Formation of Electrical Double Layer of Organic Ions during Charging/Discharging of Mesoporous Carbon Electrodes

Abstract

High surface-area microporous carbon materials have been developed for increased energy storage in capacitors, but the high diffusion resistance in micropores may limit the ion transport during charging/discharging and hence compromised current density. To improve the efficiency of high surface-area electrodes, synthesis of carbon materials with well-defined pore shape/size ranging from nano- to micro-meter scales is the recent focus. In this study, in-situ small-angle X-ray scattering (SAXS) is adopted for structural characterization and elucidation of organic ion (EMI+ and TFSI−) transport during charging/discharging cycles. Two porous carbon materials of different pore distributions, coded as SCB (unimodal) and 1227 (multimodal), are investigated. For both carbon materials, cyclic voltammetry in a window of −2 to 2 V at scan rates of 100 and 10 mV/s indicates scan rate-insensitive specific capacitance, i.e., the multimodal pore distribution does not significantly enhance ion transport at a high scan rate, as the pore sizes are much greater than the organic ions. This is consistent with the observation that the ratio of specific capacitance values between the two carbon materials lies generally in the range of 1.45−1.50, close to the ratio of 1.42 in BET-determined surface areas. Comparison between SAXS profiles of dry carbon powders with those of the carbon electrodes after 6-hr soaking in the EMI+-TFSI− electrolyte indicates a general decrease in intensity (due to decreased contrast) for shoulders representing pores of different sizes, again indicating free-filling of pores by the neutral electrolyte. Assuming a core-shell model for the double layer structure in the presence of an applied potential, the SAXS profiles show that there is always a shell (ca. 0.7 nm in thickness) of counter-ions adhering to the pore wall, hence the pore contrast is dominated by the electron density differences between the co-ion-rich core and the carbon matrix, which decreases upon negative charging and increases upon positive charging. The SAXS profiles of 1227 are first fitted in the low- to medium-q ranges by use of a power-law contribution (representing fractal-like features) and two ploydisperse-sphere form factors (23 and 9 nm, respectively, in mean diameter) with hard sphere interaction. These contributions are then subtracted from the SAXS profiles to reveal the contribution of small pores (ca. 2.3 nm in mean diameter) in the range of q = 0.075 to 0.1 A−1. For SCB, we use the integrated intensity for q = 0.02–0.03 A−1 and for q = 0.06–0.08 A−1to represent form factors of Rg = 9 nm and 2.3 nm, respectively. Both approaches consistently indicate simultaneous contrast changes in pores of different sizes during positive and negative charging at 10 mV/s for both 1227 and SCB, suggesting that the double-layer structure individually forms in pores of all sizes. On the other hand, integrated SAXS intensity in q ranges of 0.02–0.03 A−1 and the high-q extreme of 0.9–1.0 A−1 during full-window charging at 10 mV/s indicate opposite changes in intensity. This is explained in terms of the dominance of contrast between counter-ion-rich shell layer of dense packing and matrix responsible for intensity in the high-q extreme.