Journal of Electroanalytical Chemistry 658 (2011) 18–24Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem Electrodeposition of Co, Sm and SmCo from a Deep Eutectic Solvent E. Gómez a,⇑, P. Cojocaru b, L. Magagnin b, E. Valles a a b Departament de Química Fısica and Institut de Nanociència i Nanotecnologia (IN2UB), Universitat de Barcelona, 08028 Barcelona, Spain Dipartimento di Chimica, Materiali e Ingegneria Chimica ‘‘Giulio Natta’’, Politecnico di Milano, 20131 Milano, Italy a r t i c l e i n f o Article history: Received 1 March 2011 Received in revised form 15 April 2011 Accepted 19 April 2011 Available online 28 April 2011 Keywords: Electrodeposition Cobalt Samarium Deep Eutectic Solvent a b s t r a c t The suitability of 1 choline chloride:2 urea mixture, Deep Eutectic Solvent (DES) for the electrodeposition of cobalt, samarium and cobalt–samarium system has been studied. Its electrochemical window permits deposition analysis to be carried out without interference from parallel reactions. Deposition was studied at 70 °C in order to stimulate mass transfer and to lower solution viscosity. Cobalt deposits according to a nucleation and three dimensional growth mechanism, all its characteristic features do appear for all cases studied. Samarium deposition takes place through a more complex process in which a first potential range is found where the species formed limit the conductive character of the substrate. When potential becomes more negative, normal behaviour is observed. When both cobalt and samarium are present in the solution, codeposition occurs, at no potential value, any current diminution of the current was recorded under stirred conditions. Deposits of SmCo show different morphology and composition depending on applied potential. Nodular cobalt-rich deposits are obtained at low deposition potentials whereas fine grained samarium-rich ones are obtained at more negative deposition potentials. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Electrodeposition of some metals has been restricted in aqueous solutions at moderate temperatures to those presenting a standard potential less negative than that of water reduction. Some of these metals could be deposited, albeit expensively, using either organic solvents or more drastic conditions such as molten salts [1–5]. In the last decade research in ionic liquids (ionic materials with melting point below 100 °C), with a wide electrochemical window, has made it possible to deposit metals with very negative standard potentials and their alloys [6–14]. Metallic coatings that up to then had been impossible to deposit became obtainable. Different generations of ionic liquids have been developed widening the spectrum of depositable metals [15–19]. However, many of them do require a complex synthesis and a very accurate electrochemical work, since the majority of the ionic liquids present high sensibility to water or degradation by oxygen. It has been shown recently that it is possible to create an ionic fluid mixing quaternary ammonium halides with an amide, carboxylic acid or alcohol moiety [20–23]. Such mixtures are not, strictly speaking, room-temperature ionic liquids (RTIL) since in general contain an uncharged molecular component, so that, the term Deep Eutectic Solvent (DES) was coined by Abbott [24]. Unlike the ionic liquids these room-temperature eutectic mixtures ⇑ Corresponding author. E-mail address:
[email protected] (E. Gómez). 1572-6657/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2011.04.015 are easy to prepare in pure state. They are not water reactive and their biodegradability is proven. Moreover, the economic investment involved is much lower than that incurred using roomtemperature ionic liquids. However, little information is available about their physic–chemical properties [25]. The microelectronic industry employs many kinds of coatings in specific parts of the devices, magnetic materials being the most popular ones. Important advances has been achieved in the preparation of soft magnetic materials but the electrodeposition of hard magnetic ones is still restricted to a few ones [26–28]. The challenge in preparing such alloys by electrodeposition is that these alloys contain metals that have electrocatalytic character to hydrogen evolution, very negative standard potential, or both. Then, the use of water-free solvents with a wide electrochemical window is welcome in this field and its possibilities are worthy of study. In this work we select to study the cobalt–samarium electrodeposition processing possibilities in DES medium, as this alloy is potentially a hard magnetic material depending on the metal ratio [29–31]. Few studies of SmCo electrodeposition have been performed in aqueous medium [32] since samarium has a very negative standard potential leading to simultaneous hydrogen evolution. A study about the preparation of SmCo by electrodeposition in urea-acetamide-NaBr-MClx melts has been performed by Liu et al. [33]. Our interest is to analyse the possible electrodeposition of cobalt, samarium and cobalt–samarium systems using the eutectic 1 choline chloride:2 urea mixture as solvent due to its easy preparation, good working conditions and non aggressive nature. in the molar proportion 1 choline chloride:2 urea. a well defined reduction peak appeared previous to a massive reduction current (Fig. were warmed and removed constantly to achieve the liquid state of the deep eutectic solvent (DES). cleaned ultrasonically for 2 min in water and dried with air prior to be immersed in the solution. curve b).113 M solution (A) at different cathodic limits. and (B) with potential holds of: (a) t = 0 s. as corresponds to deposition process. cobalt salt was maintained in a stove at 110 °C in order to assure maximum dehydration. and (c) grey: after 50 h of first scan. Prior to dissolution. curve c). The counter electrode was a platinum spiral. For all the cathodic limits used. Results Fig. For more negative potential limits. Electrochemical experiments were carried out using an Autolab with PGSTAT30 equipment and GPES software. an increase in the oxidation charge was recorded increasing the hold time (Fig. Voltammetric experiments were carried out at 50 mV s 1.75 lm and 1. Elemental composition was determined with an X-ray analyser incorporated in Leica Stereoscan S-360 equipment and by X-ray fluorescence (XRF). 1) and to ensure reference electrode reproducibility in the DES medium.5 V. Deposits morphology were observed using Hitachi S-2300 scanning electron microscope. scanning at first to negative potentials. 1. curve a).E. Stable and reproducible values of the potential were obtained with this reference electrode. Fig. Magnetic stirrer was used when the agitation effect was tested. Holding the scan in the potential range of the first reduction peak. By reversing the scan at very low negative overpotential. 3. Solutions were prepared using the DES solvent. The reference electrode was an Ag|AgCl/NaCl 3 M mounted in a Luggin Cyclic voltammetry was selected to perform the initial electrochemical study of the deposition processes. nucleation loop was observed (Fig. Experimental Solvent was prepared using choline chloride (from Across Organics) and urea (from Merck) of analytical grade. Cyclic voltammetric scans were recorded at different negative limits. 2. Work temperature was kept constant at 70 °C to favour low viscosity and high conductivity of the solvent. Cyclic voltammograms from CoCl2 0. curve b).1. Fig. 1. (b) dashed: after two hours of first scan. the electrochemical response of the blank solution (DES solvent) was recorded to establish its electrochemical window (Fig. and cobalt chloride from Fluka both of analytical grade were used as source of electroactive species. 2A. Cobalt deposition A basic study of cobalt deposition was made using the habitual electrochemical techniques. 1 shows the coincidence between voltammetric scans performed immediate after electrode immersion and those made hours after. A wide electrochemical window in vitreous carbon electrode was observed between the reduction–oxidation processes of the solvent. the oxidation charge increase recorded was less important and a widening of the peak to positive potentials was observed (Fig.25 V and (c) t = 30 s at . Gómez et al. Before and during experiments solutions were de-aerated with argon. a single oxidation peak was recorded during the positive scan. 2. When the hold was made in the potential range of the massive reduction during the same time. Working electrodes were vitreous carbon rods (from Metrohm). The solids. (b) t = 30 s at 1. Cyclic voltammograms in DES solvent at: (a) black: immediately after desoxigenation. / Journal of Electroanalytical Chemistry 658 (2011) 18–24 19 capillary containing the DES solvent. 3. 2A. Firstly. Samarium nitrate from Aldrich. A cylindrical three electrode cell of one single compartment was used. 2B. 2B. Vitreous carbon electrode was polished to a mirror finish using alumina of different grades (3. Only one cycle was run in each voltammetric experiment.87 lm). (b) 0. Gómez et al.34 V during 60 s.113 M at the [Co(II)] = 0. Stirring the solutions. a displacement of the maximum to low deposition times and an increase in the recorded current was observed increasing the concentration (Fig.019 M. (A) j–t transients at 1. 3A).113 M. By comparing the j–t curves obtained at fixed potential from different cobalt (II) concentrations. the potentiostatic experiments were made stepping the potential from a potential value at which no current was detected to different potentials values. Voltammetric potential holds under stirred conditions from the [Sm(III)] = 0.78 and 2.52 V during 60 s and (C) 1. For a fixed concentration. For all the solutions tested. All the j–t transients recorded evolved to a maximum from which the current 8 mA cm 2 . (B) E–t transients (a) [Co(II)] = 0.20 E.2 V from different [Co(II)] solutions: (a) 0. (c) j = 16 mA cm 2 and (d) j = 32 mA cm 2.025 M solution at: (B) 1. the maximum appeared a short deposition times as the potential applied was made more negative.005 M. (b) 0. (quiescent conditions). the only difference observed being the advance in the appearance of the reduction current and the increase of the current recorded as the Co(II) concentration was increased.036 M solution curves: (b) j = 8 mA cm 2. This general behaviour was similar using solutions with different Co(II) concentration. / Journal of Electroanalytical Chemistry 658 (2011) 18–24 Fig.045 M. 4. Fig.036 M and (c) 0. (A) Cyclic voltammograms from different Sm(III) solutions: (a) 0. and from profile decays to attain a quasi stationary value. .025 M and (c) 0. the current decay after the maximum was reduced. 3. . 4B and C). / Journal of Electroanalytical Chemistry 658 (2011) 18–24 Galvanostatic experiments under stirring conditions revealed that the system initially attained a minimum negative potential value (potential spike) that evolved to less negative values as the deposition time increased (Fig. (b) j = 6.5 V and (d) 1. at stationary conditions. Whereas when the potential hold was made in the potential range of the second peak predictable behaviour was recorded: a diminution of current was observed under stationary conditions but important current increase occurred under stirring conditions (Fig. the reduction process is partially blocked for some reduction products. (C) E–t transients from [Sm(III)] = 0. All the experimental results obtained. During the positive scan no oxidation current was recorded even when very low cathodic limits were used. consecutive cyclic voltammetric scans evidenced that as the number of scans increases. Therefore. 3B. In general. The two reduction peaks were advanced. under stirring. As the applied current was made more negative. curves b–d). increasing the number of consecutive scans favours the onset of deposition process when a previous deposit is present. the first part of the curves recorded under quiescent conditions and from ⁄ symbol.2. Gómez et al. maximum in the j–t transient and nucleation spike in the E–t transients evidenced that the characteristic outputs observed in water dissolutions [35–37] continue being valid in the DES solvent demonstrating that nucleation and growth process takes place for cobalt deposition in this eutectic solvent.2 mA cm 2 under stirring. (c) j = 6. This result was corroborated by a set of voltammetric experiments holding the scan along the potential range of the first peak: both under quiescent and stirred conditions the reduction current decreased during the hold (Fig. nucleation loop. 4A). (b) a polished substrate under agitation and (c) the substrate after experiment b. (c) 1. and (f) solvent response at j = 3.E.2 mA cm 2.4 mA cm 2 quiescent solution. curve a). it seems that in the potential range of the first peak. For all Sm(III) concentrations. 4C). although in this type of solvent vigorous agitation was needed to appreciate the effect of the stirring arising difficult ion transport. This is a rather unexpected behaviour. Fig. The stirring of the solution minimises the [Co(II)] depletion near the electrode.045 M (a) j = 3. (b) 1.045 M Sm(III) at j = 1. enhanced and better defined as the Sm(III) concentration increased. In none condition oxidation current was revealed during positive scan. (A) j–t transients from the [Sm(III)] = 0.5 mA cm 2 under stirring. followed by a second peak developed previous to massive current (Fig.025 M solution at different potentials: (a) 1. potentiostatic and galvanostatic responses to the Co deposition process in the eutectic 1 choline chloride:2 urea mixture reveals the transport control of the process. as in aqueous solutions [34]. (B) E–t transients from 0. (d) j = 9.5 mA cm 2 quiescent solution. (e) j = 9.6 mA cm 2 using: (a) a polished substrate at stationary conditions. more negative potentials were associated (Fig. 3.4 V. 3B. By reversing the scan in none condition nucleation loop was recorded. voltammetric experiments revealed similar features in the negative going sweep: at 21 first a peak-band appeared.3 V. The shape of both voltammetric and potentiostatic curves and the effect of the stirring over the voltammetric.4 mA cm 2 under stirring. 5. the onset of the current appearance occurs at more negative potentials even when the negative potential limit corresponds to low overpotentials. Samarium process For all the samarium (III) solutions tested.6 V. Deposits prepared under agitation at different conditions were analysed by scanning electron microscopy. This discards the possible effect of the oxygen presence. it was not observed in any case the appearance of nucleation loop. In all conditions the potential value at which the solvent reduction takes place was very negative compared to the values where the metallic cation reduced evidencing that in all cases the main process corresponds to deposit formation (Fig. 6C). Galvanostatic transients were recorded. in case that galvanostatic experiment was repeated on a substrate. At long deposition times the deposit was cracked (Fig. Similar results were obtained when deoxygenation of solution was made during 10 h prior to the experiment. Nevertheless when the applied currents lead the system to potentials corresponding to the second voltammetric peak. thus covering stabilization potentials corresponding to the two reduction peaks. in both stationary and stirring conditions and applying wide current densities spectrum. the potential value achieved was more negative than the corresponding one to the polished substrate (Fig. SEM images of Samarium deposits obtained during a voltammetric hold potential at 1. 5C). Low or nor oxidation current was detected in the positive scan. stirring of the solution lead the potential to more negative values than that corresponding to quiescent conditions (Fig. Voltammetric scan evidenced a slight advance of the overall deposition process (Fig. the observed behaviour corresponds to a mass control transfer: the potential value achieved under stationary conditions was more negative than the corresponding under agitation (Fig. curves a and b). 7A. . curve f). Whereas if the potential applied corresponds to more negative potential. curve a) respect to that observed in the free cobalt (II) solution (Fig. Enlarging the cathodic limit to more negative limits. 7A.45 V under stirred conditions from the solution 0. Moreover. curve b). Independently of the potential.045 M Sm(III) + 0. Increasing deposition time the coverage increased and the deposit showed some cracks and a new growth was observed on the first deposit (Fig. The imaging of deposits obtained after a voltammetric hold at potential values corresponding to the first voltammetric peak. 3. in all applied potentials. 6D). 8A. the j–t transients recorded showed a previous current peak followed by a smooth current increase. 6. 7B). 8A). 5C. Gómez et al. inset as curve c). In these experiments low current were flowed and the time consuming to achieve sufficient coverage was important. which has been maintained into the solution and thus with the deposit already formed present. The j–t transients corresponded. Reversing the scan at the current onset a nucleation loop was recorded followed by an oxidation peak (Fig. 5B. At quiescent conditions no clear nucleation spikes were observed. curve c). In all potential range where codeposition occurs. However when the solution was stirred the potential increased with the increase of deposition time making in evidence the favourable stirring effect over the deposition process. more evident when the solution was stirred (Fig. and (D) SEM image of a deposit obtained at 1. it seems that this feature corresponds to cobalt deposition. to a sudden current increase followed by a current decay. As the potential applied was made more negative the current recorded was greater. a smooth current diminution was recorded. The agitation effect was also stated in the potentiostatic curves. the coating showed different morphology in which flat deposits were observed (Fig. For currents that evolved to potentials corresponding to the first voltammetric peak. revealed that: low deposition times lead to very fine grained deposits (Fig. curves c and d). positive difference in the charge recorded under agitation was observed (Fig. 6B).045 M of Sm(III) after: (A) 15 min.22 E. 6A). with the stirring of the solution the charge involved in the reduction process was enlarged (Fig. although the shape of the curve remembers the typical nucleation feature and with the time the potential dropped smoothly to more negative values. / Journal of Electroanalytical Chemistry 658 (2011) 18–24 Fig.3.9 V during 30 min under stirred conditions in a potentiostatic experiment. 7A. The deposits obtained potentiostatically in this zone present similar morphology. 5B. Samarium–cobalt deposition The effect of cobalt (II) in the samarium (III) deposition process was studied using a solution containing 0. The stirring of the solution lead to an increase in the recorded current but was not sufficient to maintain stationary value and especially when the most negative potentials were applied. Galvanostatic experiments were made applying currents sufficiently negative to allow codeposition of both metals.018 M Co(II). (B) 25 min and (C) 80 min. Scanning towards negative potential a peak was detected followed by a close peak that did not clearly developed since a current increase was observed. This deposit of 27 wt. 8. Gómez et al. or even lower. By applying similar. 8C). / Journal of Electroanalytical Chemistry 658 (2011) 18–24 23 Fig. (b) 1.6 V during 2500 s.% of Sm independently of the thickness. In order to check the possibilities to deposit on a metallic substrate a Ni/Cu/Au slide was used. the composition of the deposits corresponded always to 80 wt.8 V and (d) 1.018 M.045 M Sm(III) solution containing: (a) [Co(II)] = 0. .018 M Co(II) solution under: (a) quiescent and (b) stirred conditions. fine grained deposits with homogeneous coverage were obtained (Fig.E.045 M Sm(III) + 0.9 V and SEM pictures of deposits obtained over vitreous carbon at: (B) E = 1. Low deposition rate was obtained in this case. Deposits prepared at different potentials under stirred conditions were imaged and their samarium percentage was evaluated.6 V during 900 s and (D) E = 1. Increasing the deposition time a new growth over the first deposit was developed (Fig.% of Sm shows nodular morphology with rounded grains. 8B shows a detail of the morphology of SmCo deposit obtained in a potential near to the minimal necessary to induce codeposition.% of Sm.9 V. When the potential was made more negative.045 M Sm(III) + 0. (A) j–t transients from 0. 7.8 V. and under stirred conditions at (c) 1. Fig. Fig. deposition potentials than those over vitreous carbon. These deposits contain 75 wt.3 V during 3600 s. (A) Comparison of cyclic voltammograms obtained from 0. (C) E = 1.018 M Co(II) solution under quiescent conditions at: (a) 1. 8D). (B) Voltammograms obtained from the 0.018 M and (b) [Co(II)] = 0 M. Inset of the figure shows the voltammogram recorded reversing the scan at the initio of the deposition and (c) [Co(II)] = 0. after one hour of deposition no complete coverage was allowed. [5] D. Zein El Abedin. Chem. [36] S. C. Sanz. Electrochem. Kazuhisa Azumi.P. 154 (6) (2007) D322. 83 (2005) 51.M. D. Swain. Hope. [11] Taku Oyama. M.% of samarium are obtained. the morphology and the composition of the films prepared at low deposition potentials do not give any insight into such intermediate Sm species nature. Givord. Wilkes. M.P. Moreover.A. Fletcher. Abbott. F. Sci. P. Am. Magn. T. Electroanal. Charles L.). G. Luo. Khlopkov. . [4] E. L. Soc. [12] F. Electrochem. p.J. Q. Chem. Q. Wolf. T. Interface Anal. J. Inst. S.Q.C. intermediate samarium species formation can occur that lowers the conductive character of the electrode.M. [24] A. Nelson. Soc. J. 303 (2006) e367. 126 (2004) 9142. Phys. J. Anicai. Endress. Trans. From the Sm(III):Co(II) ratio used (around 4:1). 54 (2006) 997. samarium deposition in DES solvent shows a less standard behaviour. [18] A. [35] J. A. Gates. H. in: A. Mater. G. Rasheed. W. Chem. G. [19] J. Kirkwood. 5 (1997) 21–36. Schwartz. Capper. W. [7] Fei Xiao. Electrochem. P. K. Boland.J. [10] S. B. Thirsk. France. Bioelectron. Smirnov. (2003) 70. Marín. Proc.P. Annecy. Overcoming this potential range. ulterior work will be carried out to electrodeposit SmCo films of lower Sm percentages which could be useful as hard magnetic material. Dempsey. He. Farag. D.R. Electroanalytical Chemistry. J. [29] Y.X.L. 650 (1) (2010) 1. [31] A. G. Cobalt deposition analysis in this DES solvent reveals that cobalt deposits through a typical nucleation and three dimensional process. Munro. 2008. Chem. Lwin. G. Baizhao Zeng. 43 (2004) 3447. Zaworotko. Acta 52 (2007) 2755–2764. Po-Yu Chen. 204 (2003) 295. Visan. R.L. A. [25] Tetsuya Tsuda. even under stirring conditions. Li. [27] N. M. [16] J. 103 (2008) 043911. Chem. This result confirms also that the nucleation-growth models developed to describe the metal deposition processes could be applied in this kind of solvents. at these low potentials. Carlin. Although the agitation effect is lesser than the observed in parallel studies at similar Co(II) concentrations in aqueous solutions. Walther.G. Rasheed. Comm. T. Smirnov. vol. Welton. Fagiong Zhao. Harrison.S. Having demonstrated the possibility of obtaining CoSm deposits using a DES solvent. Curr. A. Du. G. vol. Westcott. [37] A. Masazumi Okido. H. Davies. Capper. (1992) 965. Khlopkov. K. Surf. Takeyoshi Okajima. Bousenko. 157 (8) (2010) F96. Boyd.C. [34] E. However. the deposits obtained contain high samarium percentage. Soc.A. Ryder. Commun.Z. [28] R. Tong. Bothby. [21] A. O. 333 (1992) 93. SmCo deposits can be formed because the potential deposition range of the alloy is included in the electrochemical window of the solvent. A. D. [8] Jing-Fang Huang. J. V. D.S. Germany. R. Gómez et al.A. of alloys containing some transition metal. [20] A. Abbott.L. Acta 49 (19) (2004) 3251. Wei. Electrochim. Liang. Electrochem. ECS Trans. Xu. K. Electrochem. [9] A. Acta Mater. Work is in progress in order to elucidate the nature of this film. Zhirong Mo. Xu.J. Electrochem. Acta 56 (3) (2011) 1130. G. Gemming. L. [2] A. two clear reduction regions with diverse electrochemical behaviour being detected. Etenko. N. L.Z. Magn. Commun. Section IX ‘‘MAGMAS Materials’’. Abbott. McKechine. Chem.M.K. Capper. 5.P. Susumu Kuwabata. Dempsey. in: Proceedings of the 18th Int. K. 8 (2006) 4265. Workshop on High Performance Magnets and their Applications. Schultz. R. J. 153 (1) (2006) C57. [3] M. Physica B 299 (2001) 199. P. Tambyrajah. 159 (1983) 267. Chem. Biosens.24 E. The authors wish to thank the Serveis Cientificotècnics (Universitat de Barcelona) for the use of their equipment. maximum in potentiostatic experiments and the typical spike in the galvanostatic ones.L. When both cobalt and samarium salts are present in the 1 choline chloride:2 urea eutectic mixture. 67. Gómez. Yang. Chem. Vallés. Pattanaik. Osteryoung. Finish. Electroanal. experiments at less negative potential were made. Electrochim. Taxil. [17] J.Y. Hussey. 2. This percentage is higher than that the corresponding to SmCo5 or Sm2Co17 magnetic alloys.G. L. Acta 54 (22) (2009) 5300–5306.J. 19 (2002) 97– 802. Fray. Gutfleisch. Chai-Liu Kao. Massot. 99 (1999) 2071. At low overpotentials. D. samarium deposition takes place as expected because the process is now not inhibited and solution stirring leads to an increase of the current density. Electrodeposition from Ionic Liquids. M. New York. H. Bard (Ed. during samarium deposition in this DES solvent. A. [26] G. R. Gutfleisch. Soc. Deping Mei. 144 (1999) 3881. Shchetkovskiy. on metallic substrate. Laure E. 24 (2009) 3481–3486. [15] T. Wheeler. Liu. Capper. Electrochim. 20 (2004) 295. Milchev. 42 (2010) 1271. Lisenkov. Müller. D. Halliday. Guo. J. D. [14] Hsin-Yi Huang. K. J. Yan. [32] J. Inorg. However. J. References [1] Dawei Wei. 599 (2) (2007) 288. / Journal of Electroanalytical Chemistry 658 (2011) 18–24 4. F. 12 (6) (2010) 729–732. Weinheim. M. K. Nobe. Then. Chem. Florea. W. SmCo deposits with Sm percentages that exceed 70 wt. Abbott. Marcel Dekker.P. [33] P. N. Chem. Conclusions This study demonstrates the DES capability to permit the electrodeposition. Ferreiro. Soc. Capper. Phys.S. 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Feng. 3 (14) (2007) 151–157. when metallic substrate is used or a sufficiently negative potential is applied over vitreous carbon. G. D.P. [6] A. Chamelot.). Shchetkovskiy. Rasheed. a clearly less conductive coverage than the vitreous carbon used as substrate is formed during the reduction process. 2004. [22] A. Acknowledgements This paper was supported by contract CTQ2010-20726 (subprogram BQU) from the Comisión Interministerial de Ciencia y Tecnología (CICYT). J. Met. Appl. Chem.T.-H. Gibilaro. A. Schäfer. Top. Chen. McKenzie. Electroanal. Rev.L. the observed behaviour make in evidence that the transport in this kind of systems is another parameter to consider.