The High Value-added Preparation Of Biomass Based Modified Activated Carbons And Their Application As Supercapacitor Electrode Materials

Activated carbons(ACs) with large surface area and high electrical/thermal conductity are used widely in the fields such as adsorbent, catalyst support, and electrochemistry. In this thesis, a series of ACs have been successfully produced by such methods as doping and chemical activation using biomass(glucose, prawn shells) as carbon source. The morphologies and structures of the obtained carbons have been characterized by some technologies such as scanning electron miroscope, X-ray photoelectron spectroscopy and nitrogen adsorption-desorption techniques. Their application used as supercapacitor electrode has been studied in detail. The main contents and results are as follows:Biomass-derived hydrochar(HCs) was used as the precursor to synthesize porous and nitrogen-rich carbons for supercapacitor electrodes. Porous carbon(HC/KOH-1:2) activated by potassium hydroxide was used to prepare nitrogen-rich porous carbon(HC/KOH/N) by subsequent calcination in melamine. Similarly, HC/N/KOH was obtained from nitrogen-rich carbon(HC/N-1:5) calcined in melamine by subsequent KOH activation. It is found that KOH plays an important role on the creation of porous structures and selective production of active pyridinic and pyrrolic nitrogen species. HC/KOH-1:2 samples with abundant pores are excellent as supercapacitor electrodes in basic medium, whereas HC/N/KOH samples enriched with nitrogen functionalities are especially applicable in acidic medium. Typically, HC/KOH-1:2 and HC/N/KOH show the highest specific capacitance of 279 and 492 F g-1 at the current density of 0.1 A g-1 in KOH and H2SO4 electrolytes, respectively. Their specific energy density and power density are up to 8.1-12 Wh kg-1 and about 24 W kg-1 at 0.05 A g-1 in two-electrode cell. The excellent electrochemical properties of as-made carbons may be attributed to the high specific surface(1197 m2 g-1) for the former, and to the synergistic effect of combined accessible specific surface area(566 m2 g-1) and doped nitrogen(4.38 wt.%) for the latter.Nitrogen(N), oxygen(O) and phosphorus(P) co-decorated porous carbon(PCs) was in situ synthesized for supercapacitors by H3PO4 activation of shrimp shell. The results show that shrimp shell with 4.106 at% nitrogen, 26.99 at% carbon and 27.00 at% oxygen is a good carbon precursor, and H3PO4 plays an important role on the formation of porous structure and P doping. The contents of N, O and P functional groups in the resultant carbons along with their porosities are temperature-dependent. PCs with abundant pores and heteroatom functionalities are good electrode materials for supercapacitors in basic medium. Typically, PC with moderate N, O and P contents, large surface area and hierarchical pore structure exhibits a high specific capacitance of 206 F g-1 at 0.1 A g-1 in 6 M KOH solution. Furthermore, the energy density and power density of obtained PC electrodes can be substantially improved by widening the operation voltage window with the assistance of doped P. Specifically, its energy density can increase greatly from 2.9 Wh kg-1 at 0.9 V to 5.2 Wh kg-1 at 1.1 V.N-doped carbons were synthesized using prawn shells as the carbon source and KOH as activate agent by regulating their ratio and calcination temperature. The N, O contents of the carbons decrease and the specific surface areas increase with increasing the calcination temperature. The specific surface areas can be tailored in the range of 1163~2003 m2 g-1. It is found that the carbon with the sequence of KOH activation followed removal of Ca CO3 in prawn shell gets the larger specific surface areas than that of the reverse process. The specific capacitance of the obtained carbon is as high as 356 F g-1 at the current density of 0.05 A g-1. Its energy density and power density are 8.3 Wh kg-1 and 24 W kg-1, respectively.