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Electrocatalytic behavior of thin Co–Te–O films in oxygen evolution and reduction reactions
Electrochimica Acta 52 (2007) 3794–3803
Electrocatalytic behavior of thin Co–Te–O films in oxygen evolution and reduction reactions V. Rashkova a,∗ , S. Kitova a , T. Vitanov b a
Central Laboratory of Photoprocesses “Acad. J. Malinowski”, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl. 109, Sofia 1113, Bulgaria b Institute of Electrochemistry and Energy Systems, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl. 10, Sofia 1113, Bulgaria Received 27 July 2006; received in revised form 29 September 2006; accepted 28 October 2006 Available online 30 November 2006
Abstract Co–Te–O catalytic films, obtain by vacuum co-evaporation of Co and TeO2 are investigated as electrocatalysts for oxygen reactions in alkaline media. Bifunctional gas-diffusion oxygen electrodes (gde) are prepared by direct deposition of catalyst films on gas-diffusion membranes (gdm) consisting of hydrophobized carbon blacks or hydrophobized “Ebonex” (suboxides of titanium dioxide). Thus obtained electrodes with different atomic ratio RCo/Te of the catalyst, treated at different temperatures were electrochemically tested by means of cyclic voltammetry and steady-state voltammetry. It is shown that the electrodes exhibit high catalytic activity toward oxygen evolution and reduction reaction despite very low catalyst loading of about 0.05–0.5 mg cm−2 . © 2006 Elsevier Ltd. All rights reserved. Keywords: Oxygen reduction; Oxygen evolution; Electrocatalysts; Cobalt oxides; Ebonex support
1. Introduction Bifunctional oxygen electrocatalysts still remain an attractive area of investigation, because of their possible application in oxygen-evolving and reducing gas-diffusion electrodes (gde) for metal/air and metal hydride/air rechargeable batteries and regenerative fuel cells [1,2]. The cobalt oxides have shown promising properties as electrocatalysts for oxygen evolution (oer) [1,3,4] and reduction (orr) [1,5] reactions. It is well known that the electrocatalytic properties of the metal oxides vary according to their method of preparation [6]. In the numerous investigations reported in the literature, the powder of oxides have been prepared by thermal decomposition of mixed nitrates, carbonates or hydroxides [3,6–9], sol-gel [6,10–12] or freeze drying method [13]. Thin films of cobalt spinels have been prepared by chemical spray pyrolysis [4] or by cathodic sputtering [14]. We have used a new method [15–17] for preparing cobalt oxides films by vacuum co-deposition of Co and TeO2 onto substrate held at room temperature. During the vacuum deposition,
∗
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co-evaporated substances are mixed at an atomic level, giving the possibility of obtaining catalysts with various components in the desired proportions. It has been found that during the vacuum deposition onto plane substrate (carbon coated mica or Pt) the following chemical reaction takes place between them, 2Co + TeO2 = 2CoO + Te
(1)
resulting in the formation of CoO and of elemental Te phases [15,17]. The X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) studies have shown that the reaction proceeds until the complete oxidation of Co to Co2+ in the films with atomic ratio RCo/Te < 2 and until the complete reduction of TeO2 to elemental Te in the films with RCo/Te > 2 [15,17]. It is well known the importance of catalyst support for the oxygen electrodes. Typically, the support should provide good electronic conductivity, proper physical surface necessary for achieving high surface area as well as porous structure. It should be also hydrophobic to allow gas diffusion from the gas side to the reaction surface sites through the pores. The most commonly used material for the support is a mixture of Teflon and carbon blacks [18,19]. The carbon material has good electronic conductivity and provides a suitable porous structure
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with high accessible surface area, but it is not stable enough during oxygen evolution process. In order to eliminate the disadvantages of the carbon, support we have replaced carbon blacks with Ebonex. Conductive oxide supports of particularly reduced titanium oxides have been used in electrolyzers and are important candidates for use in the oxygen electrodes of utilized regenerative fuel cell (URFC) [20]. The Ebonex (Altraverda Ltd., UK) is an electrically conductive ceramic consisting of several suboxides of titanium dioxide, mainly Ti4 O7 and Ti5 O9 , which are the most conductive compounds in homologous series of structures with general formula Tin O2n−1 (4 ≤ n ≤ 10), collectively known as Magneli phases [20]. Ebonex has a unique combination of high conductivity (σ ≈103 −1 cm−1 ), good corrosion resistance and it is electrochemically stable in both acidic and alkaline solutions [20]. Moreover, using titanium suboxides in the gdm we could expect to modulate the properties of the active layer in a way to reach a hypo-hyper-d-electronic structures leading to synergetic catalytic effect [21]. The purpose of this paper is to summarize our results on the structure and electrocatalytic properties of Co–Te–O thin films, obtained by vacuum co-deposition of Co and TeO2 directly onto gas-diffusion membrane. We have studied the influence of the atomic ratio RCo/Te , loading, thermal treatment and type of the gdm (carbon or Ebonex) on the morphology and electrocatalytic behavior of the films. 2. Experimental 2.1. Preparation of the catalyst and gas-diffusion electrodes The catalytic films were prepared by thermal co-evaporation of Co by electron gun and TeO2 by physical evaporation from Knudsen type cell under vacuum higher than 10−4 Pa. The evaporation of TeO2 was carried out from platinum crucible, while those of Co—from glassy carbon one. The condensation rates of each substance within the range 0.01–0.04 g cm−2 s−1 were controlled separately during the evaporation by quartz crystal monitor. The substances were deposited on a stationary substrate, placed above the crucibles. In most of the cases gasdiffusion membranes were used as substrates. Two types of gdm were studied—the first one, carbon-gdm, consists of hydrophobized carbon (45% Teflon and 55% acetylene carbon blacks) and the second one, Ebonex-gdm, is a sandwich system made by two layers: hydrophobized carbon blacks/hydrophobized Ebonex (20% Teflon + 80% Ebonex–300 m powder, Atraverda Ltd.). Both supports were hot pressed on Ni-screen which plays role of a current collector [22]. The system of the catalytic film condensed on the carbonor Ebonex-gdm represents the gas-diffusion electrode (carbonor Ebonex-gde), the catalyst being the active layer, while the membrane with Ni-screen serves as a current collector inside the gas-supplying layer of the electrode, the gas being supplied by diffusion. The geometric area of gde was 1 cm2 . The effective loading of the catalytic film was in the range 0.05–0.5 mg cm−2 of geometric surface, which corresponds to a nominal film thickness of about 110–1100 nm. Some of the gas-diffusion electrodes were heated for 3 h at 100–300 ◦ C in air.
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2.2. Characterization of the as-prepared catalyst The amount of each condensed substance and the atomic ratio RCo/Te at each point of the substrate were calculated from the crystal monitors data. The chemical compositions of the catalyst deposited on the substrate were controlled with an X-ray microanalyzer in scanning electron microscope (JEOL Superprobe 733 with mounted System 5000—HNU). Scanning electron microscopy (Philips 515) was applied for examining the surface morphology of the electrodes. The chemical states of the elements in the catalytic films deposited on the gdm were studied with X-ray photoelectron spectroscopy (XPS). The measurements were carried out in UHV chamber of the electron spectrometer ESCALAB MkII. The spectra were exited with an AlK␣ source with energy 1486.6 eV. The photoelectron lines of C1s, O1s, Te3d and Co2p were recorded. All spectra were calibrated using C1s line at 285.0 eV as a reference. The specific surface area was measured by low temperature nitrogen adsorption according to BET. 2.3. Electrochemical measurements The electrochemical study was carried out by means of cyclic voltammetry and steady-state galvanostatic voltammetry using equipment consisting of potentiostat–galvanostat provided with a pulse generator (Solartron 1286 electrochemical interface). The gas-diffusion electrodes were tested in a three-electrode cell at room temperature. The electrolyte was 20% KOH, prepared with double distilled water. High surface area Pt was used as a counter electrode. The potential was measured versus Hg/HgO reference electrode in 20% KOH, reaching close to working electrode by a Luggin capillary. Cyclic voltammetry was carried out in the potential region between −1000 and +650 mV versus Hg/HgO. The system was deareated by supplying Ar through the membrane and bubbling it in the electrolyte before and during CV measurements. Cathodic polarization curves were measured under a flow of pure oxygen supplied to the air side of the electrodes. 3. Results and discussion 3.1. Physical characterization of the catalytic film The results obtained indicate that the XPS spectra of films deposited on gdm are identical to those of films on plain (glass, Si or Pt) substrates [26–28]. In Fig. 1a–c are illustrated Co2p, O1s and Te3d spectra of a film with RCo/Te = 1.5, and 0.2 mg cm−2 loading (450 nm thickness), deposited on gdm with carbon blacks and treated at different temperatures in air. The Co2p spectra show typical Co2+ state. The second peak which is shifted 6.2 eV to higher binding energy from the first one is known as a “shake-up” satellite which is exclusively observed in CoO [23]. It should be noted that we have detected elemental Co only in the volume of the films with atomic ratio RCo/Te > 2 [15]. It can be seen that the thermal annealing up to 300 ◦ C does not cause any changes in the Co2p spectrum. The broad, complicated shape of the peak
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on the surface of the catalyst film during the thermal treatment in air. From Eq. (1) follows that in films with RCo/Te = 1.5 the elemental Co should be completely oxidized to Co2+ while the unreacted TeO2 should remain. We have found that the amount of Te0 increases and that of Te4+ decreases with depth profiling [15]. Te3d spectra in Fig. 1c show the presence of Te4+ and small amount of elemental Te on the surface of fresh film and that thermally treated at 100 ◦ C. Most probably, partial oxidation of elemental Te proceeds on the surface of the film in air. As seen further thermal annealing of the films at 300 ◦ C causes significant decrease in Te4+ peak intensity and disappearing of Te0 peak on the film surface. As a whole, the amount of tellurium phase on the surface decreases about twice most probably because of the evaporation of Te at that temperature or because of segregation of the cobalt on the surface. SEM micrographs in Fig. 2 illustrate the surface morphology of the same film and of bare gas-diffusion membranes for comparison. One very rough film surface made of intertwined wires following the rough surface of the membrane is seen for the as-deposited films. It was found that this morphology does not change after thermal annealing up to 300 ◦ C. It should be noted that the surface morphology shown in Fig. 2 is typical for all films deposited on the gdm independently of their thickness, atomic ratio RCo/Te , thermal treatment as well as the type of the gdm—carbon or Ebonex. It was only found that there is an increase in the “wire” diameter with increasing the film thickness. The results obtained by TEM have shown that the “wires” consist of amorphous matrix with nanosized crystals CoO embedded in it [26,27]. However, XRD spectra of the films deposited on both gdms and thermally treated at different temperatures indicate that the fresh film and that treated up to 100 ◦ C are amorphous or nanocrystalline. As seen in Fig. 3, the spectra of 450 nm thick film with RCo/Te = 1.5 on carbon blacks do not show presence of peaks different from those of the substrate. Obviously, the size of the crystals detected by TEM in as deposited film, or appearing after a possible crystallization at 100 ◦ C are very small to be detected by XRD. New peaks are observed after annealing at 200 and 300 ◦ C. The peaks can be associated with cubic and monoclinic CoO phase, as well as hexagonal tellurium. 3.2. Electrochemical measurements on carbon-gde
Fig. 1. XPS spectra of as-deposited and thermally treated at 100 and 300 ◦ C 450 nm thick films with RCo/Te = 1.5: (a) Co2p, (b) O1s and (c) Te3d spectra.
at 530.5 eV, observed in O1s spectra of the as-deposited and treated at 100 ◦ C samples is typical for polycrystalline cobalt oxides samples [24]. Such broadness is assigned to presence of non-stoichiometric oxygen whose concentration varies with the sample treatment procedure [25]. The peak’s shift of 0.5 eV to higher binding energies observed after annealing at 300 ◦ C is most probably due to an increasing of the quantity of the nonstoichiometric oxygen and/or to formation of some hydroxides
3.2.1. Steady-state measurements As far as the catalytic activity of cobalt oxides is known in the literature, the question arises: what is the role of Te in these reactions? In our previous paper [16] we have shown that TeO2 and Te themselves exhibit some activity, which is significantly lower, especially for Te, than that of the Co–Te–O films. Fig. 4 shows the potentials for oxygen evolution reaction, obtained at current density of 20 mA cm−2 versus atomic ratio RCo/Te . The values are average from measurements of four gde and the bars show maximum deviations. It is seen that gdes with RCo/Te from 1.4 to 1.8 exhibit the best characteristics for oer. The measured values of the potentials for this range of atomic ratio are almost the same, since the measurement accuracy is ±15 mV.
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Fig. 2. SEM micrographs of: (a) carbon-gdm; (b) a film with RCo/Te = 1.5 and loading of 0.2 mg cm−2 deposited on carbon-gdm; (c) Ebonex-gdm; (d) a film with the same RCo/Te and loading on Ebonex-gdm.
It should be noted that it was impossible to determine precisely the influence of the atomic ratio RCo/Te on the catalytic activity for oxygen reduction reaction because of the low reproducibility of the results obtained [28]. The small amount of catalyst has not completely covered the membrane surface.
Therefore, it can be expected that the electrolyte will be in contact with both catalyst and carbon surface area and the catalytic activity of the gde will be the sum of the activity of the membrane and the film. It is well known that carbon itself is active to oxygen reduction but the activity of carbon-gdm has very low reproducibility. For this reason the precise estimation of the cat-
Fig. 3. XRD spectra of carbon-gdm and 450 nm thick films with RCo/Te = 1.5, deposited on it, fresh and thermally treated at different temperatures.
Fig. 4. Dependence of the potential at i = 20 mA cm−2 for oer on the atomic ratio RCo/Te at catalyst loading of 0.05 mg cm−2 .
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alytic activity of the film itself toward oxygen reduction reaction is difficult to be assessed. To study how the catalyst activity toward oxygen reactions depends on the catalyst loading, we have prepared gdes with atomic ratio RCo/Te of about 1.5 and different amount of the catalyst within the range 0.05–0.5 mg cm−2 . Fig. 5 shows the potentials measured at current density of 20 mA cm−2 for oer and orr versus the amount of the catalyst deposited on the membrane. The values are again average from measurement of six gdes and the bars show maximum deviations. For easier comparison, data obtained for the gde without catalyst (gdm) are also given. The results obtained show that the increase in the catalyst loading from 0.05 to 0.5 mg cm−2 does not lead to significant increase in the catalyst activity toward oxygen evolution. As seen in Fig. 5a the difference in the measured values of the potentials for oer is about 30 mV. Since the surface of gde takes part in the electrocatalytic reactions, obviously one very high surface area can be achieved even at small amount of catalyst of about 0.05 mg cm−2 . This conclusion is confirmed by the SEM study of the gas diffusion electrodes. How the increase of the loading influences the catalytic activity toward oxygen reduction again is difficult to be estimated precisely because of the poor reproducibility of the results. Fig. 5b indicates however a tendency for improving the reproducibility of the catalytic activity with the increasing the amount of catalyst. For that reason we chose the loading of 0.2 mg cm−2 as optimum one. The anodic and cathodic polarization curves of electrodes, with optimal loading and atomic ratio, treated at different temperatures are presented in Fig. 6. The best characteristic toward
Fig. 6. Anodic (a) and cathodic (b) polarization curves of gdes with loading of 0.2 mg cm−2 and RCo/Te ≈ 1.5, treated at indicated temperatures and curves of 0.8 mg cm−2 Pt, deposited on carbon blacks “Vulkan”. Cathodic curves were measured under a flow of pure oxygen supplied to the air side of the electrodes.
Fig. 5. Dependence of the potential at i = 20 mA cm−2 for (a) oer and (b) orr on the amount of catalyst with RCo/Te ≈ 1.5.
oxygen evolution reaction shows the fresh electrode, while those treated at higher temperatures exhibit lower activity. Despite the fact that the results in the cathodic direction are not well reproducible, we have found a well defined tendency of increasing the catalytic activity after annealing at 100 ◦ C. As a rule gde with higher catalytic activity for oxygen reduction reaction exhibits lower activity for oxygen evolution reaction and vice versa. Most probably this tendency is due to the different nature of the active sites for both reactions. In the Fig. 6 the polarization curves of our electrodes are compared with the polarization curves of gde, consisting of gdm and catalytic layer of platinum on Vulcan XC72 (commercial product of E-TEK Division of DE NORA, North America). The amount of the Pt/C catalyst was calculated in a way to have 0.8 mg cm−2 Pt, which is bigger than the amount of the Co–Te–O catalyst. It can be seen that all gdes with Co–Te–O catalyst exhibit better catalytic activity for the anodic reaction and that one of them comparable characteristic with Pt/C even for the cathodic reaction. In Fig. 7 the Tafel plots for both reactions obtained for selected electrodes are depicted. Two linear segments, typically for oer and orr are obtained at low and high current densities. For oxygen evolution reaction values of b ≈ 45 mV/decade were found at low current densities. This slope is in the range of most commonly observed for the oxides (40–60 mV/decade) in the literature [29]. As seen from the figure the thermal annealing
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Fig. 7. Tafel plots for anodic and cathodic reactions of gdes with loading of 0.2 mg cm−2 and RCo/Te = 1.5, fresh and treated at 100 ◦ C.
leads to increasing in the slopes up to 90–120 mV/decade. This is probably a result of changing in the surface structure after annealing which is in agreement with some authors, pointing out that the Tafel slopes are sensitive to the surface structure of the catalysts [29]. The increasing of the Tafel slope almost twice is an indication that most probably the structure modifying leads to changing in the rate determining step [30] and/or in the surface coverage with OH− groups [31,32]. As it was mentioned above, the results for oxygen reduction reaction are not very reproducible which also reflects on the values of the Tafel slopes. The slope in the low current domain was found to vary between −50 and −80 mV/decade even higher, in rare cases. Nevertheless, a tendency of slightly decreasing of the Tafel slopes in the low current domain was found as an effect of annealing up to 200 ◦ C. 3.2.2. Cyclic voltammetry Cyclic voltammograms of gdes, fresh and thermally treated at different temperatures are given in Fig. 8a. The electrodes have optimal catalyst loading of 0.2 mg cm−2 and atomic ratio RCo/Te = 1.5. For comparison in Fig. 8b the curves of gdm, the Te and TeO2 films on gdm are given. It is seen that gdm, Te and TeO2 do not have any contribution to the peaks observed for Co–Te–O films. CV curves exhibit several very broad, not well defined peaks, prior to the oxygen evolution. The first single anodic peak a1, observed only for fresh and thermally treated at 100 ◦ C gdes has a maximum at −50 mV. Most probably a redox transition of CoO/CoOOH proceeds with calculated potential of −54 mV versus Hg/HgO [33]. The pair of peaks a2/c2 can be associated with formation of a redox couples CoO/CoO2 with calculated potential at +195 mV and/or Co(OH)2 /CoO2 at +254 mV [33]. Our detailed study showed that a third anodic peak a3 appears at +560 mV. In Fig. 8a this peak looks like a shoulder, but it becomes well depicted with increasing the scan rates. It should be noted that all peaks discussed are more clearly defined in CVs of films deposited on plane substrate (Pt). The potentials at which the peak a3 and its corresponding cathodic peak c3 appeared are an indication that the redox transition CoOOH/CoO2 at +562 mV versus Hg/HgO occurs [33]. Fig. 8a shows that the position of the main peaks is little influenced
Fig. 8. Cyclic voltammograms of: (a) gdes with Co–Te–O films, thermally treated at the indicated temperatures; (b) gdm and Te and TeO2 films deposited on it.
by the thermal treatment although a small shift to less positive values is detected for gde treated at 300 ◦ C. Most probably, this effect is due to the presence of hydroxides or non-stoichiometric oxygen on the surface as proved by the XPS spectra of electrode treated at 300 ◦ C. It is clearly seen from Fig. 8 that as a rule all peaks are broad, most probably as a result of the heterogeneity of the size distribution of the active surface sites [29] or to the high degree of porosity or/and roughness, which affects the energy of the active sites thus leading to broadening of the peak [34]. The position of the peak is indicative of the site’s chemical nature, while the area under the peak is proportional to the numbers of sites oxidized or reduced [29]. The resolution of broad, not well defined peaks is however very difficult. In this case the voltammetric charge over the whole potential range (q* ) can be taken as relative measure of the electrochemically active surface area. The values of q* obtained cannot be easily converted into an absolute value since the precise nature of surface redox transitions is unknown, but q* has proved very useful when different electrodes or different preparation procedures are being compared [29,35,36]. To estimate the actual surface area of the oxide films taking place in the surface redox reactions, we have calculated the surface charge q* by integrating the voltammetric curves applying the CV technique, proposed by Trasatti and coworkers [35,36] for thick films deposited on plane substrates. The common observation with oxide electrodes is that q* decreases as the potential
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Fig. 10. The normalized total, inner and outer voltammetric charges, as a function of the annealing temperature.
Fig. 9. Extrapolation of the voltammetric charge to: (a) ν = 0 and (b) ν = ∞ of the gdm and gde, fresh and annealed at 100 ◦ C.
scan rate ν increases. It has been shown, that q* can be extrapolated to ν = 0, thus providing the total surface charge qt∗ , as well as to ν = ∞, thus giving the outer surface charge q0∗ [36]. The difference qt∗ − q0∗ = qi∗ provides the inner surface charge, i.e. the charge associated with the less accessible surface regions (pores, cracks, grain boundaries, etc.), where protons diffuse with difficulty [37]. The extrapolation procedure for selected electrodes is illustrated in Fig. 9. For comparison the results for gdm are also given. It can be seen from Fig. 9a that the extrapolation to ν = 0 is closely linear, which is in agreement with the results for thick films on plane substrates [35,36]. Some deviation from the linearity is observed only for gdm at higher scan rates. The extrapolation to ν = ∞ is also linear for all gdes (Fig. 9b) studied. Some deviation from the linearity is only observed at the highest scan rates, which is in agreement with the results obtained for thick films [36]. Fig. 10 shows the normalized total, outer and inner voltammetric charges as a function of the annealing temperature of the gdes with optimal atomic ratio and loading. It is seen that total charge qt∗ , representing the concentration of the active sites, goes through a maximum at 100 ◦ C. The maximum at 100 ◦ C is due mainly to the increase in inner voltammetric charge while the outer voltammetric charge remains almost unchanged. This
suggests that the external morphology (macro-roughness) is not strongly dependent on the annealing temperature, as is confirmed by the SEM study. Further, we have found that the annealing at 100 ◦ C of the gas-diffusion membrane, which is known to have some catalytic activity toward oxygen reduction reaction, does not lead to any changes in the voltammetric (qt∗ , q0∗ and qi∗ ) charges. As it was mentioned above, the XPS analysis did not show any changes in the chemical state of the catalytic films annealed at temperatures up to 300 ◦ C. Therefore most probably the increase in the concentration of the active sites for electrodes annealed at 100 ◦ C is caused by full crystallization of the catalytic film but the size of the crystallites remains still very small to be detected by conventional XRD analysis. The abrupt drop in the inner active surface area of electrodes annealed at 200 and 300 ◦ C can be explained by changes, caused by both melting of the Teflon in gdm at temperatures above 200 ◦ C and the increasing size of the catalyst crystallites. According to Trasatti and coworkers [35,36], if the total charge qt∗ is related to the whole active surface, the ratio qi∗ /qt∗ can be taken as representing the “electrochemical porosity” of the electrode. For determining the porosity and/or the roughness factors Da Silva et al. [37] proposed another technique based on the dependence of the capacitive current ic on the scan rate ν, observed in short capacitive potential region of about 50 mV. In analogy with Trasatti and coworkers [35], the authors have correlated the two linear segments, observed in the low and high ν domains of the ic versus ν plot, with the morphology of highly porous or rugged films. They have attributed the change in the slope observed in the high ν domain to the exclusion of surface areas located in the more difficult-to-access regions. Thus the slopes of the two linear parts of the ic versus plot give the total Cd,t , and the external Cd,e differential capacity while the difference between them Cd,t − Cde gives the capacity Cd,i for the “internal” film regions and the ratio Cd,i /Cd,t is the morphology factor ϕ. Based on the procedure proposed by Da Silva et al. the voltammetric curves were recorded at various sweep rates. Since no clear capacitive potential region was detected on the curves (see Fig. 8) we recorded the voltammograms, covering the
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Fig. 11. Dependence of the capacitive current on sweep rate, ν for gdm, fresh and annealed at 100 ◦ C gdes with RCo/Te = 1.5 and 0.2 mg cm−2 loading. The inset shows the same dependence but the scale of the ic axis is changed in order to see the first linear segment for the fresh-gde.
potential range between 0 and 50 mV versus Hg/HgO. A representative graph is shown in Fig. 11. The gdes show two linear segments at low and high sweep rates, which is in agreement with the literature [37]. The gas-diffusion membrane shows only one linear segment (see the inset of Fig. 11) which means that gdm posses very high degree of porosity and all active centers are maximum available in the scan rates range within 2 ÷ 200 mV s−1 . The results for the electrochemical porosity and morphology factors for fresh and thermally treated electrodes with optimal loading of 0.2 mg cm−2 and RCo/Te = 1.5 are given in Table 1. For comparison the data from the BET analysis are also given. The results for the total charge are in accordance with those for the specific surface area, obtained by BET analysis which also indicate similar tendency of increasing the surface area from 25 to 27 m2 g−1 for gdm and fresh gde to 33 m2 g−1 for treated at 100 ◦ C gde. Keeping in mind the very low catalyst loading of 0.2 mg cm−2 this means that the specific surface area of the annealed film itself is much larger. Moreover, the results show that the gas-diffusion membrane itself has a little influence on the voltammetric charges, obtained for gdes. The qt∗ values of the gdm are about 10 times lower than those of the films deposited on it. As can be seen from the table, there is a contradiction between the two factors and . The electrochemical porosity increases while the morphology factor decreases for the carbon-electrode
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Fig. 12. Cyclic voltamograms of Ebonex-gdm, Ebonex- and carbon-gdes with catalytic films with RCo/Te = 1.5 and loading of 0.2 mg cm−2 .
annealed at 100 ◦ C. Both the electrochemical porosity and the morphology factor describe the ratio between the more-difficultto-access and total surface areas and can take values between 0 and 1. Their physical meaning however is not exactly the same since the electrochemical porosity includes the solid state surface redox transitions (SSSRT), while the morphology factor should be free of the SSSRT or of the minor importance [37]. Having in mind that the CV curves obtained by us have not pure capacitive region one can expect that some SSSRT, occurring in the narrow potential region should affect the values of the morphology factor. Another reason for the difference is possible dependence of Cd on the potential, which is not taken into account in calculating in the narrow potential range used. These effects are most probably the reason for the lack of correlation found between the electrochemical porosity and the morphology factor, and in our opinion make the application of the morphology factor inappropriate for our electrodes. 3.3. Electrochemical measurements on Ebonex-gde Fig. 12 compares cyclic voltammogram of Ebonex-gde with that of carbon-gde. In the electrodes catalytic films have optimal atomic ratio RCo/Te and loading. The CV of Ebonex-gdm is also given. It is seen that Ebonex-gdm does not contribute to the peaks observed for the Ebonex-gde. The comparison with carbon-gde shows that the positions of the peaks, asso-
Table 1 SBET , and for different electrodes GDE Carbon-gdm Carbon-gdm\Co–Te–O film Ebonex-gdm Ebonex-gdm\Co–Te–O film
Fresh Annealed at 100 ◦ C Fresh Annealed at 100 ◦ C
SBET (m2 g−1 )
Total charge (mC cm−2 ) qt∗
Electrochemical porosity, qi∗ /qt∗
Morphology factor, Cd,i /Cd
25 27 33 – – –
29 207 617 250 3205 1000
0.88 0.88 0.95 0.57 0.89 0.71
1 0.91 0.56 – – –
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ciated with the formation of redox couples CoO/CoO2 (a4) and CoOOH/CoO2 (a5) are slightly shifted to less positive potential values for the Ebonex-gde. These peaks are, however, not very well seen in the figure because of the peaks broadness. The CV of the Ebonex-gde exhibits several additional anodic peaks in the negative potential region (at a1 = −650, a2 = −200 mV) which can be attributed to redox transitions between Co2+ /Co3+ and Co/Co2+ [33]. A relatively well defined oxidation peak a3 observed at +49 mV, can also be connected with Co2+ /Co3+ transition [33]. The data given in Table 1 indicate that both carbon- and Ebonex-gdms have relatively low values of total voltammetric charges, i.e. number of active sites compared to those of the corresponding gdes. From the values of (Table 1) follows that the sites are distributed preferentially in the inner surface of carbon-gdm, while in the case of the Ebonex-gdm, they are uniformly distributed between the outer and inner surface. In this sense the carbon-gdm electrode exhibits more “electrochemical porosity” than the Ebonex-gdm most probably as a result of the significantly smaller particle size of the carbon powder used and higher Teflon content in it. The data given in the Table 1 clearly show that electrochemical porosity of the fresh carbonand Ebonex-electrodes is the same, but the total, outer and inner voltammetric charges obtained for the Ebonex-gde are about one order of magnitude higher than those of carbon-gde. This abrupt increase in the number of active sites participating in the surface redox reactions is most probably a result of the geometry
effect of larger particle size of Ebonex. From Table 1 follows that, unlike carbon-gde, the annealing of Ebonex-gde leads to decrease in the total charge and porosity, which is difficult for now to be explained. The anodic and cathodic polarization curves of Ebonex- and carbon-gdes are compared in Fig. 13. Despite the high number of active sites found for Ebonex-gdes they show lower catalytic activity than the carbon-gdes, especially for oer. The detailed analysis indicates, however, that for orr the Ebonex-gdes have better reproducibility and stability than carbon-gdes. The reason for the lower activity is probably the larger size of the Ebonex particles and the lower degree of hydrophobization. This allows the electrolyte to go deeper into the pores thus leading to transport hindrances of the reaction products. We hope that the optimization of the gas-diffusion membrane with Ebonex will lead to better results. 4. Conclusion The results reported in the paper show that thin electrocatalytic films of Co–Te–O, obtained by vacuum codeposition of TeO2 and Co can be used for preparing bifunctional electrodes for oxygen evolution and reduction reactions. During vacuum deposition, the co-evaporated compounds are mixed at atomic level and a chemical reaction between them takes place resulting in the formation of CoO phase and of elemental Te. This gives a possibility to obtain catalyst with various components in desired proportions and high surface/volume ratio. The results from the electrochemical measurements indicate that the films, deposited by this method have high catalytic activity toward oxygen evolution reaction even with low catalyst loading of 0.05–0.5 mg cm−2 . The as-deposited catalytic films with atomic ratio RCo/Te = 1.4–1.8 have the best characteristics for the oxygen evolution reaction. The catalytic activity of the films for the oxygen reduction reaction is difficult to be estimated precisely due to the poor reproducibility of the gdm characteristics. The fresh carbon-gdes have the best catalytic activity toward oer. Their thermal annealing at 100 ◦ C causes pronounced changes in the concentration of the active sites, participating in the surface redox reactions. This leads however to increasing the catalytic activity only toward orr. Our first results obtained for Ebonex-electrodes indicate that they have significantly higher number of active site participating in the surface redox reactions but lower catalytic activity than carbon-electrodes. We believe that an optimization of the Ebonex-gas-diffusion membrane is needed to obtain better results. Acknowledgements
Fig. 13. Anodic and catodic polarization curves of Ebonex-gdm, Ebonex- and carbon-gdes with catalytic films with RCo/Te = 1.5 and loading of 0.2 mg cm−2 .
This work was realized with the financial support of the Bulgarian National Science Fund under contract # MY-X 1402. The authors are gratefully obliged to thank to Prof. D. Mehandjiev and Dr. N. Velichkova for carrying out BET analysis; to Dr. G. Tujliev – for XPS analysis; and Tz. Iliev – for EDS analysis.
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References [1] M. Tarasevich, B. Efremov, in: S. Trasatti (Ed.), Electrodes of Conductive Metallic Oxides, Part A, Elsevier, Amsterdam, 1981. [2] S. Trasatti, Electrochim. Acta 26 (1981) 329. [3] I. Nicolov, R. Darkaoui, E. Zhecheva, R. Stoyanova, N. Dimitrov, T. Vitanov, J. Electroanal. Chem. 429 (1997) 157. [4] R. Singh, J. Koenig, G. Poillerat, P. Chartier, J. Electroanal. Chem. 314 (1991) 2411. [5] S. Jiang, Z. Lin, A. Tseung, J. Electrochem. Soc. 137 (3) (1990) 764. [6] J. Marco, J. Gancedo, M. Gracia, J. Gautier, E. Rios, F. Berry, J. Solid State Chem. 153 (2000) 74. [7] J. Haenen, W. Visscher, E. Barendrecht, J. Electroanal. Chem. 208 (1986) 273. [8] H. Carapuca, M. Pereira, F. Costa, Mater. Res. Bull. 25 (1990) 1183. [9] W. King, A. Tseung, Electrochim. Acta 19 (1974) 486. [10] F. Svegl, B. Orel, M.G. Hutchins, K. Kalcher, J. Electrochem. Soc. 143 (1996) 1532. [11] M.L. Baydi, M.G. Poillerat, J.L. Gautier, J.L. Rehspringer, J.F. Koenig, P. Chartier, J. Solid State Chem. 109 (1994) 278. [12] M.L. Baydi, S.K. Tiwari, R.N. Singh, J.L. Rehspringer, P. Chartier, J.F. Koenig, M.G. Poillerat, J. Solid State Chem. 116 (1995) 157. [13] P. Rasiyah, A. Tseung, J. Electrochem. Soc. 130 (No.12) (1983) 2384. [14] B. Nkeng, G. Poillerat, J. Koenig, P. Chartier, B. Lefez, J. Lopitaux, M. Lenglet, J. Electrochem. Soc. 142 (6) (1995) 1776. [15] S. Kitova, V. Rashkova, I. Konstantinov, Vacuum 69 (2003) 405. [16] V. Rashkova, S. Kitova, I. Konstantinov, T. Vitanov, Electrochim. Acta 47 (2002) 1555. [17] V. Rashkova, S. Kitova, L. Litynska, T. Vitanov, Vacuum 76 (2004) 147. [18] Y. Shimizu, K. Uemura, H. Matsuda, N. Miura, N. Yamazoe, J. Electrochem. Soc. 137 (11) (1990) 3430.
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[19] Z. Hou, B. Yia, H. Zhang, J. Electrochem. Solid State Lett. 6 (11) (2003) A232. [20] G. Chen, S. Bare, T. Mallouk, J. Electrochem. Soc. 149 (8) (2002) A1092. [21] M. Jaksic, J. New Mater. Electrochem. Syst. 3 (2000) 153. [22] T. Vitanov, I. Nikolov, K. Petrov, E. Zecheva, R. Stoyanova, Bulg. Patent, 101585/06.06 (1997). [23] M. Tanaka, M. Mukai, Y. Fujimori, M. Kondoh, Y. Tasaka, H. Baba, S. Usami, Thin Solid Films 281/282 (1996) 453. [24] V. Jimenez, A. Fernandez, J. Espinos, A. Gonzalez-Elipe, J. Electron Spec. Related Phen. 71 (1995) 61. [25] T. Chuang, C. Brundle, D. Rice, Surf. Sci. 59 (1976) 413. [26] V. Rashkova, S. Kitova, T. Vitanov, J. Optoelectron. Adv. Mater. 7 (No.1) (2005) 451. [27] V. Rashkova, S. Kitova, L. Litynska, T. Vitanov, in: E. Balabanova, I. Dragieva (Eds.), Nanoscience & Nanotechnology, vol. 3, Heron Press, Sofia, 2003, p. 127. [28] V. Rashkova, S. Kitova, T. Vitanov, in: E. Balabanova, I. Dragieva (Eds.), Nanoscience & Nanotechnology, vol. 4, Heron Press, Sofia, 2004, p. 184. [29] S. Trasatti, in: J. Lipkowski, P.N. Ross (Eds.), The Electrochemistry of Novel Materials, VCH, Weinheim, 1994. [30] B. Efremov, M. Tarasevich, Electrochymia XVII 11 (1981) 1672. [31] J. Bockris, T. Otagawa, J. Electrochem. Soc. 131 (2) (1984) 290. [32] J. Bockris, T. Otagawa, J. Phys. Chem. 87 (1983) 2960. [33] W. Behl, J. Toni, J. Electroanal. Chem. 31 (1971) 63. [34] M. Santana, L. De Faria, J. Boodts, Electrochim. Acta 49 (2004) 1925. [35] R. Boggio, A. Carugati, S. Trasatti, J. Appl. Electrochem. 17 (1987) 828. [36] G. Spinolo, S. Ardizzone, S. Trasatti, J. Electroanal. Chem. 423 (1997) 49. [37] L.M. Da Silva, L.A. De Faria, J.F.C. Boodts, Electrochim. Acta 47 (2001) 395.
Electrocatalytic behavior of thin Co–Te–O films in oxygen evolution and reduction reactions V. Rashkova a,∗ , S. Kitova a , T. Vitanov b a
Central Laboratory of Photoprocesses “Acad. J. Malinowski”, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl. 109, Sofia 1113, Bulgaria b Institute of Electrochemistry and Energy Systems, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl. 10, Sofia 1113, Bulgaria Received 27 July 2006; received in revised form 29 September 2006; accepted 28 October 2006 Available online 30 November 2006
Abstract Co–Te–O catalytic films, obtain by vacuum co-evaporation of Co and TeO2 are investigated as electrocatalysts for oxygen reactions in alkaline media. Bifunctional gas-diffusion oxygen electrodes (gde) are prepared by direct deposition of catalyst films on gas-diffusion membranes (gdm) consisting of hydrophobized carbon blacks or hydrophobized “Ebonex” (suboxides of titanium dioxide). Thus obtained electrodes with different atomic ratio RCo/Te of the catalyst, treated at different temperatures were electrochemically tested by means of cyclic voltammetry and steady-state voltammetry. It is shown that the electrodes exhibit high catalytic activity toward oxygen evolution and reduction reaction despite very low catalyst loading of about 0.05–0.5 mg cm−2 . © 2006 Elsevier Ltd. All rights reserved. Keywords: Oxygen reduction; Oxygen evolution; Electrocatalysts; Cobalt oxides; Ebonex support
1. Introduction Bifunctional oxygen electrocatalysts still remain an attractive area of investigation, because of their possible application in oxygen-evolving and reducing gas-diffusion electrodes (gde) for metal/air and metal hydride/air rechargeable batteries and regenerative fuel cells [1,2]. The cobalt oxides have shown promising properties as electrocatalysts for oxygen evolution (oer) [1,3,4] and reduction (orr) [1,5] reactions. It is well known that the electrocatalytic properties of the metal oxides vary according to their method of preparation [6]. In the numerous investigations reported in the literature, the powder of oxides have been prepared by thermal decomposition of mixed nitrates, carbonates or hydroxides [3,6–9], sol-gel [6,10–12] or freeze drying method [13]. Thin films of cobalt spinels have been prepared by chemical spray pyrolysis [4] or by cathodic sputtering [14]. We have used a new method [15–17] for preparing cobalt oxides films by vacuum co-deposition of Co and TeO2 onto substrate held at room temperature. During the vacuum deposition,
∗
Corresponding author. Tel.: +359 2 979 3505; fax: +359 2 722465. E-mail address: [email protected] (V. Rashkova).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.10.054
co-evaporated substances are mixed at an atomic level, giving the possibility of obtaining catalysts with various components in the desired proportions. It has been found that during the vacuum deposition onto plane substrate (carbon coated mica or Pt) the following chemical reaction takes place between them, 2Co + TeO2 = 2CoO + Te
(1)
resulting in the formation of CoO and of elemental Te phases [15,17]. The X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) studies have shown that the reaction proceeds until the complete oxidation of Co to Co2+ in the films with atomic ratio RCo/Te < 2 and until the complete reduction of TeO2 to elemental Te in the films with RCo/Te > 2 [15,17]. It is well known the importance of catalyst support for the oxygen electrodes. Typically, the support should provide good electronic conductivity, proper physical surface necessary for achieving high surface area as well as porous structure. It should be also hydrophobic to allow gas diffusion from the gas side to the reaction surface sites through the pores. The most commonly used material for the support is a mixture of Teflon and carbon blacks [18,19]. The carbon material has good electronic conductivity and provides a suitable porous structure
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with high accessible surface area, but it is not stable enough during oxygen evolution process. In order to eliminate the disadvantages of the carbon, support we have replaced carbon blacks with Ebonex. Conductive oxide supports of particularly reduced titanium oxides have been used in electrolyzers and are important candidates for use in the oxygen electrodes of utilized regenerative fuel cell (URFC) [20]. The Ebonex (Altraverda Ltd., UK) is an electrically conductive ceramic consisting of several suboxides of titanium dioxide, mainly Ti4 O7 and Ti5 O9 , which are the most conductive compounds in homologous series of structures with general formula Tin O2n−1 (4 ≤ n ≤ 10), collectively known as Magneli phases [20]. Ebonex has a unique combination of high conductivity (σ ≈103 −1 cm−1 ), good corrosion resistance and it is electrochemically stable in both acidic and alkaline solutions [20]. Moreover, using titanium suboxides in the gdm we could expect to modulate the properties of the active layer in a way to reach a hypo-hyper-d-electronic structures leading to synergetic catalytic effect [21]. The purpose of this paper is to summarize our results on the structure and electrocatalytic properties of Co–Te–O thin films, obtained by vacuum co-deposition of Co and TeO2 directly onto gas-diffusion membrane. We have studied the influence of the atomic ratio RCo/Te , loading, thermal treatment and type of the gdm (carbon or Ebonex) on the morphology and electrocatalytic behavior of the films. 2. Experimental 2.1. Preparation of the catalyst and gas-diffusion electrodes The catalytic films were prepared by thermal co-evaporation of Co by electron gun and TeO2 by physical evaporation from Knudsen type cell under vacuum higher than 10−4 Pa. The evaporation of TeO2 was carried out from platinum crucible, while those of Co—from glassy carbon one. The condensation rates of each substance within the range 0.01–0.04 g cm−2 s−1 were controlled separately during the evaporation by quartz crystal monitor. The substances were deposited on a stationary substrate, placed above the crucibles. In most of the cases gasdiffusion membranes were used as substrates. Two types of gdm were studied—the first one, carbon-gdm, consists of hydrophobized carbon (45% Teflon and 55% acetylene carbon blacks) and the second one, Ebonex-gdm, is a sandwich system made by two layers: hydrophobized carbon blacks/hydrophobized Ebonex (20% Teflon + 80% Ebonex–300 m powder, Atraverda Ltd.). Both supports were hot pressed on Ni-screen which plays role of a current collector [22]. The system of the catalytic film condensed on the carbonor Ebonex-gdm represents the gas-diffusion electrode (carbonor Ebonex-gde), the catalyst being the active layer, while the membrane with Ni-screen serves as a current collector inside the gas-supplying layer of the electrode, the gas being supplied by diffusion. The geometric area of gde was 1 cm2 . The effective loading of the catalytic film was in the range 0.05–0.5 mg cm−2 of geometric surface, which corresponds to a nominal film thickness of about 110–1100 nm. Some of the gas-diffusion electrodes were heated for 3 h at 100–300 ◦ C in air.
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2.2. Characterization of the as-prepared catalyst The amount of each condensed substance and the atomic ratio RCo/Te at each point of the substrate were calculated from the crystal monitors data. The chemical compositions of the catalyst deposited on the substrate were controlled with an X-ray microanalyzer in scanning electron microscope (JEOL Superprobe 733 with mounted System 5000—HNU). Scanning electron microscopy (Philips 515) was applied for examining the surface morphology of the electrodes. The chemical states of the elements in the catalytic films deposited on the gdm were studied with X-ray photoelectron spectroscopy (XPS). The measurements were carried out in UHV chamber of the electron spectrometer ESCALAB MkII. The spectra were exited with an AlK␣ source with energy 1486.6 eV. The photoelectron lines of C1s, O1s, Te3d and Co2p were recorded. All spectra were calibrated using C1s line at 285.0 eV as a reference. The specific surface area was measured by low temperature nitrogen adsorption according to BET. 2.3. Electrochemical measurements The electrochemical study was carried out by means of cyclic voltammetry and steady-state galvanostatic voltammetry using equipment consisting of potentiostat–galvanostat provided with a pulse generator (Solartron 1286 electrochemical interface). The gas-diffusion electrodes were tested in a three-electrode cell at room temperature. The electrolyte was 20% KOH, prepared with double distilled water. High surface area Pt was used as a counter electrode. The potential was measured versus Hg/HgO reference electrode in 20% KOH, reaching close to working electrode by a Luggin capillary. Cyclic voltammetry was carried out in the potential region between −1000 and +650 mV versus Hg/HgO. The system was deareated by supplying Ar through the membrane and bubbling it in the electrolyte before and during CV measurements. Cathodic polarization curves were measured under a flow of pure oxygen supplied to the air side of the electrodes. 3. Results and discussion 3.1. Physical characterization of the catalytic film The results obtained indicate that the XPS spectra of films deposited on gdm are identical to those of films on plain (glass, Si or Pt) substrates [26–28]. In Fig. 1a–c are illustrated Co2p, O1s and Te3d spectra of a film with RCo/Te = 1.5, and 0.2 mg cm−2 loading (450 nm thickness), deposited on gdm with carbon blacks and treated at different temperatures in air. The Co2p spectra show typical Co2+ state. The second peak which is shifted 6.2 eV to higher binding energy from the first one is known as a “shake-up” satellite which is exclusively observed in CoO [23]. It should be noted that we have detected elemental Co only in the volume of the films with atomic ratio RCo/Te > 2 [15]. It can be seen that the thermal annealing up to 300 ◦ C does not cause any changes in the Co2p spectrum. The broad, complicated shape of the peak
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on the surface of the catalyst film during the thermal treatment in air. From Eq. (1) follows that in films with RCo/Te = 1.5 the elemental Co should be completely oxidized to Co2+ while the unreacted TeO2 should remain. We have found that the amount of Te0 increases and that of Te4+ decreases with depth profiling [15]. Te3d spectra in Fig. 1c show the presence of Te4+ and small amount of elemental Te on the surface of fresh film and that thermally treated at 100 ◦ C. Most probably, partial oxidation of elemental Te proceeds on the surface of the film in air. As seen further thermal annealing of the films at 300 ◦ C causes significant decrease in Te4+ peak intensity and disappearing of Te0 peak on the film surface. As a whole, the amount of tellurium phase on the surface decreases about twice most probably because of the evaporation of Te at that temperature or because of segregation of the cobalt on the surface. SEM micrographs in Fig. 2 illustrate the surface morphology of the same film and of bare gas-diffusion membranes for comparison. One very rough film surface made of intertwined wires following the rough surface of the membrane is seen for the as-deposited films. It was found that this morphology does not change after thermal annealing up to 300 ◦ C. It should be noted that the surface morphology shown in Fig. 2 is typical for all films deposited on the gdm independently of their thickness, atomic ratio RCo/Te , thermal treatment as well as the type of the gdm—carbon or Ebonex. It was only found that there is an increase in the “wire” diameter with increasing the film thickness. The results obtained by TEM have shown that the “wires” consist of amorphous matrix with nanosized crystals CoO embedded in it [26,27]. However, XRD spectra of the films deposited on both gdms and thermally treated at different temperatures indicate that the fresh film and that treated up to 100 ◦ C are amorphous or nanocrystalline. As seen in Fig. 3, the spectra of 450 nm thick film with RCo/Te = 1.5 on carbon blacks do not show presence of peaks different from those of the substrate. Obviously, the size of the crystals detected by TEM in as deposited film, or appearing after a possible crystallization at 100 ◦ C are very small to be detected by XRD. New peaks are observed after annealing at 200 and 300 ◦ C. The peaks can be associated with cubic and monoclinic CoO phase, as well as hexagonal tellurium. 3.2. Electrochemical measurements on carbon-gde
Fig. 1. XPS spectra of as-deposited and thermally treated at 100 and 300 ◦ C 450 nm thick films with RCo/Te = 1.5: (a) Co2p, (b) O1s and (c) Te3d spectra.
at 530.5 eV, observed in O1s spectra of the as-deposited and treated at 100 ◦ C samples is typical for polycrystalline cobalt oxides samples [24]. Such broadness is assigned to presence of non-stoichiometric oxygen whose concentration varies with the sample treatment procedure [25]. The peak’s shift of 0.5 eV to higher binding energies observed after annealing at 300 ◦ C is most probably due to an increasing of the quantity of the nonstoichiometric oxygen and/or to formation of some hydroxides
3.2.1. Steady-state measurements As far as the catalytic activity of cobalt oxides is known in the literature, the question arises: what is the role of Te in these reactions? In our previous paper [16] we have shown that TeO2 and Te themselves exhibit some activity, which is significantly lower, especially for Te, than that of the Co–Te–O films. Fig. 4 shows the potentials for oxygen evolution reaction, obtained at current density of 20 mA cm−2 versus atomic ratio RCo/Te . The values are average from measurements of four gde and the bars show maximum deviations. It is seen that gdes with RCo/Te from 1.4 to 1.8 exhibit the best characteristics for oer. The measured values of the potentials for this range of atomic ratio are almost the same, since the measurement accuracy is ±15 mV.
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Fig. 2. SEM micrographs of: (a) carbon-gdm; (b) a film with RCo/Te = 1.5 and loading of 0.2 mg cm−2 deposited on carbon-gdm; (c) Ebonex-gdm; (d) a film with the same RCo/Te and loading on Ebonex-gdm.
It should be noted that it was impossible to determine precisely the influence of the atomic ratio RCo/Te on the catalytic activity for oxygen reduction reaction because of the low reproducibility of the results obtained [28]. The small amount of catalyst has not completely covered the membrane surface.
Therefore, it can be expected that the electrolyte will be in contact with both catalyst and carbon surface area and the catalytic activity of the gde will be the sum of the activity of the membrane and the film. It is well known that carbon itself is active to oxygen reduction but the activity of carbon-gdm has very low reproducibility. For this reason the precise estimation of the cat-
Fig. 3. XRD spectra of carbon-gdm and 450 nm thick films with RCo/Te = 1.5, deposited on it, fresh and thermally treated at different temperatures.
Fig. 4. Dependence of the potential at i = 20 mA cm−2 for oer on the atomic ratio RCo/Te at catalyst loading of 0.05 mg cm−2 .
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alytic activity of the film itself toward oxygen reduction reaction is difficult to be assessed. To study how the catalyst activity toward oxygen reactions depends on the catalyst loading, we have prepared gdes with atomic ratio RCo/Te of about 1.5 and different amount of the catalyst within the range 0.05–0.5 mg cm−2 . Fig. 5 shows the potentials measured at current density of 20 mA cm−2 for oer and orr versus the amount of the catalyst deposited on the membrane. The values are again average from measurement of six gdes and the bars show maximum deviations. For easier comparison, data obtained for the gde without catalyst (gdm) are also given. The results obtained show that the increase in the catalyst loading from 0.05 to 0.5 mg cm−2 does not lead to significant increase in the catalyst activity toward oxygen evolution. As seen in Fig. 5a the difference in the measured values of the potentials for oer is about 30 mV. Since the surface of gde takes part in the electrocatalytic reactions, obviously one very high surface area can be achieved even at small amount of catalyst of about 0.05 mg cm−2 . This conclusion is confirmed by the SEM study of the gas diffusion electrodes. How the increase of the loading influences the catalytic activity toward oxygen reduction again is difficult to be estimated precisely because of the poor reproducibility of the results. Fig. 5b indicates however a tendency for improving the reproducibility of the catalytic activity with the increasing the amount of catalyst. For that reason we chose the loading of 0.2 mg cm−2 as optimum one. The anodic and cathodic polarization curves of electrodes, with optimal loading and atomic ratio, treated at different temperatures are presented in Fig. 6. The best characteristic toward
Fig. 6. Anodic (a) and cathodic (b) polarization curves of gdes with loading of 0.2 mg cm−2 and RCo/Te ≈ 1.5, treated at indicated temperatures and curves of 0.8 mg cm−2 Pt, deposited on carbon blacks “Vulkan”. Cathodic curves were measured under a flow of pure oxygen supplied to the air side of the electrodes.
Fig. 5. Dependence of the potential at i = 20 mA cm−2 for (a) oer and (b) orr on the amount of catalyst with RCo/Te ≈ 1.5.
oxygen evolution reaction shows the fresh electrode, while those treated at higher temperatures exhibit lower activity. Despite the fact that the results in the cathodic direction are not well reproducible, we have found a well defined tendency of increasing the catalytic activity after annealing at 100 ◦ C. As a rule gde with higher catalytic activity for oxygen reduction reaction exhibits lower activity for oxygen evolution reaction and vice versa. Most probably this tendency is due to the different nature of the active sites for both reactions. In the Fig. 6 the polarization curves of our electrodes are compared with the polarization curves of gde, consisting of gdm and catalytic layer of platinum on Vulcan XC72 (commercial product of E-TEK Division of DE NORA, North America). The amount of the Pt/C catalyst was calculated in a way to have 0.8 mg cm−2 Pt, which is bigger than the amount of the Co–Te–O catalyst. It can be seen that all gdes with Co–Te–O catalyst exhibit better catalytic activity for the anodic reaction and that one of them comparable characteristic with Pt/C even for the cathodic reaction. In Fig. 7 the Tafel plots for both reactions obtained for selected electrodes are depicted. Two linear segments, typically for oer and orr are obtained at low and high current densities. For oxygen evolution reaction values of b ≈ 45 mV/decade were found at low current densities. This slope is in the range of most commonly observed for the oxides (40–60 mV/decade) in the literature [29]. As seen from the figure the thermal annealing
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Fig. 7. Tafel plots for anodic and cathodic reactions of gdes with loading of 0.2 mg cm−2 and RCo/Te = 1.5, fresh and treated at 100 ◦ C.
leads to increasing in the slopes up to 90–120 mV/decade. This is probably a result of changing in the surface structure after annealing which is in agreement with some authors, pointing out that the Tafel slopes are sensitive to the surface structure of the catalysts [29]. The increasing of the Tafel slope almost twice is an indication that most probably the structure modifying leads to changing in the rate determining step [30] and/or in the surface coverage with OH− groups [31,32]. As it was mentioned above, the results for oxygen reduction reaction are not very reproducible which also reflects on the values of the Tafel slopes. The slope in the low current domain was found to vary between −50 and −80 mV/decade even higher, in rare cases. Nevertheless, a tendency of slightly decreasing of the Tafel slopes in the low current domain was found as an effect of annealing up to 200 ◦ C. 3.2.2. Cyclic voltammetry Cyclic voltammograms of gdes, fresh and thermally treated at different temperatures are given in Fig. 8a. The electrodes have optimal catalyst loading of 0.2 mg cm−2 and atomic ratio RCo/Te = 1.5. For comparison in Fig. 8b the curves of gdm, the Te and TeO2 films on gdm are given. It is seen that gdm, Te and TeO2 do not have any contribution to the peaks observed for Co–Te–O films. CV curves exhibit several very broad, not well defined peaks, prior to the oxygen evolution. The first single anodic peak a1, observed only for fresh and thermally treated at 100 ◦ C gdes has a maximum at −50 mV. Most probably a redox transition of CoO/CoOOH proceeds with calculated potential of −54 mV versus Hg/HgO [33]. The pair of peaks a2/c2 can be associated with formation of a redox couples CoO/CoO2 with calculated potential at +195 mV and/or Co(OH)2 /CoO2 at +254 mV [33]. Our detailed study showed that a third anodic peak a3 appears at +560 mV. In Fig. 8a this peak looks like a shoulder, but it becomes well depicted with increasing the scan rates. It should be noted that all peaks discussed are more clearly defined in CVs of films deposited on plane substrate (Pt). The potentials at which the peak a3 and its corresponding cathodic peak c3 appeared are an indication that the redox transition CoOOH/CoO2 at +562 mV versus Hg/HgO occurs [33]. Fig. 8a shows that the position of the main peaks is little influenced
Fig. 8. Cyclic voltammograms of: (a) gdes with Co–Te–O films, thermally treated at the indicated temperatures; (b) gdm and Te and TeO2 films deposited on it.
by the thermal treatment although a small shift to less positive values is detected for gde treated at 300 ◦ C. Most probably, this effect is due to the presence of hydroxides or non-stoichiometric oxygen on the surface as proved by the XPS spectra of electrode treated at 300 ◦ C. It is clearly seen from Fig. 8 that as a rule all peaks are broad, most probably as a result of the heterogeneity of the size distribution of the active surface sites [29] or to the high degree of porosity or/and roughness, which affects the energy of the active sites thus leading to broadening of the peak [34]. The position of the peak is indicative of the site’s chemical nature, while the area under the peak is proportional to the numbers of sites oxidized or reduced [29]. The resolution of broad, not well defined peaks is however very difficult. In this case the voltammetric charge over the whole potential range (q* ) can be taken as relative measure of the electrochemically active surface area. The values of q* obtained cannot be easily converted into an absolute value since the precise nature of surface redox transitions is unknown, but q* has proved very useful when different electrodes or different preparation procedures are being compared [29,35,36]. To estimate the actual surface area of the oxide films taking place in the surface redox reactions, we have calculated the surface charge q* by integrating the voltammetric curves applying the CV technique, proposed by Trasatti and coworkers [35,36] for thick films deposited on plane substrates. The common observation with oxide electrodes is that q* decreases as the potential
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Fig. 10. The normalized total, inner and outer voltammetric charges, as a function of the annealing temperature.
Fig. 9. Extrapolation of the voltammetric charge to: (a) ν = 0 and (b) ν = ∞ of the gdm and gde, fresh and annealed at 100 ◦ C.
scan rate ν increases. It has been shown, that q* can be extrapolated to ν = 0, thus providing the total surface charge qt∗ , as well as to ν = ∞, thus giving the outer surface charge q0∗ [36]. The difference qt∗ − q0∗ = qi∗ provides the inner surface charge, i.e. the charge associated with the less accessible surface regions (pores, cracks, grain boundaries, etc.), where protons diffuse with difficulty [37]. The extrapolation procedure for selected electrodes is illustrated in Fig. 9. For comparison the results for gdm are also given. It can be seen from Fig. 9a that the extrapolation to ν = 0 is closely linear, which is in agreement with the results for thick films on plane substrates [35,36]. Some deviation from the linearity is observed only for gdm at higher scan rates. The extrapolation to ν = ∞ is also linear for all gdes (Fig. 9b) studied. Some deviation from the linearity is only observed at the highest scan rates, which is in agreement with the results obtained for thick films [36]. Fig. 10 shows the normalized total, outer and inner voltammetric charges as a function of the annealing temperature of the gdes with optimal atomic ratio and loading. It is seen that total charge qt∗ , representing the concentration of the active sites, goes through a maximum at 100 ◦ C. The maximum at 100 ◦ C is due mainly to the increase in inner voltammetric charge while the outer voltammetric charge remains almost unchanged. This
suggests that the external morphology (macro-roughness) is not strongly dependent on the annealing temperature, as is confirmed by the SEM study. Further, we have found that the annealing at 100 ◦ C of the gas-diffusion membrane, which is known to have some catalytic activity toward oxygen reduction reaction, does not lead to any changes in the voltammetric (qt∗ , q0∗ and qi∗ ) charges. As it was mentioned above, the XPS analysis did not show any changes in the chemical state of the catalytic films annealed at temperatures up to 300 ◦ C. Therefore most probably the increase in the concentration of the active sites for electrodes annealed at 100 ◦ C is caused by full crystallization of the catalytic film but the size of the crystallites remains still very small to be detected by conventional XRD analysis. The abrupt drop in the inner active surface area of electrodes annealed at 200 and 300 ◦ C can be explained by changes, caused by both melting of the Teflon in gdm at temperatures above 200 ◦ C and the increasing size of the catalyst crystallites. According to Trasatti and coworkers [35,36], if the total charge qt∗ is related to the whole active surface, the ratio qi∗ /qt∗ can be taken as representing the “electrochemical porosity” of the electrode. For determining the porosity and/or the roughness factors Da Silva et al. [37] proposed another technique based on the dependence of the capacitive current ic on the scan rate ν, observed in short capacitive potential region of about 50 mV. In analogy with Trasatti and coworkers [35], the authors have correlated the two linear segments, observed in the low and high ν domains of the ic versus ν plot, with the morphology of highly porous or rugged films. They have attributed the change in the slope observed in the high ν domain to the exclusion of surface areas located in the more difficult-to-access regions. Thus the slopes of the two linear parts of the ic versus plot give the total Cd,t , and the external Cd,e differential capacity while the difference between them Cd,t − Cde gives the capacity Cd,i for the “internal” film regions and the ratio Cd,i /Cd,t is the morphology factor ϕ. Based on the procedure proposed by Da Silva et al. the voltammetric curves were recorded at various sweep rates. Since no clear capacitive potential region was detected on the curves (see Fig. 8) we recorded the voltammograms, covering the
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Fig. 11. Dependence of the capacitive current on sweep rate, ν for gdm, fresh and annealed at 100 ◦ C gdes with RCo/Te = 1.5 and 0.2 mg cm−2 loading. The inset shows the same dependence but the scale of the ic axis is changed in order to see the first linear segment for the fresh-gde.
potential range between 0 and 50 mV versus Hg/HgO. A representative graph is shown in Fig. 11. The gdes show two linear segments at low and high sweep rates, which is in agreement with the literature [37]. The gas-diffusion membrane shows only one linear segment (see the inset of Fig. 11) which means that gdm posses very high degree of porosity and all active centers are maximum available in the scan rates range within 2 ÷ 200 mV s−1 . The results for the electrochemical porosity and morphology factors for fresh and thermally treated electrodes with optimal loading of 0.2 mg cm−2 and RCo/Te = 1.5 are given in Table 1. For comparison the data from the BET analysis are also given. The results for the total charge are in accordance with those for the specific surface area, obtained by BET analysis which also indicate similar tendency of increasing the surface area from 25 to 27 m2 g−1 for gdm and fresh gde to 33 m2 g−1 for treated at 100 ◦ C gde. Keeping in mind the very low catalyst loading of 0.2 mg cm−2 this means that the specific surface area of the annealed film itself is much larger. Moreover, the results show that the gas-diffusion membrane itself has a little influence on the voltammetric charges, obtained for gdes. The qt∗ values of the gdm are about 10 times lower than those of the films deposited on it. As can be seen from the table, there is a contradiction between the two factors and . The electrochemical porosity increases while the morphology factor decreases for the carbon-electrode
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Fig. 12. Cyclic voltamograms of Ebonex-gdm, Ebonex- and carbon-gdes with catalytic films with RCo/Te = 1.5 and loading of 0.2 mg cm−2 .
annealed at 100 ◦ C. Both the electrochemical porosity and the morphology factor describe the ratio between the more-difficultto-access and total surface areas and can take values between 0 and 1. Their physical meaning however is not exactly the same since the electrochemical porosity includes the solid state surface redox transitions (SSSRT), while the morphology factor should be free of the SSSRT or of the minor importance [37]. Having in mind that the CV curves obtained by us have not pure capacitive region one can expect that some SSSRT, occurring in the narrow potential region should affect the values of the morphology factor. Another reason for the difference is possible dependence of Cd on the potential, which is not taken into account in calculating in the narrow potential range used. These effects are most probably the reason for the lack of correlation found between the electrochemical porosity and the morphology factor, and in our opinion make the application of the morphology factor inappropriate for our electrodes. 3.3. Electrochemical measurements on Ebonex-gde Fig. 12 compares cyclic voltammogram of Ebonex-gde with that of carbon-gde. In the electrodes catalytic films have optimal atomic ratio RCo/Te and loading. The CV of Ebonex-gdm is also given. It is seen that Ebonex-gdm does not contribute to the peaks observed for the Ebonex-gde. The comparison with carbon-gde shows that the positions of the peaks, asso-
Table 1 SBET , and for different electrodes GDE Carbon-gdm Carbon-gdm\Co–Te–O film Ebonex-gdm Ebonex-gdm\Co–Te–O film
Fresh Annealed at 100 ◦ C Fresh Annealed at 100 ◦ C
SBET (m2 g−1 )
Total charge (mC cm−2 ) qt∗
Electrochemical porosity, qi∗ /qt∗
Morphology factor, Cd,i /Cd
25 27 33 – – –
29 207 617 250 3205 1000
0.88 0.88 0.95 0.57 0.89 0.71
1 0.91 0.56 – – –
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ciated with the formation of redox couples CoO/CoO2 (a4) and CoOOH/CoO2 (a5) are slightly shifted to less positive potential values for the Ebonex-gde. These peaks are, however, not very well seen in the figure because of the peaks broadness. The CV of the Ebonex-gde exhibits several additional anodic peaks in the negative potential region (at a1 = −650, a2 = −200 mV) which can be attributed to redox transitions between Co2+ /Co3+ and Co/Co2+ [33]. A relatively well defined oxidation peak a3 observed at +49 mV, can also be connected with Co2+ /Co3+ transition [33]. The data given in Table 1 indicate that both carbon- and Ebonex-gdms have relatively low values of total voltammetric charges, i.e. number of active sites compared to those of the corresponding gdes. From the values of (Table 1) follows that the sites are distributed preferentially in the inner surface of carbon-gdm, while in the case of the Ebonex-gdm, they are uniformly distributed between the outer and inner surface. In this sense the carbon-gdm electrode exhibits more “electrochemical porosity” than the Ebonex-gdm most probably as a result of the significantly smaller particle size of the carbon powder used and higher Teflon content in it. The data given in the Table 1 clearly show that electrochemical porosity of the fresh carbonand Ebonex-electrodes is the same, but the total, outer and inner voltammetric charges obtained for the Ebonex-gde are about one order of magnitude higher than those of carbon-gde. This abrupt increase in the number of active sites participating in the surface redox reactions is most probably a result of the geometry
effect of larger particle size of Ebonex. From Table 1 follows that, unlike carbon-gde, the annealing of Ebonex-gde leads to decrease in the total charge and porosity, which is difficult for now to be explained. The anodic and cathodic polarization curves of Ebonex- and carbon-gdes are compared in Fig. 13. Despite the high number of active sites found for Ebonex-gdes they show lower catalytic activity than the carbon-gdes, especially for oer. The detailed analysis indicates, however, that for orr the Ebonex-gdes have better reproducibility and stability than carbon-gdes. The reason for the lower activity is probably the larger size of the Ebonex particles and the lower degree of hydrophobization. This allows the electrolyte to go deeper into the pores thus leading to transport hindrances of the reaction products. We hope that the optimization of the gas-diffusion membrane with Ebonex will lead to better results. 4. Conclusion The results reported in the paper show that thin electrocatalytic films of Co–Te–O, obtained by vacuum codeposition of TeO2 and Co can be used for preparing bifunctional electrodes for oxygen evolution and reduction reactions. During vacuum deposition, the co-evaporated compounds are mixed at atomic level and a chemical reaction between them takes place resulting in the formation of CoO phase and of elemental Te. This gives a possibility to obtain catalyst with various components in desired proportions and high surface/volume ratio. The results from the electrochemical measurements indicate that the films, deposited by this method have high catalytic activity toward oxygen evolution reaction even with low catalyst loading of 0.05–0.5 mg cm−2 . The as-deposited catalytic films with atomic ratio RCo/Te = 1.4–1.8 have the best characteristics for the oxygen evolution reaction. The catalytic activity of the films for the oxygen reduction reaction is difficult to be estimated precisely due to the poor reproducibility of the gdm characteristics. The fresh carbon-gdes have the best catalytic activity toward oer. Their thermal annealing at 100 ◦ C causes pronounced changes in the concentration of the active sites, participating in the surface redox reactions. This leads however to increasing the catalytic activity only toward orr. Our first results obtained for Ebonex-electrodes indicate that they have significantly higher number of active site participating in the surface redox reactions but lower catalytic activity than carbon-electrodes. We believe that an optimization of the Ebonex-gas-diffusion membrane is needed to obtain better results. Acknowledgements
Fig. 13. Anodic and catodic polarization curves of Ebonex-gdm, Ebonex- and carbon-gdes with catalytic films with RCo/Te = 1.5 and loading of 0.2 mg cm−2 .
This work was realized with the financial support of the Bulgarian National Science Fund under contract # MY-X 1402. The authors are gratefully obliged to thank to Prof. D. Mehandjiev and Dr. N. Velichkova for carrying out BET analysis; to Dr. G. Tujliev – for XPS analysis; and Tz. Iliev – for EDS analysis.
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