PNNL

The Science

Removing the solvent surrounding charged molecules, or ions, in solutions during electrochemical separations is an important energy-intensive process which may be facilitated by the introduction of water repelling, or hydrophobic, domains. Ionic liquids (ILs) are molten salts with a wide range of potential applications due to their tunable hydrophobicity and distinctive electronic properties. Researchers studied the structure, stability, and ion adsorption properties of size-selected IL clusters through a synergistic experimental and theoretical approach. Electrodes were functionalized with size-tunable IL clusters with varying degrees of hydrophobicity through ion soft landing. A global optimization procedure, NWPEsSE, was used to identify the optimal cluster structures and electronic properties. The well-defined clusters control the adsorption of targeted ions from the solution during electrochemical separations.

The Impact

Electrochemical separations are a potentially transformative and energy-efficient way to selectively remove ions from solution. However, developing ways to separate targeted ions remains challenging. This study shows that size-tunable IL clusters alter the behavior of ions to enable selective adsorption onto electrodes. Specifically, functionalization with ILs improves the efficiency of electrochemical adsorption at IL-modified interfaces, thereby opening new pathways to the selective electrochemical separations central to the extraction of rare critical minerals, environmental cleanup, and production of potable water.

Summary

Functionalizing electrodes with hydrophobic regions has the potential to significantly improve the energy efficiency and selectivity of electrochemical separations. ILs, which have a broad range of compositions and properties, are leading candidates for controlling the transport, desolvation, and adsorption of targeted ions at electrochemical interfaces. A team of scientists performed a joint experimental and theoretical investigation of the structure, stability, and selective adsorption properties of 1-ethyl-3-methylimidazolium chloride [EMIM]_x[Cl]_x+1^- (x = 1 – 10) IL clusters deposited on electrodes. The structure and stability of the IL clusters were probed in the gas-phase using high-mass-resolution electrospray ionization mass spectrometry, collision-induced dissociation, and negative ion photoelectron spectroscopy. Molecular-level insight was obtained into the bonding and structural fluxionality of these species through global optimization theory and ab initio molecular dynamics simulations. The dependence of irreversible adsorption of Fe(CN)₆^4-/3- anions onto electrodes functionalized with size-selected IL clusters was established by cyclic voltammetry, along with electrochemical impedance spectroscopy measurements. The overall findings demonstrate that IL clusters with different sizes and stoichiometries increase the efficiency of electrochemical separations, opening new horizons in selective electrode functionalization.

PNNL Contact

Vassiliki-Alexandra (Vanda) Glezakou, Pacific Northwest National Laboratory, vanda.glezakou@pnnl.gov

Grant Johnson, Pacific Northwest National Laboratory, grant.johnson@pnnl.gov

Venky Prabhakaran, Pacific Northwest National Laboratory, venky@pnnl.gov

Funding

This work was supported by the Department of Energy (DOE), Office of Science, Basic Energy Sciences program, Chemical Sciences, Geosciences, and Biosciences (Interfacial Structure and Dynamics in Separations). The negative-ion photoelectron spectroscopy work was supported by the DOE, Office of Science, Basic Energy Sciences program, Division of Chemical Sciences, Geosciences, and Biosciences (Chemical Kinetics and Dynamics at Interfaces). Part of this work was performed at EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility at PNNL. PNNL is a multiprogram national laboratory operated by Battelle for DOE. Computer resources were provided by the Research Computing Division at PNNL and the National Energy Research Scientific Computing Center, a DOE Office of Science user facility.