**1. Introduction**

Electrical double-layer capacitors (EDLCs) are very attractive for use in potential energy storage devices because of their high power density, quick charge-discharge rate, and long maintenance-free operation life [1–3]. Energy storage behavior of an EDLC arises mainly from the separation of electronic and ionic charges at the interface between the electrode materials and the electrolyte solution [4]. Therefore, the electrochemical behavior of an EDLC is determined by the textural properties of the active material. Activated carbons (AC) have excellent textural properties and high electrical conductivity, which have made them the most widely used active materials for EDLCs [5–7].

Many studies have focused on the specific surface area among the textural properties of AC [5,6]; however, Baek's research showed that di fferent pore structures are also required, depending on the salt and solvent of the electrolyte [7]. In conclusion, to improve the electrochemical properties of EDLCs, it is necessary to optimize the pore structure of the AC for each electrolyte.

The pore structure of AC is known to be greatly influenced by the precursor [8–10], activation method [11–14], and pyrolysis conditions [15,16]. The final pore size distribution is mainly determined by the activation method. ACs are prepared by physical activation (gasification of a char in oxidizing gases) or by chemical activation (pyrolysis of precursor impregnated with chemical reagents).

Chemical activation is generally done by mixing carbonaceous materials with chemical activating agents (KOH, H3PO4, ZnCl2, etc.), followed by pyrolysis at 400–900 ◦C [17–19]. This process gives rise to

AC with a high specific surface area (>2000 m<sup>2</sup>/g) mainly of micropores, with some sub-mesopores [5,17]. Such a high specific surface area is ascribed to partial gasification and expansion of the interlayer spacing between crystallites through simultaneous intercalation and deintercalation [17]. However, this approach has disadvantages that include corrosiveness of the chemical agents and the washing process that is necessary to remove the chemical agents.

Physical activation is done by carbonization of carbon precursors in an inert atmosphere to remove non-carbon elements, followed by activation in the presence of suitable oxidizing gasifying agents (O2, CO2, or H2O) to develop the porosity, usually in the temperature range 600–1200 ◦C [14–16]. Pores are formed by the oxidation of crystallites by physical activation, during which the size of the crystallite affects pore characteristics as much as the activation method does [16]. Baek et al., reported that hard carbon (HC) of low crystallinity could be used to obtain AC of high specific surface area using physical activation [14,15].

In general, the textural properties (specific surface area, pore volume) of AC resulting from chemical activation are known to be better than those from physical activation [17]. This is why most AC studies for EDLC focus on chemical activation methods. However, the process cost of chemical activation is much higher than that of physical activation. Thus, research is needed to produce AC with excellent pore characteristics through physical activation.

Most commercial grade AC is derived from naturally occurring precursors such as wood [19], coal [20], and coconut shells [21]. However, naturally occurring precursors contain large quantities of ash [22]. For the production of AC, ash causes problems such as capacity reduction, gas creation, and swelling of the EDLC [23,24]. Therefore, AC produced from naturally occurring precursors requires a separate ash removal process.

Polymeric precursors have structural features similar to those in coal, but contain many fewer mineral impurities (from catalysts), which can be controlled to very low levels during their synthesis [25]. However, polymer-based AC with high specific surface area has been reported as a result of chemical activation [17,24]. Generally, a polymer-based precursor with high carbonization yield has high crystallinity; therefore, it is di fficult to produce AC having a high specific surface area by physical activation [26].

In this work, activated polymer-based hard carbon using steam activation (APHS) with a high specific surface area and mesopore-rich pore structure was prepared from polymeric precursors with low crystallinity. The pore structure of the APHS obtained was studied using N2 adsorption. The APHS thus prepared was applied as electrodes for the EDLC, and its specific capacitance was discussed in relation to the pore structure.

## **2. Experiment Details**
