Ion Distribution in Electrified Micropores and Its Role in the Anomalous Enhancement of Capacitance Guang Feng, † Rui Qiao, †, * Jingsong Huang, ‡ Bobby G. Sumpter, ‡ and Vincent Meunier ‡, * † College of Engineering & Science, Clemson University, Clemson, South Carolina 29634-0921 and ‡ Oak Ridge National Laboratory, Bethel Valley Road, Oak Ridge, Tennessee 37831-6367 E lectrical energy storage plays a criti- cal role in many high-profile energy technologies such as generation of electricity from renewable sources and as key components of all-electric vehicles. 1 Electrochemical capacitors (ECs) use the electric field in the electrical double layers (EDLs) established at the electrode/electro- lyte interfaces to store electrical energy. 2 Because of their high power density and ex- cellent cyclability, ECs are emerging as an ideal solution for many electrical energy storage applications. 3-5 The primary limita- tion of ECs is their moderate energy den- sity, which is typically less than 10 Wh/kg. To address this limitation, electrodes with high specific surface area have been devel- oped. In particular, electrodes featuring mi- cropores (pore size 20 Å) are increasingly being used, and impressive improvement in energy density has been made recently. 4 However, current knowledge of the EDLs in micropores is still rudimentary, and many experimental observations remain poorly understood. For example, the area- normalized capacitance of activated car- bon electrodes immersed in 6 M KOH solu- tion has been found to increase from 6 F/cm 2 to 12 F/cm 2 as the mean pore size of the electrode decreases from 14.5 to 10.6 Å. 6 A similar anomalous enhancement has also been reported previously for mi- cropores in organic electrolytes and in room-temperature ionic liquids. 7,8 Such an anomalous enhancement cannot be ex- plained even qualitatively by the classical EDL theories based on the Poisson-Boltzmann (PB) equation. 9,10 To rationalize the anomalous enhance- ment of capacitance in micropores, an elec- tric wire-in-cylinder capacitor (EWCC) model has recently been proposed. 11 The EWCC model captures two key effects: (1) confine- ment and (2) curvature. The main idea is that counterions form a wire along the axis of small-diameter cylindrical micropores, re- sulting in a different capacitance regime from that of mesopore structures where curvature effects are also needed for a quantitative description, even though they display smaller mean curvature as com- pared to micropores. The EWCC model has been shown to fit remarkably well to avail- able experimental data. 11 Although this model sheds light on the anomalous en- hancement of capacitance in micropores, many issues remain open. First, it is often thought that micropores are slit-shaped in- stead of cylindrical, and thus, the EWCC model may not always be applicable, de- pending on electrode materials synthesis and processing. Second, while the assump- tion that ions accumulate along the center of electrified micropores seems reasonable from a purely geometric confinement standpoint, details on the confining pro- cesses remains largely unproven in the complex electrolyte-electrode interface structure. In fact, because the classical EDL *Address correspondence to rqiao@clemson.edu, meunierv@ornl.gov. Received for review January 21, 2010 and accepted March 18, 2010. Published online April 5, 2010. 10.1021/nn100126w © 2010 American Chemical Society ABSTRACT The distribution of K  ions in electrified slit-shaped micropores with pore widths ranging from 9.36 to 14.7 Å was studied using molecular dynamics simulations. We show that, in slit pores with pore widths between 10 and 14.7 Å, the K  ion distribution differs qualitatively from that described by classical electrical double-layer (EDL) theories in that fully hydrated K  ions accumulate primarily in the central plane of the slit pores. This phenomenon disappears when the pore width is narrower than 10 Å. Ion hydration and waterwater interactions, which are rarely considered in prior EDL theories for micropores, are found to be responsible for this behavior. On the basis of these results, we have developed a new sandwich capacitance model to describe the capacitance of the EDLs formed by K  ions enclosed in slit-shaped micropores. This model is capable of predicting the anomalous enhancement of capacitance experimentally observed in micropores. KEYWORDS: electrochemical capacitor · electrical double layer · micropore · ion hydration · sandwich model ARTICLE VOL. 4 ▪ NO. 4 ▪ FENG ET AL. www.acsnano.org 2382