Time-dependent ion selectivity in capacitive charging of porous electrodes

In a combined experimental and theoretical study, we show that capacitive charging of porous electrodes in multicomponent electrolytes may lead to the phenomenon of time-dependent ion selectivity of the electrical double layers (EDLs) in the electrodes. This effect is found in experiments on capacitive deionization of water containing NaCl/CaCl(2) mixtures, when the concentration of Na(+) ions in the water is five times the Ca(2+)-ion concentration. In this experiment, after applying a voltage difference between two porous carbon electrodes, first the majority monovalent Na(+) cations are preferentially adsorbed in the EDLs, and later, they are gradually replaced by the minority, divalent Ca(2+) cations. In a process where this ion adsorption step is followed by washing the electrode with freshwater under open-circuit conditions, and subsequent release of the ions while the cell is short-circuited, a product stream is obtained which is significantly enriched in divalent ions. Repeating this process three times by taking the product concentrations of one run as the feed concentrations for the next, a final increase in the Ca(2+)/Na(+)-ratio of a factor of 300 is achieved. The phenomenon of time-dependent ion selectivity of EDLs cannot be explained by linear response theory. Therefore, a nonlinear time-dependent analysis of capacitive charging is performed for both porous and flat electrodes. Both models attribute time-dependent ion selectivity to the interplay between the transport resistance for the ions in the aqueous solution outside the EDL, and the voltage-dependent ion adsorption capacity of the EDLs. Exact analytical expressions are presented for the excess ion adsorption in planar EDLs (Gouy-Chapman theory) for mixtures containing both monovalent and divalent cations.     

The importance of ion size and electrode curvature on electrical double layers in ionic liquids

Room-temperature ionic liquids (ILs) are an emerging class of electrolytes for supercapacitors. We investigate the effects of ion size and electrode curvature on the electrical double layers (EDLs) in two ILs 1-butyl-3-methylimidazolium chloride [BMIM][Cl] and 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF(6)], using a combination of molecular dynamics (MD) and quantum density functional theory (DFT) simulations. The sizes of the counter-ion and co-ion affect the ion distribution and orientational structure of EDLs. The EDL capacitances near both planar and cylindrical electrodes were found to follow the order: [BMIM][Cl] (near the positive electrode) > [BMIM][PF(6)] (near the positive electrode) ≈ [BMIM][Cl] (near the negative electrode) ≈ [BMIM][PF(6)] (near the negative electrode). The EDL capacitance was also found to increase as the electrode curvature increases. These capacitance data can be fit to the Helmholtz model and the recently proposed exohedral electrical double-cylinder capacitor (xEDCC) model when the EDL thickness is properly parameterized, even though key features of the EDLs in ILs are not accounted for in these models. To remedy the shortcomings of existing models, we propose a "Multiple Ion Layers with Overscreening" (MILO) model for the EDLs in ILs that takes into account two critical features of such EDLs, i.e., alternating layering of counter-ions and co-ions and charge overscreening. The capacitance computed from the MILO model agrees well with the MD prediction. Although some input parameters of the MILO model must be obtained from MD simulations, the MILO model may provide a new framework for understanding many important aspects of EDLs in ILs (e.g., the variation of EDL capacitance with the electrode potential) that are difficult to interpret using classical EDL models and experiments.     

A "counter-charge layer in generalized solvents" framework for electrical double layers in neat and hybrid ionic liquid electrolytes

Room-temperature ionic liquids (RTILs) have received significant attention as electrolytes due to a number of attractive properties such as their wide electrochemical windows. Since electrical double layers (EDLs) are the cornerstone for the applications of RTILs in electrochemical systems such as supercapacitors, it is important to develop an understanding of the structure-capacitance relationships for the EDLs of these systems. Here we present a theoretical framework termed "counter-charge layer in generalized solvents" (CGS) for describing the structure and capacitance of the EDLs in neat RTILs and in RTILs mixed with different mass fractions of organic solvents. Within this framework, an EDL is made up of a counter-charge layer exactly balancing the electrode charge, and of polarized generalized solvents (in the form of layers of ion pairs, each of which has a zero net charge but has a dipole moment--the ion pairs thus can be considered as a generalized solvent) consisting of all RTILs inside the system except the counter-ions in the counter-charge layer, together with solvent molecules if present. Several key features of the EDLs that originate from the strong ion-ion correlation in RTILs, e.g., overscreening of electrode charge and alternating layering of counter-ions and co-ions, are explicitly incorporated into this framework. We show that the dielectric screening in EDLs is governed predominantly by the polarization of generalized solvents (or ion pairs) in the EDL, and the capacitance of an EDL can be related to its microstructure with few a priori assumptions or simplifications. We use this framework to understand two interesting phenomena observed in molecular dynamics simulations of EDLs in a neat IL of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF(4)]) and in a mixture of [BMIM][BF(4)] and acetonitrile (ACN): (1) the capacitance of the EDLs in the [BMIM][BF(4)]/ACN mixture increases only slightly when the mass fraction of ACN in the mixture increases from zero to 50% although the dielectric constant of bulk ACN is more than two times higher than that of neat [BMIM][BF(4)]; (2) the capacitance of EDLs near negative electrodes (with BMIM(+) ion as the counter-ion) is smaller than that near positive electrodes (with BF(4)(-) as the counter-ion) although the closest approaches of both ions to the electrode surface are nearly identical.     

Predicting ion selectivity in water purification by capacitive deionization: Electric double layer models

Ion-selective water treatment is needed to address emerging problems in an energy- and cost-efficient manner. Capacitive deionization (CDI) is a membraneless water treatment technology, which relies on storing ions in charged electric double layers (EDLs) of micropores. CDI has shown remarkable selectivity, with local density approximations (LDAs) showing some success in guiding selective separations. However, many underlying processes are represented by lumped fitting parameters in LDA models, hindering further progress. Atomistic models help unravel selectivity mechanisms, but are difficult to integrate with cell-level CDI theory. Here, we review and extend LDA models for CDI, highlight a knowledge gap in connecting between LDA and atomistic models for CDI, and emphasize and build upon analogies between micropore EDLs and nanofiltration membranes.     

石墨烯基电极材料的设计和构建及其在电容去离子中的应用

电容去离子(CDI)是一种通过静电力作用将离子从水中去除的技术,电极是整个装置中为最为核心的部件,石墨烯因具有优异的导电性和巨大的比表面积等优势成为当前CDI电极材料的研究热点之一。目前对于CDI石墨烯电极的研究主要集中于石墨烯电极的合成,然而有关CDI性能与石墨烯电极制作工艺及电极材料自身结构之间的关系,缺少相关综述。本文系统介绍了CDI的基本原理与性能指标,综述了石墨烯电极材料的研究进展与电极制作工艺,重点分析、归纳和总结了石墨烯材料的特性(孔隙结构、导电性、亲疏水性)对CDI性能的影响,最后对CDI中石墨烯电极材料今后的发展进行了总结和展望。     

Range of Validity of a Simplified Model for Diffuse Charge Dynamics

The modelling of electrochemical processes often requires the solution of the Poisson‐Nernst‐Planck (PNP) equations. In complex geometries, such as porous electrodes, that is challenging due to the presence of disparate length scales, ranging from the Debye screening length (～nm) to the device length scale (～cm). To overcome this difficulty, one often assumes that the electric double layer (EDL) is at quasi‐equilibrium to construct a simplified model that accounts for ion diffusion in the electro‐neutral bulk of the electrolyte while replacing the EDLs with appropriate boundary conditions. Various researchers have demonstrated that such an approach is valid in the asymptotic limit of a thin EDL and moderate electrode potentials. In this note, we explore the range of validity of this approximation by considering a one‐dimensional electrolytic cell with blocking electrodes subjected to a step change and time‐periodic alternations in the electrodes’ potentials by calculating the errors associated with the approximate approach as functions of the EDL thickness and electric field frequency and intensity. Additionally, we delineate numerical instabilities associated with the numerical solutions of the bulk equations with the nonlinear boundary condition peculiar to this problem.     

Modeling electrified metal/water interfaces from ab initio molecular dynamics: Structure and Helmholtz capacitance

The structure and potential distribution of electric double layers (EDLs) are of close relevance to the performance of electrode materials. In the past years, despite tremendous efforts devoted to this topic, an atomistic picture of the EDL is still lacking, let alone understanding on how the EDL structure is related to the dielectric property of interface water. In this article, we briefly review the recent progress in modeling electrified metal/water interfaces using ab initio molecular dynamics (AIMD). The ab initio methods for EDL modeling is firstly summarized, and then we discuss the structures of interface water on metal electrodes at different potential conditions. Moreover, we illustrate the potential-dependent behavior of chemisorbed water on Pt(111) surface and its relationship with the peak of the differential Helmholtz capacitance observed by experiment. At last, we give some perspective for future development in ab initio modeling of electrochemical interfaces.     

Single module flow-electrode capacitive deionization for continuous water desalination

The search for novel desalination technologies has recently led to the introduction of flow-electrodes to capacitive deionization (CDI) processes, named as flow-electrode capacitive deionization (FCDI). Unlike classical CDI, which is a discontinuous or semi-continuous process due to the need for regeneration of the electrodes within the same module, flow-electrodes offer new design opportunities which enable fully continuous desalination processes as well as easily scalable systems. Here, we describe a novel system for the continuous desalination of water based on FCDI using a single flow-electrode and a single module. The flow-electrode is based on activated carbon powder suspended in water. During continuous operation of the system, a desalination rate of a 1 g/L NaCl solution of up to 70% is achieved at water recoveries of up to 80%. Additionally we report very good current efficiencies: in case of 80% water recovery, the current efficiency is 0.93. The single flow-electrode single module process might reduce energy and investment costs and lower the threshold to a large scale implementation.     

Capacitive deionization concept based on suspension electrodes without ion exchange membranes

A new type of capacitive deionization (CDI) system, based on capacitive suspension electrodes (CSEs), was developed for the purpose of desalting brackish and seawater through the use of flowable carbon suspensions. CSEs derived from activated carbon and acetylene black demonstrated a specific capacitance of 92 F g^− 1 in a static mode in a 0.6 M NaCl solution. The novel system introduced here is a proof of concept that capacitive suspension electrodes can be envisioned to desalt water without the aid of ion exchange membranes (IEMs).     

Electrochemistry at nanoporous interfaces: new opportunity for electrocatalysis

Physical and electrochemical features of nanoporous electrodes arising from their morphology are presented in this perspective. Although nanoporous electrodes have been used to enhance electrocatalysis for several decades, the origin of their capability was understood on the basis of enlarged surface area or crystalline facet. However, considerable attention should be paid to the fact that nano-confined space of nanoporous electrodes can significantly affect electrochemical efficiency. Molecular dynamics in nano-confined spaces is capable of offering much more chances of interaction between a redox molecule and an electrode surface. The mass transport in the nanoporous electrode depends on various pore characteristics such as size, shape, charge, connectivity, and symmetry as well as molecular properties such as size, charge, and kinetics. Moreover, when the pore size is comparable to the thickness of an electric double layer (EDL), the EDLs overlap in the porous structure so that electrochemically effective surface area is not the same as that of the real electrode surface. These unique properties come from simply nanoporous structure and suggest new opportunity to innovative electrocatalysts in the future.     

Electrode Materials for Desalination of Water via Capacitive Deionization

Recent years have seen the emergence of capacitive deionization (CDI) as a promising desalination technique for converting sea and wastewater into potable water, due to its energy efficiency and eco-friendly nature. However, its low salt removal capacity and parasitic reactions have limited its effectiveness. As a result, the development of porous carbon nanomaterials as electrode materials have been explored, while taking into account of material characteristics such as morphology, wettability, high conductivity, chemical robustness, cyclic stability, specific surface area, and ease of production. To tackle the parasitic reaction issue, membrane capacitive deionization (mCDI) was proposed which utilizes ion-exchange membranes coupled to the electrode. Fabrication techniques along with the experimental parameters used to evaluate the desalination performance of different materials are discussed in this review to provide an overview of improvements made for CDI and mCDI desalination purposes     

电容去离子除氯电极的构建及其脱盐性能研究进展

电容去离子技术(Capacitive deionization,CDI)是一种新兴的脱盐技术,通过在电极两端施加较低的外加电场除去水中的带电离子和分子,由于其较低的能耗和可持续性而备受关注。基于储能电池领域近年来的迅猛发展,CDI电极材料实现了从以双电层作用机理为代表的碳材料到法拉第电极材料的跨越,使得脱盐性能有了大幅度提升。Na⁺的去除与Cl^-的去除同等重要,然而,CDI中针对氯离子高效去除的电极材料研究关注较少。本文从CDI装置的构型演变发展出发,系统地归纳与梳理了CDI中关于脱氯电极材料的分类,对比了不同类型脱氯电极材料的特点,并总结了Cl^-去除的机理,分别为基于双电层的电吸附、转化反应、离子插层和氧化还原反应。本文是首篇关于CDI阳极材料的进展综述和展望,为CDI除氯电极的后续研究提供理论基础和研究思路。     

Generalized model for time periodic electroosmotic flows with overlapping electrical double layers

In this article, an analytical model is devised for analyzing time periodic electroosmotic flows through nanochannels within the continuum regime, without presuming the validity of the Boltzmann distribution of ionic charges. The charge density distributions are obtained from the conservation considerations of the individual ionic species and other thermochemical constraints and are subsequently utilized to derive the potential distribution within the electrical double layer (EDL). This, coupled with the Navier-Stokes equation, yields a closed-form expression of the time-dependent velocity field that is valid under overlapped EDL conditions. This expression is first validated in asymptotic limits of thin EDLs, for which closed form expressions have been benchmarked in the literature. Further analyses are carried out to bring out the influences of the frequency of the electrical field on the electroosmotic flow features in the presence of overlapped EDLs.     

The Electrical Double Layer at the Bi–Ga Liquid Alloy in the Acetonitrile Electrolyte Solutions

The electrical double layer (EDL) characteristics of a Bi (0.25 at. %)–Ga liquid electrode in acetonitrile are studied. In acetonitrile, as in water, the electrode models Bi electrodes in their electrochemical properties. In contrast to aqueous solutions, in acetonitrile solutions it is possible to study the EDL structure not only at negative charges but also near the zero charge and at small positive charges. In acetonitrile, the electrode's potential of zero charge not distorted by specific ion adsorption and the corrected electrochemical work function are determined. The electrode is as lyophilic with respect to acetonitrile as Hg. Thus, the orientation of acetonitrile dipoles on Hg and Bi is identical at negative charges. The obtained data and data for a partially fused polycrystalline electrode in aqueous solutions are used to determine contributions of semimetal properties of a Bi electrode to the capacitance of the inner part of EDL and to a potential drop. The charge dependences of these contributions are found.     

Comparative study of the electrical double layer on liquid electrodes of mercury, gallium, and an In-Ga alloy in hexamethylphosphortriamide

The structure of the electrical double layer (EDL) on sp metals is studied by exploring it on liquid renewable electrodes of mercury, gallium, and an indium-gallium alloy containing 16.4 at % In. The study is performed in a solvent with a high donor number (DN), specifically, in hexamethylphosphoramide (HMPA, DN = 38.8). A very strong chemisorption interaction between the metal and HMPA is fixed on the Ga and In-Ga electrodes. It is shown that the energy of the metal-HMPA chemisorption interaction increases in the series Hg < In-Ga < Ga. The pattern revealed by the study is exactly the opposite to that previously observed on these very electrodes. The strong chemisorption interaction between the metal and HMPA does not lead to an increase in the capacitance of the inner part of the EDL and is at the same time characterized by a very large chemisorption jump of the solvent potential. The data obtained in HMPA show that, for sp metals in contact with a solvent whose DN is high enough, the effects of the metal-solvent chemisorption interaction may be commensurate with the effects previously observed on catalytically active metals. Such a result is an invincible proof, which requires no additional modeling notions, of the existence of a correlation between energies of the chemisorption interaction between a metal and a solvent and the solvent’s DN. This in turn is a convincing evidence that the specific interaction between a metal and a solvent has a donor-acceptor origin. The data obtained in HMPA make it possible to unite all the available results yielded by research into the EDL structure on the catalytically active and sp metals.     

Recent Advanced Applications of Ionic Liquid for Future Iontronics

Recently, electronic devices that make use of a state called the electric double layers (EDL) of ion have opened up a wide range of research opportunities, from novel physical phenomena in solid-state materials to next-generation low-power consumption devices. They are considered to be the future iontronics devices. EDLs behave as nanogap capacitors, resulting the high density of charge carriers is induced at semiconductor/electrolyte by applying only a few volts of the bias voltage. This enables the low-power operation of electronic devices as well as new functional devices. Furthermore, by controlling the motion of ions, ions can be used as semi-permanent charge to form electrets. In this article, we are going to introduce the recent advanced application of iontronics devices as well as energy harvesters making use of ion-based electrets, leading to the future iontronics research.     

The Electrical Double-Layer Interaction between a Spherical Particle and a Cylinder

Based on the well-known Debye-Hückel approximation and the Derjaguin's integration method, this paper presents an integral solution for the electrical double-layer (EDL) interaction between a spherical particle and a cylinder. The effects of the relative dimensions of the cylinder to the sphere on the EDL interaction are studied using this numerical solution. The detailed numerical results indicate that, in general, the curvature effect on the EDL interaction cannot be neglected at small separation distances. The widely used sphere-flat plate approximation will considerably overestimate the actual EDL interaction between a spherical particle and a cylinder. The ratio of the radius of the particle to the EDL thickness, tau=kappaa(p), also plays an important role in determining the EDL interaction at small dimensionless separation distances (相似文献   

电容去离子用功能化电极吸附材料的设计与构建

电容去离子(CDI)是近年来新兴的一种脱盐技术,由于其具有节能环保、实用性强等优势而倍受青睐。作为该技术核心的电极吸附材料,应具有高比表面积、良好导电性、亲水性、适宜孔隙结构、优异的稳定性等特点。这将有效保障该CDI器件不仅具有高CDI脱盐效率,而且拥有更强的循环稳定性。本文结合我们前期研究工作,针对吸附电极的制备、结构与性能构效关系的差异,综述了近年来多种功能化电极材料在CDI技术应用中的最新进展。     

电容去离子脱盐装置构型的研究进展

电容去离子(CDI)技术是一种新型的海水淡化技术,因其具有环境友好、操作简单和能耗低等优势而受到广大研究者的关注。在CDI技术中,电吸附的性能与装置的构型有着密切的联系。本文综述了目前常见的几种CDI装置,包括膜电容去离子(MCDI)、流动电极电容去离子(FCDI)、杂化电极电容去离子(HCDI)、反式电极电容去离子(i-CDI)以及脱盐电池(DB),对这几种装置的发展历程和装置构型进行介绍,最后,对CDI的装置构型在未来的研究发展方向进行了展望,以期为CDI装置在电脱盐领域的研究和应用提供参考。