| Supercapacitors,also known as electrochemical capacitors,have emerged as promising energy storage devices due to their high power density,fast charge and discharge rates,and long cycle life.However,their lower energy density remains a critical bottleneck that needs to be addressed in order to match or even surpass the energy density of batteries.One important approach to enhance the energy density of supercapacitors is through the development of new electrode materials.MXenes,two-dimensional transition metal nitride carbides,have emerged as ideal candidates for supercapacitor electrodes due to their high surface area,excellent conductivity,and tunable surface chemistry.In addition to developing new electrode materials,another strategy to improve the performance of supercapacitors is to use novel electrolytes.A new "water-in-salt" electrolyte(WIS)composed of high concentration salts has emerged as an effective alternative to traditional electrolytes,demonstrating significant advantages in terms of low cost,high conductivity,and wide voltage window.While the high voltage window of WIS shows great potential,most of the research to date has focused on expensive lithium-based electrolytes,making the development of a low-cost and environmentally friendly WIS particularly valuable.Furthermore,little research has been conducted on the combination of MXene electrodes and high-concentration sodium chlorate WIS electrolytes,leaving their energy storage mechanism and performance to be further explored.Therefore,the following studies have been carried out for MXene electrodes and sodium perchlorate in this paper:This article presents a systematic study of the microstructure of the highconcentration(17 mol kg-1)sodium chlorate "salt-in-water" electrolyte and its effect on widening the electrochemical voltage window.By conducting cyclic voltammetry tests at various concentrations,it was found that increasing the concentration from 1 M to 17 M widened the voltage window from 2.05 V to 2.55 V.Raman spectroscopy was used to characterize the mechanism underlying the widened voltage window,which was found to be due to the majority of water molecules being restricted around cations in high-concentration electrolytes,resulting in reduced free mobility and suppressing the electrolyte’s decomposition.Utilizing this knowledge,a high-performance symmetric supercapacitor was developed using activated carbon electrodes,which exhibited an energy density of 120.24 Wh kg-1 at a high power density of 963 W kg-1.In addition,a non-symmetric supercapacitor was constructed using NaClO4 salt-in-water electrolyte and MXene electrodes,with improved capacitance performance achieved through electrode modification.The charging mechanism of the device was analyzed and studied,and the high-concentration salt-in-water electrolyte exhibited a stable voltage window of up to 1.8 V,surpassing that of conventional water-based electrolytes(1.2 V),resulting in a significant increase in energy density.The modified 400-NaOH-MXene electrode showed superior performance compared to the untreated MXene electrode.Characterization revealed that most of the-F functional groups on the surface of the MXene electrode were converted to-OH after alkaline treatment,and the interlayer distance increased.After calcination,the functional groups on the surface were removed,the effective interlayer spacing increased,and ion transport resistance decreased.Furthermore,due to the removal of functional groups,the Ti valence state of 400-NaOH-MXene changed during the electrochemical process,resulting in a different energy storage mechanism compared to that of MXene and NaOH-MXene in the 17 M electrolyte.For the 400-NaOH-MXene electrode,energy storage was dominated by diffusioncontrolled behavior,significantly increasing the specific capacitance.As a result,the assembled device exhibited excellent energy and power density,providing a reference for the construction of wide-voltage-window water-based supercapacitors. |
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