Bipolar electrochemistry—A wireless approach for electrode reactions

Accepted Manuscript

Bipolar Electrochemistry – A Wireless Approach for Electrode Reactions Line Koefoed , Steen U. Pedersen , Kim Daasbjerg PII: DOI: Reference:

S2451-9103(16)30047-3 10.1016/j.coelec.2017.02.001 COELEC 23

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Current Opinion in Electrochemistry

Received date: Revised date: Accepted date:

23 December 2016 2 February 2017 2 February 2017

Please cite this article as: Line Koefoed , Steen U. Pedersen , Kim Daasbjerg , Bipolar Electrochemistry – A Wireless Approach for Electrode Reactions, Current Opinion in Electrochemistry (2017), doi: 10.1016/j.coelec.2017.02.001

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Highlights

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Wireless electrochemistry may be performed on particles and nanoobjects Sensing of molecules and evaluation of electrocatalytic processes are possible Electrografting and electrodeposition in patterns and with gradients are easily obtained

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Bipolar Electrochemistry – A Wireless Approach for Electrode Reactions Line Koefoed,† Steen U. Pedersen,†* Kim Daasbjerg†,‡*

‡

Department of Chemistry and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark [email protected], *[email protected]

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Carbon Dioxide Activation Center, Aarhus University, Gustav Wieds Vej 14, 8000 Aarhus C, Denmark *[email protected]

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ABSTRACT

Bipolar electrochemistry involving two feeder electrodes and a conducting object (the bipolar electrode) in an electrolytic solution has attracted a renewed interest in the last two decades due to

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its use within several fields ranging from materials science to sensing and beyond. The potential difference between the electrolyte and the bipolar electrode may drive opposite directed faradaic

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reactions (reduction/oxidation) at the cathodic and anodic sides of the bipolar electrode. The potential difference between the solution and the bipolar electrode is highest at the extremities,

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which means that the potential difference for driving the faradaic processes is always largest here.

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This wireless technique generates an asymmetric reactivity at the surface of a conducting object allowing for modification of more delicate materials such as graphene or for simultaneous

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modification of an array of electrodes. In this review, the recent applications of bipolar electrochemistry are presented focusing on sensing, electrodeposition, electrografting, and the use of graphene as a bipolar electrode.

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ATRP: atom transfer radical polymerization, bpy: 2,2ˈ-bipyridine, BE: bipolar electrode, CNT: carbon nanotube, ECL: electrogenerated chemiluminescence, ITO: indium tin oxide, MOF: metal-organic framework, NIPAM: N-isopropylacrylamide, PEDOT: poly(3,4-ethylenedioxythiophene), rGO: reduced graphene oxide, TPrA: tripropylamine

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KEYWORDS Bipolar Electrode, Electrodeposition, Graphene, Wireless Electrochemistry

INTRODUCTION Bipolar electrochemistry is a well-established technique, which has been known for many years.[1]

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In the late 1960s, Fleischmann and co-workers described fluidized bed electrodes, where electrochemical reactions at discrete conductive particles took place, when a voltage was applied between two feeder electrodes.[2] Since these early studies, bipolar fluidized bed electrodes have

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been applied to improve the efficiency of electrosyntheses,[3] photoelectrochemical cells,[4] and batteries.[5] Recently, bipolar electrochemistry has attracted renewed interest in fields ranging from site-selective electrodeposition[6] to electrogenerated chemiluminescence (ECL).[7] Today a bipolar electrode (BE) refers to any conducting object exhibiting oxidation and

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reduction reactions at the same time, i.e. it acts simultaneously as an anode and a cathode.[8] This is

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a distinct difference from a standard two- or three-electrode setup, where the cathode and the anode are physically separated. Over the last two decades bipolar electrochemistry has experienced an

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exponential growth in the number of users. In 2016 an entire issue of ChemElectroChem was devoted exclusively to bipolar electrochemistry.[9] Various reviews on bipolar electrochemistry and

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the underlying principles are already available.[1,10] The objective of this review is, first of all, to

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give an overview of some of the most recent results obtained within sensing, electrografting, and electrodeposition using bipolar electrochemistry.

SENSING The basic operating principle of bipolar electrochemistry sensing relies on electrical coupling between the sensing and the reporting poles. This is valid because the rates for the two opposite 3

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faradaic processes at the two poles are exactly equal. The magnitude of the current running through a BE can be indirectly determined by ECL as a reporting mechanism. In ECL electrochemically generated intermediates undergo a highly exergonic reaction to produce an electronically excited state, which emits light.[11] A system, which is often applied, is the oxidation of [Ru(bpy)3]2+ in the

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presence of tripropylamine (TPrA) as a co-reactant, which results in red light emission (see Figure 1). Luminol-based ECL has also been investigated,[12] and both luminophores ([Ru(bpy)3]2+ and luminol) were introduced for simultaneous ECL emission at distinct wave lengths resulting in blue and red light, respectively.[13] ECL in combination with bipolar electrochemistry was used to cancer

biomarkers,[14]

but

also

in

connection

with

chemomechanical

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detect

electrochemiluminescent motion (see Figure 1).[15,16]

Recently, ECL was generated from a suspension of multiwalled carbon nanotubes (CNTs), where each tube acted as an individual nano-emitter.[17] Usually, ECL is confined to the electrode

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surface (2D), but this intrinsic limitation was overcome by generating ECL at nano-emitters in

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solution (3D). This approach adds a new spatial dimension by switching from a surface-limited process to 3D electrogenerated light emission.[17] In another study ECL was used for sensing using

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a closed bipolar cell setup.[18] In addition to ECL, other techniques have been implemented for sensing, including fluorescence.[19,20] Furthermore, vinyl substituted [Ru(bpy)3]2+ was recently

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copolymerized with N-isopropylacrylamide (NIPAM) by indirect reductive polymerization.

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Afterwards ECL/fluorescence microscopy was used to map the polyNIPAM on the BE, and the intensity changes in the ECL was correlated with the swollen/collapsed state of the polyNIPAM.[21]

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Figure 1. Illustration of a bipolar setup showing at the same time some of the applications of bipolar electrochemistry, i.e. electrodeposition, sensing, and motion.

ELECTROGRAFTING

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Electrografting of aryldiazonium salts has recently been achieved by bipolar electrochemistry. In

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2013 Zigah and co-workers for the first time reported on the bipolar electrografting of aryldiazonium salts, more specifically 4-nitrobenzenediazonium and 4-carboxybenzenediazonium

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(see Figure 1) on a glassy carbon bead.[22] This work was further extended to include bipolar

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electrografting of the inner wall of CNTs.[23] Bipolar electrografting has also been employed to create initiator films for atom transfer radical polymerization (ATRP). Inagi and co-workers used

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indium tin oxide (ITO) as BE for the grafting of 4-(2-hydroxyethyl)benzenediazonium.[24] To further expand the use of aryldiazonium salts for bipolar electrografting, we designed a

bifunctional aryldiazonium salt for single step double deposition.[6] The novel bipolar electrografting methodology presented uses a single grafting agent, where concomitant deposition of two chemically different organic layers was achieved via simultaneous reductive and oxidative

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electrografting.[6] Furthermore, it has been shown that it is possible to electrograft not only aryldiazonium salts, but also diaryliodonium and triarylsulfonium salts.[25]

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ELECTRODEPOSITION A common use of bipolar electrochemistry is within the field of electrodeposition and/or electrodissolution of metals and organic molecules. Electrodeposition on BE beads to form asymmetrically modified particles is also possible with this technique due to its inherent wireless

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property. Reduction of a metal salt on the cathodic side of a bead afforded an asymmetrical modification of the substrate.[26] In a more recent study, particles with sophisticated surface patterns were obtained.[27] This was further explored for the deposition of inorganic and polymeric layers[28,29] as well as the synthesis and deposition of metal-organic frameworks.[30]

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Several metal compositional gradients were synthesized by bipolar electrodeposition, including

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Ni/Cu,[31,32] Ni/Mo/Co,[33] and Cu/Ni/Zn[34]. In all cases, the metal electrodeposition was facilitated from a solution of the corresponding salt such as CuSO4, NiSO4, and CoNO3‧ 6H2O.

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Bipolar electrochemistry is also useful for the formation of Au nanoparticles.[35,36]

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The possibility to use bipolar electrochemistry in the field of polymer science was extensively investigated by Inagi, Fuchigami, and co-workers with particular focus on ATRP.[37] Bipolar

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electrogeneration of CuI was used to vary the ratio of [CuI]/[CuII] along the BE.[38] In this manner the polymerization rate gradually changed with position according to the reaction mechanism of ATRP yielding 3D polymer brush gradients. In another study Kuhn and co-workers performed electropolymerization using bipolar electrochemistry for deposition of polypyrrole,[29] Also, they obtained deposits of Au at the cathodic end of a carbon microfiber BE along with a concomitant electropolymerization of polythiophene at the anodic end of the BE.[39] Inagi and co-workers

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explored the influence of switching the applied potential by applying alternating current instead of direct current.[40] In this case conducting wires of poly(3,4-ethylenedioxythiophenone) fibers were successfully used to connect Au BEs of different sizes. Application of alternating currents for indirect oxidation of aluminum was investigated by Asoh

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et al.[41] In another indirect approach it was exploited that pH may be varied around the poles of BEs through water oxidation or reduction to induce the deposition of materials.[28] Likewise, redox catalysts may be used to drive polymerizations, where the generated polymers are deposited.[21]

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GRAPHENE

One of the main advantages of bipolar electrochemistry is the wireless feature, which is particularly interesting for modification of small or delicate objects such as 2D materials. In a recent study by

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Zuccaro et al.[42] graphene was transferred to Si/SiO2 and employed as a BE to deposit Cu at one

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end and Au at the other. In another work, a graphene BE was employed to study the oxygen reduction at liquid-liquid interfaces.[43] Bipolar electrochemistry was also used to produce reduced

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graphene oxide from graphite BEs.[44]

The wireless feature of bipolar electrochemistry allows for simultaneous control over numerous

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electrodes arranged in an array by exerting potential control over the solution rather than the

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individual electrodes. In this manner bipolar electrochemistry was used to electrograft 4-bromobenzene on an array of 5  4 small graphene BEs (1 × 1 mm2).[45] This suggested that this technique may find use in the modification of graphene flakes in suspension. Table 1 provides an overview of the most recent developments and uses of bipolar electrochemistry in the three specific fields covered herein, i.e. sensing, electrografting, and electrodeposition using bipolar electrochemistry. Hence, this overview does not include important

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fields such as scanning electrochemical microscopy,[46] microwell arrays,[47] long-term water electrolysis,[48] and corrosion studies.[49] Table 1. Literature Overview of Bipolar Electrochemistry Categorized by Electrode Materials and Their Application. Sensing

Electrografting

Carbon materials

Luminol ECL [12] Fluorescence [20] Ru(bpy)32+ ECL [21]

CNTs

Ru(bpy)32+ ECL [34]

Pt Zn

Aryldiazonium salt [6,22]

Electropolymerization (polypyrrole) [29] Au and Ag [26,27] Pt [27]

Aryldiazonium salt [45]

Cu and Au [42]

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Ni Paper-basedb

Ni and Cu [31]

Fluorescence [19] Luminol ECL [16]

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Graphite

ITO

Luminol+Ru(bpy)32+ ECL [13] Ru(bpy)32+ ECL[15,17] Luminol ECL [16]

Ru(bpy)32+ ECL [18]

Electrocatalyst screening [20]

Aryldiazonium salt [23]

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DVD

Au nanovoidsc [36] Electropolymerization (PEDOT [40] and polypyrrole [29]) Au nanoparticlesd [35] Cu, Ni, and Zn [34] Silica, titanate, and electrophoretic paint [28] Pt [26] Au [26,39] Electropolymerization (polythiophene [39] and polyNIPAM [21]) Cu, Ni, and Zn [32]

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Aryldiazonium, iodonium, and sulfonium salts [25]

Graphene

Other Uses Indirect oxidation [41]

Au

Fluorine-doped tin oxidea Glassy carbon

Electrodeposition

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Bipolar Electrode Material Al

Electrocatalyst screening [31] Electrocatalyst screening [33] Motion [15,16] Reduction of Cu(II) [38]

Oxygen reduction [43] Synthesis of rGO [44]

Aryldiazonium salt [24] Electrophoretic paint [28]

Ru(bpy)32+ ECL [14] Electrophoretic paint [28] Synthesis of MOFs [30]

a

With Cr microbands at the anodic side and bi- or trimetallic systems involving Co, Fe, Ni, Mo, and W at the cathodic side. bThe actual BE is carbon ink with or without multiwalled CNTs on top. cThe Au BE was covered by polystyrene nanospheres prior to the experiments. dThiolated Au film.

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CONCLUDING REMARKS Bipolar electrochemistry is a very versatile technique with use in many areas ranging from sensing to electrodeposition and electrografting. The inherent advantages of bipolar electrochemistry are the wireless nature and the occurrence of concomitant reduction/oxidation processes at opposite ends.

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Small objects and ensembles or arrays of bipolar electrodes can be modified in parallel without a direct electrical connection. Furthermore, the technique allows formation of gradients of electrodeposited materials as well as providing a means for site-selective deposition, even on small

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particles.

ACKNOWLEDGMENT

The Danish Council for Strategic Research (DA-GATE DSF 12-131827) and the Innovation Fund Denmark (NIAGRA 58-2012-4) are acknowledged for financial support. We appreciate the

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