Communication Electrochemistry, 81(11), 890–892 (2013)

Non-electrochemical Nanobubble Formation in Ferricyanide/FerrocyanideRedox Reaction by the Cyclotron Effect under a High Magnetic FieldAtsushi SUGIYAMA,a,* Ryoichi AOGAKI,b,c Ryoichi MORIMOTO,d

Makoto MIURA,e Yoshinobu OSHIKIRI,f Miki MIURA,g

Iwao MOGI,h Yusuke YAMAUCHI,c and Tetsuya OSAKAa,i

a Institute for Nanoscience & Nanotechnology, Waseda University,Waseda-tsurumaki-cho, Shinjuku-ku, Tokyo 162-0041, Japan

b Polytechnic University, Ryogoku, Sumida-ku, Tokyo 130-0026, Japanc National Institute of Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japand Saitama Prefectural Okubo Water Filtration Plant, Saitama 338-0814, Japane Hokkaido Polytecnic College, Zenibako, Otaru 047-0292, Japanf Yamagata College of Industry Technology, Matsuei, Yamagata 990-2473, Japang Yokohama Habor Polytecnic College, Honmoku, Yokohama 231-0811, Japanh Institute for Materials Research, Tohoku University, Katahira, Sendai 980-857, Japani Department of Applied Chemistry, Faculty of Science and Engineering, Waseda University,Okubo, Shinjuku-ku, Tokyo 169-8555, Japan

*Corresponding author: sugiyama@aoni.waseda.jp

ABSTRACTFrom the observation of coalesced microbubbles arising from supersaturated nanobubbles in ferricyanide/ferro-cyanide redox reaction, the first evidence of the conversion from ionic vacancies to nanobubbles has been obtained.Since ionic vacancies are created without any electron transfer, the microbubbles are evolved in a different wayfrom ordinal electrochemical gas reactions.

© The Electrochemical Society of Japan, All rights reserved.

Keywords : Nanobubble, Ferricyanide/Ferrocyanide Redox, Magnetic Field

1. Introduction

Gas evolution reaction represented by the hydrogen- and oxygen-gas evolutions in water electrolysis is one of the most popular andsignificant electrode reactions. In recent years, on the other hand,nanobubbles have been paid much attention as the smallest gasbubbles, and their stability and physical quantities have been widelyexamined.1,2 As for electrochemical gas evolution, hydrogen oroxygen nanobubbles have been found to be generated during waterelectrolysis.3,4 If it is so, what is the origin of nanobubble? In otherword, is there something like elementary particle of nanobubble?

From about ten years ago, we have been studied ionic vacanciesformed in electrode reactions. Ionic vacancy is a negatively orpositively charged free vacuum space surrounded by positively ornegatively charged ionic cloud, of which size is of the order of0.1 nm smaller than the mean free path of gas molecules,5,6 formedfrom the momentum conservation in electrode reactions. By using anew apparatus called gravity electrode, the sizes of the vacancieswere measured by the changes in the partial molar volumes incopper electrodepositions from acidic copper sulfate solution andcopper chloride solution.5­8 As a result, in a 100molm¹3 supportingelectrolyte solution at 27°C, for ionic vacancies with one andtwo unit charges, their average diameters were determined about 0.4and 0.75 nm, respectively, which were in good agreement withtheoretical predictions.5,9 On the other hand, the lifetimes of thevacancies were measured by cyclotron MHD (magnetohydrody-namic) electrode (CMHDE),9,10 which was composed of a pair ofconcentric circular electrodes under a vertical magnetic field. Thelifetimes were obtained by measuring the circumferential velocity of

the solution driven by a Lorentz force on the free surface coveredwith ionic vacancies, e.g., ca. 1 s for ferricyanide/ferrocyanide redoxreaction.8 In all cases, the collision between ionic vacanciesdecreased the lifetime down to 1/100th of the intrinsic value. Aspredicted by the preceding papers,6,10,11 these results suggested thatnanobubbles arise from the collision of ionic vacancies. However,nanobubbles are too small to observe, so that for the visualization ofthe bubble formation, some other process to yield much largerbubbles was required.

In the present paper, using ferricyanide/ferrocyanide redoxreaction, we first introduce a new method utilizing a rotationalfluid flow generated by the cyclotron effect12 in a high magneticfield, which coalesces nanobubbles into microbubbles on the sur-face of electrode. This is a quite new type of gas evolution notthrough electron transfer. Finally, the microbubbles obtained areoptically observed.

2. Microbubble Formation by the Cyclotron Effect of VerticalMHD Flow

In electrode reactions under a vertical magnetic field, as shownin Fig. 1, a tornado-like vortex called vertical MHD (magneto-hydrodynamic) flow emerges by a Lorentz force over an electrodesurface.12 On a free surface without friction covered with ionicvacancies, the solution circulates along the same streamlines (i.e.,cyclotron effect). In the same way as CMHDE, ionic vacanciescreated on the electrode surface collide with circulating vacancies,transformed into nanobubbles. In cathodic reactions such as copperdeposition, negatively charged ionic vacancies are created, yieldingnegatively charged nanobubbles surrounded by positively chargedionic clouds. Due to positively charged outer ionic clouds,6,11The first two authors contributed equally to this work.

Electrochemistry Received: June 2, 2013Accepted: August 10, 2013Published: November 5, 2013

The Electrochemical Society of Japan http://dx.doi.org/10.5796/electrochemistry.81.890JOI:DN/JST.JSTAGE/electrochemistry/81.890

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nanobubbles are once adsorbed on the negatively charged electrodein a cathodic polarization. After supersaturation of accumulatednanobubbles, with the potential sweep in anodic direction, theywill make transition to microbubbles, coalescing with each other.Then, desorbed microbubbles are expected to circulate with thesolution.

3. Experimental

The experimental apparatus was consisted of a home-madeelectrode cell and a CCD camera (Dino-Lite Premier2 S-DINOAD7013MT, AnMo Electronics Corp.) equipped with whiteLED lights which has the flame rate of 10 fps (Fig. 2). The cell had aflat bottom covered with a optical glass, through which the surfaceof a downward-oriented circular platinum electrode was opticallyobserved. To provide a smooth surface covered with ionic vacancies,a bright platinum surface with a 1.6mm diameter surrounded by aplastic film (PTE Platinum electrode, BAS) was used. On thedownside of this working electrode, a circular platinum counterelectrode with a 12mm diameter was placed. Through the edge ofthis counter electrode, the surface of the working electrode wasobserved by the CCD camera. A saturated calomel electrode (SCE)(International Chemistry Co., Ltd.) or a platinum wire as a referenceelectrode was inserted in the neighborhood of the working electrode.

The CCD camera was capable of a magnification power of ca. 200,connected with a personal computer for in situ monitoring of thebubble evolution. The whole apparatus was settled in the bore space(with an upward-oriented magnetic field) of the 40T superconduct-ing magnet at the high magnetic field center, NIMS, Tsukuba Japanor the 15T cryocooled superconducting magnet at the High FieldLaboratory for Superconducting Materials, IMR, Tohoku University.A 500molm¹3 KCl solution with 30molm¹3 K3[Fe(CN)6] + 30molm¹3 K4[Fe(CN)6] and a 500molm¹3 KCl solution with 100molm¹3 K3[Fe(CN)6] + 300molm¹3 K4[Fe(CN)6] were used forferricyanide reduction. To protect oxygen-gas evolution from thecounter electrode, in the latter solution, three times higher concen-tration of ferrocyanide than that of ferricyanide was adopted. Undera magnetic flux density of 8 T, after potentiostatic reduction at anoverpotential of ¹200mV (+230mV vs. NHE) for 3min, for thedesorption of the positively charged ionic cloud of the nanobubbles,the electrode potential was swept to anodic side up to +400mV(+830mV vs. NHE) in a rate of 1mV s¹1. Temperature of thesolution was kept at 12 « 1°C. Prior to experiment, for the evacua-tion of dissolved oxygen, nitrogen gas bubbling was performed.Then during the experiment, the gas was continuously supplied via ahollow fiber filter (M40-200, Nagayanagi Co., Ltd.) at atmosphericpressure.

4. Results and Discussion

Experiment was first performed for a ferricyanide reduction inthe 500molm¹3 KCl solution with 30molm¹3 K3[Fe(CN)6] + 30molm¹3 K4[Fe(CN)6]. After confirming the appearance of micro-bubbles on a thin layer with refractive variation, to make clearerstatic images of the microbubbles, the solution was changed to thatof 100molm¹3 K3[Fe(CN)6] + 300molm¹3 K4[Fe(CN)6] + 500molm¹3 KCl. In Fig. 3, the current response against the potentialsweep is exhibited. Figure 4 shows the surface images observed atthe points ‘I’, ‘II’, and ‘III’ indicated in Fig. 3. Figure 4I shows theelectrode surface during the reduction at the point I, ¹166mV(+264mV vs. NHE). After starting the potential sweep, with a smallcurrent peak at the point II, about +37mV (+467mV vs. NHE), theelectrode surface was covered with a thin layer of a differentrefractive index with the accidental appearance of two white

z

x, y

Magnetic flux

Flat insulation fringe (W.E.)

Circular (disk) electrode

Sheath for electrode

Solution flow near the surface of electrode

Gravity

Figure 1. Vertical MHD flow.

CCD cameraobservation

Superconducting magnet

SCEN2 gas outlet

W.E. R.E.C.E.

Hollow fiber.

Optical glass

Pt cicular electrode with sheath

B

Figure 2. (Color online) Experimental apparatus.

0

0.1

0.2

−0.2 0.0 0.2 0.4

0.03 0.04

Overpotential / V

i / Am

sweep

at 0T

at 8T

I

II

III

II

0.005Am−2

−2

Figure 3. (Color online) Current response against the potentialsweep in anodic direction. The start overpotential is ¹200mV, andsweep rate is 1mV s¹1. I, The first observation potential corre-sponding to Fig. 4I. V = ¹166mV (+264mV vs. NHE); II, Thepotential of nanobubble-layer formation with refractive variation.V = +37mV (+467mV vs. NHE); III, The potential of micro-bubble formation. V = +122mV (+552mV vs. NHE). [KCl] =500molm¹3; K3[Fe(CN)6] = 100molm¹3; K4[Fe(CN)6] = 300molm¹3.

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globules (ca. 0.5mm diameter) coalesced by microbubbles, whichanticlockwise circulated and faded away, being ascribed to thesupersaturation of accumulated ionic vacancies (Fig. 4II). Then, atthe point III, +122mV (+552mV vs. NHE), another refractivevariation together with a current step was observed, and four whiteglobules successively emerged (Fig. 4III), which was thought asthe resulting transition of ionic vacancies to nanobubbles. Thesephenomena were followed by the sudden appearance and circulationof a white globule at +133mV (+563mV vs. NHE) (Fig. 5). Afterseveral anticlockwise circulations, the globule faded from view. Thecirculating velocity was of the order of 1 cm s¹1, which denies thepossibility of cavitation by high-speed rotation. All these eventsoccurred at electrode potentials much higher than that of hydrogenevolution and much lower than that of oxygen evolution. Thesolubilities of ferricyanide, ferrocyanide, and KCl in water at 10°C(lower than the experimental temperature) calculated from theliterature13 were 982, 527, and 5032molm¹3, respectively. In thefirst experiment to confirm the microbubble formation, the30molm¹3 equimolar concentrations much lower than above valueswere used, and then in the second experiment to obtain the clearerstatic image of the microbubbles, even during the initial 3min.potentiostatic reduction, any surface change was not observed.These results therefore deny the possibility of any salt precipitation.The variation of the solution temperature was kept within «1°C, sothat the contribution of the endothermic or exothermic effect of the

reaction on gas solubility is also denied. The magnetization energyin the present case is estimated at most of the order of 1 kJmol¹1,which is much smaller than conventional chemical activationenergies. The light source was white LED without infrared andultraviolet components, which also excludes the possibilities ofbubble evolution from other reasons.

5. Summary

The microbubbles arise from the supersaturated nanobubbles notthrough electron transfer. The nanobubbles in ferricyanide reductionhave positively charged ionic clouds, so that to detach from theelectrode surface, positively charged electrode surface is required.The cyclotron effect provides a field of the supersaturation ofnanobubbles, and also supports the formation of microbubbles.From these results, it is concluded that ionic vacancies are created inferricyanide reduction in a potassium chloride solution.

Acknowledgments

The authors thank the High Magnetic Field Center, NationalInstitute of Materials Science (NIMS), Tsukuba Japan and the HighField Laboratory for Superconducting Materials, Institute forMaterials Research (IMR), Tohoku University for financial supportand access to superconducting magnets.

References

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Figure 4. (Color online) Mirobubble evolution on the electrode. I, Electrode surface during the reduction at an overpotential V = ¹166mV(+264mV vs. NHE); II, Nanobubble-layer formation with refractive variation at V = +37mV (+467mV vs. NHE). Accidental appearanceof two globules of microbubble coalesced by microbubbles; III, Microbubble formation at V = +122mV (+552mV vs. NHE). Four globulesnewly formed. For visualization, the images are subtracted and painted by yellow.

IIIa IIIb

IIId IIIe

IIIc

IIIf

Figure 5. (Color online) Detachment and circulation of a globuleof microbubble at V = +133mV (+563mV vs. NHE). Each imagewas taken at an interval of 0.1 s. For visualization, the images aresubtracted and painted by yellow.

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