Preparation And Properties Of Bacterial Cellulose-based Composite Electrode Materials For Supercapacitors

Bacterial cellulose(BC)is an interesting and environmentally-friendly natural nanomaterial produced by some bacteria using D-glucose as the carbon source,which forms a micro-nano porous three-dimensional network structure consisting of ribbon-shaped ultrafine nanofibers with a width of 50-80 nm and a thickness of 3-8 nm.Intrinsically derived from the unique structure,BC demonstrates a series of attractive properties such as high purity,high crystallinity,high water absorbency,good mechanical stability,and excellent biological affinity.In addition,its high aspect ratio and abundant active hydroxyl groups make it suitable for the incorporation with different nanostructures by providing powerful interaction of BC with surrounding species,such as inorganic and polymeric nanoparticles and nanowires.Conducting polymers(CPs),possessing intrinsic light-weight,flexibility and good conductivity,have been widely investigated in fabricating flexible pseudocapacitor electrodes.Transition metal oxide and metal sulfides are attracting more and more interest as electrode materials for supercapacitors and have achieved excellent electrochemical performances because of their relatively high capacitance and excellent redox reversibility.In this thesis,by depositing polypyrrole(PPy)and metal oxide/sulfide on BC membranes,flexible and highly-conductive PPy/metal oxide(sulfide)/BC membranes for supercapacitor electrodes were obtained.The detailed results included:(1)We have deposited PPy and nickel sulfide(Ni S)on bacterial cellulose(BC)nanofibrous membranes to be used as flexible supercapacitor electrodes.The obtained PPy/Ni S/BC composite membranes are highly conductive,with the electrical conductivity up to 5.1 S/cm.Electrochemical measurements showed that the supercapacitors based on the PPy/Ni S/BC electrodes has a high specific capacitance of 713 F/g with a power density of 39.5 W/kg and an energy density of 239.0 Wh/kg at a current density of 0.8 m A/cm~2.With increasing the current density from 0.2 to 1.6m A/cm~2,the corresponding capacitance changes from 884 to 569 F/g.The presence of Ni S is found to considerably improve the capacitance performance,while the PPy/Ni S/BC electrodes could retain only 25% after 300 cycles.(2)In this article,polypyrrole(PPy)and cobalt sulfide(Co S)have been deposited on BC membranes to prepare the nanofibrous composite electrodes of PPy/Co S/BC with excellent flexibility for supercapacitor application.The introduction of Co S remarkably improves the pseudocapacitance and electrochemical stability of PPy/BC-based electrodes.The specific capacitance of the PPy/Co S/BC-based supercapacitors can reach to about 614 F/g at a current density of 0.8m A/cm~2(0.7A/g)with maintaining 62.4% of the original value after 300 cycles;while the PPy/BC-based device could retain only 21.7% after 300 cycles.The PPy/Co S/BC-based devices can exhibit an energy density of about 54.5 Wh/kg at a power density of 663 W/kg.This work provides a promising approach to fabricate cost-effective and flexible membranes which can be used as high-performance supercapacitor electrodes.(3)Polypyrrole(PPy)and copper sulfide(Cu S)have been successfully deposited on bacterial cellulose(BC)membranes to prepare nanofibrous composite electrodes of PPy/Cu S/BC for flexible supercapacitor applications.The introduction of Cu S remarkably improves the specific capacitance and cycling stability of BC-based electrodes.The specific capacitance of the supercapacitors based on the PPy/Cu S/BC electrodes can reach to about 580 F/g at a current density of 0.8m A/cm~2 and can retain about 73% of their initial value after 300 cycles,while the PPy/BC-based device could retain only 21.7% after 300 cycles.This work provides a promising approach to fabricate cost-effective and flexible nanofibrous composite membranes for high-performance supercapacitor electrodes.(4)We have successfully prepared bacterial cellulose(BC)membranes coated by polypyrrole(PPy)and copper oxide(Cu O)as flexible composite electrodes for supercapacitor applications.The highest electrical conductivity value of 7.4/S cm was achieved by using copper acetate aqueous solution with concentration of 1 wt%.Electrochemical measurements proved that the supercapacitors using the PPy/Cu O/BC electrodes had a specific capacitance of 601 F/g with an energy density of 48.2 Wh/kg and a power density of 85.8 W/kg at a current density of 0.8m A/cm~2.The specific capacitance was kept at 385 F/g after 300 cycles.The introduction of the Cu O nanoparticles gave rise to the improved capacitance.