Article pubs.acs.org/EF
Coconut-Shell-Based Porous Carbons with a Tunable Micro/ Mesopore Ratio for High-Performance Supercapacitors Juan Mi, Xiao-Rong Wang, Rui-Jun Fan, Wen-Hui Qu, and Wen-Cui Li* State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Faculty of Chemical, Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, P. R. China ABSTRACT: Granular micro/mesoporous carbon with a ratio of mesopore to total pore volume (Vmeso/Vtotal) greater than 75% was prepared using coconut shells as a precursor by a one-step thermal treatment, i.e., combined pyrolysis and steam activation process. The process variables, such as final activation temperature, time, and water flow rate were studied. The N2 adsorption isotherms of the samples were of type IV, indicating mesoporous characteristics. The mesoporosity of the resultant porous carbons prepared by this method is greater than the one of those prepared by the conventional two separate pyrolysis and activation processes. Experimental results showed that the yield of porous carbon was proportional to the final pyrolysis temperature and activation time. Additionally, with the increase of activation time and water flow rate, the mesoporosity increased considerably. When the activation time and water flow rate were kept constant, the mesoporosity also increased with a rise in the final pyrolysis temperature. Electrochemical tests indicate that with the increase of Vmeso/Vtotal of the porous carbons, the equivalent series resistance (ESR) decreases and the capacitance retention is of 93% at a high current density of 5 A g−1. Thereinto, the carbon electrode made from sample CS-800-0.12-60 with the highest Vmeso/Vtotal have a high capacitance of 228 F g−1 in 6.0 mol L−1 KOH electrolyte at 5 mV s−1 and the energy density of 38.5 Wh kg−1 with an ESR of 1.9 Ω at 0.5 A g−1.
1. INTRODUCTION During the past decade, there has been a great amount of interest in the use of porous carbon materials as high capacity supercapacitor electrodes. The energy storage of supercapacitors based on porous carbon materials relies on the electrostatic adsorption of positive and negative charges, thus an electric double layer storage mechanism. Hence, porous carbons including activated carbons,1,2 carbon aerogels,3,4 templated carbons,5−8 carbon fibers/nanotubes,9−11 and graphene12 applied in capacitors commonly have a tunable porous structure, high surface area, high electrical and thermal conductivities, and various forms.13 As well as the specific surface area, the wide variety of pore size distribution is significant for penetration or mobility of ions in the pore structure. Several earlier reports have also shown that high surface area carbon materials containing mesopores, which facilitate the electrolyte ion diffusion in the material, are highly desirable as a supercapacitor electrode.14−16 Some researchers have found an important role of micropores in the enhancement of capacitance.17 However, not all the micropores in a high surface area porous carbon may be effective for double-layer formation. Some report has demonstrated that the micropores (
