Origin of the Deactivation

8/14/2019 Origin of the Deactivation

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Origin of the Deactivation of Spinel CuxCo3-xO4/Ti Anodes Prepared by Thermal

Decomposition

R. Berenguer, A. La Rosa-Toro, C. Quijada, and E. Morallon*,

Departamento de Qumica Fsica e Instituto UniVersitario de Materiales, UniVersidad de Alicante, Apartado99, E-03080, Alicante, Spain, Facultad de Ciencias, UniVersidad Nacional de Ingeniera, AV. Tupac Amaru,210, Lima, Peru, and Departamento de Ingeniera Textil y Papelera, UniVersidad Politecnica de Valencia, Pza.

Ferrandiz y Carbonell, E-03801, Alcoy, Alicante, Spain.

ReceiVed: May 19, 2008; ReVised Manuscript ReceiVed: September 03, 2008

Thin films of spinel CuxCo3-xO4 with nominal composition 0.0 e x e 1.0 were supported on Ti by thethermal decomposition method. The resulting electrodes were deactivated by prolonged anodic polarizationin 1 M NaOH. The ensuing changes in the surface morphology, chemical composition, and crystalline propertieswere studied by means of scanning electron microscopy, energy dispersive X-ray microanalysis, X-raydiffraction, and X-ray photoelectron spectroscopy. The electrochemical response was inspected by cyclicvoltammetry. Surface imaging shows grain sharpening and extensive loss of the oxide coating, which becomesmore marked with the increasing proportion of lattice Cu2+ ions. Diffraction patterns show the rise in therelative intensity of the underlying Ti reflections and the loss of the cobalt spinel diffraction peaks. Lattice

Cu2+ ions dissolve preferentially and are virtually absent in deactivated mixed oxide anodes. Photoelectronspectroscopy indicates the presence of a highly hydrated Co(II)-containing surface layer. The reduction ofthe spinel oxide surface layer into an inactive hydrated CoO or a Co(OH)2 layer after oxidative electrolysisis discussed.

1. Introduction

Transition metal oxide coatings supported on valve metals(Ni, Ti, and so forth) have shown to behave as excellentelectrocatalysts for a wide variety of electrolysis processes oftechnological interest. In particular, the so-called dimensionallystable anodes, DSA, have been considered as one of thetwentieth centurys major technological breakthrough in the fieldof electrochemistry.1,2 These materials are mainly composed ofrutile-structured Ru- or Ir-based oxides supported on Ti. Theseelectrodes were originally developed by the demand of thechlorine-alkali industry; but soon, they found increasing ap-plication as electrocatalytic anodes for the electrochemicaloxidation of pollutants in wastewaters.3-9 Although much lessexpensive than the more traditional electrocatalysts consistingof bulk noble metals, DSAs are still precious-metal-basedanodes. Hence, cheaper metal oxide materials aroused a greatdeal of scientific interest during the past years.10-14 In particular,spinel cobalt oxides have emerged as a promising low-costalternative to DSAs. Spinel electrodes are known to combineadvantageously excellent catalytic activity for both the oxygenevolution reaction (OER)15-20 and the oxygen reduction reaction

(ORR)21 with outstanding long-term stability in alkaline media.Owing to their high stability, these oxides have been also testedas anodes for the oxidation of organic compounds22-25 andcyanide.25,26

In our laboratory, we have conducted detailed characterizationstudies of thermally prepared Co3O4/Ti and CuxCo3-xO4/Ti (0< x e 1.5) electrodes27 and shown that the incorporation ofcopper ions into the spinel lattice results in a substantial

enhancement of the catalytic activity for the oxidation ofcyanide.28 Indeed, binary spinel oxides of the type MxCo3-xO4(with M ) Cu, Ni, Mn, etc)20,21,29-33 and ternary spinel oxidesinvolving Ni-Cu-Co18,34,35 or Cu-Zn-Co16 have attractedlarge attention because the substitution of cobalt ions by foreigndivalent metal cations usually gives rise to improved catalyticactivity. However, the substitution of Co ions in the latticestructure can be detrimental to one of the major virtues of Co3O4electrodes, that is, the electrochemical stability. Thus, the servicelifetime, that is, the time elapsed until the material is deactivatedwhen working under service conditions, needs to be studied inorder to check for the potential use of doped cobalt spinels asefficient anodes for the electrochemical treatment of wastewaters.

Several mechanisms, involving one or both of the keyinterfaces, namely, the inner substrate/oxide coating and theouter oxide/electrolyte boundaries, have been claimed to beresponsible for the deactivation of oxide electrodes.2,36-39 Thesemechanisms include the passivation of the Ti substrate or theactive coating itself, the consumption of the oxide layer, andthe mechanical detachment of the deposit. The prevalence ofone mechanism over another is said to depend on many factors

such as the chemical composition of the coating, the surfacemorphology, and the method of preparation. However, inpractice, there is no unique cause for the final electrode failure,but the failure is due to a combination of several of the abovemechanisms.

The deactivation processes of cobalt spinel electrodes havebeen comparatively less treated than DSA-like anodes. Cobaltoxide electrodes are known to be anodically unstable in acidicmedium.15 Electrode failure is due to the dissolution of the activelayer combined with fast passivation of the supporting Ti.40 Inalkaline media, ohmic drops were found to develop in Co3O4and NiCo2O4 supported on Ti, Ni, or mild steel.41 These ohmiclosses were attributed to the growth of insulating oxide barriers

* Corresponding author. E-mail: [email protected]. Tel: 34-965909590.Fax: 34-965903537.

Universidad de Alicante. Universidad Nacional de Ingeniera. Universidad Politecnica de Valencia.

J. Phys. Chem. C2008, 112, 1694516952 16945

10.1021/jp804403x CCC: $40.75 2008 American Chemical SocietyPublished on Web 10/04/2008


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that forms at the support/oxide interface. Fradette et al.31

reported a preferential dissolution of surface copper specieswhen Ni/CuxCo3-xO4 electrodes are anodized in 1 M KOH, butthey did not examine the possible morphological or crystallinechanges and the possible growth of passivating oxides at thesupport/oxide interface.

The present work is aimed at determining the reasons forthe failure of thermally prepared CuxCo3-xO4/Ti (0 e x e 1)electrodes subjected to prolonged anodic polarization in NaOH

solution. The understanding of the deactivation phenomena andtheir relation to the loss of the catalytic activity will be ofprimary importance in the pursuit of long-term stable spineloxide electrodes suitable for the electrochemical abatement ofhazardous pollutants in waste waters. Accelerated lifetime testswere conducted to study the influence of the copper content onthe anode stability and to reach the deactivated condition. Bothelectrochemical (cyclic voltammetry experiments) and ex-situsurface analytical techniquessscanning electron microscopy(SEM), energy dispersive X-ray (EDX), X-Ray diffraction(XRD) and X-Ray Photoelectron Spectroscopy (XPS)swerecarried out in order to identify the morphological, crystal-lographic, and chemical changes involved in the deactivationof Cu-Co mixed oxide coatings.

2. Experimental Section

Binary spinel oxide electrodes of nominal compositionCuxCo3-xO4 with 0.0e x e 1.0, were prepared as thin films onTi supports by thermal decomposition of salt precursors. TheTi plates (1 1 0.05 cm, Goodfellow, 99.6%) werepreviously degreased in acetone, etched in boiling 10% oxalicacid for 1 h, and rinsed with distilled water. The salt precursorswere made up of Co(NO3)2 6H2O (ACS Aldrich) andCu(NO3)2 3H2O (Merck, p.a.) dissolved in absolute ethanol(J. T. Baker). The nitrate salts were mixed in stoichiometricamounts according to the desired nominal composition. The total

metallic cation concentration was set to 0.5 M. The salt precursorwas applied to the Ti support by the painting method. Thesolvent was dried at 70 C, and the electrode was subsequentlycalcined at 350 C for 10 min. This procedure was repeateduntil an oxide loading of 3.00-3.50 mg cm-2 was achieved.Finally, the electrodes were annealed at 350 C for 1 h.Photoelectron spectroscopy analysis showed negligible amountsof N on the catalyst surface, which ensures full precursordecomposition during the thermal step.

Accelerated life tests were carried out in order to monitorthe electrode stability and to reach the deactivated state. Spinel-oxide-coated Ti plates were anodically polarized in a 1 M NaOHsolution at a current density of 100 mA cm -2. Electrodes were

considered to become deactivated when the anode potentialincreases by 5 V over the initial stabilized potential. Thevariation with time of the anode potential measured withreference to an Ag/AgCl electrode is plotted in Figure 1. Thepotential in Figure 1 was converted into the RHE scale. Theanodic potential stays relatively constant (within ca (10%)along the accelerated lifetime experiment until it undergoes amarked rise, which denotes the onset of deactivation. Inaccordance with previous reports,27 the service lifetime, ex-pressed as the ratio of the charge passed to the oxide weight(Ah cm-2), decreases as the amount of incorporated Cu increasesfrom x ) 0.0 to x ) 1.0, although it always remains within thesame order of magnitude.

The surface morphology of the fresh and deactivated elec-

trodes was examined by scanning electron microscopy in aHitachi S-3000N electron microscope provided with a Rontec

X-ray detector for energy dispersive X-ray microanalysis (EDX).

Experimental X-ray diffraction patterns were obtained in aSeifert JSO-DEBYEFLEX 2002 diffractometer by using a Ni-filtered Cu KR radiation ( ) 1.541 ). Diffraction data pointswere recorded stepwise at a scan rate of 1 /min with a scanstep of 0.05 in 2. Diffractograms were given as the averageof five consecutive scans. Cell parameters were calculated by acomputer program using the peak position obtained after fittingthe experimental range with a pseudo-Voigt function per peakplus a background line. Line-broadening analysis was performedto determine the volume-weighed average crystallite size.

The XP spectra were obtained in a VG-Microtech Multilabelectron analyzer by using an unmonochromatized Mg KR(1253.6 eV) radiation at base pressure of 5 10-10 mbar in

the analysis chamber. Photoelectrons were collected into ahemispherical analyzer working in the constant energy modeat a pass energy of 50 eV. Binding energies were referencedagainst the main C1s line of adventitious carbon impurities at284.6 eV. Peak energies were given to an accuracy of (0.2eV.AllXPScurveswerefittedwithmixed70-30Gaussian-Lorentzianline-shape functions after nonlinear Shirley background subtrac-tion. Peak areas were normalized by using appropriate atomicsensitivity factors.

Cyclic voltammetry experiments were run in a conventionalthree-electrode cell at room temperature. The counter electrodewas a Pt electrode, and the potentials were measured withreference to a reversible hydrogen electrode (RHE) connectedwith the working solution through a Luggin capillary. Testsolutions for cyclic voltammetry (0.1 M NaOH) were preparedfrom NaOH (Merck, p.a.) and ultrapure water (Purelab Ultrafrom Elga-Vivendi, 18.2 M cm). The cyclic voltammogramswere obtained at a constant scan rate of 20 mV s-1 at roomtemperature. The current densities were calculated by consider-ing the geometric area of the electrodes (2 cm2).

3. Results and Discussion

3.1. SEM and EDX Characterization. As reported earlier,27

pure Co3O4/Ti electrodes possess a rather smooth and compactmorphology, and the surface roughness is progressively in-creased in binary Cu-Co spinel oxides as the Cu doping levelrises (Figure 2a-d, x ) 0.0, 0.2, 0.8, 1.0). The surface of

deactivated Co3O4 electrodes remains as smooth and compactas the fresh electrodes (Figure 2e), but noticeable coating

Figure 1. Electrode potential versus electrolysis time for CuxCo3-xO4/Ti (0.0 e x e 1.0) electrodes; j ) 100 mAcm-2; 1 M NaOH.

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detachment is observed near the electrode edges (not shown)leaving only isolated Co oxide aggregates onto the characteristictexture of the etched Ti substrate. In contrast, deactivatedCuxCo3-xO4/Ti electrodes undergo noticeable surface morpho-logical modifications on the whole examined area. The extentof surface damage increases with the increasing Cu amount.At x ) 0.2, surface cracks and an increased surface heterogeneityare observed (Figure 2f). At higher Cu contents, extensive lossof the coating is observed, and the remaining oxide appears asrandomly distributed grain agglomerates (Figure 2g-h). Higher

magnification micrographs show significant sharpening ofCu-Co binary oxide grains in deactivated electrodes (Figure

3d-f), whereas their fresh counterparts possess a rather compacttexture, with occasional narrow cracks and grains of globularshape (Figure 3a-c). Energy dispersive X-Ray measurements(Table 1) support the electron microscopy observations. In theundamaged area of pure Co3O4, the Ti level is of about 1%,typical of well-coated Ti electrodes. On the contrary, the amountof Co drops drastically near the edges, and a concomitantincrease of the Ti signal is observed. In deactivated CuxCo3-xO4/Ti electrodes, the Ti content is increased by several orders ofmagnitude with respect to fresh electrodes (Table 1), which is

indicative of considerable loss of the oxide active layer.Interestingly, Cu is detected at residual amounts or not detected

Figure 2. Scanning electron micrographs of CuxCo

3-

xO

4 /Ti electrodes, 0.0 e x e 1.0; fresh electrodes: (a) x ) 0.0, (b) x ) 0.2, (c) x ) 0.8, (d)

x ) 1.0; deactivated electrodes: (e) x ) 0.0, (f) x ) 0.2, (g) x ) 0.8, (h) x ) 1.0.

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at all, which suggests that copper ionic species suffer fromsevere preferential dissolution under anodic polarizationconditions.

3.2. X-ray Diffraction. The X-ray patterns of a set of freshand deactivated CuxCo3-xO4 /Ti, 0.0e x e 1.0, electrodes areshown in Figure 4. As reported earlier,27 the diffractograms offresh pure and Cu-doped cobalt oxides reveal reflections withpositions and relative intensities characteristic of a cubic spinellattice structure. There are some extra lines corresponding tothe (002) and (103) crystallographic planes of the underlyingsubstrate. The unit cell constant of the undoped fresh cobaltoxide coating was 8.093 , slightly higher than that reportedfor standard powders. This deviation is probably a consequenceof the influence of the support and the preparation method onthe crystallographic properties.31,32 The unit cell parameterundergoes an increment as the copper content rises (up to 8.130

at x ) 1.0). This behavior was interpreted as an effect of thesubstitution of Co (II) cations by larger Cu(II) cations.27

The diffraction patterns of the deactivated electrodes showvisible changes in relation to the diffraction pattern of freshelectrodes. The deactivated Co3O4 electrode still shows thetypical diffraction peaks of a cubic spinel structure, but theirrelative intensity with respect to the peaks corresponding to the(002) and (102) planes of the Ti substrate substantially decreasesafter anodic polarization. In addition, extra reflections occur thatbelong to the (102) plane of Ti. X-ray diffraction data for Co3O4deactivated electrodes suggest that prolonged anodic polarizationbrings about an extensive loss of the metal oxide coating. Inthe less stable binary Cu-Co oxide films, the loss of the coatingis so dramatic that the only surviving evidence for the residualspinel phase is the diffraction at the (311) plane. These results

are in fairly good agreement with quantitative data deduced fromEDX.

3.3. X-ray Photoelectron Spectroscopy. Figure 5 shows thedetailed Co2p photoelectronic spectra of a fresh Co3O4/Tielectrode and a set of deactivated CuxCo3-xO4/Ti samples withCu doping levels up to x ) 1.0. The spectrum of the freshcobaltite (top) shows two broad and asymmetric photoelectronicpeaks with a spin-orbit splitting of about 15 eV. The asymmetryarises from the overlapping of the line for Co(III) in octahedralspinel sites and the much weaker line for Co(II) in tetrahedralsites, which have slightly different binding energies. The mainCo 2p3/2 component is located at 779.9 eV (fwhm 3 eV) andis accompanied by a low-intensity shakeup satellite shifted by

9-10 eV to higher BEs. This shakeup feature is attributed tothe presence of high-spin Co(II) in a tetrahedral crystal field,

Figure 3. Magnified scanning electron micrographs of CuxCo3-xO4 /Ti electrodes, 0.0 < x e 1.0; fresh electrodes: (a) x ) 0.2, (b) x ) 0.8, (c) x) 1.0; deactivated electrodes: (a) x ) 0.2, (b) x ) 0.8, (c) x ) 1.0.

TABLE 1: Energy Dispersive X-ray Bulk Composition ofFresh and Deactivated CuxCo3-xO4 /Ti Electrodes Expressedas Atomic Percentage

fresh deactivated

electrode % Ti % Co % Cu % Ti % Co % Cu

Co3O4 /Ti 1 40 - 1 44 -Cu0.2Co2.8O4 /Ti 1 25 1 2 2 0Cu0.8Co2.2O4 /Ti 1 24 9 45 2 0Cu1.0Co2.0O4 /Ti 1 29 15 24 23 1



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which represents only one third of the occupied cationic sites.All these features are diagnostic characteristics of pure Co3O4with a spinel structure.32-35,42-45 As reported earlier,27 thespectra of fresh Cu-Co mixed oxides supported on Ti (notshown) display a virtually identical shape and position of the

main peaks and satellites. This fact was expected, provided thatdivalent Cu ions enter the oxide lattice to form a solid solution

with preservation of the spinel structure. In fact, curve fittingof the main Cu 2p3/2 core-level line of CuxCo3-xO4 with x e1.0 also showed that Cu(II) ions fill both tetrahedral andoctahedral sites.27

The survey spectrum (not shown) of the deactivated Co3O4electrode displays the characteristic core-level features of Co,O, Na (from the test electrolyte), and C (adventitious carboncontamination). No evidence for Ti was found, which is coherentwith the low Ti percentage determined by EDX and the lowsmall extent of film detachment evidenced by electron micros-copy. In the deactivated binary Cu-Co oxide electrodes, copperis detected at low levels (


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The shapes of O 1s XP spectra of representative Cu0.8Co2.2O4/

Ti are compared in Figure 7 for fresh and deactivated electrodes.As reported elsewhere,27 the O 1s line of a fresh CuxCo3-xO4/Ti electrode can be fitted with two contributions (Figure 7a).The most intense one (529.5 ( 0.2 eV) was assigned to latticeO2- species, and the lateral structure (531.3 ( 0.2 eV) wasmainly attributed to O-bearing carbon functionalities of adventi-tious contamination with a minor contribution of surfaceOH-.27,47 After correction of the O 1s intensity from C-relatedoxygen contamination according to the procedure describedelsewhere,27 the surface O/M ratio was just slightly above thestoichiometric value of 1.33. On the contrary, the O 1s line ismuch broader in deactivated electrodes (Figure 7b) and is bestfitted with three contributions at 530.3, 532.1, and 533.5 ( 0.2

eV. The low BE component can be related to lattice oxygenions. The intermediate BE component is by far the mostabundant oxygen atomic environment. It could be primarilyascribed to surface OH-,47,49 even though a minor contributionof C-related oxygen impurities is possible. The high BE peakcan be assigned to adsorbed water and single-bonded oxygenin adventitious impurities. From the quantitative point of view,the corrected O/Co ratio ranges from 5 to 10, which, on theone hand, confirms that oxygen related to adventitious carboncontamination is a minor contribution to the total O 1s line,and on the other hand, suggests that the Co(II)-containing layeris most likely to occur as some kind of heavily hydratedoxohydroxide at the surface.

The reduction of the spinel-like oxide at the outermost

interface under anodic conditions is an unexpected result. Earlierauthors49 interpreted XP spectra of thermally prepared Co3O4/

Ti showing shifts of the core-level lines to higher BEs andsatellite enhancement in terms of hydration of the surface whichinduces a local decomposition of Co3O4 into CoO and Co2O3.However, the extent of the Co2p3/2 energy shift reported in ref49 is three times lower than the drift observed in this work.Moreover, the photoelectronic spectra of active spinel electrodesmeasured after cyclic voltammetric experiments (see below),that is, after contact with alkaline medium (wet electrodes), didnot show any change with respect to dry electrodes (Figure 5b).

Then, an alternative explanation is that the spinel oxidizes firstto CoO2 during polarization. According to the equilibriumPourbaix diagram52 for Co, CoO2 is a thermodynamically stablespecies in alkaline media under high electrode potential.However, at open circuit, this oxide, as other high-valenttransition metal oxides, should be chemically unstable undercontact with water and may decompose to give Co(II) oxidespecies. In fact, pure bulk CoO2, obtained by electrochemicaldeintercalation of Li ions from LiCoO2 in anhydrous conditions,has been reported to be highly sensitive to moisture.50,51

Therefore, the conversion of the active Co(IV) oxide into aCo(II) oxide or hydroxide may occur during handling of theelectrode as a result of contact with residual electrolyte or

ambient moisture. Thus, it is doubtful that the formation of theCo(II) surface compound is a primary cause for electrodedeactivation.

3.4. Cyclic Voltammetry. The important changes in thesurface morphology and coating composition suffered byanodized spinel Co oxides severely affect the electrochemicalbehavior of the electrodes. Figure 8 shows a set of superimposedvoltammograms of fresh (dashed line) and deactivated (solidline) electrodes of composition CuxCo3-xO4/Ti with 0e x e 1.As reported elsewhere,16,19,30,31,53,54 fresh pure cobalt oxideelectrodes show two characteristic redox peaks, referred to asA1/C1 and A2/C2, which were assigned to Co(II)/Co(III) andCo(III)/Co(IV) surface transitions respectively. As copper isincorporated into the cobalt spinel lattice, the first redox couplevanishes, and the second transition shifts slightly to less positivepotentials. It has been pointed out by others that the Co(III)/Co(IV) transition is a highly reversible surface process.53,54 Incontrast, our voltammograms show some irreversibility, withpeak separation higher than 100 mV. This behavior is mostlikely to be associated with uncompensated ohmic drop effectsowing to the occurrence of a structure-defective TiO2 thininterlayer, which may develop from the substrate duringannealing at 350 C. Structural defects prevent the Ti oxidefrom being fully insulating and allows it to behave as an ohmicresistor. However, one should note that authors reportingreversible surface redox reactions studied Co3O4 deposited oneither a Ti support precoated with a RuO2 interlayer53 or on a

Ni substrate.54 Interlayers of metallic RuO2 are known to (i)prevent the formation of insulating TiO2 layers thanks to theintermixing of rutile phases to form a conducting RuxTi1-xO2layer41,53 and (ii) play as a dopant for the Co3O4 (p-typesemiconductor) overlayer to increase its intrinsic electricalconductivity.15 On the other hand, NiO layers growing from Niplates do not impart significant resistivity.54 Therefore, ohmicdrop effects are completely removed or largely alleviated inthese cobalt oxide electrodes and purely reversible behaviorbecomes evident.

The diagnostic redox features of cobalt spinel are only barelydiscernible in a deactivated pure cobalt oxide electrode (Figure8a, solid line), whereas the voltammogram becomes practically

featureless for Cu-containing deactivated spinels (Figure 8b-d,solid line). A common attribute to all deactivated electrodes is

TABLE 2: Photoelectron Binding Energies and PeakEnergy Gaps for the Co 2p Core-Level in Fresh andDeactivated CuxCo3-xO4 /Ti Electrodes

fresh deactivated

electrode Co 2p3/2 1a 2b Co 2p3/2 1 2

Co3O4 /Ti 779.9 15.1 9.5 781.3 16.3 5.3Cu0.2Co2.8O4 /Ti 779.9 15.1 9.2 781.4 15.7 5.3Cu0.8Co2.2O4 /Ti 779.8 15.0 9.7 781.7 15.6 5.3Cu1.0Co2.0O4 /Ti 779.8 15.1 9.6 781.4 16.2 5.3

a Spin-orbit splitting. b Main peak-shakeup satellite energy gap.

Figure 7. Representative O 1s photoelectron spectra of fresh (a) anddeactivated (b) CuxCo3-xO4 /Ti electrodes (x ) 0.8).



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the significant decrease in the transferred charge compared totheir respective fresh electrodes, which can be easily interpretedas a result of combination of three key factors, namely, theincreasing loss of active material as revealed by SEM, theconversion of the outermost layer of the oxidized active coatinginto a less conductive Co(II) oxide or hydroxide (possibly as aconsequence of contact with air moisture at open circuitpotential), and the growth of an insulating TiO2 interlayer. Theformer process leads to a decrease in the true surface area,whereas the two latter phenomena are involved in the hindranceof the charge transfer across the electrode/electrolyte interface.

As shown in Figure 1, the higher the content of Cu ions inthe Cu-Co binary spinel oxides, the shorter the service life. It

is well-documented that the intrinsic conductivity of spinelcobalt oxides is both structure and composition dependent.Therefore, it may well vary on the degree of substitution withforeign divalent ions. Poor electronic conduction leads todecreased stability because the electric field across the activeoxide layer increases until the ions become mobile and the outeroxide/electrolyte interface penetrates into the bulk of the oxidelayer, eventually reaching the substrate, to favor its passivation.Thus, one might think of a fall in the oxide conductivity withthe increasing Cu content to explain the loss of stability of mixedoxides. However, Fradette et al.31 reported that Cu0.9Co2.1O4has a resistivity of 0.047 cm, appreciably lower than theresistivity of pure Co3O4 (0.667 cm), both materials beingprepared as thin films on a glass substrate by the chemical spray

pyrolisis method. Moreover, our voltammograms in Figure 8show that the anodic and cathodic peaks for the Co(III)/Co(IV)

surface transition approach to each other in Cu-Co mixedoxides. Because a common value for the ohmic drop arisingfrom the partially conducting Ti oxide interlayer can bereasonably assumed, the increase in peak reversibility is a signof increasing oxide conductivity. Then, it follows that the servicelifetime is not affected by the oxide conductivity.

4. Conclusions

The surface texture of deactivated Co3O4 /Ti electrodesremains little disturbed when compared to fresh Co3O4/Ti.However, mixed Cu-Co spinels show progressive surfacedamage and considerable grain sharpening as the copper content

increases. X-ray microanalysis indicates that all pure and mixedspinels undergo a loss of the active oxide layer, which isparticularly severe in the highly copper-substituted electrodes.In addition, binary oxides suffer from preferential dissolutionof the Cu ionic species. These morphological and compositionalchanges are accompanied by important modification in thecrystalline properties. Diffraction studies show an increment inthe intensity of the reflections of the underlying Ti support, thusconfirming the loss of the coating under anodic polarization.The higher the Cu contents in the spinel oxide, the moredramatic the loss of the oxide layer, until extensive detachmentis eventually observed for heavily Cu-doped spinels. Photo-electron spectroscopy data also support the preferential dissolu-tion of lattice Cu and reveals the occurrence of TiO2 even at

the very surface. Moreover, the Co 2p core-level spectrum pointsto a reduction of the active spinel oxide to yield a Co(II)-

Figure 8. Cyclic voltammograms of fresh (dashed line) and deactivated (solid line) CuxCo3-xO4/Ti electrodes in 0.1 M NaOH; (a) x ) 0.0, (b) x) 0.2, (c) x ) 0.8, (d) x ) 1.0; V ) 20 mV s-1.

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containing surface layer for the whole series of CuxCo3-xO4/Tielectrodes. It can be suggest that a surface conversion of thespinel oxide into an inactive, heavily hydrated CoO or Co(OH)2layer takes place. However, it is doubtful that this mechanismis a primary cause for the deactivation process. Then, it couldbe proposed that the spinel layer is oxidized to CoO2 underhighly anodic conditions, and the Co(IV) oxide reduces chemi-cally to Co(II) oxide upon contact with ambient environment.

The electrochemical response of deactivated electrodes is

severely modified. The characteristic Co surface redox transi-tions are absent, with the exception of deactivated pure Co3O4/Ti, where they are barely distinguished. Moreover, the totalvoltammetric charge transfer suffers from a marked blockage.These changes are readily explained by the loss of the activeoxide coating and the growth of an insulating TiO2 interlayer.The occurrence of a poorly conducting Co(II) oxide or hydroxidesurface layer should have a marked effect on the voltammetricresponse, but one should recall that the formation of this passivelayer and the anodic polarization are probably unrelatedphenomena.

In summary, the deactivation of CuxCo3-xO4 /Ti electrodesunder operating conditions is a complex process involvingseveral mechanisms that include a loss of the active coatingand a change in the chemical composition, the crystallineproperties and the surface texture of the surviving material. Asexpected, the lower the stability of the tested electrodes, thehigher the extent of these changes.

Acknowledgment. Funds from the Generalitat Valenciana(Projects GV05/136 and RED ARVIV/2007/076) and Ministeriode Educacion y Ciencia-FEDER (Project MAT2007-60621) aregratefully acknowledged.

References and Notes

(1) Trasatti; S., Ed. Studies in Physical and Theoretical Chemistry. Electrodes of ConductiVe Metallic Oxides, pArts A,B. Elsevier Science

Publishers: Amsterdam, 1980.(2) Trasatti, S. Electrochim. Acta 2000, 45, 23772385.(3) Foti, G.; Gandini, D.; Comninellis, Ch. Curr. Top. Electrochem.

1997, 5, 7190.(4) Fugivara, C. S.; Sumodjo, P. T. A.; Cardoso, A. A.; Benedetti, A. V.

Analyst 1996, 121, 541545.(5) Pelegrini, R.; Peralta-Zamora, P.; de Andrade, A. R.; Reyes, J.;

Duran, N. Appl. Catal., B 1999, 22, 8390.(6) Rodgers, J. D.; Jedral, W.; Bunce, N. J. EnViron. Sci. Technol. 1999,

33, 14531457.(7) Kim, K.-W.; Lee, E.-H.; Kim, J.-S; Shin, K.-H; Jung, B.-I.

Electrochim. Acta 2002, 25252531.(8) Pelegrino, R. L.; Di Iglia, R. A.; Sanches, C. G.; Avace, L. A.;

Bertazzoli, R. J. Braz. Chem. Soc. 2002, 13, 6065.(9) Malpass, G. R. P.; Miwa, D. W.; Mortari, D. V.; Machado, S. A. S.;

Motheo, A. J. Water Res. 2007, 41, 29692977.(10) Vicent, F.; Morallon, E; Quijada, C.; Vazquez, J. L.; Aldaz, A.;

Cases, F. J. J. Appl. Electrochem. 1998, 28, 607612.(11) Montilla, F.; Morallon, E.; De Battisti, A.; Vazquez, J. L. J. Phys.Chem. B 2004, 108, 50365043.

(12) Montilla, F.; Morallon, E.; De Battisti, A.; Barison, S.; Daolio, S.;Vazquez, J. L. J. Phys. Chem. B 2004, 108, 1597615981.

(13) Montilla, F.; Morallon, E.; De Battisti, A.; Benedetti, A.; Yamashita,H.; Vazquez, J. L. J. Phys. Chem. B 2004, 108, 50445050.

(14) Montilla, F.; Morallon, E.; Vazquez, J. L. J. Electrochem. Soc. 2005,152, B421-B427.

(15) Burke, L. D.; McCarthy, M. M. J. Electrochem. Soc. 1988, 135,11751179.

(16) Lee, Y-S.; Hu, C.-C; Wen, T.-C. J. Electrochem. Soc. 1996, 143,12181225.

(17) Castro, E. B.; Gervasi, C. A. Int. J. Hydrogen Energy 2000, 25,11631170.

(18) Tavares, A. C.; Cartaxo, M. A. M.; da Silva Pereira, M. I.; Costa,F. M. J. Electroanal. Chem. 1999, 464, 187197.

(19) Svegl, F.; Orel, B.; GrabelSvegl, I.; Kaucic, V. Electrochim. Acta2000, 45, 43594371.

(20) De Koninck, M.; Poirier, S.-C; Marsan, B. J. Electrochem. Soc.2006, 153, A2103-A2110.

(21) De Koninck, M.; Poirier, S.-C; Marsan, B. J. Electrochem. Soc.2007, 154, A381-A388.

(22) Cox, P.; Pletcher, D. J. Appl. Electrochem. 1990, 20, 549553.(23) Cox, P.; Pletcher, D. J. Appl. Electrochem. 1991, 21, 1113.(24) Casella, I. G. J. Electroanal. Chem. 2002, 520, 119125.(25) Berenguer, R.; Valdes-Sols, T.; Fuertes, A. B.; Quijada, C.;

Morallon, E. J. Electrochem. Soc. 2008, 155, K110K115.(26) Stavart, A.; Lierde, A. V. J. Appl. Electrochem. 2001, 31, 469

474.(27) La Rosa-Toro, A.; Berenguer, R.; Quijada, C.; Montilla, F.;

Morallon, E.; Vazquez, J. L. J. Phys. Chem. B 2006, 110, 2402124029.(28) La Rosa-Toro, A. PhD Dissertation, Universidad de Alicante, 2008.(29) Jin, S.; Ye, S. Electrochim. Acta 1996, 41, 827834.(30) Marsan, B.; Fradette, N.; Beaudoin, G. J. Electrochem. Soc. 1992,

139, 18891896.(31) Fradette, N; Marsan, B. J. Electrochem. Soc. 1998, 145, 2320

2327.(32) Gautier, J. L.; Trollund, E.; Rios, E.; Nkeng, P.; Poillerat, G. J.

Electroanal. Chem. 1997, 428, 4756.(33) Gautier, J. L.; Rios, E.; Gracia, M.; Marco, J. F.; Gancedo, J. R.

Thin Solid Films 1997, 311, 5157.(34) Tavares, A. C.; Cartaxo, M. A. M.; da Silva Pereira, M. I.; Costa,

F. M. J. Solid State Electrochem. 2001, 5, 5767.(35) Wen, T. C.; Kang, H. M. Electrochim. Acta 1998, 43, 17291745.(36) Beck, F. Electrochim. Acta 1989, 34, 811822.(37) Martelli, G. N.; Ornelas, R.; Faita, G. Electrochim. Acta 1994, 39,

15511558.(38) Panic, V.; Dekanski, A.; MiSkovic-Stankovic, V. B.; Milonjic, S.;

Nikolic, B. J. Electroanal. Chem. 2005, 579, 6776.(39) Hu, J.-M.; Zhang, J.-Q.; Meng, H.-M.; Zhang, J.-T.; Cao, C.-N.

Electrochim. Acta 2005, 50, 53705378.(40) Da Silva, L. M.; De Faria, L. A.; Boodts, J. F. C. J. Electroanal.

Chem. 2002, 532, 141150.(41) Baronetto, D.; Kodintsev, I. M.; Trasatti, S. J. Appl. Electrochem.

1994, 24, 189194.

(42) Chuang, T. J.; Brundle, C. R.; Rice, D. W.Surf. Sci. 1976

,59

,413429.(43) Barreca, D.; Massignan, C.; Daolio, S.; Fabrizio, M.; Piccirillo,

C.; Armelao, L.; Tondello, E. Chem. Mater. 2001, 13, 588593.(44) Natile, M. M.; Glisenti, A. Chem. Mater. 2002, 14, 30903099.(45) Jimenez, V. M.; Espinos, J. P.; Gonzalez-Elipe, A. R. Surf. Interface

Anal. 1998, 26, 6271.(46) Roginskaya, Y. E.; Morozova, O. V.; Lubnin, E. N.; Ulitina, Y. U.;

Lopukhova, G. V.; Trasatti, S. Langmuir 1997, 13, 46214627.(47) Foelske, A.; Strehlblow, H. H. Surf. Interface Anal. 2000, 29, 548

555.(48) Foelske, A.; Strehlblow, H. H. Surf. Interface Anal. 2002, 34, 125

129.(49) Valeri, S.; Battaglin, G.; Lodi, G.; Trasatti, S. Colloids Surf. 1987,

19, 387398.(50) Motohashi, T.; Katsumata, Y.; Ono, T.; Kanno, R.; Karppinen, M.;

Yamauchi, H. Chem. Mater. 2007, 19 (21), 50635066.

(51) De Vaulx, C.; Julien, M.-H.; Berthier, C.; Hebert, S.; Pralong, V.;Maignan, A. Phys. ReV. Lett. 2007, 98, 246402.(52) Pourbaix, M. Atlas of Electrochemical Equilibria in aqueous

solutions, 2nd ed.; National Association of Corrosion Engineers: Houston,1974.

(53) Boggio, R.; Carugati, A.; Trasatti, S. J. Appl. Electrochem. 1987,17, 828840.

(54) Spinolo, G.; Aridzzone, S.; Trasatti, S. J. Electroanal. Chem. 1997,423, 4957.

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