Small‐Angle Neutron Scattering for In Situ Probing of Ion Adsorption Inside Micropores

Abstract

Transport and adsorption processes of electrolyte ions in porous carbon materials under an applied potential control the performance of double layer capacitors for rapidly emerging high-power energy storage applications, [1] capacitive deionization devices for water purification and desalination, [2] rotational motors for artificial muscles, microfluidic devices, and nanorobotics. [3] Electrochemical analysis techniques combined with structural and chemical characterization of porous carbon materials offer limited prognostic abilities. Several studies revealed significant impact of structural defects in carbon materials on their electrosorption properties [4] and the enhanced ion adsorption in sub-nanometer pores [5] in selected materials systems. The observed phenomena, however, were not always confirmed in other carbon materials or electrolytes, and the universality of such observations remains a topic of debates in the scientific community. The challenge comes from the difficulty of independently tuning various material properties needed for systematic experimental studies and from the high complexity of realistic materials systems for ab initio simulations.To gain better insights into adsorption phenomena, computer simulations were performed in model systems, [1a, 6] with a stronger emphasis on ionic liquid (IL) electrolytes, [1a, 6c-e] which do not contain solvent molecules and thus simplify the calculations. A recent study emphasized the importance of taking into consideration a more realistic structure of porous carbon materials. [7] While the obtained modeling results are insightful, development and adoption of complementary experimental techniques is critically needed to directly identify both the ion transport [8] and adsorption sites in a broad range of microporous solids as a function of the applied potential, verifying the molecular mechanisms previously proposed and providing guidance to future modeling efforts.Recent studies have demonstrated that small-angle neutron scattering (SANS) can provide unique, pore-size-specific information on the adsorption of confined fluids and may be used to evaluate the density of the adsorbed molecules in nanometer and sub-nanometer pores.

Neutron and X-Ray Porosimetry

“…Qualitatively similar results were obtained for liquid water saturated microporous activated carbon fibers (ACF), which represents a binder-free conductive porous carbon used as electrodes in double layer capacitors [59]. Commercially available activated carbon fibers (ACC-507-20) were purchased from Kynol (USA).…”

“…8.9c. The observed differences in the filling degree helped to understand the dramatic discrepancy between the electroadsorption of D 2 O-and H 2 O-based electrolyte ions in the smallest carbon nanopores revealed in [59]. Recent SAXS studies of the adsorption of water vapor on highly activated microporous carbons revealed physical mechanisms responsible for the degree of pore filling in these materials [60,61].…”

Individual Liquids and Liquid Solutions Under Confinement

“…A 1 M solution of tetraethylammonium tetrafluoroborate (TEATFB) salt in deuterated acetonitrile (d-AN) was used in an activated carbon with the pore size distribution similar to that used in commercial EDCLs. The SANS experimental setup and parameters were similar to those used in the previous study of the ion adsorption in aqueous electrolytes [6]. Since d-AN was used in the study, the only source of hydrogen present in the electrochemical system was N(C 2 H 5 ) 4 + cations.…”

“…The capability of SANS to provide in situ information on the pore size specific adsorption of confined fluids was utilized in [6] to directly visualize changes in the ion concentration in pores of different sizes as a function of the potential applied to nanoporous carbon electrode in aqueous electrolyte. The majority of electrolytes contain hydrogen atoms in either their solvent molecules or ions or both.…”

“…SANS and contrast-matching SANS have been utilized in [11,12] 6 -] IL was prepared following routine outlined in [13]. The pore filling of the samples with IL (named MCM_IL and SBA_IL) was achieved by physical imbibitions.…”