNCNC delivers the high reversible discharge specific capacities of ca.913(@0.1 A g-1),808(@1.0 A g-1)and 477 mAh g-1(@5.0 A g-1)after 110,600 and 1000 cycles,respectively,far superior to the counterparts of SnO2 supported on the outer surface of hNCNC and pristine SnO2.Such outstanding rate capability and cyclability mainly result from the unique structure of SnO2 nanoparticles encapsulated in the interconnected hollow nanocages of hNCNC,which not only greatly favors electron conduction and electrode kinetics,but also alleviates the stress caused by the large volume change of active species during the charge-discharge processes.