Counterion Adsorption Research Articles

Schwinger-Dyson equations for composite electrolytes governed by mixed electrostatic couplings strengths.

The electrostatic Schwinger-Dyson equations are derived and solved for an electrolyte mixture composed of monovalent and multivalent ions confined to a negatively charged nanoslit. The closure of these equations is based on an asymmetric treatment of the ionic species with respect to their electrostatic coupling strength: the weakly coupled monovalent ions are treated within a gaussian approximation, while the multivalent counterions of high coupling strength are incorporated with a strong-coupling approach. The resulting self-consistent formalism includes explicitly the interactions of the multivalent counterions with the monovalent salt. In highly charged membranes characterized by a pronounced multivalent counterion adsorption, these interactions take over the salt-membrane charge coupling. As a result, the increment of the negative membrane charge brings further salt anions into the slit pore and excludes salt cations from the pore into the reservoir. The corresponding like-charge attraction and opposite-charge repulsion effect is amplified by the pore confinement but suppressed by salt addition into the reservoir. The effect is particularly pronounced in high dielectric membranes where the attractive polarization forces lead to a dense multivalent cation layer at the membrane walls. These cation layers act as an effective positive surface charge, resulting in a total monovalent cation exclusion and a strong anion excess even in the case of neutral membrane walls.

Ion selectivity by charged spherical cavities and investigation of intersection point for average cavity density versus electric potential (a DFT study)

Ion separation and ion selectivity inside spherical cavities are studied within the framework of the density functional theory. Results show that, at lower electric potentials and depending on cavity size, counter ions accumulate at the wall to form a shell around the co ions that prefer to aggregate in the middle of the cavity. This gives rise to the formation of separated phases of co ions and counter ions inside the spherical cavity. Depending on cavity size, these phases convert to a nearly single phase of counter ions with increasing electric potential. In this situation, small spherical cavities could be exploited as a counter ion selector tool. Our study also shows that counter ion selectivity readily occurs at lower, rather than higher, concentrations of electrolytes. For electrolytes with monovalent co ions, the (+1: −1) electrolytes exhibit a better selectivity than the (+1: −2) ones at low electric potentials. Also the selective counter ion adsorption occurs at lower electric potential when the difference between size of counter ions and co ions becomes larger. The observed selectivity is the result of confinement effect, electrostatic interaction, and excluded volume. Our study of the average cavity density versus electric potential shows an intersection for different plots related to cavities of different sizes. This intersection point reveals two different behaviors of average cavity density with cavity size on the two sides of the intersection point. In other words, average cavity density increases (decreases) with cavity size for those electric potentials which are lower (higher) than that of the intersection point but it becomes independent of cavity size for the electric potential of the intersection point.

Common Source of Cryoprotection and Osmoprotection by Osmolytes.

While recent studies clarify the effect of osmolytes on Coulomb interaction at elevated concentrations of salt, little is known about the way osmolytes affect the same interaction in cryoprotection. In this Communication we explore the effect of cold on the interaction between two charged surfaces immersed in ternary solution containing salt and osmolyte and find that the effect of cold parallels that of excess salt, i.e., low temperatures increase adsorption of salt counterions to the surface, thus neutralizing it. Two osmolytes, proline and glycine-betaine, are then shown to recharge the surface by releasing the adsorbed counterions. The ability to counteract effects of both cold and excess salt on Coulomb interactions renders these known osmolytes cryoprotectants as well as osmoprotectants, explaining why plants, fish, insects and bacteria accumulate them in response to either drought or cold stress.

Energetics of counterion adsorption in the electrical double layer.

The energetics of the electrical double layer (EDL) is studied in a systematic way to define how different components of the chemical potential help or hinder cation adsorption at a negatively charged wall. Specifically, the steric (i.e., excluded-volume interactions), mean electrostatic, and screening (i.e., electrostatic correlations beyond the mean field) components were computed using classical density functional theory of the primitive model of ions (i.e., ions as charged, hard spheres in a background dielectric). The reduced physics of the primitive model allows for an extensive analysis over a large parameter space: cation valences +1, +2, and +3, cation diameters 0.15, 0.30, 0.60, and 0.90 nm, bulk concentrations ranging from 1 µM to 1M, and surface charges ranging from 0 to -0.50 C/m2. Our results show that all components are necessary to understand the physics of the EDL. The screening component is always significant; for small monovalent cations such as K+, it is generally much larger than the steric component, and for multivalent ions, charge inversion cannot occur without it. At moderate surface charges, the screening component makes the electrostatic potential less negative than in classical Poisson-Boltzmann theory, sometimes even positive (charge inversion). At high surface charges, this is overcome by the repulsive potential of the steric component as the first ion layer becomes extremely crowded. Large negative electrostatic potentials counteract this to draw even more cations into the first layer. Although we used an approximate model of the EDL, the physics inherent in these trends appears to be quite general.

Solid-state NMR and electrochemical dilatometry study of charge storage in supercapacitor with redox ionic liquid electrolyte

Redox-active electrolyte supercapacitors have been proposed to increase the capacitance and energy density of double-layer electrochemical capacitors (EDLCs) by adding a faradaic contribution to the charging mechanism. While charge storage in EDLCs has been studied extensively, the implications of having a redox-active molecule participating in charge storage are still not entirely understood. Important questions about the ability of the redox center to penetrate in the carbon micropores, on the faradaic efficiency of these centers and on the self-discharge of redox-active electrolyte supercapacitors are still open. Solid-state NMR (SS-NMR) is a powerful technique to quantify ions inside the pores of activated carbon and has been used to study the charging mechanisms in EDLCs. In this work, we demonstrate for the first time its usefulness to investigate the charge storage in an activated carbon supercapacitor based on a redox-active electrolyte. The electroactive molecule is an ionic liquid in which a triflimide anion analogue is modified with ferrocene, allowing the use of 19F NMR for its quantification in the pore and chronocoulometry to determine the charge associated with the oxidation of ferrocene. We show that the charging process of such a system involves a voltage-dependant mechanism. Charge storage on the positive electrode occurs via co-ion desorption in the low voltage range and subsequently from a combination of the faradaic reaction and counter-ion adsorption at high voltage. At the negative electrode, charging occurs exclusively by counter-ion adsorption over the studied voltage range. In-situ electrochemical dilatometry measurements, done in complement to SS-NMR, further confirm the proposed mechanism. The approach described here provides a more detailed picture of the charging mechanism of redox-active electrolyte supercapacitors than electrochemical measurements alone.

Electrostatic potential and counterion partition between flat and spherical interfaces.

While the electrostatic potential and the counterion distribution produced by interfaces with idealized geometries can be well-described by analytical models, the same does not hold true for the interaction between surfaces with different and arbitrary geometries. Besides, the geometry of a charged interface may also affect the counterion adsorption, potentially modulating the electrostatic potential and the solvent organization close to the interfaces, demanding molecular details to be taken into account. The complex electrostatics of a sodium dodecyl sulfate micelle in the presence of monolayers of the same surfactant at the water-vapor interface was assessed by a set of molecular dynamics simulations. The electrostatic potential was evaluated numerically, and its total magnitude was decomposed into contributions arising from each species comprising the system. The counterion adsorption was stronger at the flat interfaces due to the more favorable formation of sodium bridges, where the same counterion is bounded to two or more anionic heads, while water reorientation was more pronounced near the micelle. These opposing effects counteracted each other so that the overall electrostatic potential changes were similar for both interfaces. The increase in the counterion concentration between the micelle and the interface originates a double layer mediated repulsion amounting to a free energy barrier of at least 14 kJ/mol, preventing the micelle to get closer to the monolayers. It is noteworthy that the hydrophobic regions had electrostatic potential contributions as large as those arising from the hydrophilic regions, mostly due to the orderly orientation of the terminal methyl groups.

Unexpectedly Large Decay Lengths of Double-Layer Forces in Solutions of Symmetric, Multivalent Electrolytes.

Double-layer forces acting between micron-sized silica particles are measured with the atomic force microscope in solutions of symmetric, multivalent electrolytes. In particular, the 2:2 electrolytes, CuSO4 and MgSO4, and the 3:3 electrolyte LaFe(CN)6 were investigated. For the multivalent electrolytes, the measured decay lengths are substantially larger than the ones expected on the basis of simple Debye-Hückel (DH) theory. These deviations can be explained quantitatively by the formation of neutral ion pairs. The measured surface charge density decreases in magnitude with increasing valence. Both effects are caused by ion-ion correlations, which are not included in the classical DH theory. However, this theory remains applicable, provided one considers the formation of ion pairs in solution and an effective surface charge density. This effective charge is substantially smaller in magnitude than the one of the bare surface. This reduction results from adsorption of counterions, which becomes stronger with increasing valence. These observations reveal that DH theory is applicable even in the presence of multivalent ions, provided the effective parameters are chosen appropriately.

Structural and electrical properties of an electric double layer formed inside a cylindrical pore investigated by Monte Carlo and classical density functional theory

We present the properties of an electrical double layer formed by ions inside a charged cylindrical pore studied by the grand canonical Monte Carlo simulation and classical density functional theory. The cylinder radius is 3000 pm. The wall is hard, perfectly smooth. The ions are modelled by hard spheres with a point electric charge at the centre. The hard sphere diameter is fixed at 400 pm. The monovalent ions are immersed in a continuous dielectric medium of the relative permittivity er. The temperature is 298.15 K and the electrolyte concentration takes the following values: 1.0, 2.5 and 4.0 M. The surface charge density varies in the range from − 1.0 to + 1.0 C/m2. The ion singlet distribution results show adsorption of counter-ions and desorption of co-ions from the cylindrical electrode. At high electrode charges the second layer of counter-ions is formed, while for high electrolyte concentration the co-ion distribution curve has a small maximum at some distance from the electrode surface. In comparison to the planar electrode, the concave one attracts stronger the counter-ions and repels the co-ions. At high electrolyte concentration, the profiles of the volume charge density have a positive hump, while those of the mean electrostatic potential have a negative minimum, which indicates the overscreening effect. For low electrolyte concentrations, the differential capacitance curve has a minimum at σ = 0 surrounded by two maxima. With increasing concentration, the minimum transforms into a maximum. The differential capacitance curves run above the curves for the planar electrode at small electrode charges and below them for high negative and positive charges. The very good agreement of all the grand canonical Monte Carlo to the classical density functional theory results presented in the paper indicates the reliability of the latter approach in cylindrical pore as well as planar geometry.

Finite electric displacement simulations of polar ionic solid-electrolyte interfaces: Application to NaCl(111)/aqueous NaCl solution.

Tasker type III polar terminations of ionic crystals carry a net surface charge as well as a dipole moment and are fundamentally unstable. In contact with electrolytes, such polar surfaces can be stabilized by adsorption of counterions from the solution to form electric double layers. In a previous work [T. Sayer et al., J. Chem. Phys 147, 104702 (2017)], we reported on a classical force field based molecular dynamics study of a prototype model system, namely, a NaCl(111) slab interfaced with an aqueous NaCl solution on both sides. A serious hurdle in the simulation is that the finite width of the slab admits an electric field in the solid perturbing the theoretical charge balance at the interface of semi-infinite systems [half the surface charge density for NaCl(111)]. It was demonstrated that the application of a finite macroscopic field E canceling the internal electric field can recover the correct charge compensation at the interface. In the present work, we expand this method by applying a conjugate electric displacement field D. The benefits of using D instead of E as the control variable are two fold: it does not only speed up the convergence of the polarization in the simulation but also leads to a succinct expression for the biasing displacement field involving only structural parameters which are known in advance. This makes it feasible to study the charge compensating phenomenon of this prototype system with density functional theory based molecular dynamics, as shown in this work.

Salt concentration and charging velocity determine ion charge storage mechanism in nanoporous supercapacitors

A fundamental understanding of ion charge storage in nanoporous electrodes is essential to improve the performance of supercapacitors or devices for capacitive desalination. Here, we employ in situ X-ray transmission measurements on activated carbon supercapacitors to study ion concentration changes during electrochemical operation. Whereas counter-ion adsorption was found to dominate at small electrolyte salt concentrations and slow cycling speed, ion replacement prevails for high molar concentrations and/or fast cycling. Chronoamperometry measurements reveal two distinct time regimes of ion concentration changes. In the first regime the supercapacitor is charged, and counter- and co-ion concentration changes align with ion replacement and partially co-ion expulsion. In the second regime, the electrode charge remains constant, but the total ion concentration increases. We conclude that the initial fast charge neutralization in nanoporous supercapacitor electrodes leads to a non-equilibrium ion configuration. The subsequent, charge-neutral equilibration slowly increases the total ion concentration towards counter-ion adsorption.

The role of competitive counterion adsorption on the electrolyte induced surface ordering in methyl ester sulfonate surfactants at the air-water interface

The strong binding of Al3+ trivalent counterions to the anionic surfactants sodium polyethylene glycol monoalkyl ether sulfate and α-methyl ester sulfonate results in surface multilayer formation at the air-water interface. In contrast the divalent and monovalent counterions Ca2+ and Na+ result only in monolayer adsorption. Competitive counterion adsorption has been extensively studied in the context of surfactant precipitation and re-dissolution, but remains an important feature in understanding this surface ordering and how it can be manipulated.The α-methyl ester sulfonate surfactants are a promising class of anionic surfactants which have much potential for improved performance in many applications, greater tolerance to extreme solvent conditions such as water hardness, biocompatibility and sustainable production. Hence in this study we have used neutron reflectivity to extend previous studies on the surface ordering of the α-methyl ester sulfonate surfactant, sodium tetradecanoic 2-sulfo 1-methyl ester, in the presence of electrolyte to investigate the role of binary mixtures of electrolytes, AlCl3/CaCl2, and AlCl3/MgCl2.In the mixed electrolytes the evolution of the surface structure, from monolayer to multilayer with increasing AlCl3 concentration, is observed. It is broadly similar to that reported for the addition of only AlCl3. However with increasing CaCl2 concentration the structural evolution is shifted progressively to higher AlCl3 concentrations. Similar observations occur for the AlCl3/MgCl2 mixtures. However the presence of the MgCl2 results in an additional phenomenon; the partial co-adsorption of a more compact lamellar structure which exists until the highest AlCl3 concentrations. The results demonstrate the importance of the competitive adsorption of different counterions in driving and controlling the formation of surface multilayer structures with anionic surfactants. Furthermore it offers a facile route to the manipulation of these surface structures.

Modeling selective ion adsorption into cylindrical nanopores

Ion selectivity by cylindrical pores has been investigated using the primitive model of electrolytes and the classical density functional theory. It is found that a neutral pore always exhibits the preferential adsorption of large ions, regardless of the pore size and the bulk electrolyte composition. For a charged nanopore, however, the pore selectivity depends not only on the ion size but also on the pore radius as well as the surface electrical potential. While a nanopore prefers adsorption of small counterions when its radius is sufficiently large, it can be large-ion selective when its size becomes comparable to the ion diameters.

Salt-Induced Colloidal Destabilization, Separation, Drying, and Redispersion in Aqueous Phase of Cationic and Anionic Nanochitins.

This study is aimed at facilitating the use of ocean biomass for the isolation and use of derived nanostructures. Specifically, cationic and anionic nanochitins were produced from never-dried crab shells that underwent partial deacetylation (PD-NCh) or TEMPO-oxidization (TO-NCh). The effects of different electrolyte types (NaCl, CH3COONa, Na2CO3, CaCl2, AlCl3, and NH4Cl) were investigated with regards to fractionation (via colloidal destabilization and precipitation), drying, and ultimate redispersion of the nanochitins. Sodium carbonate was most effective in the case of PD-NCh processing, whereas no significant effect of salt type was noted for TO-NCh. The results are rationalized in terms of the dispersion stability that resulted from specific counterion adsorption and nanoparticle association as well as electrostatic-charge development at a given solution pH. These effects were used to limit hydrogen bonding and nonspecific interactions upon drying of the nanochitins. The weak interactions between nanochitin and monovalent Na+ and NH4+ explain the experimental observations. Aqueous dispersions reconstituted from dried PD-NCh and TO-NCh were colloidally stable and yielded highly viscous, gel-like nanochitin dispersions at mass concentrations as low as 1.5 and 3.0%, respectively. Our findings are expected to greatly facilitate green processing of nanochitin, an emerging type of biobased nanomaterial.

Molecular weight dependence of chain conformation of strong polyelectrolytes.

Using sodium polystyrene sulfonate (NaPSS) and quarternized poly 4-vinylpyridine (QP4VP) as model systems, the chain conformation of polyelectrolytes under finite salt concentrations is investigated at a single molecular level. By fluorescence correlation spectroscopy (FCS), the hydrodynamic radius (R h) of the samples with the molecular weight ranging more than one order of magnitude was measured. The variations of R h as a function of molecular weight reveal the molecular weight dependence: under moderate salt concentrations (such as 10-4 and 0.1M), the shorter chains of both NaPSS and QP4VP take the rod-like conformation, while the longer chains take the coiled conformation (random coil or swelled random coil conformation, respectively). At high enough salt levels, both the charged chains take the coiled conformations. Photon counting histogram (PCH) measurements of the local pH value at the vicinity of the NaPSS chain expose the higher extent of counterion adsorption for longer chains as well as higher salt concentrations, telling that the charge regularization process is the major governing factor.

Assessing Performance Goals and Operational Limitations for across Capacitive Deionization Technologies

Capacitive deionization (CDI) is a class of electrochemical desalination technologies which desalinate via ion storage in electric double-layers. CDI has received renewed attention in recent years due to the ability to couple energy storage with salt separation. During galvanostatic operation, a current is applied between porous carbon electrodes until a limiting voltage is reach. The cell is then discharged by applying a reverse current, generating brine and recovering stored charge. However, CDI desalination and energy efficiency can be limited by parasitic side reactions, and selective adsorption of counter-ions (anions at the positive electrode, and cations at the negative electrode). Several material additions and electrode configurations have been proposed to overcome these limitations, with the most prominent being the addition of ion exchange membranes (IEMs) promote counter ion flux out of the desalination chamber and incorporation of carbon slurry electrodes to increase system adsorption capacity. Likewise, functionalization of carbon electrodes has been studied to improve counter-ion adsorption within EDL micropores. While the incorporation of functionalized carbon, IEMs in membrane capacitive deionization (MCDI), and the use of slurry electrodes in flow capacitive deionization (FCDI) have successfully reduced energy consumption or increased ion adsorption capacity in CDI systems, these modifications are often evaluated under limited conditions on the basis of specific performance enhancements. Additional clarity is necessary to evaluate the associated performance and cost tradeoffs across the design space. In this study, an equivalent circuit (M)CDI model, with porous electrode sub-models, was used to measure the sensitivity of CDI performance to material selection, design, and operating choices. In order to investigate the performance of FCDI, pulse-flowed electrodes of high capacity and electronic resistances were incorporated into the existing model. Similarly, fixed charge in the anodic and cathodic micropores was studied to investigate functionalized carbon. Constrained system parameters were randomly selected via latin hypercube sampling (LHS) across multiple electrode geometries and influent salt concentrations. The resultant model outputs were then correlated with input values to quantify parameter sensitivity. Our sensitivity analysis shows that operating current density, electrode specific capacitance, and contact resistance were the parameters which most significantly dictated (M)CDI performance. These parameters where then used to construct an operational space for CDI, MCDI, and FCDI. The results of our operational space were then used to develop a simplified, operational model for both capacitive and faradic materials in CDI. Using this model we conducted a techno-economic analysis (TEA) of proposed improvement to CDI. From the TEA we are able more directly set operating parameters under which CDI might favorably compete with the primary technology for desalination, reverse osmosis (RO). We are able to evaluate materials lifetimes and costs necessary to economically operate CDI. Lastly, goals for operating parameters highlighted in the sensitivity analysis (specific capacitance, voltage limits, charge efficiency, and cell resistance) were set. These benchmarks will provide target for the continued development of CDI towards and economic and viable alternative to RO for brackish water desalination.

Assessing Performance Goals and Operational Limitations across Capacitive Deionization Technologies

Capacitive deionization (CDI) is a class of electrochemical desalination technologies which desalinate via ion storage in electric double-layers. CDI has received renewed attention in recent years due to the ability to couple energy storage with salt separation. During galvanostatic operation, a current is applied between porous carbon electrodes until a limiting voltage is reach. The cell is then discharged by applying a reverse current, generating brine and recovering stored charge. However, CDI desalination and energy efficiency can be limited by parasitic side reactions, and selective adsorption of counter-ions (anions at the positive electrode, and cations at the negative electrode). Several material additions and electrode configurations have been proposed to overcome these limitations, with the most prominent being the addition of ion exchange membranes (IEMs) promote counter ion flux out of the desalination chamber and incorporation of carbon slurry electrodes to increase system adsorption capacity. Likewise, functionalization of carbon electrodes has been studied to improve counter-ion adsorption within EDL micropores. While the incorporation of functionalized carbon, IEMs in membrane capacitive deionization (MCDI), and the use of slurry electrodes in flow capacitive deionization (FCDI) have successfully reduced energy consumption or increased ion adsorption capacity in CDI systems, these modifications are often evaluated under limited conditions on the basis of specific performance enhancements. Additional clarity is necessary to evaluate the associated performance and cost tradeoffs across the design space. In this study, an equivalent circuit (M)CDI model, with porous electrode sub-models, was used to measure the sensitivity of CDI performance to material selection, design, and operating choices. In order to investigate the performance of FCDI, pulse-flowed electrodes of high capacity and electronic resistances were incorporated into the existing model. Similarly, fixed charge in the anodic and cathodic micropores was studied to investigate functionalized carbon. Constrained system parameters were randomly selected via latin hypercube sampling (LHS) across multiple electrode geometries and influent salt concentrations. The resultant model outputs were then correlated with input values to quantify parameter sensitivity. Our sensitivity analysis shows that operating current density, electrode specific capacitance, contact resistance, and fixed charge density were the parameters which most significantly dictated (M)CDI performance. These parameters where then used to construct an operational space for CDI, MCDI, and FCDI. The results of the sensitivity analysis and technology comparison were used to evaluate the current effectiveness of CDI technologies as well as near-term projections for improvements in material performance. Through this analysis we are able to set targets for performance and material properties necessary for each CDI configuration to favorably operate.

Counterion binding alters surfactant self-assembly in deep eutectic solvents.

Micellisation of surfactants in deep eutectic solvents has been recently demonstrated to provide a controllable way to modify micelle morphology. Ion-pair interactions between the solvent and the surfactant headgroup were identified as affecting the micellisation by modifying the charge density of the micelle. Here we explore the micellisation of dodecylsulfate surfactants with different counterions (Li+, Cs+, Mg2+, Bmim+, Emim+, cholinium+) dissolved in two deep eutectic solvents: choline chloride:urea and choline chloride:glycerol. Surface tension results show a solvent and counterion dependence of the CMC of the surfactants. Small-angle neutron scattering was subsequently used to investigate the morphology of the micelles formed. The results show that the elongation of the micelles is strongly dependent on the solvent, showing more elongated aggregates in choline chloride:urea than in choline chloride:glycerol. The morphology of micelles in DES was also found to depend on the counterion, where the affinity of binding showed similarities to that in water.

SFG Study of the Potential-Dependent Adsorption of the p-Toluenesulfonate Anion at an Activated Carbon/Propylene Carbonate Interface

Sum frequency generation spectroscopy has been used to characterize the potential-dependent adsorption of the p-toluenesulfonate anion at the activated carbon/propylene carbonate interface of the commercial carbon YP50F. Spectra recorded from the interface between YP50F and a 1 M tetraethylammonium p-toluenesulfonate in propylene carbonate solution showed no ordered anion adsorption without an applied potential. In contrast, there is clear evidence of increasingly ordered anion adsorption with applied potential. Furthermore, there is evidence of hysteresis such that the anion remains adsorbed when the applied potential was decreased back to 0 V. Significant reversal of polarity was required before the anion signal was lost. Changes to the propylene carbonate solvent peaks during the electrochemical cycle were also observed. The data indicate that the positive electrode charges either via a counterion adsorption mechanism or via an ion-exchange mechanism.

(Invited) Capacitive Deionization of High-Salinity Water Using Ion-Exchange Membranes

It is known that capacitive deionization (CDI) is not suitable for direct desalination of seawater because of its high salinity (~ 35 g L−1) and the limit in applied potential to avoid water electrolysis and oxidation of chloride ions. It has recently been reported, however, that the potential window can be expanded with the formation of an electrode-electrolyte interphase when using highly concentrated aqueous electrolyte. Studies also have proven that increasing ion concentration can greatly enhance the ion capacity of the electrodes. In addition, ion-exchange membranes (IEMs) can be used to control the electrode performance and allow application of reverse potential for electrodesorption. Here, we present the possibility to directly desalt high-salinity water and seawater through membrane CDI (MCDI). An MCDI cell was symmetrically assembled with anion and cation exchange membranes attached respectively to the anode and cathode. Our results show that the MCDI cell equipped with mesoporous carbon can remove 66.7 mg sodium chloride per gram of carbon, while the CDI cell alone (without membranes) could only remove 8.5 mg g−1 under the same conditions. Similarly, the MCDI cell equipped with carbon aerogel demonstrated a salt removal capacity of 37.3 mg g−1 in comparison with 12.9 mg g−1 of the CDI cell. These results indicate that the MCDI cell, assisted with IEMs and overpotential, can directly desalt high-salinity water regardless of the electrode material. Moreover, IEMs can prevent coion desorption at the beginning of the adsorption stage, as well as counterion adsorption at the beginning of the desorption stage, which can increase energy efficiency. For high-salinity water desalination, electrode regeneration is challenging. By applying a reversed low potential and controlling the desorption time, electrode regeneration can become effective. The presentation will include a review of CDI approaches, motivation for overpotential CDI supported by neutron imaging of electrodes and basic electrochemical experiments, characterization of electrode materials, a discussion of overpotential CDI and MCDI results including energy requirements, and a discussion of remaining challenges for direct desalting of high-salinity water via MCDI.

Quantitative Information about Electrosorption of Ionic Liquids in Carbon Nanopores from Electrochemical Dilatometry and Quartz Crystal Microbalance Measurements

Electrochemical energy storage using nanoporous carbons and ionic liquids enables large cell voltages and is a promising way to increase the energy density of electrical double-layer capacitors. The structure of the double layer in solvent-free electrolytes is fundamentally different from other systems with organic or aqueous solvents. In our study, we investigate the physical behavior of nanoporous carbon electrodes in contact with ionic liquids with a multilength scale approach by combining electrochemical quartz-crystal microbalance and electrochemical dilatometry. Synergistic combination of both in situ methods allows one to correlate system properties on particle and electrode level. We find that the charging mechanism at low charge is characterized by the exchange of more smaller ions by fewer larger ions. At higher charges, the system is changing to preferred counterion adsorption, which is resulting in a strong increase in the electrode volume. The maximum linear strain for a bulk electrode is 2% ...