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Investigation of surface properties of Ru-based oxide electrodes containing Ti, Ce and Nb
Electrochimica Acta 48 (2003) 1885 /1891 www.elsevier.com/locate/electacta
Investigation of surface properties of Ru-based oxide electrodes containing Ti, Ce and Nb Ma´rio H.P. Santana a, Leonardo M. Da Silva a, Luiz A. De Faria b,* a
b
Departamento de Quı´mica, FFCLRP/USP, Av. Bandeirantes, 3900, CEP 14040-901 Ribeirao Preto, SP, Brazil ´ vila 2160, Santa Monica, CEP 38408-100 Uberlandia, MG, Brazil Universidad Federal de Uberlandia, Instituto de Quı´mica, Av. Joa˜o Naves de A Received 7 January 2003; received in revised form 5 March 2003; accepted 7 March 2003
Abstract Mixed oxide electrodes with nominal composition Ti/[Ru(0.3)Ti(0.6)Ce(0.1x )]O2[Nb2O5](x ), (0 5/x 5/0.1) were prepared through a process of thermal decomposition (450 8C) of chloride precursor mixture solutions. Composition effects on the surface properties were investigated and the oxide electrodes were characterized in acid solution: ex situ by XRD and in situ by open circuit potential measurements (Eoc), cyclic voltammetry (CV), the double-layer capacitance (CV and electrochemical impedance spectroscopy techniques) and roughness factor (CV). Except the Eoc, all parameters varied with the electrode composition. It was shown that the presence of CeO2 increases the electrochemical active area, but it makes the coating less stable when submitted to continuous potential cycles because of cathodic dissolution of this oxide. By adding Nb2O5 this instability can be lowered. The maximum activity (voltammetric charge) was observed with the [Ru(0.3)Ti(0.6)Ce(0.07)]O2[Nb2O5](0.03) composition. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Roughness factor; Cerium oxide; Niobium oxide; Oxide electrodes; Electrode stability
1. Introduction Oxide electrodes have acquired a significant importance in electrochemical technology due to some possible applications such as the chlor-alkali process, organic electrosynthesis, water electrolysis, fuel cells, etc. Industrial anodes have been traditionally based on a mixture of RuO2, the active component, and TiO2, the stabilizing agent [1]. In many investigations a third oxide is added either to enhance the selectivity and/or to increase the anodic stability [2,3]. De Faria has systematically investigated the surface and electrochemical properties of the RuO2/TiO2/CeO2 mixture [4,5]. Because of its high redox potential (CeIII/CeIV), CeO2 addition increased the electrocatalytic activity in the oxygen evolution reaction (OER) [4], but it showed cathodic dissolution in acid solution [5].
* Corresponding author. E-mail address: [email protected] (L.A. De Faria).
The oxide of nominal composition [Ru0.3Ti0.6Ce0.1]O2 has shown the best relation between electrocatalytic activity and stability. Following the idea of further improvement of this aspect, we decided to introduce a fourth component into this basic composition. Niobium pentoxide (Nb2O5) was chosen because it is recognized for having excellent stabilization capacity (Nb is a valve metals member). As a result, a system is expected to present an excellent resistance to anodic/cathodic corrosion [6]. Niobium oxide has also been introduced in Ru-based electrodes, but only to research optical properties [7], or in semiconductors based on TiO2 [8]. Recently, Terezo and Pereira studied the RuO2 /Nb2O5 [9] and IrO2 / Nb2O5 [10] systems, investigating the effect of the preparation parameters on the morphology and service life. The authors, however, did not investigate the electrocatalytic properties related to OER of these electrode materials. The objective of this study is to investigate the influence of the systematic substitution of CeO2 by Nb2O5 (in 1 mol.% steps), maintaining the RuO2 and TiO2 contents constant at 30 and 60 mol.%, respectively,
0013-4686/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0013-4686(03)00262-7
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on the surface and electrocatalytic properties of electrode material, having the nominal composition: [Ru(0.3)Ti(0.6)Ce(0.1x )]O2[Nb2O5](x ), with x in the range 0 5/x 5/0.1. In this paper, we report the morphological and electrochemical characterization in acid medium, while the electrocatalytic properties will be discussed in a forthcoming communication.
2. Experimental 2.1. Electrodes Mixed oxide layers deposited on a Ti support (10 / 10 /0.15 mm) were prepared by thermal decomposition (450 8C) of RuCl3 ×/n H2O (Aldrich), TiCl4 (Ventron), CeCl3 ×/7H2O (Merck), and NbCl5 (Fluka). The precursors were dissolved in concentrated aqueous HCl (Merck 1:1 v/v) in order to furnish a 0.18 mol dm 3 salt concentration. Ti-supports were degreased with isopropanol and submitted to chemical attack for 10 min in boiling 10% oxalic acid. The precursor mixture solutions, having the desired mole ratio, were spread onto both sides of the support by means of brushing. The solvent was evaporated at 90 8C and the residue was fired at 450 8C for 10 min, under an air stream, in a pre-heated oven. This procedure was repeated until the nominal thickness of 2 mm was achieved (corresponding to 0.9 /1.2 mg cm 2, depending on the composition). After obtaining the desired oxide loading, electrodes were fired at 450 8C for 1 h. Duplicated samples were prepared at 1 mol.% intervals covering 0 /10 mol.% Nb2O5 interval. To make a comparison possible, two samples of nominal composition Ru0.3Ti0.7O2 were prepared. All electrodes were assembled in fine glass tube. The extremity in contact with solution was sealed with Teflon parts and silicone glue as described previously [11]. 2.2. Solutions 0.5 mol dm 3 H2SO4 aqueous solutions were volumetrically prepared from concentrated H2SO4 (Merck) and Milli-Q quality (R /18.2 MV) water. Before and during each run, they were deaerated and stirred through ultrapure nitrogen (purity: 99.995%) bubbling. The three compartment cell used has been described previously [12]. 2.3. Reference electrode All potentials were read versus a hydrogen electrode (RHE) in the same solution. The RHE was renewed before each new experiment. The RHE was connected to
the solution via a Luggin capillary in order to minimize uncompensated ohmic drop. 2.4. Techniques and instruments Ti-supported oxide layers were characterized through X-ray analysis using a D5005 Model Siemens with Cu Ka /1.54056 nm. The surface response of the electrodes was examined by cyclic voltammetric curves, CV, recorded at 20 mV s 1 between /0.4 and /1.4 V/ RHE in 0.5 mol dm 3 H2SO4. Electrochemical characterisation of the system as function of electrode composition was also accomplished taking impedance measurements at a constant potential value of 1.0 V/ RHE located in the double layer region (0.4 /1.4 V/ RHE). Before recording the impedance spectra, the electrodes were submitted to a continuous cycling of the potential with the objective of hydrating the oxide layer and reaching a stationary condition of the surface. Impedance spectra were recorded using an a.c. perturbation signal of 5 mV (p/p), covering the 10 mHz /100 KHz frequency range. These measurements were carried out using an AUTOLAB PGSTAT20(FRA) instrumentation. All other electrochemical experiments were carried out using PAR instrumentation (Mod. 273-A) that was connected to a computer. The anodic voltammetric charge, qa, was obtained through integration of the anodic branch of the CV profiles using the M-270 software from PAR.
3. Results and discussion 3.1. X-Ray analysis Low angle XRD spectra were recorded for oxide layers deposited on Ti-supports. Fig. 1 shows some Xray spectra as representative of all compositions investigated. The main crystallographic parameters of Fig. 1 are gathered in Table 1 for the electrode composition: [Ru(0.3)Ti(0.6)Ce(0.05)]O2[Nb2O5](0.05). All spectra showed well defined Ti-peaks due to penetration of the incident beam through the oxide layer, thus reaching the metallic support. The principal TiO2 and RuO2 peaks are present in the diffractogram. For all compositions analyzed, TiO2 and RuO2 peaks did not show significant displacement and were broader and less intensive when compared to the standard peaks of pure oxides (JCPDS ASTM) [13,14], thus indicating the formation of small crystallites and poor crystallinity. This behaviour suggest that neither solid solution nor distortions of the crystal lattice occurred, which would be observed if impurities were present in the crystal structure [15] e.g., chloride proceeding from incomplete thermal decomposition,
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area. This was corroborated by the qa vs. composition behaviour (see Section 3.4). The XRD spectra of layers containing CeO2 and Nb2O5 failed to present the peaks due to the presence of these oxides. This result can be attributed to low sintering and low crystallization of the coatings and probably furthermore influenced by the low amount of these oxides in the coatings. 3.2. Open-circuit potential (Eoc) Fig. 2 shows the dependence of Eoc on nominal composition of freshly prepared electrodes. The nature of oxides as well as redox surface transitions influence the dependence. According to Pourbaix, possible redox transitions and the equilibrium potentials for the pure oxides are [6]: 2RuO2 2H 2e l Ru2 O3 H2 O E 0:95 V=RHE 2CeO2 2H 2e l Ce2 O3 H2 O Fig. 1. X-ray spectra of Ti-supported oxide films. Nominal composition: (a) Ru(0.3)Ti(0.7)O2; (b) [Ru(0.3) Ti(0.6) Ce(0.05)]O2[Nb2O5](0.05); (c) [Ru(0.3)Ti(0.6)Ce(0.1)]O2. (m) TiO2; (j) RuO2; (") Ti8.
contamination that is dependent on calcination temperature applied [16,17]. Peaks of rutile TiO2 and rutile RuO2 are very close and, in mixtures, these oxides show peaks that are the sum of the individual peaks. Consequently, peaks can be less defined and broader, for example, peaks at 2u / 27.68 and 54.08. This is consistent with finely subdivided phase mixtures rather than with well crystallized solid solutions. CeO2 and Nb2O5 addition results in a significant decrease of the TiO2 and RuO2 peak intensities, including peak disappearance at 2u /68.88. So, despite 90% of the oxide coating being composed of TiO2 and RuO2, the introduction of only 10% of a third and fourth oxide component leads to a significant decrease in intensity and makes the defining process of these peaks more difficult. The broadening of the peaks suggests decrease in crystallite size what leads to an increase of the surface
E 1:56 V=RHE 2TiO2 2H 2e l Ti2 O3 H2 O E 0:556 V=RHE Nb2 O5 2H 2e l 2NbO2 H2 O E 0:29 V=RHE
(1) (2) (3) (4)
The substitution of CeO2 by Nb2O5 did not produce significant variations in Eoc. Independent of composition, Eoc-values around 0.93 V (RHE) were observed for all electrodes. Therefore, Eoc is mainly governed by the RuIII/RuIV redox couple. Assuming that Eoc-values reflect changes in the ratio [RuIII]/[RuIV] and that this redox couple controls the surface electrochemistry, the value of this ratio can be assessed from the Nernst equation and was found to be: III aIV Ru 0:5aRu
showing that the dominant surface specie of the freshly prepared electrodes is RuIII.
Table 1 Main peaks in the X-ray spectra of oxide with nominal composition [Ru(0.3)Ti(0.6)Ce(0.05)]O2[Nb2O5](0.05) 2u Experimental
27.6 34.9 38.3 40.0 52.8 54.0 62.9
d /nm Experimental
3.238 2.569 2.351 2.252 1.731 1.696 1.478
d /nm Theoretical TiO2
RuO2
Ti8
3.247 / / / / 1.6874 /
3.182 2.557 / / / 1.689 /
/ 2.555 2.341 2.243 1.7262 / 1.4753
Fig. 2. Dependence of the open-circuit potential on nominal Nb2O5 content for two sets of freshly prepared electrodes; 0.5 mol dm 3 H2SO4; potential read after 5 min.
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3.3. Voltammetric curves All voltammetric curves were recorded between /0.4 and /1.4 V/RHE at 20 mV s 1 using 0.5 mol dm 3 H2SO4 as supporting electrolyte. Fig. 3 compares the CV for some representative compositions. The introduction of CeO2 and Nb2O5 in the binary mixture Ru0.3Ti0.7O2 caused the voltammetric curves to differ in some aspects: (1) the apparent current density is almost three times higher for quaternary oxides when compared to the binary system; (2) OER starts at less positive potentials on [Ru0.3Ti0.6Ce0.1]O2; (3) as the CeO2 content of the mixtures increases (compare curves 3A and B with 3C and D) the peaks, especially the cathodic, become better defined and the apparent current density also increases. The changes pointed in the previous paragraph can be attributed to the changes in the surface area. The higher the number of different components present in the mixture, the more difficult will be the growth of individual oxide phases [15]. This process results in oxide layers showing poor syntherization and a high dispersion of the active component (RuO2) explaining the higher effective electrode area.. The presence of CeO2 in the RuO2 /TiO2 mixture catalyses the OER, although CeO2 itself is an insulator and does not present electrocatalytic properties for OER as was shown by De Faria [18]. Due to its high redox potential [19], CeO2 favors the surface RuIII to RuIV transition (see Eq. (5)), contributing to the increase of current and the anticipation of the OER (see Fig. 3(A
Fig. 3. Voltammetric curves recorded from 0.4 to 1.4 V (RHE) in 0.5 mol dm 3 H2SO4 at 20 mV s 1: (A) [Ru(0.3)Ti(0.6)Ce(0.1)]O2; (B) [Ru(0.3)Ti(0.6)Ce(0.05)]O2[Nb2O5](0.05); (C) [Ru(0.3)Ti(0.6)]O2[Nb2O5](0.1); (D) Ru0.3Ti0.7O2.
and B)). An opposite result was observed for the Ru/ Ti/Nb oxide composition, which can be attributed to the low NbIV/NbV redox potential. Therefore, the electrochemical activity for the OER of the Ru/Ti/ Nb oxide mixture is lower compared to the Ru/Ti/Ce oxide mixture.
3.4. Voltammetric charge The anodic voltammetric charge, qa, obtained by integration of the anodic branch of the CV, can be considered proportional to the active surface area of oxide electrodes and can be used to monitor ‘in situ’ the state of the oxide surface [15]. Figs. 4 and 5 show the variation of qa with nominal composition derived from the second and twentieth cycle, respectively. All compositions, whether ternaries or quaternaries, show a higher charge than the basic composition (Ru0.3Ti0.7O2). This can be attributed to difficult crystallization, because the more complex the oxide mixture is, the more difficult its crystallization becomes. Fig. 4 shows that the anodic charge increases with CeO2 content. Since the anodic charge is proportional to the number of active surface sites and RuO2 is the only oxide component possessing electrochemical activity in the potential range investigated, the increase in qa must be attributed to an increase of active RuO2 site density. Superior qa-value of the composition containing CeO2 when compared to Nb2O5, cannot be attributed to a superior electrochemical activity of CeO2 since both CeO2 [18] and Nb2O5 [9,10] are electrochemically inactive in the potential region investigated. However, besides the change in roughness of the coating, the highest qa-value observed for the 10 mol.% CeO2 electrode can be in principle due to the possible solid state redox reaction shown below:
Fig. 4. Dependence of the voltammetric charge (qa) on Nb2O5 content. Charge measured from the second voltammetric curve for two sets of electrodes. (x )Ru0.3Ti0.7O2 electrode. Inset: dependence of the anodic/ cathodic charge ratio (qa/qc) on nominal Nb2O5 content.
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Fig. 5. Dependence of the voltammetric charge (qa) on Nb2O5 content. Charge measured from the twentieth voltammetric curve for two sets of electrodes; (x )Ru0.3Ti0.7O2 electrode. Inset: dependence of the anodic/ cathodic charge ratio (qa/qc) on nominal Nb2O5 content.
RuIII CeIV 1 * 0 RuIV CeIII (DE 0:61 V)
(5)
Thermodynamically the above reaction being viable, the presence of CeIV favours the concentration of the RuIV oxidation state explaining the OER anticipation when CeO2 is present (see Fig. 3) and the higher qa-value when compared to the behaviour of the 10 mol.% Nb2O5 containing electrode. The dependence of qa on composition changes with the number of consecutive potential cycles. Comparing Figs. 4 and 5, it is noticed that the charge of the electrode containing 10 mol.% CeO2 decreases after twenty cycles (between /0.4 and /1.4 V/RHE). As a result the maximum of qa vs. composition graph is displaced to the electrode having a 3 mol.% Nb2O5/7 mol.% CeO2 nominal composition. This reduction indicates that despite the high active surface area of the electrode containing 10 mol.% CeO2, it is unstable under the conditions investigated. De Faria et al [5] attributed this fact to CeO2 cathodic dissolution in acidic media. It is interesting to note that the introduction into the oxide mixture of only 1 mol.% Nb2O5 causes excellent electrochemical stability, as it can be seen from the constant values of the charge even after continuous cycling. This result was confirmed by a stability investigation conducted under conditions of accelerated corrosion [20]. The ratios between the anodic and the cathodic charge are constant with values close to one, independent of the number of consecutive potential cycles. This behaviour was observed in all compositions and reflects the reversibility of the surface redox transitions.
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of scan rate, n (5 5/n 5/100 mV s 1) and the current measured, iE, at constant potential (E/1.0 V/RHE). RuO2-based electrodes normally present broad faradaic peaks, a fact that difficults the choice of a purely capacitive potential region. The potential used to measure the current is located in a region of the voltammetric curve where redox transitions are minimized. However, a residual contribution of the solid state surface redox transitions influencing the apparent roughness factor (r ) cannot be totally excluded. The double-layer capacitance, obtained from the slope of the iE vs. n graphics, was used to calculate the roughness factor (r ) of oxide films. This parameter was determined by dividing the Cd-values by 80 mF cm 2, which is the theoretical value for a smooth RuO2 rutile surfaces [21]. Although the validity of this method has been discussed [22], we claim a sufficient validity for the purpose of comparison. This same methodology, using the same reference value, was published in an earlier work researching a system that was also based on RuO2 anodes [23]. Fig. 6 shows the dependence of r (left axis) and the Cd-values (right axis) on the nominal electrode composition. The roughness factor contributes to the understanding of the behaviour of the voltammetric charges as function of composition. The high voltammetric charge shown by the coatings having the highest CeO2 content is due to both the superior electrochemical activity of these coatings combined with their higher roughness, which is almost twice the roughness of the (Ru/Ti/ Nb) oxide. The maximum of r is observed for the [Ru0.3Ti0.6Ce0.07]O2[Nb2O5](0.03) composition and is concordant with the maximum of qa after 20 cycles. Comparison of the r- and qa-values of Ru0.3Ti0.7O2 and [Ru0.3Ti0.6]O2[Nb2O5](0.1) show that the latter has a higher qa-value than the former, i.e. a higher electrochemical active surface area due to a larger amount of defects (due to poor mixing on introduction of Nb2O5).
3.5. The roughness factor (r ) For all compositions, voltammetric curves were recorded between /0.9 and /1.3 V/RHE as function
Fig. 6. Dependence of the double-layer capacitance (Cd) and of the apparent roughness factor (r ) on nominal Nb2O5 content in 0.5 mol dm 3 H2SO4. (k,m) Ru0.3Ti0.7O2 electrode.
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However, the r factor shows that this composition has a somewhat lower roughness compared to Ru0.3Ti0.7O2, suggesting Nb2O5 has the capability to result in a more compact oxide layer.
In order to confirm the double-layer capacitance (Cd) results and, consequently, the roughness factor and all conclusions derived from the study obtained from CV, we investigated the capacitive domain taking impedance measurements. A representative Nyquist and Bode plot of this system are shown in Fig. 7. In the absence of faradaic reactions, smooth surfaces show ideal capacitive behaviour given by Z (v )/ 1/ (jvCd), where the double layer capacitance Cd is independent of frequency. The ideal behaviour is represented by a vertical line in the Nyquist plot. However, this is only true for the liquid mercury electrode and the impedance of most solid electrodes deviates from the purely capacitive behaviour [24]. In case of oxide electrodes, the impedance behaviour is rather complicated, and the main deviation from ideality originates from the occurrence of solid state surface redox transitions during double layer charging (pseudo capacitance phenomena) on a rugged surface having a high degree of porosity [25 /28]. As shown in Fig. 7(A), the impedance behaviour in the whole frequency range is characterised by an almost linear vertical segment in the Nyquist plot, which is normally referred to as a serial resistance /capacitance combination. The capacitive behaviour of the electrode system is better observed in Fig. 7(B) (Bode Plot),
whereas in the low frequency domain, a phase angle of /908 was found. Independent of electrode composition, the Nyquist plots obtained are characterised by two distinct regions. (i) In the high frequency domain (see inset in Fig. 7(A)), the phase angle, f , is negative due to a small inductive behaviour (/106 H). As discussed recently by Da Silva et al. [28] the main cause of this inductance in thermally prepared metallic oxide films is the disordered movement of the charge carriers through the complex micro-structure (pores, cracks, grain boundaries, etc.). At high frequencies, where f /08, the deviation from the predicted linearity is characterised by a not well developed, semicircle which does not change significantly with electrode composition. The origin of the distorted semicircle can be attributed to the solid state surface redox transitions, which is a faradaic process controlled by proton diffusion through the complex microstructure of the coating (e.g. pores, cracks), associated to the influence of pore dimension and penetration depth of a.c. signal into the pores distributed along the rugged surface [24]. Therefore, only at lower frequencies, the a.c. perturbation reaches the bottom of the pores (the more difficult-to-access region of the coating). (ii) In the low frequency domain the impedance spectra is characterised by a straight line presenting a phase angle of /908. Such result suggests a pure capacitive behaviour of the oxide/solution interface [25]. Similar results have been observed for different electrochemical systems which involve oxide electrodes presenting good conductivity [25 /27]. Fig. 8 shows the double layer capacitance, Cd, which were calculated from the next expression Z (v )/ 1/ (jvCd), using the low frequency data at 89 mHz, as a function of electrode composition. Fig. 8 gathers the Cd-values which were obtained from the impedance measurements showing that the values are close to the one obtained from the CV investigation (see Fig. 6*/right axis). Fig. 8 also shows the Cd-values before and after 20 voltammetric cycles covering the 0.4 /1.4 V/RHE interval in order to monitor changes on the surface area. In agreement with the CV-data, the
Fig. 7. Representative Nyquist (A) and Bode (B) plot recorded at 1.0 V/RHE. Electrode: [Ru(0.3)Ti(0.6)Ce(0.1)]O2 0.5 mol dm 3 H2SO4.
Fig. 8. Capacitance, Cd, as function of electrode composition, calculated at 89 mHz from impedance data.
3.6. Double layer capacitance by EIS
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only composition that showed a significant decrease in surface area after 20 potential cycles was the [Ru0.3Ti0.6Ce0.1]O2 composition. Fig. 8 reveals that the introduction of Ce results in an increase of the Cd-values. Comparing CV and electrochemical impedance spectroscopy (EIS) investigations, a good agreement is observed between the Cd vs. oxide composition profiles. Cd reaches a maximum between 9 and 5 mol.% CeO2 content. Such results support the idea that both CV and EIS techniques can be used to characterise in situ the oxide/solution interface. The applicability of voltammetry technique in the characterisation of conductive metallic oxides was discussed earlier by different authors [29 /33].
4. Conclusions All the compositions showed characteristic peaks of the components present at high concentration, i.e. RuO2 and TiO2. The XRD spectra indicated that the investigated oxides are composed of finely divided phases with poor syntherization. CeO2 and Nb2O5 peaks were not observed due to the low concentration of these components and the low crystallinity of the coatings attributed to poor interaction among the rutile structure (RuO2 and TiO2), the cubic structure (CeO2), and the amorphous structure (Nb2O5). For mixed oxide electrodes, Eoc showed that the RuIII/RuIV redox couple controls the surface electrochemistry. This is consistent with the morphology of the voltammetric curves, which are typical of the Ti/RuO2 system. The EIS investigation corroborates the CV study on the roughness factor and double layer capacitance investigations. Both showed that the addition of CeO2 and Nb2O5 clearly increased the electrochemical active surface area of the electrode, especially with high CeO2 contents. On the other hand, the CV and a.c. impedance studies indicate the CeO2 presence leads to an electrode instability when submitted to continuous potential cycling. This instability can be minimized introducing Nb2O5 into the mixture, thus qa reached a maximum for the composition [Ru0.3Ti0.6Ce0.07]O2[Nb2O5](0.03).
Acknowledgements The authors would like to thank Professor J.F.C. Boodts for helpful discussions; M.H.P. Santana is grateful to CAPES for a M.Sc. fellowship received.
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The authors acknowledge the FFCLRP/USP for the XRD analyses.
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Investigation of surface properties of Ru-based oxide electrodes containing Ti, Ce and Nb Ma´rio H.P. Santana a, Leonardo M. Da Silva a, Luiz A. De Faria b,* a
b
Departamento de Quı´mica, FFCLRP/USP, Av. Bandeirantes, 3900, CEP 14040-901 Ribeirao Preto, SP, Brazil ´ vila 2160, Santa Monica, CEP 38408-100 Uberlandia, MG, Brazil Universidad Federal de Uberlandia, Instituto de Quı´mica, Av. Joa˜o Naves de A Received 7 January 2003; received in revised form 5 March 2003; accepted 7 March 2003
Abstract Mixed oxide electrodes with nominal composition Ti/[Ru(0.3)Ti(0.6)Ce(0.1x )]O2[Nb2O5](x ), (0 5/x 5/0.1) were prepared through a process of thermal decomposition (450 8C) of chloride precursor mixture solutions. Composition effects on the surface properties were investigated and the oxide electrodes were characterized in acid solution: ex situ by XRD and in situ by open circuit potential measurements (Eoc), cyclic voltammetry (CV), the double-layer capacitance (CV and electrochemical impedance spectroscopy techniques) and roughness factor (CV). Except the Eoc, all parameters varied with the electrode composition. It was shown that the presence of CeO2 increases the electrochemical active area, but it makes the coating less stable when submitted to continuous potential cycles because of cathodic dissolution of this oxide. By adding Nb2O5 this instability can be lowered. The maximum activity (voltammetric charge) was observed with the [Ru(0.3)Ti(0.6)Ce(0.07)]O2[Nb2O5](0.03) composition. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Roughness factor; Cerium oxide; Niobium oxide; Oxide electrodes; Electrode stability
1. Introduction Oxide electrodes have acquired a significant importance in electrochemical technology due to some possible applications such as the chlor-alkali process, organic electrosynthesis, water electrolysis, fuel cells, etc. Industrial anodes have been traditionally based on a mixture of RuO2, the active component, and TiO2, the stabilizing agent [1]. In many investigations a third oxide is added either to enhance the selectivity and/or to increase the anodic stability [2,3]. De Faria has systematically investigated the surface and electrochemical properties of the RuO2/TiO2/CeO2 mixture [4,5]. Because of its high redox potential (CeIII/CeIV), CeO2 addition increased the electrocatalytic activity in the oxygen evolution reaction (OER) [4], but it showed cathodic dissolution in acid solution [5].
* Corresponding author. E-mail address: [email protected] (L.A. De Faria).
The oxide of nominal composition [Ru0.3Ti0.6Ce0.1]O2 has shown the best relation between electrocatalytic activity and stability. Following the idea of further improvement of this aspect, we decided to introduce a fourth component into this basic composition. Niobium pentoxide (Nb2O5) was chosen because it is recognized for having excellent stabilization capacity (Nb is a valve metals member). As a result, a system is expected to present an excellent resistance to anodic/cathodic corrosion [6]. Niobium oxide has also been introduced in Ru-based electrodes, but only to research optical properties [7], or in semiconductors based on TiO2 [8]. Recently, Terezo and Pereira studied the RuO2 /Nb2O5 [9] and IrO2 / Nb2O5 [10] systems, investigating the effect of the preparation parameters on the morphology and service life. The authors, however, did not investigate the electrocatalytic properties related to OER of these electrode materials. The objective of this study is to investigate the influence of the systematic substitution of CeO2 by Nb2O5 (in 1 mol.% steps), maintaining the RuO2 and TiO2 contents constant at 30 and 60 mol.%, respectively,
0013-4686/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0013-4686(03)00262-7
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on the surface and electrocatalytic properties of electrode material, having the nominal composition: [Ru(0.3)Ti(0.6)Ce(0.1x )]O2[Nb2O5](x ), with x in the range 0 5/x 5/0.1. In this paper, we report the morphological and electrochemical characterization in acid medium, while the electrocatalytic properties will be discussed in a forthcoming communication.
2. Experimental 2.1. Electrodes Mixed oxide layers deposited on a Ti support (10 / 10 /0.15 mm) were prepared by thermal decomposition (450 8C) of RuCl3 ×/n H2O (Aldrich), TiCl4 (Ventron), CeCl3 ×/7H2O (Merck), and NbCl5 (Fluka). The precursors were dissolved in concentrated aqueous HCl (Merck 1:1 v/v) in order to furnish a 0.18 mol dm 3 salt concentration. Ti-supports were degreased with isopropanol and submitted to chemical attack for 10 min in boiling 10% oxalic acid. The precursor mixture solutions, having the desired mole ratio, were spread onto both sides of the support by means of brushing. The solvent was evaporated at 90 8C and the residue was fired at 450 8C for 10 min, under an air stream, in a pre-heated oven. This procedure was repeated until the nominal thickness of 2 mm was achieved (corresponding to 0.9 /1.2 mg cm 2, depending on the composition). After obtaining the desired oxide loading, electrodes were fired at 450 8C for 1 h. Duplicated samples were prepared at 1 mol.% intervals covering 0 /10 mol.% Nb2O5 interval. To make a comparison possible, two samples of nominal composition Ru0.3Ti0.7O2 were prepared. All electrodes were assembled in fine glass tube. The extremity in contact with solution was sealed with Teflon parts and silicone glue as described previously [11]. 2.2. Solutions 0.5 mol dm 3 H2SO4 aqueous solutions were volumetrically prepared from concentrated H2SO4 (Merck) and Milli-Q quality (R /18.2 MV) water. Before and during each run, they were deaerated and stirred through ultrapure nitrogen (purity: 99.995%) bubbling. The three compartment cell used has been described previously [12]. 2.3. Reference electrode All potentials were read versus a hydrogen electrode (RHE) in the same solution. The RHE was renewed before each new experiment. The RHE was connected to
the solution via a Luggin capillary in order to minimize uncompensated ohmic drop. 2.4. Techniques and instruments Ti-supported oxide layers were characterized through X-ray analysis using a D5005 Model Siemens with Cu Ka /1.54056 nm. The surface response of the electrodes was examined by cyclic voltammetric curves, CV, recorded at 20 mV s 1 between /0.4 and /1.4 V/ RHE in 0.5 mol dm 3 H2SO4. Electrochemical characterisation of the system as function of electrode composition was also accomplished taking impedance measurements at a constant potential value of 1.0 V/ RHE located in the double layer region (0.4 /1.4 V/ RHE). Before recording the impedance spectra, the electrodes were submitted to a continuous cycling of the potential with the objective of hydrating the oxide layer and reaching a stationary condition of the surface. Impedance spectra were recorded using an a.c. perturbation signal of 5 mV (p/p), covering the 10 mHz /100 KHz frequency range. These measurements were carried out using an AUTOLAB PGSTAT20(FRA) instrumentation. All other electrochemical experiments were carried out using PAR instrumentation (Mod. 273-A) that was connected to a computer. The anodic voltammetric charge, qa, was obtained through integration of the anodic branch of the CV profiles using the M-270 software from PAR.
3. Results and discussion 3.1. X-Ray analysis Low angle XRD spectra were recorded for oxide layers deposited on Ti-supports. Fig. 1 shows some Xray spectra as representative of all compositions investigated. The main crystallographic parameters of Fig. 1 are gathered in Table 1 for the electrode composition: [Ru(0.3)Ti(0.6)Ce(0.05)]O2[Nb2O5](0.05). All spectra showed well defined Ti-peaks due to penetration of the incident beam through the oxide layer, thus reaching the metallic support. The principal TiO2 and RuO2 peaks are present in the diffractogram. For all compositions analyzed, TiO2 and RuO2 peaks did not show significant displacement and were broader and less intensive when compared to the standard peaks of pure oxides (JCPDS ASTM) [13,14], thus indicating the formation of small crystallites and poor crystallinity. This behaviour suggest that neither solid solution nor distortions of the crystal lattice occurred, which would be observed if impurities were present in the crystal structure [15] e.g., chloride proceeding from incomplete thermal decomposition,
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area. This was corroborated by the qa vs. composition behaviour (see Section 3.4). The XRD spectra of layers containing CeO2 and Nb2O5 failed to present the peaks due to the presence of these oxides. This result can be attributed to low sintering and low crystallization of the coatings and probably furthermore influenced by the low amount of these oxides in the coatings. 3.2. Open-circuit potential (Eoc) Fig. 2 shows the dependence of Eoc on nominal composition of freshly prepared electrodes. The nature of oxides as well as redox surface transitions influence the dependence. According to Pourbaix, possible redox transitions and the equilibrium potentials for the pure oxides are [6]: 2RuO2 2H 2e l Ru2 O3 H2 O E 0:95 V=RHE 2CeO2 2H 2e l Ce2 O3 H2 O Fig. 1. X-ray spectra of Ti-supported oxide films. Nominal composition: (a) Ru(0.3)Ti(0.7)O2; (b) [Ru(0.3) Ti(0.6) Ce(0.05)]O2[Nb2O5](0.05); (c) [Ru(0.3)Ti(0.6)Ce(0.1)]O2. (m) TiO2; (j) RuO2; (") Ti8.
contamination that is dependent on calcination temperature applied [16,17]. Peaks of rutile TiO2 and rutile RuO2 are very close and, in mixtures, these oxides show peaks that are the sum of the individual peaks. Consequently, peaks can be less defined and broader, for example, peaks at 2u / 27.68 and 54.08. This is consistent with finely subdivided phase mixtures rather than with well crystallized solid solutions. CeO2 and Nb2O5 addition results in a significant decrease of the TiO2 and RuO2 peak intensities, including peak disappearance at 2u /68.88. So, despite 90% of the oxide coating being composed of TiO2 and RuO2, the introduction of only 10% of a third and fourth oxide component leads to a significant decrease in intensity and makes the defining process of these peaks more difficult. The broadening of the peaks suggests decrease in crystallite size what leads to an increase of the surface
E 1:56 V=RHE 2TiO2 2H 2e l Ti2 O3 H2 O E 0:556 V=RHE Nb2 O5 2H 2e l 2NbO2 H2 O E 0:29 V=RHE
(1) (2) (3) (4)
The substitution of CeO2 by Nb2O5 did not produce significant variations in Eoc. Independent of composition, Eoc-values around 0.93 V (RHE) were observed for all electrodes. Therefore, Eoc is mainly governed by the RuIII/RuIV redox couple. Assuming that Eoc-values reflect changes in the ratio [RuIII]/[RuIV] and that this redox couple controls the surface electrochemistry, the value of this ratio can be assessed from the Nernst equation and was found to be: III aIV Ru 0:5aRu
showing that the dominant surface specie of the freshly prepared electrodes is RuIII.
Table 1 Main peaks in the X-ray spectra of oxide with nominal composition [Ru(0.3)Ti(0.6)Ce(0.05)]O2[Nb2O5](0.05) 2u Experimental
27.6 34.9 38.3 40.0 52.8 54.0 62.9
d /nm Experimental
3.238 2.569 2.351 2.252 1.731 1.696 1.478
d /nm Theoretical TiO2
RuO2
Ti8
3.247 / / / / 1.6874 /
3.182 2.557 / / / 1.689 /
/ 2.555 2.341 2.243 1.7262 / 1.4753
Fig. 2. Dependence of the open-circuit potential on nominal Nb2O5 content for two sets of freshly prepared electrodes; 0.5 mol dm 3 H2SO4; potential read after 5 min.
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3.3. Voltammetric curves All voltammetric curves were recorded between /0.4 and /1.4 V/RHE at 20 mV s 1 using 0.5 mol dm 3 H2SO4 as supporting electrolyte. Fig. 3 compares the CV for some representative compositions. The introduction of CeO2 and Nb2O5 in the binary mixture Ru0.3Ti0.7O2 caused the voltammetric curves to differ in some aspects: (1) the apparent current density is almost three times higher for quaternary oxides when compared to the binary system; (2) OER starts at less positive potentials on [Ru0.3Ti0.6Ce0.1]O2; (3) as the CeO2 content of the mixtures increases (compare curves 3A and B with 3C and D) the peaks, especially the cathodic, become better defined and the apparent current density also increases. The changes pointed in the previous paragraph can be attributed to the changes in the surface area. The higher the number of different components present in the mixture, the more difficult will be the growth of individual oxide phases [15]. This process results in oxide layers showing poor syntherization and a high dispersion of the active component (RuO2) explaining the higher effective electrode area.. The presence of CeO2 in the RuO2 /TiO2 mixture catalyses the OER, although CeO2 itself is an insulator and does not present electrocatalytic properties for OER as was shown by De Faria [18]. Due to its high redox potential [19], CeO2 favors the surface RuIII to RuIV transition (see Eq. (5)), contributing to the increase of current and the anticipation of the OER (see Fig. 3(A
Fig. 3. Voltammetric curves recorded from 0.4 to 1.4 V (RHE) in 0.5 mol dm 3 H2SO4 at 20 mV s 1: (A) [Ru(0.3)Ti(0.6)Ce(0.1)]O2; (B) [Ru(0.3)Ti(0.6)Ce(0.05)]O2[Nb2O5](0.05); (C) [Ru(0.3)Ti(0.6)]O2[Nb2O5](0.1); (D) Ru0.3Ti0.7O2.
and B)). An opposite result was observed for the Ru/ Ti/Nb oxide composition, which can be attributed to the low NbIV/NbV redox potential. Therefore, the electrochemical activity for the OER of the Ru/Ti/ Nb oxide mixture is lower compared to the Ru/Ti/Ce oxide mixture.
3.4. Voltammetric charge The anodic voltammetric charge, qa, obtained by integration of the anodic branch of the CV, can be considered proportional to the active surface area of oxide electrodes and can be used to monitor ‘in situ’ the state of the oxide surface [15]. Figs. 4 and 5 show the variation of qa with nominal composition derived from the second and twentieth cycle, respectively. All compositions, whether ternaries or quaternaries, show a higher charge than the basic composition (Ru0.3Ti0.7O2). This can be attributed to difficult crystallization, because the more complex the oxide mixture is, the more difficult its crystallization becomes. Fig. 4 shows that the anodic charge increases with CeO2 content. Since the anodic charge is proportional to the number of active surface sites and RuO2 is the only oxide component possessing electrochemical activity in the potential range investigated, the increase in qa must be attributed to an increase of active RuO2 site density. Superior qa-value of the composition containing CeO2 when compared to Nb2O5, cannot be attributed to a superior electrochemical activity of CeO2 since both CeO2 [18] and Nb2O5 [9,10] are electrochemically inactive in the potential region investigated. However, besides the change in roughness of the coating, the highest qa-value observed for the 10 mol.% CeO2 electrode can be in principle due to the possible solid state redox reaction shown below:
Fig. 4. Dependence of the voltammetric charge (qa) on Nb2O5 content. Charge measured from the second voltammetric curve for two sets of electrodes. (x )Ru0.3Ti0.7O2 electrode. Inset: dependence of the anodic/ cathodic charge ratio (qa/qc) on nominal Nb2O5 content.
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Fig. 5. Dependence of the voltammetric charge (qa) on Nb2O5 content. Charge measured from the twentieth voltammetric curve for two sets of electrodes; (x )Ru0.3Ti0.7O2 electrode. Inset: dependence of the anodic/ cathodic charge ratio (qa/qc) on nominal Nb2O5 content.
RuIII CeIV 1 * 0 RuIV CeIII (DE 0:61 V)
(5)
Thermodynamically the above reaction being viable, the presence of CeIV favours the concentration of the RuIV oxidation state explaining the OER anticipation when CeO2 is present (see Fig. 3) and the higher qa-value when compared to the behaviour of the 10 mol.% Nb2O5 containing electrode. The dependence of qa on composition changes with the number of consecutive potential cycles. Comparing Figs. 4 and 5, it is noticed that the charge of the electrode containing 10 mol.% CeO2 decreases after twenty cycles (between /0.4 and /1.4 V/RHE). As a result the maximum of qa vs. composition graph is displaced to the electrode having a 3 mol.% Nb2O5/7 mol.% CeO2 nominal composition. This reduction indicates that despite the high active surface area of the electrode containing 10 mol.% CeO2, it is unstable under the conditions investigated. De Faria et al [5] attributed this fact to CeO2 cathodic dissolution in acidic media. It is interesting to note that the introduction into the oxide mixture of only 1 mol.% Nb2O5 causes excellent electrochemical stability, as it can be seen from the constant values of the charge even after continuous cycling. This result was confirmed by a stability investigation conducted under conditions of accelerated corrosion [20]. The ratios between the anodic and the cathodic charge are constant with values close to one, independent of the number of consecutive potential cycles. This behaviour was observed in all compositions and reflects the reversibility of the surface redox transitions.
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of scan rate, n (5 5/n 5/100 mV s 1) and the current measured, iE, at constant potential (E/1.0 V/RHE). RuO2-based electrodes normally present broad faradaic peaks, a fact that difficults the choice of a purely capacitive potential region. The potential used to measure the current is located in a region of the voltammetric curve where redox transitions are minimized. However, a residual contribution of the solid state surface redox transitions influencing the apparent roughness factor (r ) cannot be totally excluded. The double-layer capacitance, obtained from the slope of the iE vs. n graphics, was used to calculate the roughness factor (r ) of oxide films. This parameter was determined by dividing the Cd-values by 80 mF cm 2, which is the theoretical value for a smooth RuO2 rutile surfaces [21]. Although the validity of this method has been discussed [22], we claim a sufficient validity for the purpose of comparison. This same methodology, using the same reference value, was published in an earlier work researching a system that was also based on RuO2 anodes [23]. Fig. 6 shows the dependence of r (left axis) and the Cd-values (right axis) on the nominal electrode composition. The roughness factor contributes to the understanding of the behaviour of the voltammetric charges as function of composition. The high voltammetric charge shown by the coatings having the highest CeO2 content is due to both the superior electrochemical activity of these coatings combined with their higher roughness, which is almost twice the roughness of the (Ru/Ti/ Nb) oxide. The maximum of r is observed for the [Ru0.3Ti0.6Ce0.07]O2[Nb2O5](0.03) composition and is concordant with the maximum of qa after 20 cycles. Comparison of the r- and qa-values of Ru0.3Ti0.7O2 and [Ru0.3Ti0.6]O2[Nb2O5](0.1) show that the latter has a higher qa-value than the former, i.e. a higher electrochemical active surface area due to a larger amount of defects (due to poor mixing on introduction of Nb2O5).
3.5. The roughness factor (r ) For all compositions, voltammetric curves were recorded between /0.9 and /1.3 V/RHE as function
Fig. 6. Dependence of the double-layer capacitance (Cd) and of the apparent roughness factor (r ) on nominal Nb2O5 content in 0.5 mol dm 3 H2SO4. (k,m) Ru0.3Ti0.7O2 electrode.
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However, the r factor shows that this composition has a somewhat lower roughness compared to Ru0.3Ti0.7O2, suggesting Nb2O5 has the capability to result in a more compact oxide layer.
In order to confirm the double-layer capacitance (Cd) results and, consequently, the roughness factor and all conclusions derived from the study obtained from CV, we investigated the capacitive domain taking impedance measurements. A representative Nyquist and Bode plot of this system are shown in Fig. 7. In the absence of faradaic reactions, smooth surfaces show ideal capacitive behaviour given by Z (v )/ 1/ (jvCd), where the double layer capacitance Cd is independent of frequency. The ideal behaviour is represented by a vertical line in the Nyquist plot. However, this is only true for the liquid mercury electrode and the impedance of most solid electrodes deviates from the purely capacitive behaviour [24]. In case of oxide electrodes, the impedance behaviour is rather complicated, and the main deviation from ideality originates from the occurrence of solid state surface redox transitions during double layer charging (pseudo capacitance phenomena) on a rugged surface having a high degree of porosity [25 /28]. As shown in Fig. 7(A), the impedance behaviour in the whole frequency range is characterised by an almost linear vertical segment in the Nyquist plot, which is normally referred to as a serial resistance /capacitance combination. The capacitive behaviour of the electrode system is better observed in Fig. 7(B) (Bode Plot),
whereas in the low frequency domain, a phase angle of /908 was found. Independent of electrode composition, the Nyquist plots obtained are characterised by two distinct regions. (i) In the high frequency domain (see inset in Fig. 7(A)), the phase angle, f , is negative due to a small inductive behaviour (/106 H). As discussed recently by Da Silva et al. [28] the main cause of this inductance in thermally prepared metallic oxide films is the disordered movement of the charge carriers through the complex micro-structure (pores, cracks, grain boundaries, etc.). At high frequencies, where f /08, the deviation from the predicted linearity is characterised by a not well developed, semicircle which does not change significantly with electrode composition. The origin of the distorted semicircle can be attributed to the solid state surface redox transitions, which is a faradaic process controlled by proton diffusion through the complex microstructure of the coating (e.g. pores, cracks), associated to the influence of pore dimension and penetration depth of a.c. signal into the pores distributed along the rugged surface [24]. Therefore, only at lower frequencies, the a.c. perturbation reaches the bottom of the pores (the more difficult-to-access region of the coating). (ii) In the low frequency domain the impedance spectra is characterised by a straight line presenting a phase angle of /908. Such result suggests a pure capacitive behaviour of the oxide/solution interface [25]. Similar results have been observed for different electrochemical systems which involve oxide electrodes presenting good conductivity [25 /27]. Fig. 8 shows the double layer capacitance, Cd, which were calculated from the next expression Z (v )/ 1/ (jvCd), using the low frequency data at 89 mHz, as a function of electrode composition. Fig. 8 gathers the Cd-values which were obtained from the impedance measurements showing that the values are close to the one obtained from the CV investigation (see Fig. 6*/right axis). Fig. 8 also shows the Cd-values before and after 20 voltammetric cycles covering the 0.4 /1.4 V/RHE interval in order to monitor changes on the surface area. In agreement with the CV-data, the
Fig. 7. Representative Nyquist (A) and Bode (B) plot recorded at 1.0 V/RHE. Electrode: [Ru(0.3)Ti(0.6)Ce(0.1)]O2 0.5 mol dm 3 H2SO4.
Fig. 8. Capacitance, Cd, as function of electrode composition, calculated at 89 mHz from impedance data.
3.6. Double layer capacitance by EIS
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only composition that showed a significant decrease in surface area after 20 potential cycles was the [Ru0.3Ti0.6Ce0.1]O2 composition. Fig. 8 reveals that the introduction of Ce results in an increase of the Cd-values. Comparing CV and electrochemical impedance spectroscopy (EIS) investigations, a good agreement is observed between the Cd vs. oxide composition profiles. Cd reaches a maximum between 9 and 5 mol.% CeO2 content. Such results support the idea that both CV and EIS techniques can be used to characterise in situ the oxide/solution interface. The applicability of voltammetry technique in the characterisation of conductive metallic oxides was discussed earlier by different authors [29 /33].
4. Conclusions All the compositions showed characteristic peaks of the components present at high concentration, i.e. RuO2 and TiO2. The XRD spectra indicated that the investigated oxides are composed of finely divided phases with poor syntherization. CeO2 and Nb2O5 peaks were not observed due to the low concentration of these components and the low crystallinity of the coatings attributed to poor interaction among the rutile structure (RuO2 and TiO2), the cubic structure (CeO2), and the amorphous structure (Nb2O5). For mixed oxide electrodes, Eoc showed that the RuIII/RuIV redox couple controls the surface electrochemistry. This is consistent with the morphology of the voltammetric curves, which are typical of the Ti/RuO2 system. The EIS investigation corroborates the CV study on the roughness factor and double layer capacitance investigations. Both showed that the addition of CeO2 and Nb2O5 clearly increased the electrochemical active surface area of the electrode, especially with high CeO2 contents. On the other hand, the CV and a.c. impedance studies indicate the CeO2 presence leads to an electrode instability when submitted to continuous potential cycling. This instability can be minimized introducing Nb2O5 into the mixture, thus qa reached a maximum for the composition [Ru0.3Ti0.6Ce0.07]O2[Nb2O5](0.03).
Acknowledgements The authors would like to thank Professor J.F.C. Boodts for helpful discussions; M.H.P. Santana is grateful to CAPES for a M.Sc. fellowship received.
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The authors acknowledge the FFCLRP/USP for the XRD analyses.
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