Investigation of the electrical properties, charging process, and passivation of RuO2–Ta2O5 oxide films

Investigation of the electrical properties, charging process, and passivation of RuO2–Ta2O5 oxide films

Journal of Electroanalytical Chemistry Journal of Electroanalytical Chemistry 592 (2006) 153–162 www.elsevier.com/locate/jelechem Investigation of t...

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Electroanalytical Chemistry Journal of Electroanalytical Chemistry 592 (2006) 153–162 www.elsevier.com/locate/jelechem

Investigation of the electrical properties, charging process, and passivation of RuO2–Ta2O5 oxide films Josimar Ribeiro, Adalgisa R. de Andrade

*

Departamento de Quı´mica, Faculdade de Filosofia Cieˆncias e Letras de Ribeira˜o Preto, Universidade de Sa˜o Paulo, 14040-901, Ribeira˜o Preto, SP, Brazil Received 23 August 2005; received in revised form 29 March 2006; accepted 1 May 2006 Available online 21 June 2006

Abstract Freshly prepared RuO2–Ta2O5 thin films containing between 10 and 80 atom% Ru have been examined and characterized by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and morphological analysis (SEM – scanning electron microscopy/ EDS – energy dispersive X-ray spectroscopy). Investigation of the electrical properties, charging process, and passivation of the electrode containing RuO2–Ta2O5 thin films was conducted as a function of electrode composition in a 0.5 mol dm3 H2SO4 solution. For potential values in the double layer region (0.2–1.0 V vs. RHE), the impedance profile observed at low frequency domain was attributed mainly to the capacitive behavior of the oxide/solution interface. As for the high frequency domain, the impedance profile gave evidence that the kinetic process is limited by supporting electrolyte/water diffusion inside the pores of the difficult-to-access oxide regions and/or the Ti/ oxide interface. The electrode passivation mechanism toward OER (oxygen evolution reaction – 1.5 V vs. RHE) was also investigated during long-term electrolysis (j = 750 mA cm2 and T = 80 C) by means of EIS at pre-established times. The SEM–EDS data give evidence of the increase in the TiOx interlayer. Moreover, the EIS data furnished complementary insight that helped our proposition of the deactivation mechanism.  2006 Elsevier B.V. All rights reserved. Keywords: Ruthenium and tantalum oxide; Impedance; Deactivation mechanism; Electrical properties

1. Introduction The DSA material consists of a metallic substrate with a ceramic coating, and its best-known composition is a mixture of RuO2 and TiO2. However, when thin films of these oxides operate under severe conditions, such as high temperature, oxidizing environment, and high overpotential, their total or partial deactivation can be observed in a short period of time [1,2]. The mechanism leading to the deactivation of these electrodes has been widely investigated and can be summarized in the following steps: (i) electrode passivation promoted by the insulating TiO2 layer formed between the metallic Ti-base and the conduct-

*

Corresponding author. Tel.: +55 16 3602 3725; fax: +55 16 3633 8151. E-mail address: ardandra@ffclrp.usp.br (A.R. de Andrade).

0022-0728/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2006.05.004

ing oxide layer [3], and (ii) partial removal of the loaded catalyst (RuO2) due to erosion and corrosion processes [4–6]. The occurrence of both processes (i) and (ii), which play different roles depending on the electrode composition and the method employed for its preparation, has also been claimed [7–9]. In order to improve the electrode lifetime obtained with TiO2-based electrodes, Vercesi et al. introduced Ta2O5 as a valve metal for oxygen production on DSA-type electrodes (Ti/IrO2–Ta2O5) because it promotes excellent protection against electrode wearing out [10]. Tantalum oxide is used not only because of its ability to function as a valve metal, but also because its semiconductor properties have also been widely investigated (band gap energy 4.3 eV) [11]. The use of ceramic films consisting of RuO2 and Ta2O5 has found many applications, specially in the technological field and electronic industry, where solid state sensors,

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dynamic memory capacitors, hybrid capacitors, and MOS (metal/oxide/semiconductor) transistors are of great interest [10–15]. Characterization of both the electrode morphology and the structure of these conducting metal oxide films is the key to understanding their electrochemical behavior. The morphology and the composition of their oxide coating bear straight relation with their electrochemical performance, such as double layer capacitance, electrocatalytic activity, voltammetric pattern, and electrode lifetime. Electrochemical impedance spectroscopy (EIS) has helped researchers gain information about substrate/oxide and oxide/electrolyte interfaces, and it has also been extremely helpful in the proposition of mechanisms for electrochemical reactions. Many dissolution, corrosion, and passivation processes have been investigated by EIS, which has helped explain the electrochemical behavior of metal oxide films [16–19]. The fact that EIS is such a powerful tool has stimulated us to use this technique in the characterization of Ru–Ta oxides thin films. In this work, we present a detailed examination of the RuO2–Ta2O5 system using 10–80 atom% Ru. Two different experimental conditions have been investigated. Firstly, we studied the double layer charging potential region (0.2– 1.0 V vs. RHE) in order to obtain a detailed understanding of the nature of the electrochemical response. Secondly, we looked into the electrode passivation process toward the region of the oxygen evolution reaction (1.5 V vs. RHE) under drastic conditions (j = 750 mA cm2 and T = 80 C). Changes in the electrical properties and lifetime parameters of RuO2–Ta2O5 anodes obtained when TiO2 was replaced with Ta2O5 are also presented. 2. Experimental 2.1. Electrode preparation Electrodes with nominal composition Ti/RuO2–Ta2O5 (Ru:Ta = 10:90, 30:70, 50:50, and 80:20 atom%) were investigated in this work. Oxide layers were prepared by thermal decomposition (Tcalcination: 450 C). Mixtures containing the precursors were prepared for each electrode composition by dissolving the appropriate amounts of RuCl3 Æ xH2O (0.072 mol dm3) and TaCl5 (0.083 mol dm3) (Aldrich) in isopropanol (Aldrich). Sandblasted Ti-supports (5 · 10 · 0.15 mm) were degreased with isopropanol and submitted to chemical attack. This attack was performed with boiling concentrated HCl for 30 min, followed by etching in boiling 10% oxalic acid for 20 min. The precursor mixtures were then dropped from a micropipette (10 lL) on both faces of a pretreated Ti-support base. The electrode was then dried at low temperature (80–90 C) in order to evaporate the solvent. The dried electrode was fired at 450 C, for 5 min, under a 5 dm3 min1 O2-flux. This procedure was repeated until the desired nominal oxide loading (1.4–1.6 mg cm2) was reached. The layers were finally annealed for 1 h, at 450 C, under O2-flux.

Duplicate samples were prepared for each electrode composition. Details of the electrodes preparation and final mounting are described elsewhere [20]. 2.2. Cell, equipment, and solutions All solutions used in this work were prepared with 18.2 MX cm water, produced and purified by a MilliporeMilli-Q system. Electrochemical studies were performed using 0.5 mol dm3 H2SO4 (Synth) as the supporting electrolyte. An experimental setup consisting of a one-compartment electrolytic cell with a main body (60 mL), two 15 cm spiral platinized platinum wires used as counter electrode, and a reference hydrogen electrode (RHE) driven close to the working electrode (WE) by means of a Luggin–Haber capillary was used throughout the experiments. Electrochemical experiments were carried out using an AUTOLAB model PGSTAT30 (GPES/FRA) instrumentation. Voltammetric curves were recorded in the 0.2– 1.2 V vs. RHE potential range. Impedance spectra were recorded at a constant potential falling between 0.2 and 1.0 V vs. RHE, and at 1.5 V vs. RHE, placed in the OER (oxygen evolution reaction) domain. EIS measurements were obtained in the 5 mHz to 100 kHz frequency interval, using the ‘single sine’ method and a sine wave amplitude of 5 mV (p/p). A software program (FRA analyzer program – version 4.9) provided by AUTOLAB was used to analyze impedance data. The inductive element observed at high frequencies was traced down; however, it was observed that it changes with the content of RuO2 loaded in the coating. We conducted several experiments using a set of dummy cells composed by resistor (R) and capacitor (C) elements, which confirmed that the inductive behavior in the high frequency domain is due to the equipment, wires, and connectors, as reported earlier [21,22]. Accelerated life test (ALT) was performed under galvanostatic conditions at high current density (750 mA cm2), and the temperature was maintained at 80 C by means of a water thermostat. The period of time necessary for the electrode potential to achieve a value of 6 V vs. RHE was termed as the electrode lifetime. The electrode was considered inactive for OER at higher potentials. All electrochemical experiments were repeated at least twice. 2.3. Morphological characterization Surface morphology, microstructure, and elemental composition of the deposited oxide films were analyzed through scanning electron microcopy (SEM) and energy dispersive X-ray spectroscopy (EDS), using a Leica-Zeiss LEO 440 model SEM coupled to an Oxford 7060 model analyzer and a Zeiss 940 microscope coupled to a ZAF 4FLF link analytical system. For analysis of the oxide transversal section, thin films were embedded in an acrylic resin. The ensemble was then

J. Ribeiro, A.R. de Andrade / Journal of Electroanalytical Chemistry 592 (2006) 153–162

submitted to polishing and the surface was covered with gold for the SEM and EDS analyses. 3. Results and discussion 3.1. SEM and EDS analyses of the RuO2–Ta2O5 electrodes Figs. 1 and 2 show representative SEM–EDS image for X-ray linescanning over the cross-section of the RuO2– Ta2O5 electrode embedded in an acrylic resin. It can be seen that the electrode is composed of four different regions: (i) region 1 corresponds to the acrylic resin used to recover the electrode; (ii) region 2 consists of a thick RuO2 and Ta2O5 oxide layer, the average thickness being the same (8 lm) before and after the ALT experiment; (iii) region 3 exhibits an intermediate composition, with a high concentration of TiOx besides the loading of thermal oxides, and the thickness of this layer significantly increases with the progress of the ALT experiments (from 5 to 13 lm); and (iv) region 4 corresponds to the metallic titanium base. EDS microanalysis performed in line over the transversal section shows occurrence of an important interdiffusion of the oxide constituents (Ru, Ta, and Ti) in the boundary face located between the substrate/oxide region (see Figs. 1B and 2B). This is in agreement with other RuO2–Ta2O5 compositions investigated by us [23]. By comparing freshly prepared anodes (Fig. 1) with those submitted to exhaustive operation (Fig. 2), it can be seen that there is a significant increase in the thickness of the TiOx interlayer. Before the ALT experiment, the mean thickness value obtained for layers 2 and 3 was around 13 lm. After the electrode was submitted to a drastic operational condition (j = 750 mA cm2, T = 80 C, acid medium: 0.5 mol dm3 H2SO4), the average thickness of these oxide layers (regions 2 and 3) increased to 22 lm (Fig. 2). Besides the increase in the thickness of the TiOx interlayer, we also observed evidences of erosion/corrosion process for Ru-rich compositions (P50 atom%). This is confirmed by the change in the color of the solution; i.e., from colorless to gray. For these sam-

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ples, the EDS-signal obtained for Ru is less intense than that of the freshly prepared samples, indicating that this metal is partially dissolved during the long-term operation over high currents. The instability of the RuO2-based coating under highly oxidizing conditions has been reported before [24]. Nevertheless, the EDS-data shows the presence of a considerable amount of Ru and Ta oxide after exhaustive electrolyses. This confirms the proposed mechanism for electrode passivation during the ALT [17,25,26]. Taking this into account, we suggest that the main reason for the inactivity of the RuO2–Ta2O5 electrode is the passivation derived from the increase in the thickness of the insulating TiOx layer under high overpotential regions (OER). In fact, the electrode lifetime can be governed by the thickness of the TiOx layer. Freshly prepared anodes exhibit a thinner TiOx layer throughout the thermal treatment step involved in the electrode preparation (Fig. 1A). However, the good solubility of RuO2 in TiOx allows the formation of a compact conductive bulk structure of Ru-doped TiOx layer. Moreover, this layer furnishes good protection for the substrate, leading to the excellent chemical and mechanical stability of the DSA-type materials [27]. The main challenge in developing materials with high service life lies on the optimization and control of the thickness of the TiOx interlayer throughout the operation. This is because this layer becomes non-conductive as it increases a few lm due to TiOx enrichment. As we have shown before [28], the RuO2–Ta2O5 coating exhibits good electrode activity because its oxide coating bears a compact and conductive bulk structure. This structure leads to better chemical and mechanical stability when compared to RuO2–TiO2 oxides. 3.2. Cyclic voltammetry Fig. 3 shows some representative cyclic voltammograms of the Ti/RuO2–Ta2O5 systems recorded in acid medium. The voltammetric curves are characterized by the presence of the Ru(III)/Ru(IV) [29–31] solid state surface redox transition, which lies between 0.5 and 1.0 V vs. RHE.

Fig. 1. (A) SEM image of the cross-section of Ti/RuO2–Ta2O5 – Ru:Ta = 50:50 atom%. (B) EDS linescanning for O, Ru, Ti and Ta before ALT. Tcalc. = 450 C; O2-flux = 5 dm3 min1.

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Fig. 2. (A) SEM image of the cross-section of Ti/RuO2–Ta2O5 – Ru:Ta = 50:50 atom%. (B) EDS linescanning for O, Ru, Ti and Ta after ALT. Tcalc = 450 C; O2-flux = 5 dm3 min1.

A

j / mA cm

-2

2

B A'

1

B'

0 -1 -2 -3 0.2

0.4

0.6

0.8

1.0

1.2

E / V vs RHE Fig. 3. Voltammetric curves at 50 mV s1 in 0.5 mol dm3 H2SO4 of the Ti/RuO2–Ta2O5 system. (- - -) Ti/RuO2–Ta2O5 (Ru:Ta = 80:20 atom%); and (––) Ti/RuO2–Ta2O5 (Ru:Ta = 30:70 atom%). A and A 0 represent the anodic peak charge density; qF, B, and B 0 represent the capacitive charge density, qdl.

The best-defined peak is observed for the electrode with 30 atom% Ru loading. The charge observed in the cyclic voltammogram is associated with both the double layer and the redox couple transition. RuO2 electrodes behave as a ‘‘protonic capacitor’’ in the potential range between the onset of OER and HER (hydrogen evolution reaction). During the potential sweep, the oxide surface is oxidized and reduced, reversibly exchanging protons with the solution, as described before [32–34] RuOx ðOHÞy þ dHþ þ de RuOxd ðOHÞyþd

06d62 ð1Þ

Taking this into account, one can decompose the total observed charge into a contribution from two main processes: (i) the anodic peak charge density, (qF), which occurs at the oxide layer hydrated portion and is related to Eq. (1), where a proton injection/ejection is involved; (ii) the capacitive charge density due to the double layer formed between the oxide layer and the solution (qdl). Using the treatment proposed before [29,35], the capacitive charge density, qdl,

and anodic peak charge density, qF, can be obtained by deconvolution of the cyclic voltammogram. Briefly, qF is obtained by taking the area under the RuO2 redox peak (areas A and A 0 in Fig. 3). The capacitive charge density described under areas B and B 0 in Fig. 3 allows us to obtain qdl. Table 1 shows the deconvolution of the data from Fig. 3 obtained for the Ti/RuO2–Ta2O5 electrodes. The qF values change with electrode composition, and it can be observed that the electrode containing 30 atom% Ru loading exhibits particularly high qF values. As confirmed by many propositions in the literature, this larger electrochemically active area reflects the greater number of active RuO2 sites in contact with the solution [28,32,36,37]. A different approach to estimate the surface characterization is to determine the morphology factor, /, defined as the ratio between the inner and total differential capacities (Cd,i/Cd), as has been proposed recently [38]. Taking this proposition into account, we obtained /-values of 0.40 for electrodes with 30 atom% Ru loading. An increase in the amount of RuO2 in the electrode composition (Ru P 50 atom%) decreased / to 0.18. These values are in agreement with the deconvolution data presented above and indicate a larger active area for the electrode containing the 30 atom% Ru loading. The number of active sites can be obtained using the equation proposed before by Nanni et al. [29]: Nsites = (qF · N)/F, where N is Avogadro’s number and F is the Faraday constant. The values obtained for the RuO2– Ta2O5 electrodes are also shown in Table 1, and they are at least 10 times higher than the values previously reported for the Ti/RuO2–TiO2 [39] and Ti/RuO2–SnO2 [29,35] electrodes. 3.3. The film charging process To further characterize the Ti/RuO2–Ta2O5 system, the AC impedance behavior of the freshly prepared electrodes was investigated. The complex impedance spectrum (Zreal vs. Zimg) and the Bode plot (h vs. log f) obtained between

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157

Table 1 Results obtained from the electrochemical characterization of the Ti/RuO2–Ta2O5 electrodes Electrode

qdl (mC cm2)

qF (mC cm2)

qa ðmC cm2 Þ

CCV (F (g RuO2)1)

CEIS (F (g RuO2)1)

1016Nsites

1016 *Nsites

10% 30% 50% 80%

3.57 16.16 27.42 48.00

0.08 9.14 3.07 2.45

3.65 25.30 30.49 50.45

20 197 17 2.7

2 60 35 2.5

0.24 5.71 1.92 1.53

0.01 3.47 7.76 2.77

Ru Ru Ru Ru

Nsites is the number of active sites calculated from CV data as: Nsites = (qF · N)/F *N sites is the number of active sites calculated from EIS data at 0.8 V vs. RHE.

[29].

qa is the anodic charge calculated from CV between 0.2 and 1.0 V vs. RHE. CCV is the specific capacitance from CV data. CEIS is the specific capacitance from EIS data at 0.8 V vs. RHE. 0.6

15.0k

A

-Zimg / Ω

0.4

0.2

-Z img / Ω

10.0k 0.0 1.4

1.6

1.8

Z real / Ω

2.0

5.0k

0.0 0.0

5.0k

10.0k

15.0k

Z real / Ω 2

4k

-Zimg / Ω

B

3k

1

0

-Z img / Ω

0.2 and 1.0 V vs. RHE for RuO2–Ta2O5 electrodes are shown in Fig. 4. The main aspect of Fig. 4 is that the impedance plots furnish a good tool to follow the different kinds of activation imposed to the electrode (e.g., potential, frequency, and composition changes). We can summarize the behavior of the RuO2–Ta2O5 electrodes in terms of the Ru content. Electrodes rich in Ru exhibit a straight line with inclination close to 90 at the low frequency domain. This behavior can be explained by considering the fact that, under these experimental conditions, the charge transfer resistance related to Eq. (1) is small, and the magnitude of the total capacitance (capacitive and pseudo-capacitive process) increases, with h reaching values close to 90 at low frequencies [1]. The shift from the ideal capacitor behavior (h = 90) is a consequence of the porous characteristic of the material [40]. Recently, this deviation has been explained as a consequence of the non-ideally polarizable behavior of the electrode attributed to the redox-related capacitance (Eq. (1)) due to proton diffusion [41]. For electrodes containing low Ru loading (10 atom%), an almost straight line with slope slightly higher than unity is observed in the whole frequency range. Analysis of Fig. 4D–F shows that their main feature is the appearance of a well-defined time constant (s) for the electrode containing low Ru content (10 atom%), which is characterized by a maximum phase angle (65) ranging from 100 to 1000 Hz. Such behavior has also been reported before for RuO2/TiO2 electrodes with low Ru loading and is the result of the interposition of various time constants with respect to frequency [1]. Besides the complex behavior shown above, one can observe a slight increase in the h-values as the potential is increased toward more positive values, at constant electrode composition and low frequency range. Once again, this is due to the pseudo-capacitance phenomena occurring at the complex oxide microstructure (see Eq. (1)). These phenomena can be related to the large number of RuO2 transition states contributing to the system charging [1,42]. In conclusion, by comparing the RuO2–Ta2O5 impedance behavior with other RuO2-based materials [41,43,44] and RuO2–TiO2 electrodes [1], one can conclude that the substitution of TiO2 for Ta2O5 does not promote any qualitatively significant change in the impedance spectrum

1

2

3

Zreal / Ω

2k

1k

0 0

1k

2k

Z real / Ω Fig. 4. Impedance diagrams in the complex plane (A–C) and Bode plot (D–F) as a function of the electrode nominal composition and applied potential: (A, D) Ti/RuO2–Ta2O5 (Ru:Ta = 10:90); (B, E) Ti/RuO2– Ta2O5 (Ru:Ta = 30:70); and (C, F) Ti/RuO2–Ta2O5 (Ru:Ta = 80:20). Insets show the high frequency part of the spectra. (-s-) 0.2 V; (-j-) 0.4 V; (-n-) 0.6 V and (-·-) 1.0 V vs. RHE.

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D

C 60 0.15

50 0.10 o

40

-θ /

-Zimg / Ω

1.5k

30

0.05

20 0.00

-Z img / Ω

10 0

-0.05 0.50

0.55

0.60

0.65

-3

0.70

-2

-1

0

1

2

3

4

5

6

log (f / Hz)

Z real / Ω

1.0k

90

E

80 70 60 50

-θ /

o

500.0

40 30 20 10 0 -3

0.0 0.0

500.0

-2

-1

0

1.0k

1

2

3

4

5

6

log (f / Hz)

Z real / Ω 90 80

F

70 60

-θ /

o

50 40 30 20 10 0 -3

-2

-1

0

1

2

3

4

5

log (f / Hz) Fig. 4 (continued)

Fig. 5. Equivalent circuit (EC) used for the fitting of the experimental results presented in Fig. 4.

profile, and the total impedance of these thin films is dominated by the pseudo-capacitive behavior of the RuO2 film. However, quantitative values such as film resistance, film capacitance, and double layer capacitance are greatly influenced by the electrode composition and the preparation method. After testing a number of different equivalent circuits, we found that the whole set of data at the double layer domain of the RuO2–Ta2O5 system can be fitted by assuming the circuit presented in Fig. 5; i.e., [Rs(CPEfRf)-

J. Ribeiro, A.R. de Andrade / Journal of Electroanalytical Chemistry 592 (2006) 153–162

5

-2

CPEPF /mFcm s

-n

6

A

4 3 2 1 0 0.2

0.4

0.6

0.8

1.0

0.8

1.0

E/ V vs RHE 150 -n

Rt /CPEPF (ΩFs )

B 100 50 0.6 0.4 0.2 0.0 0.2

0.4

0.6

E/ V vs RHE Fig. 6. Rct/CPEpc on nominal Ru-content for the different applied potentials. (-j-) Ti/RuO2–Ta2O5 (Ru:Ta = 10:90 atom%); (-e-) Ti/ RuO2–Ta2O5 (Ru:Ta = 30:70 atom%); (-n-) Ti/RuO2–Ta2O5 (Ru:Ta = 50:50 atom%); and (-d-) Ti/RuO2–Ta2O5 (Ru:Ta = 80:20 atom%).

(Cdl[RctCPEpc])]. An analogous EC has been proposed for Ta2O5 [45]. At low frequencies, the film charging process is mainly related to the oxide/solution interface. The components (Cdl[RctCPEpc]) introduced in the scheme represent the double layer process (Cdl) coupled to the charge transfer resistance (Rct) and pseudo-capacitance (CPEpc) associated with the faradaic process (Eq. (1)). The use of a CPE (constant phase element) in the equivalent circuit notation instead of C (pure capacitor) is due to the high porosity and roughness degrees of the oxide layers, which contribute to the film inhomogeneity [46]. As the frequency is enhanced, the response of the electrode is governed by the Ti/inner oxide region interface (regions 3 and 4 in Fig. 1). The charge process at the high frequency domain (CPEfRf) takes the influence of the film resistance (Rf) and capacitance on the film oxide (CPEf) into account. Comparing Fig. 6 with Fig. 3, one can observe that the CPEpc vs. E curve bears remarkable similarity with the cyclic voltammetry (CV)-curve, with a maximum CPEpcvalue around 0.8 V vs. RHE (see Fig. 6A). This has been previously observed for other oxide systems [47,48]. The pseudo-capacitive behavior of the ruthenium oxide solid state transition is confirmed through the broad region 0.5–1.0 V vs. RHE, where this processes occur [28,32,49]. As shown before (Fig. 3), this region cannot be solely associated with the pure capacitive behavior of the double layer charge, but it also reflects the faradaic process; i.e., the reversible exchange of charge/protons with the active

159

ruthenium sites [1,29,32,42]. Taking the CPEpc-values obtained by the simulation data, one can easily find the faradaic charge qpc at a fixed potential (e.g., 0.8 V vs. RHE). Table 1 shows the number of active sites (*Nsites = qpc · N/ F) obtained from the EIS-data. The number of active sites obtained from qF (CV-data) and CPEpc are of the same order of magnitude for both approaches, and they also show higher values in the case of the intermediate catalyst compositions. The agreement between dc and ac experimental values is a good indication of the attribution of the physical meaning of CPEpc. The specific capacitance values obtained for intermediate (30–50 atom%) Ru compositions (Table 1) are of the same order of magnitude of those found for RuO2 and RuO2 Æ 0.3H2O nanoparticulates [41]. The Rs-values obtained change with electrode composition, ranging from 0.6 to 1.8 X. Although many studies have shown that the ionic resistance located between the Luggin–Haber capillary and the WE is the key parameter affecting this resistance [8,17,47], we have ruled out this situation because the measurements were carried out in the same supporting electrolyte solution without changing the electrodes position. This was done in order to obtain a constant uncompensated ohmic resistance (RX). These experiments also showed that Rs changes as a function of the potential, indicating that a contribution from the apparatus (connectors, leads, and wires) for total impedance of the system cannot be dismissed at high frequencies. Therefore, the Rs values obtained by us suggest a contribution from both the non-compensated solution resistance (RX) and the apparatus resistance (connectors, leads, and wires) to the measured impedance. Furthermore, a displacement of impedance curves is observed for low Ru loading (630 atom%) as the potential is changed (0.2–1.0 V vs. RHE). This behavior is not observed for high Ru loading (see inset in Fig. 4A–C). A similar behavior has been observed for the Ru0.3Ti0.7O2graphite composite [50]. The dependence of the impedance displacement on the composition and potential can also be explained by assuming that these materials exhibit skin effect as the frequency is enhanced (>10 kHz). This means that the ac current does not circulate with a continuous movement in the inner part of the conducting material. As a consequence of this movement, the current propagates through the conductor surface, and the movement toward the centre is lower as the frequency is enhanced. Taking into consideration that the size of the crystallites increases as the Ru content is lowered [28], one can infer that, as the crystallite size increases, the system impedance increases due to the enhancement in resistance caused by the skin effect. The film resistance, Rf, shows a great dependence on the loaded catalyst. It ranges from 2.5–1725 X as the RuO2 content decreases. This suggests that a poor conducting film is formed when materials have a low catalytic content. The mobility of the charge carrier through the film is decreased due to the low content of ruthenium in the

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mixture. Also, the TiOx formed during the thermal treatment process might be less doped with Ru. The observed potential dependence can be attributed to the tunnelling resistance associated with the high energy barrier that is necessary for the electron to go through the Ti/TiOx-dop/ conducting oxide interface [45]. 3.4. Investigation of the deactivation mechanism for the RuO2–Ta2O5 system by EIS The deactivation mechanism for the RuO2–Ta2O5 system was also investigated by electrochemical impedance spectroscopy (EIS), by measuring the impedance spectrum at a pre-fixed time during the accelerated life test (ALT) (0%; 25%; 50%; 75% and 100% of ALT) at different potentials, two of them situated in the double layer region (0.8 and 1.0 V vs. RHE) and one located at 1.5 V vs. RHE, in the oxygen evolution reaction (OER) domain. Some representative Nyquist diagrams obtained at 1.5 V vs. RHE for the 30 and 80 atom% Ru loading as a function of the anodization time are shown in Fig. 7. In the low frequency region, the complex impedance spectrum for a freshly prepared electrode (ALT = 0 h – Fig. 7A) consists of a deformed semicircle, attributed to the oxygen evolution reaction. An equivalent circuit similar to the one described in Fig. 5 fits well with the experimental data. However, the attribution of the physical meaning of the circuit is differ-

ent, once the Rct now represents the charge transfer resistance for OER [1,7,17,18,25]. The Rct-values change with the ruthenium content; i.e., as the Ru loading in the film is increased, Rct decreases, which shows there is a strict dependence of the OER on the Ru content, as described for other oxides material [1,7,17,18,25]. This behavior confirms the results obtained with Tafel plots for this system [28]. Fig. 7B shows that, after the accelerated life test (ALT = 100%) of an electrode containing an intermediate Ru content (30 atom% Ru), the observed semicircle increases approximately 10 times, indicating a significant increase in the impedance of the system as a function of the electrode operational time. This increase might be explained by considering the complex deactivation process of DSA-type materials [1,7,17,18,25]. For high Ru-loading films (P50 atom% Ru), on the other hand, the formation of two semicircles is observed after ALT. A similar behavior was observed for other conductive metallic oxide systems [1,7,17,25,51], for which it is suggested that the semicircle observed at the low frequency domain is mainly controlled by the oxide/solution interface, while the second semicircle (at high frequencies domain) can be attributed to the growth of a non-conductive TiOx interlayer that becomes less doped with the catalytic oxide and/or formation of inactive RuO2 regions for OER [51]. The observation of one or two semicircles in the impedance spectrum is related

25

3

-1 -n

Rct/CPEpc ( Ω mF s )

2

-Zimg / Ω

A

24

A

1

0 0

1

2

3

4

5

23 6 3 0

6

Zreal / Ω

0

20

40

60

80

Nominal Ru content/ atom % B

400 200

20

Rf / Ω

-Zimg / Ω

40

0 0

20

40

60

80

100

Zreal / Ω Fig. 7. Nyquist diagram as a function of the anodization time: (A) freshly prepared electrode and (B) electrode after deactivation. (-j-) Ti/RuO2– Ta2O5 (Ru:Ta = 30:70 atom%); (-s-) Ti/RuO2–Ta2O5 (Ru:Ta = 80:20 atom%). Conditions: 0.5 mol dm3 H2SO4 at 0.75 A cm2; T = 80 C; E = 1.5 V vs. RHE; Tcal = 450 C.

B

14 12 10 8 6 4 2 0 0

20

40

60

80

Nominal Ru content /atom % Fig. 8. (A) Dependence of the Rct/CPEpc and (B) Rf as a function of the nominal Ru content for the different anodization times: (-n-) 0%; (-d-) 50%; and (-j-) 100% of the ALT. E = 1.5 V vs. RHE.

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with different time constants, as described before by Tilak et al. [1]. As depicted in Fig. 7, these parameters change with electrode composition and components distribution in the oxide bulk of the film. Analysis of Rct/CPEpc as a function of the electrode operation time (0%, 50%, and 100% of ALT) at 1.5 V vs. RHE (Fig. 8A) shows that the charge transfer resistance for OER remains virtually unchanged with electrode anodization. One exception is the 50 atom% Ru composition, for which the Rct/CPEpc parameter increased, although the value is still too low to explain the oxide inactivity. This is in agreement with results obtained by Hu et al. [18], who reported an enhancement in Rct values from 39 to 2500 X cm2 for the deactivation of 70% IrO2 + 30% Ta2O5 electrodes. Thus, one can infer that the external oxide layer is not responsible for electrode deactivation since it is still active for OER. The deactivation observed after ALT is mainly due to the TiOx interlayer. On the other hand, taking into account the Rf data (see Fig. 8B), one can observe a considerable increase in these values after ALT, which explains our proposition that electrode deactivation occurs mainly due to a passivation process in the inner interlayer between titanium and the deposited oxide. The Rf values obtained from Fig. 8B for electrodes with high ruthenium loading are of the same order of magnitude as those from other oxide electrodes submitted to exhaustive anodization; e.g., 25 X for 30% IrO2–70% Ta2O5 [52]; 20 X for RuO2–TiO2 [1]; 12 X for Ir0.3Ti0.7O2 [17]; 9 X for Ir0.3Ti(0.7x) SnxO2 [25]. At a first glance, these values are relatively low to explain electrode deactivation. However, when one takes the value of specific resistivity, q, (R = qL/A, where R is the film resistance; L is the thickness of the TiOx layer obtained by EDS–SEM analyses (13 lm); A is the cross-sectional area (0.5 cm2)) into account, a different conclusion can be reached. The q-values obtained for the investigated electrodes (104– 105 X cm) are of the order of magnitude of rutile TiO2; e.g., 2.9 · 103–9.1 · 104 X cm [53]. Furthermore, the CPEf value obtained from the fitting of the EIS-data ranges from 103 to 106 F cm2 sn, indicating that the internal oxide layer material becomes more resistive. 4. Conclusions We have shown that the equivalent circuit which best fits our experimental data concerning impedance measurements is [Rs(CPEfRf)(Cdl[RctCPEpc]). For potential values in the double layer region, the impedance behavior observed at the low frequency domain can be attributed mainly to the system capacitive behavior, while at the high frequency domain the kinetic process is limited by the support electrolyte/water diffusion inside the pores of the difficult-to-access oxide regions. The SEM–EDS data showed evidence of the increase in the thickness of the TiOx interlayer and Ru-loss. Moreover, the EIS data helped explain the deactivation mechanism of RuO2–Ta2O5 electrodes. In fact, the attribution of the cir-

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