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(Ti + Ru + Ce)O2-system for the oxygen evolution reaction in alkaline medium
Electrochimica Acta 51 (2006) 2809–2818
Surface, kinetics and electrocatalytic properties of the Ti/(Ti + Ru + Ce)O2-system for the oxygen evolution reaction in alkaline medium Karla C. Fernandes a , Leonardo M. Da Silva b,1 , Julien F.C. Boodts a,1 , Luiz A. De Faria a,∗ a
´ Instituto de Qu´ımica, Universidade Federal de Uberlˆandia, Campus Santa Mˆonica, Av. Jo˜ao Naves de Avila 2160, 38400-902 Uberlˆandia, MG, Brazil b Instituto de Qu´ımica, Universidade Estadual de Campinas, P.O. Box 6154, 13083-970 Campinas, SP, Brazil Received 1 March 2005; received in revised form 18 May 2005; accepted 30 July 2005 Available online 13 September 2005
Abstract Ti-supported (Ti + Ru + Ce)O2 electrodes, prepared at 450 ◦ C, were characterised by XRD, open-circuit potential (Eoc ), capacity data (C) and morphology factor (ϕ) determinations. XRD measurements showed mixed oxides present a low degree of crystallinity. Eoc -data and CV-spectra support surface electrochemistry of mixed oxides is governed by the Ru(III)/Ru(IV) redox couple. In situ surface characterisation revealed the active surface area increases on increasing nominal CeO2 -content. ϕ-Values remained in the 0.18–0.3 interval supporting the coatings have a low electrochemical porosity. Kinetics was studied recording polarisation and chronopotentiometric curves, which permitted to determine the Tafel slope and reaction order (with respect to OH− ), in the low and high overpotential domains. Tafel slope data, b, presented a dependence on overpotential and oxide composition indicating the OER electrode mechanism depends on these variables. A unit reaction order with respect to OH− was found for all electrode compositions investigated. The theoretical analysis of the electrode mechanism permitted to analyse the changes in the experimental Tafel slopes taking into account modifications in the apparent electronic transfer coefficient, αap . Analysis of the true and apparent electrocatalytic activities revealed the O2 -evolution reaction rate is affected by oxide composition due to morphologic effects. © 2005 Elsevier Ltd. All rights reserved. Keywords: Cerium; Morphology factor; Kinetics; Apparent electronic transfer coefficient; Electrocatalytic activity
1. Introduction Conductive metallic oxide electrodes, known as dimensionally stable anodes (DSA® ), are good electrocatalysts for electrode processes of technological importance, such as chlorine (ClER) and oxygen (OER) evolution reactions [1,2]. Normally, a DSA® -type electrode is constituted of a noble metal oxide (e.g. RuO2 , IrO2 ) stabilised in an inert oxide matrix (e.g. TiO2 , Ta2 O5 ) [1–5]. Since the design of the electrocatalysts using noble metals involves high costs, several ∗ 1
Corresponding author. Tel.: +55 34 32394143; fax: +55 34 32394208. E-mail address: [email protected] (L.A. De Faria). ISE member.
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.07.056
contributions have appeared in the literature reporting the electrochemical behaviour of different oxide electrodes for processes, such as OER and ClER [1,3,6–17]. The electrocatalytic activity of oxide electrodes for the OER can be improved controlling the electrode preparation parameters and selecting appropriate oxide components [18]. This is due to the fact both electrode stability and electrocatalytic properties are related to the relative position of the surface redox transitions of the active oxide component [3,6]. From an experimental point of view addition of a modulating oxide (e.g. SnO2 , CeO2 ) can be used to modify the redox level, thus permitting to modulate the electronic properties of the electrocatalyst [19,20]. As discussed in several contributions [6,7,14,18,20], CeO2 is a promising candidate to
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modulate the electrocatalytic properties of oxide electrodes due to its high Ce(III)/Ce(IV) redox potential. Surface properties, kinetics and electrocatalytic activity of Ti/(Ru + Ti + Ce)O2 -electrodes for the OER and ClER in acid medium were investigated by De Faria et al. [6,7,20]. These authors showed substitution of Ti by Ce in Ru + Ti mixed oxides changes both the surface properties (geometric effects) and electrocatalytic activity (electronic properties) for the O2 and Cl2 evolution reactions. A recent EIS study [18] showed that the electrocatalytic activity of Ti/(Ru + Ti + Ce)O2 electrodes for the OER in alkaline medium strongly depends on Ce-content and anodisation time. This paper reports the investigation of the influence of nominal cerium dioxide concentration [CeO2 ]N , on surface, kinetics and electrocatalytic properties of Ti/(Ti + Ru + Ce)O2 -electrodes for the OER in alkaline medium.
2. Experimental procedure 2.1. Electrodes Electrodes of nominal composition Ti/[Ru0.3 Ti(0.7 − x) Cex ]O2 , with 0 ≤ x ≤ 0.7, were prepared by thermal decomposition (T = 450 ◦ C) of RuCl3 ·nH2 O (Stream Chemicals), TiCl3 (Vetec) and CeCl3 ·7H2 O (Fluka) precursor solutions dissolved in HCl (1:1, v/v). Sandblasted Tisupports (5 mm × 10 mm × 0.12 mm) were degreased with isopropanol and etched for 5 min with boiling 10% (w/w) oxalic acid. Precursor mixtures were applied by brush to both faces of the Ti-support; the solvent evaporated at 80–90 ◦ C and the residue calcinated, in air, at 450 ◦ C for 10 min in a preheated oven. This procedure was repeated until the desired oxide load was obtained (2.4–2.8 mg cm−2 , depending on composition). A 1-h final annealing at the same temperature completed the treatment in all cases. Electrodes were mounted in a glass tube and sealed using silicone glue. In all cases the average nominal oxide coating thickness is 2 m. 2.2. Cell A three-compartment all-glass cell (0.2 dm3 ) was used throughout. Ohmic drop was minimised using a Luggin capillary approaching the working electrode from below, while two heavily platinised platinum counter electrodes ensured uniformity of the current on the two opposite faces of the sample. Electrode potential was read against a Hg/HgO, NaOH (1.0 mol dm−3 ) reference electrode, separated from the main body of the cell by a coarse glass frit. 2.3. Procedures and equipment 2.3.1. XRD Low angle X-ray diffraction spectra were recorded using Ti-supported films, prepared under the same conditions as the coatings described in Section 2.1, by means of a
Model D5005 Siemens instrument using Cu(K␣) radiation ˚ (λ = 1.54056 A). 2.3.2. Open circuit potential (Eoc ) The Eoc of freshly prepared electrodes was measured during 20 min after electrode immersion in the electrolyte. In all cases, a stationary Eoc -value was reached after ∼15 min. 2.3.3. Cyclic voltammetry Cyclic voltammograms were recorded at 20 mV s−1 covering the −0.6 to 0.5 V (versus Hg/HgO, NaOH (1.0 mol dm−3 )) pseudocapacitive potential interval [18]. “In situ” surface characterisation was carried out recording voltammetric curves at different sweep rates following the procedure proposed by Da Silva et al. [21]. To ensure stationary conditions, 20 consecutive voltammetric curves were recorded covering the −0.1 to 0 V potential interval. Total, CT , and external, CE , capacity values were obtained from the slopes of the two straight segments in the j versus ν plots, observed in the low and high ν-domains, respectively. To ensure the pseudocapacitive current obeys the j ∼ = Cd ν-relation, current data extracted from the E versus j profiles were measured at a potential close to Eλ,a (E = −0.02 V). Internal capacities, CI , were calculated using CI = CT − CE , while the morphology factor, ϕ, was calculated using next equation ϕ = CI /CT (for more details see Ref. [21]). 2.3.4. Quasi-stationary polarisation curves Using the potentiostatic mode, forwards and backwards polarisation curves were recorded twice sweeping the electrode potential, under quasi-stationary conditions (ν = 0.5 mV s−1 ), covering the potential window interval of 0.3–0.88 V, which corresponds to a current density interval of 0.1–100 mA cm−2 . Experimental data extracted from the backwards scan of the second consecutive polarisation curve were used in the kinetic study. 2.3.5. Reaction order Reaction order experiments with respect to OH− ion were carried out by systematic substitution of NaOH by NaClO4 keeping the ionic strength constant at 1.0 mol dm−3 . Chronoamperometric curves (j versus t) were recorded during 25 min at 0.55 V (low overpotential domain) and 0.75 V (high overpotential domain). These electrode potentials were selected because they fall in the linear current domain of the Tafel curves. Considering E-values are affected by ohmic drop, IR , especially in the high current domain, the distance between the WE and RE was kept constant at ∼2 mm in between the recording of the j versus t-curves. An AUTOLAB (Eco Chemie, The Netherlands) electrochemical system (GPES), model PGSTAT20, was used throughout.
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2.4. Solutions Electrochemical studies (CV and polarisation curves) were carried out at 25 ◦ C in 1.0 mol dm−3 NaOH (Merck) prepared volumetrically using Milli-Qplus quality water. Solutions were deaerated during experiments using a strong nitrogen flux (99.995% purity).
3. Results and discussion 3.1. XRD study Fig. 1 shows representative XRD diffractograms obtained from Ti-supported mixed oxides. The peak positions, referred to the 2θ-scale, are in agreement with Refs. [22,23] revealing substitution of titanium by cerium does not provoke significant distortions in the unitary cell. Such behaviour indicates solid solution formation between CeO2 and TiO2 + RuO2 is unlikely since in this case a systematic change of the lattice parameters of the matrix phase with mixture composition is expected. These findings indicate individual phases are dispersed in the oxide matrix phase which is in agreement with Ref. [22] reporting that cerium oxide prepared by thermal decomposition at 450 ◦ C, as thin coating or as powder, does not form solid solutions with the rutile phase (e.g. RuO2 and TiO2 ), acting solely as a
Fig. 1. Representative diffractograms [CeO2 ]N : (A) x = 0; (B) x = 0.4.
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dispersant matrix for this phase. So, mixed oxides constituted of (Ru + Ti + Ce)O2 are constituted by very small crystallites which is expected to lead to a strong increase of the electrochemically active surface area [18,23] (see Section 3.3). The XRD-spectra as function of nominal Ce-content show diffractograms are characterised by broad peaks presenting a low intensity and by differences in the intensity of diffraction peaks (see Fig. 1). According to the literature this behaviour, presented by thin coatings and powders constituted of mixed oxides prepared by thermal decomposition, originates from next factors [22–25]: (i) differences in the lattice parameters of oxides; (ii) oxygen concentration in the furnace; (iii) chemical nature of the precursor reagent mixture; (iv) calcination temperature; (v) difference in crystalline orientation. 3.2. Open-circuit potential and voltammetric curves The nature of the redox couple governing the surface electrochemical response of freshly prepared electrodes was investigated by Eoc measurements and voltammetric curves (E versus j) recorded covering the −0.6 to 0.5 V (versus Hg/HgO, NaOH (1.0 mol dm−3 )) pseudocapacitive potential interval [18]. Eoc -data show (see Fig. 2A) introduction of cerium into the binary RuO2 + TiO2
Fig. 2. Influence of [CeO2 ]N on the open circuit potential (A) and voltammetric profile (B). Electrolyte: 1.0 mol dm−3 NaOH. Ti/[Ru(0.3) Ti(0.7 − x) Cex ]O2 : (A) x = 0; (B) x = 0.6; (C) x = 0.7. T = 25 ◦ C.
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mixture causes a significant change in Eoc . The theoretical Eo -value of the Ru(III)/Ru(IV) redox couple in alkaline medium ([OH− ] = 1.0 mol dm−3 ) is 0.264 V (versus Hg/HgO, NaOH (1.0 mol dm−3 )) [26]. Theoretical Eo -values for the Ti(III)/Ti(IV) and Ce(III)/Ce(IV) redox couples in alkaline medium are −1.242 and 0.87 V (versus Hg/HgO, NaOH (1.0 mol dm−3 )), respectively, suggesting the Eoc of 0.08 V (versus Hg/HgO, NaOH (1.0 mol dm−3 )), obtained for the cerium containing oxide electrodes (see Fig. 2A), is controlled by the Ru(III)/Ru(IV) redox couple. Considering the Ru(IV)/Ru(III) surface redox transition behaves reversible, the surface activity ratio of this redox couple in the case of ternary electrodes is aRu(IV) /aRu(III) = 1.32 × 10−3 . Applying the same treatment to the Ti/(Ru + Ti)O2 electrode (Eoc = 0.13 V) one obtains aRu(IV) /aRu(III) = 9.12 × 10−3 . Fig. 2B shows representative voltammetric profiles for different oxide compositions. Substitution of only 5 mol.% TiO2 by CeO2 already leads to a significant increase in the voltammetric current density and is accompanied with the disappearance of the peak located at ∼0.4 V. While the band located at ∼0.15 V is attributed to the Ru(III)/Ru(IV) redox couple, the later peak is attributed to further oxidation of the Ru-sites to higher oxidation states [3,18,20]. As discussed in Ref. [20], the non-symmetric profile of the broad bands located in the −0.6 to −0.15 V interval, which are also present in acid medium, can be correlated with a slow irreversible Faradaic reduction occurring on the oxide surface. Although a small electrode instability associated with the apparent irreversibility of the surface solid-state redox transitions is denounced by the non-symmetric profile of the broad bands located in the more cathodic region [20,23], when integration of the anodic and cathodic branches of the voltammetric profile is limited to the more anodic potential interval, qa /qc -ratios ≥ 0.96 are obtained for all oxide compositions, revealing the oxide surface is stable for the conditions used in the present work and therefore cathodic dissolution of the oxides during cyclisation of the electrode potential comprises only a minor process. This behaviour is rather different form the one observed in acid medium where Ce(III) cathodic dissolution is significant. The disappearance of the peak at 0.4 V on introduction of CeO2 into the oxide composition is probably due to its displacement towards more anodic potentials. Fig. 3 shows the anodic voltammetric charge, qa , as function of nominal Ce-content. Fig. 3 shows replacement of Ti by Ce provokes an important increase in qa -values, which is an extensive parameter representative of the electrochemically active surface area [3]. Consistent with the XRD results, this behaviour can be attributed to the low interaction between the CeO2 (cubic) and RuO2 + TiO2 (rutile) phases. As a result the roughness factor of the cerium containing electrodes is increased. While in the 10–50 mol.% CeO2 interval qa -values are almost constant, minimum and maximum qa -values are presented by the electrodes containing 0 and 70 mol.% CeO2 , respectively. This result suggests CeO2 is a better dispersant than TiO2 , which
Fig. 3. Dependence of the anodic voltammetric charge, qa , on [CeO2 ]N .
can be attributed to the fact CeO2 crystallizes as a different structure (cubic) causing a higher dispersion of the RuO2 thus increasing the electrochemically active surface area. 3.3. Capacity data and morphology factor, ϕ Information about the contribution to the total active surface area of the different regions that constitute the porous/rugged electrode surface (inner and outer surfaces), can be obtained “in situ” by voltammetric measurements [27]. Applying the experimental procedure recently proposed in Refs. [21,28] it is possible to calculate the “electrochemical porosity” of the oxide surface that combined with CT permits to obtain a “finger print” of the coating. Fig. 4 shows the influence of [CeO2 ]N on the extensive (internal, CI , external, CE , and total, CT , differential capacities) and intensive, ϕ (=CI /CT ), surface parameters. Fig. 4A clearly shows the introduction into the binary (RuO2 + TiO2 ) oxide mixture of a small CeO2 -content significantly influences the microstructure of the coating (inner, CI , and outer, CE , surface areas). CT -values reveal substitution of Ti by Ce initially strongly increases the electrode surface roughness which on further increase of the CeO2 content slowly reaches a maximum for [CeO2 ]N = 70 mol.%. Fig. 4A also shows the main contribution to CT comes from the outer surface area described by CE . Fig. 4B shows the dependence of the morphology factor, ϕ, on [CeO2 ]N . Comparison of Fig. 4A and B clearly shows the strong increase in electrode surface roughness represented by CT is not accompanied by an increase in ϕ-values. Experimentally several CT and ϕ limiting cases can be found which can be attributed to different oxide topographies associated with the non-uniform distribution of the surface irregularities [21,27]. The behaviour shown in Fig. 4 is characterised by low ϕ-values (<0.3) combined with low CT -values (∼7 to 28 mF cm−2 ), thus indicating the inner surface regions (bottom of the pores, deep cracks, inter-granular contacts) present a low contribution to the total active electrode surface area.
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Fig. 5. Representative Tafel plot of the OER. Electrode: Ti/[Ru(0.3) Ti(0.3) Ce(0.4) ]O2 . Electrolyte: 1.0 mol dm−3 NaOH. T = 25 ◦ C.
Fig. 4. Surface parameters (CT , CE , CI ) (A) and morphology factor (ϕ) (B) as function of [CeO2 ]N . Electrolyte: 1.0 mol dm−3 NaOH. T = 25 ◦ C.
These results are consistent with the existence of a low number of irregularities resulting in a more closed structure (e.g. narrow pores, narrow cracks), causing the internal electrode surface area to be easily excluded from the electrochemical response [27]. 3.4. Kinetics: analysis of the polarisation curves To investigate the influence of [CeO2 ]N on the OER kinetics in alkaline medium, polarisation curves were recorded at 25 ◦ C under quasi-stationary conditions covering the ∼0.1 to 100 mA cm−2 current density interval. To ensure the hysteresis phenomenon, sometimes observed in the electrode response, is negligible each potential scan (backward/forward) was recorded twice without interruption. Fig. 5 shows a representative Tafel plot before (raw data) and after ohmic drop correction carried out using the treatment proposed by Shub and Reznik [29]. The influence of ohmic drop, IR , on the polarisation curve can be written as: E − IR = a + b log j
(1)
where R is the total ohmic resistance, E the electrode potential, a the constant and b is the Tafel slope. The optimum
R -value is obtained when, using as criterion the correlation coefficient, r, of the current data in the high overpotential domain, an r-value very close to unity is obtained [9]. Depending on the quality of the experimental data, a very good linearisation of the corrected Tafel curve can be obtained (see Fig. 5). All Tafel curves, after ohmic drop correction, presented r > 0.9998. For the several compositions investigated, R -values (=Rsolution + Rfilm ) in the 0.2–1.0 cm2 range were obtained. These values are in excellent agreement with R -data obtained for conductive oxide electrodes immersed in strong electrolytes [9,18]. After IR -correction, Tafel curves presented two linear segments, revealing the deviation from linearity observed in the experimental curve (raw data) is due to ohmic resistance combined with changes in the electrode kinetics (e.g. change in rate determining step and/or apparent electronic transfer coefficient [6,9,18]). Fig. 6 gathers Tafel slope data, b, determined in the low and high overpotential domains as function of [CeO2 ]N . Tafel data reveal the influence of [CeO2 ]N on OER kinetics strongly depends on the overpotential domain. This behaviour is in agreement with literature reports [1,5,6,9,10] on oxide electrodes containing noble metals (e.g. Ru and Ir) as the active component. Fig. 6 clearly shows Tafel slopes in the low overpotential domain, blow , are little affected by the [CeO2 ]N , presenting a slight decrease on increasing Cecontent. Values start around ∼50 mV for the Ti/(Ru + Ti)O2 electrode and reach a minimum of ∼35 mV for the Ti/(Ru + Ce)O2 -electrode. In the high overpotential domain Tafel slope data, bhigh , are strongly influenced, decreasing from 120 to 60 mV with increasing [CeO2 ]N . The same figure also shows that in the ∼15 mol.% < [CeO2 ]N ≤ 50 mol.% composition interval Tafel slopes are practically constant (∼100 mV), revealing electrode kinetics are independent on Ce-content over this rather large composition interval. Interestingly, for these electrode compositions the surface parameters (qa and ϕ) remain almost constant with changing Ce-
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Fig. 6. Dependence of Tafel slope on [CeO2 ]N in the low, blow , and high, bhigh , overpotential domains. Electrolyte: 1.0 mol dm−3 NaOH. T = 25 ◦ C.
content, supporting OER kinetics, outside this composition span, is influenced by electrode morphology. Literature reports show the Tafel slope is very sensitive to the physico-chemical nature of the electrode/electrolyte interface [1], being affected by factors, such as: (i) oxide composition; (ii) overpotential domain; (iii) electrode preparation conditions; (iv) electrolyte composition [1,3,9,20,27]. Experimental Tafel slope (bexp ) frequently differ from the theoretical value (bt ) obtained from the kinetic analysis of different electrode mechanisms, especially for gas evolving reactions at rugged/porous electrodes [17,27,30]. The behaviour associated with bexp = bt can be understood in the light of the theoretical analysis based on the concept of the apparent (effective) electronic transfer coefficient [17,27]. It should be emphasised this analysis is only applicable for Tafel curves already corrected for ohmic drop and presenting high quality data (linear segment(s) characterised by an excellent correlation coefficient (r ≥ 0.9998) of the data—see Section 3.6.1). Darowicki and Orlikowski [31] presented similar theoretical approach using the concept of the effective electronic transfer coefficient for the OER on composite electrodes. 3.5. Reaction order with respect to OH− , ζOH− Since different Tafel slopes were reported for the OER at Ru-based oxide electrodes in alkaline medium [1,10], reaction order determination provides useful information to elucidate the electrode mechanism. The reaction order with respect to OH− , ζOH− , was determined in the both low and high overpotential domains as function of nominal oxide composition. To be significant reaction orders must be determined at constant ionic strength to avoid diffuse double layer effects [32]. To accomplish this, NaOH concentration was varied between 0.01 and 0.1 mol dm−3 keeping the ionic strength constant at 1.0 mol dm−3 by systematic substitution of OH− by ClO4 − ions.
Fig. 7. (A) Dependence of current density, j, on pOH (E = 0.55 V). Ti/[Ru(0.3) Ti(0.7 − x) Cex ]O2 : (1) x = 0.05; (2) x = 0.4. (B) Influence of [CeO2 ]N on reaction order with respect to OH− , ζOH− . Electrolyte: [NaClO4 ] + [NaOH] = 1.0 mol dm−3 .
Keeping constant the electrode potential and considering negligible changes in water activity [10], the reaction order with respect to OH− , ζOH− , for conditions of the high field approximation, η ≥ 0.1 V, was determined using next equation: ∂ log j ζOH− = − (2) ∂pOH E,T where pOH = −log[OH− ]. Fig. 7 gathers the experimental results obtained in the low (0.55 V), (ζOH− )low , and high (0.75 V), (ζOH− )high , overpotential domains. Fig. 7A clearly shows the linear segments in the log j versus pOH-plots are parallel, revealing ζOH− -values are independent on [CeO2 ]N . Fig. 7B presents the ζOH− -data as function of [CeO2 ]N showing the data are randomly scattered around unity on changing [CeO2 ]N (ζOH− ∼ = 1 in all cases). Although for the system investigated experimental ζOH− -values are unit, it is worthwhile to mention that kinetic complications arising from the complex microstructure of the oxide/electrolyte interface, in association with the acid–base properties of oxide electrodes, sometimes affect the elec-
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trode process resulting in non-integer (fractional) ζ-values [3,6,9,10,23].
Tafel coefficient of 40 mV is theoretically predicted in case step (II) is r.d.s.
3.6. Theoretical analysis of the OER reaction mechanism in alkaline medium
3.6.1. Interpretation of the experimental Tafel slopes based on the analysis of the apparent electronic transfer coefficient Confronting our experimental Tafel slopes (see Fig. 6) with the theoretical values predicted by the analysis presented in the previous section, a discrepancy is observed for the vast majority of the experimental data. As recently discussed by Da Silva et al. [17,27,30], this behaviour can be understood considering the true (absolute) electronic transfer coefficient of 0.5 is only observed for an electron transfer process at a flat electrode surface in the absence of the adsorption effects. The experimental Tafel slope for electrode processes occurring at solid electrodes is affected by a combination of geometrical and chemical effects, thus resulting in an apparent (effective) electronic transfer coefficient, αap , different from 0.5 [34,36–39]. Using the concept of the apparent electronic transfer coefficient, steps (II) and (I) can be used to calculate αap -values for the low, blow , and high, bhigh , overpotential domains as function of [CeO2 ]N . So, the influence of [CeO2 ]N on OER kinetics can be analysed in the light of αap , a parameter which describes the effectiveness of the applied overpotential on the reduction of the energy barrier for electron transfer and the apparent heat of activation. However, as already mentioned in Section 3.4, a kinetic study based on this approach is only possible when the scatter of the experimental kinetic data is negligible, or-be-it, if high quality experimental data are available. The αap -values with respect to steps (I) and (II) were calculated from the experimental b-values using Eqs. (3) and (4), respectively. Fig. 8 shows the dependence of αap in the low, αap-low , and high, αap-high , overpotential domains on [CeO2 ]N .
Different electrode mechanisms have been proposed for the OER in alkaline medium [1,10]. Considering our experimental b-values, reaction order data and the theoretical treatment based on the effective electronic transfer coefficient (see Section 3.6.1), the next mechanism proposed by Yeager [33] adequately describes the OER in all cases. k1
SZ + OH(aq) − ←→ ≡ S Z –OH + e− kII
SZ − OH←→ ≡ S Z+1 –OH + e− kIII
2 SZ+1 –OH− + 2OH− ←→2 SZ + 2H2 O + O2 ↑
(I) (II) (III)
where SZ represents an active surface site. As pointed out by Krishtalik [34], the individual steps of an electrochemical reaction behave as a serial resistor association of an electrical circuit. In the light of this proposal the step of the electrode mechanism presenting a minimum conductance governs the overall electrode process. Considering step (I) of the electrode mechanism as the r.d.s., and applying the kinetic treatment proposed by B¨ockris [35], for conditions of the high field approximation, η ≥ 0.1 V, and considering that the forward (kI ) and the backward (k−I ) process are close enough to zero, next relation is obtained for the reaction rate: α1 Fη j = 2FAkI [OH− ] exp (3) RT where kI is the forward rate constant for step (I), A the electrode surface area, [OH− ] the concentration of the hydroxyl ion, α1 the electronic transfer coefficient with respect to step (I) and η = E − Eo is the anodic overpotential. The other symbols have their usual meaning. Eq. (3) predicts a unit reaction order with respect to OH− ion. After rearranging, using the definition b ≡ (∂η/∂log j)T , the Tafel slope associated with step (I), bI , is given by 2.303RT/α1 F. So, considering α1 = 0.5 and T = 25 ◦ C a 120 mV Tafel slope is theoretically predicted if step (I) is the r.d.s. On the other hand, considering step (II) as r.d.s. of the overall reaction mechanism, and considering quasi-equilibrium conditions for step (I) [35], the reaction rate is described by next equation: (1 + α2 )Fη − j = 2FAkII [OH ] exp (4) RT where kII is the forward rate constant for step (II) and α2 is the electronic transfer coefficient with respect to step (II). Eq. (4) predicts an unit reaction order with respect to OH− and the Tafel slope associated with step (II), bII , is given by 2.302RT/(1 + α2 )F. Considering α2 = 0.5 and T = 25 ◦ C, a
Fig. 8. Dependence of the apparent electronic transfer coefficient, αap , calculated for the low, αap-low , and high, αap-high , overpotential domains on [CeO2 ]N . Electrolyte: 1.0 mol dm−3 NaOH. T = 25 ◦ C.
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The change in αap -values is consistent with the literature dealing with the OER at conductive oxide electrodes [17,27,30,39] and other materials [31] that report αap -values are affected by overpotential, electrode composition and electrolyte nature. In the light of these considerations, the results of the kinetic analysis presented in this work (see Fig. 8) strongly support Yeager’s electrode mechanism adequately describes the OER at Ti(Ru + Ce + Ti)O2 -electrodes. The changes in αap-low values can be attributed to modifications in the energy barrier of the electron transfer due to changes in electronic properties of the activated surface complex formed during oxidation of the active surface site, represented by next reaction: SZ –OH → SZ + 1 –OH + e− . In the case of the high overpotential domain, modifications in αap-high values are associated with changes in the electronic interaction between the acceptors orbitals located in the active surface site ( SZ ) and the donating orbitals of the OH− -groups. As a result the electric component of the effective energy barrier tends to decrease on increasing αap -values, thus decreasing the Tafel slope in both low and high overpotential domain on increasing nominal Ce-content (see Fig. 6). In the low overpotential domain, αap -values increase with increasing CeO2 -content starting at ∼0.1 reaching the maximum value of ∼0.6 for [CeO2 ]N = 70 mol.% (see Fig. 8). These results suggest the nominal oxide composition (albeit the chemical environment sensed by the active surface sites) affects the magnitude of the activation barrier with respect to step (II) as a consequence of changes in the SZ –OH bond strength. From a theoretical point of view this change can find its origin in several effects: (i) change in the [Ru]/[Ce] surface concentration ratio with changing composition; (ii) interaction at the electronic level (synergetic effects) between the surface Ce and Ru-sites; (iii) influence of the oxide morphology on the distribution of the surface electric filed causing a modification in the nature of the active sites (edge effect). Although it is very difficult to separate the contribution to the change in αap , of each of the above-mentioned effects, a hint can be obtained from the analysis of Figs. 3, 4 and 8. These figures show the main changes in αap -values are accompanied by a concomitant modification (increase) in the active surface area. Besides, the maximum ␣ap -value is observed for the binary Ti/[Ru(0.3) Ce(0.7) ]O2 electrode which presents the maximum CT -value. These results suggest a significant contribution to the change in αap can be attributed to the dependence of the edge effect on the oxide morphology (effect iii), which leads to modification in surface properties during OER due to the non-uniform distribution of the surface electric field because of surface roughness [37]. In the high overpotential domain and CeO2 -contents in the 0–50 mol.% interval αap -values present much less fluctuation (0.48–0.65), suggesting the influence of the above-mentioned effects is less pronounced in the case of high η-values and only exerts a weak influence on the oxidation of the surface OH− groups. However, for [CeO2 ]N ≥ 60 mol.% αap values increase significantly reaching a maximum of 0.98
for the Ti/[Ru(0.3) Ce(0.7) ]O2 suggesting a high CeO2 -content, strongly promotes a decrease in the effective energy barrier for electron transfer under conditions of the high electric field. 3.7. Electrocatalytic activity for the OER Electrocatalytic activity depends on both electronic and geometric factors [3]. Since the voltammetric charge is a function of the oxide composition (see Fig. 3), true electrocatalytic effects can be found normalising the Faradaic current by the voltammetric charge [3,9,17,18]. Fig. 9 shows a plot of the apparent, j, and true, j/qa , electrocatalytic activities measured at 0.62 V as function of the [CeO2 ]N . qa -Values were extracted from Fig. 3 and j-values were obtained from Tafel plots corrected for ohmic drop. Fig. 9 clearly shows nominal oxide composition affects both the apparent and true electrocatalytic activities for the OER. Analysis of Fig. 9A reveals the apparent (effective) electrocatalytic activity is constant for electrode compositions in the 0 mol.% < [CeO2 ]N < 50 mol.% interval composition. However, in agreement with the discussion presented in the previous section where it was found that a high CeO2 -content leads to a decrease in the apparent (effective) energy barrier for electron transfer, the increase in j-values on increasing Ce-content observed for [CeO2 ]N ≥ 50 mol.%
Fig. 9. Dependence of the current density, j, (A) and normalised current density, j/qa , (B) on [CeO2 ]N . j-Values were extracted from Tafel plots, corrected for IR , at 0.62 V. qa -Data were extracted from Fig. 3.
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reveals the maximum performance for the OER in alkaline medium is presented by the oxide composition containing Ti/[Ru(0.3) Ce(0.7) ]O2 . Fig. 9B clearly shows the true electrocatalytic activity is maximum for the Ti/[Ru(0.3) Ti(0.7) ]O2 composition, leading to the conclusion the increase observed in the apparent electrocatalytic activity for mixed oxides containing cerium (see Fig. 9A) is mainly due to the influence of the morphological effect (high degree of roughness and/or porosity). The fact CeO2 depresses the “true electrocatalytic activity” for the OER in alkaline medium, when introduced in the binary Ru0.3 Ti0.7 O2 composition, can be attributed to the inferior electrocatalytic activity presented by the superficial Ru-sites attached at ternary oxide particles containing Ce.
4. Conclusions XRD study shows replacement of Ti by Ce does not change the lattice parameters, which is consistent with the low probability of interaction between the different oxide phases (rutile and cubic structures). As a result, CeO2 acts as a dispersant and causes a strong increase in the oxide surface roughness on increasing Ce-content. “In situ” studies of the conductive metallic oxide electrodes containing cerium, ruthenium and titanium reveals the replacement of TiO2 with CeO2 , keeping the RuO2 constant at 30 mol.%, changes the electrode morphology and surface properties, increasing the electrode surface area and decreasing the open circuit potential. Surface characterisation revealed electrochemical surface porosity is little affected by oxide composition. The kinetic study of the OER revealed both oxide composition and overpotential domain affect the electrode process. Introduction of cerium leads to a reduction in the Tafel slope values in the high overpotential domain, which presents the minimum value for the nominal oxide composition containing 70 mol.% CeO2 . Experimental Tafel slope and reaction order data with respect to hydroxyl anion, combined with the theoretical analysis based on the concept of the apparent electronic transfer coefficient, support the electrode mechanism proposed by Yeager adequately describes the OER in alkaline medium. Analysis of the apparent electrocatalytic activity revealed the electrode performance for the OER considerable increases on increasing the nominal CeO2 content of the oxide mixture, contrary to the true electrocatalytic activity (in the absence of morphologic effects) which is maximum for the Ru(0.3) Ti(0.7) O2 electrode. Rationalisation of these findings permits to propose the superior performance presented by oxide electrodes containing cerium is due to the low interaction between the individual oxide components possessing different crystallographic structures, which leads to a strong increase in the electrochemically active surface area. The fact that the presence of CeO2 depresses the “true electrocatalytic activity” can be attributed to the inferior electrocat-
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alytic activity presented by the superficial Ru-sites attached at ternary oxide particles.
Acknowledgements K.C. Fernandes wishes to thank a M.Sc. fellowship received from the CAPES Foundation. L.M. Da Silva wishes to thank a Post-Doctoral fellowship granted by the FAPESP Foundation. J.F.C. Boodts acknowledges a Visiting Researcher Fellowship granted by the FAPEMIG Foundation. L.A. De Faria acknowledges the CAPES and FAPEMIG Foundations.
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[28] S. Ardizzone, G. Fregonara, S. Trasatti, Electrochim. Acta 35 (1990) 263. ´ [29] D.M. Shub, M.F. Reznik, Elektrokhimiya 21 (1985) 855. [30] L.M. Da Silva, L.A. De Faria, J.F.C. Boodts, Electrochim. Acta 48 (2003) 699. [31] K. Darowicki, J. Orlikowski, J. Electrochem. Soc. 146 (1999) 663. [32] V. Consonni, S. Trasatti, F. Pollak, W.E. O’Grady, J. Electroanal. Chem. 228 (1987) 393. [33] E. Yeager, in: A.D. Franklin (Ed.), Electrocatalysis on Nonmetallic Surfaces, U.S. Government Printing Office, Washington D.C, 1976.
[34] L.I. Krishtalik, in: J.O’M. B¨ockris, B.E. Conway, E. Yeager, R.E. White (Eds.), Comprehensive Treatise of Electrochemistry, vol. 7, Plenum Press, 1983. [35] J.O’M. B¨ockris, J. Chem. Phys. 24 (1956) 817. [36] P. R¨uetschi, J. Electrochem. Soc. 106 (1959) 819. [37] M. Filoche, B. Sapoval, Electrochim. Acta 46 (2000) 213. [38] W.H. Mulder, J.H. Sluyters, L. Nyklos, J. Electroanal. Chem. 285 (1990) 103. [39] C.N. Ho, B.J. Hwang, J. Electroanal. Chem. 377 (1994) 177.
Surface, kinetics and electrocatalytic properties of the Ti/(Ti + Ru + Ce)O2-system for the oxygen evolution reaction in alkaline medium Karla C. Fernandes a , Leonardo M. Da Silva b,1 , Julien F.C. Boodts a,1 , Luiz A. De Faria a,∗ a
´ Instituto de Qu´ımica, Universidade Federal de Uberlˆandia, Campus Santa Mˆonica, Av. Jo˜ao Naves de Avila 2160, 38400-902 Uberlˆandia, MG, Brazil b Instituto de Qu´ımica, Universidade Estadual de Campinas, P.O. Box 6154, 13083-970 Campinas, SP, Brazil Received 1 March 2005; received in revised form 18 May 2005; accepted 30 July 2005 Available online 13 September 2005
Abstract Ti-supported (Ti + Ru + Ce)O2 electrodes, prepared at 450 ◦ C, were characterised by XRD, open-circuit potential (Eoc ), capacity data (C) and morphology factor (ϕ) determinations. XRD measurements showed mixed oxides present a low degree of crystallinity. Eoc -data and CV-spectra support surface electrochemistry of mixed oxides is governed by the Ru(III)/Ru(IV) redox couple. In situ surface characterisation revealed the active surface area increases on increasing nominal CeO2 -content. ϕ-Values remained in the 0.18–0.3 interval supporting the coatings have a low electrochemical porosity. Kinetics was studied recording polarisation and chronopotentiometric curves, which permitted to determine the Tafel slope and reaction order (with respect to OH− ), in the low and high overpotential domains. Tafel slope data, b, presented a dependence on overpotential and oxide composition indicating the OER electrode mechanism depends on these variables. A unit reaction order with respect to OH− was found for all electrode compositions investigated. The theoretical analysis of the electrode mechanism permitted to analyse the changes in the experimental Tafel slopes taking into account modifications in the apparent electronic transfer coefficient, αap . Analysis of the true and apparent electrocatalytic activities revealed the O2 -evolution reaction rate is affected by oxide composition due to morphologic effects. © 2005 Elsevier Ltd. All rights reserved. Keywords: Cerium; Morphology factor; Kinetics; Apparent electronic transfer coefficient; Electrocatalytic activity
1. Introduction Conductive metallic oxide electrodes, known as dimensionally stable anodes (DSA® ), are good electrocatalysts for electrode processes of technological importance, such as chlorine (ClER) and oxygen (OER) evolution reactions [1,2]. Normally, a DSA® -type electrode is constituted of a noble metal oxide (e.g. RuO2 , IrO2 ) stabilised in an inert oxide matrix (e.g. TiO2 , Ta2 O5 ) [1–5]. Since the design of the electrocatalysts using noble metals involves high costs, several ∗ 1
Corresponding author. Tel.: +55 34 32394143; fax: +55 34 32394208. E-mail address: [email protected] (L.A. De Faria). ISE member.
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.07.056
contributions have appeared in the literature reporting the electrochemical behaviour of different oxide electrodes for processes, such as OER and ClER [1,3,6–17]. The electrocatalytic activity of oxide electrodes for the OER can be improved controlling the electrode preparation parameters and selecting appropriate oxide components [18]. This is due to the fact both electrode stability and electrocatalytic properties are related to the relative position of the surface redox transitions of the active oxide component [3,6]. From an experimental point of view addition of a modulating oxide (e.g. SnO2 , CeO2 ) can be used to modify the redox level, thus permitting to modulate the electronic properties of the electrocatalyst [19,20]. As discussed in several contributions [6,7,14,18,20], CeO2 is a promising candidate to
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modulate the electrocatalytic properties of oxide electrodes due to its high Ce(III)/Ce(IV) redox potential. Surface properties, kinetics and electrocatalytic activity of Ti/(Ru + Ti + Ce)O2 -electrodes for the OER and ClER in acid medium were investigated by De Faria et al. [6,7,20]. These authors showed substitution of Ti by Ce in Ru + Ti mixed oxides changes both the surface properties (geometric effects) and electrocatalytic activity (electronic properties) for the O2 and Cl2 evolution reactions. A recent EIS study [18] showed that the electrocatalytic activity of Ti/(Ru + Ti + Ce)O2 electrodes for the OER in alkaline medium strongly depends on Ce-content and anodisation time. This paper reports the investigation of the influence of nominal cerium dioxide concentration [CeO2 ]N , on surface, kinetics and electrocatalytic properties of Ti/(Ti + Ru + Ce)O2 -electrodes for the OER in alkaline medium.
2. Experimental procedure 2.1. Electrodes Electrodes of nominal composition Ti/[Ru0.3 Ti(0.7 − x) Cex ]O2 , with 0 ≤ x ≤ 0.7, were prepared by thermal decomposition (T = 450 ◦ C) of RuCl3 ·nH2 O (Stream Chemicals), TiCl3 (Vetec) and CeCl3 ·7H2 O (Fluka) precursor solutions dissolved in HCl (1:1, v/v). Sandblasted Tisupports (5 mm × 10 mm × 0.12 mm) were degreased with isopropanol and etched for 5 min with boiling 10% (w/w) oxalic acid. Precursor mixtures were applied by brush to both faces of the Ti-support; the solvent evaporated at 80–90 ◦ C and the residue calcinated, in air, at 450 ◦ C for 10 min in a preheated oven. This procedure was repeated until the desired oxide load was obtained (2.4–2.8 mg cm−2 , depending on composition). A 1-h final annealing at the same temperature completed the treatment in all cases. Electrodes were mounted in a glass tube and sealed using silicone glue. In all cases the average nominal oxide coating thickness is 2 m. 2.2. Cell A three-compartment all-glass cell (0.2 dm3 ) was used throughout. Ohmic drop was minimised using a Luggin capillary approaching the working electrode from below, while two heavily platinised platinum counter electrodes ensured uniformity of the current on the two opposite faces of the sample. Electrode potential was read against a Hg/HgO, NaOH (1.0 mol dm−3 ) reference electrode, separated from the main body of the cell by a coarse glass frit. 2.3. Procedures and equipment 2.3.1. XRD Low angle X-ray diffraction spectra were recorded using Ti-supported films, prepared under the same conditions as the coatings described in Section 2.1, by means of a
Model D5005 Siemens instrument using Cu(K␣) radiation ˚ (λ = 1.54056 A). 2.3.2. Open circuit potential (Eoc ) The Eoc of freshly prepared electrodes was measured during 20 min after electrode immersion in the electrolyte. In all cases, a stationary Eoc -value was reached after ∼15 min. 2.3.3. Cyclic voltammetry Cyclic voltammograms were recorded at 20 mV s−1 covering the −0.6 to 0.5 V (versus Hg/HgO, NaOH (1.0 mol dm−3 )) pseudocapacitive potential interval [18]. “In situ” surface characterisation was carried out recording voltammetric curves at different sweep rates following the procedure proposed by Da Silva et al. [21]. To ensure stationary conditions, 20 consecutive voltammetric curves were recorded covering the −0.1 to 0 V potential interval. Total, CT , and external, CE , capacity values were obtained from the slopes of the two straight segments in the j versus ν plots, observed in the low and high ν-domains, respectively. To ensure the pseudocapacitive current obeys the j ∼ = Cd ν-relation, current data extracted from the E versus j profiles were measured at a potential close to Eλ,a (E = −0.02 V). Internal capacities, CI , were calculated using CI = CT − CE , while the morphology factor, ϕ, was calculated using next equation ϕ = CI /CT (for more details see Ref. [21]). 2.3.4. Quasi-stationary polarisation curves Using the potentiostatic mode, forwards and backwards polarisation curves were recorded twice sweeping the electrode potential, under quasi-stationary conditions (ν = 0.5 mV s−1 ), covering the potential window interval of 0.3–0.88 V, which corresponds to a current density interval of 0.1–100 mA cm−2 . Experimental data extracted from the backwards scan of the second consecutive polarisation curve were used in the kinetic study. 2.3.5. Reaction order Reaction order experiments with respect to OH− ion were carried out by systematic substitution of NaOH by NaClO4 keeping the ionic strength constant at 1.0 mol dm−3 . Chronoamperometric curves (j versus t) were recorded during 25 min at 0.55 V (low overpotential domain) and 0.75 V (high overpotential domain). These electrode potentials were selected because they fall in the linear current domain of the Tafel curves. Considering E-values are affected by ohmic drop, IR , especially in the high current domain, the distance between the WE and RE was kept constant at ∼2 mm in between the recording of the j versus t-curves. An AUTOLAB (Eco Chemie, The Netherlands) electrochemical system (GPES), model PGSTAT20, was used throughout.
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2.4. Solutions Electrochemical studies (CV and polarisation curves) were carried out at 25 ◦ C in 1.0 mol dm−3 NaOH (Merck) prepared volumetrically using Milli-Qplus quality water. Solutions were deaerated during experiments using a strong nitrogen flux (99.995% purity).
3. Results and discussion 3.1. XRD study Fig. 1 shows representative XRD diffractograms obtained from Ti-supported mixed oxides. The peak positions, referred to the 2θ-scale, are in agreement with Refs. [22,23] revealing substitution of titanium by cerium does not provoke significant distortions in the unitary cell. Such behaviour indicates solid solution formation between CeO2 and TiO2 + RuO2 is unlikely since in this case a systematic change of the lattice parameters of the matrix phase with mixture composition is expected. These findings indicate individual phases are dispersed in the oxide matrix phase which is in agreement with Ref. [22] reporting that cerium oxide prepared by thermal decomposition at 450 ◦ C, as thin coating or as powder, does not form solid solutions with the rutile phase (e.g. RuO2 and TiO2 ), acting solely as a
Fig. 1. Representative diffractograms [CeO2 ]N : (A) x = 0; (B) x = 0.4.
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dispersant matrix for this phase. So, mixed oxides constituted of (Ru + Ti + Ce)O2 are constituted by very small crystallites which is expected to lead to a strong increase of the electrochemically active surface area [18,23] (see Section 3.3). The XRD-spectra as function of nominal Ce-content show diffractograms are characterised by broad peaks presenting a low intensity and by differences in the intensity of diffraction peaks (see Fig. 1). According to the literature this behaviour, presented by thin coatings and powders constituted of mixed oxides prepared by thermal decomposition, originates from next factors [22–25]: (i) differences in the lattice parameters of oxides; (ii) oxygen concentration in the furnace; (iii) chemical nature of the precursor reagent mixture; (iv) calcination temperature; (v) difference in crystalline orientation. 3.2. Open-circuit potential and voltammetric curves The nature of the redox couple governing the surface electrochemical response of freshly prepared electrodes was investigated by Eoc measurements and voltammetric curves (E versus j) recorded covering the −0.6 to 0.5 V (versus Hg/HgO, NaOH (1.0 mol dm−3 )) pseudocapacitive potential interval [18]. Eoc -data show (see Fig. 2A) introduction of cerium into the binary RuO2 + TiO2
Fig. 2. Influence of [CeO2 ]N on the open circuit potential (A) and voltammetric profile (B). Electrolyte: 1.0 mol dm−3 NaOH. Ti/[Ru(0.3) Ti(0.7 − x) Cex ]O2 : (A) x = 0; (B) x = 0.6; (C) x = 0.7. T = 25 ◦ C.
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mixture causes a significant change in Eoc . The theoretical Eo -value of the Ru(III)/Ru(IV) redox couple in alkaline medium ([OH− ] = 1.0 mol dm−3 ) is 0.264 V (versus Hg/HgO, NaOH (1.0 mol dm−3 )) [26]. Theoretical Eo -values for the Ti(III)/Ti(IV) and Ce(III)/Ce(IV) redox couples in alkaline medium are −1.242 and 0.87 V (versus Hg/HgO, NaOH (1.0 mol dm−3 )), respectively, suggesting the Eoc of 0.08 V (versus Hg/HgO, NaOH (1.0 mol dm−3 )), obtained for the cerium containing oxide electrodes (see Fig. 2A), is controlled by the Ru(III)/Ru(IV) redox couple. Considering the Ru(IV)/Ru(III) surface redox transition behaves reversible, the surface activity ratio of this redox couple in the case of ternary electrodes is aRu(IV) /aRu(III) = 1.32 × 10−3 . Applying the same treatment to the Ti/(Ru + Ti)O2 electrode (Eoc = 0.13 V) one obtains aRu(IV) /aRu(III) = 9.12 × 10−3 . Fig. 2B shows representative voltammetric profiles for different oxide compositions. Substitution of only 5 mol.% TiO2 by CeO2 already leads to a significant increase in the voltammetric current density and is accompanied with the disappearance of the peak located at ∼0.4 V. While the band located at ∼0.15 V is attributed to the Ru(III)/Ru(IV) redox couple, the later peak is attributed to further oxidation of the Ru-sites to higher oxidation states [3,18,20]. As discussed in Ref. [20], the non-symmetric profile of the broad bands located in the −0.6 to −0.15 V interval, which are also present in acid medium, can be correlated with a slow irreversible Faradaic reduction occurring on the oxide surface. Although a small electrode instability associated with the apparent irreversibility of the surface solid-state redox transitions is denounced by the non-symmetric profile of the broad bands located in the more cathodic region [20,23], when integration of the anodic and cathodic branches of the voltammetric profile is limited to the more anodic potential interval, qa /qc -ratios ≥ 0.96 are obtained for all oxide compositions, revealing the oxide surface is stable for the conditions used in the present work and therefore cathodic dissolution of the oxides during cyclisation of the electrode potential comprises only a minor process. This behaviour is rather different form the one observed in acid medium where Ce(III) cathodic dissolution is significant. The disappearance of the peak at 0.4 V on introduction of CeO2 into the oxide composition is probably due to its displacement towards more anodic potentials. Fig. 3 shows the anodic voltammetric charge, qa , as function of nominal Ce-content. Fig. 3 shows replacement of Ti by Ce provokes an important increase in qa -values, which is an extensive parameter representative of the electrochemically active surface area [3]. Consistent with the XRD results, this behaviour can be attributed to the low interaction between the CeO2 (cubic) and RuO2 + TiO2 (rutile) phases. As a result the roughness factor of the cerium containing electrodes is increased. While in the 10–50 mol.% CeO2 interval qa -values are almost constant, minimum and maximum qa -values are presented by the electrodes containing 0 and 70 mol.% CeO2 , respectively. This result suggests CeO2 is a better dispersant than TiO2 , which
Fig. 3. Dependence of the anodic voltammetric charge, qa , on [CeO2 ]N .
can be attributed to the fact CeO2 crystallizes as a different structure (cubic) causing a higher dispersion of the RuO2 thus increasing the electrochemically active surface area. 3.3. Capacity data and morphology factor, ϕ Information about the contribution to the total active surface area of the different regions that constitute the porous/rugged electrode surface (inner and outer surfaces), can be obtained “in situ” by voltammetric measurements [27]. Applying the experimental procedure recently proposed in Refs. [21,28] it is possible to calculate the “electrochemical porosity” of the oxide surface that combined with CT permits to obtain a “finger print” of the coating. Fig. 4 shows the influence of [CeO2 ]N on the extensive (internal, CI , external, CE , and total, CT , differential capacities) and intensive, ϕ (=CI /CT ), surface parameters. Fig. 4A clearly shows the introduction into the binary (RuO2 + TiO2 ) oxide mixture of a small CeO2 -content significantly influences the microstructure of the coating (inner, CI , and outer, CE , surface areas). CT -values reveal substitution of Ti by Ce initially strongly increases the electrode surface roughness which on further increase of the CeO2 content slowly reaches a maximum for [CeO2 ]N = 70 mol.%. Fig. 4A also shows the main contribution to CT comes from the outer surface area described by CE . Fig. 4B shows the dependence of the morphology factor, ϕ, on [CeO2 ]N . Comparison of Fig. 4A and B clearly shows the strong increase in electrode surface roughness represented by CT is not accompanied by an increase in ϕ-values. Experimentally several CT and ϕ limiting cases can be found which can be attributed to different oxide topographies associated with the non-uniform distribution of the surface irregularities [21,27]. The behaviour shown in Fig. 4 is characterised by low ϕ-values (<0.3) combined with low CT -values (∼7 to 28 mF cm−2 ), thus indicating the inner surface regions (bottom of the pores, deep cracks, inter-granular contacts) present a low contribution to the total active electrode surface area.
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Fig. 5. Representative Tafel plot of the OER. Electrode: Ti/[Ru(0.3) Ti(0.3) Ce(0.4) ]O2 . Electrolyte: 1.0 mol dm−3 NaOH. T = 25 ◦ C.
Fig. 4. Surface parameters (CT , CE , CI ) (A) and morphology factor (ϕ) (B) as function of [CeO2 ]N . Electrolyte: 1.0 mol dm−3 NaOH. T = 25 ◦ C.
These results are consistent with the existence of a low number of irregularities resulting in a more closed structure (e.g. narrow pores, narrow cracks), causing the internal electrode surface area to be easily excluded from the electrochemical response [27]. 3.4. Kinetics: analysis of the polarisation curves To investigate the influence of [CeO2 ]N on the OER kinetics in alkaline medium, polarisation curves were recorded at 25 ◦ C under quasi-stationary conditions covering the ∼0.1 to 100 mA cm−2 current density interval. To ensure the hysteresis phenomenon, sometimes observed in the electrode response, is negligible each potential scan (backward/forward) was recorded twice without interruption. Fig. 5 shows a representative Tafel plot before (raw data) and after ohmic drop correction carried out using the treatment proposed by Shub and Reznik [29]. The influence of ohmic drop, IR , on the polarisation curve can be written as: E − IR = a + b log j
(1)
where R is the total ohmic resistance, E the electrode potential, a the constant and b is the Tafel slope. The optimum
R -value is obtained when, using as criterion the correlation coefficient, r, of the current data in the high overpotential domain, an r-value very close to unity is obtained [9]. Depending on the quality of the experimental data, a very good linearisation of the corrected Tafel curve can be obtained (see Fig. 5). All Tafel curves, after ohmic drop correction, presented r > 0.9998. For the several compositions investigated, R -values (=Rsolution + Rfilm ) in the 0.2–1.0 cm2 range were obtained. These values are in excellent agreement with R -data obtained for conductive oxide electrodes immersed in strong electrolytes [9,18]. After IR -correction, Tafel curves presented two linear segments, revealing the deviation from linearity observed in the experimental curve (raw data) is due to ohmic resistance combined with changes in the electrode kinetics (e.g. change in rate determining step and/or apparent electronic transfer coefficient [6,9,18]). Fig. 6 gathers Tafel slope data, b, determined in the low and high overpotential domains as function of [CeO2 ]N . Tafel data reveal the influence of [CeO2 ]N on OER kinetics strongly depends on the overpotential domain. This behaviour is in agreement with literature reports [1,5,6,9,10] on oxide electrodes containing noble metals (e.g. Ru and Ir) as the active component. Fig. 6 clearly shows Tafel slopes in the low overpotential domain, blow , are little affected by the [CeO2 ]N , presenting a slight decrease on increasing Cecontent. Values start around ∼50 mV for the Ti/(Ru + Ti)O2 electrode and reach a minimum of ∼35 mV for the Ti/(Ru + Ce)O2 -electrode. In the high overpotential domain Tafel slope data, bhigh , are strongly influenced, decreasing from 120 to 60 mV with increasing [CeO2 ]N . The same figure also shows that in the ∼15 mol.% < [CeO2 ]N ≤ 50 mol.% composition interval Tafel slopes are practically constant (∼100 mV), revealing electrode kinetics are independent on Ce-content over this rather large composition interval. Interestingly, for these electrode compositions the surface parameters (qa and ϕ) remain almost constant with changing Ce-
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Fig. 6. Dependence of Tafel slope on [CeO2 ]N in the low, blow , and high, bhigh , overpotential domains. Electrolyte: 1.0 mol dm−3 NaOH. T = 25 ◦ C.
content, supporting OER kinetics, outside this composition span, is influenced by electrode morphology. Literature reports show the Tafel slope is very sensitive to the physico-chemical nature of the electrode/electrolyte interface [1], being affected by factors, such as: (i) oxide composition; (ii) overpotential domain; (iii) electrode preparation conditions; (iv) electrolyte composition [1,3,9,20,27]. Experimental Tafel slope (bexp ) frequently differ from the theoretical value (bt ) obtained from the kinetic analysis of different electrode mechanisms, especially for gas evolving reactions at rugged/porous electrodes [17,27,30]. The behaviour associated with bexp = bt can be understood in the light of the theoretical analysis based on the concept of the apparent (effective) electronic transfer coefficient [17,27]. It should be emphasised this analysis is only applicable for Tafel curves already corrected for ohmic drop and presenting high quality data (linear segment(s) characterised by an excellent correlation coefficient (r ≥ 0.9998) of the data—see Section 3.6.1). Darowicki and Orlikowski [31] presented similar theoretical approach using the concept of the effective electronic transfer coefficient for the OER on composite electrodes. 3.5. Reaction order with respect to OH− , ζOH− Since different Tafel slopes were reported for the OER at Ru-based oxide electrodes in alkaline medium [1,10], reaction order determination provides useful information to elucidate the electrode mechanism. The reaction order with respect to OH− , ζOH− , was determined in the both low and high overpotential domains as function of nominal oxide composition. To be significant reaction orders must be determined at constant ionic strength to avoid diffuse double layer effects [32]. To accomplish this, NaOH concentration was varied between 0.01 and 0.1 mol dm−3 keeping the ionic strength constant at 1.0 mol dm−3 by systematic substitution of OH− by ClO4 − ions.
Fig. 7. (A) Dependence of current density, j, on pOH (E = 0.55 V). Ti/[Ru(0.3) Ti(0.7 − x) Cex ]O2 : (1) x = 0.05; (2) x = 0.4. (B) Influence of [CeO2 ]N on reaction order with respect to OH− , ζOH− . Electrolyte: [NaClO4 ] + [NaOH] = 1.0 mol dm−3 .
Keeping constant the electrode potential and considering negligible changes in water activity [10], the reaction order with respect to OH− , ζOH− , for conditions of the high field approximation, η ≥ 0.1 V, was determined using next equation: ∂ log j ζOH− = − (2) ∂pOH E,T where pOH = −log[OH− ]. Fig. 7 gathers the experimental results obtained in the low (0.55 V), (ζOH− )low , and high (0.75 V), (ζOH− )high , overpotential domains. Fig. 7A clearly shows the linear segments in the log j versus pOH-plots are parallel, revealing ζOH− -values are independent on [CeO2 ]N . Fig. 7B presents the ζOH− -data as function of [CeO2 ]N showing the data are randomly scattered around unity on changing [CeO2 ]N (ζOH− ∼ = 1 in all cases). Although for the system investigated experimental ζOH− -values are unit, it is worthwhile to mention that kinetic complications arising from the complex microstructure of the oxide/electrolyte interface, in association with the acid–base properties of oxide electrodes, sometimes affect the elec-
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trode process resulting in non-integer (fractional) ζ-values [3,6,9,10,23].
Tafel coefficient of 40 mV is theoretically predicted in case step (II) is r.d.s.
3.6. Theoretical analysis of the OER reaction mechanism in alkaline medium
3.6.1. Interpretation of the experimental Tafel slopes based on the analysis of the apparent electronic transfer coefficient Confronting our experimental Tafel slopes (see Fig. 6) with the theoretical values predicted by the analysis presented in the previous section, a discrepancy is observed for the vast majority of the experimental data. As recently discussed by Da Silva et al. [17,27,30], this behaviour can be understood considering the true (absolute) electronic transfer coefficient of 0.5 is only observed for an electron transfer process at a flat electrode surface in the absence of the adsorption effects. The experimental Tafel slope for electrode processes occurring at solid electrodes is affected by a combination of geometrical and chemical effects, thus resulting in an apparent (effective) electronic transfer coefficient, αap , different from 0.5 [34,36–39]. Using the concept of the apparent electronic transfer coefficient, steps (II) and (I) can be used to calculate αap -values for the low, blow , and high, bhigh , overpotential domains as function of [CeO2 ]N . So, the influence of [CeO2 ]N on OER kinetics can be analysed in the light of αap , a parameter which describes the effectiveness of the applied overpotential on the reduction of the energy barrier for electron transfer and the apparent heat of activation. However, as already mentioned in Section 3.4, a kinetic study based on this approach is only possible when the scatter of the experimental kinetic data is negligible, or-be-it, if high quality experimental data are available. The αap -values with respect to steps (I) and (II) were calculated from the experimental b-values using Eqs. (3) and (4), respectively. Fig. 8 shows the dependence of αap in the low, αap-low , and high, αap-high , overpotential domains on [CeO2 ]N .
Different electrode mechanisms have been proposed for the OER in alkaline medium [1,10]. Considering our experimental b-values, reaction order data and the theoretical treatment based on the effective electronic transfer coefficient (see Section 3.6.1), the next mechanism proposed by Yeager [33] adequately describes the OER in all cases. k1
SZ + OH(aq) − ←→ ≡ S Z –OH + e− kII
SZ − OH←→ ≡ S Z+1 –OH + e− kIII
2 SZ+1 –OH− + 2OH− ←→2 SZ + 2H2 O + O2 ↑
(I) (II) (III)
where SZ represents an active surface site. As pointed out by Krishtalik [34], the individual steps of an electrochemical reaction behave as a serial resistor association of an electrical circuit. In the light of this proposal the step of the electrode mechanism presenting a minimum conductance governs the overall electrode process. Considering step (I) of the electrode mechanism as the r.d.s., and applying the kinetic treatment proposed by B¨ockris [35], for conditions of the high field approximation, η ≥ 0.1 V, and considering that the forward (kI ) and the backward (k−I ) process are close enough to zero, next relation is obtained for the reaction rate: α1 Fη j = 2FAkI [OH− ] exp (3) RT where kI is the forward rate constant for step (I), A the electrode surface area, [OH− ] the concentration of the hydroxyl ion, α1 the electronic transfer coefficient with respect to step (I) and η = E − Eo is the anodic overpotential. The other symbols have their usual meaning. Eq. (3) predicts a unit reaction order with respect to OH− ion. After rearranging, using the definition b ≡ (∂η/∂log j)T , the Tafel slope associated with step (I), bI , is given by 2.303RT/α1 F. So, considering α1 = 0.5 and T = 25 ◦ C a 120 mV Tafel slope is theoretically predicted if step (I) is the r.d.s. On the other hand, considering step (II) as r.d.s. of the overall reaction mechanism, and considering quasi-equilibrium conditions for step (I) [35], the reaction rate is described by next equation: (1 + α2 )Fη − j = 2FAkII [OH ] exp (4) RT where kII is the forward rate constant for step (II) and α2 is the electronic transfer coefficient with respect to step (II). Eq. (4) predicts an unit reaction order with respect to OH− and the Tafel slope associated with step (II), bII , is given by 2.302RT/(1 + α2 )F. Considering α2 = 0.5 and T = 25 ◦ C, a
Fig. 8. Dependence of the apparent electronic transfer coefficient, αap , calculated for the low, αap-low , and high, αap-high , overpotential domains on [CeO2 ]N . Electrolyte: 1.0 mol dm−3 NaOH. T = 25 ◦ C.
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The change in αap -values is consistent with the literature dealing with the OER at conductive oxide electrodes [17,27,30,39] and other materials [31] that report αap -values are affected by overpotential, electrode composition and electrolyte nature. In the light of these considerations, the results of the kinetic analysis presented in this work (see Fig. 8) strongly support Yeager’s electrode mechanism adequately describes the OER at Ti(Ru + Ce + Ti)O2 -electrodes. The changes in αap-low values can be attributed to modifications in the energy barrier of the electron transfer due to changes in electronic properties of the activated surface complex formed during oxidation of the active surface site, represented by next reaction: SZ –OH → SZ + 1 –OH + e− . In the case of the high overpotential domain, modifications in αap-high values are associated with changes in the electronic interaction between the acceptors orbitals located in the active surface site ( SZ ) and the donating orbitals of the OH− -groups. As a result the electric component of the effective energy barrier tends to decrease on increasing αap -values, thus decreasing the Tafel slope in both low and high overpotential domain on increasing nominal Ce-content (see Fig. 6). In the low overpotential domain, αap -values increase with increasing CeO2 -content starting at ∼0.1 reaching the maximum value of ∼0.6 for [CeO2 ]N = 70 mol.% (see Fig. 8). These results suggest the nominal oxide composition (albeit the chemical environment sensed by the active surface sites) affects the magnitude of the activation barrier with respect to step (II) as a consequence of changes in the SZ –OH bond strength. From a theoretical point of view this change can find its origin in several effects: (i) change in the [Ru]/[Ce] surface concentration ratio with changing composition; (ii) interaction at the electronic level (synergetic effects) between the surface Ce and Ru-sites; (iii) influence of the oxide morphology on the distribution of the surface electric filed causing a modification in the nature of the active sites (edge effect). Although it is very difficult to separate the contribution to the change in αap , of each of the above-mentioned effects, a hint can be obtained from the analysis of Figs. 3, 4 and 8. These figures show the main changes in αap -values are accompanied by a concomitant modification (increase) in the active surface area. Besides, the maximum ␣ap -value is observed for the binary Ti/[Ru(0.3) Ce(0.7) ]O2 electrode which presents the maximum CT -value. These results suggest a significant contribution to the change in αap can be attributed to the dependence of the edge effect on the oxide morphology (effect iii), which leads to modification in surface properties during OER due to the non-uniform distribution of the surface electric field because of surface roughness [37]. In the high overpotential domain and CeO2 -contents in the 0–50 mol.% interval αap -values present much less fluctuation (0.48–0.65), suggesting the influence of the above-mentioned effects is less pronounced in the case of high η-values and only exerts a weak influence on the oxidation of the surface OH− groups. However, for [CeO2 ]N ≥ 60 mol.% αap values increase significantly reaching a maximum of 0.98
for the Ti/[Ru(0.3) Ce(0.7) ]O2 suggesting a high CeO2 -content, strongly promotes a decrease in the effective energy barrier for electron transfer under conditions of the high electric field. 3.7. Electrocatalytic activity for the OER Electrocatalytic activity depends on both electronic and geometric factors [3]. Since the voltammetric charge is a function of the oxide composition (see Fig. 3), true electrocatalytic effects can be found normalising the Faradaic current by the voltammetric charge [3,9,17,18]. Fig. 9 shows a plot of the apparent, j, and true, j/qa , electrocatalytic activities measured at 0.62 V as function of the [CeO2 ]N . qa -Values were extracted from Fig. 3 and j-values were obtained from Tafel plots corrected for ohmic drop. Fig. 9 clearly shows nominal oxide composition affects both the apparent and true electrocatalytic activities for the OER. Analysis of Fig. 9A reveals the apparent (effective) electrocatalytic activity is constant for electrode compositions in the 0 mol.% < [CeO2 ]N < 50 mol.% interval composition. However, in agreement with the discussion presented in the previous section where it was found that a high CeO2 -content leads to a decrease in the apparent (effective) energy barrier for electron transfer, the increase in j-values on increasing Ce-content observed for [CeO2 ]N ≥ 50 mol.%
Fig. 9. Dependence of the current density, j, (A) and normalised current density, j/qa , (B) on [CeO2 ]N . j-Values were extracted from Tafel plots, corrected for IR , at 0.62 V. qa -Data were extracted from Fig. 3.
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reveals the maximum performance for the OER in alkaline medium is presented by the oxide composition containing Ti/[Ru(0.3) Ce(0.7) ]O2 . Fig. 9B clearly shows the true electrocatalytic activity is maximum for the Ti/[Ru(0.3) Ti(0.7) ]O2 composition, leading to the conclusion the increase observed in the apparent electrocatalytic activity for mixed oxides containing cerium (see Fig. 9A) is mainly due to the influence of the morphological effect (high degree of roughness and/or porosity). The fact CeO2 depresses the “true electrocatalytic activity” for the OER in alkaline medium, when introduced in the binary Ru0.3 Ti0.7 O2 composition, can be attributed to the inferior electrocatalytic activity presented by the superficial Ru-sites attached at ternary oxide particles containing Ce.
4. Conclusions XRD study shows replacement of Ti by Ce does not change the lattice parameters, which is consistent with the low probability of interaction between the different oxide phases (rutile and cubic structures). As a result, CeO2 acts as a dispersant and causes a strong increase in the oxide surface roughness on increasing Ce-content. “In situ” studies of the conductive metallic oxide electrodes containing cerium, ruthenium and titanium reveals the replacement of TiO2 with CeO2 , keeping the RuO2 constant at 30 mol.%, changes the electrode morphology and surface properties, increasing the electrode surface area and decreasing the open circuit potential. Surface characterisation revealed electrochemical surface porosity is little affected by oxide composition. The kinetic study of the OER revealed both oxide composition and overpotential domain affect the electrode process. Introduction of cerium leads to a reduction in the Tafel slope values in the high overpotential domain, which presents the minimum value for the nominal oxide composition containing 70 mol.% CeO2 . Experimental Tafel slope and reaction order data with respect to hydroxyl anion, combined with the theoretical analysis based on the concept of the apparent electronic transfer coefficient, support the electrode mechanism proposed by Yeager adequately describes the OER in alkaline medium. Analysis of the apparent electrocatalytic activity revealed the electrode performance for the OER considerable increases on increasing the nominal CeO2 content of the oxide mixture, contrary to the true electrocatalytic activity (in the absence of morphologic effects) which is maximum for the Ru(0.3) Ti(0.7) O2 electrode. Rationalisation of these findings permits to propose the superior performance presented by oxide electrodes containing cerium is due to the low interaction between the individual oxide components possessing different crystallographic structures, which leads to a strong increase in the electrochemically active surface area. The fact that the presence of CeO2 depresses the “true electrocatalytic activity” can be attributed to the inferior electrocat-
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alytic activity presented by the superficial Ru-sites attached at ternary oxide particles.
Acknowledgements K.C. Fernandes wishes to thank a M.Sc. fellowship received from the CAPES Foundation. L.M. Da Silva wishes to thank a Post-Doctoral fellowship granted by the FAPESP Foundation. J.F.C. Boodts acknowledges a Visiting Researcher Fellowship granted by the FAPEMIG Foundation. L.A. De Faria acknowledges the CAPES and FAPEMIG Foundations.
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