Electrochimica Acta 49 (2004) 1105–1112 Small-angle neutron scattering and cyclic voltammetry study on electrochemically oxidized and reduced pyrolytic carbon A. Braun a,b,∗ , J. Kohlbrecher c , M. Bärtsch a , B. Schnyder a , R. Kötz a,1 , O. Haas a , A. Wokaun a,b a Electrochemistry Section, General Energy Research Department, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland b Department of Chemical Engineering and Industrial Chemistry, Swiss Federal Institute of Technology, ETH Zentrum, CH-8092 Zürich, Switzerland c ASQ, Spallation Source Division, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland Received 25 April 2003; received in revised form 23 September 2003; accepted 26 October 2003 Abstract The electrochemical double layer capacitance and internal surface area of a pyrolytic carbon material after electrochemical oxidation and subsequent reduction was studied with cyclic voltammetry and small-angle neutron scattering. Oxidation yields an enhanced internal surface area (activation), and subsequent reduction causes a decrease of this internal surface area. The change of the Porod constant, as obtained from small-angle neutron scattering, reveals that the decrease in internal surface area is not caused merely by a closing or narrowing of the pores, but by a partial collapse of the pore network. © 2003 Elsevier Ltd. All rights reserved. Keywords: Graphite; Activated carbon; Double layer; Supercapacitors; Small-angle scattering 1. Introduction Based on the pioneering work by Miklos et al. [1], we have recently published several studies on the oxidation of glassy carbon (GC) for supercapacitor applications [2–14]. GC is a hard carbon material obtained by pyrolysis of phe- nolic resins and frequently referred to as pyrolytic carbon or vitreous carbon. Subsequent annealing at high temperatures, which may range from 800 to 3000 ◦ C, has significant influ- ence on the structure of the material [15]. It contains voids of about 1–2 nm dia meter, which are closed and separated against each other [15–17]. The voids (pores) can be opened (activation), for instance, by thermochemical gas phase oxi- dation [1,6–8] or by electrochemical oxidation [2,3]. Oxida- tion, in this context, means the creation of an open porosity with a very large internal surface, which is often referred to ∗ Corresponding author. Present address: Consortium for Fossil Fuel Science, University of Kentucky, Suite 107 Sam Whalen Bldg., 533 South Limestone Street, Lexington, KY 40506, USA. Tel.: +1-859-257-6087; fax: +1-859-257-7215. E-mail address: artur.braun@alumni.ethz.ch (A. Braun). 1 ISE member. as activation. The terms activation and oxidation are used equivocally in this work. The activation process causes a film with open pores to grow on the surface of the GC. The film growth law for ther- mochemical activation exhibits a complex sigmoidal char- acteristics [9,10]. For electrochemical activation, the film growth seems to have a simple linear growth characteristics [3,10]. The opened pores are accessible to gases or liquids and provide a huge specific internal surface area, which can be utilized as an electrode material for electrochemical dou- ble layer capacitors [1,4,5,7,8,14]. Differential electrochemical mass spectrometry per- formed on the G-type GC had revealed that CO 2 is the main reaction product during activation at 2.2 V versus Pd/H 2 , and to a lesser extent, H 2 . The evolution of CO 2 means then, that carbon is burned off. Evolution of oxygen could be ruled out. Reduction at 0.0 V versus Pd/H 2 causes evolu- tion of CO 2 , most likely due to the reduction of the surface groups created during oxidation. Upon further reduction, H 2 evolution occurs. X-ray photoelectron spectroscopy with respect to the carbon K-shell showed that activation creates carbonyl and/or quinone, and carboxyl surface groups, a well observed and studied phenomena on oxidized carbon 0013-4686/$ – see front matter © 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2003.10.022