J. Phys. Chem. C 2008, 112, 16945–16952
16945
Origin of the Deactivation of Spinel CuxCo3-xO4/Ti Anodes Prepared by Thermal Decomposition R. Berenguer,† A. La Rosa-Toro,‡ C. Quijada,§ and E. Morallo´n*,† Departamento de Quı´mica Fı´sica e Instituto UniVersitario de Materiales, UniVersidad de Alicante, Apartado 99, E-03080, Alicante, Spain, Facultad de Ciencias, UniVersidad Nacional de Ingenierı´a, AV. Tupac Amaru, 210, Lima, Peru´, and Departamento de Ingenierı´a Textil y Papelera, UniVersidad Polite´cnica de Valencia, Pza. Ferra´ndiz 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 the thermal decomposition method. The resulting electrodes were deactivated by prolonged anodic polarization in 1 M NaOH. The ensuing changes in the surface morphology, chemical composition, and crystalline properties were studied by means of scanning electron microscopy, energy dispersive X-ray microanalysis, X-ray diffraction, and X-ray photoelectron spectroscopy. The electrochemical response was inspected by cyclic voltammetry. Surface imaging shows grain sharpening and extensive loss of the oxide coating, which becomes more marked with the increasing proportion of lattice Cu2+ ions. Diffraction patterns show the rise in the relative 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. Photoelectron spectroscopy indicates the presence of a highly hydrated Co(II)-containing surface layer. The reduction of the spinel oxide surface layer into an inactive hydrated CoO or a Co(OH)2 layer after oxidative electrolysis is discussed. 1. Introduction Transition metal oxide coatings supported on valve metals (Ni, Ti, and so forth) have shown to behave as excellent electrocatalysts for a wide variety of electrolysis processes of technological interest. In particular, the so-called dimensionally stable anodes, DSA, have been considered as one of the twentieth century’s major technological breakthrough in the field of electrochemistry.1,2 These materials are mainly composed of rutile-structured Ru- or Ir-based oxides supported on Ti. These electrodes were originally developed by the demand of the chlorine-alkali industry; but soon, they found increasing application as electrocatalytic anodes for the electrochemical oxidation of pollutants in wastewaters.3-9 Although much less expensive than the more traditional electrocatalysts consisting of bulk noble metals, DSAs are still precious-metal-based anodes. Hence, cheaper metal oxide materials aroused a great deal of scientific interest during the past years.10-14 In particular, spinel cobalt oxides have emerged as a promising low-cost alternative to DSAs. Spinel electrodes are known to combine advantageously excellent catalytic activity for both the oxygen evolution 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 tested as anodes for the oxidation of organic compounds22-25 and cyanide.25,26 In our laboratory, we have conducted detailed characterization studies of thermally prepared Co3O4/Ti and CuxCo3-xO4/Ti (0 < x e 1.5) electrodes27 and shown that the incorporation of copper ions into the spinel lattice results in a substantial * Corresponding author. E-mail:
[email protected]. Tel: 34-965909590. Fax: 34-965903537. † Universidad de Alicante. ‡ Universidad Nacional de Ingenierı´a. § Universidad Polite ´ cnica de Valencia.
enhancement of the catalytic activity for the oxidation of cyanide.28 Indeed, binary spinel oxides of the type MxCo3-xO4 (with M ) Cu, Ni, Mn, etc)20,21,29-33 and ternary spinel oxides involving Ni-Cu-Co18,34,35 or Cu-Zn-Co16 have attracted large attention because the substitution of cobalt ions by foreign divalent metal cations usually gives rise to improved catalytic activity. However, the substitution of Co ions in the lattice structure can be detrimental to one of the major virtues of Co3O4 electrodes, that is, the electrochemical stability. Thus, the service lifetime, that is, the time elapsed until the material is deactivated when working under service conditions, needs to be studied in order to check for the potential use of doped cobalt spinels as efficient anodes for the electrochemical treatment of wastewaters. Several mechanisms, involving one or both of the key interfaces, namely, the inner substrate/oxide coating and the outer oxide/electrolyte boundaries, have been claimed to be responsible for the deactivation of oxide electrodes.2,36-39 These mechanisms include the passivation of the Ti substrate or the active coating itself, the consumption of the oxide layer, and the mechanical detachment of the deposit. The prevalence of one mechanism over another is said to depend on many factors such as the chemical composition of the coating, the surface morphology, and the method of preparation. However, in practice, there is no unique cause for the final electrode failure, but the failure is due to a combination of several of the above mechanisms. The deactivation processes of cobalt spinel electrodes have been comparatively less treated than DSA-like anodes. Cobalt oxide electrodes are known to be anodically unstable in acidic medium.15 Electrode failure is due to the dissolution of the active layer combined with fast passivation of the supporting Ti.40 In alkaline media, ohmic drops were found to develop in Co3O4 and NiCo2O4 supported on Ti, Ni, or mild steel.41 These ohmic losses were attributed to the growth of insulating oxide barriers
10.1021/jp804403x CCC: $40.75 2008 American Chemical Society Published on Web 10/04/2008
16946 J. Phys. Chem. C, Vol. 112, No. 43, 2008 that forms at the support/oxide interface. Fradette et al.31 reported a preferential dissolution of surface copper species when Ni/CuxCo3-xO4 electrodes are anodized in 1 M KOH, but they did not examine the possible morphological or crystalline changes and the possible growth of passivating oxides at the support/oxide interface. The present work is aimed at determining the reasons for the 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 and their relation to the loss of the catalytic activity will be of primary importance in the pursuit of long-term stable spinel oxide electrodes suitable for the electrochemical abatement of hazardous pollutants in waste waters. Accelerated lifetime tests were conducted to study the influence of the copper content on the anode stability and to reach the deactivated condition. Both electrochemical (cyclic voltammetry experiments) and ex-situ surface analytical techniquessscanning electron microscopy (SEM), energy dispersive X-ray (EDX), X-Ray diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS)swere carried out in order to identify the morphological, crystallographic, and chemical changes involved in the deactivation of Cu-Co mixed oxide coatings. 2. Experimental Section Binary spinel oxide electrodes of nominal composition CuxCo3-xO4 with 0.0 e x e 1.0, were prepared as thin films on Ti supports by thermal decomposition of salt precursors. The Ti plates (1 × 1 × 0.05 cm, Goodfellow, 99.6%) were previously degreased in acetone, etched in boiling 10% oxalic acid for 1 h, and rinsed with distilled water. The salt precursors were made up of Co(NO3)2 · 6H2O (ACS Aldrich) and Cu(NO3)2 · 3H2O (Merck, p.a.) dissolved in absolute ethanol (J. T. Baker). The nitrate salts were mixed in stoichiometric amounts according to the desired nominal composition. The total metallic cation concentration was set to 0.5 M. The salt precursor was applied to the Ti support by the painting method. The solvent was dried at 70 °C, and the electrode was subsequently calcined at 350 °C for 10 min. This procedure was repeated until 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 amounts of N on the catalyst surface, which ensures full precursor decomposition during the thermal step. Accelerated life tests were carried out in order to monitor the electrode stability and to reach the deactivated state. Spineloxide-coated Ti plates were anodically polarized in a 1 M NaOH solution at a current density of 100 mA cm-2. Electrodes were considered to become deactivated when the anode potential increases by ∼5 V over the initial stabilized potential. The variation with time of the anode potential measured with reference to an Ag/AgCl electrode is plotted in Figure 1. The potential in Figure 1 was converted into the RHE scale. The anodic potential stays relatively constant (within ca (10%) along the accelerated lifetime experiment until it undergoes a marked rise, which denotes the onset of deactivation. In accordance with previous reports,27 the service lifetime, expressed as the ratio of the charge passed to the oxide weight (Ah cm-2), decreases as the amount of incorporated Cu increases from x ) 0.0 to x ) 1.0, although it always remains within the same order of magnitude. The surface morphology of the fresh and deactivated electrodes was examined by scanning electron microscopy in a Hitachi S-3000N electron microscope provided with a Rontec
Berenguer et al.
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.
X-ray detector for energy dispersive X-ray microanalysis (EDX). Experimental X-ray diffraction patterns were obtained in a Seifert JSO-DEBYEFLEX 2002 diffractometer by using a Nifiltered Cu KR radiation (λ ) 1.541 Å). Diffraction data points were recorded stepwise at a scan rate of 1°/min with a scan step of 0.05° in 2θ. Diffractograms were given as the average of five consecutive scans. Cell parameters were calculated by a computer program using the peak position obtained after fitting the experimental range with a pseudo-Voigt function per peak plus a background line. Line-broadening analysis was performed to determine the volume-weighed average crystallite size. The XP spectra were obtained in a VG-Microtech Multilab electron 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 a hemispherical analyzer working in the constant energy mode at a pass energy of 50 eV. Binding energies were referenced against the main C1s line of adventitious carbon impurities at 284.6 eV. Peak energies were given to an accuracy of (0.2 eV.AllXPScurveswerefittedwithmixed70-30Gaussian-Lorentzian line-shape functions after nonlinear Shirley background subtraction. Peak areas were normalized by using appropriate atomic sensitivity factors. Cyclic voltammetry experiments were run in a conventional three-electrode cell at room temperature. The counter electrode was a Pt electrode, and the potentials were measured with reference to a reversible hydrogen electrode (RHE) connected with the working solution through a Luggin capillary. Test solutions for cyclic voltammetry (0.1 M NaOH) were prepared from NaOH (Merck, p.a.) and ultrapure water (Purelab Ultra from Elga-Vivendi, 18.2 MΩ · cm). The cyclic voltammograms were obtained at a constant scan rate of 20 mV s-1 at room temperature. The current densities were calculated by considering 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 compact morphology, and the surface roughness is progressively increased in binary Cu-Co spinel oxides as the Cu doping level rises (Figure 2a-d, x ) 0.0, 0.2, 0.8, 1.0). The surface of deactivated Co3O4 electrodes remains as smooth and compact as the fresh electrodes (Figure 2e), but noticeable coating
Deactivation of Spinel CuxCo3-xO4/Ti Anodes
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Figure 2. Scanning electron micrographs of CuxCo3-xO4/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.
detachment is observed near the electrode edges (not shown) leaving only isolated Co oxide aggregates onto the characteristic texture of the etched Ti substrate. In contrast, deactivated CuxCo3-xO4/Ti electrodes undergo noticeable surface morphological modifications on the whole examined area. The extent of surface damage increases with the increasing Cu amount. At x ) 0.2, surface cracks and an increased surface heterogeneity are observed (Figure 2f). At higher Cu contents, extensive loss of the coating is observed, and the remaining oxide appears as randomly distributed grain agglomerates (Figure 2g-h). Higher magnification micrographs show significant sharpening of Cu-Co binary oxide grains in deactivated electrodes (Figure
3d-f), whereas their fresh counterparts possess a rather compact texture, with occasional narrow cracks and grains of globular shape (Figure 3a-c). Energy dispersive X-Ray measurements (Table 1) support the electron microscopy observations. In the undamaged area of pure Co3O4, the Ti level is of about 1%, typical of well-coated Ti electrodes. On the contrary, the amount of Co drops drastically near the edges, and a concomitant increase of the Ti signal is observed. In deactivated CuxCo3-xO4/ Ti electrodes, the Ti content is increased by several orders of magnitude 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
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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 of Fresh and Deactivated CuxCo3-xO4/Ti Electrodes Expressed as Atomic Percentage fresh
deactivated
electrode
% Ti
% Co
% Cu
% Ti
% Co
% Cu
Co3O4/Ti Cu0.2Co2.8O4/Ti Cu0.8Co2.2O4/Ti Cu1.0Co2.0O4/Ti
1 1 1 1
40 25 24 29
1 9 15
1 2 45 24
44 2 2 23
0 0 1
at all, which suggests that copper ionic species suffer from severe preferential dissolution under anodic polarization conditions. 3.2. X-ray Diffraction. The X-ray patterns of a set of fresh and deactivated CuxCo3-xO4/Ti, 0.0 e x e 1.0, electrodes are shown in Figure 4. As reported earlier,27 the diffractograms of fresh pure and Cu-doped cobalt oxides reveal reflections with positions and relative intensities characteristic of a cubic spinel lattice structure. There are some extra lines corresponding to the (002) and (103) crystallographic planes of the underlying substrate. The unit cell constant of the undoped fresh cobalt oxide coating was 8.093 Å, slightly higher than that reported for standard powders. This deviation is probably a consequence of the influence of the support and the preparation method on the crystallographic properties.31,32 The unit cell parameter undergoes an increment as the copper content rises (up to 8.130 Å at x ) 1.0). This behavior was interpreted as an effect of the substitution of Co (II) cations by larger Cu(II) cations.27
The diffraction patterns of the deactivated electrodes show visible changes in relation to the diffraction pattern of fresh electrodes. The deactivated Co3O4 electrode still shows the typical diffraction peaks of a cubic spinel structure, but their relative intensity with respect to the peaks corresponding to the (002) and (102) planes of the Ti substrate substantially decreases after anodic polarization. In addition, extra reflections occur that belong to the (102) plane of Ti. X-ray diffraction data for Co3O4 deactivated electrodes suggest that prolonged anodic polarization brings about an extensive loss of the metal oxide coating. In the less stable binary Cu-Co oxide films, the loss of the coating is so dramatic that the only surviving evidence for the residual spinel phase is the diffraction at the (311) plane. These results are in fairly good agreement with quantitative data deduced from EDX. 3.3. X-ray Photoelectron Spectroscopy. Figure 5 shows the detailed Co2p photoelectronic spectra of a fresh Co3O4/Ti electrode and a set of deactivated CuxCo3-xO4/Ti samples with Cu doping levels up to x ) 1.0. The spectrum of the fresh cobaltite (top) shows two broad and asymmetric photoelectronic peaks with a spin-orbit splitting of about 15 eV. The asymmetry arises from the overlapping of the line for Co(III) in octahedral spinel sites and the much weaker line for Co(II) in tetrahedral sites, which have slightly different binding energies. The main Co 2p3/2 component is located at 779.9 eV (fwhm ≈ 3 eV) and is accompanied by a low-intensity shakeup satellite shifted by 9-10 eV to higher BEs. This shakeup feature is attributed to the presence of high-spin Co(II) in a tetrahedral crystal field,
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Figure 6. Representative Ti 2p photoelectron spectrum of deactivated CuxCo3-xO4/Ti electrodes (x ) 0.8).
Figure 4. X-ray diffraction patterns of fresh and deactivated CuxCo3-xO4/ Ti electrodes; (b) spinel phase, (g) Ti support.
Figure 5. Co2p photoelectron spectra of fresh dry Co3O4/Ti (a), fresh wet Cu1.0Co2.0O4 (b), and deactivated CuxCo3-xO4/Ti electrodes with (c) x ) 0.0, (d) x ) 0.2, (e) x ) 0.8, and (f) x ) 1.0.
which represents only one third of the occupied cationic sites. All these features are diagnostic characteristics of pure Co3O4 with a spinel structure.32-35,42-45 As reported earlier,27 the spectra of fresh Cu-Co mixed oxides supported on Ti (not shown) display a virtually identical shape and position of the main peaks and satellites. This fact was expected, provided that divalent Cu ions enter the oxide lattice to form a solid solution
with preservation of the spinel structure. In fact, curve fitting of the main Cu 2p3/2 core-level line of CuxCo3-xO4 with x e 1.0 also showed that Cu(II) ions fill both tetrahedral and octahedral sites.27 The survey spectrum (not shown) of the deactivated Co3O4 electrode displays the characteristic core-level features of Co, O, Na (from the test electrolyte), and C (adventitious carbon contamination). No evidence for Ti was found, which is coherent with the low Ti percentage determined by EDX and the low small extent of film detachment evidenced by electron microscopy. In the deactivated binary Cu-Co oxide electrodes, copper is detected at low levels (
