Configuration And Modulation Of Novel Carbon-Based Composites For Supercapacitors

Owing to the integrated merits of sound safety in use,high power density and long cycle span,supercapacitors involving the surface/interface reactions,as the important representative of new energy storage devices,have received great attention and intensive study of researchers,thus displaying wide application prospects.However,during the charging/discharging process,especially at large current densities,the sluggish electron-transfer and electrolyte-transport behaviors always happen,serving as the bottleneck issue for efficient and stable charge storage.Therefore,devising highly active electrode materials,effectively enhancing the charge-transfer and mass transport dynamics,and thus realizing the co-enhancement of capacitance,rate performance and cycle performance become the key issues in recent years.In this thesis,based on multiple synthesis routes and advanced characterization techniques,we have realized the controllable configuration of a series of carbon-based transition metal nanocomposites,and precisely modulated the microstructure,electronic structure and coordination environment.Moreover,we have clarified the dynamic evolution mechanisms of electrode materials under electrochemical environment and their true active sites,then systematically decoupled the underlying relationship between the electrochemical performance and the structure properties under the atomic and nano/micrometer scale.The main results are as follows:With the assistance of MnO₂ as the bridging and structure-directing agents,NiCo carbonate hydroxides(NiCo-CH)nanowires and NiCoMn layered double hydroxides(NiCoMn-LDH)nanosheets were in-situ integrated onto carbon fiber via an one-step hydrothermal process,yielding three-dimension porous composites CP-M-LDH-CH.It has been revealed that the modulation and inducing effects of MnO₂ can effectively optimize the reaction reversibility and stability,and the open structure can enable the fast and efficient mass transport and charge transfer,ultimately resulting in the super-high initial Coulombic efficiency(up to 96%)and superior rate performance(90%@30 A g^-1).The asymmetric supercapacitor made with the CP-M-LDH-CH and the commercial activated carbon(AC)can deliver a high energy density up to 59.8 Wh kg^-1.Carbon fiber-coupled Co(OH)₂ was formed by constant-voltage electrodeposition and used as the precursor.Subsequently,the fast and drastic reconstruction of Co(OH)₂ was realized by the converse voltage-enabled controllable oxidation,in-situ yielding carbon-coupled EACoOOH micro structure with abundant defective sites and numerous mesopores.It has been identified that the effective incorporation of internal defects can positively modulate the redox activity and boost ions transport,serving as the key of the superior charge storage capability.The as-formed carbon-coupled EA-CoOOH electrode can well work at 1 V s^-1,and demonstrates a sound capacitance retention rate of 78%at 200 A g^-1.The asymmetric supercapacitor made with the carbon-based EA-CoOOH as the positive electrode and AC as the negative electrode can achieve an energy density of 65.4 Wh kg^-1.The 'converse voltage'strategy can be applied to in-situ modulate a series of transition metal-based composites,especially for fast&efficient regulation of phase components and electronic structure.With graphene and carbon nanotube as the conductive species,carbon-coupled MnOOH composite film was formed by the vacuum filtration and used as the precursor.Then based on the low-temperature solid-phase phosphorization strategy,the carbon coupled phosphorusmodulated Mn₃O₄(P-Mn₃O₄/C)composite films have been successfully developed.As identified by the multiple characterization techniques(such as synchrotron radiation and in-situ Raman),there exists the unique modulation effect of phosphorus species(P)toward surface chemistry and coordination environment of Mn₃O₄,Accordingly,the P-modulated Mn₃O₄(PMn₃O₄)can realize the fast transformation to highly active phase MnO₂(birnessite)with abundant edge sites and oxygen vacancies(20 times decrease in the required time is achieved),thus acting as the highly active species to enhance the charge-storage dynamics.Moreover,the high-mass-loading(44.2 mg cm^-2)P-Mn₃O₄ composite film was successfully configured,presenting the high areal capacitance of 8743 mF cm^-2.This phosphorus species modulation strategy is universal to trigger fast reconstruction,which was identified by NiCo-based oxides.Carbon fiber-coupled MnAl-LDH was formed by constant-potential electrodeposition and used as the precursor.Then based on the electrochemistry-induced dynamic etching of Al and structure regulation,the low-crystalline and oxygen vacancies-enriched birnessite-type MnO₂(AK-MnO₂)was developed.With the assistance of soft X-ray absorption spectroscopy(sXAS),Raman mapping and other advanced techniques,it was found that the irreversible oxidation of Mn²⁺ and the incorporation of K+ act as the major reasons for fast reconstruction from MnAlLDH to AK-MnO₂.According to the theoretical calculation results,the abundant oxygen vacancies on the surface/near surface regions can help improve the conductivity and the adsorption capability toward electrolyte ions.The as-formed carbon-based AK-MnO₂ composites can well work at a high scan rate of 1 V s^-1,and can display a high capacitance of 356 F g^-1 at a current density of 1 A g^-1.This electrochemistry-induced dynamic etching of Al strategy can be applied to modulate a series of M²⁺ Al-LDH,where the reductibility of M²⁺ was proved to be vital for the reconstruction rate.