Study On The Molybdenum-based Electrode Materials For Asymmetric Supercapacitors

The rapid advances of new energy-related industries have led to the extensive applications of supercapacitors and batteries,representatives of modern energy storage devices,in many aspects of daily lives,including portable electronic products,sustainable transportation,and renewable/green energy.Compared to the battery technologies,the supercapacitors possess unique advantages of high-power density,fast charging,and long cycle life.However,the practicality of the state-of-the-art supercapacitors are largely restricted by the capacities and energy densities.This thesis aims to study novel molybdenum-based active material systems,and improve their electrochemical performance through methods of material innovation,component adjustment,and structure optimization,to create more possibilities for building new supercapacitors.To improve the electrochemical performance,a core-shell structured NiCo₂S₄@CoMoO₄electrode material on the carbon cloth was fabricated using a hydrothermal method combined with the post annealing treatment.While the core-shell structure provides open network that improves the holistic mechanical stability of the system,the unique interaction between NiCo₂S₄ nanowires（stable support）and CoMoO₄ nanosheets（protective coating）improves the cyclic stability.The synergistic effects of NiCo₂S₄ and CoMoO₄,the large specific surface area,and mesoporous features not only raises the contact between the active material and the electrolyte,but also promotes the rapid transfer and accumulation of charges,leading to the significantly increased capacitance of the supercapacitor.An asymmetric supercapacitor was assembled with NiCo₂S₄@CoMoO₄ and MoS₂/KB as positive and negative electrodes,which exhibits a superior energy density of 50.7 Wh kg^-1.CoMoO₄@NiCo-LDH/CC nanowire composite material was synthesized where CoMoO₄nanowires as the"core"material and NiCo-LDH nanosheets as the"shell"material.Taking advantage of the synergistic effect of strong interfacial interactions in heterostructures,the material structure is optimized while its rate performance maintained.Besides,as a shell material,the NiCo-LDH nanosheets can provide additional charge transfer paths for the electrode,accelerating the transfer of electrons and ions,and providing more active sites.The structure of core-shell material can be further controlled by adjusting the reaction time of NiCo-LDH nanosheets,enabling meet the needs of specific performance.Further applying as a positive electrode for an asymmetric supercapacitor,the supercapacitor can achieve a high energy density of 57.2 Wh kg^-1.Due to the large specific surface and multi-metal active sites of MOFs materials,the method of the CC@CoMoO₄-Co（OH）₂ growth on carbon cloth was developed using Co-MOF as a template,via a high-efficiency and low-cost chemical deposition process and heating refluxing route.The electrode material has a closely connected nanosheet structure,which enhances the contact between the electrode active material and electrolyte.It is able to shorten the ion diffusion distance and accelerate the electron transfer,thereby improving the capacitance performance.By adjusting the reflux reaction time,the differences between the morphology and structure of the composite nanosheets were investigated to obtain the optimal reaction conditions.The electrochemical behaviors of different samples were studied by using the according electrochemical analysis.An asymmetric supercapacitor was successfully assembled with CC@CoMoO₄-Co（OH）₂ and active carbon as positive and negative electrodes,respectively,which can achieve a high energy density of 61 Wh kg^-1 at 910.4 W kg^-1.The CoP/Mo-NiCoP was successfully prepared on carbon cloth,with the Co-MOF utilized as the precursor template.The influence of Modoping on the phase,structure and performance of the products was studied.The as-prepared CoP/Mo-NiCoP has a hierarchical nanosheet structure to provide large open space and abundant active sites,which is desired for maximizing the utilization efficiency of active materials and the contact between ions and electrodes,thereby reducing ion transmission resistance.The results shown that Modoping can change the surface electronic structure,promote electron transformation,and enhance conductivity,leading to significantly enhanced capacitance performance.Finally,the as-assembled CoP/Mo-NiCoP//AC supercapacitor delivered an ultrahigh energy density of 64.7 Wh kg^-1 at a power density of 1225.5 W kg^-1.After 10000 cycles,the specific capacitance retained 88.5% of the initial value.