Preparation Of Green Onion-derived Activated Carbon And Its Application In Environment-friendly New-energy-resource Field

With the rapid development of the global economy and the growth ofpopulation, the lack of energy resources and environmental crisis have beensignificant problems for our society. So there is an urgent need for efficient,clean, and sustainable resources of energy. Supercapacitor and direct formic fuelcell are two important energy conversion and storage devices that have beenpaid much attention by governments. The development of new energy resourcesdepends on the discovery of new materials. As a kind of porous carbon material,activated carbon has been widely applied, and its preparation process has been aresearch focus these years.In this dissertation, porous carbon with pore sizes of approx.0.6～1.2nm and3～5nm were synthesized by using green onion leaves, a kind of biomass, via afacile one-step carbonization and activation process. Furthermore, itselectrochemical behaviors in supercapacitor and direct formic fuel cell wereinvestigated. Scanning electron microscope (SEM), field-emission scanningelectron microscope (FE-SEM), energy dispersive X-ray detector (EDX), flameatomic absorption spectrometry (FAAS), X-ray diffraction (XRD),thermo-gravimetric analysis (TGA) and nitrogen (N2) adsorption-desorptionisothermal analysis were applied to characterize mesoporous carbon’smorphology, surface elements, specific surface area as well as pore size. Cyclicvoltammetry (CV), electrochemical impedance spectroscopy (EIS) andgalvanostatic charge/discharge (GCD) were carried out to evaluate theirelectrochemical behaviors. The main conclusions can be shown as follows: (1) The initial mineral substances in green onion leaves such as calcium (Ca)and potassium (K) play an essential role in the carbonization and activationprocess. They worked as porogens. Under the temperatures of600℃,700℃and800℃, activated carbon with a specific surface area of230.5m2·g-1,348.4m2·g-1and551.7m2·g-1were abtained by a one-step carbonization andactivation process. The value of the specific surface area is increasing with theincreasing of the reaction temperature. Furthermore, samples synthesized under800℃delivers the best performance, and has a more concentrated pore sizedistribution.(2) Green onion-derived activated carbons (GOC) performed well assupercapacitor’s electrode materials. The cyclic voltammetry curves of GOC arenearly rectangular-shaped, and the galvanostatic charge/discharge curves ofGOC are linear and nearly symmetrical with no obvious iR drop, Nyquist plotsof GOC show well-defined lines in the low frequency region, implying thatGOC are suitable to be applied in supercapacitors. GOC synthesized underdiffereent temperatures have a different capacitor performance. The larger theirspecific surface area are and the more concentrated their pore size distributionsare,the better they performed in capacitance.(3) Effective areal capacitance (EAC) is introduced, and mutiple biomassderived carbons’ EAC are caculated and compared. The EAC of GOCsynthesized under800℃could reach to28.8μF·cm-2at0.2A·g-1，which isnearly twice higher than others, revealing that GOC is more suitable for somesmall electronic devices.(4) As PdCu catalyst carrier, GOC performed well in formic acid catalysis.The amount of copper affects the behavior of PdCu catalyst. With the additionof copper, the electrocatalytic performance of PdCu catalyst increased first andthen decreased, implying that a suitable addition of copper could reduce theamount of palladium used and increase the catalytic behavior of catalysts.