Microwave synthesis of micro-mesoporous activated carbon xerogels for high performance supercapacitors E.G. Calvo, N. Ferrera-Lorenzo, J.A. Menéndez, A. Arenillas ⇑ Instituto Nacional del Carbón, CSIC, Apartado 73, 33080 Oviedo, Spain article info Article history: Received 17 August 2012 Accepted 15 October 2012 Available online 23 October 2012 Keywords: Microwave technology Chemical activation Porous carbon xerogels Energy storage abstract This work illustrates the production of porous carbon xerogels by means of a chemical activation method based on microwave radiation. The evolution of textural properties and the electrochemical performance of the materials synthesized, in relation to activation time and temperature, were investigated. The study of the activation time revealed that carbon xerogels with a remarkable micro-mesoporosity development (S BET around 2200 m 2 g À1 ) can be produced in a time range of 6–30 min. However, the prolongation of microwaves exposure, i.e. the increase in the activation time, leads to a decrease in microporosity and reduces the contribution of the precursor material mesoporosity. The results derived from the study of different activation temperatures (i.e. 700, 600 and 500 °C) revealed that the most suitable temperature for synthesizing carbon xerogel with a high surface area is 700 °C. Electrochemical capacitors assembled with carbon xerogels as electrode material and H 2 SO 4 (1 M) as electrolyte, were characterized by cyclic voltammetry and galvanostatic techniques. Carbon xerogels synthesized in the laboratory displayed spe- cific capacitance values of about 170 F g À1 , higher values than those of various commercial activated car- bons for this specific application. The best energy storage value was achieved with the xerogel activated for just 6 min, probably as a result of the increase in the volume of ultramicropores from 0.4 to 0.7 nm. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction In recent years, carbon gels have attracted widespread attention for energy applications due to a number of interesting features such as: a unique three-dimensional nano network, a pore texture tailored according to the synthesis protocol, a high electrical con- ductivity and the possibility of being used without any binding substances [1,2]. Despite these advantages, the main drawback of this kind of carbonaceous material lies in the synthesis process because, by means of conventional methods, at least 24 h are required to produce materials with a significant textural develop- ment [3]. The limitation of such slow and uncompetitive synthesis method has recently been solved through the use of microwave technology, as evidenced by several published works [2,3]. These studies demonstrated that it is possible to prepare carbon xerogels analogous to those conventionally synthesized but with a substan- tial saving of time (about 5 h as opposed to several days in conven- tional processes). Another problem that needs to be addressed is the development of carbon xerogels microporosity. The specific surface area of or- ganic xerogels is about 200 m 2 g À1 , a value that can be increased to 600–700 m 2 g À1 after the pyrolysis stage under certain operat- ing conditions [2,4–6]. However, this porosity is well below that exhibited by activated carbons used as electrode material in sup- ercapacitors. Microporosity can be increased to surface area values of almost 2000 m 2 g À1 by means of activation processes [7–10] and, in particular, by chemical activation, which is the process studied in this work. Many variables are involved in chemical activation and this makes it possible to design the porosity of carbon xerogels by selecting specific activation parameters. Variables such as activat- ing agent (A) and precursor (P) used, A/P mass ratio or time and temperature of activation, have a very noticeable effect on the properties of the resulting material [9,11,12]. In this work, both the activating agent (potassium hydroxide) and the precursor material (resorcinol–formaldehyde organic xerogel) used were the same in all cases. Only the activation temperature (T a ) and time (t a ) were modified in order to produce materials with different mi- cro/mesoporosity and evaluate the effect of this porosity on the en- ergy storage capacitance of the carbon xerogels synthesized. Traditionally, chemical activation processes have been carried out by means of conventional heating mechanisms, at tempera- tures between 400 and 950 °C and with activation times ranging between 0.5 and 5 h [7,9,12,13]. However, microwave heating has emerged as an alternative activation technique in recent years [14–18]. Some published works report the use of microwaves for producing activated carbons from biomass residues, with a consid- erable saving of energy and short processing times. For example, Foo and Hameed [16,19,20] have recently published several papers 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.10.008 ⇑ Corresponding author. Tel.: +34 985119090; fax: +34 985297662. E-mail address: aapuente@incar.csic.es (A. Arenillas). Microporous and Mesoporous Materials 168 (2013) 206–212 Contents lists available at SciVerse ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso