- Email: [email protected]
In situ Confocal Microscopy of Electrochemical Generation and Collision of Emulsion Droplets in Bromide Redox System
Electrochimica Acta 252 (2017) 164–170
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Research Paper
In situ Confocal Microscopy of Electrochemical Generation and Collision of Emulsion Droplets in Bromide Redox System Dong Hyeop Hana,1, Sangmee Parkb,1, Eun Joong Kima , Taek Dong Chunga,b,c,* a
Department of Chemistry, Seoul National University, Seoul 08826, Republic of Korea Program in Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Suwon-Si, Gyeonggi-do 16229, Republic of Korea c Advanced Institutes of Convergence Technology, Suwon-Si, Gyeonggi-do 16229, Republic of Korea b
A R T I C L E I N F O
Article history: Received 24 April 2017 Received in revised form 18 August 2017 Accepted 22 August 2017 Available online 26 August 2017 Keywords: Redox flow batteries Emulsion droplet Particle impact chronoamperometry Collision Confocal microscopy
A B S T R A C T
For redox flow battery, 1-ethyl-1-methylpyrrolidinium bromide (MEPBr) is a promising brominecomplexing agent that forms insoluble organic phase of MEPBr3. A series of optical images acquired by in-situ confocal microscopy with tens of millisecond interval visualize how MEPBr3 emulsion droplets are electrochemically generated and collide with the electrode surface. Two types of electrodes, i.e. a Pt microdisk of 10 mm diameter and a 2.5 mm long Pt wire of 25 mm diameter, show clear correlation between electrochemical behavior and optical images. The droplets starts growing on the electrode surface at the potential at which oxidative faradaic current starts flowing. As the overpotential increases, the droplets become larger adhered at the electrode surface, from which small droplets start detaching at 0.925 V or higher potential. Some of the droplets leave the surface, move back and collide with the electrode surface, thus producing current spikes, which are detected by chronoamperometry simultaneously. In-situ confocal microscopy in this study confirms that the droplets are heterogeneously generated by electrochemical reaction and that individual droplets collide back onto the electrode surface, thus providing a better understanding of the phenomena that happen at redox flow battery electrode surfaces. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Emulsion droplets have been an interesting research theme for various aspects in industrial processes. What we can access to investigate droplets is mostly ensemble properties. Lack of proper analytical methods to characterize individual particles is the cause of such limited knowledge about them. In this regard, ‘particleimpact chronoamperometry’ is getting a new spotlight recently [1,2]. This technique is a simple but informative way to detect electrochemical signals originated by individual particles [1]. In the presence of particles near the electrode, chronoamperometry typically gives current spikes [3], which are current surges that happen when the particles contact the electrode surface [2]. Such current spikes contain information about size [3], concentration
* Corresponding author at: Seoul National University, Seoul 00826, Republic of Korea. E-mail addresses: [email protected], [email protected] (T.D. Chung). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.electacta.2017.08.132 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
[4], and aggregation state [5] of the particles and electron transfer associated with them [6–8]. This method is applied to study of, not only metal nanoparticles but also ‘soft particles’, such as liposomes [9–12], viruses [13], single macromolecules [14] and emulsion droplets containing redox species [15–17]. 1-ethyl-1-methylpyrrolidinium bromide (MEPBr) is one of quaternary ammonium bromides (QBr’s) widely used in redox flow batteries (RFBs)[18–21]. It captures electrochemically produced bromine (Br2), during electro-oxidation of Br, to induce the formation of insoluble organic emulsion droplets of MEPBr3. Formation of such emulsion prevents from permeation through the battery membrane and self-discharging of electrogenerated Br2, which is one of the key causes bringing about malfunction of RFB systems [22–28]. Reportedly, MEPBr3 droplets participate in Br electrooxidation rather than behaving merely as bromine complexing agents [29], making the system even more complex. The electrochemistry involving MEPBr and MEPBr3, however, remains unknown. For instance, it is uncertain whether the MEPBr3 emulsions are homogenously created in aqueous solution or heterogeneously formed on the Pt electrode. There has been few
D.H. Han et al. / Electrochimica Acta 252 (2017) 164–170
previous study on movement and behavior of electrochemically generated MEPBr3 emulsion as a function of time. The recent simulation work reported a model based on the assumption of static geometry, telling little concerning dynamic emulation. [29,31] In addition, it is still unclear what causes the characteristic spikes in chronoamperograms, requiring lack of direct experimental evidence. It is also unknown why the oxidative current of MEPBr is larger than currents from aqueous Br oxidation without MEP+. In this study, we investigate the electro-oxidation of Br in MEPBr3 droplet through comparison between electrochemical analysis and in-situ confocal microscopy. Previous reports showed collision events of particles such as nanoparticles, fluorescent beads and some soft particles through fluorescence [30,31] and plasmonic signals [32–36] that are produced when particles collide with the electrode. Unfortunately, it is difficult to directly observe the generation and behavior of particles in the solution. This led us to seek an alternative way of observing the MEPBr3 droplets, i.e. by using in-situ confocal microscopy. Droplets are predicted to be a few mm in diameter, for which confocal microscopy can provide direct optical images with sufficient resolution. The optical images can show the change in droplets at the focused range used. Using cylindrical wire electrode helps with monitoring the stereoscopic movement of droplet and ultramicroelectrode (UME) giving low background current suits for simultaneous acquisition of the optical images and transient current caused by a droplet collision moment. Therefore, in-situ confocal microscopy allows us to see directly generation, diffusion, and collision of the droplets at the electrode, and how they change as potential varies. These are expected to provide a better understanding of the contribution of MEPBr3 droplets to the oxidation of Br in redox flow batteries. 2. Experimental 2.1. Materials and reagents 1-ethyl-1-methylpyrrolidinium bromide and potassium bromide were acquired from Sigma-Aldrich. Sulfuric acid (95%) and potassium sulfate were purchased from Deajung and Alfa Aesar, respectively. All the aqueous solutions in these experiments were prepared with ultrapure deionized water produced by a Barnstead Nanopure system from Thermo Scientific. 2.2. Electrochemical measurements Cyclic voltammograms and chronoamperograms were obtained with a CHI 750 electrochemical workstation from CHI instruments. The electrochemical cell consisted of three
165
electrodes. The working electrodes were either a Pt UMEs (diameter: d = 10 mm) or a Pt microwire electrode (diameter: d = 25 mm). A silver/silver chloride (3 M potassium chloride) and a Pt wire were used as reference and counter electrode, respectively. The pH of the solutions comprising 0.5 M potassium sulfate and 1-ethyl-1-methylpyrrolidinium bromide or potassium bromide, was adjusted with sulfuric acid. 2.3. Confocal microscopy imaging Imaging was carried out with a Zeiss LSM880 confocal laser scanning microscope equipped with a laser argon-multiline for RGB and a water-immersion objective (20x, N.A. = 1.0; Zeiss). Series of images were acquired with the interval of about 15 150 ms and were sharpened to clearly see the shape and movement of the droplet using a ZEN (black edition; Zeiss) software. 3. Results and discussion The homogeneous complexation process of MEPBr3 proposed in the literature is represented as follows [29]. Br 3 þ 2e fi 3Br
ð1Þ
MEP þ þ Br 3 fi MEPBr3
ð2Þ
This suggested mechanism proposes that Br3, which is generated on the electrode surface in reaction (1), encounters the 1-ethyl-1-methylpyrrolidinium cation, MEP+, to form a complex homogeneously, MEPBr3, as described in reaction (2). This is supported by in-situ FT-IR investigations[26,27]. Multiple insoluble MEPBr3 complexes agglomerate themselves to create a droplet in the aqueous solution. When the droplet hanging around the solution happens to reach the electrode surface, additional spike current is produced in voltammograms and chronoamperograms. From the electrochemical analysis and simulation study in MEPBr3 droplet, it was predicted that Br is enriched ca. 7.5 M in the droplet and causes the spike current. [29,37]. This is a proposed scenario, although highly probable as implied by a couple of indirect observations, which has not been proved by direct imaging and thereby the respective steps of the previously mentioned mechanism remains veiled to date. To address this issue, we introduce confocal microscopy to monitor what occurs at the electrode during the electrochemical measurements. Fig. 1 shows the experimental scheme for simultaneous measurements using a potentiostat and a confocal microscope. Three electrodes, including a Pt wire or a UME as working electrodes, constitute the electrochemical system
Fig. 1. Schematic diagram of electrochemical measurement and confocal imaging. (a) Pt wire (25 mm in diameter) insulated except the part exposed to the electrolyte (ca. 2.5 mm long) (b) Pt UME (10 mm in diameter) utilized in electrochemical cell.
166
D.H. Han et al. / Electrochimica Acta 252 (2017) 164–170
Fig. 2. (a) The focus is on the uppermost surface of the cylindrical wire electrode. (b) The focus is on a plane perpendicular to the sideward electrode surface. The left schemes are the planes focused (red box) and the right is actual images at 1.0 V. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
installed on the mount of an upright type confocal microscope. The Pt wire of 25 mm in diameter provides a cylindrical electrode where its length exposed to the solution is about 2.5 mm. This wire electrode helps see the stereoscopic movement of droplets through the images in both vertical and horizontal views from the electrode surface. (Fig. 2) Its large surface area leads to correspondingly high background current in the current transient curve and the cyclic voltammogram so that it is difficult to observe distinct spikes clearly. UME is a good solution to this problem (Fig. 1b). It allows discrimination of discrete current spikes at a constant potential as to investigate whether droplet collision and current spike coincide with each other. An objective lens immersed in solution enables clean optical high-resolution images to be obtained. The dipping lens is sufficiently stable within the pH
Fig. 3. Cyclic voltammograms recorded in 72 mM MEPBr + 0.5 M K2SO4 at pH 5.1 (red line) and 72 mM KBr + 0.5 M K2SO4 solution at pH 5.1 (black line). A Pt UME was used as working electrode and scan rate is 10 mV s1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
range between 5 and 8. For this reason, the supporting electrolyte solution is used 0.5 M K2SO4 solution at pH 5.1. Figs. 3 and S1 show the effects of MEP+ to the electrochemical redox behavior of Br. In Fig. 3 the cyclic voltammogram of 72 mM aqueous KBr at Pt UME (black line) tells that sufficiently high overpotential gives rise to diffusion-controlled Br oxidation in the absence of MEP+. The faradaic current is smaller than that in the presence of MEP+ and no spike appears. A 72 mM aqueous MEPBr at pH 5.1 (red line) starts giving oxidation current at around 0.85 V and reaches diffusion-controlled state at a potential higher than 1.0 V. Overall oxidative current is larger than that of KBr solution, and current spikes randomly. At the same potential, the current during the reverse scan is much larger than that during the forward one, indicating that the redox process is not a simple diffusion system. This implies that the products of electrochemical oxidation accumulate on the electrode to accelerate further oxidation rather than blocking reactant diffusion towards the electrode. Unlike KBr, MEPBr solution undergoes reduction in the reverse scan at the Pt UME. The reduction peak would be correlate with the reduction of Br3 in droplets adsorbed on the electrode [23]. (Fig. S2) The area of the reduction peak is proportional to the total charge for oxidation. However, the charge of oxidation is 1.4 times larger than the amount of reduction charge. This tells that all of the oxidized species do not come back to the electrode. The cyclic voltammogram from the Pt wire electrode is shown in Fig. S1. Because the shape and size, thus mass transport near it, are different from those of the UME, the cyclic voltammogram from the Pt wire is not perfectly sigmoidal (black line). Still, the sharp reduction peak during reverse scan is observed in the presence of MEP+, reminiscent of voltammetric response coming from reducible species confined on the electrode surface (red line). This is similar behavior as that found from the UME. In order to further assess the fundamental electrochemical behavior of this system, we examine the Pt wire electrode system in the potential range of 0.8 to 1.0 V by confocal microscopy. (Fig. 4) Neither oxidation current nor droplet is observed at 0.8 V. (Fig. 4a) At 0.85 V, when the oxidation current begins to increase, small droplets start appearing and merging into larger ones on the electrode surface. (Fig. 4b) Since the chemically synthesized MEPBr3 droplets have brownish color [29], the droplets in the
D.H. Han et al. / Electrochimica Acta 252 (2017) 164–170
167
Fig. 4. Confocal images taken 15 s after applying constant potentials to Pt wire electrode in 72 mM MEPBr + 0.5 M K2SO4 at pH 5.1. (a) 0.8 V, (b) 0.85 V, (c) 0.925 V, (d) 1.0 V. Scale bar = 5 mm.
confocal images appear black. The droplets keep growing as the potential increases, and are still adhered to the electrode surface. At 0.925 V, small droplets are generated on the electrode surface and are released to the bulk solution. Some droplets, adsorbed on the surface of the electrode, gradually aggregate themselves to form a larger hemispherical one. Detached droplets are much smaller than the droplets still attached to the surface. (Fig. 4c) Upon higher overpotential, the size of the droplets attached to the electrode surface decreases and a greater number of droplets leave the surface to the bulk solution. On the electrode surface at 1.0 V, it is even hard to see growth of droplets. Rather, small droplets quickly emerge and some of them aggregate in the solution, not on the very electrode surface. (Fig. 4d, Video 1 and 2) The emulsion consists of extremely small droplets with a haze-like appearance. Interestingly, a few droplets hovering over the electrode surface move back to the surface. In order to understand why the droplets move back to the electrode again and collides, we compared the size of the droplets hovering over the electrode surface (Left image of Fig. 4d) with the size of those diffusing away from the electrode (Right image of Fig. 4d). 20 droplets hovering over are 3.20 0.84 mm in diameter. For 30 droplets diffusing away from the electrode surface, the diameters are 1.84 0.51 mm. The UME, which is immersed inversely in the solution, i.e. facing downward, shows no significant change in the number or shape of the spikes due to collisions of droplets. Therefore, it is improbable that weight of droplet is responsible for collision with the electrode. Confocal microscopic images at Pt wire tell that electrogenerated droplets randomly move and hit others. (Figs. S3, S4) Stochastic collisions
among droplets appear to make them drift in arbitrary directions. As a consequence, some of the droplets contact the electrode surface to produce spikes. Fig. 5 (Video 3) shows magnifications of the images of Fig. 4d, focused at 1 mm distance range from the Pt wire surface. Thus, the droplets in these images are located very close to the electrode. These droplets are not adsorbed on the electrode surface but seem to drift over it (Fig. 5a). As soon as a droplet collides with the electrode, it spreads out over the surface (Fig. 5b) and becomes flat to adhere to the surface in 0.06 s (Fig. 5c). Thereafter, the droplet on the electrode gradually fades away to become a hazy shape (Fig. 5d–f). Presumably, the droplet disintegrates in miniscule pieces. There are other examples of similar events when droplets collide with the Pt wire electrode surface. (Figs. S5–S8 and Video 4–7) The collision in confocal microscopic images is observed at potentials above 1.0 V. This is same as the potential range where the characteristic spikes appear in the corresponding cyclic voltammograms in Fig. 3. The current transient curves of a Pt UME of 10 mm diameter, which produces a lower background current than the Pt wire electrode, allow us to demonstrate that the droplet collisions cause the current spikes. Fig. 6 shows the chronoamperogram of 32 mM MEPBr diluted with 0.5 M K2SO4 at pH 5.1 and 1.0 V. Dilution of the solution makes the number of spikes small enough to compare the confocal microscopic images with individual spikes in the current transient curve, which were simultaneously acquired. The red dots in Fig. 6 indicate the droplets collision time with the electrode, which are determined
168
D.H. Han et al. / Electrochimica Acta 252 (2017) 164–170
Fig. 5. Images at the moment the droplet collides with surface of a Pt wire electrode. White arrows indicate the position of the targeted droplet. The time elapsed at 1.0 V (a) 16.96 s, (b) 16.99 s, (c) 17.02 s, (d) 17.62 s, (e) 18.04 s, (f) 18.34 s, respectively. Scale bar = 2 mm.
based on the confocal microscopic images continuously taken. In Fig. 6, successive droplet collision starting at 20.21 seconds are shown in Fig. S9 and Video 8. The elapsed times of red dots from the images coincide with the spikes on the chronoamperogram as seen in Fig. 6. Some spikes observed in the chronoamperogram, however, are invisible under the microscope. Smaller spikes following immediately after a large one, e.g. at 22.24 s, are not visible in the confocal microscopy images. For a large spike, corresponding collided droplet covers a wide, sometimes even entire, area of the electrode so that the whole image appears entirely black and it is not possible to see small spots (Fig. S10). Fig. 7 and Video 9 show how a droplet collides with the UME. The collision of droplet occurs at 20.21 s and corresponds to the red dot mark at the same time in Fig. 6. This agrees with the result observed at the Pt wire electrode shown in Fig. 5. Fig. 7a and b
shows a droplet approaching and colliding with the electrode surface, respectively. The droplet spreads out and sticks to the electrode surface and gradually fade away (Fig. 7c–f). There are other examples of droplet collision with the Pt UME in Figs. S11–13 and Videos 10 to 12. From 0.85 V to 0.9 V, where the current starts to increase on cyclic voltammogram (Figs. 3 and S1), the droplets tend to grow at the electrode heterogeneously. (Figs. S2 and S14) In this potential range, the current is larger than that of KBr solution. At 1.0 V, the droplet collides with the electrode and generates a current spike. There are some explanations about the current increase phenomenon caused by droplets. The first is that Br diffuses from the aqueous solution and is concentrated in the droplet. This is supported by the studies on current spikes using bulk electrolysis model and simulation, although they provide indirect evidences.
D.H. Han et al. / Electrochimica Acta 252 (2017) 164–170
169
collides on the electrode surface, which is reminiscent of insoluble redox film on the electrode or Hg deposited on the UME. [40–43] Overall, this issue is still open, requiring further study. Consequently, in-situ confocal microscopy allows us to see how emulsion droplet behaves near the electrode surface with several mm and tens of ms of spatial and temporal resolutions, respectively, which are sufficient for investigating Br electrochemistry. 4. Conclusion
Fig. 6. Chronoamperogram from a Pt UME monitored by confocal microscopy. (black line) Red dots are the moments droplets collide with the electrode surface. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
[29,37] In Figs. 6 and 7, it is apparent that spike current appears when the droplets collide with electrode, implying that higher concentration of Br makes current increase. Another possibility is the acceleration of transport process by the rapid exchange reaction of the Br-Br3 couple. The diffusion of Br can be accelerated by Br3 owing to the equilibrium Br3 fi Br2 + Br. [38,39] Since the MEPBr3 droplet has a large number of Br3, it is highly probable that the diffusion of Br in the droplet is accelerated and thereby current increases. The third is the effect of enlarged electrode area by the droplets. We can imagine that the effective electrode area increases when the droplet grows or
We used for the first time confocal microscopy to investigate formation, diffusion, and collision of MEPBr3 droplets in the electrochemical system. Confocal microscopy images and voltammograms from a wire and a UME of Pt unambiguously told that the droplets are generated heterogeneously on the electrode surface and adsorbed onto the electrode. A droplet emulsion appears in solution at 0.925 V or higher potentials and some of the resulting droplets collide back with the Pt wire surface. The confocal images and chronoamperogram at 1.0 V, simultaneously acquired from the Pt UME, ensure that collisions of droplets produce the spikes. Oxidative current increases whenever a MEPBr3 droplet contacts the electrode surface, regardless of whether it is by adsorption or collision. Droplet collision with the electrode surface leads to current surge up to 7.38 times from the baseline of the chronoamperogram. These results are compelling evidences supporting that electrochemistry at the interface between the electrode and the droplet makes significant contribution to overall Br oxidation reaction. This may suggest paths to better bromine-complex agents for the redox flow batteries. The observations in this work are expected to be a cornerstone to achieve deeper understanding of the Br oxidation in the MEPBr3 droplet, eventually resulting in advance of the redox flow batteries.
Fig. 7. Images of the moment a droplet collides with the surface of a Pt UME. White arrows indicate the position of the droplet traced. The time elapsed at 1.0 V is (a) 20.19 s, (b) 20.21 s, (c) 20.23 s, (d) 20.31 s, (e) 20.36 s, (f) 20.42 s, respectively. Scale bar = 2 mm.
170
D.H. Han et al. / Electrochimica Acta 252 (2017) 164–170
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1A2A1A13001897), a grant to CABMC (Control of Animal Brain using MEMS Chip) funded by Defense Acquisition Program Administration (UD140069ID), Nano-Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2011-0030268) Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta.2017. 08.132. References [1] N.V. Rees, Electrochemical insight from nanoparticle collisions with electrodes: A mini-review, Electrochem. Commun. 43 (2014) 83–86. [2] W. Cheng, R.G. Compton, Electrochemical detection of nanoparticles by ‘nanoimpact’ methods, TrAC Trends Anal. Chem. 58 (2014) 79–89. [3] Y.-G. Zhou, N.V. Rees, R.G. Compton, The Electrochemical Detection and Characterization of Silver Nanoparticles in Aqueous Solution, Angew. Chem. Int. Ed. 50 (2011) 4219–4221. [4] E.J.E. Stuart, Y.-G. Zhou, N.V. Rees, R.G. Compton, Determining unknown concentrations of nanoparticles: the particle-impact electrochemistry of nickel and silver, RSC Adv. 2 (2012) 6879–6884. [5] J. Ellison, K. Tschulik, E.J.E. Stuart, K. Jurkschat, D. Omanovi c, M. Uhlemann, A. Crossley, R.G. Compton, Get More Out of Your Data: A New Approach to Agglomeration and Aggregation Studies Using Nanoparticle Impact Experiments, ChemistryOpen 2 (2013) 69–75. [6] S.J. Kwon, F.-R.F. Fan, A.J. Bard, Observing Iridium Oxide (IrOx) Single Nanoparticle Collisions at Ultramicroelectrodes, J. Am. Chem. Soc. 132 (2010) 13165–13167. [7] X. Xiao, A.J. Bard, Observing Single Nanoparticle Collisions at an Ultramicroelectrode by Electrocatalytic Amplification, J. Am. Chem. Soc. 129 (2007) 9610–9612. [8] X. Xiao, F.-R.F. Fan, J. Zhou, A.J. Bard, Current Transients in Single Nanoparticle Collision Events, J. Am. Chem. Soc. 130 (2008) 16669–16677. [9] D. Hellberg, F. Scholz, F. Schauer, W. Weitschies, Bursting and spreading of liposomes on the surface of a static mercury drop electrode, Electrochem. Commun. 4 (2002) 305–309. [10] W. Cheng, R.G. Compton, Investigation of Single-Drug-Encapsulating Liposomes using the Nano-Impact Method, Angew. Chem. Int. Ed. 53 (2014) 13928–13930. [11] J. Dunevall, H. Fathali, N. Najafinobar, J. Lovric, J. Wigström, A.-S. Cans, A.G. Ewing, Characterizing the Catecholamine Content of Single Mammalian Vesicles by Collision–Adsorption Events at an Electrode, J. Am. Chem. Soc 137 (2015) 4344–4346. [12] A. Oleinick, F. Lemaitre, M.G. Collignon, I. Svir, C. Amatore, Vesicular release of neurotransmitters: converting amperometric measurements into size, dynamics and energetics of initial fusion pores, Faraday Discuss. 164 (2013) 33–55. [13] J.E. Dick, A.T. Hilterbrand, A. Boika, J.W. Upton, A.J. Bard, Electrochemical detection of a single cytomegalovirus at an ultramicroelectrode and its antibody anchoring, Proc. Natl. Acad. Sci. 112 (2015) 5303–5308. [14] J.E. Dick, C. Renault, A.J. Bard, Observation of Single-Protein and DNA Macromolecule Collisions on Ultramicroelectrodes, J. Am. Chem. Soc. 137 (2015) 8376–8379. [15] B.-K. Kim, A. Boika, J. Kim, J.E. Dick, A.J. Bard, Characterizing Emulsions by Observation of Single Droplet Collisions—Attoliter Electrochemical Reactors, J. Am. Chem. Soc. 136 (2014) 4849–4852. [16] B.-K. Kim, J. Kim, A.J. Bard, Electrochemistry of a Single Attoliter Emulsion Droplet in Collisions, J. Am. Chem. Soc. 137 (2015) 2343–2349.
[17] Y. Li, H. Deng, J.E. Dick, A.J. Bard, Analyzing Benzene and Cyclohexane Emulsion Droplet Collisions on Ultramicroelectrodes, Anal. Chem. 87 (2015) 11013– 11021. [18] Z. Yang, J. Zhang, M.C.W. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemmon, J. Liu, Electrochemical Energy Storage for Green Grid, Chem. Rev. 111 (2011) 3577– 3613. [19] M. Skyllas-Kazacos, M.H. Chakrabarti, S.A. Hajimolana, F.S. Mjalli, M. Saleem, Progress in Flow Battery Research and Development, J. Electrochem. Soc. 158 (2011) R55–R79. [20] A.Z. Weber, M.M. Mench, J.P. Meyers, P.N. Ross, J.T. Gostick, Q. Liu, Redox flow batteries: a review, J. Appl. Electrochem. 41 (2011) 1137. [21] P. Alotto, M. Guarnieri, F. Moro, Redox flow batteries for the storage of renewable energy: A review, Renewable Sustainable Energy Rev. 29 (2014) 325–335. [22] D.J. Eustace, Bromine Complexation in Zinc-Bromine Circulating Batteries, J. Electrochem. Soc. 127 (1980) 528–532. [23] M. Mastragostino, S. Valcher, Polymeric salt as bromine complexing agent in a Zn-Br2 model battery, Electrochim. Acta 28 (1983) 501–505. [24] M. Skyllas-Kazacos, G. Kazacos, G. Poon, H. Verseema, Recent advances with UNSW vanadium-based redox flow batteries, Int. J. Energy Res. 34 (2010) 182– 189. [25] E.E. Kalu, R.E. White, Zn/Br2 cell: Effects of plated zinc and complexing organic phase, AIChE J. 37 (1991) 1164–1174. [26] W. Kautek, A. Conradi, M. Sahre, C. Fabjan, J. Drobits, G. Bauer, P. Schuster, In Situ Investigations of Bromine-Storing Complex Formation in a Zinc-Flow Battery at Gold Electrodes, J. Electrochem. Soc. 146 (1999) 3211–3216. [27] W. Kautek, A. Conradi, C. Fabjan, G. Bauer, In situ FTIR spectroscopy of the Zn– Br battery bromine storage complex at glassy carbon electrodes, Electrochim. Acta 47 (2001) 815–823. [28] E. Lancry, B.-Z. Magnes, I. Ben-David, M. Freiberg, New Bromine Complexing Agents for Bromide Based Batteries, ECS Trans. 53 (2013) 107–115. [29] S. Park, H. Kim, J. Chae, J. Chang, Electrochemical Generation of Single Emulsion Droplets and In Situ Observation of Collisions on an Ultramicroelectrode, J. Phys. Chem. C 120 (2016) 3922–3928. [30] J.E. Dick, C. Renault, B.-K. Kim, A.J. Bard, Simultaneous Detection of Single Attoliter Droplet Collisions by Electrochemical and Electrogenerated Chemiluminescent Responses, Angew. Chem. Int. Ed. 53 (2014) 11859–11862. [31] S.E. Fosdick, M.J. Anderson, E.G. Nettleton, R.M. Crooks, Correlated Electrochemical and Optical Tracking of Discrete Collision Events, J. Am. Chem. Soc. 135 (2013) 5994–5997. [32] X. Shan, U. Patel, S. Wang, R. Iglesias, N. Tao, Imaging Local Electrochemical Current via Surface Plasmon Resonance, Science 327 (2010) 1363–1366. [33] X. Shan, I. Diez-Perez, L. Wang, P. Wiktor, Y. Gu, L. Zhang, W. Wang, J. Lu, S. Wang, Q. Gong, J. Li, N. Tao, Imaging the electrocatalytic activity of single nanoparticles, Nat. Nanotechnol. 7 (2012) 668–672. [34] Y. Fang, W. Wang, X. Wo, Y. Luo, S. Yin, Y. Wang, X. Shan, N. Tao, Plasmonic Imaging of Electrochemical Oxidation of Single Nanoparticles, J. Am. Chem. Soc. 136 (2014) 12584–12587. [35] C.M. Hill, S. Pan, A Dark-Field Scattering Spectroelectrochemical Technique for Tracking the Electrodeposition of Single Silver Nanoparticles, J. Am. Chem. Soc. 135 (2013) 17250–17253. [36] Y. Wang, X. Shan, F. Cui, J. Li, S. Wang, N. Tao, Electrochemical Reactions in Subfemtoliter-Droplets Studied with Plasmonics-Based Electrochemical Current Microscopy, Anal. Chem. 87 (2015) 494–498. [37] S. Park, S. Shin, D. Jung, J. Chae, J. Chang, Understanding Br- transfer into electrochemically generated discrete quarternary ammonium polybromide droplet on Pt ultermicroelectrode, J. Electroanal. Chem. 797 (2017) 97–106. [38] I. Ruff, V.J. Friedrich, K. Csillag, Transfer diffusion. III. Kinetics and mechanism of the triiodide-iodide exchange reaction, J. Phys. Chem. 76 (1972) 162–165. [39] I. Ruff, V.J. Friedrich, Tansfer diffusion. V. Kinetics and mechanism of the exchange reaction between bromide and tribromide ions, J. Phys. Chem. 76 (1972) 2957–2958. [40] B. Scharifker, C. Wehrmann, Phase formation phenomena during electrodeposition of benzyl and heptyl viologen bromides, J. Electroanal. Chem. 185 (1985) 93–108. [41] C. Lee, Y.K. Lee, Y. Lee, I.C. Jeon, Electrochemical study of methylalkylviologens using an electrochemical quartz crystal microbalance, J. Electroanal. Chem. 463 (1999) 224–231. [42] J. Mauzeroll, E.A.A.J. Hueske Bard, Scanning Electrochemical Microscopy. 48. Hg/Pt Hemispherical Ultramicroelectrodes, Fabrication and Characterization 75 (2003) 3880–3889. [43] B. Scharifker, G. Hills, Electrochemical kinetics at microscopically small electrodes, J. Electroanal. Chem. 130 (1981) 81–97.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Research Paper
In situ Confocal Microscopy of Electrochemical Generation and Collision of Emulsion Droplets in Bromide Redox System Dong Hyeop Hana,1, Sangmee Parkb,1, Eun Joong Kima , Taek Dong Chunga,b,c,* a
Department of Chemistry, Seoul National University, Seoul 08826, Republic of Korea Program in Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Suwon-Si, Gyeonggi-do 16229, Republic of Korea c Advanced Institutes of Convergence Technology, Suwon-Si, Gyeonggi-do 16229, Republic of Korea b
A R T I C L E I N F O
Article history: Received 24 April 2017 Received in revised form 18 August 2017 Accepted 22 August 2017 Available online 26 August 2017 Keywords: Redox flow batteries Emulsion droplet Particle impact chronoamperometry Collision Confocal microscopy
A B S T R A C T
For redox flow battery, 1-ethyl-1-methylpyrrolidinium bromide (MEPBr) is a promising brominecomplexing agent that forms insoluble organic phase of MEPBr3. A series of optical images acquired by in-situ confocal microscopy with tens of millisecond interval visualize how MEPBr3 emulsion droplets are electrochemically generated and collide with the electrode surface. Two types of electrodes, i.e. a Pt microdisk of 10 mm diameter and a 2.5 mm long Pt wire of 25 mm diameter, show clear correlation between electrochemical behavior and optical images. The droplets starts growing on the electrode surface at the potential at which oxidative faradaic current starts flowing. As the overpotential increases, the droplets become larger adhered at the electrode surface, from which small droplets start detaching at 0.925 V or higher potential. Some of the droplets leave the surface, move back and collide with the electrode surface, thus producing current spikes, which are detected by chronoamperometry simultaneously. In-situ confocal microscopy in this study confirms that the droplets are heterogeneously generated by electrochemical reaction and that individual droplets collide back onto the electrode surface, thus providing a better understanding of the phenomena that happen at redox flow battery electrode surfaces. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Emulsion droplets have been an interesting research theme for various aspects in industrial processes. What we can access to investigate droplets is mostly ensemble properties. Lack of proper analytical methods to characterize individual particles is the cause of such limited knowledge about them. In this regard, ‘particleimpact chronoamperometry’ is getting a new spotlight recently [1,2]. This technique is a simple but informative way to detect electrochemical signals originated by individual particles [1]. In the presence of particles near the electrode, chronoamperometry typically gives current spikes [3], which are current surges that happen when the particles contact the electrode surface [2]. Such current spikes contain information about size [3], concentration
* Corresponding author at: Seoul National University, Seoul 00826, Republic of Korea. E-mail addresses: [email protected], [email protected] (T.D. Chung). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.electacta.2017.08.132 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
[4], and aggregation state [5] of the particles and electron transfer associated with them [6–8]. This method is applied to study of, not only metal nanoparticles but also ‘soft particles’, such as liposomes [9–12], viruses [13], single macromolecules [14] and emulsion droplets containing redox species [15–17]. 1-ethyl-1-methylpyrrolidinium bromide (MEPBr) is one of quaternary ammonium bromides (QBr’s) widely used in redox flow batteries (RFBs)[18–21]. It captures electrochemically produced bromine (Br2), during electro-oxidation of Br, to induce the formation of insoluble organic emulsion droplets of MEPBr3. Formation of such emulsion prevents from permeation through the battery membrane and self-discharging of electrogenerated Br2, which is one of the key causes bringing about malfunction of RFB systems [22–28]. Reportedly, MEPBr3 droplets participate in Br electrooxidation rather than behaving merely as bromine complexing agents [29], making the system even more complex. The electrochemistry involving MEPBr and MEPBr3, however, remains unknown. For instance, it is uncertain whether the MEPBr3 emulsions are homogenously created in aqueous solution or heterogeneously formed on the Pt electrode. There has been few
D.H. Han et al. / Electrochimica Acta 252 (2017) 164–170
previous study on movement and behavior of electrochemically generated MEPBr3 emulsion as a function of time. The recent simulation work reported a model based on the assumption of static geometry, telling little concerning dynamic emulation. [29,31] In addition, it is still unclear what causes the characteristic spikes in chronoamperograms, requiring lack of direct experimental evidence. It is also unknown why the oxidative current of MEPBr is larger than currents from aqueous Br oxidation without MEP+. In this study, we investigate the electro-oxidation of Br in MEPBr3 droplet through comparison between electrochemical analysis and in-situ confocal microscopy. Previous reports showed collision events of particles such as nanoparticles, fluorescent beads and some soft particles through fluorescence [30,31] and plasmonic signals [32–36] that are produced when particles collide with the electrode. Unfortunately, it is difficult to directly observe the generation and behavior of particles in the solution. This led us to seek an alternative way of observing the MEPBr3 droplets, i.e. by using in-situ confocal microscopy. Droplets are predicted to be a few mm in diameter, for which confocal microscopy can provide direct optical images with sufficient resolution. The optical images can show the change in droplets at the focused range used. Using cylindrical wire electrode helps with monitoring the stereoscopic movement of droplet and ultramicroelectrode (UME) giving low background current suits for simultaneous acquisition of the optical images and transient current caused by a droplet collision moment. Therefore, in-situ confocal microscopy allows us to see directly generation, diffusion, and collision of the droplets at the electrode, and how they change as potential varies. These are expected to provide a better understanding of the contribution of MEPBr3 droplets to the oxidation of Br in redox flow batteries. 2. Experimental 2.1. Materials and reagents 1-ethyl-1-methylpyrrolidinium bromide and potassium bromide were acquired from Sigma-Aldrich. Sulfuric acid (95%) and potassium sulfate were purchased from Deajung and Alfa Aesar, respectively. All the aqueous solutions in these experiments were prepared with ultrapure deionized water produced by a Barnstead Nanopure system from Thermo Scientific. 2.2. Electrochemical measurements Cyclic voltammograms and chronoamperograms were obtained with a CHI 750 electrochemical workstation from CHI instruments. The electrochemical cell consisted of three
165
electrodes. The working electrodes were either a Pt UMEs (diameter: d = 10 mm) or a Pt microwire electrode (diameter: d = 25 mm). A silver/silver chloride (3 M potassium chloride) and a Pt wire were used as reference and counter electrode, respectively. The pH of the solutions comprising 0.5 M potassium sulfate and 1-ethyl-1-methylpyrrolidinium bromide or potassium bromide, was adjusted with sulfuric acid. 2.3. Confocal microscopy imaging Imaging was carried out with a Zeiss LSM880 confocal laser scanning microscope equipped with a laser argon-multiline for RGB and a water-immersion objective (20x, N.A. = 1.0; Zeiss). Series of images were acquired with the interval of about 15 150 ms and were sharpened to clearly see the shape and movement of the droplet using a ZEN (black edition; Zeiss) software. 3. Results and discussion The homogeneous complexation process of MEPBr3 proposed in the literature is represented as follows [29]. Br 3 þ 2e fi 3Br
ð1Þ
MEP þ þ Br 3 fi MEPBr3
ð2Þ
This suggested mechanism proposes that Br3, which is generated on the electrode surface in reaction (1), encounters the 1-ethyl-1-methylpyrrolidinium cation, MEP+, to form a complex homogeneously, MEPBr3, as described in reaction (2). This is supported by in-situ FT-IR investigations[26,27]. Multiple insoluble MEPBr3 complexes agglomerate themselves to create a droplet in the aqueous solution. When the droplet hanging around the solution happens to reach the electrode surface, additional spike current is produced in voltammograms and chronoamperograms. From the electrochemical analysis and simulation study in MEPBr3 droplet, it was predicted that Br is enriched ca. 7.5 M in the droplet and causes the spike current. [29,37]. This is a proposed scenario, although highly probable as implied by a couple of indirect observations, which has not been proved by direct imaging and thereby the respective steps of the previously mentioned mechanism remains veiled to date. To address this issue, we introduce confocal microscopy to monitor what occurs at the electrode during the electrochemical measurements. Fig. 1 shows the experimental scheme for simultaneous measurements using a potentiostat and a confocal microscope. Three electrodes, including a Pt wire or a UME as working electrodes, constitute the electrochemical system
Fig. 1. Schematic diagram of electrochemical measurement and confocal imaging. (a) Pt wire (25 mm in diameter) insulated except the part exposed to the electrolyte (ca. 2.5 mm long) (b) Pt UME (10 mm in diameter) utilized in electrochemical cell.
166
D.H. Han et al. / Electrochimica Acta 252 (2017) 164–170
Fig. 2. (a) The focus is on the uppermost surface of the cylindrical wire electrode. (b) The focus is on a plane perpendicular to the sideward electrode surface. The left schemes are the planes focused (red box) and the right is actual images at 1.0 V. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
installed on the mount of an upright type confocal microscope. The Pt wire of 25 mm in diameter provides a cylindrical electrode where its length exposed to the solution is about 2.5 mm. This wire electrode helps see the stereoscopic movement of droplets through the images in both vertical and horizontal views from the electrode surface. (Fig. 2) Its large surface area leads to correspondingly high background current in the current transient curve and the cyclic voltammogram so that it is difficult to observe distinct spikes clearly. UME is a good solution to this problem (Fig. 1b). It allows discrimination of discrete current spikes at a constant potential as to investigate whether droplet collision and current spike coincide with each other. An objective lens immersed in solution enables clean optical high-resolution images to be obtained. The dipping lens is sufficiently stable within the pH
Fig. 3. Cyclic voltammograms recorded in 72 mM MEPBr + 0.5 M K2SO4 at pH 5.1 (red line) and 72 mM KBr + 0.5 M K2SO4 solution at pH 5.1 (black line). A Pt UME was used as working electrode and scan rate is 10 mV s1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
range between 5 and 8. For this reason, the supporting electrolyte solution is used 0.5 M K2SO4 solution at pH 5.1. Figs. 3 and S1 show the effects of MEP+ to the electrochemical redox behavior of Br. In Fig. 3 the cyclic voltammogram of 72 mM aqueous KBr at Pt UME (black line) tells that sufficiently high overpotential gives rise to diffusion-controlled Br oxidation in the absence of MEP+. The faradaic current is smaller than that in the presence of MEP+ and no spike appears. A 72 mM aqueous MEPBr at pH 5.1 (red line) starts giving oxidation current at around 0.85 V and reaches diffusion-controlled state at a potential higher than 1.0 V. Overall oxidative current is larger than that of KBr solution, and current spikes randomly. At the same potential, the current during the reverse scan is much larger than that during the forward one, indicating that the redox process is not a simple diffusion system. This implies that the products of electrochemical oxidation accumulate on the electrode to accelerate further oxidation rather than blocking reactant diffusion towards the electrode. Unlike KBr, MEPBr solution undergoes reduction in the reverse scan at the Pt UME. The reduction peak would be correlate with the reduction of Br3 in droplets adsorbed on the electrode [23]. (Fig. S2) The area of the reduction peak is proportional to the total charge for oxidation. However, the charge of oxidation is 1.4 times larger than the amount of reduction charge. This tells that all of the oxidized species do not come back to the electrode. The cyclic voltammogram from the Pt wire electrode is shown in Fig. S1. Because the shape and size, thus mass transport near it, are different from those of the UME, the cyclic voltammogram from the Pt wire is not perfectly sigmoidal (black line). Still, the sharp reduction peak during reverse scan is observed in the presence of MEP+, reminiscent of voltammetric response coming from reducible species confined on the electrode surface (red line). This is similar behavior as that found from the UME. In order to further assess the fundamental electrochemical behavior of this system, we examine the Pt wire electrode system in the potential range of 0.8 to 1.0 V by confocal microscopy. (Fig. 4) Neither oxidation current nor droplet is observed at 0.8 V. (Fig. 4a) At 0.85 V, when the oxidation current begins to increase, small droplets start appearing and merging into larger ones on the electrode surface. (Fig. 4b) Since the chemically synthesized MEPBr3 droplets have brownish color [29], the droplets in the
D.H. Han et al. / Electrochimica Acta 252 (2017) 164–170
167
Fig. 4. Confocal images taken 15 s after applying constant potentials to Pt wire electrode in 72 mM MEPBr + 0.5 M K2SO4 at pH 5.1. (a) 0.8 V, (b) 0.85 V, (c) 0.925 V, (d) 1.0 V. Scale bar = 5 mm.
confocal images appear black. The droplets keep growing as the potential increases, and are still adhered to the electrode surface. At 0.925 V, small droplets are generated on the electrode surface and are released to the bulk solution. Some droplets, adsorbed on the surface of the electrode, gradually aggregate themselves to form a larger hemispherical one. Detached droplets are much smaller than the droplets still attached to the surface. (Fig. 4c) Upon higher overpotential, the size of the droplets attached to the electrode surface decreases and a greater number of droplets leave the surface to the bulk solution. On the electrode surface at 1.0 V, it is even hard to see growth of droplets. Rather, small droplets quickly emerge and some of them aggregate in the solution, not on the very electrode surface. (Fig. 4d, Video 1 and 2) The emulsion consists of extremely small droplets with a haze-like appearance. Interestingly, a few droplets hovering over the electrode surface move back to the surface. In order to understand why the droplets move back to the electrode again and collides, we compared the size of the droplets hovering over the electrode surface (Left image of Fig. 4d) with the size of those diffusing away from the electrode (Right image of Fig. 4d). 20 droplets hovering over are 3.20 0.84 mm in diameter. For 30 droplets diffusing away from the electrode surface, the diameters are 1.84 0.51 mm. The UME, which is immersed inversely in the solution, i.e. facing downward, shows no significant change in the number or shape of the spikes due to collisions of droplets. Therefore, it is improbable that weight of droplet is responsible for collision with the electrode. Confocal microscopic images at Pt wire tell that electrogenerated droplets randomly move and hit others. (Figs. S3, S4) Stochastic collisions
among droplets appear to make them drift in arbitrary directions. As a consequence, some of the droplets contact the electrode surface to produce spikes. Fig. 5 (Video 3) shows magnifications of the images of Fig. 4d, focused at 1 mm distance range from the Pt wire surface. Thus, the droplets in these images are located very close to the electrode. These droplets are not adsorbed on the electrode surface but seem to drift over it (Fig. 5a). As soon as a droplet collides with the electrode, it spreads out over the surface (Fig. 5b) and becomes flat to adhere to the surface in 0.06 s (Fig. 5c). Thereafter, the droplet on the electrode gradually fades away to become a hazy shape (Fig. 5d–f). Presumably, the droplet disintegrates in miniscule pieces. There are other examples of similar events when droplets collide with the Pt wire electrode surface. (Figs. S5–S8 and Video 4–7) The collision in confocal microscopic images is observed at potentials above 1.0 V. This is same as the potential range where the characteristic spikes appear in the corresponding cyclic voltammograms in Fig. 3. The current transient curves of a Pt UME of 10 mm diameter, which produces a lower background current than the Pt wire electrode, allow us to demonstrate that the droplet collisions cause the current spikes. Fig. 6 shows the chronoamperogram of 32 mM MEPBr diluted with 0.5 M K2SO4 at pH 5.1 and 1.0 V. Dilution of the solution makes the number of spikes small enough to compare the confocal microscopic images with individual spikes in the current transient curve, which were simultaneously acquired. The red dots in Fig. 6 indicate the droplets collision time with the electrode, which are determined
168
D.H. Han et al. / Electrochimica Acta 252 (2017) 164–170
Fig. 5. Images at the moment the droplet collides with surface of a Pt wire electrode. White arrows indicate the position of the targeted droplet. The time elapsed at 1.0 V (a) 16.96 s, (b) 16.99 s, (c) 17.02 s, (d) 17.62 s, (e) 18.04 s, (f) 18.34 s, respectively. Scale bar = 2 mm.
based on the confocal microscopic images continuously taken. In Fig. 6, successive droplet collision starting at 20.21 seconds are shown in Fig. S9 and Video 8. The elapsed times of red dots from the images coincide with the spikes on the chronoamperogram as seen in Fig. 6. Some spikes observed in the chronoamperogram, however, are invisible under the microscope. Smaller spikes following immediately after a large one, e.g. at 22.24 s, are not visible in the confocal microscopy images. For a large spike, corresponding collided droplet covers a wide, sometimes even entire, area of the electrode so that the whole image appears entirely black and it is not possible to see small spots (Fig. S10). Fig. 7 and Video 9 show how a droplet collides with the UME. The collision of droplet occurs at 20.21 s and corresponds to the red dot mark at the same time in Fig. 6. This agrees with the result observed at the Pt wire electrode shown in Fig. 5. Fig. 7a and b
shows a droplet approaching and colliding with the electrode surface, respectively. The droplet spreads out and sticks to the electrode surface and gradually fade away (Fig. 7c–f). There are other examples of droplet collision with the Pt UME in Figs. S11–13 and Videos 10 to 12. From 0.85 V to 0.9 V, where the current starts to increase on cyclic voltammogram (Figs. 3 and S1), the droplets tend to grow at the electrode heterogeneously. (Figs. S2 and S14) In this potential range, the current is larger than that of KBr solution. At 1.0 V, the droplet collides with the electrode and generates a current spike. There are some explanations about the current increase phenomenon caused by droplets. The first is that Br diffuses from the aqueous solution and is concentrated in the droplet. This is supported by the studies on current spikes using bulk electrolysis model and simulation, although they provide indirect evidences.
D.H. Han et al. / Electrochimica Acta 252 (2017) 164–170
169
collides on the electrode surface, which is reminiscent of insoluble redox film on the electrode or Hg deposited on the UME. [40–43] Overall, this issue is still open, requiring further study. Consequently, in-situ confocal microscopy allows us to see how emulsion droplet behaves near the electrode surface with several mm and tens of ms of spatial and temporal resolutions, respectively, which are sufficient for investigating Br electrochemistry. 4. Conclusion
Fig. 6. Chronoamperogram from a Pt UME monitored by confocal microscopy. (black line) Red dots are the moments droplets collide with the electrode surface. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
[29,37] In Figs. 6 and 7, it is apparent that spike current appears when the droplets collide with electrode, implying that higher concentration of Br makes current increase. Another possibility is the acceleration of transport process by the rapid exchange reaction of the Br-Br3 couple. The diffusion of Br can be accelerated by Br3 owing to the equilibrium Br3 fi Br2 + Br. [38,39] Since the MEPBr3 droplet has a large number of Br3, it is highly probable that the diffusion of Br in the droplet is accelerated and thereby current increases. The third is the effect of enlarged electrode area by the droplets. We can imagine that the effective electrode area increases when the droplet grows or
We used for the first time confocal microscopy to investigate formation, diffusion, and collision of MEPBr3 droplets in the electrochemical system. Confocal microscopy images and voltammograms from a wire and a UME of Pt unambiguously told that the droplets are generated heterogeneously on the electrode surface and adsorbed onto the electrode. A droplet emulsion appears in solution at 0.925 V or higher potentials and some of the resulting droplets collide back with the Pt wire surface. The confocal images and chronoamperogram at 1.0 V, simultaneously acquired from the Pt UME, ensure that collisions of droplets produce the spikes. Oxidative current increases whenever a MEPBr3 droplet contacts the electrode surface, regardless of whether it is by adsorption or collision. Droplet collision with the electrode surface leads to current surge up to 7.38 times from the baseline of the chronoamperogram. These results are compelling evidences supporting that electrochemistry at the interface between the electrode and the droplet makes significant contribution to overall Br oxidation reaction. This may suggest paths to better bromine-complex agents for the redox flow batteries. The observations in this work are expected to be a cornerstone to achieve deeper understanding of the Br oxidation in the MEPBr3 droplet, eventually resulting in advance of the redox flow batteries.
Fig. 7. Images of the moment a droplet collides with the surface of a Pt UME. White arrows indicate the position of the droplet traced. The time elapsed at 1.0 V is (a) 20.19 s, (b) 20.21 s, (c) 20.23 s, (d) 20.31 s, (e) 20.36 s, (f) 20.42 s, respectively. Scale bar = 2 mm.
170
D.H. Han et al. / Electrochimica Acta 252 (2017) 164–170
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1A2A1A13001897), a grant to CABMC (Control of Animal Brain using MEMS Chip) funded by Defense Acquisition Program Administration (UD140069ID), Nano-Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2011-0030268) Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta.2017. 08.132. References [1] N.V. Rees, Electrochemical insight from nanoparticle collisions with electrodes: A mini-review, Electrochem. Commun. 43 (2014) 83–86. [2] W. Cheng, R.G. Compton, Electrochemical detection of nanoparticles by ‘nanoimpact’ methods, TrAC Trends Anal. Chem. 58 (2014) 79–89. [3] Y.-G. Zhou, N.V. Rees, R.G. Compton, The Electrochemical Detection and Characterization of Silver Nanoparticles in Aqueous Solution, Angew. Chem. Int. Ed. 50 (2011) 4219–4221. [4] E.J.E. Stuart, Y.-G. Zhou, N.V. Rees, R.G. Compton, Determining unknown concentrations of nanoparticles: the particle-impact electrochemistry of nickel and silver, RSC Adv. 2 (2012) 6879–6884. [5] J. Ellison, K. Tschulik, E.J.E. Stuart, K. Jurkschat, D. Omanovi c, M. Uhlemann, A. Crossley, R.G. Compton, Get More Out of Your Data: A New Approach to Agglomeration and Aggregation Studies Using Nanoparticle Impact Experiments, ChemistryOpen 2 (2013) 69–75. [6] S.J. Kwon, F.-R.F. Fan, A.J. Bard, Observing Iridium Oxide (IrOx) Single Nanoparticle Collisions at Ultramicroelectrodes, J. Am. Chem. Soc. 132 (2010) 13165–13167. [7] X. Xiao, A.J. Bard, Observing Single Nanoparticle Collisions at an Ultramicroelectrode by Electrocatalytic Amplification, J. Am. Chem. Soc. 129 (2007) 9610–9612. [8] X. Xiao, F.-R.F. Fan, J. Zhou, A.J. Bard, Current Transients in Single Nanoparticle Collision Events, J. Am. Chem. Soc. 130 (2008) 16669–16677. [9] D. Hellberg, F. Scholz, F. Schauer, W. Weitschies, Bursting and spreading of liposomes on the surface of a static mercury drop electrode, Electrochem. Commun. 4 (2002) 305–309. [10] W. Cheng, R.G. Compton, Investigation of Single-Drug-Encapsulating Liposomes using the Nano-Impact Method, Angew. Chem. Int. Ed. 53 (2014) 13928–13930. [11] J. Dunevall, H. Fathali, N. Najafinobar, J. Lovric, J. Wigström, A.-S. Cans, A.G. Ewing, Characterizing the Catecholamine Content of Single Mammalian Vesicles by Collision–Adsorption Events at an Electrode, J. Am. Chem. Soc 137 (2015) 4344–4346. [12] A. Oleinick, F. Lemaitre, M.G. Collignon, I. Svir, C. Amatore, Vesicular release of neurotransmitters: converting amperometric measurements into size, dynamics and energetics of initial fusion pores, Faraday Discuss. 164 (2013) 33–55. [13] J.E. Dick, A.T. Hilterbrand, A. Boika, J.W. Upton, A.J. Bard, Electrochemical detection of a single cytomegalovirus at an ultramicroelectrode and its antibody anchoring, Proc. Natl. Acad. Sci. 112 (2015) 5303–5308. [14] J.E. Dick, C. Renault, A.J. Bard, Observation of Single-Protein and DNA Macromolecule Collisions on Ultramicroelectrodes, J. Am. Chem. Soc. 137 (2015) 8376–8379. [15] B.-K. Kim, A. Boika, J. Kim, J.E. Dick, A.J. Bard, Characterizing Emulsions by Observation of Single Droplet Collisions—Attoliter Electrochemical Reactors, J. Am. Chem. Soc. 136 (2014) 4849–4852. [16] B.-K. Kim, J. Kim, A.J. Bard, Electrochemistry of a Single Attoliter Emulsion Droplet in Collisions, J. Am. Chem. Soc. 137 (2015) 2343–2349.
[17] Y. Li, H. Deng, J.E. Dick, A.J. Bard, Analyzing Benzene and Cyclohexane Emulsion Droplet Collisions on Ultramicroelectrodes, Anal. Chem. 87 (2015) 11013– 11021. [18] Z. Yang, J. Zhang, M.C.W. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemmon, J. Liu, Electrochemical Energy Storage for Green Grid, Chem. Rev. 111 (2011) 3577– 3613. [19] M. Skyllas-Kazacos, M.H. Chakrabarti, S.A. Hajimolana, F.S. Mjalli, M. Saleem, Progress in Flow Battery Research and Development, J. Electrochem. Soc. 158 (2011) R55–R79. [20] A.Z. Weber, M.M. Mench, J.P. Meyers, P.N. Ross, J.T. Gostick, Q. Liu, Redox flow batteries: a review, J. Appl. Electrochem. 41 (2011) 1137. [21] P. Alotto, M. Guarnieri, F. Moro, Redox flow batteries for the storage of renewable energy: A review, Renewable Sustainable Energy Rev. 29 (2014) 325–335. [22] D.J. Eustace, Bromine Complexation in Zinc-Bromine Circulating Batteries, J. Electrochem. Soc. 127 (1980) 528–532. [23] M. Mastragostino, S. Valcher, Polymeric salt as bromine complexing agent in a Zn-Br2 model battery, Electrochim. Acta 28 (1983) 501–505. [24] M. Skyllas-Kazacos, G. Kazacos, G. Poon, H. Verseema, Recent advances with UNSW vanadium-based redox flow batteries, Int. J. Energy Res. 34 (2010) 182– 189. [25] E.E. Kalu, R.E. White, Zn/Br2 cell: Effects of plated zinc and complexing organic phase, AIChE J. 37 (1991) 1164–1174. [26] W. Kautek, A. Conradi, M. Sahre, C. Fabjan, J. Drobits, G. Bauer, P. Schuster, In Situ Investigations of Bromine-Storing Complex Formation in a Zinc-Flow Battery at Gold Electrodes, J. Electrochem. Soc. 146 (1999) 3211–3216. [27] W. Kautek, A. Conradi, C. Fabjan, G. Bauer, In situ FTIR spectroscopy of the Zn– Br battery bromine storage complex at glassy carbon electrodes, Electrochim. Acta 47 (2001) 815–823. [28] E. Lancry, B.-Z. Magnes, I. Ben-David, M. Freiberg, New Bromine Complexing Agents for Bromide Based Batteries, ECS Trans. 53 (2013) 107–115. [29] S. Park, H. Kim, J. Chae, J. Chang, Electrochemical Generation of Single Emulsion Droplets and In Situ Observation of Collisions on an Ultramicroelectrode, J. Phys. Chem. C 120 (2016) 3922–3928. [30] J.E. Dick, C. Renault, B.-K. Kim, A.J. Bard, Simultaneous Detection of Single Attoliter Droplet Collisions by Electrochemical and Electrogenerated Chemiluminescent Responses, Angew. Chem. Int. Ed. 53 (2014) 11859–11862. [31] S.E. Fosdick, M.J. Anderson, E.G. Nettleton, R.M. Crooks, Correlated Electrochemical and Optical Tracking of Discrete Collision Events, J. Am. Chem. Soc. 135 (2013) 5994–5997. [32] X. Shan, U. Patel, S. Wang, R. Iglesias, N. Tao, Imaging Local Electrochemical Current via Surface Plasmon Resonance, Science 327 (2010) 1363–1366. [33] X. Shan, I. Diez-Perez, L. Wang, P. Wiktor, Y. Gu, L. Zhang, W. Wang, J. Lu, S. Wang, Q. Gong, J. Li, N. Tao, Imaging the electrocatalytic activity of single nanoparticles, Nat. Nanotechnol. 7 (2012) 668–672. [34] Y. Fang, W. Wang, X. Wo, Y. Luo, S. Yin, Y. Wang, X. Shan, N. Tao, Plasmonic Imaging of Electrochemical Oxidation of Single Nanoparticles, J. Am. Chem. Soc. 136 (2014) 12584–12587. [35] C.M. Hill, S. Pan, A Dark-Field Scattering Spectroelectrochemical Technique for Tracking the Electrodeposition of Single Silver Nanoparticles, J. Am. Chem. Soc. 135 (2013) 17250–17253. [36] Y. Wang, X. Shan, F. Cui, J. Li, S. Wang, N. Tao, Electrochemical Reactions in Subfemtoliter-Droplets Studied with Plasmonics-Based Electrochemical Current Microscopy, Anal. Chem. 87 (2015) 494–498. [37] S. Park, S. Shin, D. Jung, J. Chae, J. Chang, Understanding Br- transfer into electrochemically generated discrete quarternary ammonium polybromide droplet on Pt ultermicroelectrode, J. Electroanal. Chem. 797 (2017) 97–106. [38] I. Ruff, V.J. Friedrich, K. Csillag, Transfer diffusion. III. Kinetics and mechanism of the triiodide-iodide exchange reaction, J. Phys. Chem. 76 (1972) 162–165. [39] I. Ruff, V.J. Friedrich, Tansfer diffusion. V. Kinetics and mechanism of the exchange reaction between bromide and tribromide ions, J. Phys. Chem. 76 (1972) 2957–2958. [40] B. Scharifker, C. Wehrmann, Phase formation phenomena during electrodeposition of benzyl and heptyl viologen bromides, J. Electroanal. Chem. 185 (1985) 93–108. [41] C. Lee, Y.K. Lee, Y. Lee, I.C. Jeon, Electrochemical study of methylalkylviologens using an electrochemical quartz crystal microbalance, J. Electroanal. Chem. 463 (1999) 224–231. [42] J. Mauzeroll, E.A.A.J. Hueske Bard, Scanning Electrochemical Microscopy. 48. Hg/Pt Hemispherical Ultramicroelectrodes, Fabrication and Characterization 75 (2003) 3880–3889. [43] B. Scharifker, G. Hills, Electrochemical kinetics at microscopically small electrodes, J. Electroanal. Chem. 130 (1981) 81–97.