Application Of Active Electrolyte And Electrode Modification In Supercapacitors

Electrode and electrolyte are the two most important factors that determine the performance of supercapacitors.Transition metal oxides have become an ideal materials for supercapacitors due to their high specific capacitance and electrochemical activity.Among them,cobalt-based oxides have attracted widespread attention due to their environmental friendliness,lower cost,and high theoretical specific capacitance.Cobalt-based oxides also suffer from poor cycle stability and insufficient rate performance.Most Strategies were about rationally designing morphology and combining different materials in previous studies.Few metal doping have been reported to improve the material defects by one-step hydrothermal methods.In this paper,we designed a one-step hydrothermal method to prepare copper-doped porous self-assembled NiCo₂O₄(Cu-NiCo₂O₄).The Cu-NiCo₂O₄ exhibits superior performance as an electrode compared to NiCo₂O₄.Redox electrolyte additives have been widely studied because they can provide redox couples to enhance the specific capacitance of the device.However,the redox electrolyte additive enhances the specific capacitance performance by increasing the charge/discharge time of the electrode simultaneously and cannot acquire an ideal fast charge/slow discharge effect.Therefore,we designed a new type of electrolyte additive K₃Fe(CN)₆/K₂S₂O₈,which was applied to KOH electrolyte.Then,Co O was used as electrode to study the behavior of the whole device.The specific research is as follows:(1)In the aspect of material modification,copper doped NiCo₂O₄ electrode was prepared for the defects of poor cycling stability and rate performance of NiCo₂O₄ electrode.Doped metal atoms tend to occupy the position of the original crystal structure,thus changing the charge state of the original crystal and regulating its band structure,providing more active sites.We doped Cu into NiCo₂O₄ by one-step hydrothermal method to prepare a porous self-supporting Cu-NiCo₂O₄ nanosheet.The porous structure is conducive to the diffusion of electrolyte ions and the self-supporting structure also enhances the contact with the substrate to reduce the resistance.The doping of Cu effectively improved the capacity,cycle stability and rate performance of the NiCo₂O₄ electrode.In a three-electrode system,the capacitance retention of Cu-NiCo₂O₄ is 53.4%at current density from 1 A g^-1 to 10 A g^-1,while the pure NiCo₂O₄ capacitance retention is only 36.6%.After 3000 cycles at 1 A g^-1,the capacity retention rate of Cu-NiCo₂O₄ is 78.8%and the cycle retention rate of NiCo₂O₄ material is 48.1%.The specific capacitance of Cu-NiCo₂O₄ is 1512 F g^-1 and the specific capacitance of NiCo₂O₄ is600 F g^-1 at a current density of 1 A g^-1.(2)K₃Fe(CN)₆/K₂S₂O₈ was applied as active additives for KOH electrolyte.Traditional redox electrolyte additives only can prolong the charge and discharge time of electrodes at the same time.The strong oxidizing additives can accelerate the oxidation process of low-cost electrodes during charge process and shorten the charge time.The additives is partially reversible so the strategy can extending the discharge time.Fe(CN)₆^3-and S₂O₈^2-ions can accelerate Co²⁺oxidation to Co³⁺,referring to the standard electrode potentials of Fe(CN)₆^3-(0.36 V)and S₂O₈^2-(2.05 V)versus Co³⁺(0.17 V).The Co O as electrode and studied its electrochemical properties in the alkaline electrolyte of KOH with or without electrolyte additives while gave a mechanism explanation.In a three-electrode system,a Co O-based supercapacitor with an electrolyte additive can achieve a fast charge of 939 s and a slow discharge of 1699 s at a current density of 1 A g^-1.Even at a high current density of 10 A g^-1,the electrode still shows a fast charge of46 s and a slow discharge of 62 s.