Preparation Of Porous Graphene And NiCo LDH Electrode Materials And Assembly Of Flexible Devices

Supercapacitors,also known as electrochemical capacitors,have been widely used in a various fields,including electric vehicles,emergency power and astronautics,due to their high power density,excellent charge-discharge rate and outstanding cycling stability.In order to meet the growing demand for portable electronic devices,developing lightweight,safe and flexible supercapacitor is highly desirable.The electrode plays an crucial role in determining the main performance of a supercapacitor.In this work,we systematically study and characterize the morphology,structure and capacitive performance of graphene-based electrode materials and NiCo LDH materials.In order to prepare flexible supercapacitor,we used commercial carbon cloth and textile carbon cloth as flexible substrates.Flexible electrodes were prepared by direct coating graphene-based materials on commercial carbon cloth and electrochemical depositing NiCo LDH on textile carbon cloth.With these electrode and PVA/KOH gel electrolyte,flexible supercapacitors were fabricated and their capacitive performances were systematically investigated.The results are listed as following:We present a facile one-step pyrolysis procedure for preparing hierarchical graphene/porous carbon network by directly pyrolyzing the mixture of ethylene diamine tetraacetic acid tripotassium salt(EDTA-3K)and graphene oxide(GO).The electrochemical behaviors of the prepared hybrid materials were measured in a traditional supercapacitor.The PCG-based supercapacitor shows a large specific capacitance of 220 F g-1 in 6.0 M KOH electrolyte at 0.5 A g-1 and 92%of its initial capacitance has been retained as the current density increased to 30 A g-1.Moreover,the PCG electrode also exhibits surprisingly short relaxation time constant of 0.74 s due to the high diffusion rate of electrolyte ions into the inner of PCG electrode.When measured in 1.0 M TEABF4/AN electrolyte,the voltage window of PCG-capacitor can be expanded to 2.5 V.PCG electrode in TEABF4/AN exhibits a high specific capacitance of 145 F g-1 and a better capacitance retention.Moreover,the cycling stability of PCG electrode in aqueous and organic electrolyte reaches 97%and 91%,respectively,demonstrating the superior cycling stability of PCG electrode.When PCG electrode is assembled into a flexible supercapacitor,the device can deliver a high capacitance of 197 F g-1,which is close to the capacitance of traditional supercapacitor.When the current density increased from 1 A g-1 to 30 A g-1,73.6%of its initial capacitance is still retained.Furthermore,the PCG-based flexible supercapacitor exhibits a good capacitive performance at different bending states.We directly utilize the carbonized ATC as flexible substrate to grow NiCo-LDH through an electrochemical deposition technique.By changing the deposition time,NiCo LDH with different amount were deposited on ATC.The amount and sheet thickness of NiCo LDH on ATC were found to increase upon extending the deposition time.When the deposition time is 60 minutes,the areal and volumetric capacitance of ATC-NiCo LDH is 9.16 F cm-2 and 458 F cm-3,respectively.In order to well balance the charge between positive and negative electrodes,we used ATC-NiCo-10m as positive electrode,ATC-AC as negative electrode and PVA/KOH as gel electrolyte to fabricate an asymmetric flexible supercapacitor.The device has an extended voltage of 1.6 V and delivers a volumetric capacitance of 23 F cm-3 at 6 mA cm-2.The capacitance retention reaches 78%as the current density increases to 100 mA cm-2.Moreover,the flexible supercapacitor also exhibits a good capacitive performance at different bending angles.The device exhibits a high energy density of 7.9 mWh cm-3,meeting the energy requirements of flexible and wearable devices.Furthermore,the capacitance retention can reach 91%after 3000 cycling charge-discharge,demonstrating an excellent cycling stability.