Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Full Length Article The interfacial surface of an electrode for a supercapacitor as a factor affecting the capacitance and energy density D.S. Dmitriev ⁎ , A.V. Nashchekin, V.I. Popkov Ioffe Institute, 26, Polytechnicheskaya St., St. Petersburg 194021, Russian Federation ARTICLE INFO Keywords: Supercapacitor Activated carbon Specific surface Functional groups Specific capacity Energy density ABSTRACT The present paper proposes a criterion-based approach for selecting carbon material for supercapacitors, which is based on the type of surface functional groups and the micro/mesopore ratio. The activated carbons DLC Supra 30, W35, SX1G and CAP Super WJ (Cabot Corp.) with different porous structure and surface composition have been used as model objects. The BJH, DFT and t-plot methods have been used to calculate the specific surface area and the porosity of activated carbons, characterizing them as micro- and mesoporous materials with a surface area from 845 to 1855 m 2 ·g -1 . The SEM micrographs of electrodes allowed estimating the size and agglomeration degree of carbon particles and width of pores on surface. The combined results of FTIR spec- troscopy and Boehm titration showed the presence of phenolic, carbonyl and carboxyl functional groups on the surface, the concentration of which depended on the activated carbon grade. By comparing these results with the data of cyclic voltammetry, the influence of the micro/mesopore surface area ratio on the specific capacitance was established. Also, a correlation between the ionic character of the functional groups and the value of energy density has been noted. The results obtained by these methods can serve as a criterion for choosing the activated carbon as an electrode material for supercapacitors. 1. Introduction Supercapacitors (SC) are devices that are widely used as accumu- lators and concentrators of large amounts of electricity. The charge accumulation capability is due to the developed surface of the materials used as the main component of the active electrode mass. Such mate- rials include activated carbons (AC), which are characterized by a specific surface area of up to 2500 m 2 ·g -1 , high electrical conductivity and lyophilicity to organic electrolytes. The use of aprotic liquids such as acetonitrile or propylene carbonate as solvents for electrolytes en- sures the system stability over a wide potential range by increasing the window of thermodynamic stability. The electrically conductive com- ponent of the electrolyte is the R 4 NBF 4 salts (R = C n H 2n +1 ), which have a large charge due to big cations [1–12]. The SC application conditionally separates the system operation into two modes: charge/discharge mode within a long time, which re- quires a large capacity of electrode material (energy storage mode), and pulsed charge-discharge operation, providing for high power (energy concentrator mode). Today, selection of the electrode material for each of the modes is done empirically. A significant contribution to the SC capacitance is made by the electrode specific surface area. According to the expression = C εε S d 0 (1) capacitance C is directly proportional to the surface area S when such electrolyte parameters as the dielectric constant ε and thickness of the electrical double layer d, are fixed. In the case with AC, a large specific surface area is provided by pores sized from 2 to 20 A (micropores) and from 20 to 500 A (mesopores) [13,14]. Also important is the type of the pores in question. Pores can be continuous, blind and isolated. For the blind pores, condensation of electrolyte in the channels is possible, which leads to isolation of a part of the material surface and reduction of capacitive characteristics [15–17]. Surface properties of porous carbon materials are studied by the low-temperature gas adsorption, which allows determination of the specific area size, and the pores volume and area distribution according to their diameter. In order to analyze the AC electrochemical char- acteristics, which depend on the surface area and pore size (capacity, equivalent resistance), it is important to choose an objective mathe- matical model for calculating the porous structure parameters. The Langmuir, BET (Brunauer, Emmett, Teller) and t-plot methods are used to determine the specific surface. The Langmuir method supposes the absence of meso- and macropores with the main contribution to the https://doi.org/10.1016/j.apsusc.2019.144216 Received 9 July 2019; Received in revised form 9 September 2019; Accepted 28 September 2019 ⁎ Corresponding authors at: Ioffe Institute, Laboratory of Materials and Processes of Hydrogen Energy, Russian Federation. E-mail address: electrochemist@mail.ioffe.ru (D.S. Dmitriev). Applied Surface Science 501 (2020) 144216 Available online 08 October 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved. T