RuO2–Sb2O5–SnO2 electrodes for O2 evolution

RuO2–Sb2O5–SnO2 electrodes for O2 evolution

Electrochimica Acta 50 (2005) 4155–4159 Stable Ti/RuO2–Sb2O5–SnO2 electrodes for O2 evolution Xueming Chen, Guohua Chen ∗ Department of Chemical Engi...

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Electrochimica Acta 50 (2005) 4155–4159

Stable Ti/RuO2–Sb2O5–SnO2 electrodes for O2 evolution Xueming Chen, Guohua Chen ∗ Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Received 9 October 2004; received in revised form 19 January 2005; accepted 22 January 2005 Available online 26 February 2005

Abstract RuO2 -based electrodes are generally known to be unstable for O2 evolution. In this paper, a stable type of RuO2 -based electrode, Ti/RuO2 –Sb2 O5 –SnO2 , is demonstrated for O2 evolution. In the ternary oxide coating, RuO2 serves as the catalyst, SnO2 as the dispersing agent, and Sb2 O5 as the dopant. The accelerated life test showed that the Ti/RuO2 –Sb2 O5 –SnO2 electrode containing 12.2 molar percent of RuO2 nominally in the coating had a service life of 307 h in 3 M H2 SO4 solution under a current density of 0.5 A cm−2 at 25 ◦ C, which is more than 15 times longer than other types of RuO2 -based electrodes. Instrumental analysis indicated that RuO2 –Sb2 O5 –SnO2 was a solid solution with a compact structure, which contributed to the stable nature of the electrode. © 2005 Elsevier Ltd. All rights reserved. Keywords: Dimensionally stable anode; Electroflotation; Electroplating; Electrolysis; O2 evolution

1. Introduction The dimensionally stable anodes (DSA) invented by Beer in the late 1960s have become the most important electrodes in electrochemical engineering. Beer’s invention led to a technical leap in the chlorine–alkali industry. In addition to Cl2 evolution, considerable efforts have been devoted to engineering the evolution O2 , which is a common anodic reaction occurring in many electrochemical processes such as electroplating, metal electrowinning, water electrolysis, and electroflotation for wastewater treatment. Generally, DSA-type electrodes use conductive precious metal oxides as electrocatalysts. RuO2 is known to be the most attractive electrocatalyst [1–5]. This oxide has exhibited excellent activity in both Cl2 and O2 evolutions. The Tafel slope for O2 evolution is only 0.031–0.041 V dec−1 in the low potential region and 0.042–0.066 V dec−1 in the high potential region [6,7]. Unfortunately, RuO2 and the widely used RuO2 –TiO2 in the chlorine–alkali industry are not stable for O2 evolution in acidic environments. Their service lives are below 4 h at a current density of 0.5 A cm−2 in 0.25–0.5 M H2 SO4 solutions [8,9]. The electrochemical stability of these electrodes is sig∗

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nificantly improved by selecting proper dispersing agents. Burke and McCarthy [10] increased the service life of the electrode by a factor of 5.3 by adding 20 molar percent of ZrO2 to the RuO2 layer. Iwakura and Sakamoto [9] studied the effect of addition of SnO2 to RuO2 on the service life, and found that the electrode with a molar ratio of Ru:Sn = 30:70 had a service life of about 12 h, four times longer than that of the pure RuO2 -coated electrode, under accelerated life test conditions (0.5 A cm−2 , 0.5 M H2 SO4 , 30 ◦ C). Investigation of RuO2 -based DSA for oxygen evolution has continued in recent years. Oxide mixtures of interest include RuO2 –Nb2 O5 [11], RuO2 –PbO2 [12,13], and RuO2 –Co3 O4 [14]. Despite their good activity for oxygen evolution, none of them has a service life over 20 h in accelerated life tests. Obviously, these electrocatalysts lack sufficient stability for industrial applications. The poor electrochemical stability of RuO2 -based DSA for O2 evolution is principally due to the easy conversion of the ruthenium from stable dioxide into unstable tetraoxide at a high electrical potential [15]. In our previous works [16,17], we successfully developed a stable ternary IrO2 –Sb2 O5 –SnO2 electrocatalyst for O2 evolution. In this oxide mixture, IrO2 serves as the catalyst for O2 evolution, SnO2 as the dispersing agent, and Sb2 O5 as the dopant for conductivity improvement. The service life of a Ti/IrO2 –SnO2 –Sb2 O5 electrode containing

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only 10 molar percent of IrO2 in the oxide coating is as high as 1600 h at a current density of 1 A cm−2 in a 3 M H2 SO4 solution. Our success in developing a stable ternary IrO2 –SnO2 –Sb2 O5 electrocatalyst motivated us to extend our investigation to RuO2 –Sb2 O5 –SnO2 because Ru is much cheaper and more active than Ir in oxygen evolution. The experimental results show that RuO2 –Sb2 O5 –SnO2 can be a very active and stable electrode for O2 evolution as discussed subsequently. The physical and electrochemical properties of Ti/RuO2 –Sb2 O5 –SnO2 electrodes are examined and reported in this paper.

2. Experimental 2.1. Precursor preparation A combination of 0.161 g RuCl3 ·H2 O (47.0% Ru, Aldrich, MO), 0.171 g SbCl3 (99+%, Acros, NJ), and 1.227 g SnCl4 ·5H2 O (98+%, Acros, NJ) was dissolved in a solvent consisting of 10 mL iso-propanol (99.7%, Lab-scan, Thailand) and 0.5 mL hydrochloric acid (37%, Riedel-deHaen, Germany). The molar ratio of this precursor solution was 15:15:70 [Ru:Sb:Sn]. It should be noted that the precursor solution must be freshly prepared before coating. 2.2. Electrode preparation Titanium disks, 2 mm thick and 12.7 mm in diameter with an effective area of 1.27 cm2 , were machined from a titanium rod (99.2%, Grade 2, McMaster-CARR, CA) and used as substrates. The electrodes were prepared by a thermal decomposition method. Prior to coating, the titanium substrates underwent sandblasting, tap water washing, 10 min of ultrasonic cleaning in deionized water, 2 min of etching in boiling 37% hydrochloric acid, and another 10 min of ultrasonic cleaning in deionized water. After pretreatment, the titanium substrates were first brushed at room temperature with the precursor solution, dried at 80 ◦ C for 5 min to allow the solvents to vaporize, and then calcinated at 550 ◦ C for 5 min. This procedure was repeated about 20 times. Finally, the electrodes were annealed at 550 ◦ C for an hour. The oxide coating loading was about 1.5 mg cm−2 . 2.3. Characterization Surface composition of the coating was analyzed by Xray photoelectron spectroscopy (XPS, PHI 5600, Physical Electronics, USA). The average composition was obtained by dissolving the coating on a quartz substrate in boiling hydrochloric acid (37%) and then measuring the concentrations of the individual metal ions in the solution using an inductive coupled plasma spectrometer (ICP-AMS, OPTIMA 3000XL, Perkin Elmer). The microstructure and morphology of the coating were examined by X-ray diffraction (XRD, PW1830, Philips, The Netherlands) and scanning electron microscopy

(SEM, JSM-6300F, Jeol, Japan), respectively. The cyclic voltammetric behavior was investigated using a potentiostat/galvanostat (PGSTAT 100, Autolab, The Netherlands). Pt wire was used as a counter-electrode, and Ag/AgCl/saturated KCl (0.222 V versus NHE) with a Luggin capillary was used as a reference electrode. The resistances between the working electrode and the reference electrode were measured using the frequency response analyzer of the potentiostat/galvanostat. The ohmic drops in the solutions were compensated. Before experiments, the solutions were purged with nitrogen gas. 2.4. Accelerated life test In order to reduce the experimental time, the accelerated life test following the procedure of Hutchings et al. [18] was adopted to assess the stability of the electrode. The electrolyte was 3 M H2 SO4 , and the cell temperature was controlled at about 25 ◦ C. A dc power supply (PD110-5AD, Kenwood, Japan) provided a constant anodic current density of 0.5 A cm−2 . The potential of the working electrode was periodically monitored. Due to the generation of a large amount of bubbles, the Luggin capillary was not used. However, the reference electrode was placed as close as possible to the working electrode. Since the ohmic drops from the solution were not compensated in this part of the test, the real potentials could be a bit smaller than the values reported.

3. Results and discussion 3.1. Coating composition Fig. 1 shows the XPS spectrum in terms of emitted electron intensity against the binding energy. The peak positions show that Ru4+ , Sb5+ and Sn4+ are the major oxidation states of the Ru, Sb, Sn species, respectively. Therefore, the ternary ruthenium, antimony and tin oxide mixture is designated as RuO2 –Sb2 O5 –SnO2 in this paper. C was detected mainly from the organic solvent, i.e., iso-propanol. In addition, a very weak Cl− signal was observed, implying that a considerably small amount of Cl− ions substituted for oxygen ions in the oxide lattice. However, no titanium species was detected, suggesting that the titanium substrate was completely covered with the coating film. The average coating composition, in terms of contents of Ru, Sb and Sn, was 12.2, 22.2 and 65.6 mol.%, respectively, indicating that the coating had lower molar percentages of Ru and Sn, but a higher molar percentage of Sb than the precursor solution. This could be attributed to the yield differences of the individual metal oxides. The lower molar content of Sn is related to the high volatility of the SnCl4 precursor [19].

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Table 1 Cell unit parameter comparison of RuO2 –Sb2 O5 –SnO2 with SnO2 Sample

˚ a = b (A)

˚ c (A)

RuO2 –Sb2 O5 –SnO2 Cassiterite (SnO2 )

4.6824 4.7382

3.1628 3.1871

mum difference is 13%, smaller than the Hume–Rothery limit for successful substitution, i.e., 15% [21]. This explains why the ternary oxide mixture of RuO2 –Sb2 O5 –SnO2 existed as a solid solution. Table 1 compares the cell unit parameters of the RuO2 –Sb2 O5 –SnO2 film and cassiterite. The cell unit parameters of the ternary oxide film are smaller than those of the cassiterite, indicating the deformation of the crystal unit after partial replacement of Sn4+ in the SnO2 lattices by Ru4+ and Sb5+ . This is understandable because Ru4+ and Sb5+ have smaller ionic radii than Sn4+ has. 3.3. Surface morphology Fig. 1. XPS spectrum of Ti/RuO2 –Sb2 O5 –SnO2 .

3.2. Microstructure Fig. 2 shows the XRD pattern of the RuO2 –SnO2 –Sb2 O5 film coated on a quartz substrate with a dimension of 25 mm × 25 mm × 3 mm. Only a single set of broad and symmetric peaks was found with the peak positions deviating from those of cassiterite (SnO2 ). Neither RuO2 -rich peaks nor Sb2 O5 -rich peaks were detected. These facts reveal that various components in RuO2 –Sb2 O5 –SnO2 were highly intermixed. In other words, RuO2 –Sb2 O5 –SnO2 existed in the form of a solid solution like IrO2 –Sb2 O5 –SnO2 [16]. The average crystalline size of the RuO2 –Sb2 O5 –SnO2 film measured by XRD was only 4 nm. Theoretically, whether or not an oxide mixture can form a solid solution is highly dependent on the ionic radius difference in the elements. The ionic radii of Ru4+ , Sb5+ and Sn4+ are 0.062, 0.060 and 0.069 nm, respectively [20]. Their maxi-

Fig. 2. XRD pattern of Ti/RuOx –Sb2 O5 –SnO2 at a scan speed of 0.002 s−1 .

Fig. 3 presents a typical SEM image of a Ti/RuO2 –Sb2 O5 –SnO2 electrode. Unlike RuO2 films, which generally have mud-like morphologies, the RuO2 –Sb2 O5 –SnO2 film had rare cracks. This indicates that incorporation of Sn and Sb components can improve the coating structure effectively. 3.4. Voltammetric behavior in 0.5 M H2 SO4 solution Fig. 4 shows the cyclic voltammograms obtained on the Ti/RuO2 –Sb2 O5 –SnO2 electrode in a 0.5 M H2 SO4 solution. In the first a few scans, the voltammogram changed dramatically as observed on the Ti/SnO2 –Sb2 O5 and Ti/RuO2 electrodes, probably as a consequence of hydration of the coating surface [22,23]. The shape of the voltammogram then quickly became consistent. After 60 cycles, the voltammograms were almost identical. As observed on the Ti/RuO2 electrodes [6], the anodic current peaks around 0.75 and 1.15 V versus NHE and the

Fig. 3. SEM image of Ti/RuO2 –Sb2 O5 –SnO2 .

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Fig. 4. Cyclic voltammograms obtained in a 0.5 M H2 SO4 solution at a scan rate of 0.1 V s−1 at 25 ◦ C.

corresponding cathodic current peaks at slightly lower potentials were detected on the Ti/RuO2 –Sb2 O5 –SnO2 electrode during the potential scan. These peaks resulted from the redox reactions of the Ru species. The quick increase in current beyond 1.4 V versus NHE is attributed to O2 evolution. 3.5. Potential dependence on current density The dependence of potential on current density is shown in Fig. 5. Like the Ti/RuO2 electrode [6,7], the Ti/RuO2 –Sb2 O5 –SnO2 electrode has two Tafel slopes, i.e., 0.063 V and 0.094 V dec−1 in the low and high potential regions, respectively, which are larger than those obtained on the Ti/RuO2 electrode, i.e. 0.031–0.041 V dec−1 in the low potential region and 0.042–0.066 V dec−1 in the high potential region [6,7]. This is associated with the use of a low Ru molar ratio and a high calcination temperature in preparing the Ti/RuO2 –Sb2 O5 –SnO2 electrode in the present study.

Fig. 6. Potential variation vs. time in the accelerated life test performed in a 3 M H2 SO4 solution under 0.5 A cm−2 at 25 ◦ C.

3.6. Stability Fig. 6 shows the variation in potential versus time in the accelerated life test. The potential increased very slowly in the initial period of time. After 250 h, the potential increased quickly. A sharp increase in the potential was observed during the last few hours, indicating the failure of the Ti/RuO2 –Sb2 O5 –SnO2 electrode. The service life of the Ti/RuO2 –Sb2 O5 –SnO2 electrode is about 307 h under accelerated life test conditions, which is over 15 times longer than the reported service lives of any other RuO2 -based electrodes [8–14]. Electrode failure can be the consequence of several mechanisms, including metal base passivation, coating consumption, coating detachment, and mechanical damage [24]. At the end of the accelerated life test, the weight loss of the coating was measured to be over 70%. This indicates that the failure of Ti/RuO2 –Sb2 O5 –SnO2 electrode can be attributed to the coating consumption. It should be noted that the service life mentioned above was obtained in the accelerated life test. Under normal operating conditions, an electrode will have a much longer service life. A simple relationship between the electrode service life (SL) and the current density (i) was proposed in our previous study [16] as follows: SL ∝

Fig. 5. Dependence of potential on current density obtained potentiostatically under quasi-stationary states at a scan rate of 0.002 V s−1 in a 0.5 M H2 SO4 solution at 25 ◦ C.

1 , in

(1)

where n ranges from 1.4 to 2.0. By assuming an average n of 1.7 for the Ti/RuO2 –Sb2 O5 –SnO2 electrode, we predict its service life to be 27 and 8.3 years in strong acidic solutions at current densities of 0.01 and 0.02 A cm−2 , respectively. Therefore, Ti/RuO2 –Sb2 O5 –SnO2 electrodes are stable enough for applications where low current densities are used. The much longer service life of the Ti/RuO2 –Sb2 O5 – Sn-O2 electrode than that of other RuO2 -based oxide

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electrodes is primarily attributed to the formation of a solid solution of RuO2 –Sb2 O5 –SnO2 . Since both SnO2 and Sb2 O5 are very stable chemically, the homogenous mixing of RuO2 with SnO2 and Sb2 O5 by forming a solid solution decreases the rate of RuO2 dissolution. This leads to a significant increase in the electrode service life. In addition, the RuO2 –SnO2 –Sb2 O5 film has a compact structure as shown in Fig. 3, which further enhances the electrode stability. We need to point out that an electrocatalyst whose performance deteriorates with time is technologically less interesting than a material with lower electrocatalytic activity but much better long-term stability [25]. Moreover, the potential difference between a Ti/RuO2 –Sb2 O5 –SnO2 and a Ti/RuO2 electrode is estimated to be smaller than 0.1 V at current densities below 0.02 A m−2 . Therefore, it is believed that the RuO2 –Sb2 O5 –SnO2 electrocatalyst is superior to RuO2 and other common RuO2 -based electrocatalysts for oxygen evolution because of its much longer service life.

References [1] [2] [3] [4] [5]

[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

4. Conclusions

[17] [18]

RuO2 –Sb2 O5 –SnO2 is a promising electrocatalyst for O2 evolution. It has a compact microstructure with its metal oxides existing in a solid solution. The accelerated life test showed that the Ti/RuO2 –Sb2 O5 –SnO2 electrode containing 12.2 molar percent of RuO2 nominally in the coating had a service life of 307 h in a 3 M H2 SO4 solution under a current density of 0.5 A cm−2 at 25 ◦ C, over 15 times longer than other typical RuO2 -based electrodes. Ti/RuO2 –Sb2 O5 –SnO2 electrodes are believed to be stable enough for applications in which low current densities (<0.02 A cm−2 ) are required.

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[19] [20] [21] [22] [23] [24] [25]

H.B. Beer, US Patent 3,632,498 (1972). H.B. Beer, US Patent 3,771,385 (1973). H.B. Beer, J. Electrochem. Soc. 127 (1980) 303C. H.B. Beer, J.M. Hinden, US Patent 4,331,528 (1982). D.M. Novak, B.V. Tilak, B.E. Conway, in: J.O. Bockris, B.E. Conway, R.E. White (Eds.), Modern Aspects of Electrochemistry, vol. 14, Plenum Press, New York, 1982. R.S. Yeo, J. Orehotsky, W. Visscher, S. Srinivasan, J. Electrochem. Soc. 9 (1981) 1900. J. Melsheimer, D. Ziegler, Thin Solid Films 163 (1988) 301. T. Loucka, J. Appl. Electrochem. 7 (1977) 211. C. Iwakura, K. Sakamoto, J. Electrochem. Soc. 132 (1985) 2420. L.D. Burke, M. McCarthy, Eletrochim. Acta 29 (1984) 211. A.J. Terezo, E.C. Pereira, Electrochim. Acta 44 (1999) 4507. M. Musiani, F. Furlanetto, R. Bertoncello, J. Electroanal. Chem. 465 (1999) 160. R. Bertoncello, S. Cattarin, I. Frateur, M. Musiani, J. Electroanal. Chem. 492 (2000) 145. L.M. Da Silva, L.A. De Faria, J.F.C. Boodts, J. Electroanal. Chem. 532 (2002) 141. F. Hine, M. Yasuda, T. Noda, T. Yoshida, J. Okuda, J. Electrochem. Soc. 126 (1979) 1439. X.M. Chen, G.H. Chen, P.L. Yue, J. Phys. Chem. B 105 (2001) 4623. G.H. Chen, X.M. Chen, P.L. Yue, J. Phys. Chem. B 106 (2002) 4364. R. Hutchings, K. Muller, F. Kotz, S. Stucki, J. Mater. Sci. 19 (1984) 3987. C. Comninellis, G.P. Vercesi, J. Appl. Electrochem. 21 (1991) 335. D.R. Lide, CRC Handbook of Chemistry and Physics, 83th ed., CRC Press, Boca Raton, FL, 2002. W. Hume-Rothery, The Structure of Metals and Alloys, Institute of Metals, London, 1936. B. Correa, C. Comninellis, A.D. Battisti, J. Appl. Electrochem. 26 (1996) 683. S. Ardizzone, G. Fregonara, S. Trasatti, J. Electroanal. Chem. 266 (1989) 191. G.N. Martelli, R. Ornelas, G. Faita, Electrochim. Acta 39 (1994) 1551. S. Trasatti, Electrochim. Acta 29 (1984) 1503.