Preparation and Performance Investigation of Flexible Supercapacitor Electrode Based on Bacterial Cellulose

Author:Ma Li Na

Supervisor:huang yu dong


Degree Year:2018





The current development trend of wearable and portable electronics has promoted the demand for flexible,thin,lightweight,and hi ghly efficient energy storage devices.Supercapacitors are widely considered as a class of state–of–the–art energy storage devices due to high charge and discharge rates,high power densities,moderate energy densities and long lifetimes.One of challenges existing in flexible supercapacitors is to design bendable electrodes with both high areal capacitance and favorable mechanical properties.Benefiting from the ultrafine nanosized three–dimensional(3D)fibrous networks structure and remarkable mechanical properties,Bacterial cellulose(BC)has shown exciting potential as an ideal substrate for flexible electrodes.In this research,the BC was designed for preparing high areal performance and mechanically tough flexible electrodes.We explored and studied the synthesis process,electrochemical performances and physical properties of the flexible electrodes.At last,we provided the structure model of the flexible electrode based on the microtopography.Nitrogen–doped carbon nanofiber networks(N–CNFs)/graphene(RGO)/BC freestanding paper is first designed as a high–performance,mechanically tough and bendable electrode for supercapacitor.The BC is exploited as both supporting substrate and biomass precursor for N–CNFs.The one–step carbonization treatment not only fabricates the N–doped three–dimensional(3D)nanostructured carbon composite materials,but also forms the reduction of the GO sheets at the same time.The fabricated paper electrode exhibits an ultrahigh areal capacitance of 2.11 F cm–2(263 F g–1)in KOH electrolyte and 2.54 F cm–2(318 F g–1)in H2SO4 electrolyte,exceptional cycling stability(100%retentions after 20,000 cycles),and excellent tensile strength(40.7 MPa).The symmetric supercapacitor delivers a high energy density(0.11 mWh cm–2in KOH and 0.29 mWh cm–2in H2SO4)and a maximum power density(27 m W cm–2in KOH and 37.5 mW cm–2in H2SO4).Further improvement should be conducted to enhance the specific capacitance for practical application,A simple and low–cost approach toward flexible and freestanding electrode is developed.The method involves coating the polypyrrole(PPY)encapsulated RGO composites on BC,from which large mass loading of 8.93 mg cm–2is obtained.The flexible PPY/RGO/BC electrode achieves a high areal capacitance(2.10 F cm–22 at 2 mA cm–2),good rate performance(1.57 F cm–2retention at 50 mA cm–2)and good stability(64.7%after 5000 cycles).To improve the capacitive performance with high specific capacitance and good stability,another freestanding conductive film is designed and prepared by PPY/BC composites in combination with RGO through a simple in–situ polymerization and filtering method.The porous and flexible BC nanofiber is used as a substrate and template for successive polymerization of PPY,which is responsible for such a large areal mass of 13.5 mg cm–2.Thus,the high areal capacitance of 3.66 F cm–2at 1 mA cm–2and 2.59 F cm–2at 50 mA cm–2are achieved,moreover,the electrode shows good cycling property for 65.4%after 8000 cycles.A general approach toward flexible supercapacitor electrode based on metal hydroxide is also developed,which allows ultrahigh areal capacitance without compromising their gravimetric capacitance and mechanical properties.As a prototype,bendable freestanding film is fabricated by coating RGO–wrapped flowery Ni(OH)2 on BC with a rational combination of hydrothermal method and filtration technology.This as–assembled hierarchically structured flexible electrode is characterized by remarkable areal capacitance of 10.44 F cm–2(877.1 F g–1)at a large mass loading of 11.9 mg cm–2,excellent cycling stability with 93.6%capacitance retention after 15,000 cycles,high flexibility by bending to arbitrary angles(even 180°)and prominent tensile strength(48.8 MPa at we t state).Furthermore,it is hoped that the typical method can be applied for realizing other metal oxide/hydroxide flexible electrodes.These results indicate that the BC as substrate for flexible electrode can be applied for the vast majority of active materials,including carbon material,metal oxide and conducting polymer.And importantly,both high electrochemical performance and mechanically properties for flexible electrode can be achieved.This study provides efficient technological approaches for developing high performance flexible electrodes,which are promising candidates for the application of flexible supercapacitors.