Research On High-Frequency And High-Capacitance Graphene-Based Devices And Their Functional Applications

The scale-down of transistors on the intergrated circuits is developed by following the Moore’s law,while the common capacitors make slowly downscale progress and no significant breakthrough is obtained.For example,the key filter capacitors（FCs）in power rectifier conversion circuit are generally used with the rigid and large-volume aluminum electrolytic capacitors,hindering the development of more compact electronic circuits.In addition,the rapid development of wearable electronic systems also puts forward the miniaturized capacitors for energy storage to realize the integrated and portable application.Recently,the micro-supercapacitors（MSCs）based on the effect of electric double-layer（EDL）have gained widely attention owing to the excellent characteristics of fast charge/discharge ability,high power density and long cycle life.Particularly,the miniaturized,flexible and intergratable features enable capacitors to develop along the Moore’s law.Two-dimensional carbon-based materials,such as graphene,are characterized by high carrier mobility,large specific surface area and rich surface physical and chemical information,hence the as-prepared MSCs have extremely high charge storage capacity.However,such MSCs have poor frequency response under hundred Hertz with the limition of electrode structure and characteristics,which is failed to satisfy the requirements of high frequency and high capacitance.At the same time,the power density and energy density of MSCs are still required to improve for energy storage.Based on the above background,this dissertation aims to realize the miniaturization of high voltage,high capacitance and high frequency capacitors,so ao to solve the key problems about narrow voltage window,poor frequency characteristic,low energy and power density.It is comprehensively investigated about graphene based capacitive devices that from five aspects including material design,device structure optimization,key mechanism exploration,performance improvement and functionalization development.The main research achievements are shown as follows:1.A novel three-dimensional（3D）nitrogen-doped graphene and its heterostrcture with pseudocapacitive NiMoO₄ materials（3D NG/NiMoO₄）were proposed,and a super high specific capacitance of 1913 F g^-1 was obtained based on this kide electrode materials,which represented the advanced international level in the same period.In order to solve the lower energy density limited by the pure graphene based EDL capacitors,the 3D NG was prepared as high conductive framework to provide the large surface area,shorten ions transport pathway and decrease the ionic resistance.The nitrogen-doped surface could adsorb the pseudocapacitive ions to ensure the uniform deposition of NiMoO₄ nanosheets.Finally,an open and honeycomb structure was obtained,which could contribute to the adequate ions’adsorption and fast reversible redox.Finally,an asymmetric supercapacitor was assembled using 3D NG/NiMoO₄ as positive electrode and active carbon as negative electrode,displaying a wide voltage window of 1.8 V and max energy density of 22.2 Wh kg^-1.This chapter builds the experimental foundation for the miniaturization of supercapacitors by studying the material structure design,assembled devices and charge storage mechanism.2.A new method for preparing miniaturized 3D graphene network based all-solid-state micro-supercapacitors was achieved by the assistance of custom mask for the first time.By combining with the transfer technology of graphene network,the as-prepared 3D graphene network based MSCs（GMSCs）displayed a high power feature of 1.1 W cm^-3.Then the 3D graphene/NiOOH/Ni（OH）2 composite was constructed to improve the energy density of MSCs.Finally,this MSCs exhibited an areal capacitance of 0.75 mF cm^-2 and volumetric energy density of 1.04 mWh cm^-3,which could be contributed to the phase transformation of Ni（OH）2 and NiOOH in both positive and negative when charging and discharging.3.A novel multifunctional and high-capacitance MSC with electromagnetic interference（EMI）shielding was firstly demonstrated based on the cation-intercalated Ti₃C₂Tx MXene,exhibiting a high total shielding effectiveness of 44.3 dB in X-band.Although the 3D graphene-based supercapacitors show excellent power density owing to the high conductivity and porous structure,it still need to build heterostructure to afford the high energy density.By virtue of superb intrinsic electrical conductivity and abundant functional groups（-OH,-O,and-F）,the Ti₃C₂Tx MXene,similar with graphene-based layered materials,has been considered as promising active material for the construction of high-capacitive devices with the fast surface redox.However,the problem of restacking between nanosheets causes low accessible surface area,limiting its pseudocapacitive performance.In this regard,the strategy of cation intercalation was developed to enlarge the interlayer spacing,increase electrolyte accessibility,and provide sufficient ions transport pathway.By density functional theory（DFT）simulation,it was found that the cation was inclined to be bonding with oxygen-contained terminal by the orbital hybridization to enhance the conductity and pseudocapacitance.Hence the assembled MSCs displayed a high areal capacitance of 87 mF cm^-2 and remarkable energy density of 11.8 mWh cm^-3.Furthermore,the interspace modulated Ti₃C₂Tx based MSCs within an EMIMBF4 gel electrolyte was proposed to enable layered transport of large-sized ions.It achieved a wide voltage windows of 2.5 V and a high areal energy density of 21.5 μWh cm^-2,providing insightful guidance for building MSCs with high voltage and high energy density.4.High-frequency Ti₃C₂Tx MXene based MSCs（M-MSCs）with wafer-size were demonstrated for AC line-filtering application by photolithography technology.By optimizing device structure,M-MSCs exhibited the over kHz frequency characteristic（f0）,which could reach 5.6 kHz in aqueous electrolyte and the highest power density was 2570 W cm^-3,which was the highest level ever reported in the world.The f0 could be further pushed to 6.6 kHz relying on the EMIMBF4 ionic liquid as electrolyte.Moreover,the MMSCs could maintaining quasi-rectangular cyclic voltammogram curves at 5000 V s^-1,displaying the stable operating characteristic.To verify the intergratable ability,the M-MSCs arrays in series and parallel were constructed.The areal capacitance and operating voltage were increased monotonically with the number of MSC units,suggesting that the MSCs arrays hold great potential for various requirements of tailored voltage,capacitance and frequency.Finally,the MSCs array was utilized as the FC in AC line-filtering circuits,realizing the stable DC transform with various AC inputs from low-frequency（60 Hz）to high-frequency（5000 Hz）.