Electrochemical Generation of Single Emulsion Droplets and In Situ

Article pubs.acs.org/JPCC

Electrochemical Generation of Single Emulsion Droplets and In Situ Observation of Collisions on an Ultramicroelectrode Sangmee Park, Hyunju Kim, Junghyun Chae,* and Jinho Chang* Department of Chemistry and Center for NanoBio Applied Technology, Sungshin Women’s University, 55 Dobong-ro 76 ga-gil, Gangbuk-gu, Seoul 142-732, Korea S Supporting Information *

ABSTRACT: The Br−/Br2 redox couple in aqueous solution has been often employed for redox flow batteries along with N-methyl-N-ethyl pyrrolidinium bromide (MEPBr) as a bromine-complexing agent, which forms insoluble organic droplets of MEPBr3 complexes during electro-oxidation of Br−. We, for the first time, report the electrochemistry of Br− electro-oxidation in electrochemically generated single droplets of MEPBr3 using the current transient method on an ultramicroelectrode (UME). Current spikes were observed in the chronoamperogram of the aqueous solutions containing more than 32 mM of MEPBr, and they correspond to electro-oxidation of Br− in MEPBr3. The voltammetric behavior of Br− electro-oxidation in single droplets of MEPBr3 was similar to that in the aqueous phase. The maximum concentration of Br− in the MEPBr3 droplets was estimated to be ∼7.5 M by fitting the observed current transient curves to the simulation using a bulk electrolysis model. Our study reveals that MEPBr3 also plays a vital role as an electrochemical reaction medium for Br− electro-oxidation in the Br−/Br2 redox system.

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INTRODUCTION A Br−/Br2 redox reaction in aqueous solutions has been widely used as a half-cell reaction in redox flow batteries (RFBs)1−4 such as Zn−Br,5−9 V−Br,10,11 Cr−Br, and Br−polysulfide systems.9,12,13 In these batteries, undesirable self-discharge often occurs because the electrochemically generated Br2 crosses over the membrane to a counter side chamber, thereby significantly lowering the cell efficiency. Quaternary ammonium bromides (QBr’s) are added in the electrolyte to capture the generated Br2 and to form a water-insoluble organic droplet, QBr2n+1. Thus, they function as bromine complexing agents; they not only suppress the cross contamination by Br2 but also prevent the vaporization of Br 2 , helping safe battery operation.6,7,11,14−17 The most commonly used QBr is Nmethyl-N-ethyl pyrrolidinium bromide (MEPBr), and it is known to form the MEPBr3 complex.7,15,16 It has long been believed that the role of the MEPBr3 organic phase is limited to Br2 capture, and electrochemistry associated with a Br−/Br2 redox reaction occurs solely in the aqueous phase. From a different point of view, however, the MEPBr3 phase, instead of being a bystander, participates in the electrochemistry as it is polar enough and the Br− in the aqueous phase can be diffused into MEPBr3. Then, the whole electrochemistry would be affected by both the Br−/Br2 redox reaction in the aqueous phase and electrochemical reactions occurring in the MEPBr3 phase. Therefore, electroanalysis for electrochemistry in MEPBr3 is important in order to fully understand the overall electrochemistry of Br-related RFB systems. However, there have been no reports to show evidence that the emulsion © 2016 American Chemical Society

droplets of the MEPBr3 phase can be reservoirs for electroactive species, where faradaic processes occur on an electrode. Here, we report the electro-oxidation of Br− in electrochemically generated single droplets of MEPBr3 by particleimpact chronoamperometry using a Pt ultramicroelectrode (UME) and thereby elucidate the previously unnoticed role of MEPBr3. Particle-impact electrochemistry18−72 has been a powerful method for electrochemically analyzing both “hard” metallic particles and “soft” particles. The reported examples of soft particles are mostly micelles18 containing substances such as vitamin C,49 catecholamine hormones,56 vitamin B12,61 Br−,62 and ferrocyanide,72 and recently, emulsion droplets containing redox species50,60,65 have been reported. When we applied the particle-impact method to our Br−/Br2 redox system containing MEPBr, and we observed the individual current spikes. Because these spikes belong to the electrochemical reaction in single droplets of MEPBr3, the signals associated with Br− electro-oxidation in the MEPBr3 phase can be differentiated from those in the aqueous phase. Thus, in situ electrochemical analysis of the reaction occurring in MEPBr3 is possible without isolating MEPBr3 from the aqueous phase.

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RESULTS AND DISCUSSION A proposed reaction pathway of Br− electro-oxidation in a single MEPBr3 droplet is described in Figure 1. Because the Received: December 9, 2015 Revised: January 25, 2016 Published: January 26, 2016 3922

DOI: 10.1021/acs.jpcc.5b12029 J. Phys. Chem. C 2016, 120, 3922−3928

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The Journal of Physical Chemistry C

formation of MEPBr3 is a homogeneous complexation process between MEP+ and electrochemically generated Br3− in aqueous solution,15,16 the following mechanism has been proposed Br3− + 2e− ⇋ 3Br −

(1)

MEP+ + Br3− → MEPBr3

(2)

After a MEPBr3 droplet is formed through reactions 1 and 2, both Br− and MEP+ in the aqueous phase are diffused into a MEPBr3 droplet. When an emulsion droplet of MEPBr3 reaches the surface of the Pt electrode, Br− in a MEPBr3 droplet is electro-oxidized to Br3−, which is subsequently combined with free MEP+ in a MEPBr3 droplet to generate additional MEPBr3. The electrolysis ends when all Br− in a MEPBr3 droplet is converted to Br3−. Meanwhile, the oily droplets in the system are mainly composed of MEPBr3. Other possible MEPBr2n+1 (n

Figure 1. A schematic representation of a proposed pathway of Br− electro-oxidation occurring in an electrochemically generated single droplet of MEPBr3.

Figure 2. CVs measured in (a) 75 mM NaBr (red line) and the same concentration of MEPBr aqueous solutions (black line), (b) 5 mM MEPBr + 5 mM NaBr (black line), 10 mM NaBr (red line) and 10 mM MEPBr (blue) aqueous solutions, and (c) 32, 52, 72, and 92 mM MEPBr (black) and NaBr (red) solutions on a Pt UME (a = 5 μm) at 0.02 V/s. All of the aqueous solutions contain 0.5 M H2SO4. 3923

DOI: 10.1021/acs.jpcc.5b12029 J. Phys. Chem. C 2016, 120, 3922−3928

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The Journal of Physical Chemistry C ≥ 2) species, which are derived from MEPBr3 and Br2,7 are thought to be absent because electro-oxidation of Br3− to Br2 is not likely to occur in our potential range as Br3− strongly binds to MEP+. This was supported by the polarization curve obtained from single MEPBr3 droplets in Figure 4 (vide infra).7 Figure 2a shows cyclic voltammograms (CVs) obtained from an aqueous NaBr solution (CNaBr(aq) = 75 mM, red line) and an aqueous MEPBr solution (CMEPBr(aq) = 75 mM, black line), both of which contain 0.5 M H2SO4. From the CV measured in the aqueous NaBr solution, a typical, diffusion-controlled voltammetric behavior was observed. However, the CV from the aqueous MEPBr solution shows considerably different voltammetric behaviors; 3.23 times higher electro-oxidative current and a large broad reductive peak were observed. The higher oxidative currents in the MEPBr solution are mainly attributed to electro-oxidation of Br− contained in MEPBr3 droplets, which are generated when CMEPBr(aq) ≥ 32 mM (see Figure 5). The reductive peak in the reverse scan would be associated with the reduction of Br3− to Br− in MEPBr3 residually adsorbed on the electrode.7 Both of the two steady-state currents (iss) from CVs of MEPBr and NaBr solutions in Figure 2b were identical at a concentration as low as 10 mM, but they became significantly different as the concentration increased from 32 to 92 mM, as shown in Figure 2c. This implies that both MEPBr and NaBr behave similarly in electrochemically oxidizing Br− in a dilute aqueous solution, and MEPBr3 droplets start to form above the concentration of 32 mM. The electro-oxidation of Br− in single MEPBr3 droplets was monitored using chronoamperometry, which has been proven to be an effective tool to study stochastic single electrochemical events on UME.50,60 While a featureless i−t curve was observed from a chronoamperogram (CA) of the aqueous NaBr solution (CNaBr(aq) = 75 mM) (Figure 3a), many current spikes with

Figure 4. (a) CAs performed by stepping potentials from 0.5 up to 1.2 V. (b) A CV of aqueous MEPBr solution (CMEPBr(aq) = 32 mM) (black line) and ispike‑avg obtained from individual current spikes in CAs of aqueous MEPBr solution (CMEPBr(aq) = 32 mM) at different Eox (red square).

bigger average peak currents, ispike‑avg were measured (see Figure 3c for the meaning of ispike). When the ispike‑avg was plotted against the applied potential, ranging from 0.8 to 1.3 V (see Figure S4 for more details), a sigmoidal-shaped curve was obtained, where ispike‑avg increased rapidly from 0.925 to 1.05 V and then leveled off at 1.1 V (Figure 4b, red square). This curve can be considered as a polarization curve of the electro-oxidation of Br− in MEPBr3 because ispike‑avg is entirely from MEPBr3 droplets. Interestingly, this curve is fairly well correlated to a CV of MEPBr solutions (CMEPBr(aq) = 32 mM) (Figure 4b, black line). It suggests that electro-oxidation of Br− on Pt UME occurs similarly in both the aqueous phase and MEPBr3 droplets, although the onset potential for the electro-oxidation of Br− in MEPBr3 droplets is 0.075 V higher than that in the aqueous phase (0.925 vs 0.85 V). The slightly higher onset potential of the polarization curve in MEPBr3 can be explained by the fact that it is more difficult to electro-oxidize Br− in the stabilized MEPBr3 phase than it is in the aqueous phase, which was similarly observed in the micelle containing Br− reported by Compton et al.62 Meanwhile, the second consecutive voltammetric wave in MEPBr3 droplets was not observed in our potential range, implying that further electro-oxidation of 2Br3− to 3Br2 via 2e− transfer73,74 does not take place, and consequently, chemical species such as MEPBr5 are not likely to form7 (vide supra).

Figure 3. CAs of (a) an aqueous NaBr solution (CNaBr(aq) = 75 mM) and (b) an aqueous MEPBr solution (CMEPBr(aq) = 75 mM) on Pt UME (a = 5 μm). (c) The i−t curve of a single current spike from (b). All of the CAs were carried out by stepping potentials from 0.5 to 1.05 V.

individual current transient behaviors appeared on a CA of the aqueous MEPBr solution (CMEPBr(aq) = 75 mM) (Figure 3b). These current spikes are generated whenever a single emulsion droplet of MEPBr3 collides on the surface of Pt UME, which is very similar to the cases reported previously.49,50,60−62 The individual current spikes turned out to be dependent on the applied potentials (Figure 4a). Various potentials (Eox) from 0.875 to 1.2 V were applied to the MEPBr solutions (CMEPBr(aq) = 32 mM) for 60 s, where water oxidation did not occur (Figure S3). The current spikes were not detected at Eox lower than 0.95 V. As Eox became higher, more current spikes appeared with a slight increase of the background current, and 3924

DOI: 10.1021/acs.jpcc.5b12029 J. Phys. Chem. C 2016, 120, 3922−3928

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The Journal of Physical Chemistry C The observed current spikes were strongly related to the concentration of MEPBr in the aqueous solution (CMEPBr(aq)). The plot between ispike‑avg and CMEPBr(aq) is shown in Figure 5.

Figure 5. Average current peaks, ispike‑avg, obtained from individual current spikes from CAs performed by stepping potentials from 0.5 to 1.1 V for 60 s in different CMEPBr(aq).

Figure 6. Four different current spikes (black line) with different ispike values attributed to an electro-oxidation reaction of Br− in single MEPBr3 droplets from a CA of the MEPBr solution (CMEPBr(aq) = 72 mM) and their corresponding theoretical values (blue triangle) derived from the bulk electrolysis model.

Each value of ispike‑avg was measured from CAs (Figure S5) obtained from the solutions of different CMEPBr(aq) at 1.1 V, beyond which ispike‑avg no longer increases due to the masstransfer limit of Br− into MEPBr3 (Figure 4b). As the curve presents, the current spikes were not detected at low concentrations (CMEPBr(aq) ≤ 28.1 mM), and they began to be observed at 32 mM MEPBr, which is, we believe, the critical concentration of MEPBr3 formation in 0.5 M H2SO4 aqueous solution. Such a critical concentration is similarly observed in the micelle formation containing Br−,62 and H2,75 N276 bubble nucleation. The ispike‑avg increased in proportion to CMEPBr(aq) from 32.8 to 52 mM, after which it leveled off. This indicates that CBr−(MEPBr3) increased as CBr−(aq) became higher, and finally, CBr−(MEPBr3) was saturated when CBr−(aq) reached 52 mM. Because the single MEPBr3 droplet can be considered as a small electrochemical reactor and Br− in the droplet can be fully electrolyzed, a bulk electrolysis model was applied.50,60 For the simulation, we assumed that a MEPBr3 droplet collides and adsorbs on the Pt UME surface as a hemispherical shape,77,78 and the electrolysis of Br− in MEPBr3 is faster than influx of Br− from the aqueous to MEPBr3 phase. Several individual current spikes from a CA of aqueous MEPBr solution (CMEPBr(aq) = 72 mM) were selected and overlapped with their corresponding theoretical values in Figure 6. These current spikes having different ispike values are derived from different sizes of single MEPBr3 droplets. Each current decay as a function of time (t) was compared with that of corresponding theoretical values (blue triangle), which were calculated from following equations50,60 i(t ) = ispikee−(mA / V )t m=

be 1.08 × 10−6 cm2/s by the following Stokes−Einstein equation DBr−(MEPBr3) =

(5)

where kB is the Boltzmann constant, T is the temperature, η is the viscosity of MEPBr3 (∼0.011 Pa·s) at 25 °C,6 and rBr− is the ionic radius of Br− (182 pm). Current transient behaviors in each current spike were in good agreement with simulated ones, as shown in Figure 6. Furthermore, an average size of re is estimated to be 1.7(±0.7) μm by fitting the simulation curves into the eight experimental current spikes. In addition, the maximum CBr−(MEPBr3) was estimated to be 7.5(±3.2) M based on the estimated V and the integrated charges obtained from the eight individual current spikes (see Figure S6 and Table S1 for further information). The relatively large standard deviation of Br− concentration in MEPBr3 droplets might be mainly due to the discrepancy in the two geometrical shapes of hypothetical77,78 and actual MEPBr3 droplets. Such a high concentration implies that Br− is highly soluble in electrochemically generated MEPBr3 droplets. Because CBr−(MEPBr3) is considered to be much higher than CBr−(aq) within a diffusion layer on Pt UME, the observed iss from the CV shown in Figure 2 is mainly governed by CBr−(MEPBr3). Thus, iss can be calculated by the following equation79

(3)

4DBr−(MEPBr3) πre

kBT 6πηrBr−

iss = 4nFC Br−(MEPBr3)DBr −(MEPBr3)rUME

(4)

(6)

where CBr−(MEPBr3) is 7.5 M, n is the electron number per Br− (2/3), F is the Faraday constant, and rUME is the radius of the Pt UME (5 μm). The iss was calculated to be 1.0 μA, which is close to the measured one, 0.9 μA at 1.1 V in the CV (Figure 2). This implies that the estimation of CBr−(MEPBr3) based on the bulk electrolysis model is valid.

where re is the contact radius of a single MEPBr3 droplet on the Pt UME surface, m is the mass-transfer coefficient of Br− in hemispherical MEPBr3 droplets,79 A is the contact area of the Pt surface with a MEPBr3 droplet (πre2), and V is the volume of the hemispherical MEPBr3 (2/3 × πre3). DBr−(MEPBr3) is the diffusion coefficient of Br− in MEPBr3, which was estimated to 3925

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CONCLUSIONS In summary, a Br− electro-oxidation reaction in electrochemically generated single droplets of MEPBr3 was first observed by the current transient method using UME. The current spikes showed up when MEPBr3 droplets formed (CMEPBr(aq) ≥ 32 mM), implying that they originated from Br− electro-oxidation in MEPBr3 droplets. The voltammetric behavior of a Br− electro-oxidation in single MEPBr3 droplets was similar to that in the aqueous phase, and Br− in MEPBr3 became saturated as CBr−(aq) was more than 52 mM. The current transient curves of individual current spikes from a CA of the MEPBr solution (CMEPBr(aq) = 72 mM) were well-fitted by the simulated plots based on the bulk electrolysis model. Therefore, our study demonstrates that MEPBr3 serves not only as bromine capturing complexes but also as a reaction medium for Br− electro-oxidation. Our in situ electroanalytical method can be further applied to the elucidation of electrochemistry in aqueous systems using various QBr, thereby providing a very useful analytic tool for the Zn−Br and other related RFB systems, where Br−/Br2 is employed as a half-redox reaction and QBr is a necessary component.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.Chae). *E-mail: [email protected] (J.Chang). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (No.20152010103390). We are very grateful to Dr. Tae Hyuk Kang and his group at Lotte Chemical Corp for kind collaboration on the Zn−Br RFB project and to Dr. Heung Chan Lee for valuable discussion.

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EXPERIMENTAL METHODS Chemicals. All reagents and solvents were purchased from commercial vendors and used without further purification. All solutions were prepared with deionized Milli-Q water. Instruments and Measurements. 1H and 13C NMR spectra were recorded on a 500 MHz spectrometer (Varian VNMRS 500), and LC-MS spectra were recorded using ESI mode (Hewlett-Packard Series 1100 and Agilent Technologies 6130). A CHI-600e potentiostat (CH Instruments, Austin, TX) was used for all electrochemical measurements. Three electrodes were used in an electrochemical cell, Pt UMEs (radius: a = 5 μm) as working electrodes, Ag/AgCl (1 M KCl) as a reference electrode, and Pt wire as a counter electrode. All electrodes were purchased and used from CH Instruments. The all-aqueous solutions contained 0.5 M H2SO4 under a deaerated condition with Ar. Synthesis and Characterization of MEPBr (CAS No. 69227-51-6).80 To a solution of 1-methylpyrrolidine (8.5 g, 100 mmol) in ethyl acetate (20 mL) was added ethyl bromide (9.2 mL, 120 mmol) dropwise in an ice bath. Then, the mixture was allowed to stir at room temperature for 6 h. The solid product was filtered, washed with ethyl acetate 3 times, and dried in vacuum to give the product as a white solid (17.7 g, 91%). 1H NMR (500 MHz, DMSO) δ 1.26 (t, J = 7.2 Hz, 3H), 2.07 (m, 4H), 2.95 (s, 3H), 3.35−3.52 (m, 6H); 13C NMR (125 MHz, DMSO) δ 63.58, 59.03, 47.59, 21.77, 9.58; MS (m/ z) ES+ 114.2; ES− 79.2 (Figure S1 in Supporting Information)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b12029. 1 H NMR and 13C NMR of MEPBr, pictures of bulk MEPBr3 droplets formed by the addition of Br2 into a 0.5 M H2SO4 aqueous solution containing MEPBr, frequency counts vs ispike associated with a single MEPBr3 droplet’s impact on Pt UME in different Eox and MEPBr concentrations, respectively, and the experimental and corresponding simulation data based on the bulk electrolysis model in more details (PDF) 3926

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The Journal of Physical Chemistry C

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DOI: 10.1021/acs.jpcc.5b12029 J. Phys. Chem. C 2016, 120, 3922−3928