Visualized In-Situ Characterization Of Ion Diffusion And Adsorption Behavior In Supercapacitors

As an important energy storage device,electrochemical capacitors(ECs),also called supercapacitors,have attracted extensive attention from academia and industry mainly due to their superior performance capability in contrast to secondary batteries,such as large power density,fast charge and discharge rates as well as long cycle life.The past decade has witnessed the extensive use of in-situ electron microscopies(EM)in the study of Lithium-ion batteries(LiBs),which have helped us to gain fundamental insight into the structural and chemical evolution of a variety of electrode materials in real time.However,the working electrode of supercapacitor does not exhibit significant dimensional and structural changes like lithium-ion batteries during charging and discharging,so it is rarely observed and studied based on in-situ electron microscopy.In this paper,we built a miniature supercapacitor in a scanning electron microscope and a transmission electron microscope,and used X-ray energy dispersive spectroscopy(EDS)to visualize the diffusion and absorption of sulfate ions on the carbon-based nanoelectrode.And the electrochemical properties of the miniature supercapacitors composed of ionic liquid and carbon-based materials were tested.The main results of the thesis are summarized as below:The EDS element mapping can "see" the diffusion processes of sulfate ions between both gyraphene electrodes and solid electrolytes driven by external bias or the ion concentration gradient.These results confirm that the applied voltage has a dominant influence on the SO42-diffusion and distribution in the region around the electrolyte/graphene interface.In addition,the electrochemical properties of the supercapacitors compose of ionic liquid and composite carbon nanofibers under scanning electron microscopy showed a nearly rectangular cyclic voltammetry;and further demonstrated that appropriate increase of the effective area of electrolytes and electrode materials could improve the capacitanceFor the first time,a supercapacitor composed of a single composite carbon nanofiber was constructed inside a TEM;and the effects of different metal and metal oxide particles in carbon fiber on ion distribution were studied.It was found that the SO42-was preferentially adsorbed on the composite carbon nanofiber with the Ni or NiO.The selective adsorption of sulfate ions on carbon nanofibers can also be revealed,which would be helpful for understanding the microscopic mechanism of pseudocapacity.In addition,when the reverse voltage was applied to the sample,it made no obvious change to the S distribution on the fiber.This can explains why pseudocapacitors suffer from inferior cycling stability and fast capacity decay during cycling.More importantly,we believe that elemental imaging based on in-situ electron microscopy will be of great significance not only for the study of supercapacitors,but also for the study of ion diffusion and adsorption processes in other related systems.