Operando SANS Study on Ion Adsorption in Conductive Porous MOF Electrodes

Insights into the molecular mechanisms of transport and adsorption of electrolyte ions in porous materials under applied potentials are essential to control the performance of double layer capacitors for rapidly emerging high power energy storage, water purification and desalination. Here we present an extensive small-angle neutron scattering (SANS) characterization of ions with different sizes and chemical characteristics adsorbed into a conductive metal-organic framework (MOF) under operating conditions. The scattering curve of dry MOF revealed that the structure was divided into two regimes. The regime I consisted mainly of cylindrical mesopores of radius of ~24 Å. The correlation peak at 0.35 Å^-1 was attributed to ordered arrangement of micropores in regime II. Significant drop of scattering intensity at low q region and disappearance of correlation peak at high q region upon the addition of dueterated dimethylformamide (DMF) implied that the majority of the pores were accessible to the solvent. The ions diffused into mesopores immediately after the salts were added, which was evidenced by the increased scattering intensity. The negligible changes in scattering intensity between charging and discharging seemingly suggested the anion-cation exchange mechanism in mesopores.