Redox-electrodes for selective electrochemical separations

    Redox-electrodes for selective electrochemical separations Xiao Su, T. Alan Hatton PII: DOI: Reference:

S0001-8686(16)30253-6 doi: 10.1016/j.cis.2016.09.001 CIS 1690

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Advances in Colloid and Interface Science

Received date: Revised date: Accepted date:

18 December 2015 1 September 2016 5 September 2016

Please cite this article as: Su Xiao, Hatton T. Alan, Redox-electrodes for selective electrochemical separations, Advances in Colloid and Interface Science (2016), doi: 10.1016/j.cis.2016.09.001

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ACCEPTED MANUSCRIPT Redox-electrodes for selective electrochemical separations

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Xiao Su and T. Alan Hatton* Department of Chemical Engineering Massachusetts Institute of Technology, MA

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Redox-active materials hold great promise as platforms for selective liquid-phase separations. In contrast to capacitive electrodes that rely purely on double-layer charge for deionization, redox-modified electrodes can be used to control Faradaic reactions at the interface to selectively bind various charged and uncharged molecules, thus modulating surface interactions through electrochemical potential solely. These electrodes can be composed of a range of functional materials, from organic and organometallic polymers to inorganic crystalline compounds, each relying on its own distinct ion-exchange process. Often, redox electrochemical systems can serve as pseudocapacitors or batteries, thus offering an advantageous combination of adsorption selectivity and energy storage/recovery. This review summarizes redox-interfaces for electrosorption and release, outlines methods for preparation and synthesis, discusses the diverse mechanisms for interaction, and gives a perspective on the future of redox-mediated separations.

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Keywords: Electrochemical separation, redox electrodes, ion selectivity, pseudocapacitor, electrosorption , capacitive deionization, ion-exchange

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Graphical abstract

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ACCEPTED MANUSCRIPT Outline Introduction ..................................................................................................................................... 4 Faradaic Processes for Deionization....................................................................................... 5 Faradaic Processes for Electrosorption ................................................................................. 8 3.1. Cations............................................................................................................................................ 9 3.3. Anions............................................................................................................................................. 2 3.4. Specific Chemical Interactions ........................................................................................... 18 4. Synthesis and preparation of redox-electrodes .................................................................. 21 4.1. Redox-active Organic Conducting Polymers ................................................................ 21 4.2. Redox-active Organometallic Surfaces ........................................................................... 22 4.3. Composites of conductive materials and redox-active species ............................ 23 4.4. Inorganic Conductive Electrodes: Metal-oxides and crystalline clusters ......... 24 5. Perspective: chemical specificity and energy recovery ................................................... 25 5.1. Challenges in ion-specific separation: cost, selectivity and diverse applications ................................................................................................................................................................ 25 5.2. Energy integration: battery and pseudocapacitive separation systems ........... 26 6. Conclusions........................................................................................................................................ 26 References .............................................................................................................................................. 27

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ACCEPTED MANUSCRIPT 1. Introduction

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Faradaic processes are at the core of many electrochemical applications, from electrocatalysis and sensing to energy storage. An electrocatalyst transforms substrates into value-added products by promoting electron transfer through changes in its oxidation state. Electrochemically-responsive receptors rely on shifts in the oxidation or reduction potential to monitor chemical identity and concentration of target analytes [1]. Redox reactions accompanying ion intercalation govern charge/discharge potentials and rates in batteries [2-4]; and provide pseudocapacitance in addition to the usual double-layer capacitance for improved energy storage performance [5]. For all these applications, redox species at the interface of the solid electrode interact with ions in solution, be they supporting electrolytes or specific target analytes. Molecular recognition in particular has been the principal application in which the goal is to enhance selective binding of solutes [1, 6]. Target analytes for electrochemical redox-responsive receptors have ranged from ions to neutral species and even particles and biomacromolecules. The interaction of analytes in the liquid phase is coupled with voltage and current responses from the redoxcomponents, which can be either molecular species in homogeneous solution or redox-species immobilized on the electrode interface, thus allowing for identification and quantitation of the analytes in nanomolar concentrations [7]. Selective binding activated by redox reactions can go beyond sensing – heterogeneous redox-species can impart selectivity and act as an electrochemicallymodulated adsorbents for ion-extraction processes from the liquid phase, where ion capacity, selectivity and kinetics are all major design components for a successful electrochemical separation process. The large focus of recent electrochemical separations has been on deionization problems with carbon-based materials reliant on double-layer charge, in which ion-selectivity, though desirable, is neglected in favor of total ion capacity. Redox-modified electrodes, on the other hand, offer an opportunity to enhance selectivity for selective ion-separations and, in some cases, even increase ion uptake over high surface area carbon-based materials. In terms of the fundamental science of ion-doping, electroactive organic and organometallic films have been used for binding various cations and anions selectively and reversibly through ion-exchange since the early 80s. Many of these studies have focused on the basic electrochemistry of the conducting polymers, which have resulted in a greater understanding of ion transport and redox-binding mechanisms. Over the past decade, with developments in high-surface area electrodes for energy storage, there has been renewed interest in redox-materials for ion-separations in practical applications. Charged molecules have been the prime targets for these earlier redox systems due to their electrostatically-driven attraction, such as electrochemically switched ion-exchange systems (ESIX) developed at the Pacific Northwest National Lab [8, 9] to target cationic pollutants in waste streams [10-12]. More recently, battery and hybrid-type deionization processes have taken advantage of Faradaic-type electrodes to improve desalination efficiency [13]. The combination of surface polarization and redox activity enables

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binding with higher selectivity and faster kinetics than observed with traditional, solvent-based chromatographic methods. The inherent reversibility of the redox reaction allows for efficient recovery of adsorbed molecules without the need for extra solvent or change in the liquid-phase conditions [14]. In redox separation systems, selection of the appropriate materials chemistry for the electrodes is of paramount importance for successful electrochemical and separation performance. Fortunately, the explosive growth in the development of redox materials from various fields offers a large library from which to choose and improve, ranging from fully organic systems to pure metal electrodes. In particular, the discovery and implementation of innovative binding mechanisms is at the core of the development of next-generation redox-separation electrodes. In this case, the challenge of designing proper binding mechanisms for redox-mediated separations can benefit greatly from developments in molecular sensing and recognition. The targeting of specific, high-value or high-risk analytes, both organic and inorganic, in dilute concentrations is of crucial concern in the chemical industries and in environmental remediation. Some examples include organic synthesis products, which are highly desirable to target over buffers and background ions, or heavy metal cations such as mercury and arsenic from industrial waste. Chemical interactions based on donor-acceptor interactions or dispersion and hydrophobicity are but some of the alternatives which can be designed into a redox electrode and applied for electrochemical separations. In the current review, we summarize the application of redox-electrodes for electrochemical separations, both in deionization and selective electrosorption of ions. We discuss both the inorganic crystalline electrodes used for cation exchange as well as the range of polymeric organic and organometallic materials that have been used for ion-doping, and present some novel developments in the field that exploit chemical interactions for ion-selective extraction and release. We also review broadly the synthesis and preparation methods of redox-functionalized surfaces, and conclude by sharing perspectives for future materials and process design.

2. Faradaic Processes for Deionization Water purification has been a key area of interest for researchers in electrochemical separations, due to both its scientific challenges and societal importance. New technologies are needed in order to address limited resources in many parts of the world, with water resources constrained by geography and affected by anthropogenic pollution [15]. A range of competing desalination techniques exist, mainly membrane-based such as reverse-osmosis (RO). With the development of high surface area conductive materials, electrochemical processes such as capacitive deionization (CDI) have narrowed the gap with membrane processes in terms of costs and energetic requirements [16-18]. In CDI, porous conductive electrodes are charged by applied potential and thus accumulate significant quantities of ions close to the electrode interface [15]. The mechanism

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for charging relies on the formation of an electrical double layer, with the electrodes acting as plate capacitors (Figure 1a). CDI systems have the additional advantage of being able to store energy during the charging step, which can then be recovered during the discharge process. This serves to decrease the overall cost of the separation processes. Depending on the particular method, this energy recovery can range from 20% to 80% for membrane-capacitive deionization. One of the methods to increase performance is to utilize a cation/anion exchange membrane (Figure 1b), which can serve both to select desired anions if required, but most importantly, to increase electrochemical performance by preventing co-ions from leaving the electrode-structure [17, 19]. However, due to the electric field-based mechanism for charge accumulation in the double-layer, standard CDI technology does not provide an inherently high ionic selectivity. The presence of selective Faradaic reactions at the electrode similar to those in a battery material (Figure 1c) can provide ionic-selectivity directly through chemical means. Ionic intercalation into inorganic matrices, which is at the core of battery technologies, is one of the most studied redox-processes in recent decades [20-23]. Redox-electrodes from battery-type materials can be designed for separation of salts, with the added selectivity that accompanies the intercalation reactions, which is the conceptual basis for battery desalination[24] (Figure 2). Selectivity is desirable for a wide range of applications, such as chemical process purification from organic synthesis, or removal of particularly dangerous contaminants. The main difference between these Faradaic-based systems and standard CDI processes, as noted, is that instead of the charge being stored in the EDLC, the charge is stored through the chemical reactions that occur on intercalation of the ionic species within the electrodes [24, 25]. In one of the earliest examples of a battery-deionization system, the cathode was made of sodium-manganese oxide (Na2Mn5O10) nanorods, while the anode was made of silver[24]. The separation system works by operating the two-electrode system under sea-water to produce fresh water, and releasing the captured salts to a brine stream to regenerate the adsorbent (Figure 2). The cathode was found to be highly selective towards Na+ through preferential intercalation over other cations (Mg2+, Ca2+, K+), and the anode was equally selective towards chloride over sulfate due to the more favorable reduction potential for the formation of the AgCl composite. The Coulombic efficiency of battery deionization (0.29 WhL-1) was reported to be comparable to that observed in reverse osmosis, making it a highly competitive desalination system.

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Figure 1. (a) Conventional capacitive deionization. Reproduced with permission from [17], (b) Membrane capacitive deionization (MCDI/IEM) reproduced with permission from [17] and (c) hybrid capacitive deionization. Reproduced with permission from [26].

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An improvement on the original desalination system was proposed based on a hybrid anion-exchange/battery system[26] (Figure 1c). The cathode consisted of the battery material for selective cation insertion (Na4Mn9O18) whereas the anode was composed of conductive porous carbon with an anion exchange membrane at the interface to trap chloride in the double layer. This hybrid system was shown to have a higher capacity (31 mg/g) than a corresponding purely capacitive system. In the case of metal hexacyanoferrates, oxidation/reduction of the framework and simultaneous intercalation of various cations (both monovalent and multivalent) has been found to vary depending on valency, resulting in a number of insertion modes depending on the nature of the ion (Figure 3) [27]. The investigation of these mechanisms is expected to have potential applications in energy storage and desalination systems. Also, as will be discussed later, electrodes based on ferrocyanates (Prussian-blue type materials), have been shown to have remarkable cation selectivity [28-30].

Figure 2. Schematic (a) Schematic example showing a desalination battery cycledesalinated in which desalinated water Figure 2. (a) example showing a desalination battery cycle in which water is produced + and Cl- ions due to preferential - ionsNa is produced by adsorbing all ions.isSelectivity is achieved by adsorbing all ions. Selectivity achieved with Na+ and Clwith due to preferential intercalation. (b) SEM image (b) of Na used onnanorods the negative electrode. with permission from intercalation. SEM of Na2Mn used on theReproduced negative electrode. Reproduced 5O10 2Mnimage 5O10 nanorods with[24]. permission from [24].

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Figure 3. (a) Schematic example showing ion intercalation into open-framework Prussian Blue electrode, with both smaller vacancy sites as well as larger vacancy sites. (b) Galvanostatic cycling of trivalent ions showing retention of specific capacity at high charge/discharge rates, an important quality for energy storage and desalination materials. Reproduced with permission from [27].

Faradaic Processes for Electrosorption

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Selective ion sorption onto modified redox-electrodes was explored as early as the 1980s through ion-exchange voltammetry (IEV), in which differential voltammetric responses were obtained depending on the up-concentration of a particular ion inside a polymeric layer at the tip of a sensor [31, 32] [33]. The process of electrically controlled ion-exchange at the surface of an electrode has been formulated [32]: if P- is the ion-exchange center of the polymer or substrate, Cm+ is the desired cation to be exchanged and X+ is an inert supporting electrolyte which can easily be released, the binding reaction can be written as: Cm+ + m(P-X+) <-> P-mCm++ mX+

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The anionic counterpart is: An- + n(P+X-) <-> P+nCn++ nX- (2) Note that this formulation assumes a monovalent counter-ion to the initial film and a potentially multivalent (m+ or n-) coordinating ion. The selectivity of a certain species X+ over another, Y+, depends on a complex combination of factors, including their relative concentrations, chemical identities, and properties of the electroactive film, among others. Electrostatics, hydrophobicity, chemical interaction and solvation are some of the factors that

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contribute to ion-specificity. In this section, we discuss a variety of systems which take advantage of an electrochemically-switched ion-exchange process to bind various targets. We discuss cation-based redox separation systems based on the nature of the cations being targeted, which are usually heavy-metal pollutants but, with recent advances in biomedical research, could also be alkaline cations of physiological relevance. For anions, on the other hand, we discuss each case based on the nature of the electrode material and the redox-species, guided by the extensive studies of various classes of anion-exchange materials. In general, cation separation processes are closer to industrial implementation due to their importance in water treatment and environmental remediation, whereas anion separation still presents a broad range of fundamental problems that require elucidation, due to the similarities between the anions to be separated that make selective sorption a challenging process to address.

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Figure 4. (a) Redox-mediated cesium-selective system for capture of cesium at a nickel ferrocyanate inorganic Reproduced withcesium-selective permission from [43] (b) Redox-mediated selective Figure 4.electrode. (a) Redox-mediated system for capture copper of cesium at process a nickel with the LCTP (lowest critical transfer potential) and HCTP (highest critical transfer potential) illustrated. ferrocyanate inorganic electrode. Reproduced with permission from [43]. (b) Redox-mediated Reproduced with permission from [49].

copper selective process with the LCTP (lowest critical transfer potential) and HCTP (highest critical transfer potential) illustrated. Reproduced with permission from [49].

3.1. Cations Cations are one of the primary targets for ion-separation and sensing due to the health and environmental issues associated with heavy metal compounds as well as their biological importance as physiological salts and enzyme constituents [34, 35]. From an environmental standpoint, heavy metals are often cationic in nature, both mono- and multivalent, and their toxic effects at high concentration range from carcinogenic to acute neural and motor toxicity [36], and in more extreme cases, to radioactivity such as with cesium [37, 38] and uranium [39]. One of the first electrochemical ion-exchange systems developed targeted cesium[9], which is a nucleotide with a radioactive half-life of 30.17 years [8, 40]. The selective 9

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removal of cesium can dramatically reduce the radioactivity of aqueous media from nuclear processing and is crucial for sustainable nuclear power. Nickel hexaferrocyanate has been used as the redox-species to ion-exchange cesium (Figure 4)[41-43] through ionic intercalation similar to that in battery deionization devices. The ion-exchange properties have been found to be quite selective over even monovalent ions (Cs+>Rb+>K+>Na+>Li+) [43-45], with selectivity towards cesium over a wide range of Cs+:Na+ ratios, even with 2000 fold excess Na+ [41-43]. A two-electrode system configuration with an anion exchange membrane was used in which one electrode adsorbed Cs+ and the other one released it (Figure 4). The process was found to be highly selective and efficient for the removal of Cs+ at 5 mM over Na+ at 50 mM, with a separation factor of 123, in the presence of no supporting electrolyte. Depending on the material, stability of the electrodes can be quite high – nickel hexaferrocyanate nanotubes were found to retain more than 92% of their (c)

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Figure 5. Electrochemically controlled ion exchange selectivity of (a) Nickel and (b) Copper,

Figure 5. Electrochemically controlled exchange selectivity of (a)oxidation Nickel, of reproduced with reproduced with permission from [49] (c)ion Proton exchange by electrolytic copper. permission from [47] and Reproduced (b) Copper,with reproduced with permission from [49] and (c) Proton permission from [50]. exchange by electrolytic oxidation of copper. Reproduced with permission from [50].

ion-exchange capacity even after 500 cycles [41].

Nickel has been another target of electrochemically switched ion-exchange processes. In its various forms, both soluble and metallic, nickel can have severe toxic effects on biological organisms, including being carcinogenic [46]. An inorganic/organic composite of polyaniline and a layered alpha-zirconium phosphate nanosheet was used to selectively remove nickel (Figure 5a) [47]. The electrodes adsorbed the cations at -0.2 V (reference Ag/AgCl) and were regenerated at +0.8 V, with equilibrium uptakes of up to 100 mg/g. The system was found to be highly selective over other cations, including Na+, K+, Cd2+ and Pb2+, the selectivity 10

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being affected by the ion valency and size [47]. Similarly, recent work with the use of the polypyrrole-ferricyanide system [48] shows an ion selective electrochemically-switched system for nickel over other cations, including both multivalent and monovalent. Another heavy metal ion that has been removed through similar concepts is copper (Cu2+) [49], by poly (2,6-pyridinedicarboxylic acid) (PDDA), an electroactive proton exchange polymer, deposited on a quartz crystal electrode through electropolymerization through pulse deposition in 0.5 M KCl solution (Figure 5b). The PDDA film was oxidized at 1 V and reduced at 0.2 V in 0.1 M Cu(NO3)2 aqueous solution, as well as under a series of other conditions with interfering cations. The system was found to be highly selective for cupric ions over both monovalent and divalent competing cations, and operated through a simple potential swing between the LCPT (lowest critical transfer potential) and the HCTP (highest critical transfer potential) (see Figure 4b), in which the proton self-exchange processes took place to allow for the divalent metal to be incorporated into the film. Electrochemicallyswitched adsorption and desorption of copper was also accomplished with pyridine-based polymers such as poly-4-vinylpyridine (Figure 4c), in which the lone pair on the pyridine was used in its neutral state to chelate with Cu2+ and was reversibly ion-exchanged with protons in solution upon application of an oxidative potential of 1 V [50, 51]. The protonation occurs through the oxidative degradation of water, with a localized pH switch close to the protonated interface of the electrode. Cation exchange systems can also be created with polymers that would normally be considered as anion-exchange materials through the use of a polyampholytic copolymer (positive and negative units) in which only one of the units is electroactive [7, 52]. A sulfonate-ferrocene copolymer has been used to load various cations such as methylviologen [7] in its original state (neutral ferrocene), so that the bound cation neutralizes the sulfonate charge - then upon oxidation of the system the bound cation is electroexpelled (Figure 6). This mechanism of iongating through a redox switch was found to be widely applicable [52, 53]. Dopamine was able to be taken up during reduction of a poly(N-methylpyrrolydium)/ poly(styrenesulfonate) copolymer at -0.4 V, and subsequently released upon stepping the electrode to +0.5 V [54].

Figure 6. Ion-gating mechanism for controlled redox-mediated electrorelease. Reproduced with permission from [52].

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The same concept with organic conducting polymers has been implemented with pyrrole-based polymers, in which anionic dopants are incorporated and, can be “cation-exchanged” through application of a potential [55]. During conventional polymerization of pyrrole (as will be detailed later), the following mechanism of anion doping occurs to ensure charge compensation:

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However, if a large entrapped anion with selective properties, such as a large catechol or hydroquinone species, is present, the following reactions can occur:

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PPyn+nX- + n/2 M2+ + ne- -> [PPy nX- n/2 M2+] [reduction] PPy nX- n/2 M2+ -ne_ -> PPyn+nX- + n/2 M2+ [oxidation]

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The large entrapped species (X-, as opposed to the mobile A-) is initially coordinated with the film itself, but, upon reduction, allows for the binding and capture of a mobile cation from solution [55]. With catechol and quinones as the large entrapped anions, potential switching between +0.4 mV and -0.4 mV vs SCE allowed for incorporation and release of Ni(II), Pb(II), Cd(II), and Co(II) in the presence of a large excess of sodium ions. In all the above cases, cations were extracted through electrochemicallycontrolled binding with a redox-moiety immobilized on the electrode. Though we have presented some interesting systems here, electrochemically-switched separation for cations is a fast growing field, with many challenges remaining to be explored, both in materials development and in applications to real, practical cation systems. Homogeneous redox receptor systems for cation recognition are much more diversified and chemically complex than the limited applications seen so far with heterogeneous electrodes [56-59], and some of these systems have the potential to bridge the gap between molecular sensing and heterogeneous ionextraction, with possible applications in biological diagnostics, such as calciumselective redox ligands for up-concentration of physiologically important ions [60].

3.3. Anions Anion recognition, when compared to its cation counterpart, is a relatively recent field, and is, in some ways, more challenging due to greater similarities in charge and size between negatively charged ions and greater solvation effects that can take place [61]. There is an extensive literature on supramolecular recognition of anions, especially redox-active organic and organometallic receptors [61-63]. Single12

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molecule recognition shares much in common with ion-specific separation by electrodes, since the selectivity is often granted by molecular-level interactions. However, macroscopic effects of the structure of the surface can play an important role in determining mass-transport properties which are absent in homogeneous sensing. Electrosorption of anions onto immobilized redox-films has held great interest for electrochemical characterization, both on the design of electrochemically-responsive sensors and on the incorporation of anions within cationic conducting films [64]. Organic electroactive conducting polymers, in particular, have played a crucial role in the development of the field. Polypyrrole (PPY) is one of the most well-studied ion-exchange electro-active polymers for both the breadth of its applications as well as its ease of synthesis [65]. This polymer has been proposed both as a component of batteries [66] and as a component of pseudocapacitive systems in combination with electroactive electrolytes [67]. Its redox-recognition properties have been utilized for biological and chemical sensing [68-70]. The mechanism of sensing/anion-exchange relies on a starting bound-anion inherent present due to the synthesis procedures. Parallel redox-processes can occur depending on the potential. Under reduction, the anion can be exchanged and at the same time cations ingress and neutralize the charge (Figure 7) [64, 71]. For example, in the case of the polypyrrole-bromide conjugate (PPy+/Br-), under potentials greater than -0.2 V vs SCE, incorporation of cations was found to be favored, while at lower reduction potentials, anion ejection was preferred.[71] Through cycling (oxidation and reduction steps), selective incorporation of a certain anion with more affinity for the film could be achieved. The anion capacity was reported to be 7.1 x 10-4 mol/g and the exchange selectivity was dictated by the counter-ion incorporated in the synthesis: for a polypyrrole with Cl-, the anion selectivity is Br- > SCN- > SO42- > I- > CrO42- and with perchlorate, SCN- > Br- > I- > SO42- > CrO42- [64]. Anion structure plays an important role in ion-exchange in polypyrrole [72], with the ion association being stronger for smaller anions, which increase its charging ability. Anion selectivity can be attributed to the size of the anion and the interplanar distance for anion intercalation within the polymer [73]. A recent study on aromatic sulfonate-modified PPy films showed that small, singlycharged anions and even doubly-charged sulfate anions could easily replace the original doped sulfonate anions, due to their multivalency and high electrostatic attraction [74]. Polypyrrole films were also found to have high affinity for Fe(CN)63anions, with desorption of these dopants only possible at potentials lower than -0.4 V vs SCE, and with the dopants not being readily exchangeable with other anions tested; during the cation incorporation step, the films were found to be more selective towards Cs+ than Li+ [75].

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Figure 7. (a) Polypyrrole electrochemical redox reactions (adapted from reference[71] (b) Polypyrrole doping with various anions[74].

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Poly(aniline) (PANI) and its derivatives are also efficient anion-exchangers through the proton-exchange of the aniline group and the incorporation of negative counter-ions [76]. This polymer has many intrinsic redox states (Figure 8) [77], which have also been utilized to enhance pseudocapacitance in energy storage systems [78]. The speciation of anions, uptake capacity, and relation to charge transfer in poly(aniline) films were studied by Maranhao and Torresi [76], who found that during electropolymerization, the affinity of a dopant anion for the growing polyaniline film was dictated primarily by the size of the anion and its solvation. Smaller anions allow for solvent ingression and uptake by the film and greater electrolyte movement, resulting in greater electroactivity. Modified poly(aniline) electrodes have recently been described for selective removal of fluoride from aqueous solutions [79]. The modified electrodes were created by electropolymerization under acidic conditions with chloride as the counter-ion – on application of a positive potential, the chloride was found to be able to exchange fluoride selectively (Figure 9). At 1.5 V, the maximum fluoride uptake was found to be 20.49 mg/g [79]. Regeneration was achieved by reversing the potential and reducing the film in 0.1 M HCl solution to ion-exchange again with chloride. The system was found to preserve 89% of uptake efficiency after 5 cycles of oxidation and reduction. Composities of poly(aniline) with other conductive polymers such as poly(vinyl)sulfonate can dramatically modify its anion-exchange behavior [80]. With both sulfonate and aniline playing a role, a PANI/PVS composite was shown to undergo cation/proton exchange at pH>2, and below 2, anion exchange. What is particularly interesting is that the system becomes an efficient anion-exchanger in non-aqueous solution, with most of the sulfonic acid undissociated. Finally, a range of other organic conducting materials have presented fast ion self-exchange, such as polythiophenes and their derivatives [81, 82]. However, whether they are as competitive as pyrrole or aniline systems are in terms of anion-exchange capabilities still remains to be seen.

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Figure 8. Cyclic voltammogram (CV) showing the redox behavior of poly(aniline) in HCl and various oxidation/protonation states of the polymer film. Reproduced with permission from[77].

Figure 9. Defluoridation of an aqueous system using an immobilized polyaniline system for ionexchange with chlorine, reproduced with permission from [79].

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Organometallic redox-active polymers are perhaps the most interesting and versatile class of anion exchange species. The design of the metal-ligand systems allows for tuning of the potentials, electron-transfer rates and electronic structures of the redox units. Electrochemically responsive organometallics are at the frontier of receptor chemistry, with metallocenes, bipyridines, porphyrins, and a range of other ligand systems having been used for both cationic and anionic recognition [6, 83, 84]. Metallocenes in particular are suitable candidates as active centers due to their stable one-electron transfer redox processes, and the facile derivatization of a wide variety of surfaces with these compounds. Heterogeneous organometallic systems can either be in polymeric form or the electrodes can be post-synthetically functionalized with homogeneous units. Some examples of organometallic centers used for anion recognition are shown in Figure 10.

Figure 10. Ferrocene, cobaltocene and osmium-bipyridine are some of the examples of redox organometallics that can be used in ion-selective separations.

An early work on the selective adsorption of anions was that of Simon et al. [85] who used surface-bound redox polymers of cobaltocenium. Using the precursor 1,1’-bis(30triethoxysilyl)propyl)amino)-carbonyl]cobaltocenium(I), they functionalized Pt and SnO2 electrodes with redox Co(II)/(III) systems by chemical or electrochemical deposition. The electron-transfer was found to be highly reversible with electrochemical stabilities up to 48 hrs in deoxygenated aqueous electrolyte. This work stands out in terms of the careful study of selective anion binding in the presence of various ratios of Fe(CN)63-/Cl- under different conditions for ionexchange. Strong binding of anions was established, with an ordering of Mo(CN)84- > Fe(CN)63- ≥ Fe(CN)63- > IrCl6- >>Cl-. This ion-exchange at the interface was found to be highly endothermic (Ho=+12 kcal/mol) for Fe(CN)63-/Cl-, but driven forward by a large entropic effect. Ferrocene or ferrocene-derivatized surfaces are perhaps one of the most interesting and well-studied redox-systems in terms of their electrochemical behavior and interaction with anionic species [86, 87]. The charging and counter16

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ion association phenomena in poly(vinyl)ferrocene (PVF) are quite complex, with both kinetic and thermodynamic effects playing important roles. Kinetic film oxidation shows an increased transport rate of tetrafluoroborate over perchlorate and hexafluorophosphate due to more facile diffusion within the film [88]. Smaller anions suffer less transport resistance than do larger anions during doping of the films, and their greater mobility within the redox-film allows for a higher oxidation current and electrochemical charge during PVF charging. Hexafluorophosphate was the largest anion, and therefore suffered most from counter-ion trapping. In both water and organic solvent, poly(vinyl)ferrocene systems exhibit highly reversible redox peaks, considerable stability and selective anion adsorption behavior. Ingression of a dianion electrolyte into PVF films under oxidation has been shown to be more favorable than of a mono-anion and solvent due to stronger electrical field effects [86], although in the case of multivalent anions, cation ingression and repulsion also play a role to ensure electroneutrality. The anion exchange properties of polyvinylferrocene films containing perchlorate were studied with a number of electrolytes, including iodide, cyanide and thiocyanate, with all anions being able to readily displace perchlorate, but with varying rates and reversibility, with iodide doping being a significantly more reversible process than cyanide binding [89]. The extent of oxidation of ferrocene films is also of interest, as kinetic trapping can often lead to unused sites during voltammetric cycling. The doping of non-interacting anions (ReO4-) was studied with X-ray absorption showing that over 75% of ferrocene moieties can be oxidized when a positive potential is applied [90]. Mixed-metal polymers derivatized with bipyridine ligands (such as Os(bipy)2) have been shown to have complex ion-exchange behavior for both cations and anions [91, 92]. The osmium complexes can be incorporated into copolymers with anionic groups such as poly(styrene-sulfonate)[93] or directly coordinated on the backbone of an aromatic polymer [93, 94]. These polymers, like their homogeneous counterparts, have shown remarkable electroactivity There has been a variety of other work on the interaction of immobilized electroactive species with various anions with distinct flavors, some in which the redox-species can actually be exchanged itself. One example is given by the polyelectrolyte brushes of poly(methacryloyloxy)ethyl-trimethyl-ammonium chloride (PMTAC), which is initially positively charged with chloride as the counterion. The surface can be easily functionalized by ion-exchange with a redox-probe ([Fe(CN)6]3-)[95] that forms strong pairs with the quaternary ammonium groups; upon oxidation and reduction, the redox-probe itself can exchange with other anions such as perchlorate due to stronger hydrophobic interactions of the latter anion with the brush. Finally, a very complete series of anion-exchange studies was carried out by Bruce and Wrighton [96] using surface Auger spectroscopy and electrochemical techniques on a surface confined electroactive polymer (N,N'bis(3(trimethoxysilyl)propyl)-4,4'-bipyridinium) dibromide (I)) . The incorporation of a series of anions was studied, with the initial bromide anion being exchangeable with iodide, and chemically reversible species such as ferricyanide being readily incorporated into the film. An ordering of binding was found to be I->SCN- ~ ClO4- ~ SO4- > Br- > Cl- ~ p-toluene sulfonate, with I- exhibiting the strongest binding. Large 17

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electroactive, substitution inert anions bound significantly tightly than did nonelectroactive anions: Mo(CN)84- > Fe(CN)64->IrCl62->>Cl-, in both single anion and competitive binding studies.

3.4. Specific Chemical Interactions

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For most of the systems previously discussed, the anion or cation adsorption interaction has been based on electrostatic or size exclusion effects. However, in supramolecular recognition, specific chemical interactions such as donor-acceptor effects and charge transfer play an important role, through hydrogen bonding or π-π interactions. A redox-active polymer composed of viologen units (4,4-bipyridinium) has been used to confer anion-exchange properties to a conductive surface for electrorelease [97]. In its oxidized state, the polymeric viologen film was shown to form favorable charge-transfer adducts with various anionic dopants, including the redox-active molecules (Mo(CN)83-, Fe(CN)83- and ABTS; these dopants were released on reduction of the redox-polymer film. Incorporation of these dopants within the film was shown to be possible, with up to 37% of the film sorbing ABTS at 0.3 V oxidation, and with full release at -1.6 V. With a potential of only -0.9 V, about 25% of the ABTS was released, showing that the release mechanism is potentialdependent. Recently, Su et al. [14] have shown that specific interactions between oxidized ferrocene can be used to adsorb and release carboxylates selectively. This is one of the first examples of redox-mediated separation based on a functional group recognition, with the degree of adsorption between the carboxylates dictated by the H-bonding donor properties of the carboxylates themselves (Figure 11) [14]. A poly(vinyl)ferrocene/CNT composite was used as the working electrode to adsorb dilute carboxylates (~3 mM) over excess competing electrolyte (100 mM of perchlorate or hexafluorophosphate) in both aqueous and organic media, and the electrosorption and release was found to be 1:1 stoichiometric between the number of ferrocenium sites deposited and the carboxylate adsorbed. The underlying mechanism of specific interaction was found through both electronic structure modeling as well as NMR spectroscopy to be a hydrogen-bond between bidentate carboxylate anions and the hydrogens of the ligand, and is one of the first reported cases of C-H hydrogen bonding through redox-mediated electrochemical control. From a practical standpoint, the poly(vinyl)ferrocene system takes advantage of redox-activated chemical interactions to selectively extract anions based on chemical group affinity, with the distinction of also being fully reversible.

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Figure 11. (a) Redox-mediated electrosorption of carboxylate anions through reversible Hbonding. Reproduced with permission from [14]. An example of using specific chemical interactions to achieve selectivity over size and charge. (b) Tuning of ferrocene-functionalized electrode through redox activity for the solubilization of organics. Reproduced with permission from [98]

Another interesting application of ferrocene is its use for the controlled solubilization of organics (Figure 11b) [98]. A co-polymer of hydroxybutyl methacrylate (HBMA) and vinylferrocene (VF) was utilized to extract butanol from water due to the hydrophobic nature of the system in the uncharged state. Upon oxidation, counter-ions ingress into the system carrying solvated water, which increases its hydrophilicity and decreases its butanol content. Thus, this is an 19

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example in which the distribution coefficient of neutral species between the two phases can be tuned through oxidation and reduction of the film, solely by changing the hydrophilicity of the polymer environment. Similarly, it was found in these systems that transfer of electrons through the redox polymer layer was the limiting mechanism for redox, with electron hopping from one redox center to another being the primary mode of electron diffusion [99].

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Figure 12. (a) Anion affinity sequence of a poly(aniline-co-p-aminobenzoic acid) conducting polymer through electrochemical control. Reproduced with permission from [100]. (b) Electrochemically-mediated binding of Pb2+ through the use of a cationic redox-responsive ligand. Reproduced with permission from [105].

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Another interesting target of electro-release is that of the cationic organic molecule dopamine, which was accomplished with both pyrrole platforms[54] as well as a relatively rarer quinone redox-species [101, 102]. Quinones are able to form anions upon reduction and thus adsorb cations selectively. The same group has used other electropolymerized systems for releasing oligonucleotides [103]. Specific interactions can also be found in the organic polymer mentioned above – in particular, when immersed in a non-aqueous solvent, protonation does not play an important role and H-bonding can dominate as the main mechanism of interaction in quinone systems [104]. Finally, copolymerization in the presence of an imprinting anion can be a exploited to change the intrinsic ion-affinity properties of a polymer [100], such as the copolymerization of aniline and p-benzoic acid in H2SO4 (Figure 12a), with an anion-exchange affinity sequence of SO42- > ClO4- >Br- >NO3->Cl-; this sequence is uncommon when compared with other electrochemical ion-exchange polymers. The specific interactions were postulated to be dictated partially by the spatial structure of the polymer film which has more selectivity for the originally imprinted molecule (SO42-) than for anions based on size [100]. Finally, redox-active organic-ligands have been used for the heterogeneous mediated adsorption of heavy-metal ions, in particular lead Pb2+ (Figure 12b) [105]. In a heterogeneously-bound tetrathiafulvalene (TTF) framework, Pb2+ ions were adsorbed by a reduced ligand (TTF0 state) and released upon oxidation of this ligand (TTF2+ state). This work is interesting in that it utilizes the strong binding event of the TTF ligand to recognize and capture Pb2+ anions selectively over other species, and to release them based on electrostatic repulsion.

4. Synthesis and preparation of redox-electrodes There have been extensive reviews on the synthesis and preparation of the organic, inorganic, organometallic and composite redox-materials presented in the current review [106-108]. A brief overview of the main methods for preparation will be given, while referring the reader to the original methodology for detailed recipes and conditions.

4.1. Redox-active Organic Conducting Polymers In general, there is a wide variety of methods in which the same polymer or surface coating can be prepared, ranging from self-assembly, electrodeposition, vapor deposition, spin-coating, drop-casting and electrografting; some of these coatings can be prepared through all of these methods [109]. One such example is polypyrrole, which can be prepared under a range of various conditions, both chemical and electrochemical [106, 110-112]. A list of various chemical oxidants, additives and their yields can be found in literature[106], as well as detailed mechanisms for pyrrole electropolymerization [112]. Electrochemically, aqueous or organic solutions of pyrroles can be oxidized at relatively low oxidation potentials and the radical cation formed can subsequently form the polymer layer on a wide variety of materials, including carbon, ITO, and platinum, among others. Cyclic 21

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voltammetry[113], chronoamperometry [113, 114] and coulometry [113] are all widely used for the electropolymerization of pyrroles, and, depending on the method, various degrees of porosity and conductivity can be achieved. Copolymers of pyrroles and anilines can be prepared by reactivating the polymer and thus creating composites with more complex electrochemical behavior (Figure 13) [106].

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Figure 13. Formation of a block co-polymer of pyrrole and aniline through reactivation of original polymer chain. Reproduced with permission from [106].

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As with polypyrrole, polyaniline has received much attention in the optimization and control of various synthesis procedures to achieve higher conductivities and surface morphologies, both chemically [115-120] and electrochemically [121-123]. Polyaniline can be electrodeposited from acidic aqueous solutions onto a wide variety of substrates, with both electrochemical parameters as well as pH and counter-ion playing an important role on the final conductivity and film stability.

4.2. Redox-active Organometallic Surfaces As noted above, redox organometallics are promising materials for electrosorption and anion recognition, due to the careful control of their electronic structure through the ligand metal coordination [84, 124, 125]. The creation of organometallic functionalized surfaces is closely related to the formation of the films, which can either be by in-situ electropolymerization or post-synthetic modification. Poly(viny)ferrocene (PVF) was one of the earliest metallocene polymers proposed, synthesized via radical and cationic chain growth polymerization of vinylferrocene [107]. PVF itself can then be deposited anodically onto conductive surfaces [126] or simply through dip-coating of a conductive substrate [127]. A wealth of various derivatized metallocenes has been described, both derivatized ferrocene systems as well as with other metal centers, such as cobalt, ruthenium, manganese and chromium metallocenes [107]. A series of ferrocene copolymers has also been

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synthesized, including those with sulfonates, pyrroles among others as the complementary units. In addition to polymeric forms, redox organometallics on surfaces can be modified post-synthetically such as by click-chemistry (Figure 14) [128]. Other ligand systems such as phthalocyanines and porphyrins can be created through electropolymerization or chemical incorporation with other conducting units [129-132]. For example, a series of thiophene transition metal surfaces can be created by electropolymerization of thiophene monomers with the electroactive ligands [133].

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Figure 14. Click-chemistry for ferrocene functionalization on a monolayer. Reproduced with permission from[128].

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4.3. Composites of conductive materials and redox-active species Electrodes are often composed of two or more different type of materials to improve performance and offer complementary properties (e.g. a conductive substrate as a molecular wire alongside a redox-active center). Nanoconjugates with carbon nanotubes are often a smart-design choice to increase conductivity and accelerate electrokinetics of various redox-active processes, in both molecular and heterogeneous systems [134]. Polymer-decorated carbon nanotubes prepared either by post-synthetic functionalization through carboxylic acid chemistry or by non-covalent functionalization have proven to be efficient materials for energy storage and sensing. Carbon nanotubes serve a molecular wire to facilitate the electron transfer from a surface to an immobilized polymer, such composites of multi-walled carbon nanotubes (MWCNTs) and poly(vinyl)ferrocene which have been applied for glucose sensing [134]. Composites with MWCNTs on a glassycarbon electrode surface served as substrates for vinyl(ferrocene) electropolymerization in acetonitrile, with the procedure being continuous scanning of the MWCNT electrode between -1.2 and 1.0 V in 3 M vinylferrocene in acetonitrile, then followed by holding the potential constant at +0.7 V for varying times (Figure 15) [135]. The resulting electrochemical sensitivity for glucose detection was as low as 41 uM and a sensitivity of 0.0095 M A-1. Similar combinations of electroactive polymers such as polypyrrole and CNTs have been

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prepared by electrolysis at positive potentials to make redox-active surfaces for supercapacitors and desalination [136-139]. More recently, a non-covalent PVF/CNT composite mixture was prepared by solution processing in chloroform to offer a facile method for preparation of pseudocapacitors, with a dramatic increase in pseudocapacitance owing to the presence of the ferrocene units (Figure 16) [140]. . (b)

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Figure 15. (a) MWCNT and functionalized MWCNT/PVF, (b) Cyclic voltammogram of MWCNT/PVF after 0.1 V/s in phosphate buffer. Reproduced with permission from [135].

Figure 16. Non-covalent functionalization of electrode surface with dispersed PVF-CNT composite mixture. Reproduced with permission from[140].

4.4. Inorganic Conductive Electrodes: Metal-oxides and crystalline clusters Despite the more extensive interest in organic substrates, inorganic structures have been the mainstay in battery technology, and, as an extension, in systems for redoxmediated separation. They were shown to be especially effective as open framework matrices for cation insertion [30]. Prussian blue, in its natural form, defined as ferric ferricyanide, or ferric hexacyanoferrate [141], is but one famous example of a host of transition metal cyanides. They have a distinct crystalline structure, and have been extensively studied; in the electron transfer processes, cations intercalate 24

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Fe4III[FeII(CN)6)3 + 4 e- + 4K+ K4Fe4II(FeII(CN)6)3 [reduction and entrapment of cations] Fe4III(FeII(CN)6)3 + 3e- +3A- Fe4III (FeII(CN)6A)3 [oxidation and entrapment of anions]

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Cyanates formed from other metals, especially nickel hexaferrocyanate, have been touted as efficient electrochemically switchable ion-exchange materials in thin-film form. Electrosynthesis of films of metal hexacyanoferrate mostly relies on potential cycling of a working electrode in the presence of a liquid mixture containing the metal precursor (Mn+) and ferricyanide [29, 48, 49]. The growth of the film has been reported for various metal substrates such as gold, copper and platinum, while glassy carbon and graphite have also been used [30]. Nickel hexaferrocyanate can be synthesized in combinatorial fashion between the nickel source, the ferricyanide source and water by chemical nucleation [142]. Structural variants have been created depending on the ratio of each component, with the electro-activity of the film being highly dependent on an appropriate stoichiometry.

5. Perspective: chemical specificity and energy recovery

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5.1. Challenges in ion-specific separation: cost, selectivity and diverse applications For redox-active surfaces to be applied in commercial applications, economic and technical challenges must be overcome. Due to their abundance and low cost, conductive-carbon based materials still form the largest component of electrode materials [143, 144]. However, as seen extensively in this review, the degree of chemical specificity that carbon-based materials can achieve can be limited when compared to heteroatom redox species. Organic, inorganic, and organometallic composite materials offer a wealth of Faradaic reactions and specificity that can be easily tuned to various mixture properties and target analytes. With the integration of simple yet selective redox-units on a surface, the increase in selectivity can be tremendous, yet at still at relatively low cost, especially if earth abundant metals or organic conductive units are used. One area of separation that may benefit tremendously from the selectivity granted by redox electrodes is the purification of high-value neutral and charged products from organic synthesis [145, 146]. Often these value-added chemicals are produced in dilute concentrations and in the presence of many competing species, and to be able to remove them specifically based on functional group affinity over catalysts, buffers and other salts is of great importance. The use of organic media in many of these reactions poses another electrochemical challenge – most of the focus

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so far for deionization and dilute pollutant removal has been on aqueous solutions. However, many value-added synthesis processes such as carbon dioxide hydrogenation occur in organic or mixed media [147-149], and selective separation of these compounds under a variety of pH, solvent and ionic strength conditions can be greatly enhanced with the use of specifically designed redox-electrodes. Among the modes of implementation for these types of separations, electrochemicallymodulated chromatography (EMLC) has been proposed[150-155] to modulate the retention of various analytes, ranging from complex aromatic compounds to metal ions. In EMLC, the application of potential onto a conductive surface changes significantly the adsorption of non-ionic and ionic adsorbates, based on effects such as dipole interactions of polarized molecules with the electrified interface. Redoxenhanced surfaces may play an important role in the future to further improve chromatographic resolution in these types of electrochemical flow-systems.

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5.2. Energy integration: battery and pseudocapacitive separation systems

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One of the greatest challenges for electrochemical systems in both deionization and small-molecule separations is to reduce energy costs. Energy minimization recovery is a main priority for deionization systems [18, 156, 157]. During charging steps, energy is accumulated for deionization, and during discharge, that energy can be recovered. Pseudocapacitive systems rely on Faradaic reactions to provide a better combination of power and energy densities, much faster response times and longer life cycles than obtained with either battery or supercapacitors [158, 159] and as such, when coupled with a separation process, provide an efficient process as well as a naturally coupled strategy for energy minimization. Furthermore, such systems, due to their electrochemical nature and the energy recovery from charge and discharge cycles, can be created to be much more modular than other bulkier desalination systems, and be potentially used to address purification systems in remote locations.

6. Conclusions

A broad range of redox-materials exist that can be used to enhance ion-selective adsorption processes. Redox-processes enhance both selectivity and capacity, and depending on the nature of the charged species, can be used either in the anode or in the cathode for the selective separation of anions and cations. Inorganic batterytype electrodes can intercalate ions selectively and eject them upon reversing potential. Similarly, pseudocapacitive materials and redox-polymers can reversibly bind and desorb select ions based on a variety of electrochemical and chemical mechanisms. The synthesis of redox-electrodes can vary dramatically depending on whether the structure is organic or inorganic. Electro-polymerization onto a conductive substrate seems to be a facile and reproducible method for quickly generating redox-responsive organic surfaces from redox monomers, whereas

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solvothermal synthesis and crystal growth are the most used methods for preparing inorganic electrodes. The integration of the energy storage and the electrochemical separation process seems a promising route for further development of these redoxmediated systems, as they seem to be an ideal platform for implementing sustainable processes for chemical and environmental applications.

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ACCEPTED MANUSCRIPT Highlights Redox-electrodes as promising platforms for electrochemically-modulated selective separation.

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Efficient ion-selective extraction of both cations and anions from liquidphase.

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Higher electrochemical performance and ion-selectivity than traditional, carbon-based conductive materials.

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Design of chemical specificity towards charged functional groups.

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Review of functional materials preparation, including organic, inorganic and composite, for redox-electrodes used in electrochemical separations.

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