Composite Electrodes Derived From Bacterial Cellulose Nanofibers And Their Supercapacitors

Bacterial cellulose has received more and more attention due to its high crystallinity,ultrafine network structure,high elastic modulus and tensile strength,high water retention capacity,good biocompatibility and biodegradability.The fast charge and discharge,high efficiency,low cost and high stability of supercapacitors have gradually found their widespread application in the energy field.The unique characteristics of bacterial cellulose make its utilization in supercapacitors more and more diverse.In this thesis,bacterial cellulose nanofibers were used as the basic material to prepare various electrodes,via the modification or treatment of the bacterial cellulose,and the resulted free-standing porous electrodes were combined with other redox active materials to fabricate supercapacitors with higher electrochemical performance.Firstly,polypyrrole was in-situ deposited on the surface of bacterial cellulose nanofibers by a chemical deposition method.By studying the effect of polypyrrole content on the electrochemical performance of the resulted BC/PPY electrode,the optimized electrochemical electrode material was obtained.It is found that when the polypyrrole content is 85%,its specific capacitance measured at current density of 0.5 A·g^-1 is 325 F·g^-1.The effect of the concentrations of Fe^3+/2+redox additives in the electrolyte on the capacitance performance of the resulted electrodes was also investigated.Results indicated that when the concentration of Fe^3+/2+in the electrolyte is 0.1 M and measured at a current density of 0.5 A·g^-1,the specific capacitance of the symmetrical capacitor assembled with the optimized BC/PPY electrodes is 219.5 F·g^-1,and it has114%capacitance retention rate after 5000 charge-discharge cycles.Theses indicated that the presence of the Fe^3+/2+redox additives in the electrolyte can effectively improve the electrochemical capacitive performance of BC/PPY electrodes and their supercapacitors.Secondly,the bacterial cellulose nanofiber was modified by the grafting of sulfonic groups on their surface to prepare the sulfonated bacterial cellulose（SBC）,which was further compounded with polypyrrole to prepare SBC/PPY electrode material.The results show that under the same polypyrrole（85%）content,the electrochemical performance of SBC/PPY electrode is improved in comparison with that of BC/PPY electrode,and its specific capacitance is measured to be 339 F·g^-1 at a current density of 0.5 A·g^-1.At the same time,a hydrothermal reaction was used to prepare MoO₃ nanobelts,which were mixed with SBC and CNTs to prepare SBC/Mo O₃/CNTs thin films electrodes.Their electrochemical behavior in 1 M H₂SO₄ was studied and its specific capacitance measured at 1 A·g^-1 is 215.7 F·g^-1.Finally,it was used as a negative electrode to assemble asymmetric supercapacitors（ASC）where the optimized SBC/PPY film was utilized as a positive electrode.The resulted ASC displayed specific capacitance of 92.7 F·g^-1 in the voltage range of 0-1.3V at a current density is 0.5 A·g^-1.At the same time,when the power density is 1.2 KW·kg^-1,the energy density 77.9 Wh·kg^-1 of could be achieved.Moreover,its capacitance remained to be 98%after 5000 charge discharge cycles.Finally,quasi-solid-state dual-asymmetric supercapacitors（DASC）with high capacitive performance were developed based on the aforementioned results.Free-standing carbonized bacterial cellulose（CBC）aerogels were fabricated by pyrolyzing the BC aerogels,which was prepared by freezing drying the BC hydrogels.They were directly utilized as positive electrodes and combined with iron ions based redox active gel electrolyte(PVA/H₂SO₄/Fe^3+/2+)for the positive compartment of the capacitors.Mo O₃ nanobelts were used for the fabrication of negative electrodes,which were assembled with conventional PVA/H₂SO₄ gel electrolyte for the negative part of the capacitors.In the assembled DASC,the two compartments were separated by a Nafion proton exchange membrane.The assembled DASC with the configuration of CBC-PVA/H₂SO₄/0.7M Fe^3+/2+|PVA/H₂SO₄-Mo O₃ displayed a remarkable specific capacitance of 560.8 F·g^-1 at 1 A·g^-1 and a high energy density of 131.6 Wh·kg^-1,which could be one of the highest energy density among the carbon aerogel based supercapacitors to our best knowledge.Moreover,the optimized DASC exhibited a remarkable long-term cycling stability that retains 101.3%of the original capacitance together with a coulombic efficiency of 98.8%after 30000 cycles.This work on the design and assembly of the battery-like supercapacitor with such a dual-asymmetric configuration combined with the utilization of the excellent hydrogel electrolyte system affords a new concept to fabricate the optimized devices with high performance in the energy storage and conversion.