Desalination 199 (2006) 59–61

Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy.

Role of water splitting in development of electroconvectionin ion-exchange membrane systems

Elena Belovaa, Galina Lopatkovaa, Natalia Pismenskayaa, Victor Nikonenkoa*, Christian Larchetb

aPhysical Chemistry Department, Kuban State University, Stavropolskaya 149, Krasnodar, Russiaemail: v_nikonenko@mail.ru

bLaboratoire des Matériaux Echangeurs d’Ions, Université Paris 12, av. du Général de Gaulle,Créteil 94010, France

Received 18 October 2005; accepted 2 March 2006

1. Introduction

Ion-exchange membranes are widely used inelectrochemical separation processes as well asin other applications such as fuel cells. Toimprove these processes, it is important to betterunderstand the mechanisms of mass transfernear membrane/solution interface as dependingon physico-chemical surface properties. Inten-sive current modes present a special interest asthey are accompanied by several coupledeffects: water splitting and coupled convection.Electroconvection (electroosmotic slip of thesecond kind) occurs due to the action ofimposed electric field on the space electriccharge produced by the same field. The courseof electroconvection does not directly depend onthe membrane position in the gravity field.Gravitational convection is caused by the inter-action of Archimedean and gravity forces. In theposition when diluted (hence lighter) diffusionlayer is under the membrane, the gravitationalconvection is absent. It occurs in other positions

attaining maximum when the lighter layer isover the horizontal membrane.

2. Results and discussion

To make clearer the role of water splittingin development of electroconvection, twoanion-exchange membranes were studied. Aheterogeneous MA-40 membrane has ternaryand secondary ammonium groups that are char-acterized by a strong catalytic activity regardingwater splitting. The experimental MA-40Mmembrane has been produced from a MA-40membrane by a surface modification; it has thesame surface structure as the origin MA-40,but quaternary ammonium groups in a layer nearthe surface instead of the ternary and secondaryones. The quaternary groups are characterizedby a weak catalytic water splitting activity.Current–voltage curves (CVC) and chronopo-tentiograms (ChP) of the both membranes in a0.005 M NaCl solution as well as pH near thedepleting interface are measured in a cell [1]where the distance between two neighboringmembranes is 7.0 mm and the flow velocity*Corresponding author.

doi:10.1016/j.desal.2006.03.1420011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.

60 E. Belova et al. / Desalination 199 (2006) 59–61

0.31 cm/s. Fig 1. shows that the replacement offixed groups with strong catalytic activity bythat with weak ones leads to a reduction in thewater splitting rate resulting in smaller variationof pH near the membrane. Fig. 1 presents alsothe overpotential drop Dj¢ (the total potentialminus iR, where the ohmic resistance R isdetermined by the initial slope of the CVC)through the MA-40 and MA-40M membranes

as a function of the current density applied. Asit can be seen, Dj¢ varies only slightly whenchanging the membrane position in the gravityfield. This signifies that in conditions of theexperiment (a dilute solution, a relatively smallinter-membrane distance, and a considerableforced convection), the influence of gravitationalconvection is negligible. In the range 1.0 < i/ilim

< 2.0, DjMA-40M > DjMA-40 because of higherwater splitting rate at the MA-40 interface,which gives there more new current carriersthan at the MA-40M; the coupled convection issmall under these conditions. The electroconvec-tion appears in both membrane systems whenthe overpotential becomes close to 1.5 V.There is a threshold (» 1.5 V) when severaloscillations of potential drop occur before Dj¢attains a stationary value (Fig. 2a). Then, in theMA-40M system under a little bit higher currentdensity (i/ilim = 2.7), regular periodic oscillationstake place (Fig. 2b), which are changed intochaotic ones under i/ilim > 3.8 (Fig. 2c). Thescenario of the oscillation development withgrowing current is in an excellent accordance withthat predicted by Rubinstein et al. [2]. However,intensive electroconvection is observed only inthe case of MA-40M membrane where low H+

(OH–) generation is produced. In the case of

0

0.5

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1.5

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0i, mA/cm2

Δϕ −

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, V

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7pHilim

pHMA-40

pHMA-40M

MA-40M

MA–40

1

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2

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Fig. 1. The overpotential drop and pH near the depletinginterface vs. current density in vertical position (1) andhorizontal position with the lighter solution under themembrane (2).

0

0.5

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1.5

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2.5

0 604020t, s

0 604020t, s

0 4020t, s

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, V

i/ilim = 2.3

MA-40vertical position

MA-40horizontal position

(a) (b) (c)

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i/ilim = 2.7 i/ilim = 3.8

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MA-40vertical position

0

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hm, V

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MA-40Mvertical position

Fig. 2. Chronopotentiograms of MA-40 and MA-40M systems in vertical and horizontal position with the lighter solu-tion layer under the membrane.

E. Belova et al. / Desalination 199 (2006) 59–61 61

MA-40 membrane the electroconvection seems tobe suppressed by the water splitting, as a conse-quence, Dj¢ over this membrane becomes higherif i/ilim > 2.1.

3. Conclusion

It is found that under certain conditions (adilute solution, a relatively small inter-mem-brane distance, and a significant forced convec-tion), the contribution of the gravitationalconvection in the ion transfer is negligible.Considerable electroconvective mixing of thedepleted solution starts at an overpotentialthreshold close to 1.5 V. With increasing currentdensity, intensive potential oscillations appearin a MA-40M membrane system where thewater splitting is weak. The scenario of oscilla-tion development there is in an excellent agree-ment with that predicted theoretically by I.

Rubinstein. It seems that the intensive watersplitting occurring at a MA-40M membranesuppresses the electroconvection.

Acknowledgement

This study is supported by INTAS, grant no.04-83-3878, and RFBR, grant no 04-03-32365.

References

[1] E. Volodina, N. Pismenskaya, V. Nikonenko,C. Larchet and G. Pourcelly, Ion transfer acrossion-exchange membranes with homogeneous andheterogeneous surface, J. Colloid Interface Sci.,285 (2005) 247–258.

[2] I. Rubinstein, B. Zaltsman, I. Prets and K. Linder,Experimental verification of electroosmotic mech-anism of overlimiting current conductance througha cation exchange electrodialysis membrane, Rus.J. Electrochem., 38 (2002) 853–864.