DOI: 10.1007/s00339-005-3440-9 Appl. Phys. A 82, 607–613 (2006) Materials Science & Processing Applied Physics A l. eliad e. pollak n. levy g. salitra a. soffer ✉ d. aurbach Assessing optimal pore-to-ion size relations in the design of porous poly(vinylidene chloride) carbons for EDL capacitors Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel Received: 21 September 2004/Accepted: 17 August 2005 Published online: 24 November 2005 • © Springer-Verlag 2005 ABSTRACT In order to maximize the capacitance of electrical double layer (EDL) capacitors per unit electrode volume, higher surface areas are required. This leads to an increasingly greater subdivision of the carbon electrode, namely to pore systems of smaller pore size. When the pore size approaches the ion size, the EDL charging kinetics lows down because of multiple in- teractions of the ions with the surrounding pore walls. On the other hand, the ion electroadsorption capacity increases just be- cause of this enhanced interaction. Therefore, there is a conflict between improving discharge kinetics and improving capacity. Knowing the effective ion size relative to the pore size can be helpful in optimizing the pore system design in electrodes for EDL capacitors. A thorough technique based on the adsorption of molecular probes in the gas phase and the electroadsorp- tion of different ions was developed to assess pore dimensions. In this report, the technique is applied to the unique case of polyvinylidene chloride (PVDC) based carbon electrodes in an attempt to elucidate its extraordinary high EDL capacitance, in terms of the relation between effective ion size and pore size. PACS 82.47.Uv; 73.22.-f; 73.30.+y; 82.75.-z 1 Introduction The electrical double layer (EDL) capacitors, like other electric capacitors, are electrostatic energy storage de- vices and are probably the most recent development in the field of energy storage and conversion [1]. In contrast to batteries in which energy conversion is obtained by electro- chemical reactions, EDL capacitors (EDLC) are based on the storage and release of electrical charge stored in the electri- cal double layer, i.e., at the interface between an electrode and the electrolytic solution. A special advantage of these de- vices is their remarkable reversibility. Because electrostatic interactions are significantly less detrimental to the electrode and to the solution stability, EDL capacitors can be charged- discharged hundreds of thousands of times [1]. EDL capacitors are usually based on highly microporous, high surface area carbon electrodes. Compared with recharge- able batteries, these devices are superior in power density but ✉ Fax: 972-3-535 1250, E-mail: asoffer@bgu.ac.il inferior in energy density. In an attempt to improve both pa- rameters, we are currently carrying out a detailed study on the relation between pore size and ion size and their influence on the EDL charge capacity and the charging rates of porous carbons. It has been demonstrated [1, 2] that within the potential window of the electric double layer, the ions enter pores whose size matches the size of the hydrated state of the ions (this may be denoted as the effective ion size). We may reason- ably assume that for pores that are significantly larger than the effective ion size, the specific EDL capacity of porous elec- trodes (F/g or μ F/cm 2 ) is proportional to the overall surface area of the pore system. Therefore, it is of interest to have the highest surface area per unit electrode volume. This necessi- tates higher subdivisions of the solid, leading to smaller pore dimensions. Obviously, there is a limit as to how small the pores can be. When the average pore size is smaller than the ion size, no ion electroadsorption can take place. As is the case with adsorption from the gas phase, an ideal situation can be at- tained when the average pore size is slightly larger than the ion size [3]. This enables maximal (about two fold in the case of slit-shaped pores) interaction of the ions in the EDL with the pore walls, thus increasing the capacity without slowing down the electroadsorption and desorption kinetics. Hence, it is of great importance to study as precisely as possible the ion- pore interaction of EDL electrodes using electrolytes that are suitable for EDLCs. In this article, we briefly review experimental data on our method of assessing the pores and ions sizes [3–5], which are then used as a background to elucidate the behavior of polyvinylidene chloride (PVDC) based carbons. Attention should be paid to the extraordinary high EDL capacity that was obtained by Endo et al. [6], and reproduced herein by the application of special activation modes. 2 Experimental 2.1 Preparation of carbon samples Carbon cloth, originating from cellulose fabric, was produced by the temperature-programmed pyrolysis (up to 1050 ◦ C) of commercial cotton cloth under a nitrogen flow. Ammonium chloride was used as a carbonization moderator (about 10 wt % vs. cellulose) to produce carbon with a yield