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Copper-doped cobalt oxide electrodes for oxygen evolution reaction prepared by magnetron sputtering
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 8 2 2 e8 3 0
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Copper-doped cobalt oxide electrodes for oxygen evolution reaction prepared by magnetron sputtering Q. Zhang a,b, Z.D. Wei a,b,*, C. Liu a, X. Liu a, X.Q. Qi a,b, S.G. Chen a, W. Ding a, Y. Ma a, F. Shi a, Y.M. Zhou a a
State Key Laboratory of Power Transmission Equipment & System Security and New Technology, School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China b School of Material Science and Engineering, Chongqing University, Chongqing 400044, China
article info
abstract
Article history:
The Copper-doped cobalt oxide films have been prepared onto titanium support by reactive
Received 11 December 2010
DC magnetron sputtering. The deposits are characterized by X-ray diffraction (XRD),
Received in revised form
energy-dispersive spectrometry (EDS), Ultroviolet (UV) and scanning electron microscope
5 April 2011
(SEM). The electrochemical characteristics of deposits are explored by cyclic voltammetry
Accepted 7 April 2011
(CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS).
Available online 19 May 2011
The effect of copper content in the CueCo oxides on the surface morphology, electrochemical and crystallographic properties has been investigated. The introduction of Cu
Keywords:
element makes the CueCo oxides deposit fine and further lead to high roughness of
CuO
deposits, which is helpful for the oxides’ electrochemical performance. Based on the
Co3O4
electrochemical determination, the binary oxide electrodes exhibit better catalysis activity
Spinel
than that of CuO and Co3O4 electrodes for oxygen evolution reaction (OER). The increase
Oxygen evolution
OER activity has been attributed to the enlarged surface roughness and the new active sites
Magnetron sputtering
of deposits, according to the results of EIS.
Electrocatalysis
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The electrocatalysis of oxygen evolution reaction (OER) on cobalt oxide (Co3O4) electrodes has been a topic of great interest in electrochemistry for a long time. It is known that OER overpotentials are efficiently decreased by the application of cobalt doped complex oxides because substitution of cobalt ions by foreign divalent metal ions can result in a new spinel structures with novel physicochemical properties, which may exhibit more excellent performances as compared to a single oxide [1]. To date, the cobalt-containing spinel oxides MCo2O4 (M ¼ Ni, Cu, Zn, Mg, Mn, Cd, etc.) [2e11] have been extensively studied in order to establish the defined correlations refer to their properties, composition and structure. Among those complex oxides,
copper-doped cobaltite spinels show relatively high stability and activity with low cost and availability [2,13]. Up to now, diverse synthetic routes have been adopted to prepared copperdoped spinel cobaltites, such as: chemical coprecipitation [13,14], thermal decomposition [5,6,12,15e19], spray pyrolysis [6,7], and solegel routes [2]. However, the catalytic activity and physicochemical properties of the prepared oxides are rather different because they are profoundly affected by the preparation methods. Moreover, the catalytic activity reported was not consistent, even contrary, despite the same preparation method was adopted [20,21]. The annealing atmosphere, the chemical nature of the precursor and substrate, even the man-made factor, are also the reasons for the differences in the literatures. Therefore, the characterization of thin films prepared
* Corresponding author. Tel.: þ86 23 65102531; fax: þ86 23 65112134. E-mail address: [email protected] (Z.D. Wei). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.051
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 8 2 2 e8 3 0
Fig. 1 e X-ray diffraction patterns for CueCo oxides and Co3O4 thin films deposited on glass.
with a particular method and under concrete experimental conditions is necessary to understand their electrochemical performance. In this work, the reactive DC magnetron sputtering method (RDCMS) is adopted to prepare the CueCo complex oxides with Co/Cu ratio from 0.5 to 3.4. Cu and Co sources are ultrapure copper and cobalt targets. Thus the so-called precursor effect can be avoided. The stable atmosphere and annealing condition also ensure the accuracy and reproducibility of the experiment. This work is aimed at establishment of relationship among the catalyst activity and morphology, composition, crystallinity and electric properties of the CoeCu complex oxides, which has been not yet clear.
2.
Experimental section
2.1.
Preparation of the samples
and cobalt (99.99%) disks of 60 mm diameter and 3 mm thick were used as the sputter targets. Argon of 99.999% purity was used as the sputter gas and oxygen of 99.999% purity as the reactive gas. Before deposition, the substrate was heated to 500 C and the sputtering chamber was first pumped down from atmospheric pressure to a base pressure of 1 103 Pa. The Cu and Co targets were pre-sputtered in turn at 60 W in pure argon atmosphere for 5 min to remove any oxide layer or contaminant on the target surface. Furthermore, all oxide films deposited at a total pressure of 1.0 Pa, during which the argon and oxygen were introduced into the sputtering chamber at gas flux of 4 and 22 sccm, respectively. The substrates were fixed on the anode and the distance between target and substrate was 120 mm. In sputtering, the Co target sputtering power was fixed at 100 W while the Cu target sputtering power changed from 15, 30, 45, 60 to 75 W. The pure oxide phase (CuO and Co3O4) obtained by single target sputtering only. All of the films were deposited in half an hour and cooled to the room temperature in the high vacuum atmosphere. For comparison, the RuO2 catalyst with loading of 1 mg cm2 was prepared on Ti sheet by a pyrolysis method [22].
2.2.
Characterization of samples
The morphology and chemical compositions of the prepared films on titanium sheet were characterized by scanning electron microscopy (SEM, FEI Nova 400 FEG) and energydispersive spectrometry (EDS, OXFORD LinkdISIS-300). X-ray diffraction (XRD) of the films on glass was conducted on an X-ray diffractometer (XRD-6000, Shimadzu) with filtered Cu Ka radiation at a wavelength of 0.15428 nm, and the grain size was calculated using Scherrer’s formula. UVevis transmission spectra (Cintra-10e, Australia GBS) were also used to characterize the oxide films on glass.
2.3.
The oxide thin films were deposited on titanium sheet (exposed area 1 1 cm2) or microscope glass slides (2.5 3 cm2) by means of dc magnetron sputtering system (JGP-450, SKY Technology Development Co., Ltd., China). The high purity copper (99.99%)
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Electrochemical measurements
All the electrochemical measurements were performed on Autolab electrochemical analyzer (PGSTAT302N) in a conventional three-electrode glass cell at room temperature (25 C).
Table 1 e Lattice parameters and unit cell volume for CueCo oxides on glass calculated from diffractogram. Lattice parameters Electrodes
˚) a (A
˚ 3) V (A
Co3O4(JCPDS) Co3O4/glass Cu0.68Co2.32O4 Cu1.25Co1.75O4 Cu1.45Co1.55O4 Cu1.45CoO3 Cu1.97CoO3 CuO
8.084 8.094 8.112 8.119 8.130
528.20 530.39 533.79 535.13 537.47
˚) Crystallite size (A 360 425 339 266 62 55 119
Fig. 2 e UV spectra of CueCo oxides, CuO and Co3O4 thin films deposited on glass.
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Fig. 3 e SEM images of CueCo oxides, single Co3O4 and CuO deposited on Ti sheet: (a) Co3O4 [Co:100W], (b) CuO [Cu:100W], (c) Cu1.25Co1.75O4 [Cu:30W;Co:100W], (d) Cu1.45Co1.55O4 [Cu:45W;Co:100W], (e) Cu1.45CoO3 [Cu:60W;Co:100W], (f) Cu1.97CoO3 [Cu:75W;Co:100W].
A platinum wire and an Ag/AgCl electrode (0.20 V vs. NHE) were used as the counter electrode and reference electrode, respectively. The deposited oxide thin films onto titanium sheet (1 cm2) were used as working electrodes for the catalysis of the OER in 1 M KOH solution. Electrochemical impedance spectroscopy measurements were performed at different bias potentials with the excitation amplitude of 5 mV. The impedance spectra were recorded in a range of frequency from 100 kHz to 0.05 Hz, and before recording each spectrum, the electrode was first equilibrated at the corresponding bias potential for 100 s. The experimental impedance data was fitted to an appropriate equivalent circuit using Zview software.
3.
Results and discussion
3.1. Oxides composition vs. their crystalline structure and morphology In order to avoid cover-effect of Ti substrate and obtain the vivid XRD peaks of the deposits, the microscope glass slides were employed as the substrate for oxides thin films deposition. Fig. 1 shows the X-ray diffraction patterns of CueCo complex oxides electrodes with different Co/Cu ratios (0.5e3.4). For comparison, the figure also includes the XRD spectra of the Co3O4 and CuO on glass substrate. The XRD for sample Co3O4/glass shows
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 8 2 2 e8 3 0
Fig. 4 e CVs plots of CueCo oxides, single Co3O4 and CuO deposited on Ti sheet at a scan rate 50 mV sL1 in 1 M KOH.
reflections at 2q of 19.08, 36.87, 38.51 and 59.25 that could be indexed to a cubic spinel lattice of the single Co3O4 phase (JCPDS 42-1467). The XRD for sample CuO/glass shows reflections at 2q of 32.53, 35.70 and 38.71 that could be indexed to a monoclinic CuO phase (JCPDS 80-0076). Upon increasing the Cu target power, i.e., increasing the Cu content in the oxides, the XRD patterns keep changed, but no peaks corresponding to CuO was observed. No CuO crystalline phase was found either at any Cu content by XRD even when its stoichiometric amount was superfluous. This phenomenon could be assigned to that the CuO exists in form of an amorphous phase or a crystalline phase but with a very small size, making it difficult to be detected by XRD. The relevant crystalline phases and chemical composition of the oxide films can be distinguished by integrative analysis of XRD and EDS, and the results are as follows: Cu0.68Co2.32O4[Cu:15W,Co:100W], Cu1.25Co1.75O4 [Cu:30W,Co:100W], and Cu1.45Co1.55O4[Cu:45W,Co:100W] show
Fig. 5 e LSVs plots of CueCo oxides, single Co3O4 and CuO deposited on Ti sheet at a scan rate 10 mV sL1 in 1 M KOH.
825
the spinel structure of CuCo2O4 (JCPDS 37-0878), while Cu1.45CoO3[Cu:60W,Co:100W] and Cu1.97CoO3[Cu:75W,Co:100W] have Cu2CoO3 crystalline structure (JCPDS 21-0288). The unit cell parameter of the spinel CueCo oxides has been calculated using its main indexed planes appeared in the diffractogram. The lattice parameters and the unit cell volume as well as the mean crystallite size are listed in Table 1. The unit cell parameter of spinel oxides continuously increases with the copper content. It is believed that the increase in unit cell parameter is caused by the substitution of large Cu(II) cations for small Co (II) cations. The crystallite size decrease with the copper content indicates that the increment of Cu content benefits the crystallite size reduction. This results derived from XRD are also confirmed by SEM images displayed in Fig. 3. That is, the doping of Cu ions favors the formation of fine grain CueCo oxides. It would be good for obtaining an electrode with a high surface area. Not seeing the air doesn’t mean there is no air. For further ascertainment of CuO phase, the optical property of the above oxide films was studied by UV adsorption spectrum. Among the curves in Fig. 2, curves 1 and 2 belong to pure Co3O4 and CuO, curves 3e7 to CueCo oxides. The UV adsorption behavior of Co3O4 and CuO is so different from each other. The Curve 2 demonstrates the particular and unique adsorption behavior of single CuO phase that make it easy to confirm the appearance of CuO in the oxide films. It’s well known that spinel oxides have the AB2O4 composition [1], where A ions are generally bivalent metal ions occupying the tetrahedral sites, and B ions are trivalent ions occupying the octahedral sites. As for single Co3O4, both the two sites are occupied by cobalt ions. In the case of CuCo2O4, the doped Cu ions would replace parts of Co ions at octahedral and tetrahedral sites. The substitution of Co sites in Co3O4 structure could reduce its energy band-gap by removing the orbital degeneracy and adding new orbital energy levels, resulting in a continuous adsorption of light [25]. The UV adsorption behavior of CueCo oxides continuously changes with the copper content. No typical UV adsorption behavior of monocline CuO appears until the copper content exceeds the stoichiometric CuCo2O4 spinel. It is easy to confirm that the independent CuO phase surely exists in Cu1.45Co1.55O4, Cu1.45CoO3 and Cu1.97CoO3 according
Fig. 6 e OER on Cu1.97CoO3 and RuO2.
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Table 2 e Simulated parameters of the elements in equivalent circuits for oxygen evolution of CueCo oxides, RuO2, single Co3O4 and CuO deposited on Ti sheet at 0.70, 0.75, 0.77 and 0.8V in 1 M KOH. Oxides
E/V Rs (U cm2) R0 (U cm2) Q1 104 (S cm2 sn) n1 Rt (U cm2) Q2 103 (S cm2 sn) n2 Cdl 103 (F cm2)
Rf
CuO
0.70 0.75 0.77 0.80
1.25 1.25 1.24 1.25
7.88 7.49 7.48 7.41
0.78 0.79 0.93 0.88
0.81 0.81 0.80 0.80
13500.00 1070.00 466.00 158.00
1.43 1.66 1.73 1.74
0.84 0.83 0.83 0.82
0.43 0.43 0.45 0.45
7.16 7.16 7.50 7.50
Co3O4
0.70 0.75 0.77 0.80
0.75 0.75 0.76 0.77
4.28 4.07 3.97 3.79
1.51 1.44 1.33 1.14
0.83 0.83 0.84 0.85
4242.00 416.20 164.90 46.47
1.58 1.38 1.31 1.29
0.91 0.92 0.92 0.91
0.81 0.76 0.72 0.65
13.65 12.67 12.00 10.83
Cu0.68Co2.32O4 0.70 0.75 0.77 0.80
0.74 0.75 0.77 0.78
0.80 0.79 0.78 0.78
8.86 10.56 12.58 10.74
0.71 0.70 0.68 0.69
1502.0 186.30 71.12 22.98
3.70 3.38 3.14 3.10
0.92 0.91 0.91 0.90
2.16 1.91 1.89 1.74
36.00 31.83 31.50 28.30
Cu1.25Co1.75O4 0.70 0.75 0.77 0.80
0.81 0.81 0.82 0.83
0.74 0.73 0.73 0.74
3.07 4.01 2.99 2.38
0.80 0.78 0.81 0.83
377.80 46.35 22.48 8.10
7.84 7.55 7.18 7.11
0.93 0.92 0.92 0.91
5.32 4.84 4.58 4.24
88.67 80.67 76.30 70.60
Cu1.45Co1.55O4 0.70 0.75 0.77 0.80
0.70 0.70 0.71 0.72
0.93 0.91 0.91 0.89
3.32 3.15 2.88 2.45
0.81 0.81 0.82 0.84
302.20 37.78 18.67 6.91
14.08 13.12 12.15 11.97
0.91 0.90 0.90 0.89
8.91 7.76 7.14 6.57
148.50 129.33 119.00 109.50
Cu1.45CoO3
0.70 0.75 0.77 0.80
0.74 0.75 0.75 0.76
1.21 1.14 1.13 1.10
1.93 1.96 1.84 1.61
0.83 0.83 0.84 0.85
183.9 26.3 13.71 5.57
14.71 13.95 13.39 13.08
0.91 0.89 0.89 0.88
9.40 7.91 7.53 6.85
156.67 131.83 125.50 114.16
Cu1.97CoO3
0.70 0.75 0.77 0.80
0.80 0.82 0.82 0.78
1.39 1.37 1.37 1.36
0.94 0.92 0.91 0.90
0.80 0.82 0.83 0.83
119.90 19.47 10.32 4.76
17.62 16.66 16.18 16.50
0.89 0.88 0.87 0.86
10.39 9.23 8.47 7.92
173.16 153.80 141.16 132.00
RuO2
0.70 0.75 0.77 0.80
1.24 1.31 1.35 1.37
0.23 0.69 0.41 0.40
264.87 501.59 543.80 253.08
0.58 0.47 0.48 0.59
28.73 6.98 3.033 2.344
21.27 20.86 20.40 20.15
0.97 0.98 0.97 0.98
19.24 19.52 17.93 18.70
320.61 325.26 298.82 311.61
to the UV adsorption curves. In addition, the unique broad peaks (370e450 nm) of Curves 6 and 7 also indicate some new chemical bond formed in these oxides.
3.2.
Performance in catalysis of oxygen evolution
The CVs obtained from the Co3O4, CuO and CueCo oxides electrodes in the potential range of 0.0e0.8 V are shown in Fig. 4. The CV of Co3O4 electrode exhibits a couple of reversible redox peaks at about 0.63 V, while no peak appears in the CV related to CuO. The anodic peak at about 0.63 V for Co3O4 electrodes can be assigned to the Co(IV)/Co(III) redox couple [21]. It is noteworthy that the peaks of CueCo oxide electrodes are broad and not well defined compared to that of Co3O4 electrode. The broadness of the peaks could be connected with roughness of the electrode. As suggested by Filoche and Sapoval [23], materials with a high degree of roughness present a wide potential distribution throughout the surface. The corresponding LSVs of the above electrodes for the oxygen evolution reaction are shown in Fig. 5. The catalytic activity of the electrodes for the oxygen evolution reaction increases sharply from Co3O4 to Cu0.68Co2.32O4 and Cu1.25Co1.75O4 in spinel, and then to increase slightly from Cu1.45Co1.55O4 to Cu1.45CoO3 and Cu1.97CoO3 in the Cu2CoO3 crystalline phase.
These catalytic activity changes parallel the modifications observed in the oxides morphology (Fig. 3), and hence they could be attributed to the growth of surface-active area besides the effect of transition in crystal structure. The foreign metal oxide segregated at the surface may caused a decrease in the catalytic activity for the doped cobalt oxides [4,7,24]. In our case, the copper-rich Cu2CoO3 and Cu1.45Co1.55O4 with segregated CuO phase show even better performance than that of the pure spinel, indicating the CuO phase may homogeneously distribute in the oxide bulk and benefit the increase of active surface. The comparison of performance between Cu1.97CoO3 and RuO2 are depicted in Fig. 6. In order to reveal the exact performance of these catalysts, the LSVs data of the Cu1.97CoO3 and RuO2 were normalized by their roughness factor Rf in Table 2 before the comparison. After eliminating the effect of surface roughness, it was found that the Cu1.97CoO3 (the best CueCo oxide) and RuO2 showed almost the same catalytic activity to the OER.
3.3.
Catalytic mechanism based on EIS analysis
In order to gain an insight into the behavior of the system during oxygen evolution, the EIS have been carried out on the oxide electrodes at potentials chosen in the potential range,
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Fig. 7 e Equivalent circuit-Rs(RoQ1)(RtQ2) and Nyquist plots (experimental and simulated) for CueCo oxides, single Co3O4 and CuO deposited on Ti sheet at 0.7, 0.75, 0.77 and 0.8V in 1M KOH.
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0.7e0.8 V, in 1 M KOH. As it can be seen in Fig. 7, each complexplane diagram (Nyquist) shows two semicircles. One diameter decreases with increasing the potential in the low frequency domain, while the other has the constant diameter at the medium and high frequency region regardless of the potential high or low. The former can be ascribed to the OER, while the latter is related to the solid oxide. The equivalent circuit was found to match most closely the observed impedance diagrams in form of Rs(R0Q1)(RtQ2) (Fig. 7), where Rs, R0, Rt and Q represent solution resistance, oxide film resistance, charge transfer resistance of OER, constant phase element (CPE) corresponding to the oxide mass (Q1) and the interface between film and electrolyte (Q2). The CPE(ZCPE ¼ [Q( ju)n]1) contains two parameters: Q, the frequency independent parameter and n(0 < n < 1), the deviation from the ideal behavior, being n ¼ 1 for perfect capacitors and n ¼ 0 for pure resistance. By using CPE instead of pure capacitor, the simulated results are in good agreement with the experimental data as shown in Fig. 7. Estimates of the circuit parameters are given in Table 2. The roughness factor (Rf) of the oxides electrodes has also been calculated using Cdl/60(mF cm2) [26,27], where Cdl represents double layer capacitance of the oxide film/solution interface and can be estimated by equation of Q2 ¼ (Cd1)n[(Rs)1 þ (Rt)1] [28]. Table 2 illustrates that Cu doping can decrease the resistance of the oxides film till the formation of Cu1.25Co1.75O4, and then increase gradually from Cu1.45Co1.55O4 to Cu1.45CoO3 and Cu1.97CoO3. In addition, the charge transfer resistance (Rt) of OER decreased sharply for all the oxide electrodes with the applied potential increased. At a given potential, the CueCo oxide electrodes exhibited the lowest Rt of all oxides electrodes, indicating the CueCo oxide electrodes show the best catalysis for OER. The slightly decrease of Rf with electrode potential increase is in line with previous reports [29e31]. The fact, roughness Rf of CueCo oxide electrodes increases with the increase of Cu content, is in accord with the results observed in Fig. 3. Based on the exponentially decreasing trend of Rt (Table 2) with overpotential (h) [32], the apparent current density ( j ) was calculated using equation Rt ¼ RT/nFj [32,33], where R, F, j and n represent gas constant, faraday constant, apparent current density and electron transfer number (here, n ¼ 4). The linear trend of the data (h vs. log ( j )) is displayed in Fig. 8. The Tafel slope was estimated. The results are listed in Table 3. The exchange current density ( j0) was also calculated according to the Tafel slope. For comparison, the exchange current density, average value of oxide film resistance (R0) and roughness factor (Rf) are also listed in Table 3. For the oxides deposited in this work, two transition processes among the crystal structures are confirmed by XRD, one refers to transition from the pure oxides to spinel structure (CuCo2O4), while the other refers to that from spinel to Cu2CoO3 structure. During each transition, the Cu0.68Co2.32O4 or Cu1.45CoO3 obtained with unsaturated composition compared to the pure crystal phase. For the first transition, the Tafel slope increases twice with the almost same amplitude. The Tafel slope increase means the change of mechanism for OER on these oxide electrodes. As reported in the literatures [33], the species of active site and the OH concentration are two major factors that affect the OER mechanism, particularly, the determining step in the reaction. Taking into account the
Fig. 8 e Apparent current density (j) e overpotential (h) trend for CueCo oxides, single Co3O4 and CuO deposited on Ti sheet calculated from EIS data in table 2.
solution (1 M KOH) being used in the experiments, the change of crystal structure, i.e., the formation of new active site should be responsible for the modification of slope. It is undoubted the slope has a sensitivity to the crystal structure according to the data in Table 3.Upon increasing Cu content, the supersaturated Cu1.45Co1.55O4 in spinel structure formed but did not show effect on slope, although the Rf increase obviously. This phenomenon revealed that the excessive Cu doped for saturated crystal would not cause the change of mechanism for OER until a new crystal appeared. The second transition exhibited a trend of slope modification similar to the former. The above results indicated the doped Cu possesses two kinds of actions for the binary oxides, one refers to the formation of new active site by ion substitution in the crystal phase, while the other refers to the increase of surface area before the amount of Cu can arise transition of crystal structures. The exchange current density (in Table 3) shows increasing trend depend on the modification of crystal phase which illuminates that the Cu doping can effectively enhance the rate of kinetic performance of the CueCo oxides. In addition, although oxide film resistance (R0) decreased with Cu doping, it seems that R0 have no obviously effect on the electrochemical performance of these oxides.
Table 3 e The value of the kinetic parameters for OER and the average value of R0 and Rf of CueCo oxides, single Co3O4 and CuO deposited on Ti sheet. Oxides
R0/U
Rf
CuO Co3O4 Cu0.68Co2.32O4 Cu1.25Co1.75O4 Cu1.45Co1.55O4 Cu1.45CoO3 Cu1.97CoO3
7.57 7.33 4.02 12.29 0.79 31.91 0.74 79.06 0.91 126.58 1.15 132.04 1.37 150.03
Slope/(mV/dec) j0 109 (mA cm2) 50.95 50.79 54.50 59.40 60.49 64.98 70.15
0.70 1.97 13.78 163.19 253.90 909.44 3056.13
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4.
Conclusion [9]
The CueCo complex oxides were deposited by reactive DC magnetron sputtering. The SEM images show that the doping of Cu ions not only makes the grain size reduced, but also favors the formation of the fine grain. Although XRD spectra analysis shows no peak of CuO phase due to its too small size, even when the copper content exceeds the stoichiometric CuCo2O4 spinel, the UV adsorption of CueCo oxides with a high Cu content clearly shows the presence of CuO. The oxides deposited in crystal structures experience two transitions with Cu content increase, one from the pure oxides to spinel structure (CuCo2O4), another from spinel to monoclinic structure (Cu2CoO3). The catalytic activity of CueCo oxide electrodes changes with Cu content. The Copper-rich Cu2CoO3 oxides show better performance for the OER than that of spinel CueCo oxides.
[10]
[11]
[12]
[13] [14]
[15]
Acknowledgments This work was financially supported by NSFC of China (Grant Nos 20806096, 20936008 and 20906107), by Innovative Talent Training Project, Chongqing University (S-2010532 and S-09013), by the Fundamental Research Funds for the Central University (Nos. CDJXS10221141 and 11132229), and by National Innovation Experiment Program for University Students (101061136).
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MnxCo3xO4(1 x 0) spinel films prepared by lowtemperature spray pyrolysis. Thin Solid Films 1997;311:51e7. Tiwari SK, Samuel S, Singh RN, Poillerat G, Koenig JF, Chartier P. Active thin NiCo2O4 film prepared on nickel by spray pyrolysis for oxygen evolution. Int J Hydrogen Energy 1995;20:9e15. Nikolov I, Darkaoui R, Zhecheva E, Stoyanova R, Dimitrov N, Vitanov T. Electrocatalytic activity of spinel related cobaltites MxCo3xO4 (M ¼ Li, Ni, Cu) in the oxygen evolution reaction. J Electroanal Chem 1997;429:157e68. Chi B, Lin H, Li JB, Wang N, Yang J. Comparison of three preparation methods of NiCo2O4 electrodes. Int J Hydrogen Energy 2006;31:1210e4. Tavares AC, Cartaxo MAM, da Silva Pereira MI, Costa FM. Effect of the partial replacement of Ni or Co by Cu on the electrocatalytic activity of the NiCo2O4 spinel oxide. J Electroanal Chem 1999;464:187e97. Li GH, Dai LZ, Lu DS, Peng SY. Characterization of copper cobalt mixed oxide. J Solid State Chem 1990;89:167e73. Fierro G, Lo Jacono M, Inversi M, Dragone R, Porta P. TPR and XPS study of cobaltecopper mixed oxide catalysts: evidence of a strong CoeCu interaction. Top Catal 2000;10:39e48. Tavares AC, da Silva Pereira MI, Mendocu¨a MH, Nunes MR, Costa FM, Sa´ CM. XPS and voltammetric studies on Ni1xCuxCo2O4 spinel oxide electrodes. J Electroanal Chem 1998;449:91e100. Wen TC, Kang HM. CoeNieCu ternary spinel oxide-coated electrodes for oxygen evolution in alkaline solution. Electrochim Acta 1998;43:1729e45. Lee YS, Hu CC, Wen TC. Oxygen evolution on CoeCueZn ternary spinel oxide-coated electrodes in alkaline solution. J Electrochem Soc 1996;143:1218e25. Angelov S, Zhecheva E, Petrov K, Menandjiev D. The properties of a spinel copper cobaltite prepared at low temperatures and normal pressure. Mater Res Bull 1982;17:235e40. Angelov S, Tyuliev G, Marinova Ts. XPS study of surface composition of polycrystalline CuxCo3xO4 (0 x < 1) obtained by thermal decomposition of nitrate mixtures. Appl Surf Sci 1987;27:381e92. La Rosa-Toro A, Berenguer R, Quijada C, Montilla F, Morallon E, Vazquez JL. Preparation and characterization of copper-doped cobalt oxide electrodes. J Phys Chem B 2006; 110:24021e9. Chi B, Lin H, Li J-B. Cations distribution of CuxCo3xO4 and its electrocatalytic activities for oxygen evolution reaction. Int J Hydrogen Energy 2008;33:4763e8. Subramanyan V. Effect of cations of alkali and alkaline-earth metal chlorides for chlorine evolution reaction. Ind Eng Chem Res 2008;47:976e9. Filoche M, Sapoval B. Shape-dependency of current through nonlinear irregular electrodes. Electrochim Acta 2000;46:213e20. Miedzinska KME, Hollebone BR, Cook JG. The quantum Boltzmann equation and some applications to transport phenomena. J Phys Chem Solids 1987;48:639e49. Bockris JO’M, Otagawa T. The electrocatalysis of oxygen evolution on perovskites. J Electrochem Soc 1984;131: 290e302. Tiwari SK, Chartier P, Singh RN. Preparation of perovskitetype oxides of cobalt by the malic acid aided process and their electrocatalytic surface properties in relation to oxygen evolution. J Electrochem Soc 1995;142: 148e53. Brug GJ, Van den Eeden ALG, Rehbach MS, Sluyters JH. The analysis of electrode impedances complicated by the presence of a constant phase element. J Electroanal Chem 1984;176:275e95. Lassali TAF, Boodts JFC, Bulhoes LOS. Charging processes and electrocatalytic properties of IrO2/TiO2/SnO2 oxide films
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investigated by in situ AC impedance measurements. Electrochim Acta 1999;44:4203e16. [29] Da Silva LA, Alves VA, Da Silva MAP, Trasatti S, Boodts JFC. Oxygen evolution in acid solution on IrO2 þ TiO2 ceramic films. A study by impedance, voltammetry and SEM. Electrochim Acta 1997;42:271e81. [30] Palmas S, Ferrara F, Vacca A, Mascia M, Polcaro AM. Behavior of cobalt oxide electrodes during oxidative processes in alkaline medium. Electrochim Acta 2007;53:400e6.
[31] Bocca C, Cerisola G, Magnone E, Barbucci A. Oxygen evolution on Co3O4 and Li-doped Co3O4 coated electrodes in an alkaline solution. Int J Hydrogen Energy 1999;24:699e707. ic V. Characterization of [32] Svegl F, Orel B, Grabec-Svegl I, Kauc spinel Co3O4 and Li-doped Co3O4 thin film electrocatalysts prepared by the solegel route. Electrochim Acta 2000;45:4359e71. [33] Sh-X Jin, Ye SY. Oxygen evolution on titanium anodes coated with conductive metallic oxides: kinetics and mechanism in alkaline solution. Electrochim Acta 1996;41:827e34.
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Copper-doped cobalt oxide electrodes for oxygen evolution reaction prepared by magnetron sputtering Q. Zhang a,b, Z.D. Wei a,b,*, C. Liu a, X. Liu a, X.Q. Qi a,b, S.G. Chen a, W. Ding a, Y. Ma a, F. Shi a, Y.M. Zhou a a
State Key Laboratory of Power Transmission Equipment & System Security and New Technology, School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China b School of Material Science and Engineering, Chongqing University, Chongqing 400044, China
article info
abstract
Article history:
The Copper-doped cobalt oxide films have been prepared onto titanium support by reactive
Received 11 December 2010
DC magnetron sputtering. The deposits are characterized by X-ray diffraction (XRD),
Received in revised form
energy-dispersive spectrometry (EDS), Ultroviolet (UV) and scanning electron microscope
5 April 2011
(SEM). The electrochemical characteristics of deposits are explored by cyclic voltammetry
Accepted 7 April 2011
(CV), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS).
Available online 19 May 2011
The effect of copper content in the CueCo oxides on the surface morphology, electrochemical and crystallographic properties has been investigated. The introduction of Cu
Keywords:
element makes the CueCo oxides deposit fine and further lead to high roughness of
CuO
deposits, which is helpful for the oxides’ electrochemical performance. Based on the
Co3O4
electrochemical determination, the binary oxide electrodes exhibit better catalysis activity
Spinel
than that of CuO and Co3O4 electrodes for oxygen evolution reaction (OER). The increase
Oxygen evolution
OER activity has been attributed to the enlarged surface roughness and the new active sites
Magnetron sputtering
of deposits, according to the results of EIS.
Electrocatalysis
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The electrocatalysis of oxygen evolution reaction (OER) on cobalt oxide (Co3O4) electrodes has been a topic of great interest in electrochemistry for a long time. It is known that OER overpotentials are efficiently decreased by the application of cobalt doped complex oxides because substitution of cobalt ions by foreign divalent metal ions can result in a new spinel structures with novel physicochemical properties, which may exhibit more excellent performances as compared to a single oxide [1]. To date, the cobalt-containing spinel oxides MCo2O4 (M ¼ Ni, Cu, Zn, Mg, Mn, Cd, etc.) [2e11] have been extensively studied in order to establish the defined correlations refer to their properties, composition and structure. Among those complex oxides,
copper-doped cobaltite spinels show relatively high stability and activity with low cost and availability [2,13]. Up to now, diverse synthetic routes have been adopted to prepared copperdoped spinel cobaltites, such as: chemical coprecipitation [13,14], thermal decomposition [5,6,12,15e19], spray pyrolysis [6,7], and solegel routes [2]. However, the catalytic activity and physicochemical properties of the prepared oxides are rather different because they are profoundly affected by the preparation methods. Moreover, the catalytic activity reported was not consistent, even contrary, despite the same preparation method was adopted [20,21]. The annealing atmosphere, the chemical nature of the precursor and substrate, even the man-made factor, are also the reasons for the differences in the literatures. Therefore, the characterization of thin films prepared
* Corresponding author. Tel.: þ86 23 65102531; fax: þ86 23 65112134. E-mail address: [email protected] (Z.D. Wei). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.051
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 8 2 2 e8 3 0
Fig. 1 e X-ray diffraction patterns for CueCo oxides and Co3O4 thin films deposited on glass.
with a particular method and under concrete experimental conditions is necessary to understand their electrochemical performance. In this work, the reactive DC magnetron sputtering method (RDCMS) is adopted to prepare the CueCo complex oxides with Co/Cu ratio from 0.5 to 3.4. Cu and Co sources are ultrapure copper and cobalt targets. Thus the so-called precursor effect can be avoided. The stable atmosphere and annealing condition also ensure the accuracy and reproducibility of the experiment. This work is aimed at establishment of relationship among the catalyst activity and morphology, composition, crystallinity and electric properties of the CoeCu complex oxides, which has been not yet clear.
2.
Experimental section
2.1.
Preparation of the samples
and cobalt (99.99%) disks of 60 mm diameter and 3 mm thick were used as the sputter targets. Argon of 99.999% purity was used as the sputter gas and oxygen of 99.999% purity as the reactive gas. Before deposition, the substrate was heated to 500 C and the sputtering chamber was first pumped down from atmospheric pressure to a base pressure of 1 103 Pa. The Cu and Co targets were pre-sputtered in turn at 60 W in pure argon atmosphere for 5 min to remove any oxide layer or contaminant on the target surface. Furthermore, all oxide films deposited at a total pressure of 1.0 Pa, during which the argon and oxygen were introduced into the sputtering chamber at gas flux of 4 and 22 sccm, respectively. The substrates were fixed on the anode and the distance between target and substrate was 120 mm. In sputtering, the Co target sputtering power was fixed at 100 W while the Cu target sputtering power changed from 15, 30, 45, 60 to 75 W. The pure oxide phase (CuO and Co3O4) obtained by single target sputtering only. All of the films were deposited in half an hour and cooled to the room temperature in the high vacuum atmosphere. For comparison, the RuO2 catalyst with loading of 1 mg cm2 was prepared on Ti sheet by a pyrolysis method [22].
2.2.
Characterization of samples
The morphology and chemical compositions of the prepared films on titanium sheet were characterized by scanning electron microscopy (SEM, FEI Nova 400 FEG) and energydispersive spectrometry (EDS, OXFORD LinkdISIS-300). X-ray diffraction (XRD) of the films on glass was conducted on an X-ray diffractometer (XRD-6000, Shimadzu) with filtered Cu Ka radiation at a wavelength of 0.15428 nm, and the grain size was calculated using Scherrer’s formula. UVevis transmission spectra (Cintra-10e, Australia GBS) were also used to characterize the oxide films on glass.
2.3.
The oxide thin films were deposited on titanium sheet (exposed area 1 1 cm2) or microscope glass slides (2.5 3 cm2) by means of dc magnetron sputtering system (JGP-450, SKY Technology Development Co., Ltd., China). The high purity copper (99.99%)
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Electrochemical measurements
All the electrochemical measurements were performed on Autolab electrochemical analyzer (PGSTAT302N) in a conventional three-electrode glass cell at room temperature (25 C).
Table 1 e Lattice parameters and unit cell volume for CueCo oxides on glass calculated from diffractogram. Lattice parameters Electrodes
˚) a (A
˚ 3) V (A
Co3O4(JCPDS) Co3O4/glass Cu0.68Co2.32O4 Cu1.25Co1.75O4 Cu1.45Co1.55O4 Cu1.45CoO3 Cu1.97CoO3 CuO
8.084 8.094 8.112 8.119 8.130
528.20 530.39 533.79 535.13 537.47
˚) Crystallite size (A 360 425 339 266 62 55 119
Fig. 2 e UV spectra of CueCo oxides, CuO and Co3O4 thin films deposited on glass.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 8 2 2 e8 3 0
Fig. 3 e SEM images of CueCo oxides, single Co3O4 and CuO deposited on Ti sheet: (a) Co3O4 [Co:100W], (b) CuO [Cu:100W], (c) Cu1.25Co1.75O4 [Cu:30W;Co:100W], (d) Cu1.45Co1.55O4 [Cu:45W;Co:100W], (e) Cu1.45CoO3 [Cu:60W;Co:100W], (f) Cu1.97CoO3 [Cu:75W;Co:100W].
A platinum wire and an Ag/AgCl electrode (0.20 V vs. NHE) were used as the counter electrode and reference electrode, respectively. The deposited oxide thin films onto titanium sheet (1 cm2) were used as working electrodes for the catalysis of the OER in 1 M KOH solution. Electrochemical impedance spectroscopy measurements were performed at different bias potentials with the excitation amplitude of 5 mV. The impedance spectra were recorded in a range of frequency from 100 kHz to 0.05 Hz, and before recording each spectrum, the electrode was first equilibrated at the corresponding bias potential for 100 s. The experimental impedance data was fitted to an appropriate equivalent circuit using Zview software.
3.
Results and discussion
3.1. Oxides composition vs. their crystalline structure and morphology In order to avoid cover-effect of Ti substrate and obtain the vivid XRD peaks of the deposits, the microscope glass slides were employed as the substrate for oxides thin films deposition. Fig. 1 shows the X-ray diffraction patterns of CueCo complex oxides electrodes with different Co/Cu ratios (0.5e3.4). For comparison, the figure also includes the XRD spectra of the Co3O4 and CuO on glass substrate. The XRD for sample Co3O4/glass shows
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 8 2 2 e8 3 0
Fig. 4 e CVs plots of CueCo oxides, single Co3O4 and CuO deposited on Ti sheet at a scan rate 50 mV sL1 in 1 M KOH.
reflections at 2q of 19.08, 36.87, 38.51 and 59.25 that could be indexed to a cubic spinel lattice of the single Co3O4 phase (JCPDS 42-1467). The XRD for sample CuO/glass shows reflections at 2q of 32.53, 35.70 and 38.71 that could be indexed to a monoclinic CuO phase (JCPDS 80-0076). Upon increasing the Cu target power, i.e., increasing the Cu content in the oxides, the XRD patterns keep changed, but no peaks corresponding to CuO was observed. No CuO crystalline phase was found either at any Cu content by XRD even when its stoichiometric amount was superfluous. This phenomenon could be assigned to that the CuO exists in form of an amorphous phase or a crystalline phase but with a very small size, making it difficult to be detected by XRD. The relevant crystalline phases and chemical composition of the oxide films can be distinguished by integrative analysis of XRD and EDS, and the results are as follows: Cu0.68Co2.32O4[Cu:15W,Co:100W], Cu1.25Co1.75O4 [Cu:30W,Co:100W], and Cu1.45Co1.55O4[Cu:45W,Co:100W] show
Fig. 5 e LSVs plots of CueCo oxides, single Co3O4 and CuO deposited on Ti sheet at a scan rate 10 mV sL1 in 1 M KOH.
825
the spinel structure of CuCo2O4 (JCPDS 37-0878), while Cu1.45CoO3[Cu:60W,Co:100W] and Cu1.97CoO3[Cu:75W,Co:100W] have Cu2CoO3 crystalline structure (JCPDS 21-0288). The unit cell parameter of the spinel CueCo oxides has been calculated using its main indexed planes appeared in the diffractogram. The lattice parameters and the unit cell volume as well as the mean crystallite size are listed in Table 1. The unit cell parameter of spinel oxides continuously increases with the copper content. It is believed that the increase in unit cell parameter is caused by the substitution of large Cu(II) cations for small Co (II) cations. The crystallite size decrease with the copper content indicates that the increment of Cu content benefits the crystallite size reduction. This results derived from XRD are also confirmed by SEM images displayed in Fig. 3. That is, the doping of Cu ions favors the formation of fine grain CueCo oxides. It would be good for obtaining an electrode with a high surface area. Not seeing the air doesn’t mean there is no air. For further ascertainment of CuO phase, the optical property of the above oxide films was studied by UV adsorption spectrum. Among the curves in Fig. 2, curves 1 and 2 belong to pure Co3O4 and CuO, curves 3e7 to CueCo oxides. The UV adsorption behavior of Co3O4 and CuO is so different from each other. The Curve 2 demonstrates the particular and unique adsorption behavior of single CuO phase that make it easy to confirm the appearance of CuO in the oxide films. It’s well known that spinel oxides have the AB2O4 composition [1], where A ions are generally bivalent metal ions occupying the tetrahedral sites, and B ions are trivalent ions occupying the octahedral sites. As for single Co3O4, both the two sites are occupied by cobalt ions. In the case of CuCo2O4, the doped Cu ions would replace parts of Co ions at octahedral and tetrahedral sites. The substitution of Co sites in Co3O4 structure could reduce its energy band-gap by removing the orbital degeneracy and adding new orbital energy levels, resulting in a continuous adsorption of light [25]. The UV adsorption behavior of CueCo oxides continuously changes with the copper content. No typical UV adsorption behavior of monocline CuO appears until the copper content exceeds the stoichiometric CuCo2O4 spinel. It is easy to confirm that the independent CuO phase surely exists in Cu1.45Co1.55O4, Cu1.45CoO3 and Cu1.97CoO3 according
Fig. 6 e OER on Cu1.97CoO3 and RuO2.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 8 2 2 e8 3 0
Table 2 e Simulated parameters of the elements in equivalent circuits for oxygen evolution of CueCo oxides, RuO2, single Co3O4 and CuO deposited on Ti sheet at 0.70, 0.75, 0.77 and 0.8V in 1 M KOH. Oxides
E/V Rs (U cm2) R0 (U cm2) Q1 104 (S cm2 sn) n1 Rt (U cm2) Q2 103 (S cm2 sn) n2 Cdl 103 (F cm2)
Rf
CuO
0.70 0.75 0.77 0.80
1.25 1.25 1.24 1.25
7.88 7.49 7.48 7.41
0.78 0.79 0.93 0.88
0.81 0.81 0.80 0.80
13500.00 1070.00 466.00 158.00
1.43 1.66 1.73 1.74
0.84 0.83 0.83 0.82
0.43 0.43 0.45 0.45
7.16 7.16 7.50 7.50
Co3O4
0.70 0.75 0.77 0.80
0.75 0.75 0.76 0.77
4.28 4.07 3.97 3.79
1.51 1.44 1.33 1.14
0.83 0.83 0.84 0.85
4242.00 416.20 164.90 46.47
1.58 1.38 1.31 1.29
0.91 0.92 0.92 0.91
0.81 0.76 0.72 0.65
13.65 12.67 12.00 10.83
Cu0.68Co2.32O4 0.70 0.75 0.77 0.80
0.74 0.75 0.77 0.78
0.80 0.79 0.78 0.78
8.86 10.56 12.58 10.74
0.71 0.70 0.68 0.69
1502.0 186.30 71.12 22.98
3.70 3.38 3.14 3.10
0.92 0.91 0.91 0.90
2.16 1.91 1.89 1.74
36.00 31.83 31.50 28.30
Cu1.25Co1.75O4 0.70 0.75 0.77 0.80
0.81 0.81 0.82 0.83
0.74 0.73 0.73 0.74
3.07 4.01 2.99 2.38
0.80 0.78 0.81 0.83
377.80 46.35 22.48 8.10
7.84 7.55 7.18 7.11
0.93 0.92 0.92 0.91
5.32 4.84 4.58 4.24
88.67 80.67 76.30 70.60
Cu1.45Co1.55O4 0.70 0.75 0.77 0.80
0.70 0.70 0.71 0.72
0.93 0.91 0.91 0.89
3.32 3.15 2.88 2.45
0.81 0.81 0.82 0.84
302.20 37.78 18.67 6.91
14.08 13.12 12.15 11.97
0.91 0.90 0.90 0.89
8.91 7.76 7.14 6.57
148.50 129.33 119.00 109.50
Cu1.45CoO3
0.70 0.75 0.77 0.80
0.74 0.75 0.75 0.76
1.21 1.14 1.13 1.10
1.93 1.96 1.84 1.61
0.83 0.83 0.84 0.85
183.9 26.3 13.71 5.57
14.71 13.95 13.39 13.08
0.91 0.89 0.89 0.88
9.40 7.91 7.53 6.85
156.67 131.83 125.50 114.16
Cu1.97CoO3
0.70 0.75 0.77 0.80
0.80 0.82 0.82 0.78
1.39 1.37 1.37 1.36
0.94 0.92 0.91 0.90
0.80 0.82 0.83 0.83
119.90 19.47 10.32 4.76
17.62 16.66 16.18 16.50
0.89 0.88 0.87 0.86
10.39 9.23 8.47 7.92
173.16 153.80 141.16 132.00
RuO2
0.70 0.75 0.77 0.80
1.24 1.31 1.35 1.37
0.23 0.69 0.41 0.40
264.87 501.59 543.80 253.08
0.58 0.47 0.48 0.59
28.73 6.98 3.033 2.344
21.27 20.86 20.40 20.15
0.97 0.98 0.97 0.98
19.24 19.52 17.93 18.70
320.61 325.26 298.82 311.61
to the UV adsorption curves. In addition, the unique broad peaks (370e450 nm) of Curves 6 and 7 also indicate some new chemical bond formed in these oxides.
3.2.
Performance in catalysis of oxygen evolution
The CVs obtained from the Co3O4, CuO and CueCo oxides electrodes in the potential range of 0.0e0.8 V are shown in Fig. 4. The CV of Co3O4 electrode exhibits a couple of reversible redox peaks at about 0.63 V, while no peak appears in the CV related to CuO. The anodic peak at about 0.63 V for Co3O4 electrodes can be assigned to the Co(IV)/Co(III) redox couple [21]. It is noteworthy that the peaks of CueCo oxide electrodes are broad and not well defined compared to that of Co3O4 electrode. The broadness of the peaks could be connected with roughness of the electrode. As suggested by Filoche and Sapoval [23], materials with a high degree of roughness present a wide potential distribution throughout the surface. The corresponding LSVs of the above electrodes for the oxygen evolution reaction are shown in Fig. 5. The catalytic activity of the electrodes for the oxygen evolution reaction increases sharply from Co3O4 to Cu0.68Co2.32O4 and Cu1.25Co1.75O4 in spinel, and then to increase slightly from Cu1.45Co1.55O4 to Cu1.45CoO3 and Cu1.97CoO3 in the Cu2CoO3 crystalline phase.
These catalytic activity changes parallel the modifications observed in the oxides morphology (Fig. 3), and hence they could be attributed to the growth of surface-active area besides the effect of transition in crystal structure. The foreign metal oxide segregated at the surface may caused a decrease in the catalytic activity for the doped cobalt oxides [4,7,24]. In our case, the copper-rich Cu2CoO3 and Cu1.45Co1.55O4 with segregated CuO phase show even better performance than that of the pure spinel, indicating the CuO phase may homogeneously distribute in the oxide bulk and benefit the increase of active surface. The comparison of performance between Cu1.97CoO3 and RuO2 are depicted in Fig. 6. In order to reveal the exact performance of these catalysts, the LSVs data of the Cu1.97CoO3 and RuO2 were normalized by their roughness factor Rf in Table 2 before the comparison. After eliminating the effect of surface roughness, it was found that the Cu1.97CoO3 (the best CueCo oxide) and RuO2 showed almost the same catalytic activity to the OER.
3.3.
Catalytic mechanism based on EIS analysis
In order to gain an insight into the behavior of the system during oxygen evolution, the EIS have been carried out on the oxide electrodes at potentials chosen in the potential range,
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 8 2 2 e8 3 0
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Fig. 7 e Equivalent circuit-Rs(RoQ1)(RtQ2) and Nyquist plots (experimental and simulated) for CueCo oxides, single Co3O4 and CuO deposited on Ti sheet at 0.7, 0.75, 0.77 and 0.8V in 1M KOH.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 8 2 2 e8 3 0
0.7e0.8 V, in 1 M KOH. As it can be seen in Fig. 7, each complexplane diagram (Nyquist) shows two semicircles. One diameter decreases with increasing the potential in the low frequency domain, while the other has the constant diameter at the medium and high frequency region regardless of the potential high or low. The former can be ascribed to the OER, while the latter is related to the solid oxide. The equivalent circuit was found to match most closely the observed impedance diagrams in form of Rs(R0Q1)(RtQ2) (Fig. 7), where Rs, R0, Rt and Q represent solution resistance, oxide film resistance, charge transfer resistance of OER, constant phase element (CPE) corresponding to the oxide mass (Q1) and the interface between film and electrolyte (Q2). The CPE(ZCPE ¼ [Q( ju)n]1) contains two parameters: Q, the frequency independent parameter and n(0 < n < 1), the deviation from the ideal behavior, being n ¼ 1 for perfect capacitors and n ¼ 0 for pure resistance. By using CPE instead of pure capacitor, the simulated results are in good agreement with the experimental data as shown in Fig. 7. Estimates of the circuit parameters are given in Table 2. The roughness factor (Rf) of the oxides electrodes has also been calculated using Cdl/60(mF cm2) [26,27], where Cdl represents double layer capacitance of the oxide film/solution interface and can be estimated by equation of Q2 ¼ (Cd1)n[(Rs)1 þ (Rt)1] [28]. Table 2 illustrates that Cu doping can decrease the resistance of the oxides film till the formation of Cu1.25Co1.75O4, and then increase gradually from Cu1.45Co1.55O4 to Cu1.45CoO3 and Cu1.97CoO3. In addition, the charge transfer resistance (Rt) of OER decreased sharply for all the oxide electrodes with the applied potential increased. At a given potential, the CueCo oxide electrodes exhibited the lowest Rt of all oxides electrodes, indicating the CueCo oxide electrodes show the best catalysis for OER. The slightly decrease of Rf with electrode potential increase is in line with previous reports [29e31]. The fact, roughness Rf of CueCo oxide electrodes increases with the increase of Cu content, is in accord with the results observed in Fig. 3. Based on the exponentially decreasing trend of Rt (Table 2) with overpotential (h) [32], the apparent current density ( j ) was calculated using equation Rt ¼ RT/nFj [32,33], where R, F, j and n represent gas constant, faraday constant, apparent current density and electron transfer number (here, n ¼ 4). The linear trend of the data (h vs. log ( j )) is displayed in Fig. 8. The Tafel slope was estimated. The results are listed in Table 3. The exchange current density ( j0) was also calculated according to the Tafel slope. For comparison, the exchange current density, average value of oxide film resistance (R0) and roughness factor (Rf) are also listed in Table 3. For the oxides deposited in this work, two transition processes among the crystal structures are confirmed by XRD, one refers to transition from the pure oxides to spinel structure (CuCo2O4), while the other refers to that from spinel to Cu2CoO3 structure. During each transition, the Cu0.68Co2.32O4 or Cu1.45CoO3 obtained with unsaturated composition compared to the pure crystal phase. For the first transition, the Tafel slope increases twice with the almost same amplitude. The Tafel slope increase means the change of mechanism for OER on these oxide electrodes. As reported in the literatures [33], the species of active site and the OH concentration are two major factors that affect the OER mechanism, particularly, the determining step in the reaction. Taking into account the
Fig. 8 e Apparent current density (j) e overpotential (h) trend for CueCo oxides, single Co3O4 and CuO deposited on Ti sheet calculated from EIS data in table 2.
solution (1 M KOH) being used in the experiments, the change of crystal structure, i.e., the formation of new active site should be responsible for the modification of slope. It is undoubted the slope has a sensitivity to the crystal structure according to the data in Table 3.Upon increasing Cu content, the supersaturated Cu1.45Co1.55O4 in spinel structure formed but did not show effect on slope, although the Rf increase obviously. This phenomenon revealed that the excessive Cu doped for saturated crystal would not cause the change of mechanism for OER until a new crystal appeared. The second transition exhibited a trend of slope modification similar to the former. The above results indicated the doped Cu possesses two kinds of actions for the binary oxides, one refers to the formation of new active site by ion substitution in the crystal phase, while the other refers to the increase of surface area before the amount of Cu can arise transition of crystal structures. The exchange current density (in Table 3) shows increasing trend depend on the modification of crystal phase which illuminates that the Cu doping can effectively enhance the rate of kinetic performance of the CueCo oxides. In addition, although oxide film resistance (R0) decreased with Cu doping, it seems that R0 have no obviously effect on the electrochemical performance of these oxides.
Table 3 e The value of the kinetic parameters for OER and the average value of R0 and Rf of CueCo oxides, single Co3O4 and CuO deposited on Ti sheet. Oxides
R0/U
Rf
CuO Co3O4 Cu0.68Co2.32O4 Cu1.25Co1.75O4 Cu1.45Co1.55O4 Cu1.45CoO3 Cu1.97CoO3
7.57 7.33 4.02 12.29 0.79 31.91 0.74 79.06 0.91 126.58 1.15 132.04 1.37 150.03
Slope/(mV/dec) j0 109 (mA cm2) 50.95 50.79 54.50 59.40 60.49 64.98 70.15
0.70 1.97 13.78 163.19 253.90 909.44 3056.13
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 8 2 2 e8 3 0
4.
Conclusion [9]
The CueCo complex oxides were deposited by reactive DC magnetron sputtering. The SEM images show that the doping of Cu ions not only makes the grain size reduced, but also favors the formation of the fine grain. Although XRD spectra analysis shows no peak of CuO phase due to its too small size, even when the copper content exceeds the stoichiometric CuCo2O4 spinel, the UV adsorption of CueCo oxides with a high Cu content clearly shows the presence of CuO. The oxides deposited in crystal structures experience two transitions with Cu content increase, one from the pure oxides to spinel structure (CuCo2O4), another from spinel to monoclinic structure (Cu2CoO3). The catalytic activity of CueCo oxide electrodes changes with Cu content. The Copper-rich Cu2CoO3 oxides show better performance for the OER than that of spinel CueCo oxides.
[10]
[11]
[12]
[13] [14]
[15]
Acknowledgments This work was financially supported by NSFC of China (Grant Nos 20806096, 20936008 and 20906107), by Innovative Talent Training Project, Chongqing University (S-2010532 and S-09013), by the Fundamental Research Funds for the Central University (Nos. CDJXS10221141 and 11132229), and by National Innovation Experiment Program for University Students (101061136).
[16]
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