Fabrication Of Bacterial Cellulose-based Carbon Fibers And Their Application In Supercapacitors

Compared with other electrode materials of supercapacitor, carbon fiber has higher conductivity, higher mechanical strength, larger specific surface area and more stable structure. However, according to the working principle of the supercapacitor, carbon materials have less specific capacitance than ideal. Researches show both activation modification and doping into active functional groups can improve its specific capacitance. The bacterial cellulose(BC) is a kind of secretion of the bacteria, and it has high crystallinity, high surface area and high degree of polymerization. BC meets requirements of supercapacitor electrode materials both in the structure and sources. Therefore, this paper selected BC as a precursor of carbon fiber to study. Then we tried to modify the prepared carbon fiber and observed the change of its electrochemical properties. Main researches and results of this paper are as follows:(1) The nano-carbon-fiber material was obtained by carbonization with different temperature within 800 ~ 1000 ℃, which is prepared by freeze-dried BC. From SEM graph we know that the carbon fiber material is consisted of nanometer sized fiber-silk and displays three-dimensional net structure. XRD picture shows that this material is amorphous carbon. Electrochemical tests show that this material carbonized by 900 ℃ has the best electrochemical performance. Its specific capacitance droped from 85.777 F/g to 66.0223 F/g with its current density increased from 0.5 A/g to 6 A/g. These results reflecte that the material has good rate capability and cycle stability.(2) Activation modification. Activated nano-carbon-fiber material was prepared by one-step carbonization activation. KOH and H3PO4 were selected as activating agent, respectively. SEM gragh shows that the nano-carbon-fiber prepared by KOH-one-step carbonization activation is composed of nano-size fibers. The result of BET shows that this nano-carbon-fiber’s specific surface area was increased from 385.43 m2/g to 477.05 m2/g. The material modified by KOH shows 114.44 F/g with 0.5 A/g while by H3PO4 shows 101.08 F/g at the same situation. These materials’ capacitance retention rate are both above 98.60% after 500 times of charge-discharge cycle with 4 A/g.(3) Doping modification. I:Nano-carbon-fiber material was prepared by one-step carbonization doping process. Urea and NH4H2PO4 were selected as doping source, respectively. II: Polypyrrole was situ-synthesized on the BC matrix, taking BC homogenate as raw materials. And then soaked in H3PO4,which is used as P source and activator. At last, we obtained nitrogen and phosphorus co-doped activated nano-carbon-fiber through one-step carbonization activation and doping process. SEM shows the carbon fiber producted by situ-PPy/H3PO4 modification was coated by a uniform layer. The layer is consisted of various-sizes particles with impurity elements. The infrared spectra and XPS analysis both show that the materials prepared by these three methods all contain nitrogen functional groups(N-5, N-6 and N-Q). Materials made by last two methods have phosphorus functional groups(C-PO3, C2-PO2 and C-O-P). The specific capacitance of N-CBC, NP-CBC-51 and Py-P-CBC-52 were improved in 6 M KOH electrolyte. The specific capacitance of them at 0.5 A/g were 122.20 F/g, 150.37 F/g and 240 F/g, respectively. And their capacitance retention rate were 99.19%, 96.64% and 97.2% after 500 times of charge-discharge cycle with 4 A/g.