Ion Distribution in Electrified Micropores and Its Role in the Anomalous Enhancement of Capacitance

The distribution of K+ ions in electrified slit-shaped micropores with pore widths ranging from 9.36 to 14.7 angstrom was studied using molecular dynamics simulations. We show that, in slit pores with pore widths between 10 and 14.7 angstrom, 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 angstrom. 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.