Design Of Heterogeneous Structure And Interface Towards High Performance Carbon-Based Supercapacitors

Supercapacitor is an energy storage device with fast charge/discharge rate and long cycle life.Nevertheless,there is a gap between the basic experimental research and commercial devices,due to the different requirement and evaluations in two different situations.An urgent issue is to keep a decent performance for a high-mass-loading electrode from a practical application standpoint.Porous carbons have been widely used as electrode materials for supercapacitors but the further increase in the performance is restricted by the challenge of precisely tuning the porosity in the thick carbon electrodes.Specifically,the deteriorated electronic/ionic kinetics is a bottleneck for high-mass-loading electrodes,requiring a rational design of electronic/ionic interface in the electrodes.In thesis,the effect of heterogeneous interfaces on the ionic behavior has been studied by designing stacking carbons with different pore structures,and in this way the ion storage capacity in high-mass-loading electrodes improved.Furthermore,an optimized interface between electrode and electrolyte was studied to reduce the self-discharge attenuation of carbon based supercapacitors.Firstly,activated carbon(YP50F)mainly with micropores,activated microwave exfoliated graphene oxide(aMEGO)mainly with small mesopores and reduced graphene oxide(rGO)mainly with large mesopores have been selected as the model materials for the design of heterogeneous electrodes,by assembling carbons with differential pore features in different stacking orders.It is found that the charge storage performance is sensitive to the stacking order of these films along the thickness direction.Specifically,rGO being placed in the middle layer can efficiently reduce the ionic diffusion resistance.When aMEGO with high specific surface area is exposed to the electrolyte,the higher electrochemical capacitance is obtained.The interfacial effects of caused by different pore structures have been confirmed by finite element and molecular dynamics simulations.Secondly,the performance of high-mass-loading heterogeneous electrode has been further improved by selecting materials featured with ion transport channels and tuning the electrode preparation method and the combination ratio of heterogeneous electrodes.When YP50F,rGO and aMEGO,each with a thickness of 70 μm,were sequentially stacked in the electrode with aMEGO close to the separator,the 210-μm-thick electrode(～12.7 mg cm-2)has delivered superior capacitance performances(171.4 F g-1,102.8 F cm-3,2.2 F cm-2).Compared with other materials,microwave exfoliated graphene oxide(MEGO)with high density is more suitable for achieving the high loading.When the thickness of aMEGO,MEGO and aMEGO respectively were 100 μm,70 μm,and 50μm,this electrode with a～11.6 mg cm-2 area loading yielded capacitances of 112.5 F g-1,56.3 F cm-3 and 1.3 F cm-2.Lastly,to improve the self-discharge of supercapacitors,the interface optimization has been performed by adding a nematic liquid crystal,E7,as an additive to the electrolyte.The results show that,as the electrode is charged,the electrolyte viscosity increases while the ion diffusion rate decreases.Thus,the charge redistribution on the electrode surface is blocked,and the ion transport at the interface is dynamically changed due to the electrorheological effect of E7.Benefitting from the contribution of E7,the open circuit potential of 2 mg electrodes drops from 3 V to 1.4 V after 24 h,superior to that in the case without E7(dropping to 1.1 V).The use of liquid crystal provides a useful strategy to alleviate mitigate the self-discharge of carbon based electrodes.