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Oxygen and chlorine evolution on RuO2 + TiO2 + CeO2 + Nb2O5 mixed oxide electrodes
Electrochimica Acta 51 (2006) 3578–3585
Oxygen and chlorine evolution on RuO2 + TiO2 + CeO2 + Nb2O5 mixed oxide electrodes M´ario H.P. Santana, Luiz A. De Faria ∗ ´ Instituto de Qu´ımica, UFU, Av. Jo˜ao Naves de Avila, 2160, Campus Sta Mˆonica, CEP 38400-902, Uberlˆandia/MG, Brazil Received 27 July 2005; received in revised form 28 September 2005; accepted 29 September 2005 Available online 23 November 2005
Abstract A systematic investigation was conducted on the mechanism and electrocatalytic properties of O2 and Cl2 evolution on mixed oxide electrodes of nominal composition: Ti/[Ru(0.3) Ti(0.6) Ce(0.1−x) ]O2 [Nb2 O5 ](x) (0 ≤ x ≤ 0.1). For the oxygen evolution, a 30 mV Tafel slope is obtained in the presence of CeO2 , while in its absence a 40 mV coefficient is observed. The intrinsic electrocatalytic activity is mainly due to electronic factors, as result of the synergism between Ru and Ce oxides. For chlorine evolution, the Tafel slope (30 mV) is independent on oxide composition. The best global electrocatalytic activity for ClER was observed in the absence of Nb2 O5 additive. Variation of the voltammetric charge throughout the experiments confirms high CeO2 content compositions are fragile, due mainly to the porosity caused by CeO2 presence. On the other hand, Nb2 O5 addition decreases considerably this instability. © 2005 Elsevier Ltd. All rights reserved. Keywords: Ru + Ti + Ce + Nb oxides; Oxygen evolution; Chlorine evolution; Ruthenium oxide; Electrocatalysis
1. Introduction DSA® -type electrodes have been well succeeded for almost 40 years due to their versatile electrocatalytic properties and stability [1]. These electrodes have shown significant activity for all common gas evolution reactions (O2 , Cl2 ). Up to now, the most widely industrial application of these electrodes is for Cl2 production [1]. In this field, the electrode material plays an important role in both Cl2 yield and purity, since the chlorine evolution reaction, ClER, is affected by the presence of O2 from the oxygen evolution reaction, OER [2,3]. To achieve high Cl2 current efficiency, it is necessary to work at low pH values of the electrolyte and with electrode materials that show, in principle, significant overpotentials for the OER [2]. Thus, electrode selectivity for the ClER remains a problem of technological interest, although a broad fundamental and applied research of ClER is available [1–5]. OER is the main anodic reaction in the majority of the cathodic processes and it is an important side reaction in processes such as production of strong oxidants and combus∗
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0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.09.050
tion of organics [1]. Development of fuel cells and the socalled Chemical Green Processes [6] has also stimulated OER investigations. The most active oxides for OER and ClER are RuO2 and IrO2 , or their mixtures, and a great number of the investigations are concerned with oxides of rutile structure [7–12]. The global electrode activity of a pure oxide is dependent on the nature of the oxide and the electrode preparation procedure. However, for a single oxide, these variables affect the activity to a limited extend. As alternative, the use of mixed oxides can lead to synergetic effects improving the electrocatalytic properties and the stability/selectivity of the electrode [5,9,10,13–15]. De Faria et al. [16] demonstrated partial substitution of TiO2 by CeO2 in a RuO2 -based electrode material improves the electrocatalytic activity for OER, but decreases the mechanical stability of the mixture. In a recent paper [17], we reported the surface properties of the RuO2 + TiO2 + CeO2 + Nb2 O5 system and showed the instability caused by CeO2 can be reduced introducing a small Nb2 O5 content. In this work, we investigate the electrocatalytic properties of [Ru0.3 Ti0.6 Ce(0.1−x) ]O2 [Nb2 O5 ](x) with (0 ≤ x ≤ 0.1) for OER and ClER and the stability of these oxide electrodes throughout the experiments.
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2. Experimental 2.1. Electrodes Mixed oxide layers deposited on a Ti support (10 mm × 10 mm × 0.15 mm; geometric area = 2 cm2 ) were prepared by thermal decomposition (450 ◦ C) of RuCl3 ·nH2 O (Aldrich), TiCl4 (Ventron), CeCl3 ·7H2 O (Merck), and NbCl5 (Fluka). Precursor solutions were prepared (0.18 mol dm−3 ) using aqueous HCl (Merck 1:1 v/v) as solvent. Ti-supports were degreased with isopropanol and submitted to chemical attack for 10 min in boiling 10% oxalic acid. Precursor mixtures, presenting the desired mole ratio, were spread onto both sides of the support by brushing. The solvent was evaporated at 90 ◦ C and the residue calcinated at 450 ◦ C for 10 min, under an air stream, in a pre-heated oven. This procedure was repeated until the nominal thickness of 2 m was achieved (corresponding to 0.9–1.2 mg cm−2 , depending on the composition). A final calcination, at 450 ◦ C for 1 h, completed the procedure. Duplicated samples of [Ru0.3 Ti0.6 Ce(0.1−x) ]O2 [Nb2 O5 ](x) were prepared at 1 mol% intervals covering the 0–10 mol% Nb2 O5 interval. RuO2 and TiO2 contents were kept constant at 30 and 60 mol%, respectively. For comparison, two samples of nominal composition Ru0.3 Ti0.7 O2 were prepared. All electrodes were mounted in a Teflon holder as described previously [13,18]. 2.2. Solutions The OER study was executed in 0.5 M H2 SO4 volumetrically prepared from concentrated H2 SO4 (Merck) and Milli-Q quality (R > 18.2 M) water. The ClER study was executed in 5.0 mol dm−3 NaCl + 0.01 mol dm−3 HCl solutions. Before and during each run, the electrolyte was deaerated and stirred using ultrapure nitrogen bubbling. The three compartment cell used has been described previously [19]. Ohmic drop was minimized using a Luggin capillary, while two heavily platinized platinum electrodes were used as counter electrodes. 2.3. Potentials For OER studies, all potentials were measured and reported against a hydrogen electrode containing the working solution (RHE). For ClER studies, the electrode potentials were measured and reported against a sodium saturated calomel electrode (SSCE). 2.4. Techniques PAR-273 instrumentation was used throughout. Electrodes were studied by cyclic voltammetric (CV) and steady-state potential sweep curves. CV’s were recorded at 20 mV s−1 covering the 0.4–1.4 V/RHE range for OER and 0.0–1.0 V/SSCE interval for ClER studies. The voltammetric charges spent in these potential ranges were determined by integration of the i/E profiles using PAR software (M-270).
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In the OER studies, polarisation curves were recorded in 0.5 M H2 SO4 keeping the electrode at 1.3 V/RHE for 15 min, then moving the potential anodically, at ν = 56 V s−1 , until the current reached a value around 100 mA. In ClER, the same procedure was maintained, but the electrode preconditioned at 0.95 V/SSCE for 15 min. in 5.0 mol dm−3 NaCl + 0.01 mol dm−3 HCl. 2.5. OER The reaction order with respect to H+ , νH + , was determined fixing the potential at 1.25 V/RHE for 5 min, stepping to 1.51 V where it was hold an additional 5 min and finally stepped to 1.45 V, and the current measured after 15 min. Electrolytes with different pH values were prepared by systematic substitution of H2 SO4 by Na2 SO4 keeping the ionic strength constant at 1.5 mol dm−3 . 2.6. ClER The reaction order with respect to Cl− , νCl − , was determined covering the 0.5–5.0 mol dm−3 NaCl concentration interval keeping the ionic strength constant by substitution of NaCl by NaClO4 at constant pH 2. The reaction order with respect to H+ was obtained changing the [H+ ] between 0.1 and 0.01 mol dm−3 by partial substitution of NaCl with HCl. Reaction orders were determined after conditioning the electrode at 1.0 V/SSCE for 5 min, stepping to 1.28 V/SSCE for additional 5 min and stepped to 1.25 V, when currents were recorded after 15 min. 3. Results and discussion 3.1. Oxygen evolution 3.1.1. Tafel lines A typical voltammetric curve, recorded between +0.4 and +1.4 V/RHE at 20 mV s−1 using 0.5 mol dm−3 H2 SO4 as supporting electrolyte, is shown in Fig. 1. The large peak around +0.7 V is characteristic of the solid state Ru(III)/Ru(IV) transition, while the start of the OER, normally observed close to +1.30 V/HER, becomes less pronounced with increasing Nb2 O5 content of the coating. (For more details see [17]). Fig. 2 shows a typical quasi-stationary potential sweep curve before and after IR correction. For each electrode, polarisation curves were recorded twice (forward and backward scans) presenting negligible hysteresis between the scans. The results shown are the second potential sweep. All experimental Tafel curves showed, at high overpotentials, a deviation from linearity, requiring correction of the curves for ohmic drop, IR. This correction was based on the procedure originally proposed by Shub and Reznik [20]. R -values randomly distributed in the 0.3–1.3 interval were obtained. The R -values are in good agreement with other IrO2 and RuO2 -based oxide electrodes submerged in similar supporting electrolyte [14–16]. As the resistance of the film is negligible due to the metallic nature of RuO2 and its thin thickness (2 m),
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Fig. 1. Voltammetric curve recorded from 0.4 to 1.4 V (RHE) in 0.5 mol dm−3 H2 SO4 at 20 mV s−1 . Electrode composition: [Ru(0.3) Ti(0.6) Ce(0.05) ]O2 [Nb2 O5 ](0.05) .
the R -values can be attributed only to the solution resistance between the electrode surface and the Luggin capillary. After the ohmic drop correction, all curves show two Tafel slopes. Fig. 3 shows the Tafel slope dependence on nominal oxide composition. At low overpotentials, Tafel slopes close to 30 mV was observed for all compositions containing CeO2 while 40 mV values were obtained for the binary Ru0.3 Ti0.7 O2 and ternary [Ru0.3 Ti0.6 O2 ][Nb2 O5 ]0.1 compositions. The 40 mV value for the binary composition agrees with literature data [16,19]. The addition of only 1 mol% CeO2 already reduces the Tafel slope to 30 mV. The substitution of TiO2 by 10 mol% Nb2 O5 in Ru0.3 Ti0.7 O2 does not affect the 40 mV slope, confirming the inert character of Nb2 O5 , with no electrochemical activity in the OER region [17,21,22]. At high overpotentials, after IR correction, a second Tafel slope is observed, randomly distributed around 120 mV for all compositions. The change of the Tafel coefficient indicates deviation of the experimental data from linearity is due to ohmic drop
Fig. 2. Representative quasi-stationary current-potential curve for OER: () before and () after IR correction. Electrolyte: 0.5 mol dm−3 H2 SO4 . Electrode composition: [Ru0,3 Ti0,6 ]O2 [Nb2 O5 ]0,1 .
Fig. 3. Tafel slopes as function of nominal composition for OER. Electrolyte: 0.5 mol dm−3 H2 SO4 . (䊉) low and () high overpotentials; (, ) Ru0.3 Ti0.7 O2 .
resistance combined with a change in the electrode mechanism (or its rate determining step, rds). 3.1.2. Reaction order Fig. 4 shows the dependence of the current density on H+ concentration for two electrodes, measured at 1.45 V/RHE. For all electrodes the current density is independent on pH, showing the reaction order at constant overpotential is zero, irrespective of electrode composition. However, the reference electrode used was a RHE, containing the working solution, whose potential varies with pH solution. The reaction order at constant overpotential is not chemically significant [23] and must be converted to the reaction order at constant potential by: η vpH
= E vpH − γ
(1)
where η is the overpotential, ν the reaction order and γ the factor in the denominator of the Tafel slope (RT/γF) [24]. Since for all
Fig. 4. Dependence of current density on pH for OER. Electrolyte: 0.5 mol dm−3 [H2 SO4 + Na2 SO4 ]. (1) [Ru0.3 Ti0.6 Ce0.05 ]O2 [Nb2 O5 ]0.05 ; (2) [Ru0.3 Ti0.6 Ce0.09 ]O2 [Nb2 O5 ]0.01 . Eap = 1.45 V/RHE.
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electrodes η vpH = 0 was obtained, E vpH
=γ
(2)
For a Tafel slope of 40 mV, E vpH = 1.5 is calculated, while for Tafel slope of 30 mV, E vpH = 2 is obtained (see Section 3.1.3). Although the predicted and observed reaction orders coincide for the 30 mV Tafel slope, the value of 1.5 differs from the predicted value of 1 for the 40 mV Tafel slope. Fractional reaction orders are not uncommon with oxides and are related to double layer effects [23] due to the surface charge of oxides being a function of pH [25,26]. Briefly, the electrical potential at the surface sites becomes a function of pH: the higher the acidity, the more positive (or the less negative) the potential at the oxide surface. As demonstrated earlier [15], the reaction order with respect to pH turns out to be (1 + α); or-be-it, 1.5 considering α = 0.5. 3.1.3. Mechanism According to the Tafel slopes and reaction orders, next general electrode mechanism for the OER from acid medium can be employed (S is an active surface site) to describe the response of the system investigated:
(a) (b) (b ) (c)
S + H2 O → S OH + H+ + e S OH → S O + H+ + e S OH + S OH → S O + S + H2 O S O + S O → 2S + O2
b (mV) 120 40 30 15
In this mechanism, steps (b) and (b ) are alternative, i.e., they occur in parallel, the occurrence of one or the other depending on the adsorption strength of the intermediate. The 40 mV slope, obtained from electrodes without CeO2 content, suggests that the step known as electrochemical oxide path [27] is the rds: S OH → S O + H+ + e
(b)
Film compositions containing CeO2 show b = 30 mV, which suggests the recombination step (chemical step) [27] is the rds: 2S OH → S O + S + H2 O
(b )
The rds of the electrode mechanism depends on the strength of the adsorption of the intermediate (directly related to the oxide layer composition). Using a similar analysis developed by Parsons [28], one can argue that the rds changes with film composition as a consequence of the variation in the strength of the S OH bond. The presence of CeO2 seems to strengthen this bond, leading to an increase in the surface coverage by OH intermediate. Hence, the rds becomes step (b ) in the above mechanism. Our results confirm the data of a previous paper [16], where Faria et al. also concluded the introduction of CeO2 strengthen the S OH surface bond. Although Tafel slope is a kinetic parameter giving information about the rds of the electrode process, it can be influenced by the degree of porosity of the electrode. As pointed out by Trasatti [29] the Tafel coefficient can be sensitive, besides other
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factors, to the surface structure of the electrocatalysts, showing lower values for very rugged coatings [30]. Fig. 6 shows the voltammetric charge of recently prepared electrodes is very sensitive to composition and coating porosity/roughness. If these parameters would have affected the kinetics of the electrode process a change in Tafel coefficient would have been observed in between the potential sweeps, since the voltammetric charge present a significant change after the backwards potential scan. Indeed a constant Tafel values of 30 mV was observed for the OER at CeO2 containing electrodes. Besides this effect, the presence of CeO2 in RuO2 –TiO2 mixtures catalyses the OER due to a synergetic process [11,13,17], possible by means of a solid state redox reaction between Ce4+ and Ru3+ , which was discussed in a previous paper [17]. Nb2 O5 is an electrochemically inactive oxide in this anodic potential range and, as expected, did not influenced the kinetics of the RuO2 –TiO2 system. In the high overpotential domain, the second Tafel coefficient shows values around 120 mV, which is consistent with the primary water discharge, via step (a) in the above mechanism [11,16,29]. In this potential domain, the intense gas evolution inside the fragile oxide structure causes erosion on the surface coating. However, it does not affect the adsorption strength, which in turns governs the charge transfer. Indeed, as recently discussed [22], in the particular case of OER, the electroactive specie is the water molecule with concentration about 50 mol dm−3 , which avoids mass control and furnishes meaningful b-values, even with possible changes in active surface area in high current densities. 3.1.4. Electrocatalytic activity To analyse the electrode activity, it is necessary to separate electronic effects (true electrocatalysis) from geometric effects (related to surface area). Although the true surface area is not known, the anodic voltammetric charge, qa , can be taken as proportional to the surface concentration of active sites, or-be-it, of the electrochemically active surface area (EASA) and is a function of oxide composition [14,15,17,22]. Both oxide composition and surface area affect the current, a measure of the electrode activity. So, normalisation of the activity to the unit surface charge eliminates the area effect from the data while plotting (i/qa ) versus nominal surface composition permits to evaluate the influence of the oxide nature. Fig. 5 shows the behaviour of the real electrocatalytic activity, i/qa , as function of oxide composition. Current data were extracted from Tafel curves while qa -values were obtained by integration of the anodic branch of CV’s, already reported [17]. Fig. 5 shows the real electrocatalytic activity, in general, diminishes with increasing Nb2 O5 content (or with decreasing CeO2 content), with a maximum at 10 mol% CeO2 nominal composition. Since the qa -values are higher for compositions with increasing CeO2 content of the mixture [17], the electrocatalytic activity of the system (i/qa ) is mainly governed by electronic factors. The change of the EASA was followed by determining qa for freshly prepared electrodes, after the Tafel slope and reaction
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Tafel experiments. Since the experimental conditions used in the reaction order experiments (much lower current densities are observed), qa -values do change much less. Oxide electrodes with high CeO2 contents present a more pronounced decrease of the EASA than films with high Nb2 O5 contents. Since oxide coatings with higher Nb2 O5 contents present a more compact oxide layer [17,21,31], these electrodes are more stable. While CeO2 provides a more active oxide mixture, Nb2 O5 turned it into a more compact and wear resistant layer [17]. 3.2. Chlorine evolution
Fig. 5. Dependence of normalised current density (i/qa ) at 1.45 V/RHE, on nominal oxide composition: 0.5 mol dm−3 H2 SO4 ; () Ru0.3 Ti0.7 O2 . Data extracted from Tafel lines.
order’s determinations. Fig. 6 shows qa as function of Nb2 O5 content after each group of experiments. The EASA presents a remarkable decrease after the Tafel slope determination for all compositions. For fresh electrodes, the qa versus composition profile is constant up to 3 mol% Nb2 O5 + 7 mol% CeO2 , showing a significant linear decrease for higher Nb2 O5 content compositions. After Tafel and reaction order experiments, qa -values become much smaller, showing a slight linear increase with Nb2 O5 content. The authors showed in a previous paper [17], based on XRD and electrochemical characterisation, that the oxide layer is composed of finely divided particles, more porous/rugged for films with the highest CeO2 contents. This particular structure caused by CeO2 addition was already observed for other systems [11,13,16]. Thus, during the polarisation curves, gas evolution inside the pores causes mechanical stress, explaining the decrease of qa -values after
Fig. 6. Voltammetric charge as function of oxide composition: () fresh electrodes; () after Tafel curves; () after reaction order experiment 0.5 mol dm−3 H2 SO4 .
3.2.1. Tafel lines Literature reports show the ClER is an “easy” electrode process (low overpotential) characterised by a close to theoretical Tafel coefficient (30–40 mV) [8,9,32]. Normally, the ClER does not show a significant dependence on the nature of the electrode material [33], though the kinetics can present a dependence on such parameters as the electrode morphology and the electrolyte pH [34]. For each electrode current-potential curves were recorded (a forward and a backward scan) and hysteresis between scans was negligible. All experimental Tafel curves showed, at high overpotentials, a deviation from linearity, requiring correction for ohmic drop in order to permit an appropriate interpretation. Tafel curve, before and after IR correction, representative of the system behaviour, is shown in Fig. 7. The ohmic drop correction was calculated using the same methodology employed for the Tafel lines of the OER. R -values in the range 0.5–1.5 were obtained which are in good agreement with similar systems made up of thin conductive coatings [15,16,35]. Thus, the main contribution to the total resistance of the system comes from the uncompensated solution resistance between the working electrode and the Luggin capillary. Independent of coating composition, after IR correction, a single linear segment is obtained (see Fig. 7) showing kinetics of the ClER is independent of potential. This behaviour is
Fig. 7. Representative quasi-stationary curve for ClER: () before and () after IR correction. Electrolyte: 5.0 mol dm−3 NaCl + 0.01 mol dm−3 HCl. Electrode composition: [Ru0.3 Ti0.6 ]O2 [Nb2 O5 ]0.1 .
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Fig. 8. Tafel slopes (medium values) as function of nominal composition for ClER. Electrolyte: 5.0 mol dm−3 NaCl + 0.01 mol dm−3 HCl; () Ru0.3 Ti0.7 O2 .
rather different from the OER, for which a strong dependence on potential and nominal electrode composition is observed. Fig. 8 shows the dependence of the Tafel coefficient (b) on nominal oxide composition. The Tafel slope is independent of composition, showing a randomly distributed value around 30 mV for all electrodes. Although the average Tafel slope for RuO2 -based oxides is 40 mV, the value of 30 mV has also been reported in the literature [5,29,32]. 3.2.2. Reaction orders with respect to H+ and Cl− Tafel slope data obtained in this work are consistent with several different electrode mechanisms [4,7,32]. So, determination of the reaction order (i) with respect to Cl− at constant pH and (ii) with respect to H+ at constant Cl− concentration becomes a crucial factor to discriminate between the various possibilities. Fig. 9 shows a representative log i versus log(CCl− ) profile (i-values were read at 1.25 V/SSCE). The ionic strength of the
Fig. 10. Dependence of current density on pH at [Ru0.3 Ti0.6 Ce0.07 ] O2 [Nb2 O5 ]0.03 . E = 1.25 V/SSCE. Electrolyte: 5.0 mol dm−3 NaCl + 0.01 mol dm−3 HCl.
solution was kept constant to avoid diffuse double layer effects [8]. The slope of the lines is independent of electrode composition, presenting values around unity. Fig. 10 shows a representative plot of the influence of pH on current measured at 1.25 V/SSCE. Although no influence of the solution acidity on ClER is expected on the basis of purely thermodynamic considerations [8] some literature reports [4,9,32,36], investigating different oxide materials, show the reaction rate is depressed by increasing acidity. However, with the title system, as shown in Fig. 10, no significant influence of the pH on the ClER rate is observed, independently of the oxide composition. 3.2.3. Mechanism Several different electrode mechanisms have been proposed for the ClER process [4,8,32,36]. Considering the experimental data: b-values, the dependence on the chloride ion concentration and the independence on pH, the mechanism proposed by Erenburg et al. [37] adequately describes the ClER:
(d) (e) (f)
Fig. 9. Dependence of current density on Cl− concentration at [Ru0.3 Ti0.6 ] O2 [Nb2 O5 ]0.1 . E = 1.25 V/SSCE. Electrolyte: 5.0 mol dm−3 NaCl + 0.01 mol dm−3 HCl.
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S + Cl− → S Cl + e S Cl → S Cl+ + e S Cl+ + Cl− → S + Cl2
b (mV) 120 40 30
where S represents an active surface site. Although considering step (f) as rds agrees with the experimental Tafel slope, it predicts a reaction order with respect to Cl− of 2, which is not consistent with the experimental value of 1. The above mechanism does not predict any dependency of the reaction rate on pH, in agreement with the experimental results. Considering the experimental reaction order with respect to Cl− and step (e) as the rds, the reaction rate, i, at η ≥ 0.1 V can be written as: (1 + α)F (E − Eo ) − i = k[Cl ] exp (3) RT
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cates the intrinsic electrocatalytic activity for ClER is mainly due to electronic factors. 4. Conclusions
Fig. 11. Dependence of apparent (i) and normalised current density (i/qa ) at 1.25 V/SSCE on oxide composition from 5.0 mol dm−3 NaCl + 0.01 mol dm−3 HCl; (, ) Ru0.3 Ti0.7 O2 . Data extracted from Tafel lines.
Rearranging and applying the definition b ≡ (∂E/∂ log i)T , the theoretical Tafel slope is given by: b =
2.303RT F (1 + α)
(4)
According to Eq. (4), Tafel slopes of 30 and 40 mV (T = 25 ◦ C) require the electronic transfer coefficient, α, to assume values of 1.0 and 0.5, respectively. As discussed by Da Silva et al. [5], α = 0.5 represents a perfect symmetry between the activation energy curves with respect to oxidised and reduced species while α = 1.0 represents an electron transfer occurring without activation in the electric component of the total activation energy barrier [2,38]. According to Krishtalik [39], a value of α = 1.0 is not necessarily associated with a complete disappearance of the energy barrier (this process may be called quasibarrierless), and is predicted on the basis of the quantum mechanical theory of an elementary act. It was shown that certain reactions, particularly the ClER, are indeed quasibarrierless [5,40]. Generally, changes in α-value can be used to correlate the influence of the electrode material on the electron transfer process [41], so α can be correlated to changes in S Cl bond strength [5]. The experimental b-values, close to 30 mV, indicate an α-value around 0.8, independent on nominal electrode composition. This can be understood if we keep in mind that the active oxide (RuO2 ) content is constant at 30 mol% and it is shown that Cl2 evolution on RuO2 -based electrodes presents almost constant kinetic and activity down to 20% RuO2 [29]. 3.2.4. Electrocatalytic activity Fig. 11 shows a plot of the apparent, i, and true electrocatalytic activity, i/qa , measured at 1.25 V/SSCE as function of the nominal oxide composition. qa -values were extracted from CV between 0.0 and 1.0 V/SSCE in 5.0 M NaCl + 0.01 M HCl. Both i and i/qa versus composition plots show that the electrocatalytic activity is higher for low Nb2 O5 contents (high CeO2 contents). The electrodes employed for the ClER studies were the same employed for the OER investigations. After the wear imposed by the OER experiments, the qa -values increase with Nb2 O5 content (see Fig. 6). So, the behaviour of Fig. 11 indi-
For OER, both the Tafel coefficient and reaction order with respect to H+ show a dependence on composition and overpotential. The presence of CeO2 reduces the Tafel slope from 40 to 30 mV, while Nb2 O5 exerts no influence on the electrode kinetics. The highest global electrocatalytic activity observed for higher CeO2 contents is attributed to an electronic and geometric factors, while the true electrocatalytic activity depends on the electronic factor. For ClER, only a single rds, independent of overpotential interval and electrode composition, is observed. Experimental data support the electrode mechanism proposed by Erenburg et al., with an α-value close to unity, in agreement with some literature reports shows ClER can be characterised as a quasibarrierless reaction. Analysis of both global and true electrocatalytic activity show the main contribution comes from the electronic nature, confirming the ClER is little affected by the electrode morphology. The electrode stability is closely related to the composition. Comparison of the anodic voltammetric charges obtained before and after each set of experiments denounces significant electrode wear during the OER. The higher the CeO2 content, the less stable the coating, probably related to the coating fragility of the porous structure of CeO2 containing oxide mixture. On the other hand, introduction of Nb2 O5 causes a stabilisation of the oxide mixture due to a more compact layer. Acknowledgements M´ario H.P. Santana wishes to thank a M.Sc. Fellowship from the CAPES Foundation. Luiz A. De Faria wishes to thank financial support from the FAPEMIG/CNPq Foundations. Authors thank the helpful discussions provided by Julien F.C. Boodts. References [1] S. Trasatti, Electrochim. Acta 45 (2000) 2377. [2] S. Trasatti, G. Lodi, in: S. Trasatti (Ed.), Electrodes of Conductive Metallic Oxides, Part B, Elsevier, Amsterdam, 1981, p. 535. [3] S. Trasatti, Electrochim. Acta 36 (1991) 225. [4] J.L. Fern´andez, M.R.G. De Chialvo, A.C. Chialvo, Electrochim. Acta 47 (2002) 1145. [5] L.M. Da Silva, J.F.C. Boddts, L.A. De Faria, J. Braz. Chem. Soc. 14 (2003) 388. [6] L.M. Da Silva, L.A. De Faria, J.F.C. Boddts, Pure Appl. Chem. 73 (2001) 1871. [7] L. Tomcs´anyi, A. De Battisti, G. Hirschberg, K. Varga, J. Liszi, Electrochim. Acta 44 (1999) 2463. [8] V. Consonni, S. Trasatti, F. Pollak, W.E. O’Grady, J. Electroanal. Chem. 228 (1987) 393. [9] L.A. De Faria, J.F.C. Boodts, S. Trasatti, Electrochim. Acta 42 (1997) 3525. [10] T. Arikado, C. Iwakura, H. Tamura, Electrochim. Acta 23 (1978) 9. [11] V.A. Alves, L.A. Da Silva, J.F.C. Boodts, S. Trasatti, Electrochim. Acta 39 (1994) 1585.
M.H.P. Santana, L.A. De Faria / Electrochimica Acta 51 (2006) 3578–3585 [12] R. Mr´az, V. Srb, S. Tich´y, Electrochim. Acta 18 (1973) 551. [13] L.A. De Faria, J.F.C. Boodts, S. Trasatti, Electrochim. Acta 37 (1992) 2511. [14] C.P. De Pauli, S. Trasatti, J. Electroanal. Chem. 538 (2002) 145. [15] L.M. Da Silva, J.F.C. Boodts, L.A. De Faria, Electrochim. Acta 46 (2001) 1369. [16] L.A. De Faria, J.F.C. Boodts, S. Trasatti, J. Appl. Electrochem. 26 (1996) 1195. [17] M.H.P. Santana, L.M. Da Silva, L.A. De Faria, Electrochim. Acta 48 (2003) 1885. [18] R. Garavaglia, C.M. Mari, S. Trasatti, Surf. Technol. 23 (1984) 41. [19] G. Lodi, E. Sivieri, A. De Battisti, S. Trasatti, J. Appl. Electrochem. 8 (1978) 135. [20] D.M. Shub, M.F. Reznik, V.V. Shalaginov, Elektrokhimiya 21 (1985) 937. [21] A.J. Terezo, E.C. Pereira, Electrochim. Acta 44 (1999) 4507. [22] M.H.P. Santana, L.A. De Faria, J.F.C. Boodts, Electrochim. Acta 49 (2004) 1925. [23] B.E. Conway, M. Salomon, Electrochim. Acta 9 (1964) 1599. [24] R. Parsons, Pure Appl. Chem. 52 (1987) 233. [25] L.I. Krishtalik, Electrochim. Acta 26 (1981) 329. [26] C.A. Angelinetta, M. Falciola, S. Trasatti, J. Electroanal. Chem. 205 (1986) 347. [27] A. Damjanovic, in: J.O.’M. Bockris, B.E. Conway (Eds.), Modern Aspects of Electrochemistry, vol. 5, Butterworths, London, 1969, p. 369.
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[28] R. Parsons, Trans. Faraday Soc. 54 (1958) 1053. [29] S. Trasatti, in: J. Lipkowski, P.N. Ross, J. Lipowski (Eds.), Frontiers of Electrochemistry: The Electrochemistry of Novel Materials, John Wiley & Sons, NY, 1994, p. 239, 243, 248. [30] M. Filoche, B. Sapoval, Electrochim. Acta 46 (2000) 213. [31] A.J. Terezo, E.C. Pereira, Electrochim. Acta 45 (2000) 4351. [32] R. Boggio, A. Carugati, G. Lodi, S. Trasatti, J. Appl. Electrochem. 15 (1985) 335. [33] S. Trasatti, G. Lodi, in: S. Trasatti (Ed.), Electrodes of Conductive Metallic Oxides, Elsevier, Amsterdam, 1981 (chapter 10). [34] S. Trasatti, Electrochim. Acta 32 (1987) 369. [35] L.A. Da Silva, V.A. Alves, M.A.P. Da Silva, S. Trasatti, J.F.C. Boodts, Can. J. Chem. 75 (1997) 1483. [36] A. Cornell, B. Hakansson, G. Lindbergh, Electrochim. Acta 48 (2003) 473. [37] R.G. Erenburg, L.I. Krishtalik, I.P. Yaroshevskaya, Elektrokhimiya 11 (1975) 1236. [38] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, John Wiley & Sons, N.Y., 1980, 96. [39] L.I. Krishtalik, in: J.O.’M. Bockris, B.E. Conway, E. Yeager, R.E. White (Eds.), Comprehensive Treatise of Electrochemistry, vol. 7, 1981, p. 426. [40] L.I. Krishtalik, Electrochim. Acta 13 (1968) 1045. [41] J.O.’M. Bockris, A.K.N. Reddy, Modern Electrochemistry, Plenum Press, New York, NY, 1977, 918.
Oxygen and chlorine evolution on RuO2 + TiO2 + CeO2 + Nb2O5 mixed oxide electrodes M´ario H.P. Santana, Luiz A. De Faria ∗ ´ Instituto de Qu´ımica, UFU, Av. Jo˜ao Naves de Avila, 2160, Campus Sta Mˆonica, CEP 38400-902, Uberlˆandia/MG, Brazil Received 27 July 2005; received in revised form 28 September 2005; accepted 29 September 2005 Available online 23 November 2005
Abstract A systematic investigation was conducted on the mechanism and electrocatalytic properties of O2 and Cl2 evolution on mixed oxide electrodes of nominal composition: Ti/[Ru(0.3) Ti(0.6) Ce(0.1−x) ]O2 [Nb2 O5 ](x) (0 ≤ x ≤ 0.1). For the oxygen evolution, a 30 mV Tafel slope is obtained in the presence of CeO2 , while in its absence a 40 mV coefficient is observed. The intrinsic electrocatalytic activity is mainly due to electronic factors, as result of the synergism between Ru and Ce oxides. For chlorine evolution, the Tafel slope (30 mV) is independent on oxide composition. The best global electrocatalytic activity for ClER was observed in the absence of Nb2 O5 additive. Variation of the voltammetric charge throughout the experiments confirms high CeO2 content compositions are fragile, due mainly to the porosity caused by CeO2 presence. On the other hand, Nb2 O5 addition decreases considerably this instability. © 2005 Elsevier Ltd. All rights reserved. Keywords: Ru + Ti + Ce + Nb oxides; Oxygen evolution; Chlorine evolution; Ruthenium oxide; Electrocatalysis
1. Introduction DSA® -type electrodes have been well succeeded for almost 40 years due to their versatile electrocatalytic properties and stability [1]. These electrodes have shown significant activity for all common gas evolution reactions (O2 , Cl2 ). Up to now, the most widely industrial application of these electrodes is for Cl2 production [1]. In this field, the electrode material plays an important role in both Cl2 yield and purity, since the chlorine evolution reaction, ClER, is affected by the presence of O2 from the oxygen evolution reaction, OER [2,3]. To achieve high Cl2 current efficiency, it is necessary to work at low pH values of the electrolyte and with electrode materials that show, in principle, significant overpotentials for the OER [2]. Thus, electrode selectivity for the ClER remains a problem of technological interest, although a broad fundamental and applied research of ClER is available [1–5]. OER is the main anodic reaction in the majority of the cathodic processes and it is an important side reaction in processes such as production of strong oxidants and combus∗
Corresponding author. Tel.: +55 34 3239 4143; fax: +55 34 3239 4208. E-mail address: [email protected] (L.A. De Faria).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.09.050
tion of organics [1]. Development of fuel cells and the socalled Chemical Green Processes [6] has also stimulated OER investigations. The most active oxides for OER and ClER are RuO2 and IrO2 , or their mixtures, and a great number of the investigations are concerned with oxides of rutile structure [7–12]. The global electrode activity of a pure oxide is dependent on the nature of the oxide and the electrode preparation procedure. However, for a single oxide, these variables affect the activity to a limited extend. As alternative, the use of mixed oxides can lead to synergetic effects improving the electrocatalytic properties and the stability/selectivity of the electrode [5,9,10,13–15]. De Faria et al. [16] demonstrated partial substitution of TiO2 by CeO2 in a RuO2 -based electrode material improves the electrocatalytic activity for OER, but decreases the mechanical stability of the mixture. In a recent paper [17], we reported the surface properties of the RuO2 + TiO2 + CeO2 + Nb2 O5 system and showed the instability caused by CeO2 can be reduced introducing a small Nb2 O5 content. In this work, we investigate the electrocatalytic properties of [Ru0.3 Ti0.6 Ce(0.1−x) ]O2 [Nb2 O5 ](x) with (0 ≤ x ≤ 0.1) for OER and ClER and the stability of these oxide electrodes throughout the experiments.
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2. Experimental 2.1. Electrodes Mixed oxide layers deposited on a Ti support (10 mm × 10 mm × 0.15 mm; geometric area = 2 cm2 ) were prepared by thermal decomposition (450 ◦ C) of RuCl3 ·nH2 O (Aldrich), TiCl4 (Ventron), CeCl3 ·7H2 O (Merck), and NbCl5 (Fluka). Precursor solutions were prepared (0.18 mol dm−3 ) using aqueous HCl (Merck 1:1 v/v) as solvent. Ti-supports were degreased with isopropanol and submitted to chemical attack for 10 min in boiling 10% oxalic acid. Precursor mixtures, presenting the desired mole ratio, were spread onto both sides of the support by brushing. The solvent was evaporated at 90 ◦ C and the residue calcinated at 450 ◦ C for 10 min, under an air stream, in a pre-heated oven. This procedure was repeated until the nominal thickness of 2 m was achieved (corresponding to 0.9–1.2 mg cm−2 , depending on the composition). A final calcination, at 450 ◦ C for 1 h, completed the procedure. Duplicated samples of [Ru0.3 Ti0.6 Ce(0.1−x) ]O2 [Nb2 O5 ](x) were prepared at 1 mol% intervals covering the 0–10 mol% Nb2 O5 interval. RuO2 and TiO2 contents were kept constant at 30 and 60 mol%, respectively. For comparison, two samples of nominal composition Ru0.3 Ti0.7 O2 were prepared. All electrodes were mounted in a Teflon holder as described previously [13,18]. 2.2. Solutions The OER study was executed in 0.5 M H2 SO4 volumetrically prepared from concentrated H2 SO4 (Merck) and Milli-Q quality (R > 18.2 M) water. The ClER study was executed in 5.0 mol dm−3 NaCl + 0.01 mol dm−3 HCl solutions. Before and during each run, the electrolyte was deaerated and stirred using ultrapure nitrogen bubbling. The three compartment cell used has been described previously [19]. Ohmic drop was minimized using a Luggin capillary, while two heavily platinized platinum electrodes were used as counter electrodes. 2.3. Potentials For OER studies, all potentials were measured and reported against a hydrogen electrode containing the working solution (RHE). For ClER studies, the electrode potentials were measured and reported against a sodium saturated calomel electrode (SSCE). 2.4. Techniques PAR-273 instrumentation was used throughout. Electrodes were studied by cyclic voltammetric (CV) and steady-state potential sweep curves. CV’s were recorded at 20 mV s−1 covering the 0.4–1.4 V/RHE range for OER and 0.0–1.0 V/SSCE interval for ClER studies. The voltammetric charges spent in these potential ranges were determined by integration of the i/E profiles using PAR software (M-270).
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In the OER studies, polarisation curves were recorded in 0.5 M H2 SO4 keeping the electrode at 1.3 V/RHE for 15 min, then moving the potential anodically, at ν = 56 V s−1 , until the current reached a value around 100 mA. In ClER, the same procedure was maintained, but the electrode preconditioned at 0.95 V/SSCE for 15 min. in 5.0 mol dm−3 NaCl + 0.01 mol dm−3 HCl. 2.5. OER The reaction order with respect to H+ , νH + , was determined fixing the potential at 1.25 V/RHE for 5 min, stepping to 1.51 V where it was hold an additional 5 min and finally stepped to 1.45 V, and the current measured after 15 min. Electrolytes with different pH values were prepared by systematic substitution of H2 SO4 by Na2 SO4 keeping the ionic strength constant at 1.5 mol dm−3 . 2.6. ClER The reaction order with respect to Cl− , νCl − , was determined covering the 0.5–5.0 mol dm−3 NaCl concentration interval keeping the ionic strength constant by substitution of NaCl by NaClO4 at constant pH 2. The reaction order with respect to H+ was obtained changing the [H+ ] between 0.1 and 0.01 mol dm−3 by partial substitution of NaCl with HCl. Reaction orders were determined after conditioning the electrode at 1.0 V/SSCE for 5 min, stepping to 1.28 V/SSCE for additional 5 min and stepped to 1.25 V, when currents were recorded after 15 min. 3. Results and discussion 3.1. Oxygen evolution 3.1.1. Tafel lines A typical voltammetric curve, recorded between +0.4 and +1.4 V/RHE at 20 mV s−1 using 0.5 mol dm−3 H2 SO4 as supporting electrolyte, is shown in Fig. 1. The large peak around +0.7 V is characteristic of the solid state Ru(III)/Ru(IV) transition, while the start of the OER, normally observed close to +1.30 V/HER, becomes less pronounced with increasing Nb2 O5 content of the coating. (For more details see [17]). Fig. 2 shows a typical quasi-stationary potential sweep curve before and after IR correction. For each electrode, polarisation curves were recorded twice (forward and backward scans) presenting negligible hysteresis between the scans. The results shown are the second potential sweep. All experimental Tafel curves showed, at high overpotentials, a deviation from linearity, requiring correction of the curves for ohmic drop, IR. This correction was based on the procedure originally proposed by Shub and Reznik [20]. R -values randomly distributed in the 0.3–1.3 interval were obtained. The R -values are in good agreement with other IrO2 and RuO2 -based oxide electrodes submerged in similar supporting electrolyte [14–16]. As the resistance of the film is negligible due to the metallic nature of RuO2 and its thin thickness (2 m),
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Fig. 1. Voltammetric curve recorded from 0.4 to 1.4 V (RHE) in 0.5 mol dm−3 H2 SO4 at 20 mV s−1 . Electrode composition: [Ru(0.3) Ti(0.6) Ce(0.05) ]O2 [Nb2 O5 ](0.05) .
the R -values can be attributed only to the solution resistance between the electrode surface and the Luggin capillary. After the ohmic drop correction, all curves show two Tafel slopes. Fig. 3 shows the Tafel slope dependence on nominal oxide composition. At low overpotentials, Tafel slopes close to 30 mV was observed for all compositions containing CeO2 while 40 mV values were obtained for the binary Ru0.3 Ti0.7 O2 and ternary [Ru0.3 Ti0.6 O2 ][Nb2 O5 ]0.1 compositions. The 40 mV value for the binary composition agrees with literature data [16,19]. The addition of only 1 mol% CeO2 already reduces the Tafel slope to 30 mV. The substitution of TiO2 by 10 mol% Nb2 O5 in Ru0.3 Ti0.7 O2 does not affect the 40 mV slope, confirming the inert character of Nb2 O5 , with no electrochemical activity in the OER region [17,21,22]. At high overpotentials, after IR correction, a second Tafel slope is observed, randomly distributed around 120 mV for all compositions. The change of the Tafel coefficient indicates deviation of the experimental data from linearity is due to ohmic drop
Fig. 2. Representative quasi-stationary current-potential curve for OER: () before and () after IR correction. Electrolyte: 0.5 mol dm−3 H2 SO4 . Electrode composition: [Ru0,3 Ti0,6 ]O2 [Nb2 O5 ]0,1 .
Fig. 3. Tafel slopes as function of nominal composition for OER. Electrolyte: 0.5 mol dm−3 H2 SO4 . (䊉) low and () high overpotentials; (, ) Ru0.3 Ti0.7 O2 .
resistance combined with a change in the electrode mechanism (or its rate determining step, rds). 3.1.2. Reaction order Fig. 4 shows the dependence of the current density on H+ concentration for two electrodes, measured at 1.45 V/RHE. For all electrodes the current density is independent on pH, showing the reaction order at constant overpotential is zero, irrespective of electrode composition. However, the reference electrode used was a RHE, containing the working solution, whose potential varies with pH solution. The reaction order at constant overpotential is not chemically significant [23] and must be converted to the reaction order at constant potential by: η vpH
= E vpH − γ
(1)
where η is the overpotential, ν the reaction order and γ the factor in the denominator of the Tafel slope (RT/γF) [24]. Since for all
Fig. 4. Dependence of current density on pH for OER. Electrolyte: 0.5 mol dm−3 [H2 SO4 + Na2 SO4 ]. (1) [Ru0.3 Ti0.6 Ce0.05 ]O2 [Nb2 O5 ]0.05 ; (2) [Ru0.3 Ti0.6 Ce0.09 ]O2 [Nb2 O5 ]0.01 . Eap = 1.45 V/RHE.
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electrodes η vpH = 0 was obtained, E vpH
=γ
(2)
For a Tafel slope of 40 mV, E vpH = 1.5 is calculated, while for Tafel slope of 30 mV, E vpH = 2 is obtained (see Section 3.1.3). Although the predicted and observed reaction orders coincide for the 30 mV Tafel slope, the value of 1.5 differs from the predicted value of 1 for the 40 mV Tafel slope. Fractional reaction orders are not uncommon with oxides and are related to double layer effects [23] due to the surface charge of oxides being a function of pH [25,26]. Briefly, the electrical potential at the surface sites becomes a function of pH: the higher the acidity, the more positive (or the less negative) the potential at the oxide surface. As demonstrated earlier [15], the reaction order with respect to pH turns out to be (1 + α); or-be-it, 1.5 considering α = 0.5. 3.1.3. Mechanism According to the Tafel slopes and reaction orders, next general electrode mechanism for the OER from acid medium can be employed (S is an active surface site) to describe the response of the system investigated:
(a) (b) (b ) (c)
S + H2 O → S OH + H+ + e S OH → S O + H+ + e S OH + S OH → S O + S + H2 O S O + S O → 2S + O2
b (mV) 120 40 30 15
In this mechanism, steps (b) and (b ) are alternative, i.e., they occur in parallel, the occurrence of one or the other depending on the adsorption strength of the intermediate. The 40 mV slope, obtained from electrodes without CeO2 content, suggests that the step known as electrochemical oxide path [27] is the rds: S OH → S O + H+ + e
(b)
Film compositions containing CeO2 show b = 30 mV, which suggests the recombination step (chemical step) [27] is the rds: 2S OH → S O + S + H2 O
(b )
The rds of the electrode mechanism depends on the strength of the adsorption of the intermediate (directly related to the oxide layer composition). Using a similar analysis developed by Parsons [28], one can argue that the rds changes with film composition as a consequence of the variation in the strength of the S OH bond. The presence of CeO2 seems to strengthen this bond, leading to an increase in the surface coverage by OH intermediate. Hence, the rds becomes step (b ) in the above mechanism. Our results confirm the data of a previous paper [16], where Faria et al. also concluded the introduction of CeO2 strengthen the S OH surface bond. Although Tafel slope is a kinetic parameter giving information about the rds of the electrode process, it can be influenced by the degree of porosity of the electrode. As pointed out by Trasatti [29] the Tafel coefficient can be sensitive, besides other
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factors, to the surface structure of the electrocatalysts, showing lower values for very rugged coatings [30]. Fig. 6 shows the voltammetric charge of recently prepared electrodes is very sensitive to composition and coating porosity/roughness. If these parameters would have affected the kinetics of the electrode process a change in Tafel coefficient would have been observed in between the potential sweeps, since the voltammetric charge present a significant change after the backwards potential scan. Indeed a constant Tafel values of 30 mV was observed for the OER at CeO2 containing electrodes. Besides this effect, the presence of CeO2 in RuO2 –TiO2 mixtures catalyses the OER due to a synergetic process [11,13,17], possible by means of a solid state redox reaction between Ce4+ and Ru3+ , which was discussed in a previous paper [17]. Nb2 O5 is an electrochemically inactive oxide in this anodic potential range and, as expected, did not influenced the kinetics of the RuO2 –TiO2 system. In the high overpotential domain, the second Tafel coefficient shows values around 120 mV, which is consistent with the primary water discharge, via step (a) in the above mechanism [11,16,29]. In this potential domain, the intense gas evolution inside the fragile oxide structure causes erosion on the surface coating. However, it does not affect the adsorption strength, which in turns governs the charge transfer. Indeed, as recently discussed [22], in the particular case of OER, the electroactive specie is the water molecule with concentration about 50 mol dm−3 , which avoids mass control and furnishes meaningful b-values, even with possible changes in active surface area in high current densities. 3.1.4. Electrocatalytic activity To analyse the electrode activity, it is necessary to separate electronic effects (true electrocatalysis) from geometric effects (related to surface area). Although the true surface area is not known, the anodic voltammetric charge, qa , can be taken as proportional to the surface concentration of active sites, or-be-it, of the electrochemically active surface area (EASA) and is a function of oxide composition [14,15,17,22]. Both oxide composition and surface area affect the current, a measure of the electrode activity. So, normalisation of the activity to the unit surface charge eliminates the area effect from the data while plotting (i/qa ) versus nominal surface composition permits to evaluate the influence of the oxide nature. Fig. 5 shows the behaviour of the real electrocatalytic activity, i/qa , as function of oxide composition. Current data were extracted from Tafel curves while qa -values were obtained by integration of the anodic branch of CV’s, already reported [17]. Fig. 5 shows the real electrocatalytic activity, in general, diminishes with increasing Nb2 O5 content (or with decreasing CeO2 content), with a maximum at 10 mol% CeO2 nominal composition. Since the qa -values are higher for compositions with increasing CeO2 content of the mixture [17], the electrocatalytic activity of the system (i/qa ) is mainly governed by electronic factors. The change of the EASA was followed by determining qa for freshly prepared electrodes, after the Tafel slope and reaction
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Tafel experiments. Since the experimental conditions used in the reaction order experiments (much lower current densities are observed), qa -values do change much less. Oxide electrodes with high CeO2 contents present a more pronounced decrease of the EASA than films with high Nb2 O5 contents. Since oxide coatings with higher Nb2 O5 contents present a more compact oxide layer [17,21,31], these electrodes are more stable. While CeO2 provides a more active oxide mixture, Nb2 O5 turned it into a more compact and wear resistant layer [17]. 3.2. Chlorine evolution
Fig. 5. Dependence of normalised current density (i/qa ) at 1.45 V/RHE, on nominal oxide composition: 0.5 mol dm−3 H2 SO4 ; () Ru0.3 Ti0.7 O2 . Data extracted from Tafel lines.
order’s determinations. Fig. 6 shows qa as function of Nb2 O5 content after each group of experiments. The EASA presents a remarkable decrease after the Tafel slope determination for all compositions. For fresh electrodes, the qa versus composition profile is constant up to 3 mol% Nb2 O5 + 7 mol% CeO2 , showing a significant linear decrease for higher Nb2 O5 content compositions. After Tafel and reaction order experiments, qa -values become much smaller, showing a slight linear increase with Nb2 O5 content. The authors showed in a previous paper [17], based on XRD and electrochemical characterisation, that the oxide layer is composed of finely divided particles, more porous/rugged for films with the highest CeO2 contents. This particular structure caused by CeO2 addition was already observed for other systems [11,13,16]. Thus, during the polarisation curves, gas evolution inside the pores causes mechanical stress, explaining the decrease of qa -values after
Fig. 6. Voltammetric charge as function of oxide composition: () fresh electrodes; () after Tafel curves; () after reaction order experiment 0.5 mol dm−3 H2 SO4 .
3.2.1. Tafel lines Literature reports show the ClER is an “easy” electrode process (low overpotential) characterised by a close to theoretical Tafel coefficient (30–40 mV) [8,9,32]. Normally, the ClER does not show a significant dependence on the nature of the electrode material [33], though the kinetics can present a dependence on such parameters as the electrode morphology and the electrolyte pH [34]. For each electrode current-potential curves were recorded (a forward and a backward scan) and hysteresis between scans was negligible. All experimental Tafel curves showed, at high overpotentials, a deviation from linearity, requiring correction for ohmic drop in order to permit an appropriate interpretation. Tafel curve, before and after IR correction, representative of the system behaviour, is shown in Fig. 7. The ohmic drop correction was calculated using the same methodology employed for the Tafel lines of the OER. R -values in the range 0.5–1.5 were obtained which are in good agreement with similar systems made up of thin conductive coatings [15,16,35]. Thus, the main contribution to the total resistance of the system comes from the uncompensated solution resistance between the working electrode and the Luggin capillary. Independent of coating composition, after IR correction, a single linear segment is obtained (see Fig. 7) showing kinetics of the ClER is independent of potential. This behaviour is
Fig. 7. Representative quasi-stationary curve for ClER: () before and () after IR correction. Electrolyte: 5.0 mol dm−3 NaCl + 0.01 mol dm−3 HCl. Electrode composition: [Ru0.3 Ti0.6 ]O2 [Nb2 O5 ]0.1 .
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Fig. 8. Tafel slopes (medium values) as function of nominal composition for ClER. Electrolyte: 5.0 mol dm−3 NaCl + 0.01 mol dm−3 HCl; () Ru0.3 Ti0.7 O2 .
rather different from the OER, for which a strong dependence on potential and nominal electrode composition is observed. Fig. 8 shows the dependence of the Tafel coefficient (b) on nominal oxide composition. The Tafel slope is independent of composition, showing a randomly distributed value around 30 mV for all electrodes. Although the average Tafel slope for RuO2 -based oxides is 40 mV, the value of 30 mV has also been reported in the literature [5,29,32]. 3.2.2. Reaction orders with respect to H+ and Cl− Tafel slope data obtained in this work are consistent with several different electrode mechanisms [4,7,32]. So, determination of the reaction order (i) with respect to Cl− at constant pH and (ii) with respect to H+ at constant Cl− concentration becomes a crucial factor to discriminate between the various possibilities. Fig. 9 shows a representative log i versus log(CCl− ) profile (i-values were read at 1.25 V/SSCE). The ionic strength of the
Fig. 10. Dependence of current density on pH at [Ru0.3 Ti0.6 Ce0.07 ] O2 [Nb2 O5 ]0.03 . E = 1.25 V/SSCE. Electrolyte: 5.0 mol dm−3 NaCl + 0.01 mol dm−3 HCl.
solution was kept constant to avoid diffuse double layer effects [8]. The slope of the lines is independent of electrode composition, presenting values around unity. Fig. 10 shows a representative plot of the influence of pH on current measured at 1.25 V/SSCE. Although no influence of the solution acidity on ClER is expected on the basis of purely thermodynamic considerations [8] some literature reports [4,9,32,36], investigating different oxide materials, show the reaction rate is depressed by increasing acidity. However, with the title system, as shown in Fig. 10, no significant influence of the pH on the ClER rate is observed, independently of the oxide composition. 3.2.3. Mechanism Several different electrode mechanisms have been proposed for the ClER process [4,8,32,36]. Considering the experimental data: b-values, the dependence on the chloride ion concentration and the independence on pH, the mechanism proposed by Erenburg et al. [37] adequately describes the ClER:
(d) (e) (f)
Fig. 9. Dependence of current density on Cl− concentration at [Ru0.3 Ti0.6 ] O2 [Nb2 O5 ]0.1 . E = 1.25 V/SSCE. Electrolyte: 5.0 mol dm−3 NaCl + 0.01 mol dm−3 HCl.
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S + Cl− → S Cl + e S Cl → S Cl+ + e S Cl+ + Cl− → S + Cl2
b (mV) 120 40 30
where S represents an active surface site. Although considering step (f) as rds agrees with the experimental Tafel slope, it predicts a reaction order with respect to Cl− of 2, which is not consistent with the experimental value of 1. The above mechanism does not predict any dependency of the reaction rate on pH, in agreement with the experimental results. Considering the experimental reaction order with respect to Cl− and step (e) as the rds, the reaction rate, i, at η ≥ 0.1 V can be written as: (1 + α)F (E − Eo ) − i = k[Cl ] exp (3) RT
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cates the intrinsic electrocatalytic activity for ClER is mainly due to electronic factors. 4. Conclusions
Fig. 11. Dependence of apparent (i) and normalised current density (i/qa ) at 1.25 V/SSCE on oxide composition from 5.0 mol dm−3 NaCl + 0.01 mol dm−3 HCl; (, ) Ru0.3 Ti0.7 O2 . Data extracted from Tafel lines.
Rearranging and applying the definition b ≡ (∂E/∂ log i)T , the theoretical Tafel slope is given by: b =
2.303RT F (1 + α)
(4)
According to Eq. (4), Tafel slopes of 30 and 40 mV (T = 25 ◦ C) require the electronic transfer coefficient, α, to assume values of 1.0 and 0.5, respectively. As discussed by Da Silva et al. [5], α = 0.5 represents a perfect symmetry between the activation energy curves with respect to oxidised and reduced species while α = 1.0 represents an electron transfer occurring without activation in the electric component of the total activation energy barrier [2,38]. According to Krishtalik [39], a value of α = 1.0 is not necessarily associated with a complete disappearance of the energy barrier (this process may be called quasibarrierless), and is predicted on the basis of the quantum mechanical theory of an elementary act. It was shown that certain reactions, particularly the ClER, are indeed quasibarrierless [5,40]. Generally, changes in α-value can be used to correlate the influence of the electrode material on the electron transfer process [41], so α can be correlated to changes in S Cl bond strength [5]. The experimental b-values, close to 30 mV, indicate an α-value around 0.8, independent on nominal electrode composition. This can be understood if we keep in mind that the active oxide (RuO2 ) content is constant at 30 mol% and it is shown that Cl2 evolution on RuO2 -based electrodes presents almost constant kinetic and activity down to 20% RuO2 [29]. 3.2.4. Electrocatalytic activity Fig. 11 shows a plot of the apparent, i, and true electrocatalytic activity, i/qa , measured at 1.25 V/SSCE as function of the nominal oxide composition. qa -values were extracted from CV between 0.0 and 1.0 V/SSCE in 5.0 M NaCl + 0.01 M HCl. Both i and i/qa versus composition plots show that the electrocatalytic activity is higher for low Nb2 O5 contents (high CeO2 contents). The electrodes employed for the ClER studies were the same employed for the OER investigations. After the wear imposed by the OER experiments, the qa -values increase with Nb2 O5 content (see Fig. 6). So, the behaviour of Fig. 11 indi-
For OER, both the Tafel coefficient and reaction order with respect to H+ show a dependence on composition and overpotential. The presence of CeO2 reduces the Tafel slope from 40 to 30 mV, while Nb2 O5 exerts no influence on the electrode kinetics. The highest global electrocatalytic activity observed for higher CeO2 contents is attributed to an electronic and geometric factors, while the true electrocatalytic activity depends on the electronic factor. For ClER, only a single rds, independent of overpotential interval and electrode composition, is observed. Experimental data support the electrode mechanism proposed by Erenburg et al., with an α-value close to unity, in agreement with some literature reports shows ClER can be characterised as a quasibarrierless reaction. Analysis of both global and true electrocatalytic activity show the main contribution comes from the electronic nature, confirming the ClER is little affected by the electrode morphology. The electrode stability is closely related to the composition. Comparison of the anodic voltammetric charges obtained before and after each set of experiments denounces significant electrode wear during the OER. The higher the CeO2 content, the less stable the coating, probably related to the coating fragility of the porous structure of CeO2 containing oxide mixture. On the other hand, introduction of Nb2 O5 causes a stabilisation of the oxide mixture due to a more compact layer. Acknowledgements M´ario H.P. Santana wishes to thank a M.Sc. Fellowship from the CAPES Foundation. Luiz A. De Faria wishes to thank financial support from the FAPEMIG/CNPq Foundations. Authors thank the helpful discussions provided by Julien F.C. Boodts. References [1] S. Trasatti, Electrochim. Acta 45 (2000) 2377. [2] S. Trasatti, G. Lodi, in: S. Trasatti (Ed.), Electrodes of Conductive Metallic Oxides, Part B, Elsevier, Amsterdam, 1981, p. 535. [3] S. Trasatti, Electrochim. Acta 36 (1991) 225. [4] J.L. Fern´andez, M.R.G. De Chialvo, A.C. Chialvo, Electrochim. Acta 47 (2002) 1145. [5] L.M. Da Silva, J.F.C. Boddts, L.A. De Faria, J. Braz. Chem. Soc. 14 (2003) 388. [6] L.M. Da Silva, L.A. De Faria, J.F.C. Boddts, Pure Appl. Chem. 73 (2001) 1871. [7] L. Tomcs´anyi, A. De Battisti, G. Hirschberg, K. Varga, J. Liszi, Electrochim. Acta 44 (1999) 2463. [8] V. Consonni, S. Trasatti, F. Pollak, W.E. O’Grady, J. Electroanal. Chem. 228 (1987) 393. [9] L.A. De Faria, J.F.C. Boodts, S. Trasatti, Electrochim. Acta 42 (1997) 3525. [10] T. Arikado, C. Iwakura, H. Tamura, Electrochim. Acta 23 (1978) 9. [11] V.A. Alves, L.A. Da Silva, J.F.C. Boodts, S. Trasatti, Electrochim. Acta 39 (1994) 1585.
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