- Email: [email protected]
Preparation and characterization of RuO2–IrO2–SnO2 ternary mixtures for advanced electrochemical technology
Applied Catalysis B: Environmental 67 (2006) 34–40 www.elsevier.com/locate/apcatb
Preparation and characterization of RuO2–IrO2–SnO2 ternary mixtures for advanced electrochemical technology Lourdes Vazquez-Gomez, Sergio Ferro, Achille De Battisti * Department of Chemistry, University of Ferrara, via L. Borsari 46, 44100 Ferrara, Italy Received 1 December 2005; received in revised form 10 March 2006; accepted 24 March 2006 Available online 16 May 2006
Abstract Electrochemical methods have proved to be particularly effective both for water detoxification (abatement of heavy metals and organic impurities) and sterilization, from natural resources to utilization points, combining these features with low costs and easier handling of equipments. The efficiency of electrochemical methods strongly depends on electrode nature, anodes being of particular importance because they must stand much more severe polarization conditions and have to exhibit good catalytic activity for complex reactions like oxygen and chlorine evolution. The present work is devoted to the preparation and characterization of mixed-oxide electrode coatings based on IrO2, RuO2 and SnO2, with the scope of finding optimal compositions guaranteeing longer service-life under the critical conditions where oxygen evolution is concomitant with chloride oxidation. # 2006 Elsevier B.V. All rights reserved. Keywords: Electrocatalysis; Oxide electrodes; Chlorine evolution reaction; Oxygen evolution reaction; Water sterilization
1. Introduction The challenge of the increasing demand for quality water can be met only applying all suitable scientific and technical methods to increase the productivity of process technology for the conversion of low-grade water into pure water. Sterilization and elimination of organic micro-impurities from drinkable water and water for surgery-medical use is of utmost importance [1]. Many different methods for improving the water quality are already established: these include membrane-based methods, ultra- and micro-filtration, biological, chemical, physicochemical treatments. More traditional purification methods, although generally quite effective for the destruction of bacterial and fungine pollutants, as well as for organic/inorganic chemical impurities, may result relatively ineffective for the elimination of viral components and residual organic substrates [2]. A solution to these problems can be sought in advanced electrochemical technologies based on electric field and Faradaic effects in specially designed electrochemical cells, readily installed at the consumption sites. Two main types of treatment can be considered [3]:
* Corresponding author. Tel.: +39 0532 291124; fax: +39 0532 240709. E-mail address: [email protected] (A. De Battisti). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.03.023
1. Drinkable waters suffering degradation of properties (due to lengthy routes, pipe damages, uphill-treatment problems) may be subjected to a ‘‘direct’’ electrochemical treatment, i.e., the water is passed through anodic and cathodic compartments of one or more electrochemical cells with specific features. The process depends on applied electric field, pH ‘‘shock’’ and Faradaic processes (anodic oxidation, cathodic reduction) and results in the elimination of biological and chemical impurities (traces of heavy metals) and in the abatement of the redox potential to 0.10 (vs. SCE). 2. Good quality water for surgery, food industry, etc. may be obtained through an ‘‘indirect’’ treatment, adding suitable amounts of so-called ‘‘neutral anolytes’’, which are produced by electrolyzing diluted brines; under optimal conditions, a complete elimination of the biological and chemical impurities can be obtained. Both treatments prove to be effective, requiring low energy consumption while ensuring long service-life and low costs. Concerning the disadvantages, the possible need for frequent maintenance applies in particular for the direct-treatment concept. The above technology has already found practical applications and its further development requires improvement of cell
L. Vazquez-Gomez et al. / Applied Catalysis B: Environmental 67 (2006) 34–40
engineering [4] and electrode materials, of the anodes in particular. The indirect, chloride-mediated water sterilization/detoxification involves a quite complex set of reactions, at the electrode surface as well as in the bulk of the solution, which are mostly related with the electrochemical reactivity of Cl and the chemical/electrochemical reactivity of some of its oxidation products. Following the rich literature available on the subject, the anodic oxidation of Cl can be represented, in principle, by two relatively simple mechanisms, both involving a discharge as a first step, with adsorbed radical formation, and differing in the successive radical desorption step: chemical or electrochemical. The explicit involvement of adsorbed intermediates in the process makes it an important example of catalysis in an electrochemical reaction and assigns the electrode material the utmost importance. The highly positive potentials, at which the reaction takes place, inevitably cause changes in the degree of oxidation of the electrode surface, which justify further mechanistic hypotheses, including hydroxyl radical formation as a kinetic step preliminary to chloride oxidation. The competition with the oxygen evolution reaction, at extents that depend on pH, further enhances the importance of the nature of the electrode catalyst not only in terms of activity, but also of selectivity and stability. In the present work, it has been thought of interest to start a research on the latter point, studying electrode materials which could respond to requirements of stability and lower cost with respect to the available technology. Thus, the research concerns the development of new anode materials based on RuO2–IrO2– SnO2 ternary mixtures [5,6], deposited on Ti supports. Simpler systems, based on RuO2–SnO2 [7,8] or IrO2–SnO2 [9,10], are less suitable, either for the poor service-life, in the first case, or for the cost in the second. 2. Experimental The oxide coatings were prepared by a thermal decomposition method described in [11], requiring an oxidative pyrolysis step of precursor–salt deposits at 480 8C, for 20 min. Noble-metal precursor salts were, as often suggested in the literature, hydrated ruthenium trichloride and tetrachloroiridic acid, which were dissolved in water, with addition of acetic acid, up to the appropriate concentration. The tin–precursor– salt was an aceto-chloro-complex of Sn(IV) [11], which does not undergo any volatilization during the drying and successive thermal decomposition stages. The synthesized coating compositions were: IrxRu0.34xSn0.66O2 (x nominal values: 1.7, 3.7, 7.3, 11.6, 17.9, 23.3, 28.4, 31.6 and 33.5%). Prior to use, Ti plates were cleaned in 10% aqueous oxalic acid, kept at a temperature of 90 8C, carefully rinsed in triply distilled water, then kept in hot concentrated NaOH for a few minutes, rinsed with water again and eventually dried in an oven at 120 8C [12]. The microstructure, surface morphology and composition of these mixed-oxide electrodes were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis; electrochemical surface characterization was carried out by linear sweep
35
voltammetry, cycling the working electrode between 0.15 and 1.15 V vs. SCE. A double-walled, saturated calomel electrode (SCE), with an intermediate saturated NaNO3 solution, minimizing the occurrence of insoluble-salt precipitation inside the frit, was used as the reference in a conventional three-electrode cell. For these measures, a 1 M HClO4 test solution was chosen. In consideration of the importance of the chloride anodic oxidation (e.g., in neutral anolyte production), a study of chlorine evolution reaction was also carried out, making use of quasi-steady polarization data. Potentiostatic curves were recorded in 0.01 M HCl, in the presence of 4.25, 1 or 0.1 mol dm3 NaCl; all solutions were saturated with gaseous chlorine, and potential values were corrected for the uncompensated ohmic resistance. Finally, in order to ascertain the long term stability of the different anode materials, service-life tests were carried out in 0.5 M Na2SO4, at 0.5 A cm2; in these experiments, anodes were considered as passivated upon a 2 V increase of their potential under the applied current density. 3. Results and discussion XRD measurements have proved the formation of a rutilestructured RuO2–IrO2–SnO2 solid solution as the only crystalline component of the coatings, through the whole composition range explored. The average crystallite size was in the range 3.0–4.0 nm, moderately increasing with increasing the Ir/Ru ratio. Small measurable effects could be observed, in the opposite direction, also for the a = b cell parameter, while the c parameter remained substantially unchanged. SEM pictures for an as-prepared electrode, with a nominal iridium concentration of 23.3 mol%, are shown in Fig. 1. Beside the well-known and scarcely diagnostic ‘‘cracked-mud’’ texture, no other significant features could be recognized by SEM. This may be an indirect evidence of a homogeneous distribution of microporosity across the film surface. Increasing the iridium content in the ternary mixture, the metal tends to accumulate in the inner layers (enrichment in the cracks between islands), as evidenced by EDX; in contrast, ruthenium seems to maintain a better dispersion. More quantitative data cannot be obtained by EDX, the analytical answer depending on different factors, including the sample thickness and topography, and the atomic number of each component. These considerations suggest that data may be taken as proofs of the presence (or absence) of a given element, while quantitative evaluations should be obtained by different procedures. Fig. 2 shows cyclic voltammograms, recorded at a potential scan rate s of 100 mV s1, for the two limiting compositions, i.e., the ternary mixtures with the lowest and the highest iridium concentration. The shape of the voltammograms is quite symmetrical and anodic and cathodic voltammetric areas are identical, within the limits of experimental error, indicating the absence of hysteresis in the anodic and cathodic charging processes. On the basis of the latter evidence, the analysis of the dependence of voltammetric charge on s and electrode composition has been hereafter carried out only for one of
36
L. Vazquez-Gomez et al. / Applied Catalysis B: Environmental 67 (2006) 34–40
the two components, i.e., the anodic one. As widely discussed in the literature [8–10,13], qa is an important in situ diagnostic parameter in the study of oxide-film electrodes, supplying indication on the effective surface area and porosity and on the existence of accessible oxidation states for one or more of the metal cations in the oxide-film structure. In practice, the central event is just the ability to store charge through electron/proton transfer reactions of the type: ½MOa ðOHÞb xH2 O þ dHþ þ de @ ½MOad ðOHÞbþd xH2 O
Fig. 1. SEM image of an as prepared mixed-oxide electrode, with an Ir content of 23.3 atoms%. Magnification: 300 (A) and 5000 (B).
Fig. 2. LCV curves recorded in 1 M HClO4, at 100 mV s1, for electrodes with the lowest (Ir 1.7%, solid line) and highest (Ir 33.5%, broken line) iridium content.
(1)
where the concentration of electroactive sites (concentration of noble-metal ion species) and their accessibility to protons (oxide film porosity) decide of the amount of charge stored within a given potential scan range. The oxide film texture is related in turn to the features of the reactivity of precursor salts used in the film preparation, as well as other variables, like film thickness. Clearly, the complexity of the film synthesis may involve significant irreproducibility in such delicate features, because strict control of the precursor path and amount of oxide or mixed-oxide is quite difficult. Still charge-storage capacity maintains a paramount importance as a parameter, allowing a first-approach prediction of apparent catalytic activity of a given electrode, toward a number of electrochemical reactions whose adsorbed intermediates may interact with noble-metal-ion sites at the electrode surface. As a consequence of the above described charge-storage mechanism, qa decreases with increasing s; this supplies further diagnostic data allowing approximate evaluation of outer and inner contributions to the charge-storage capacity. In [13], total voltammetric charge and ‘‘outer’’ voltammetric charge have been estimated by extrapolation procedures. In the present work, the two quantities have been estimated making use of a low-s (10 mV s1) and high-s value (600 mV s1), bearing in mind the semi-quantitative character of the obtained values. In Table 1, qa data recorded at the two above potential-scan-rates are shown, indicating that higher qa values are associated with larger Ir contents. The shape of the dependence on Ir concentration is substantially the same for the total and external voltammetric charges. Basing on a very rough preliminary assumption that all of the voltammetric charge is due to proton-assisted redox processes of the type in (1), and that three oxidation state changes occur in the potential range scanned, a number of electroactive sites per (apparent) square centimeter as large as 1.6 1017 would be estimated. This is about three orders of magnitudes higher than the number of metal-ions per effective square centimeter in a rutile-structure oxide surface (from 1014 to 1015, depending on the orientation of the crystal plane [7]). This proves an important involvement of the ‘‘inner surface’’ of the oxide film or, in other words, a very good dispersion of the catalyst in the film texture. The modest difference between total and ‘‘external’’ voltammetric charges is also an evidence in favor of the accessibility of electroactive sites and of the relative easiness of their rearrangement during oxidation-state changes.
L. Vazquez-Gomez et al. / Applied Catalysis B: Environmental 67 (2006) 34–40
37
Table 1 Effect of electrode composition on voltammetric charges Electrode composition Ir%
Ru%
Sn%
1.7 3.7 7.3 11.6 17.9 23.3 28.4 31.6 33.5
31.7 29.9 26.5 22.4 16.5 11.3 6.5 3.5 1.7
66.6 66.4 66.2 66.0 65.6 65.4 65.1 64.9 64.8
CV area (Qan,tot) mC cm2
CV area (Qan,ext) mC cm2
CV area (Qan,int) mC cm2
26.5 0.6 28.9 2.0 35.6 2.2 40.9 1.7 49.4 0.9 81.8 3.5 68.4 0.8 78.8 2.1 70.2 0.4
22.4 0.8 23.8 0.5 29.7 1.5 33.2 1.0 40.3 0.2 63.3 3.2 55.0 0.5 62.9 1.4 55.4 0.9
4.0 0.2 5.2 1.5 5.9 0.7 7.7 0.7 9.1 1.2 18.5 0.4 13.5 0.4 15.9 0.7 14.9 0.5
In consideration of the fact that the electrolytic production of chlorine and hypochlorite from chloride could be used for water disinfection, the electrocatalytic activity of the different ternary mixtures was investigated by studying the model chlorine evolution reaction. Polarization curves recorded in a Cl2saturated brine solution (4.25 M NaCl) are shown as Tafel plot in Fig. 3; in order to obtain current–density values reflecting the effective extension of the electrode surface, normalization with respect to the external voltammetric charge has been carried out [14]. Data in Fig. 3 show that, in spite of the normalization to qa, accounting for effective or ‘‘electrochemical’’ surface area, significant differences still remain among j values at constant electrode overpotential, which are also observed in 1 M and 0.1 M NaCl solutions. This evidence witnesses for an influence of the Ir/Ru mol ratio on the ‘‘intrinsic’’ catalytic activity of the sites at which chloride oxidation takes place. The literature on Cl anodic oxidation at most common oxide electrodes reports Tafel slopes around 30–40 mV, indicating that desorption is the rate-determining step of the reaction (electrochemical/low uCl or chemical/high uCl) [15]. Considering our experiments, the lack of satisfactorily defined linear regions in Fig. 3 could be due to high uCl and to its dependence on electrode potential. This possibility rules out the
hypothesis of electrochemical desorption, and only the chemical desorption alternative remains. Conway and Novak [16] developed a kinetic analysis for this specific case: k1
Cl þ S Ð SCl þ e
(I)
k1
k2
2SCl !Cl2
(II)
The proposed method can describe the kinetic behavior associated with the step sequence (I)–(II) over the whole range of uCl and corresponding currents up to ilim. The current density for (II) is written as: i2 ¼ 2Fk2 u2Cl
(2)
If (II) is rate-controlling, uCl may be represented as a function of h; as a first approach to the discussion of the current–potential behavior, the Langmuir electrochemical adsorption isotherm can be applied: uCl ¼
K1 cCl expðhF=RTÞ 1 þ K1 cCl expðhF=RTÞ
(3)
Replacing (3) in Eq. (2), the expression for i2 for the whole range of uCl (that is itself a function of h) can be obtained. It is convenient to rearrange the new expression at the next form: expðhF=RTÞ 1=2 i2
¼
1 ð2Fk2 Þ
1=2
K1 cCl
þ
expðhF=RTÞ ð2Fk2 Þ1=2 1=2
Fig. 3. Current–overpotential curves for chlorine evolution in 4.25 M NaCl + 0.01 M HCl. Electrode compositions: (a) Ir 1.7%; (b) Ir 7.3%; (c) Ir 11.6%; (d) Ir 17.9%; (e) Ir 23.3%; (f) Ir 28.4%; (g) Ir 33.5%.
(4)
Hence, a plot of expðhF=RTÞ=i2 vs. exp(hF/RT) should give a straight line of slope (2Fk2)1/2 and an intercept ð2Fk2 Þ1=2 =K1 cCl . Eq. (4) was applied to experimental results for the three NaCl concentrations; curves obtained for the highest one (4.25 M NaCl) gave the plot shown in Fig. 4. Good linearity is obtained over almost the whole range of h, indicating applicability of the recombination-controlled mechanism. The bending at low h is due to the increasing contribution
38
L. Vazquez-Gomez et al. / Applied Catalysis B: Environmental 67 (2006) 34–40
Fig. 4. Test of Cl recombination for the different anode materials in 4.25 M NaCl + 0.01 M HCl (Cl2 saturated solution). Electrode compositions as reported in Fig. 3.
of the cathodic reaction, while h approaches zero, and should not be considered; conversely, this plot shows that the reduction of Cl2 is not important just for overpotentials larger than, e.g., 25 mV. For the other NaCl concentrations, the same trend was obtained, witnessing for an increase of uCl with the applied overpotential, until up to the saturation, even in diluted chloride solutions (0.1 M NaCl). Moreover, the Langmuir isotherm seems to be a good approximation in Cl electrosorption at IrO2–RuO2–SnO2 mixed-oxide electrodes. From Figs. 3 and 4, as expectable, the difference in the chlorine evolution rate at the different electrodes is not very large. However, electrodes with lower Ir/Ru mol ratio exhibit a higher catalytic activity, apparently related with higher k2 values (Fig. 5A), related in turn with the interaction between adsorbed Cl species and the active sites at the mixed-oxide surface. On the other hand, K1 values (Fig. 5B) show a significant dependence on chloride ion concentration, increasing with decreasing this variable. In this connection, we have here to remind that in the constant-overpotential-analysis, like that applied in the present work, the physical electrode potential depends on the relative concentration of oxidized and reduced species. Since the oxidized one is kept constant (in our case), inevitably the Cl oxidation takes place at an electrode potential more and more positive, when the concentration of the reduced form in the test solution is decreased. As a further comment on the general kinetic evidence obtained, the equilibrium constant of the Volmer step (formation of the adsorbed chlorine radical, step (I)) is considerably high in value, symptomatic of the easy formation of Cl. The above result is substantially independent from the iridium content in the oxide mixture. Moreover, electrosorbed radicals are so stabilized that the second step is not only ratecontrolling but significantly sluggish also with respect to different RuO2-containing electrode materials [17]. Such a behavior may be beneficial in that the electrode surface is not so available for the adsorption of other kind of radicals, thus hindering or inhibiting other electrode reactions; this could be particularly advantageous in the case of the electrochemical sterilization of low Cl-containing waters and/or for the oxidative elimination of water pollutants [18].
Fig. 5. Values of k2 (A) and K1 (B) for the Cl2 evolution mechanism, as a function of Ir concentration, at different Cl concentrations (4.25 M NaCl, square; 1 M NaCl, circle; 0.1 M NaCl, triangle).
As a further characterization, the selectivity of the different electrode catalysts has been evaluated, measuring the active chlorine production from aqueous solutions containing 0.5 g dm3 NaCl (which corresponds to 300 mg dm3 of chlorides, i.e., a situation compatible with that encountered in the usual tap water composition). Each test has been carried out in a single-body cell with a volume of 0.25 dm3, polarizing the working electrode at 50 mA for 10 min, and subsequently measuring the active chlorine content by means of a traditional iodometric titration. In all cases, a 5 min pre-electrolysis, under the same experimental conditions, has been performed to avoid the interference of potential-reducing substances in the water samples (the pre-electrolysis contribution has been subtracted from the final active chlorine amount; after this pre-electrolysis, the solution volume is 0.2 dm3). As shown in Fig. 6, the production of active chlorine depends with good approximation linearly on the ternary mixture composition, decreasing at higher iridium concentrations (which correspond to lower amounts of ruthenium). Interestingly, an RuO2–SnO2 binary mixture showed an unexpectedly low efficiency in the production of active chlorine, which has to be combined with a very low service-life, due to the well known poor wear resistance of RuO2 under oxygen evolution conditions. It is also worth mentioning that higher active-chlorine-production yields
L. Vazquez-Gomez et al. / Applied Catalysis B: Environmental 67 (2006) 34–40
Fig. 6. Dependence of active chlorine production on the Ir content in the ternary mixture.
were obtained at oxide coatings having a low iridium content, at which higher k2 values were estimated (Fig. 5A). As shown in the previously developed analysis of voltammetric charge, larger qa, and therefore larger effective surface areas, are associated with the Ir-rich anodes (Table 1). This implies, in turn, effective lower current densities for the latter coating compositions and would favor the chloride oxidation with respect to the oxygen evolution (see again [18]). The lower active-chlorine production has therefore to be related with effective differences in catalytic activity. As anticipated in the experimental part, the electrochemical behavior of each ternary mixture was finally tested through an accelerated service-life test, polarizing the electrode at a fixed current density (0.5 A cm2) in 0.5 M Na2SO4, till deactivation. In each case, the passed charge (Ah) was calculated from the working time (h), normalized to the unit of electrode surface (m2), and plotted against the iridium content in the film (g m2), as shown in Fig. 7. For comparison, data pertaining to a sample based on the RuO2–SnO2 binary mixture has also been taken into account in the figure. Data in Fig. 7 do not include the
39
two compositions having the highest Ir content (nominal content of 31.6 and 33.5%), for which a passivation time higher than 2200 h should be predicted. Due to the poor stability of RuO2 under oxygen evolution, and the well-known better performance of IrO2, this plot is a quantitative description of expected results. Clearly, a decrease of Ir content to about 20% is still sufficient to allow a good electrochemical behavior and some advantage from the point of view of the coating cost. For instance, a Ir0.23Ru0.11Sn0.66O2 coating could work for almost 1800 h (charge passed: 0.9 107 Ah m2 with only 3.44 g m2 Ir). Analogous results were obtained by Gorodetskii and coworkers for electrode materials containing titanium dioxide as inert or stabilizing component [19]. However, the latter materials exhibit an intrinsically poor stability under oxygen evolution, compared with SnO2-based coatings. 4. Conclusions The iridium–ruthenium–tin oxide mixture represents a suitable alternative to more expensive oxide coatings, generally based on IrO2. The concomitant presence of IrO2 and RuO2 allows to optimize both the stability (service life) and the Faradaic efficiency for the active chlorine production. The latter feature may result from a substantial decrease of the catalytic activity of the electrode coating toward the more ‘‘difficult’’ of the two competing reactions [20], i.e., the oxygen evolution, when the ruthenium oxide concentration is decreased. A compromise between catalytic properties and wear resistance can be found with coatings containing about 20% of iridium (hence 15% of ruthenium), thus allowing some advantages in terms of coating costs. Acknowledgements This work was partly financed by the University of Ferrara (nano & nano project). LVG gratefully acknowledges the CONACYT for financial support of her PhD. References
Fig. 7. Influence of the Ir loading on the total charge passed by Ir–Ru–Sn electrodes.
[1] J. La˚ngmark, M.V. Storey, N.J. Ashbolt, T.A. Stenstro¨m, Wat. Res. 38 (2004) 740. [2] J. Grimm, D. Bessarabov, R. Sanderson, Desalination 115 (1998) 285. [3] V.M. Bakhir, S.A. Panicheva, Y.G. Zadorozhni, US Patent no. 6843895, 2005. [4] K. Ju¨ttner, U. Galla, H. Schmieder, Electrochim. Acta 45 (2000) 2575. [5] R. Hutchings, K. Mu¨ller, R. Ko¨tz, S. Stucki, J. Mater. Sci. 19 (1984) 3987. [6] S.-M. Lin, T.-C. Wen, J. Electrochem. Soc. 140 (1993) 2265. [7] L. Nanni, S. Polizzi, A. Benedetti, A. De Battisti, J. Electrochem. Soc. 146 (1999) 220. [8] J. Gaudet, A.C. Tavares, S. Trasatti, D. Guay, Chem. Mater. 17 (2005) 1570. [9] C.P. De Pauli, S. Trasatti, J. Electroanal. Chem. 396 (1995) 161. [10] C.P. De Pauli, S. Trasatti, J. Electroanal. Chem. 538 (2002) 145. [11] A. Morozov, A. De Battisti, S. Ferro, G. Martelli, WO 2005/014885 A1, 2005. [12] A. Morozov, A. De Battisti, S. Ferro, G. Martelli, WO 2005/014884 A1, 2005. [13] S. Ardizzone, G. Fregonara, S. Trasatti, Electrochim. Acta 35 (1990) 263.
40
L. Vazquez-Gomez et al. / Applied Catalysis B: Environmental 67 (2006) 34–40
[14] A. Marshall, B. Børresen, G. Hagen, M. Tsypkin, R. Tunold, Electrochim. Acta 51 (2006) 3161. [15] S. Trasatti, Electrochim. Acta. 32 (1987) 369. [16] B.E. Conway, D.M. Novak, J. Electroanal. Chem. 99 (1979) 133. [17] B.E. Conway, G. Ping, A. De Battisti, A. Barbieri, G. Battaglin, J. Mater. Chem. 1 (1991) 725.
[18] C.A. Martı´nez-Huitle, S. Ferro, A. De Battisti, Electrochem. Solid-State Lett. 8 (2005) D35. [19] V.V. Gorodetskii, V.A. Neburchilov, M.M. Pecherskii, Elektrokhimiya 30 (1994) 1013. [20] S. Trasatti, G. Lodi, in: S. Trasatti (Ed.), Electrodes of Conductive Metal Oxides, Part B, Elsevier, Amsterdam, 1981, p. 521.
Preparation and characterization of RuO2–IrO2–SnO2 ternary mixtures for advanced electrochemical technology Lourdes Vazquez-Gomez, Sergio Ferro, Achille De Battisti * Department of Chemistry, University of Ferrara, via L. Borsari 46, 44100 Ferrara, Italy Received 1 December 2005; received in revised form 10 March 2006; accepted 24 March 2006 Available online 16 May 2006
Abstract Electrochemical methods have proved to be particularly effective both for water detoxification (abatement of heavy metals and organic impurities) and sterilization, from natural resources to utilization points, combining these features with low costs and easier handling of equipments. The efficiency of electrochemical methods strongly depends on electrode nature, anodes being of particular importance because they must stand much more severe polarization conditions and have to exhibit good catalytic activity for complex reactions like oxygen and chlorine evolution. The present work is devoted to the preparation and characterization of mixed-oxide electrode coatings based on IrO2, RuO2 and SnO2, with the scope of finding optimal compositions guaranteeing longer service-life under the critical conditions where oxygen evolution is concomitant with chloride oxidation. # 2006 Elsevier B.V. All rights reserved. Keywords: Electrocatalysis; Oxide electrodes; Chlorine evolution reaction; Oxygen evolution reaction; Water sterilization
1. Introduction The challenge of the increasing demand for quality water can be met only applying all suitable scientific and technical methods to increase the productivity of process technology for the conversion of low-grade water into pure water. Sterilization and elimination of organic micro-impurities from drinkable water and water for surgery-medical use is of utmost importance [1]. Many different methods for improving the water quality are already established: these include membrane-based methods, ultra- and micro-filtration, biological, chemical, physicochemical treatments. More traditional purification methods, although generally quite effective for the destruction of bacterial and fungine pollutants, as well as for organic/inorganic chemical impurities, may result relatively ineffective for the elimination of viral components and residual organic substrates [2]. A solution to these problems can be sought in advanced electrochemical technologies based on electric field and Faradaic effects in specially designed electrochemical cells, readily installed at the consumption sites. Two main types of treatment can be considered [3]:
* Corresponding author. Tel.: +39 0532 291124; fax: +39 0532 240709. E-mail address: [email protected] (A. De Battisti). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.03.023
1. Drinkable waters suffering degradation of properties (due to lengthy routes, pipe damages, uphill-treatment problems) may be subjected to a ‘‘direct’’ electrochemical treatment, i.e., the water is passed through anodic and cathodic compartments of one or more electrochemical cells with specific features. The process depends on applied electric field, pH ‘‘shock’’ and Faradaic processes (anodic oxidation, cathodic reduction) and results in the elimination of biological and chemical impurities (traces of heavy metals) and in the abatement of the redox potential to 0.10 (vs. SCE). 2. Good quality water for surgery, food industry, etc. may be obtained through an ‘‘indirect’’ treatment, adding suitable amounts of so-called ‘‘neutral anolytes’’, which are produced by electrolyzing diluted brines; under optimal conditions, a complete elimination of the biological and chemical impurities can be obtained. Both treatments prove to be effective, requiring low energy consumption while ensuring long service-life and low costs. Concerning the disadvantages, the possible need for frequent maintenance applies in particular for the direct-treatment concept. The above technology has already found practical applications and its further development requires improvement of cell
L. Vazquez-Gomez et al. / Applied Catalysis B: Environmental 67 (2006) 34–40
engineering [4] and electrode materials, of the anodes in particular. The indirect, chloride-mediated water sterilization/detoxification involves a quite complex set of reactions, at the electrode surface as well as in the bulk of the solution, which are mostly related with the electrochemical reactivity of Cl and the chemical/electrochemical reactivity of some of its oxidation products. Following the rich literature available on the subject, the anodic oxidation of Cl can be represented, in principle, by two relatively simple mechanisms, both involving a discharge as a first step, with adsorbed radical formation, and differing in the successive radical desorption step: chemical or electrochemical. The explicit involvement of adsorbed intermediates in the process makes it an important example of catalysis in an electrochemical reaction and assigns the electrode material the utmost importance. The highly positive potentials, at which the reaction takes place, inevitably cause changes in the degree of oxidation of the electrode surface, which justify further mechanistic hypotheses, including hydroxyl radical formation as a kinetic step preliminary to chloride oxidation. The competition with the oxygen evolution reaction, at extents that depend on pH, further enhances the importance of the nature of the electrode catalyst not only in terms of activity, but also of selectivity and stability. In the present work, it has been thought of interest to start a research on the latter point, studying electrode materials which could respond to requirements of stability and lower cost with respect to the available technology. Thus, the research concerns the development of new anode materials based on RuO2–IrO2– SnO2 ternary mixtures [5,6], deposited on Ti supports. Simpler systems, based on RuO2–SnO2 [7,8] or IrO2–SnO2 [9,10], are less suitable, either for the poor service-life, in the first case, or for the cost in the second. 2. Experimental The oxide coatings were prepared by a thermal decomposition method described in [11], requiring an oxidative pyrolysis step of precursor–salt deposits at 480 8C, for 20 min. Noble-metal precursor salts were, as often suggested in the literature, hydrated ruthenium trichloride and tetrachloroiridic acid, which were dissolved in water, with addition of acetic acid, up to the appropriate concentration. The tin–precursor– salt was an aceto-chloro-complex of Sn(IV) [11], which does not undergo any volatilization during the drying and successive thermal decomposition stages. The synthesized coating compositions were: IrxRu0.34xSn0.66O2 (x nominal values: 1.7, 3.7, 7.3, 11.6, 17.9, 23.3, 28.4, 31.6 and 33.5%). Prior to use, Ti plates were cleaned in 10% aqueous oxalic acid, kept at a temperature of 90 8C, carefully rinsed in triply distilled water, then kept in hot concentrated NaOH for a few minutes, rinsed with water again and eventually dried in an oven at 120 8C [12]. The microstructure, surface morphology and composition of these mixed-oxide electrodes were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis; electrochemical surface characterization was carried out by linear sweep
35
voltammetry, cycling the working electrode between 0.15 and 1.15 V vs. SCE. A double-walled, saturated calomel electrode (SCE), with an intermediate saturated NaNO3 solution, minimizing the occurrence of insoluble-salt precipitation inside the frit, was used as the reference in a conventional three-electrode cell. For these measures, a 1 M HClO4 test solution was chosen. In consideration of the importance of the chloride anodic oxidation (e.g., in neutral anolyte production), a study of chlorine evolution reaction was also carried out, making use of quasi-steady polarization data. Potentiostatic curves were recorded in 0.01 M HCl, in the presence of 4.25, 1 or 0.1 mol dm3 NaCl; all solutions were saturated with gaseous chlorine, and potential values were corrected for the uncompensated ohmic resistance. Finally, in order to ascertain the long term stability of the different anode materials, service-life tests were carried out in 0.5 M Na2SO4, at 0.5 A cm2; in these experiments, anodes were considered as passivated upon a 2 V increase of their potential under the applied current density. 3. Results and discussion XRD measurements have proved the formation of a rutilestructured RuO2–IrO2–SnO2 solid solution as the only crystalline component of the coatings, through the whole composition range explored. The average crystallite size was in the range 3.0–4.0 nm, moderately increasing with increasing the Ir/Ru ratio. Small measurable effects could be observed, in the opposite direction, also for the a = b cell parameter, while the c parameter remained substantially unchanged. SEM pictures for an as-prepared electrode, with a nominal iridium concentration of 23.3 mol%, are shown in Fig. 1. Beside the well-known and scarcely diagnostic ‘‘cracked-mud’’ texture, no other significant features could be recognized by SEM. This may be an indirect evidence of a homogeneous distribution of microporosity across the film surface. Increasing the iridium content in the ternary mixture, the metal tends to accumulate in the inner layers (enrichment in the cracks between islands), as evidenced by EDX; in contrast, ruthenium seems to maintain a better dispersion. More quantitative data cannot be obtained by EDX, the analytical answer depending on different factors, including the sample thickness and topography, and the atomic number of each component. These considerations suggest that data may be taken as proofs of the presence (or absence) of a given element, while quantitative evaluations should be obtained by different procedures. Fig. 2 shows cyclic voltammograms, recorded at a potential scan rate s of 100 mV s1, for the two limiting compositions, i.e., the ternary mixtures with the lowest and the highest iridium concentration. The shape of the voltammograms is quite symmetrical and anodic and cathodic voltammetric areas are identical, within the limits of experimental error, indicating the absence of hysteresis in the anodic and cathodic charging processes. On the basis of the latter evidence, the analysis of the dependence of voltammetric charge on s and electrode composition has been hereafter carried out only for one of
36
L. Vazquez-Gomez et al. / Applied Catalysis B: Environmental 67 (2006) 34–40
the two components, i.e., the anodic one. As widely discussed in the literature [8–10,13], qa is an important in situ diagnostic parameter in the study of oxide-film electrodes, supplying indication on the effective surface area and porosity and on the existence of accessible oxidation states for one or more of the metal cations in the oxide-film structure. In practice, the central event is just the ability to store charge through electron/proton transfer reactions of the type: ½MOa ðOHÞb xH2 O þ dHþ þ de @ ½MOad ðOHÞbþd xH2 O
Fig. 1. SEM image of an as prepared mixed-oxide electrode, with an Ir content of 23.3 atoms%. Magnification: 300 (A) and 5000 (B).
Fig. 2. LCV curves recorded in 1 M HClO4, at 100 mV s1, for electrodes with the lowest (Ir 1.7%, solid line) and highest (Ir 33.5%, broken line) iridium content.
(1)
where the concentration of electroactive sites (concentration of noble-metal ion species) and their accessibility to protons (oxide film porosity) decide of the amount of charge stored within a given potential scan range. The oxide film texture is related in turn to the features of the reactivity of precursor salts used in the film preparation, as well as other variables, like film thickness. Clearly, the complexity of the film synthesis may involve significant irreproducibility in such delicate features, because strict control of the precursor path and amount of oxide or mixed-oxide is quite difficult. Still charge-storage capacity maintains a paramount importance as a parameter, allowing a first-approach prediction of apparent catalytic activity of a given electrode, toward a number of electrochemical reactions whose adsorbed intermediates may interact with noble-metal-ion sites at the electrode surface. As a consequence of the above described charge-storage mechanism, qa decreases with increasing s; this supplies further diagnostic data allowing approximate evaluation of outer and inner contributions to the charge-storage capacity. In [13], total voltammetric charge and ‘‘outer’’ voltammetric charge have been estimated by extrapolation procedures. In the present work, the two quantities have been estimated making use of a low-s (10 mV s1) and high-s value (600 mV s1), bearing in mind the semi-quantitative character of the obtained values. In Table 1, qa data recorded at the two above potential-scan-rates are shown, indicating that higher qa values are associated with larger Ir contents. The shape of the dependence on Ir concentration is substantially the same for the total and external voltammetric charges. Basing on a very rough preliminary assumption that all of the voltammetric charge is due to proton-assisted redox processes of the type in (1), and that three oxidation state changes occur in the potential range scanned, a number of electroactive sites per (apparent) square centimeter as large as 1.6 1017 would be estimated. This is about three orders of magnitudes higher than the number of metal-ions per effective square centimeter in a rutile-structure oxide surface (from 1014 to 1015, depending on the orientation of the crystal plane [7]). This proves an important involvement of the ‘‘inner surface’’ of the oxide film or, in other words, a very good dispersion of the catalyst in the film texture. The modest difference between total and ‘‘external’’ voltammetric charges is also an evidence in favor of the accessibility of electroactive sites and of the relative easiness of their rearrangement during oxidation-state changes.
L. Vazquez-Gomez et al. / Applied Catalysis B: Environmental 67 (2006) 34–40
37
Table 1 Effect of electrode composition on voltammetric charges Electrode composition Ir%
Ru%
Sn%
1.7 3.7 7.3 11.6 17.9 23.3 28.4 31.6 33.5
31.7 29.9 26.5 22.4 16.5 11.3 6.5 3.5 1.7
66.6 66.4 66.2 66.0 65.6 65.4 65.1 64.9 64.8
CV area (Qan,tot) mC cm2
CV area (Qan,ext) mC cm2
CV area (Qan,int) mC cm2
26.5 0.6 28.9 2.0 35.6 2.2 40.9 1.7 49.4 0.9 81.8 3.5 68.4 0.8 78.8 2.1 70.2 0.4
22.4 0.8 23.8 0.5 29.7 1.5 33.2 1.0 40.3 0.2 63.3 3.2 55.0 0.5 62.9 1.4 55.4 0.9
4.0 0.2 5.2 1.5 5.9 0.7 7.7 0.7 9.1 1.2 18.5 0.4 13.5 0.4 15.9 0.7 14.9 0.5
In consideration of the fact that the electrolytic production of chlorine and hypochlorite from chloride could be used for water disinfection, the electrocatalytic activity of the different ternary mixtures was investigated by studying the model chlorine evolution reaction. Polarization curves recorded in a Cl2saturated brine solution (4.25 M NaCl) are shown as Tafel plot in Fig. 3; in order to obtain current–density values reflecting the effective extension of the electrode surface, normalization with respect to the external voltammetric charge has been carried out [14]. Data in Fig. 3 show that, in spite of the normalization to qa, accounting for effective or ‘‘electrochemical’’ surface area, significant differences still remain among j values at constant electrode overpotential, which are also observed in 1 M and 0.1 M NaCl solutions. This evidence witnesses for an influence of the Ir/Ru mol ratio on the ‘‘intrinsic’’ catalytic activity of the sites at which chloride oxidation takes place. The literature on Cl anodic oxidation at most common oxide electrodes reports Tafel slopes around 30–40 mV, indicating that desorption is the rate-determining step of the reaction (electrochemical/low uCl or chemical/high uCl) [15]. Considering our experiments, the lack of satisfactorily defined linear regions in Fig. 3 could be due to high uCl and to its dependence on electrode potential. This possibility rules out the
hypothesis of electrochemical desorption, and only the chemical desorption alternative remains. Conway and Novak [16] developed a kinetic analysis for this specific case: k1
Cl þ S Ð SCl þ e
(I)
k1
k2
2SCl !Cl2
(II)
The proposed method can describe the kinetic behavior associated with the step sequence (I)–(II) over the whole range of uCl and corresponding currents up to ilim. The current density for (II) is written as: i2 ¼ 2Fk2 u2Cl
(2)
If (II) is rate-controlling, uCl may be represented as a function of h; as a first approach to the discussion of the current–potential behavior, the Langmuir electrochemical adsorption isotherm can be applied: uCl ¼
K1 cCl expðhF=RTÞ 1 þ K1 cCl expðhF=RTÞ
(3)
Replacing (3) in Eq. (2), the expression for i2 for the whole range of uCl (that is itself a function of h) can be obtained. It is convenient to rearrange the new expression at the next form: expðhF=RTÞ 1=2 i2
¼
1 ð2Fk2 Þ
1=2
K1 cCl
þ
expðhF=RTÞ ð2Fk2 Þ1=2 1=2
Fig. 3. Current–overpotential curves for chlorine evolution in 4.25 M NaCl + 0.01 M HCl. Electrode compositions: (a) Ir 1.7%; (b) Ir 7.3%; (c) Ir 11.6%; (d) Ir 17.9%; (e) Ir 23.3%; (f) Ir 28.4%; (g) Ir 33.5%.
(4)
Hence, a plot of expðhF=RTÞ=i2 vs. exp(hF/RT) should give a straight line of slope (2Fk2)1/2 and an intercept ð2Fk2 Þ1=2 =K1 cCl . Eq. (4) was applied to experimental results for the three NaCl concentrations; curves obtained for the highest one (4.25 M NaCl) gave the plot shown in Fig. 4. Good linearity is obtained over almost the whole range of h, indicating applicability of the recombination-controlled mechanism. The bending at low h is due to the increasing contribution
38
L. Vazquez-Gomez et al. / Applied Catalysis B: Environmental 67 (2006) 34–40
Fig. 4. Test of Cl recombination for the different anode materials in 4.25 M NaCl + 0.01 M HCl (Cl2 saturated solution). Electrode compositions as reported in Fig. 3.
of the cathodic reaction, while h approaches zero, and should not be considered; conversely, this plot shows that the reduction of Cl2 is not important just for overpotentials larger than, e.g., 25 mV. For the other NaCl concentrations, the same trend was obtained, witnessing for an increase of uCl with the applied overpotential, until up to the saturation, even in diluted chloride solutions (0.1 M NaCl). Moreover, the Langmuir isotherm seems to be a good approximation in Cl electrosorption at IrO2–RuO2–SnO2 mixed-oxide electrodes. From Figs. 3 and 4, as expectable, the difference in the chlorine evolution rate at the different electrodes is not very large. However, electrodes with lower Ir/Ru mol ratio exhibit a higher catalytic activity, apparently related with higher k2 values (Fig. 5A), related in turn with the interaction between adsorbed Cl species and the active sites at the mixed-oxide surface. On the other hand, K1 values (Fig. 5B) show a significant dependence on chloride ion concentration, increasing with decreasing this variable. In this connection, we have here to remind that in the constant-overpotential-analysis, like that applied in the present work, the physical electrode potential depends on the relative concentration of oxidized and reduced species. Since the oxidized one is kept constant (in our case), inevitably the Cl oxidation takes place at an electrode potential more and more positive, when the concentration of the reduced form in the test solution is decreased. As a further comment on the general kinetic evidence obtained, the equilibrium constant of the Volmer step (formation of the adsorbed chlorine radical, step (I)) is considerably high in value, symptomatic of the easy formation of Cl. The above result is substantially independent from the iridium content in the oxide mixture. Moreover, electrosorbed radicals are so stabilized that the second step is not only ratecontrolling but significantly sluggish also with respect to different RuO2-containing electrode materials [17]. Such a behavior may be beneficial in that the electrode surface is not so available for the adsorption of other kind of radicals, thus hindering or inhibiting other electrode reactions; this could be particularly advantageous in the case of the electrochemical sterilization of low Cl-containing waters and/or for the oxidative elimination of water pollutants [18].
Fig. 5. Values of k2 (A) and K1 (B) for the Cl2 evolution mechanism, as a function of Ir concentration, at different Cl concentrations (4.25 M NaCl, square; 1 M NaCl, circle; 0.1 M NaCl, triangle).
As a further characterization, the selectivity of the different electrode catalysts has been evaluated, measuring the active chlorine production from aqueous solutions containing 0.5 g dm3 NaCl (which corresponds to 300 mg dm3 of chlorides, i.e., a situation compatible with that encountered in the usual tap water composition). Each test has been carried out in a single-body cell with a volume of 0.25 dm3, polarizing the working electrode at 50 mA for 10 min, and subsequently measuring the active chlorine content by means of a traditional iodometric titration. In all cases, a 5 min pre-electrolysis, under the same experimental conditions, has been performed to avoid the interference of potential-reducing substances in the water samples (the pre-electrolysis contribution has been subtracted from the final active chlorine amount; after this pre-electrolysis, the solution volume is 0.2 dm3). As shown in Fig. 6, the production of active chlorine depends with good approximation linearly on the ternary mixture composition, decreasing at higher iridium concentrations (which correspond to lower amounts of ruthenium). Interestingly, an RuO2–SnO2 binary mixture showed an unexpectedly low efficiency in the production of active chlorine, which has to be combined with a very low service-life, due to the well known poor wear resistance of RuO2 under oxygen evolution conditions. It is also worth mentioning that higher active-chlorine-production yields
L. Vazquez-Gomez et al. / Applied Catalysis B: Environmental 67 (2006) 34–40
Fig. 6. Dependence of active chlorine production on the Ir content in the ternary mixture.
were obtained at oxide coatings having a low iridium content, at which higher k2 values were estimated (Fig. 5A). As shown in the previously developed analysis of voltammetric charge, larger qa, and therefore larger effective surface areas, are associated with the Ir-rich anodes (Table 1). This implies, in turn, effective lower current densities for the latter coating compositions and would favor the chloride oxidation with respect to the oxygen evolution (see again [18]). The lower active-chlorine production has therefore to be related with effective differences in catalytic activity. As anticipated in the experimental part, the electrochemical behavior of each ternary mixture was finally tested through an accelerated service-life test, polarizing the electrode at a fixed current density (0.5 A cm2) in 0.5 M Na2SO4, till deactivation. In each case, the passed charge (Ah) was calculated from the working time (h), normalized to the unit of electrode surface (m2), and plotted against the iridium content in the film (g m2), as shown in Fig. 7. For comparison, data pertaining to a sample based on the RuO2–SnO2 binary mixture has also been taken into account in the figure. Data in Fig. 7 do not include the
39
two compositions having the highest Ir content (nominal content of 31.6 and 33.5%), for which a passivation time higher than 2200 h should be predicted. Due to the poor stability of RuO2 under oxygen evolution, and the well-known better performance of IrO2, this plot is a quantitative description of expected results. Clearly, a decrease of Ir content to about 20% is still sufficient to allow a good electrochemical behavior and some advantage from the point of view of the coating cost. For instance, a Ir0.23Ru0.11Sn0.66O2 coating could work for almost 1800 h (charge passed: 0.9 107 Ah m2 with only 3.44 g m2 Ir). Analogous results were obtained by Gorodetskii and coworkers for electrode materials containing titanium dioxide as inert or stabilizing component [19]. However, the latter materials exhibit an intrinsically poor stability under oxygen evolution, compared with SnO2-based coatings. 4. Conclusions The iridium–ruthenium–tin oxide mixture represents a suitable alternative to more expensive oxide coatings, generally based on IrO2. The concomitant presence of IrO2 and RuO2 allows to optimize both the stability (service life) and the Faradaic efficiency for the active chlorine production. The latter feature may result from a substantial decrease of the catalytic activity of the electrode coating toward the more ‘‘difficult’’ of the two competing reactions [20], i.e., the oxygen evolution, when the ruthenium oxide concentration is decreased. A compromise between catalytic properties and wear resistance can be found with coatings containing about 20% of iridium (hence 15% of ruthenium), thus allowing some advantages in terms of coating costs. Acknowledgements This work was partly financed by the University of Ferrara (nano & nano project). LVG gratefully acknowledges the CONACYT for financial support of her PhD. References
Fig. 7. Influence of the Ir loading on the total charge passed by Ir–Ru–Sn electrodes.
[1] J. La˚ngmark, M.V. Storey, N.J. Ashbolt, T.A. Stenstro¨m, Wat. Res. 38 (2004) 740. [2] J. Grimm, D. Bessarabov, R. Sanderson, Desalination 115 (1998) 285. [3] V.M. Bakhir, S.A. Panicheva, Y.G. Zadorozhni, US Patent no. 6843895, 2005. [4] K. Ju¨ttner, U. Galla, H. Schmieder, Electrochim. Acta 45 (2000) 2575. [5] R. Hutchings, K. Mu¨ller, R. Ko¨tz, S. Stucki, J. Mater. Sci. 19 (1984) 3987. [6] S.-M. Lin, T.-C. Wen, J. Electrochem. Soc. 140 (1993) 2265. [7] L. Nanni, S. Polizzi, A. Benedetti, A. De Battisti, J. Electrochem. Soc. 146 (1999) 220. [8] J. Gaudet, A.C. Tavares, S. Trasatti, D. Guay, Chem. Mater. 17 (2005) 1570. [9] C.P. De Pauli, S. Trasatti, J. Electroanal. Chem. 396 (1995) 161. [10] C.P. De Pauli, S. Trasatti, J. Electroanal. Chem. 538 (2002) 145. [11] A. Morozov, A. De Battisti, S. Ferro, G. Martelli, WO 2005/014885 A1, 2005. [12] A. Morozov, A. De Battisti, S. Ferro, G. Martelli, WO 2005/014884 A1, 2005. [13] S. Ardizzone, G. Fregonara, S. Trasatti, Electrochim. Acta 35 (1990) 263.
40
L. Vazquez-Gomez et al. / Applied Catalysis B: Environmental 67 (2006) 34–40
[14] A. Marshall, B. Børresen, G. Hagen, M. Tsypkin, R. Tunold, Electrochim. Acta 51 (2006) 3161. [15] S. Trasatti, Electrochim. Acta. 32 (1987) 369. [16] B.E. Conway, D.M. Novak, J. Electroanal. Chem. 99 (1979) 133. [17] B.E. Conway, G. Ping, A. De Battisti, A. Barbieri, G. Battaglin, J. Mater. Chem. 1 (1991) 725.
[18] C.A. Martı´nez-Huitle, S. Ferro, A. De Battisti, Electrochem. Solid-State Lett. 8 (2005) D35. [19] V.V. Gorodetskii, V.A. Neburchilov, M.M. Pecherskii, Elektrokhimiya 30 (1994) 1013. [20] S. Trasatti, G. Lodi, in: S. Trasatti (Ed.), Electrodes of Conductive Metal Oxides, Part B, Elsevier, Amsterdam, 1981, p. 521.