Hydrothermal carbons from hemicellulose derived aqueous hydrolysis products as electrode materials for supercapacitors

J.M. SIEBEN; C. FALCO; M. SEVILLA; M.M. TITIRICI; E. MORALLÓN; D. CAZORLA-AMORÓS

Cracovia

Congreso; Carbon 2012; 2012

The Polish Carbon Society

The effective conversion of a cellulosic substrate into biofuels through the developedsynthetic routes is only possible when the lignocellulosic biomass is pre-treated in such a way asto separate the useful cellulose fraction. However, all the pre-treatments generate considerableamounts of by-products, such as the isolated lignin fraction and hemicellulose derived aqueoushydrolysis products which are abundant but they have so far found very limited use.Hydrothermal carbonization has demonstrated to be an effective synthetic route for theproduction of functionalized carbonaceous materials from simple monosaccharides, such asglucose and xylose. Therefore, hemicellulose derived aqueous hydrolysis products can be used aspotential carbon precursor for the hydrothermal carbonization (HTC) process.In this work, the carbonaceous materials obtained from the hydrothermal treatment of thehydrolysis products of spruce (sample E1) and corn cobs (sample E2), after KOH activation at700 °C, have been tested for supercapacitors applications.The apparent surface area, porous texture and elemental composition for each sample weremeasured. The BET surface area and pore size distributions of both samples are very similar.However, there are differences in their surface chemistry, being the amount of oxygen andnitrogen in sample E2 higher than in sample E1 (16.99 vs. 11.53 O% and 1.80 vs. 1.52 N%).However, the sulphur content of sample E2 is lower than that of sample E1 (0.34 vs. 0.60 S%).Both carbon materials have been electrochemically characterized in acid aqueous electrolytein a three-electrode cell configuration by cyclic voltammetry and galvanostatic experiments. Thematerials exhibit high specific capacitances (Cs) due to their high surface area and to the presenceof oxygen, nitrogen and sulphur functionalities. For example the specific capacitances of E1 andE2, for a voltage window of 0.8 V, are 267 and 300 F g-1 respectively at a current density of 250mAg-1 and above 400 F/g in both cases at a current density of 2 mA/g.Galvanostatic charge/discharge experiments at a wide current density range were also carriedout in a two-electrode cell using a sandwich type construction. In both materials Cs decreaseswith similar loss of capacitance when the current density increases, being the capacitance of thesymmetric E2/E2 capacitor higher in the whole range of current density. The symmetriccapacitors E1/E1 and E2/E2 develop good energy densities, with values of 6.4 and 7 W h kg-1respectively and power densities of 0.76 and 1.08 kW kg-1 in a voltage window of 0.8 V andcurrent density of 250 mA/g, with high coulombic efficiencies (> 99 %).