Construction And Electrochemical Performance Of High-Voltage Carbon Fiber-Based Supercapacitors

With the development of wearable electronics,the requirements for energy storage devices have become more diverse.Supercapacitors are gradually becoming the most promising energy storage devices due to their fast charge/discharge rates,high safety and long cycle stability.However,its low energy density is still the main factor limiting its practical application.Carbon fiber cloth is a commonly used flexible substrate for supercapacitors.The loaded active material is limited by the sluggish long-distance electron/ion transport and inevitable agglomeration of nanoscale materials,of which mass loading is usually 1ess than 2 mg/cm2,making it difficult to obtain a satisfactory energy storage density.At the same time,the low electrochemical activity of the carbon cloth itself and the fact that only its surface can be utilized are also important constraints.Besides,for aqueous supercapacitors,the voltage window is constrained by the low stabilization potential(1.23 V),which further hinders the improvement of their energy density.In this study,a layer-void manganese dioxide cathode was in-situ constructed on the surface of carbon cloth by anodic overpotential deposition,which realized high mass loading and the "multilayering" of the active material simultaneously.In addition to manganese dioxide with high mass loading,its high pseudo-capacitance and conductivity can be retained,thus realizing the efficient utilization of the active material.The strong oxygen evolution reaction during the deposition process also allows in-situ activation of the carbon substrate.The electrode can be applied in ionic liquids and aqueous systems,which exhibits excellent electrochemical properties and mechanical flexibility.Based on this,hierarchical nanostructures of manganese dioxide were designed to further expose the active surface and shorten the electron/ion transport paths to reduce the diffusion barrier.Manganese dioxide with an open shell structure is induced by a simple acidification method with a composite of nanoparticles and nanoneedles.The formation of one-dimensional nanoneedles not only avoids agglomeration of nanomaterials but also accelerates the charge transfer at the electrode/electrolyte interface.The reaction is accompanied by a shift in the manganese valence state,which reduces the formation of metastable trivalent manganese ions,and thus increases their oxygen evolution overpotential and broadens the upper voltage range to 1.2 V.The assembled device delivers a maximum volumetric energy density of 4.3 mWh/cm3.Both the single electrode and the assembled device exhibit excellent capacity retention after 20,000 cycles.Most previous work uses excessive activated carbon as the negative electrode with a wide range of pore size and usage of additional conductive agent and binder,leads to its low utilization.Based on this,ZIF-67 as a template precursor was pyrolyzed to produce a foam-like shell with high conductivity and multiple pores,and the charge storage density of the electrode was improved by using a combination of electrical etching and reduction to unblock the pore channels while introducing polarizing groups to reduce the transport resistance of electrolyte ions within the pores.The sodium ions introduced in the reduction process can effectively hinder the adsorption of hydrogen ions and reduce the hydrogen evolution activity,which broaden the lower limit of potential.The device with layered manganese dioxide/carbon cloth as positive electrode achieves a stable voltage window from 0 to 2.3 V and a high volumetric energy density(10.07 mWh/cm3).On this basis,the flexible carbon cloth substrate is directly optimized.Using the "ion etching" method,a porous structure is constructed on the outer surface of the carbon cloth,followed by in-situ modification in a mild neutral electrolyte.The carbon cloth self-supported electrodes show wide lower voltage limit(-1.3 V)and high area capacitance(1089 mF/cm2),and this preparation model provides a useful reference for the development of other flexible carbon-based materials,which opens up new avenues for the design of high-performance carbon cloth-based electrodes.