A carbon nanopore model to quantify structure and kinetics of ion electrosorption with in situ small-angle X-ray scattering.

A new carbon model derived from in situ small-angle X-ray scattering (SAXS) enables a quantitative description of the voltage-dependent arrangement and transport of ions within the nanopores of carbon-based electric double-layer capacitors. In the first step, ex situ SAXS data for nanoporous carbon-based electrodes are used to generate a three-dimensional real-space model of the nanopore structure using the concept of Gaussian random fields. This pore model is used to derive important pore size characteristics, which are cross-validated against the corresponding values from gas sorption analysis. In the second step, simulated in situ SAXS patterns are generated after filling the model pore structure with an aqueous electrolyte and rearranging the ions via a Monte Carlo simulation for different applied electrical potentials. These simulated SAXS patterns are compared with in situ SAXS patterns recorded during voltage cycling. Experiments with different cyclic voltammetry scan rates revealed a systematic time lag between ion transport processes and the applied voltage signal. Global transport into and out of nanopores was found to be faster than the accommodation of the local equilibrium arrangement in favor of sites with a high degree of confinement.

Improved capacitive deionization performance of mixed hydrophobic/hydrophilic activated carbon electrodes.

Capacitive deionization (CDI) is a promising salt removal technology with high energy efficiency when applied to low molar concentration aqueous electrolytes. As an interfacial process, ion electrosorption during CDI operation is sensitive to the pore structure and the total pore volume of carbon electrodes limits the maximum salt adsorption capacity (SAC). Thus, activation of carbons as a widely used method to enhance the porosity of a material should also be highly attractive for improving SAC values. In our study, we use easy-to-scale and facile-to-apply CO2-activation at temperatures between 950 °C and 1020 °C to increase the porosity of commercially available activated carbon. While the pore volume and surface area can be significantly increased up to 1.51 cm(3) g(-1) and 2113 m(2) g(-1), this comes at the expense of making the carbon more hydrophobic. We present a novel strategy to capitalize on the improved pore structure by admixing as received (more hydrophilic) carbon with CO2-treated (more hydrophobic) carbon for CDI electrodes without using membranes. This translates into an enhanced charge storage ability in high and low molar concentrations (1 M and 5 mM NaCl) and significantly improved CDI performance (at 5 mM NaCl). In particular, we obtain stable CDI performance at 0.86 charge efficiency with 13.1 mg g(-1) SAC for an optimized 2:1 mixture (by mass).