Bipolar Electrochemistry: A Powerful Tool for Electrifying Functional

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Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

Bipolar Electrochemistry: A Powerful Tool for Electrifying Functional Material Synthesis Published as part of the Accounts of Chemical Research special issue “Electrifying Synthesis”. Naoki Shida,† Yaqian Zhou,† and Shinsuke Inagi*,†,§ †

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Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan § PRESTO, Japan Science and Technology Agency (JST) 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan CONSPECTUS: Electrosynthesis is a powerful method for the synthesis of organic, inorganic, and polymeric materials based on electron-transfer-driven reactions at the substrate/electrode interface. The use of electricity for synthetic reactions without the need for hazardous chemical oxidants and reductants is recognized as a green and sustainable method. Other advantages include control of the reaction selectivity by tuning the electrode potentials. A different mode for driving electrochemical reactions has recently been proposed, in which bipolar electrodes (BPEs) are available as wireless electrodes that undergo anodic and cathodic reactions simultaneously. Bipolar electrochemistry is an old technology that has recently garnered renewed attention because of the interesting features of BPEs: (i) the wireless nature of a BPE is useful for sensors and material synthesis; (ii) the gradient potential distribution on BPEs is a powerful tool for the preparation of gradient surfaces and materials; and (iii) electrophoresis is available for effective electrolysis. In addition to these unique features, a BPE system only requires a small amount of supporting electrolyte in principle, whereas a large amount of electrolyte is necessary in conventional electrochemistry. Hence, bipolar electrochemistry is an inherently green and sustainable chemical process for the synthesis of materials. In this Account, recent progress in bipolar electrochemistry for the electrosynthesis of functional materials is summarized. The wireless nature of BPEs was utilized for symmetry breaking to produce anisotropic materials based on the site-selective modification of conductive objects by electrodeposition and electropolymerization. Potential gradients on a BPE interface have been successfully used as controllable templates to form molecular or polymeric gradient materials, which are potentially applicable for high throughput analytical equipment or as biomimetic materials. The electric field necessary to drive BPEs is also potentially useful to induce the directed migration of charged species. The synergetic effects of electrophoresis and electrolysis were also successfully demonstrated to obtain various functional materials. These features of bipolar electrochemistry and the various combinations of techniques have the potential to change the methodologies of material synthesis. Furthermore, the fundamental principle of bipolar electrochemistry infers that very small amounts of supporting electrolyte are necessary for an electrode system, which is expected to lead new methods of sustainable organic electrosynthesis.

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INTRODUCTION

electronically connected to a power supply, and a BPE embedded in between the driving electrodes. For BPEs, no electrical connections are required, no limitation in shape and size of materials exists as long as it is conductive (carbon, metals, conductive metal oxides, etc.), and a number of BPEs can be placed at the same time. These features give significant freedom to design reaction systems. Since a BPE is driven by the influence of an electric field generated in an electrolytic solution from driving electrodes, a limited amount of supporting electrolyte is used. Principles and detailed mechanistic discussions on bipolar electrochemistry is nicely summarized in the review by Crooks and co-workers.6

Electrosynthesis is a powerful method for the synthesis of organic,1 inorganic,2 and polymeric materials3 based on electron-transfer-driven reactions at the substrate/electrode interface. The use of electricity for synthetic reactions without the need for hazardous chemical oxidants and reductants is recognized as a green method.4 Other advantages include control of the reaction selectivity by tuning electrode potentials. In addition to classical electrochemical setups, new types of electrochemical reactors and cells have recently been developed.5 A different mode to drive electrochemical reactions has also recently been proposed, in which bipolar electrodes (BPEs)6 are available as wireless electrodes that undergo anodic and cathodic reactions simultaneously. As shown in Figure 1b, a general setup for bipolar electrochemistry consists of a pair of driving electrodes, which are © XXXX American Chemical Society

Received: June 25, 2019

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DOI: 10.1021/acs.accounts.9b00337 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

potentials are sufficiently high (Figure 1a).7 On the other hand, in a low concentration of supporting electrolytes, electrical double layers are not fully formed and an electric field is instead created in the bulk of the electrolytic solution. In such a configuration, redox reactions do not readily proceed at the electrode surfaces; however, a conductor placed in between the electrodes can act as a BPE that simultaneously involves oxidation and reduction at its terminals (Figure 1b).8 The sum of the interface potential differences at both edges of a conductor (ΔVBPE) is an important factor to drive redox reactions of substrates in solution, that is, ΔVBPE should be larger than the potential difference between the target redox reactions. ΔVBPE is proportional to the length of the BPE and the strength of the electric field, and the latter is experimentally measurable with a simple setup. Bipolar electrochemistry is a relatively old technology9 that has recently garnered renewed attention because of the interesting features of BPEs10 because (i) the wireless nature of a BPE is useful for sensors11 and material synthesis,12 (ii) the gradient potential distribution on BPEs is a powerful tool for the preparation of gradient surfaces13 and materials,14 and (iii) electrophoresis is available for effective electrolysis.15 In this Account, recent progress in bipolar electrochemistry for the electrosynthesis of functional materials is summarized. Here, we discuss wireless electrochemical reactions to modify conductive objects, the use of the gradient potential distribution on BPEs for modification of conducting polymer films, and the use of electrophoretic effects in electrolysis.

Figure 1. Comparison of electrolytic systems for (a) conventional and (b) bipolar electrolysis. Red dotted line represents an ideal electric field generated between driving electrodes.

Here, we highlight the features of bipolar electrochemistry through the comparison with the conventional electrochemistry. In a conventional electrolytic system that contains a high concentration of supporting electrolytes (>0.1 M), a voltage applied between two electrodes generates electrical double layers upon accumulating ions at the electrode/solution interfaces. Significantly steep electric fields generated in a couple of nanometer thickness of the layers can drive redox reactions at the anode and cathode, if the applied electrode

Figure 2. (a) General scheme for the oxidative electropolymerization of heteroaromatic monomers. (b) Schematic illustration of the setup for bipolar electropolymerization of pyrrole on a CNT as a BPE. (c) SEM images of CNTs modified with PPy after bipolar electropolymerization. Adapted with permission from ref 23. Copyright 2005 Springer. (d) Bipolar electropolymerization of pyrrole and reduction of Cu2+ on a CNT as a BPE in a capillary electrophoresis cell. (e) SEM image of a modified Janus CNT with PPy and Cu. B


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WIRELESS MODIFICATION OF CONDUCTIVE OBJECTS BPEs work without need of wiring to a power supply. Besides, multiple BPEs function equally in the same external electric field. On the basis of these features, bipolar electrochemistry has been regarded as a means to expand effective surface area for electrochemical reactions, as can be seen in the actual industrial process.9 In the modern bipolar electrochemistry, in addition to these advantages, more emphasis has been placed on its symmetry-breaking nature for material synthesis. Bipolar electrochemistry would be a straightforward method to create anisotropic materials.

Bradley, Gogotsi, and co-workers demonstrated that a carbon nanotube (CNT) can function as a BPE for the electropolymerization of pyrrole to form a polypyrrole (PPy) deposition at one terminal of the CNT.23 CNTs were immobilized on a porous polyester film from a dispersed toluene solution and were then immersed in an acetonitrile (MeCN)/toluene solution containing sodium p-toluenesulfonate and pyrrole. A pulsed voltage was applied between two platinum (Pt) wires (driving electrodes) to generate an electric field between them (Figure 2b). Randomly oriented CNTs behaved as BPEs when a sufficient ΔVBPE was applied between their terminals, whereby the anodic polymerization of pyrrole and reduction of electrolyte/solvent/protons occurred simultaneously. PPy was thus successfully deposited in a site-selective manner (Figure 2c). For application, the partially attached hydrophilic PPy acted as a guide for water to enter inside the CNT wall, similar to a nanopipette, which was evidenced by SEM analysis.24 Kuhn and co-workers employed a capillary electrophoresis procedure in which CNTs were successfully passed through a reaction chamber with alignment.25 When a high voltage was applied from both sides of a silica capillary containing an electrolyte, electroosmotic flow was generated inside the capillary, which transported the CNT suspension from the anodic capillary inlet toward the cathodic compartment while maintaining the orientation of the CNTs. As shown in Figure 2d, a water suspension of CNTs/pyrrole monomer/copper iodide (CuI) was introduced into the chamber, where the CNTs act as BPEs for the electropolymerization of pyrrole on one side and for the electroreductive reaction of Cu+ to Cu0 on the other side during the course of the capillary electrophoresis. As a result, the CNTs were modified with PPy and Cu0 clusters on either end (Figure 2e).26 Similarly, another Janus-type modification of carbon microfibers with polythiophene and gold (Au) in a MeCN solution was reported.27 To summarize this section, two important points should be emphasized. One is that all the BPEs in the same electric field behave equally as long as the size of BPEs are the same. This feature is also the key to create BPE-based sensing systems.28 The second is that no wiring is required to the target material despite the electrochemical reactions on BPEs. These features make bipolar electrochemistry as an ideal method for bulk anisotropic surface modification of micro-objects.

Inorganic and Organic Modification

Kuhn and co-workers reported the synthesis of Janus particles, where one hemisphere has a distinct property to the other hemisphere, using bipolar electrochemistry.16−18 Conductive carbon beads were used as BPEs, and the key of this technology is to fix BPEs in an electrolyte to suppress movement of the BPEs during electrolysis. Agarose-based hydrogel was found to be useful, and the fixing of carbon beads in a glass capillary also worked for this purpose. On the basis of this concept, they reported the modification of SiO2 and TiO2 via electrogenerated acid-induced sol−gel reaction,16 metal deposition,17 and the introduction of covalently bonded organic groups via the electrografting of diazonium salts.18 Further modification via the introduced linkage was also achieved to introduce functionalities, which demonstrates the wide range of applicability of this technology. Our group prepared Au- and Pt-modified Janus carbon beads at each electrode face using bipolar electrochemistry and used the micro-objects for the self-locomotion system in a H2O2 fuel solution.19 We, then, proposed the use of bipolar electrochemistry under application of an alternating current (AC).20 Under AC conditions, a BPE gains symmetric anisotropy, in contrast to the unidirectional anisotropy along the IR drop under direct current (DC) conditions. When an AC voltage was applied to carbon beads fixed in agarose gel containing HAuCl4 or H2PtCl6 under bipolar electrochemical conditions, symmetrical and site-selective metal-deposition around both poles of the particles was achieved.21 The resulting bifunctionalized beads were characterized using scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDX). Tetrafunctionalization was also achieved using a cross electrochemical cell with two pairs of driving electrodes.

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GRADIENT MATERIAL SYNTHESIS One beneficial feature of BPEs is that they have a gradient distribution of electrical potential on their surface, derived from the linear IR drop in solution. The slope of this IR drop can be easily manipulated by changing the applied voltage between a pair of driving electrodes. Thus, this potential gradient can be regarded as an in situ generated controllable template for gradient surface modification. Transcribing this gradient into the material surface can easily result in various gradient materials, which is potentially applicable for highthroughput analytical equipment or biomimetic materials.29

Electropolymerization

Electrooxidative coupling polymerization of aromatic monomers affords π-conjugated polymers via aromatic C−H coupling, as shown in Figure 2a.3 They are intrinsically semiconductive but become conductive upon a chemical or electrochemical doping process. Such conducting polymers are generally insoluble in solvents and are, thus, deposited on the surface of an anode during polymerization. In a bipolar electrochemical system, the electropolymerization of aromatic monomers on one terminal of a BPE is possible to give a conductive object that is site-selectively modified with a conducting polymer film. Therefore, bipolar electropolymerization is a next-generation electropolymerization method beyond conventional electropolymerization that forms a uniform thin film on an anode surface.22

Gradient Monolayer Formation

The most straightforward modification is the gradient modification of a bipolar electrode itself. A variety of redoxtriggered methods have been reported for the modification of bipolar electrodes. A pioneering work was reported by Björefors and co-workers,13 where an alkanethiol-modified gold electrode was prepared using HS-C2H4-(OC2H4)6-OCH3 C


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Accounts of Chemical Research and subjected to bipolar electrochemical conditions. The reductive cleavage of Au−S bonds proceeded in response to the external electric field, as confirmed by ellipsometry measurements. The pristine Au-surface was then backfilled with HS-C2H4-(OC2H4)8-COOH to provide binding properties for biorelevant molecules. The surface was incubated in lysozymes for 30 min, which enabled the preferable binding of proteins at the carboxylic-acid modified region in a gradient manner. As expected, the thickness profile showed no change on the surface orthogonal to the external electric field. Similar molecular gradient was prepared by Bouffier and co-workers for modulation of wetting properties of the modified surface.30 These results demonstrate that bipolar electrochemistry is a facile and straightforward way to create various molecular gradients. Our group reported the modification of an electrode surface with a molecular gradient via the bipolar reduction of aryl diazonium salts.31 To analyze the in-plane molecular distribution in detail, a surface-initiated polymerization was conducted to amplify the information on the modified thin layer. Height analysis of the polymer brush provided evidence for the formation of a molecular gradient at the cathodic surface, and the modification of organic groups was also determined to become saturated under the application of a high voltage. The molecular gradient based on aryl diazonium chemistry was also successful in two-phase bipolar electrochemical method.32

Figure 3. (a) Scheme for the electroclick reaction of an azidofunctionalized polymer film on a BPE with an alkyne derivative using an electrogenerated Cu(I) species. (b) Weight ratio of fluorine and sulfur atoms of the treated film at each position as determined by EDX measurements. (c) Contact angle of water droplets (1 μL) at different positions on the treated film.

An electrochemically mediated click reaction, so-called the electro-click reaction,33 was combined with bipolar electrochemistry. In the electro-click reaction, electrogenerated Cu(I) catalyzes azide−alkyne cycloaddition (click reaction). In the bipolar electro-click system, the azide-functionalized conducting polymer was fixed on an indium tin oxide (ITO) surface and placed in a U-type cell filled with water/t-BuOH (1/1) solution containing copper sulfate (10 mM) and alkyne with a perfluoroalkyl group. The copper sulfate acted both as the supporting electrolyte and the source of a Cu(I) catalyst (Figure 3a). Introduction of the perfluoroalkyl group was confirmed by tracing fluorine atoms with EDX, proving the formation of the molecular gradient across the entire film following the potential gradient on the BPE (Figure 3b).34 The in-plane molecular gradient of the perfluoroalkyl group induced a gradual change of a surface property. The contact angle of a water droplet was changed depending on the amount of perfluoroalkyl group introduced, indicating the gradual change in surface free energy within the polymer surface (Figure 3c).

[Cu(I)]/[Cu(II)]; therefore, the polymerization rate is tuned by changing this ratio by electrochemical reactions (Figure 4b). Under the bipolar conditions, generation of Cu(I) is faster at the position closer to the cathodic edge. As a consequence, the gradient of [Cu(I)]/[Cu(II)] is built in the space within the microgap. Therefore, the rate of surface-initiated ATRP changes depending on the positions to give a polymer brush with gradient height profile (Figure 4c). The polymerization was conducted using N-isopropylacrylamide (NIPAM) monomer. The electrolyte solution containing 3.2 M NIPAM, H2O/MeOH (1/1), CuCl2 (2 mM), and N,N,N′,N′,N″-pentamethyl-diethylenetriamine (PMDETA) was used for the reaction. The microgap D was fixed to 360 mm, and ΔVBPE = 1.4 V was applied for 60 min under ambient conditions. The thickness of the resulting polymer brush was measured at various positions using a stylus-type tester to generate a height profile. As expected, the 3D gradient profile of poly(NIPAM) brush was verified. The control experiments revealed that the thickness of the polymer brush (i.e., steepness of the gradient) can be tuned through parameters such as the applied voltage (ΔVBPE) and the microgap (D). Time versus film thickness plot revealed that the polymerization proceeds in a living manner at each position of the substrate, whereas the polymerization rate differs depending on the position.

Fabrication of Gradient Polymer Brushes

Gradient Doping of Conducting Polymer Films

The electrochemically generated Cu(I) was also used for the catalyst of atom-transfer radical polymerization (ATRP). In the electrochemically mediated ATRP (eATRP), the living radical polymerization can be controlled by electrochemistry.35 Because of the highly controllable and environmentally benign nature of electrochemistry, this idea has attracted significant attention. Bipolar electrochemistry was used to generate a concentration gradient for eATRP envisioning a formation of a 3D gradient polymer brush.36 The setup for bipolar eATRP is composed of a glassy carbon (GC) BPE and an initiator-modified substrate, and these were sandwiched leaving microgap (D) in between (Figure 4a). Polymerization rate of ATRP is generally in proportional to

Electron transfer reactions of a conducting polymer generate polaron or bipolaron in its repeating unit (known as electrochemical doping), which vary the physical properties of polymers. The electrochemical doping imparts electrical conductivity concomitant with a significant color change.37 The doping event creates mobile charge carriers that are not associated with a particular atom or functional group. The charges generated along the polymer must be compensated by the addition of neighboring ions derived from the electrolyte (dopants). The electrochemical doping process is reversible in general. The application of a gradient potential on a BPE enables a gradient doping of conducting polymer films. A poly(3-

Electro-Click Modification of Conducting Polymer Surface

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Figure 4. (a) Schematic illustration of the setup for surface-initiated eATRP using a BPE system. (b) General scheme for eATRP using a Cu catalyst. (c) Representation of a gradient poly(NIPAM) brush propagated from an initiator-modified glass substrate. Adapted with permission from ref 36. Copyright 2015 Wiley.

methylthiophene) (P3MT) film fixed on an ITO plate was placed into a U-type cell containing 5 mM Bu4NPF6/MeCN and equipped with a Pt anode and a Pt cathode connected to a power supply (Figure 5a).38 P3MT is a red-colored film in the

underwent a significant color change from deep blue to rather transparent at the anodic surface. Bipolar doping of polyaniline (PANI) was performed in 5 mM H2SO4, giving a multicolored gradient depending on the potential applied to the BPE. Gradient Reactions of Conducting Polymer Films

Although a doped state of conducting polymers is stable in the presence of noncoordinating dopants, overoxidation or overreduction (i.e., undesirable reactions with nucleophiles or electrophiles, respectively) is also conceivable, which form a nonconducting polymer. In other words, the appropriate choice of dopant potentially leads to the selective reaction to achieve precise functionalization of conducting polymers via an electrochemical reaction. On the basis of this concept, we have developed electrochemical polymer reactions, where the conjugated polymer chains are modified through the electrochemical doping and following bond formation.40 The above-mentioned gradient doping through bipolar electrochemistry implies that the gradient reaction is possible by the use of reactive (i.e., nucleophilic) dopants. As a proof of concept, the oxidative chlorination of P3MT on an ITO, which works as BPE, was performed in 5 mM Et4NCl/MeCN (Figure 6a).41 EDX measurement evidenced the introduction of the chlorine atom in a gradient manner from anodic edge reflecting the potential gradient on BPE.39 Chlorine-substitution of poly(thiophene) derivatives lowers the highest occupied molecular orbital (HOMO) energy level, which results in the enhancement of the tolerance toward oxidative reactions.42 Therefore, the chlorine-modified P3MT film possessed a gradient of HOMO energy level across the film, confirmed by the anodic doping of the prepared chlorine-gradient film in 0.1 M Bu4NPF6/MeCN (Figure 6b). When 0.8 V of potential (vs SCE) was applied on the chlorinated film, only the nonchlorinated area was doped to change its color into blue. The application of far more anodic potential (1.4 V vs SCE) induced the doping of the entire film.

Figure 5. (a) Schematic illustration of a U-type electrolytic cell containing a pair of driving electrodes and a BPE covered with a conducting polymer film. (b) Photographs of P3MT, PEDOT, and PANI films gradually doped on the BPE.

neutral state, whereas the color changes to blue upon the anodic oxidation. Upon the passage of constant current between the driving electrodes in the U-type cell, the P3MT film worked electrochemically doped on the BPE and its color changed into blue at the anodic pole (Figure 5b). Absorption spectra of the treated P3MT film showed a gradient change from anodic to the cathodic surface, indicating that the doping level continuously varied across the polymer film. Depending on the current passed between the driving electrodes, the steepness of the potential slope generated on the BPE was controlled.39 EDX analysis also supported the compositional gradient in a sigmoidal shape according to the potential distribution in the U-type cell.38,39 In the same manner, a poly(3,4-ethylenedioxythiophene) (PEDOT) film on the BPE E


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Figure 6. (a) Photograph of the gradually chlorinated P3MT film on a BPE and profile showing the amount of chlorine atoms across the film. (b) Photographs of the gradually chlorinated P3MT film during the course of anodic doping at different potentials. Adapted with permission from ref 41. Copyright 2010 Wiley.

Figure 7. (a) Anodic and cathodic reactions of P(CHOH) to give P(CO) and P(CH2), respectively. (b) Photograph of the conducting polymer film treated by bipolar electrolysis, under UV irradiation (λ = 365 nm). Adapted with permission from ref 43. Copyright 2013 Wiley.

Parallel synthesis is an idea in electrosynthesis to produce the desired products using both anodic and cathodic reactions. Parallel synthesis is applicable for the electrochemical polymer reaction using a conjugated polymer which is capable of accepting both the oxidative and reductive transformations. For example, a film of a polyfluorene derivative bearing 9hydroxyfluorene moiety (P(CHOH)) fixed on the electrode gives a polyfluorenone derivative (P(CO)) upon the oxidation, whereas a polyfluorene derivative (P(CH2)) is obtained by reduction (Figure 7a).43 The concept of parallel synthesis is highly compatible with bipolar electrochemistry and it is expected that the divergent transformation of the 9hydroxyfluorene-based precursor polymer can be achieved on both poles of a BPE, giving a multicompositional gradient film. A P(CHOH) film was formed on the boron-doped diamond plate, and it was placed in the electrolytic cell equipped with a pair of stainless driving electrodes. Five millimolar Et4NOTs/ 2-propanol was used as an electrolyte. After electrolysis, the photoluminescence color of the polymer film consisted of a color gradation from yellow to blue to dark orange from the cathodic pole to the anodic pole, which corresponded to the colors of P(CH2), P(CHOH), and P(CO), respectively (Figure 7b).

composition of an Ag−Au alloy thus deposited is dependent on the applied potential.46 Diao and co-workers reported the deposition of gold nanoparticles (NPs) onto a BPE surface.47 Gold nanoseeds were prepared by the chemical reduction of HAuCl4 using NaBH4. The gold nanoseeds were capped with a cetyltrimethlyammonium (CTA) bilayer. An ITO substrate was pretreated with 11-aminoundecanoic acid (AUA) solution to modify an anchoring group onto the surface. The ITO plate was placed in a growth solution consisting of 20 mM CTA bromide, 0.2 mM HAuCl4, 0.3 mM ascorbic acid, and 0.08 mM KI and an external electric field was applied. SEM analysis of the resultant substrate revealed that the nanostructural shape of the gold NPs was changed depending on the position on the substrate. Gold triangular nanoprisms and triangular, hexagonal, and polygonal nanoplates were mainly observed with an increase in the potential. We investigated the electrochemical oxidation of pillar[6]arene (P6A) on a BPE (ITO plate). On the anodic side of the BPE, the P6A hydroquinone (HQ) form underwent oxidative partial transformation to its benzoquinone (BQ) form, triggering its self-assembly via charge transfer (CT) interaction and following hexagonal cylinder deposition.48 The SEM observation of the surface of the BPE at different positions revealed that the size and shape of the cylindrical structures were gradually changed depending on the potential applied on the BPE.

Gradient Electrodeposition

Shannon and co-workers reported the reductive electrodeposition of Cd and CdS on a BPE.44 An Au wire electrode placed in an aqueous solution containing 0.02 M Cd(NO3)2, 0.05 M Na2S2O3, and 0.10 M KNO3 was used as a BPE under the application of an external electric field. The resulting material showed three distinct colors at each region. The region closest to the cathodic pole was silver/gray in color (Cd enriched CdS), the adjacent region was orange (CdS), and the next region was yellow (S). The same group also reported the gradual formation of an Au−Ag alloy based on bipolar electrochemistry.45 Ag-deposition is thermodynamically favored, while Au-deposition is kinetically favored; therefore, the

Site-Controlled Application of Potential on a BPE

As implied in the electrochemical bipolar doping of conjugated polymers in the U-type cell, a careful design of the electrolytic cell enables modulation of the electric field. Build on this idea, cylindrical setup was designed envisioning the site-controlled potential application onto a BPE surface.49 This spot bipolar configuration is shown in Figure 8a. A conducting polymer film on an ITO plate was used as a BPE, and it was placed in a F


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shape of the cylinder. Similar procedure was also applied for a PANI film, which showed the complete recovery of the local color upon the application of reversed polarity (dedoping).50 Spot bipolar electrochemical polymer reaction was also reported using the nucleophilic chloride salt (5 mM Et4NCl/ MeCN). P3MT film showed an irreversible color change to yellow with circular shape due to C−Cl bond formation at 4position of 3-methylthiophene repeating unit. Line patterning was achieved by moving the cylinder system during the bipolar electrolysis (Figure 8c). The spot bipolar technology was used to generate a Cu(I) catalyst as well and was utilized for site-selective eATRP of the NIPAM monomer. On the top of the initiator-modified glass, a Pt-mesh electrode was placed with incorporating microgap of 130 μm. The insulating cylinder was set above the Pt-mesh and the electrolysis was conducted by applying constant voltage between driving electrodes. The poly(NIPAM) brush was observed as a circular pattern on the glass substrate. The diameter of the patterned circles corresponds to that of the cylinder. Rinsing the substrate with water left a water droplet stayed on the poly(NIPAM) pattern due to the hydrophobic nature of the polymer, whereas the brush became hydrophobic at 60 °C due to the phase separation of the poly(NIPAM) in water.36 In this section, we have introduced various applications of BPEs to fabricate gradient materials using a potential gradient on a BPE as an in situ generated template. The bipolar electrochemical methods do not require the use of any conventional template or complex equipment for fabricating gradient materials. Not only a linear-shaped potential distribution but also sigmoidal or even circular shaped potential distribution can be prepared by using an appropriately designed electrolytic cell, suggesting the great potential of bipolar electrochemical techniques.

Figure 8. (a) Schematic illustration of the setup for bipolar pattering using an insulating cylinder. (b) Photographs of the conducting polymer films showing the site-selectively doped PEDOT and P3MT using a cylinder with a diameter of 6.0 mm. (c) Photograph of the line-patterned P3MT film by anodic chlorination using a cylinder with a diameter of 1.0 mm.

container equipped with a plastic shielding cylinder with an external cathode wire and an anode ring. Upon the passage of current between the driving electrodes, an anodic region appeared under the cylinder concomitant with the surrounding cathodic region. The steep potential gradient was generated in between these regions. The spot bipolar electrochemistry was visualized by the color change of conducting polymers fixed on the ITO as a result of the electrochemical doping. Figure 8b shows photographs of PEDOT and P3MT49 films after spot bipolar doping treatment. Site-selective doping of these polymers were accomplished with reflecting the circular

Figure 9. (a) Schematic illustration of AC-bipolar electropolymerization system using Au wires as BPEs. Optical microscopy image of the obtained (b) PEDOT fibers and (c) PEDOT films. (d) TEM image of the PEDOT:Pt NPs hybrid fibers obtained by the reduction of Pt2+. G


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Metal Ion Migration-Assisted Electroplating

USE OF ELECTROPHORETIC EFFECT IN ELECTROCHEMICAL REACTIONS The electric field generated can induce the directed migration of ions, that is, electrophoresis. Bipolar electrochemical conditions generate an electric field across the electrolyte; therefore, electrophoresis is simultaneously triggered. Some groups have proposed novel electrochemical systems for material synthesis by exploiting the synergetic effect of electrolysis and electrophoresis.

Kuhn and co-workers reported the quite unique electrodeposition of metal ions onto conductive beads by bipolar electrolysis. They observed the ring-shaped deposition of Au NPs onto BPE microbeads.55 The use of HAuCl4 as a metal source was the key for this technology. In this system, the metal anions migrated along the direction from the driving cathode to the anode. Thus, the AuCl4− anions contacted the anodic pole of the BPE first, and then migrated to the cathodic side. The migrated anions were then reduced and deposited once they reached the cathodic area, which had sufficient potential to reduce the AuCl4− anions. The anions were consumed before reaching the cathodic pole; therefore, this resulted in a ring-shaped deposition of gold NPs. In addition, the ions initially present around the cathodic pole migrated away immediately after application of the external potential, which made electrodeposition at the cathodic pole much more difficult. On the contrary, when cationic metal ions were used, the metal cations contacted the cathodic pole of the BPE first, so that electrodeposition occurred entirely at the cathodic side. COMSOL simulation also supported that the selection of anionic or cationic metal sources determines the shape of the metal deposition on the BPE surface. We have also designed the electrophoresis-assisted synthesis of densely packed metal nanorods in conjunction with bipolar electrochemistry (Figure 10).15 Porous anodic aluminum oxide

Conducting Polymer Fiber/Film Growth by AC-Bipolar Electroolymeriation

We have demonstrated that AC bipolar electropolymerization of EDOT as a monomer resulted in microfiber formation of the corresponding PEDOT from both terminals of the BPEs.51 Figure 9a shows that Au wires were placed in between Pt driving electrodes in an electrolytic cell containing EDOT, BQ, and tetrabutylammonium perchlorate (Bu4NClO4). An AC voltage was applied between the driving electrodes to activate the Au wires as BPEs, which induced the electropolymerization of EDOT and sacrificial reduction of BQ simultaneously at both poles. As a consequence of such AC electrolysis, both terminals of the Au wires were modified with PEDOT; however, the PEDOT microfibers (5 μm diameter) propagated dendritically from the tips of the BPEs parallel to the direction of the electric field. The two Au wires were then finally connected with the PEDOT fibers (1 mm gap closed by PEDOT microfibers after 90 s) (Figure 9b). Optimization of the electrolytic conditions, such as the applied voltage, frequency, solvent, and electrolyte concentration, revealed that the electrophoretic effect has a key role in the formation of the PEDOT fiber structure. Electrogenerated PEDOT or corresponding oligomers should have cationic charges in the polymer chains (p-doped state) that move toward the driving cathode along with the direction of the electric field, which results in the anisotropic deposition of the PEDOT fibers. This methodology is a quite a new approach to obtain conducting polymer fibers using an electric field as a template. We have also demonstrated the dimensional control of PEDOT fibers by employing the microspace around the BPE terminal.52 Under a condition that limits the supply of EDOT monomer to the active BPE tip, quasi-one-dimensional PEDOT fibers were obtained, which are potentially useful for circuit wiring applications. During optimization of the parameters for electropolymerization (concentration of monomers and electrolyte, applied frequency), the in-plane growth of PEDOT thin films (∼3 μm thick) on a substrate material (glass, plastics, etc.) was inadvertently discovered (Figure 9c).53 Such AC-bipolar electropolymerization employed a sacrificial cathodic reaction of BQ to HQ. We next reported the paired electrosynthesis using oxidative electropolymerization and the reduction of metal ions to afford metal NPs simultaneously. 5 4 The replacement of BQ with hexachloroplatinic(IV) acid (H2PtCl6) did not significantly affect the propagation behavior of the PEDOT fibers; however, well-dispersed Pt NPs (5−10 nm diameter) were incorporated within the PEDOT fibers, as evidenced by field emission (FE)SEM and transmission electron microscopy (TEM) analyses (Figure 9d).

Figure 10. Schematic illustration of the electrochemical setup for templated bipolar electrolysis, including the electroreduction of Co2+ and Pt2+, and the sacrificial oxidation of HQ. SEM images of the electrodeposited Co and Pt nanorods prepared under optimized conditions. Adapted with permission from ref 15. Copyright 2018 Royal Society of Chemistry.

(AAO) membranes are a popular template for the fabrication of 1D nanostructures by electrodeposition. In AAO membranes, however, hollow materials tend to be obtained due to the limited supply of ions or monomers, which results in mechanical weakness of the 1D structures. We reported the formation of dense Co and Pt nanorods onto a bipolar electrode surface fixed with an AAO membrane under the application of 20 V/cm between driving electrodes. The resultant nanorods maintained their structure, even after removal of the template, which indicates the mechanical strength was also maintained. Control experiments under an electric field of 5 V/cm or with a lower concentration of metal ions gave collapsed structures after removal of the template. H



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These results clearly suggest the importance of the synergetic effect of electrolysis and electrophoresis in bipolar electrochemistry for dense 1D nanomaterial synthesis. In this section, the synergetic effects of electrochemical reactions and electrophoresis in bipolar electrochemical systems have been described. In a conventional electrochemical system with a high concentration of electrolyte, electrophoresis is not usually considered since the electric field is barely formed in the bulk solution. Electrophoresis has been widely used as an analytical method, but not frequently used for electrochemical reactions. Thus, a synergetic effect of these two events represents a highly unique aspect of bipolar electrochemistry. Since the electrochemical reaction is heterogeneous, the mass transfer of substrates can be crucial, one can imagine that boosting the ion migration by electrophoresis can significantly change the outcome. Examples shown here clearly demonstrate that the careful design of reaction systems realizes unique or highly efficient modification bipolar electrochemistry.

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Article

AUTHOR INFORMATION

Corresponding Author

*Phone: +81-45-924-5407. Fax: +81-45-924-5407. E-mail: [email protected]. ORCID

Shinsuke Inagi: 0000-0002-9867-1210 Notes

The authors declare no competing financial interest. Biographies Naoki Shida received his PhD from Tokyo Institute of Technology in 2016. He then worked as a postdoctoral scholar (Research Fellowship for Young Scientists of the Japan Society for the Promotion of Science, JSPS) at both Tokyo University of Agriculture and Technology and California Institute of Technology. Afterward, he joined the group of Prof. Shinsuke Inagi as a Specially Appointed Assistant Professor in 2018. His research interest is based on electrosynthesis and covers the areas of organic chemistry, polymer synthesis, and inorganic chemistry. Yaqian Zhou received her M.S. in Polymer Chemistry and Physics from Xi’an Jiaotong University in 2018, where she worked in the lab of Prof. Demei Yu. She is currently a PhD candidate in the group of Prof. Shinsuke Inagi at Tokyo Institute of Technology. Her research interests include preparation of one-dimensional functional materials by bipolar electrolysis.

CONCLUDING REMARKS AND PERSPECTIVE

The revival of bipolar electrochemistry has resulted in a renaissance that has had a strong impact on a wide variety of fields, such as materials chemistry and organic electrochemistry. In this Account, recent progress in bipolar electrochemistry for the synthesis of functional materials has been summarized. The wireless nature of BPEs was utilized for symmetry-breaking to produce anisotropic materials based on the site-selective modification of conductive objects by electrodeposition and electropolymerization. Potential gradients on a BPE interface have been successfully used as controllable templates to form gradient materials, which are potentially applicable for high throughput analytical equipment or as biomimetic materials. The electric field necessary to drive BPEs is also potentially useful to induce the directed migration of charged species. The synergetic effects of electrophoresis and electrolysis were also successfully demonstrated to obtain various functional materials. These features of bipolar electrochemistry and the various combinations of techniques have the potential to change the methodologies of material synthesis. One clear direction of this technology for the next step is its application to the controlled nanomaterial synthesis, while one can imagine that applying enough voltage to nano-objects is a great challenge. One way to “activate” such a small object as a BPE without applying too large external electric field is to sequentially activate the small objects by the connection with larger BPEs, as demonstrated in our previous study.51 The combination of electric fields and light-driven reactions well matches for the modification of nanosized semiconducting TiO2 particles as reported by Kuhn and co-workers.56 Another direction of this field is to apply the bipolar chemistry for the anisotropic modification of nonconductive materials, which will significantly expand the possibility of this technology. Furthermore, the fundamental principle of bipolar electrochemistry infers that very small amounts of supporting electrolyte are necessary for an electrode system, which is expected to lead new methods of sustainable organic electrosynthesis.57,58

Shinsuke Inagi received his PhD from Kyoto University in 2007. After a JSPS postdoctoral research fellowship at Kyoto University, he joined the group of Prof. Toshio Fuchigami as an Assistant Professor at Tokyo Institute of Technology in 2007. He was promoted to Lecturer in 2011, then to Associate Professor in 2015. He is concurrently a PRESTO researcher of Japan Science and Technology Agency (JST) from 2018. His current research interests include electrosynthesis of functional materials by means of bipolar electrochemistry. He is the recipient of the Tajima Prize of the International Society of Electrochemistry (ISE) in 2019.

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ACKNOWLEDGMENTS This research was supported by a Kakenhi Grant-in-Aid (No. JP17H03095) from the Japan Society for the Promotion of Science (JSPS), and JST, PRESTO (No. JPMJPR18T3).

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Bipolar Electrochemistry: A Powerful Tool for Electrifying Functional

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