Electrochemistry DOI: 10.1002/anie.201300947 Bipolar .
.AngewandteReviewsR. M. Crooks et al.DOI: 10.1002/anie.201300947ElectrochemistryBipolar ElectrochemistryStephen E. Fosdick, Kyle N. Knust, Karen Scida, and Richard M. Crooks*Keywords:electrochemistry · materials science ·sensorsIn memory of Su-Moon ParkAngewandteChemie10438 www.angewandte.org 2013 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimAngew. Chem. Int. Ed. 2013, 52, 10438 – 10456
AngewandteChemieBipolar ElectrochemistryA bipolar electrode (BPE) is an electrically conductive material thatpromotes electrochemical reactions at its extremities (poles) even in theabsence of a direct ohmic contact. More specifically, when sufficientvoltage is applied to an electrolyte solution in which a BPE isimmersed, the potential difference between the BPE and the solutiondrives oxidation and reduction reactions. Because no direct electricalconnection is required to activate redox reactions, large arrays ofelectrodes can be controlled with just a single DC power supply oreven a battery. The wireless aspect of BPEs also makes it possible toelectrosynthesize and screen novel materials for a wide variety of applications. Finally, bipolar electrochemistry enables mobile electrodes,dubbed microswimmers, that are able to move freely in solution.1. IntroductionThe objective of this Review is to introduce biological andphysical scientists to the concept of bipolar electrochemistrywith a view toward expanding its scope in new and interestingdirections. This is a worthwhile goal, because the broadadoption of electrochemical methods by those working inother subdisciplines of science in recent years has underscored some shortcomings of existing techniques, apparatuses,and theory. These include the difficulty of making directelectrical contact to nanoscale electrodes, maintaining controlover and reading out very large arrays of electrodes simultaneously, controlling electrodes that are mobile in solution,maintaining a non-uniform potential difference over thesurface of an electrode, and using electrodes to control localsolution potentials. In this Review, we will show that all ofthese aspects of electrochemistry can be addressed, at leastpartially, using bipolar electrodes (BPEs).A more detailed summary of the principles underpinningbipolar electrochemistry will be presented in Section 2, but itis instructive to provide a very brief overview now. Scheme 1Scheme 1.Angew. Chem. Int. Ed. 2013, 52, 10438 – 10456From the Contents1. Introduction104392. Fundamentals of BipolarElectrochemistry104403. Materials Preparation andFabrication104444. Sensing and ScreeningApplications104465. Bipolar Electrode Focusing104496. Microswimmers104527. Summary and Outlook10454shows a typical experimental configuration used for carryingout bipolar electrochemistry. Here, the driving electrodesapply a uniform electric field across the electrolyte solution,and the resulting faradaic electrochemical reactions at theBPE are shown occurring at the anodic (blue arrow) andcathodic (red arrow) poles of the BPE. As discussed later, theinterfacial potential difference between the solution and BPEis highest at the ends of the electrode, so faradaic processesare always observed there first.Consider the following simple thought experiment basedon the electrochemical cell shown in Scheme 1, a platinumBPE, and an aqueous solution containing a dilute, inertelectrolyte. When the power supply is turned on to, forexample, 1 V, no faradaic reactions are observed at either thedriving electrodes or the BPE. However, at a critical voltagethat depends on a number of experimental factors, bubblesare observed at the poles of the BPE. Analysis would showthat those at the cathodic pole are hydrogen and those at theanodic pole are oxygen. In other words, even though the BPEis an equipotential surface (or nearly so), the electrolysis ofwater is occurring at its two poles. Importantly, charge mustbe conserved at the BPE, and therefore the rates of formationof 1 2 O2 and H2 are the same. Faradaic reactions might alsooccur at the driving electrodes, but although this is usuallya nuisance it does not directly affect the BPE. We wish toemphasize that this is an oversimplified version of bipolarelectrochemistry, but more details and experimental nuanceswill be discussed later.The main focus of the present article is on interestingfundamentals and applications of bipolar electrochemistry[*] S. E. Fosdick, K. N. Knust, K. Scida, Prof. R. M. CrooksDepartment of Chemistry and Biochemistry and the Center for Nanoand Molecular Science and TechnologyThe University of Texas at Austin105 E. 24th St., Stop A5300, Austin, TX 78712-1224 (USA)E-mail: crooks@cm.utexas.eduHomepage: http://rcrooks.cm.utexas.edu/research/index.html 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim10439
.AngewandteReviewsR. M. Crooks et al.that have emerged since about the year 2000, but it isimportant to note that bipolar electrochemistry has beenaround for many years. Beginning in the 1960s, Fleischmann,Goodridge, Wright, and co-workers described fluidized bedelectrodes, where a voltage applied between two drivingelectrodes enables electrochemical reactions at discreteconductive particles.[1–6] Since these early studies, bipolarfluidized bed electrodes have been used in applicationsranging from improving the efficiency of electrosyntheses,[7–9]photoelectrochemical cells,[10, 11] and even batteries.[12] Bipolarplate technology is critical for polymeric electrolyte membrane (PEM) fuel cells where the plates form a series ofBPEs.[13, 14] Additionally, neuronal behavior can also bemimicked using short-circuited microbands which act asa BPE, forming logic gates.[15]During the past 15 years or so, many interesting bipolarelectrochemical experiments have been presented in theliterature, and some of these are discussed in a recentlypublished review article.[124] Our own group has exploreda number of fundamental aspects and applications of bipolarelectrochemistry. For example, Figure 1 a is an optical micrograph of a microfabricated BPE array consisting of 1000electrodes.[16] When a sufficiently large driving voltage isapplied to the array, electrogenerated chemiluminescence(ECL) is produced at the anodic pole of each BPE (Figure 1 b). The important result of this experiment is that thewireless capabilities of BPEs allow arbitrarily large arrays ofelectrodes to be powered in a very simple setup.A number of groups have shown that bipolar electrochemical reactions can be used to induce motion inobjects.[17–20] An illustrative example from Kuhn s group isshown in Figure 2.[21–23] Here, bubbles produced by electrochemical reactions (i.e., H2 or O2) generate sufficient10440 www.angewandte.orgpropulsion to induce the movement of small BPEs. Compositional or chemical gradients can be synthesized on BPEs, ashas been elegantly demonstrated by Inagi, Fuchigami, and coworkers, who carried out position-dependent doping ofelectroactive polymers (Figure 3).[24, 25] Our group has alsoinvestigated BPEs as a means of enriching and separatingcharged species electrokinetically. In Figure 4, a single AuBPE is used to locally enrich several different chargedmarkers in a single microchannel.[26] This approach makes itpossible to concentrate analytes up to 500 000-fold in a highlycontrolled zone.[27–33] Finally, arrays of BPEs can be used fora variety of sensing and screening applications,[16, 34–44] whichwe will discuss in Section 4. Further explanation and discussion of these types of bipolar electrochemical experimentsare included in later sections.2. Fundamentals of Bipolar Electrochemistry2.1. Relationship Between Driving Electrodes and a BPEA key point for understanding wireless bipolar electrochemistry is that the poles of a BPE are oriented in theopposite polarity of the driving electrodes (Scheme 2 a). Thiswas clearly demonstrated by Manz and co-workers whoplaced a Pt wire in a weighing boat filled with a pH indicatorsolution.[45] As shown in Figure 5 a, application of 30 Vbetween the red and blue wires (driving electrodes) causesthe pH of the solution at the positive driving electrode(anode, red) to decrease (orange color) due to wateroxidation and concomitant production of H . Likewise,water reduction at the negative driving electrode (cathode,blue) leads to formation of OH , an increase in pH, and theRichard M. Crooks is presently the Robert A.Welch Chair in Materials Chemistry at TheUniversity of Texas at Austin. His scientificinterests include electrochemistry, chemicalsensing, and catalysis. He has publishednearly 250 research papers and is the recipient of several awards including the CarlWagner Memorial Award of the Electrochemical Society and the American Chemical Society Electrochemistry Award.Kyle N. Knust earned his B.S. in chemistryfrom the University of Evansville in 2009.He also performed research in the lab ofProf. Dennis G. Peters at Indiana University.He is currently a graduate student in thegroup of Prof. Richard M. Crooks at TheUniversity of Texas at Austin with researchfocused on bipolar electrochemistry and thedevelopment of lab-on-a-chip technologies.Stephen E. Fosdick received his B.S. inChemistry from Iowa State University in2009, where he worked in the lab of Prof.Emily A. Smith. He is currently a graduatestudent in the group of Prof. Richard M.Crooks, where his current research focuseson the development of electrocatalyst screening platforms using bipolar electrochemistry.Karen Scida studied chemistry at the University of Texas San Antonio where she workedin the lab of Prof. Carlos Garcia. She iscurrently a graduate student in the group ofProf. Richard M. Crooks at the University ofTexas at Austin. Her scientific interestsinclude developing paper-based sensors aspoint-of-care technology and the design ofbipolar electrodes for their analytical application in microfluidics. 2013 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimAngew. Chem. Int. Ed. 2013, 52, 10438 – 10456
AngewandteChemieBipolar ElectrochemistryFigure 1. a) Optical micrograph of an array of 1000 Au BPEs. Each BPEis 500 mm long and 50 mm wide, and the electrodes are spaced by50 mm vertically and 200 mm horizontally. b) Luminescence micrographshowing the ECL response of 5 mm [Ru(bpy)3]2 and 25 mm tri-npropylamine (TPrA) in 0.10 m phosphate buffer (pH 6.9) whenEtot 85.0 V. c) Photograph of the bipolar electrochemical cell showingthe BPE array immersed in electrolyte solution contained withina poly(dimethylsiloxane) (PDMS) reservoir. The two alligator clips areattached to stainless steel driving electrodes that span the array.d) Plot of ECL intensity vs. pixel number showing the uniformity of theECL response over the rows of BPEs indicated by the white arrows in(b). The ECL intensity varies by no more than 10 %, indicating thatDEelec is uniform over the entire array. Reprinted (adapted) withpermission from Ref. [16]. Copyright 2008 American Chemical Society.Figure 2. a) Schematic illustration of bipolar electrochemical watersplitting on a conductive microbead. b) Optical micrograph of a 1 mmdiameter stainless steel bead experiencing an electric field of1.6 V mm 1 in 24 mm H2SO4. The cathodic pole is located on the leftside of the bead and the scale bar indicates 250 mm. c) Motion ofa 285 mm diameter glassy carbon microbead in a microchannelexposed to a 5.3 V mm 1 electric field in 7 mm H2SO4 (scalebar 100 mm). d) Scheme showing proton reduction and hydroquinone oxidation on a BPE sphere. e) Motion of a 1 mm diameterstainless steel bead exposed to a 1.3 V mm 1 electric field in 24 mmHCl and 48 mm hydroquinone (scale bar 1 mm). f) Motion ofa 275 mm diameter glassy carbon microbead in a microchannel witha 4.3 V mm 1 electric field in 7 mm HCl and 14 mm hydroquinone(scale bar 100 mm). Reprinted by permission from Macmillan Publishers Ltd, Nature Communications, Ref. [21], copyright 2011.Scheme 2.Angew. Chem. Int. Ed. 2013, 52, 10438 – 10456observed purple color of the indicator. In Figure 5 b, a Ushaped Pt wire (BPE) has been inserted between the drivingelectrodes, and the change in color of the indicator demonstrates that water electrolysis occurs at its ends, even though itis not in direct electrical contact with the power supply.Moreover, the relative positions of the colors reveal the polesof the BPE are oriented in the opposite polarity of the drivingelectrodes. 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheimwww.angewandte.org10441
.AngewandteReviewsR. M. Crooks et al.2.2. Potential Differences in Bipolar ElectrochemistryFigure 3. Gradient doping of conducting polymers. a) Poly(3-methylthiophene) (P3MT), b) poly(3,4-ethylenedioxythiophene) (PEDOT), andc) poly(aniline) (PANI). Adapted with permission from Ref. [25]. Copyright 2011 American Chemical Society.Figure 4. a) Optical micrograph of a 500 mm long Au BPE in a microfluidic channel. b) Fluorescence micrograph demonstrating simultaneous enrichment and separation of the anionic tracers za-s-indacene-2,6-disulfonic acid(BODIPY2 ) and 8-methoxypyrene-1,3,6-trisulfonic acid (MPTS3 ) 200 safter application of Etot 40 V. The white dashed lines indicate theedges of the BPE, and the gray dotted lines indicate the walls of themicrochannel. (c) Fluorescence micrographs showing the enrichmentand separation of MPTS3 and 1,3,6,8-pyrene tetrasulfonic acid (PTS4 )400 s after the application of Etot 60 V. Adapted with permission fromRef. [26]. Copyright 2009 American Chemical Society.Figure 5. a) A plastic weighing boat outfitted with two Pt drivingelectrodes embedded in its walls and filled with a universal pHindicator solution. 30 V was applied between the two driving electrodes and the indicator solution shows the changes in local pH due tothe water oxidation (left) and water reduction (right). b) When a Ushaped Pt wire is added to the weighing boat it acts as a BPE, andfaradaic reactions occur at its surface. The positive driving electrode(red) induces a cathode at the nearest pole of the BPE and thenegative driving electrode (blue) induces an anode. Adapted withpermission from Ref. [45]. Copyright 2001 American Chemical Society.10442 www.angewandte.orgCells for carrying out bipolar electrochemistry can beconfigured to accommodate a range of applications frompreparative-scale electrosynthesis to microanalysis. For example, Scheme 2 a is an illustration showing a simple microscalecell design used by our group. In this case, a BPE, or an arrayconsisting of multiple BPEs, is embedded within a microfluidicchannel that has a height of tens of microns, a width ofhundreds of microns, and a length of perhaps a centimeter.Potential contaminants electrogenerated at the driving electrodes do not interfere with the BPE in this design, because ofthe macroscale length of the channel.The voltage applied between the two driving electrodes(Etot) results in an electric field in solution that causes theBPE to float to an equilibrium potential (Eelec) that dependson its position in the field and the composition of theelectrolyte solution. Because the electrode is a conductor, itspotential (Eelec) is the same (or nearly so) everywhere on itssurface. However, the interfacial potential difference betweenthe BPE and the solution varies along its length due to thepresence of an electric field in solution. It is these anodic andcathodic overpotentials,[36] han and hcat, respectively, that driveelectrochemical reactions at the poles of a BPE. Scheme 2 bshows that the magnitudes of the overpotentials depend onjust two experimental variables: the magnitude of Etot and thelength of the BPE. The location on the BPE that defines theboundary between its two poles, and therefore itself has zerooverpotential with respect to the solution, is defined as x0.Although x0 is represented as being at the center of the BPEin Scheme 2 b, its actual location depends on the nature of thefaradaic processes occurring at the poles.[36]As mentioned earlier, the magnitudes of the overpotentials vary along the length of the BPEs, with the highestoverpotentials occurring at its extremities. This is in contrastto the working electrode in a traditional three-electrode cellconfiguration, wherein the interfacial potential difference isgenerally considered to be uniform (although this depends onhow the cell is designed, the placement of the three electrodesrelative to one another, and the resistivity of the electrolytesolution).[46] In either case, however, it is this interfacialpotential difference that drives electrochemical reactions.[47]Importantly, the non-uniformity of the interfacial potentialdifferences along the length of a BPE can be quite beneficial.For example, as we will show later, it can be used tosimultaneously drive reactions at different rates or tosynthesize materials and thin films having a graded composition or density.2.3. Controlling the Electric Field and Current PathsThe electric field that powers bipolar electrochemistry istypically applied by a pair (or more) of driving electrodes,which can be metallic (e.g., Au, Ag, Pt, or stainless steel),carbonaceous (e.g., glassy carbon or graphite), or nonpolarizable (e.g., Ag/AgCl reference electrode). The nature of theelectric field formed between the driving electrodes dependson the cell geometry and the conductivity of the electrolyte 2013 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimAngew. Chem. Int. Ed. 2013, 52, 10438 – 10456
AngewandteChemieBipolar Electrochemistrysolution. In some cases a linear electric field is generated byrestricting the cross-sectional area of the solution between thedriving electrodes, thereby increasing its resistance. This canbe achieved by embedding the BPE in a microchannel havinga small cross-sectional area (e.g., Scheme 2 a) or by limitingthe volume of electrolyte solution over a BPE in an openchannel. We,[36, 48] along with Duval and co-workers, havediscussed many of the parameters that control bipolarelectrochemical processes.[47, 49–53] As alluded to earlier, theseparameters include Etot, the distance separating the drivingelectrodes, lchannel, and the length of the BPE, lelec. The fractionof Etot that is dropped over a BPE, which we refer to as DEelec,can be estimated using Equation (1).[34, 47, 48] DEelec ¼ Etotlelec lchannelð1ÞThe value of DEelec is a critical parameter for analyzingelectrochemical processes at BPEs. The simple relationshipexpressed in Equation (1) incorporates a number of assumptions that may be possible to ignore for a particular system,but not others. For example, it assumes that an active BPEdoes not significantly affect the electric field in the solution,which is often not the case.The foregoing point can be understood in terms of theequivalent circuits shown in Scheme 3, which are reasonablyRelec, which can be achieved by lowering the concentrationof the electrolyte, substantial current flows through the BPE.This can cause either an increase or decrease in the localelectric field strength in solution, which is proportional toitot RS2 (itot is the total current flowing in the cell), thusresulting in a nonlinear electric field in the solution above theBPE. Duval and co-workers call this effect faradaic depolarization, and its magnitude depends on the strength of theelectric field, the concentration of the supporting electrolyte,and the electrochemical properties of the electroactivespecies in the system.[49–53] It is also important to note thatEquation (1) does not account for any potential dropping atthe driving electrodes. The fractional loss of Etot within theelectrochemical double layers can, under some circumstances,be substantial.[36] Other electrochemical process can alsomitigate Equation (1), but these are far more minor than thetwo discussed here and are, therefore, beyond the scope ofthis Review.2.4. Open and Closed BPEsThe majority of this Review focuses on the use of “open”BPEs, where current can flow through both the electrolyteand the BPE. As discussed in Sect
Electrochemistry DOI: 10.1002/anie.201300947 Bipolar Electrochemistry Stephen E. Fosdick, Kyle N. Knust, Karen Scida, and Richard M. Crooks* Angewandte Chemie Keywords: electrochemistry · materials science · sensors In memory of Su-Moon Park Angewandte. Reviews R. M. Crooks et al. 10438 www.angewandte.org 2013 Wiley-VCH Verlag GmbH & Co. KGaA .
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