Electrodeposited Co-Doped Fe3O4 Thin Films as Efficient Catalysts for the Oxygen Evolution Reaction

Electrochimica Acta 210 (2016) 942–949

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Electrodeposited Co-Doped Fe3O4 Thin Films as Efficient Catalysts for the Oxygen Evolution Reaction Shan Hana,b , Suqin Liua , Shengjie Yina , Lei Chena , Zhen Hea,* a b

College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, PR China Innovation Base of Energy and Chemical Materials for Graduate Students Training, Central South University, Changsha, Hunan 410083, PR China

A R T I C L E I N F O

Article history: Received 14 February 2016 Received in revised form 22 April 2016 Accepted 29 May 2016 Available online 3 June 2016 Keywords: electrodeposition CoxFe3-xO4 catalytic activity oxygen evolution reaction thin film

A B S T R A C T

The development of highly active electrocatalysts at a low cost is essential to the process of generating hydrogen fuel through electrochemical and photoelectrochemical water splitting. Here, we report a comprehensive investigation of the one-step electrodeposited cobalt-doped magnetite (CoxFe3-xO4, 0 < x < 1) thin films as active and stable OER catalysts in alkaline solutions. The CoxFe3-xO4 thin film electrode can be fabricated in minutes or even tens of seconds. The Co-doping concentration, thickness, and orientation of the CoxFe3-xO4 films can be simply controlled by varying the deposition potential, deposition time, and the substrate, respectively. The dependences of the catalytic activity of the CoxFe3xO4 films on the composition, thickness, and orientation of growth are explored. The CoxFe3-xO4 films exhibit greatly enhanced catalytic activities toward the OER compared to the Fe3O4 thin film. The polycrystalline CoxFe3-xO4 film deposited at 0.88 VAg/AgCl for 150 seconds exhibits the highest catalytic activity with an overpotential of 0.42 V at a current density of 10 mA cm 2, a Tafel slope of 53 mV dec 1, and an exchange current density of about 2.39  10 10 A cm 2, which are comparable to those of Co3O4. Besides, the CoxFe3-xO4 films possess good stability during the long-term electrolysis at a current density of 10 mA cm 2 in 1 M NaOH. The satisfactory catalytic activity and stability combined with the simplicity of fabrication make the electrodeposited CoxFe3-xO4 films economically and environmentally preferable compared to Co3O4 as catalysts for the oxygen evolution reaction. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction The splitting of water into hydrogen and oxygen is an intriguing way to convert the intermittent renewable energy, such as solar and wind energy, into a clean, storable, and transportable fuel. The kinetically sluggish four-electron oxidation reaction, i.e., the oxygen evolution reaction (OER), is the bottleneck of electrochemical (EC) and photoelectrochemical (PEC) water splitting [1,2]. Thus, the development of highly active, low-cost, and robust electrocatalysts for the OER is essential for this energy conversion technology [3,4]. Although ruthenium oxides and iridium oxides are well-known to be the most active OER catalysts, their scarcity and high cost have limited their large-scale applications in water splitting [5–8]. Hence, intensive efforts have been dedicated to exploring efficient and stable OER catalysts based on earthabundant elements [9–15]. Among the electrocatalysts based on non-precious metals, the spinel-type cobalt oxide (Co3O4) exhibits

* Corresponding author. Tel.: +86 13574857624; fax: +86 731 88879616. E-mail address: [email protected] (Z. He). http://dx.doi.org/10.1016/j.electacta.2016.05.194 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.

promising OER activity and corrosion-resistance in alkaline solutions [16–18]. More recently, mixed metal oxides and (oxy) hydroxides produced by partially replacing Co in CoOx and CoOxHy with more eco-friendly and/or less expensive metallic elements (e.g., Zn, Fe, Cu, and Ni) have been reported to show the OER catalytic activity comparable to or even higher than CoOx and CoOxHy [19–28]. These results motivate the development of facile synthesis methods of the Co-based mixed metal oxide/(oxy) hydroxide electrodes as well as thorough understanding of the relationships between their OER catalytic activities and chemical/ physical properties, e.g., composition, thickness, morphology, and crystalline orientation. In this study, we report a comprehensive investigation of the electrodeposited cobalt-doped magnetite (CoxFe3-xO4, 0 < x < 1) thin films as active and stable OER catalysts in alkaline solutions. The CoxFe3-xO4 thin films in this study are fabricated by a facile one-step electrodeposition method on conductive substrates (e.g., Au and Cu), which could also be used to produce various binary and ternary metal oxide electrodes [29–32]. Compared to the electrodes fabricated by loading the powdery active materials onto current collectors using additional adhesives, the electrodes

S. Han et al. / Electrochimica Acta 210 (2016) 942–949

produced by directly electrodepositing active materials onto conducting or semiconducting substrates have some advantages. For example, the electrodeposition is a bottom-up technique, i.e., the growth of the active materials starts right from the surfaces of the substrates. This could provide direct paths for electrons flowing through the interfaces of the active materials and the substrates, and thus could lower the resistance at the interfaces. Besides, the electrodes made by direct electrodeposition method eliminate the additional resistance imposed by the use of the adhesives. Moreover, electrodeposition offers many degrees of freedom (e.g., applied potential/current, temperature, composition, pH, and additives of the deposition solution), which allows the researchers to control the growth of the deposits including nucleation density, orientation, and morphology [33]. In this study, the Co-doping concentration, thickness, and orientation of the CoxFe3-xO4 films can be simply tuned by varying the deposition potential, deposition time, and the orientation of the substrate, respectively. This allows us to study the influence of each of the properties of the active materials on the OER performance independently. For example, the CoxFe3-xO4 films with the same thickness, morphology, and orientation but different Co:Fe ratios could be electrodeposited and tested for exploring the relationship between the composition and the OER performance of the films. It is critical, in terms of making meaningful comparisons, that the two electrodes compared for the OER performance are prepared from the same or similar synthesis methods and have comparable or same properties except for the targeted one for study [19]. Based on this principle, the dependence of the catalytic activity of the CoxFe3-xO4 films on the composition, thickness, and orientation of growth are explored. 2. Experimental 2.1. Preparation of the deposition solution The electrolyte for the electrodeposition of CoxFe3-xO4 films contained 50 mM Co(II), 50 mM Fe(III), 100 mM triethanolamine (TEA, Vetec by Sigma Aldrich, 97%), and 2 M NaOH (Xilong Scientific, 96%) [30]. To prepare the deposition solution, approximately 1.50 g of Fe2(SO4)3
xH2O (Vetec by Sigma Aldrich, 97%) was dissolved in an 8 mL prepared 1 M TEA solution at 50  C (controlled by a water bath) under stirring, resulting in a reddish brown Fe(III)-TEA complex solution. This Fe(III)-TEA complex was then slowly added (at a rate of about three to four drops per second) into a 135 mL concentrated NaOH solution containing about 12 g NaOH under vigorous stirring, forming an alkaline Fe (III)-TEA solution. Approximately 2.18 g of Co(NO3)2
6H2O (Sinopharm, 99.0%) was dissolved in a 7 mL prepared 1 M TEA solution at room-temperature, resulting in a Co(II)-TEA complex solution. This Co(II)-TEA solution was then added into the prepared alkaline Fe(III)-TEA solution under vigorous stirring to produce a 150 mL of final electrolyte for the electrodeposition of CoxFe3-xO4 films. 2.2. Electrodeposition of CoxFe3-xO4 films A polycrystalline Au-disk electrode (manufactured in PTFE with a fixed area of 0.196 cm2), five pieces of polycrystalline Au sheets (20  10  0.5 mm), and three Cu single crystals (10  10  0.5 mm) with different orientations (i.e., Cu(100), (110), and (111)) were used as the substrates for the electrodeposition of CoxFe3-xO4 films. All the substrates were mechanically polished and cleaned by sonication in acetone, anhydrous ethanol, and deionized water (D. I. H2O) successively before electrodeposition. The CoxFe3-xO4 films were electrodeposited onto the substrates at 80  C by applying constant potentials using a CHI440C electrochemical workstation, which is similar to what we reported in previous work [30]. On the

943

Au substrates, the potential range for the electrodeposition was between 0.88 and 0.96 V vs Ag/AgCl (VAg/AgCl) at increments of 0.02 VAg/AgCl. On the Cu substrates, the deposition potential was 0.88 VAg/AgCl. The freshly deposited films were thoroughly rinsed by D. I. H2O and dried under ambient conditions for further characterizations. 2.3. XRD, ICP-OES, SEM, EDS, and XPS characterizations The crystal structure of the CoxFe3-xO4 films (on Au sheet substrates) electrodeposited at different potentials was analyzed by using a Rigaku D/Max 2500 X-ray diffractometer with a Cu Ka radiation (l = 0.154184 nm) at 40 kV and 250 mA. The u-2u scans were obtained in the range of 5 –80 with a scan rate of 8 min 1 using the linear focal spot mode with a graphite monochromator. The atomic Co:Fe ratios of the CoxFe3-xO4 films were determined by using a PerkinElmer Optima 5300 DV inductively coupled plasma optical emission spectrometer (ICP-OES). The ICP sample was prepared by dissolving the electrodeposited CoxFe3xO4 film in approximately 2 mL concentrated hydrochloric acid (HCl, 35–38 wt%) and then diluting with D. I. H2O to 25.00 mL in a volumetric flask. The sample injection rate was 1.5 mL min 1. The plasma power was 1300 W. The detection limitations of the instrument for elemental Co and Fe were about 7.0 and 6.2 ppb, respectively. The surface morphologies of the CoxFe3-xO4 films electrodeposited at different potentials on the polycrystalline Au sheets were studied by a scanning electron microscope (SEM, Nova NanoSEM230) equipped with an energy dispersive spectroscopy (EDS) system at an accelerating voltage of 15 kV. In addition to the ICP-OES measurement as described above, the atomic Co:Fe ratio of the CoxFe3-xO4 films was also determined by using EDS. The EDSbased Co:Fe ratio was reported by averaging the Co:Fe values from 5 different spots of each sample. Surface analysis of the CoxFe3-xO4 films (on Au sheet substrates) deposited at different potentials was done on a Thermo Scientific ESCALAB 250 Xi X-ray photoelectron spectrometer (XPS) using monochromatic Al Ka radiation energy (l = 1486.6 eV). 2.4. Electrochemical measurements The catalytic properties and stability of the electrodeposited CoxFe3-xO4 films were studied in 1 M NaOH at room-temperature using a CHI440C electrochemical workstation (with an electrochemical quartz crystal microbalance (EQCM) accessory) in a standard three-electrode cell with the CoxFe3-xO4 films as the working electrode, a Ag/AgCl electrode (in saturated KCl) as the reference electrode, and a Pt mesh (20  10 mm) as the counter electrode. The distances between the three electrodes were fixed for all the tests. Linear sweep voltammetry (LSV) and steady-state polarization (Tafel plots) were used to estimate the catalytic activities of the films toward the OER, whereas the long-term electrolysis was used to evaluate the stability of the films under the conditions of the OER. The overpotentials (h) were calculated referring to the thermodynamic potential of the oxygen evolution reaction at pH 14 (i.e., 0.401 V vs NHE). The stability of the film under the OER conditions was also studied by using EQCM to monitor the in situ mass change of the film. For the EQCM study, a CoxFe3-xO4 film was deposited at 0.88 VAg/AgCl (in the deposition solution described above) onto a Au-coated 8 MHz EQCM electrode (with a Au-disk area of 0.205 cm2). The film was then used for the OER under the current density of 10 mA cm 2 for 20 h in a static 1 M NaOH solution. The mass change of the film was thus calculated from the measured resonance frequency change during the process of the OER based

944

S. Han et al. / Electrochimica Acta 210 (2016) 942–949

The electrodeposition of the spinel-type Co-doped Fe3O4 (CoxFe3-xO4) films follows an electrochemical-chemical (EC) mechanism as we proposed in our previous work [29,30]. In this study, the potential window for the electrodeposition was determined to be between 0.88 and 0.96 VAg/AgCl by LSVs (Fig. A.1 in the Appendix). Five CoxFe3-xO4 films were electrodeposited at 0.88, 0.90, 0.92, 0.94, and 0.96 VAg/AgCl, respectively. All the XRD patterns (Fig. A.2 in the Appendix) of these films match well with the spinel-type cobalt ferrite. The XPS measurements show that the Co mostly exists as Co2+, whereas Fe probably exists with mixed valence states of Fe3+ (as the major species) and Fe2+ (as the minor species) (see the discussion on Fig. A.3 and Table A.1 in the Appendix). This result agrees well with the EC mechanism we proposed for the deposition of CoxFe3-xO4, according to which the cathodic deposition of CoxFe3-xO4 is triggered in an initial solution containing Fe3+ and Co2+ by partially reducing the Fe3+ to Fe2+ (see discussion on Fig. A.1 in the Appendix). The Co:Fe ratios in these films were determined by both ICP-OES and EDS measurements. Fig. 1 shows the atomic concentration of Co (metal-only basis) as a function of the deposition potential. Although the ICP-OES-based and EDS-based data are not exactly the same, they agree well with each other on the trend of the Co content in the films as a function of the deposition potential. In addition, it has been reported that the increase of the Fe content in Co1-xFex(OOH) films causes the Co2 + /Co3+ redox peaks to shift anodically due to the strong electronic interaction between the Co and Fe that makes the Co2+ oxidation

more difficult [26]. Similar phenomenon was observed when we scanned the potential near the Co2+/Co3+ redox region on the CoxFe3-xO4 films deposited at different potentials (as shown in Fig. A.4 in the Appendix). Approximately, the Co2+/Co3+ redox peaks shift anodically for the films deposited at more negative potentials, suggesting a higher Fe (or lower Co) content in the films deposited at more negative potentials. The results above show the successful synthesis of spinel-type CoxFe3-xO4 films and indicate that the Co content in the electrodeposited CoxFe3-xO4 films increases as the deposition potential is driven to more positive, which is consistent with our previously reported result [30]. The growth rates of the CoxFe3-xO4 films at different potentials were investigated. Fig. 2 shows the thickness of the CoxFe3-xO4 film as a function of the deposition time at different deposition potentials. At each deposition potential, five CoxFe3-xO4 films were deposited for different time. These films were dissolved and the amounts of Co and Fe in the films were measured by ICP-OES. The thickness of each film was calculated by assuming the molecular formula of the film is M3O4 (M = Fe + Co) and the density of the film is nearly equal to that of the stoichiometric cobalt ferrite (CoFe2O4), i.e., 5.29 g cm 3. The result shows that the thickness of the film basically increases linearly with the deposition time. The film grows faster at a more negative potential (corresponding to a larger overpotential of deposition). The inset of Fig. 2 shows a magnified plot in the time range of 0 to 200 s. Based on the linear fitting lines in the inset of Fig. 2, the deposition time for a 550 nm thick CoxFe3xO4 film were determined to be about 150, 100, 75, 55, and 45 s at the deposition potentials of 0.88, 0.90, 0.92, 0.94, and 0.96 VAg/AgCl, respectively. These results shows that by using the one-step electrodeposition method, a CoxFe3-xO4 electrode can be fabricated and ready for use in minutes or even tens of seconds, which is much faster compared to the approaches involving the synthesis of the target materials and loading the materials onto the surface of a conducting substrate. The catalytic activities of the electrodeposited CoxFe3-xO4 films toward the OER were evaluated by LSV and Tafel measurements. To study the influence of the amount of cobalt doping on the OER catalytic activity of the CoxFe3-xO4 film, a series of the CoxFe3-xO4 films with the same thickness (550 nm) and different Co contents were electrodeposited on the polycrystalline Au-disk electrode at

Fig. 1. The atomic Co contents (at% Co) in the CoxFe3-xO4 films deposited at different potentials measured by EDS (black spheres) and ICP-OES (red spheres). The Co contents (metal-only basis) are calculated by assuming the sum of the atomic contents of Co and Fe in the CoxFe3-xO4 films is 100%. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. Plots of the film thickness as a function of deposition time for the CoxFe3-xO4 films deposited at 0.88 VAg/AgCl (black squares), 0.90 VAg/AgCl (red spheres), 0.92 VAg/AgCl (blue triangles), 0.94 VAg/AgCl (pink inverted triangles), and 0.96 VAg/AgCl (green diamonds). The dashed lines are linear fits to the data points at different deposition potentials, respectively. The inset is a magnified plot in the time range of 0 to 200 s, which shows the fitted growth rates of the CoxFe3-xO4 films in the first 200 s during the deposition at different potentials. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

on the Sauerbrey equation. The mass change is 0.14 ng for 0.1 Hz frequency change in our case. The electrochemical capacitance of the CoxFe3-xO4 films was estimated to compare the roughness of the films electrodeposited at different potentials by determining the scan-rate dependence of the current in cyclic voltammetry (CV) measurements. The CV measurements were carried out on each film by sweeping the potential across the nonfaradaic region ( 20 mV vs the open circuit potential of the system) at different scan rates (0.005, 0.01, 0.025, 0.05, 0.1, 0.2, 0.4, and 0.8 V s 1) in a static 1 M NaOH solution. 3. Results and discussion

S. Han et al. / Electrochimica Acta 210 (2016) 942–949

the potentials of 0.88, 0.90, 0.92, 0.94, and 0.96 VAg/AgCl, respectively. For comparison, a Fe3O4 film and a Co3O4 film (with a thickness of 550 nm) were also electrodeposited on the Au-disk electrode using the same methods reported in our previous work [17,29,32]. Linear sweep voltammograms (LSVs) at a scan rate of 1 mV s 1 on the electrodeposited CoxFe3-xO4, Fe3O4, and Co3O4 films, as well as on the bare Au-disk substrate in unstirred 1 M NaOH at room-temperature are shown in Fig. 3a. The onset potentials of the OER on the Au substrate and the Fe3O4 film are beyond 1.73 V vs RHE (corresponding to an overpotential of about 0.5 V with respect to the thermodynamic potential of the OER at pH 14, i.e., 0.401 vs NHE), suggesting that they are not very active OER catalysts. However, by doping Co into Fe3O4, the resulting CoxFe3xO4 films show greatly enhanced catalytic activities toward the OER compared to the Fe3O4 film. The onset potentials of the OER on the CoxFe3-xO4 films significantly shift to more negative potentials (corresponding to smaller overpotentials toward the OER). Besides,

Fig. 3. Catalytic activities of the CoxFe3-xO4 films with different amounts of Co doping toward the OER. (a) Linear sweep voltammograms (LSVs) measured on the bare Au substrate (purple), the electrodeposited Fe3O4 film (navy), electrodeposited Co3O4 film (orange), and the CoxFe3-xO4 films electrodeposited at 0.88 (black), 0.90 (red), 0.92 (blue), 0.94 (pink), and 0.96 VAg/AgCl (green). (b) Overpotential (h) on the electrodeposited CoxFe3-xO4 films as a function of Co content in the films at current densities of 10 (blue spheres) and 100 mA cm 2 (red diamonds). All the films were about 550 nm thick. The x values of the CoxFe3-xO4 films deposited at different potentials (labelled right next to the corresponding data points in the plot) are calculated based on the ICP-OES result presented in Fig. 1 above. The overpotentials were calculated with respect to the thermodynamic potential of the OER at pH 14, 0.401 V vs NHE. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

945

the OER catalytic activity of the CoxFe3-xO4 film varies with the amount of Co doping in the film. The plots of the overpotential (of the OER) on the CoxFe3-xO4 films at current densities of 10 and 100 mA cm 2 as a function of the amount of Co doping are shown in Fig. 3b. As the Co concentration in the CoxFe3-xO4 film increases, the overpotentials of the OER corresponding to the current densities of 10 and 100 mA cm 2 both decrease, which implies a better OER catalytic activity of the film. In order to exclude the possibility that this Co content-dependent OER activity of the CoxFe3-xO4 films electrodeposited at different potentials is simply due to the surface morphology/roughness variation of the films, SEM (Fig. A.5 in the Appendix) and electrochemical capacitance (Fig. A.6 in the Appendix) characterizations were carried out on each CoxFe3-xO4 film. The surface features of the CoxFe3-xO4 films electrodeposited at different potentials look similar, suggesting that the surface roughness of these films should be close [19]. The specific double layer capacitances of the CoxFe3-xO4 films determined from the plot of current density versus scan rate (Fig. A.6 in the Appendix) are very similar (in the range of 0.35-0.40 mF cm 2), also suggesting the roughness factors of these CoxFe3-xO4 films should be similar. Hence, the variation of the OER activities of these CoxFe3-xO4 films is more likely due to the composition difference (i.e., different Co:Fe ratios) in the films. This result is also reasonable considering that Co3O4 is a well-known active OER catalyst, whereas Fe3O4 is not [17]. The CoxFe3-xO4 films deposited at 0.88 to 0.92 VAg/AgCl (the black, red, and blue LSV curves in Fig. 3a) show better OER catalytic activities than the electrodeposited Co3O4 on the same Au-disk electrode (the orange LSV curve in Fig. 3a) under the same experimental conditions in this study. Dr. Boettcher’s group has reported similar results that the introduction of Fe (with proper contents) could significantly enhance the OER activity of Co and Ni oxides/(oxy)hydroxides due to the electronic interaction between Co/Ni and Fe, which could activate the Fe site for OER [25,26]. However, the OER activity of the CoxFe3-xO4 films deposited at 0.88 VAg/AgCl is slightly lower compared to the electrodeposited Co3O4 films on stainless steel and Pt coated glass substrates [17,19]. The variation of the apparent OER activities for the electrodeposited Co3O4 thin films on different substrates could be due to the difference of the interface interactions between the Co3O4 films and the substrates under alkaline OER conditions, or due to the morphology (roughness) difference of the Co3O4 films deposited on different substrates [34]. Based on our results, further increase of the Co doping into Fe3O4 might further enhance the OER catalytic activity. However, by using the synthesis method in this study, the CoxFe3-xO4 film with the highest Co content is obtained at 0.88 VAg/AgCl. A CoxFe3-xO4 film with an even higher Co content could only be electrodeposited at a potential more positive than 0.88 VAg/AgCl, at which nucleation becomes a problem [30]. One thing worth to be mentioned is that further increase of the Co doping would also increase the resistivity of the CoxFe3-xO4 film [30], which might impair its overall performance when used as an OER catalyst. The OER catalytic activities of the electrodeposited CoxFe3-xO4 films were also characterized by Tafel analyses in an unstirred 1 M NaOH electrolyte at room-temperature. Fig. 4 shows the steadystate polarization curves (see more details in Fig. A.7 in the Appendix) measured on the CoxFe3-xO4 films (with the same thickness and different amounts of Co doping) electrodeposited at different potentials. The Tafel slopes of these CoxFe3-xO4 films are close to 60 mV dec 1, suggesting that the OER on these CoxFe3-xO4 films follows a mechanism involving one electrochemical preequilibrium step preceding a rate-limiting chemical step [35]. The exchange current densities of these CoxFe3-xO4 films are extrapolated from the Tafel plots (Fig. A.8 in the Appendix) and listed in Table A.2 (in the Appendix). The CoxFe3-xO4 films deposited in the potential range of 0.88 to 0.92 VAg/AgCl have exchange current

946

S. Han et al. / Electrochimica Acta 210 (2016) 942–949

Fig. 4. Steady-state Tafel plots measured in 1 M NaOH for the CoxFe3-xO4 films deposited at 0.88 (black squares), 0.90 (red spheres), 0.92 (blue triangles), 0.94 (pink inverted triangles), and 0.96 VAg/AgCl (green diamonds). The dashed line is a fit to the linear portion of the curve. The overpotential was calculated with respect to the thermodynamic potential of the OER at pH 14, 0.401 V vs NHE. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

densities close to that of the crystalline Co3O4 film (2  10 10 A cm 2) [17]. The CoxFe3-xO4 film deposited at 0.88 VAg/AgCl has the highest exchange current density (2.39  10 10 A cm 2) of all the tested CoxFe3-xO4 films, which suggests that this film has a higher catalytic activity at zero overpotential. Besides, based on the Tafel plots, the overpotential on these CoxFe3-xO4 films at the same current density decreases as the Co doping in the film increases. This indicates that the film deposited at 0.88 VAg/AgCl has the best OER catalytic activity among these CoxFe3-xO4 films, which is in consistent with the dynamic polarization characterizations by LSV. The effect of the thickness of the electrodeposited CoxFe3-xO4 films on the OER catalytic activity was also investigated. Five CoxFe3-xO4 films were deposited at 0.88 VAg/AgCl for 150, 300, 450, 600, and 750 s, respectively. Based on the measured growth rate of the film deposited at 0.88 VAg/AgCl (in Fig. 2), the thicknesses of these five films are about 550, 1210, 1835, 2440, and 3060 nm. Fig. 5a shows the LSVs at a scan rate of 1 mV s 1 in 1 M NaOH on these CoxFe3-xO4 films. The results show that unlike the composition (i.e., Co:Fe ratio in the film) the film thickness does not significantly affect the catalytic activity of the CoxFe3-xO4 films toward the OER. At the current density of 10 mA cm 2, the CoxFe3xO4 film with a thickness of 550 nm exhibits the lowest overpotential (0.42 V), whereas thicker films all show a similar activity with the overpotentials in the range of 0.439 to 0.447 V (Fig. 5b). A slightly larger difference of the overpotential is observed on these films in the LSV tests when the current density reaches 100 mA cm 2. This might be due to the resistance of the films, which has a more pronounced effect on the overpotential at higher current densities. A CoxFe3-xO4 film with a thickness less than 550 nm was also tested for the catalytic activity toward the OER (pink dashed curve in Fig. 5a). However, a too thin film (with a thickness of about 480 nm) could not completely cover the surface of the substrate (see Fig. A.9 in the Appendix) and thus exhibits a lower activity (with an overpotential of 0.477 V at 10 mA cm 2) compared to the 550 nm-thick film (Figs. 5a & b). Since the thin CoxFe3-xO4 films (with 100% coverage of the substrate) would consume less materials, require less time for preparation, and possess a similar or even better catalytic activity, they would be a better choice compared to thick CoxFe3-xO4 films as the catalytic electrodes for the OER.

Fig. 5. Catalytic activities of the CoxFe3-xO4 films with different thicknesses toward the OER. (a) Linear sweep voltammograms (LSVs) measured on the CoxFe3-xO4 films electrodeposited at 0.88 VAg/AgCl with a thickness of about 480 (pink dashed curve), 550 (black curve), 1210 (red curve), 1835 (green curve), 2440 (blue curve), and 3060 nm (cyan curve), respectively. (b) The overpotential on the electrodeposited CoxFe3-xO4 films as a function of film thickness at the current densities of 10 (blue spheres) and 100 mA cm 2 (red diamonds), respectively. The 480 nm-thick CoxFe3-xO4 film does not completely cover the Au substrate and therefore is marked as incomplete film in the plot. The overpotentials were calculated with respect to the thermodynamic potential of the OER, 0.401 V vs NHE. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The stability of the CoxFe3-xO4 films with different Co:Fe ratios was tested at 10 mA cm 2 for 2 h in 1 M NaOH under mild stirring (Fig. 6a). The overpotentials on these CoxFe3-xO4 films do not show significant changes over time, indicating that these CoxFe3-xO4 films are stable under the tested conditions of electrolysis. A longterm stability test was carried out on a 550 nm-thick CoxFe3-xO4 film (deposited at 0.88 VAg/AgCl) under the same conditions for 24 h (Fig. 6b). The film still shows a great stability without a significant change of the overpotential. The Co:Fe ratios in the CoxFe3-xO4 film before (0.29) and after (0.30) the long-term OER are reasonably close based on the ICP-OES measurement, suggesting that the composition of the film could keep constant during the OER. The concentrations of Co and Fe ions in the electrolyte (i.e., 150 mL of 1 M NaOH) were also characterized (by ICP-OES) before and after the long-term stability test to detect if the CoxFe3-xO4 film was under dissolution during the electrolysis.

S. Han et al. / Electrochimica Acta 210 (2016) 942–949

Fig. 6. Stability tests of the electrodeposited CoxFe3-xO4 films under the conditions of the OER. (a) Plot of the overpotential as a function of time (in 2 h) on CoxFe3-xO4 films deposited at 0.88 VAg/AgCl (black line), 0.90 VAg/AgCl (red line), 0.92 VAg/AgCl (blue line), 0.94 VAg/AgCl (pink line), and 0.96 VAg/AgCl (green line) at a current density of 10 mA cm 2 measured in 1 M NaOH. (b) Plot of the overpotential as a function of time (in 24 h) on the CoxFe3-xO4 film deposited at 0.88 VAg/AgCl at a current density of 10 mA cm 2 measured in 1 M NaOH. The overpotential was calculated with respect to the thermodynamic potential of the OER at pH 14, 0.401 V vs NHE. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Based on our calculation, the dissolution of 5 wt% of the CoxFe3-xO4 film could result in increases of the ion concentration in the electrolyte by about 3.8 ppb for Co and 10.0 ppb for Fe. However, the ICP-OES data show that the concentration changes of the Co and Fe elements in the electrolyte before and after the long-term electrolysis are below the detect limitations of the instrument (i.e., 7.0 ppb for Co and 6.2 ppb for Fe) and therefore are probably negligible. The in situ mass change of the CoxFe3-xO4 film under the OER conditions of 10 mA cm 2 in 1 M NaOH for 20 h was also monitored by using EQCM measurement (as shown in Fig. A.10 in the Appendix). Less than 0.5% mass change ( 0.2 mg) of the CoxFe3-xO4 film was observed in the EQCM study, indicating the dissolution of the CoxFe3-xO4 film during the OER is negligible. We also studied the structural change of a CoxFe3-xO4 film (deposited at 0.88 VAg/AgCl) under the same OER conditions. There is no obvious difference between the XRD patterns of the film before and after the long-term OER (Fig. A.11 in the Appendix), suggesting that the spinel structure of the electrodeposited CoxFe3-xO4 film almost remains. These results show that the electrodeposited CoxFe3-xO4

947

films possess good stability in terms of both catalytic activity and structural integrity as OER catalysts. The catalytic activities of the CoxFe3-xO4 films with different preferred out-of-plane orientations were studied and compared. It has been reported that different crystalline planes have different arrangements of ions, which could result in different catalytic and electrochemical properties, as well as chirality and biocompatibility [36–39]. For example, different crystalline planes of Co3O4 have been shown to possess various catalytic activities toward CO oxidation, methane combustion, ethylene oxidation, and hydrodesulfurization [40–43]. We intend to study the catalytic activity of different planes of CoxFe3-xO4 toward the OER by electrodepositing CoxFe3-xO4 films with controlled growth orientations. Three CoxFe3-xO4 films were electrodeposited at 0.88 VAg/AgCl for 150 s on Cu(100), Cu(110) and Cu(111) single crystals, respectively. The XRD patterns of these CoxFe3-xO4 films are shown in Fig. 7. It should be mentioned that the most intense diffraction peak in the XRD pattern of the spinel cobalt ferrite without any preferred orientation of growth should be (311). However, the relative intensity of the (311) peak in the XRD patterns of the CoxFe3-xO4 films deposited on the Cu single crystals is significantly reduced. In each XRD pattern, except for the (311) peak the rest diffraction peaks of the film belong to a family of planes. That is, {100} for the CoxFe3-xO4 film on Cu(100), {110} for the CoxFe3-xO4 film on Cu (110), and {111} for the CoxFe3-xO4 film on Cu(111). This result suggests that the preferred out-of-plane orientations of these CoxFe3-xO4 films are [100] on Cu(100), [110] on Cu(110), and [111] on Cu(111). The catalytic activities of the three CoxFe3-xO4 films were studied by LSV in 1 M NaOH (Fig. 8). The overpotentials on these CoxFe3-xO4 films at the same current density are different, indicating that these films have different catalytic activities toward the OER. The order of the catalytic activity toward the OER from high to low is, CoxFe3-xO4(110) film > CoxFe3-xO4(111) film > CoxFe3xO4(100) film. It should be mentioned that the out-of-plane orientations of these films are probably not the same as the orientations of the real exposed crystalline planes on the surface of the CoxFe3-xO4 films. However, considering these three CoxFe3-xO4 films have the same composition and thickness, the difference of their catalytic activities toward the OER is probably due to the difference with respect to the exposed planes. Further studies on the surface topography of the films combined with the XRD analysis might be able to provide more information about the exact

Fig. 7. X-ray diffraction characterization of the CoxFe3-xO4 films electrodeposited at 0.88 VAg/AgCl for 150 s on Cu single crystals (100), (110), and (111).

948

S. Han et al. / Electrochimica Acta 210 (2016) 942–949

References

Fig. 8. Linear sweep voltammograms on the CoxFe3-xO4 films deposited at 0.88 VAg/AgCl on Cu(100) (black curve), Cu(110) (red curve), and Cu(111) (blue curve) scanned at 1 mV s 1 in 1 M NaOH at room-temperature. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

exposed planes on the surface of the CoxFe3-xO4 films and their activities toward the OER. 4. Conclusions Co-doped Fe3O4 (CoxFe3-xO4, 0 < x < 1) thin films were synthesized by using a facile one-step electrodeposition method. The CoxFe3-xO4 films exhibit greatly enhanced catalytic activities toward the OER compared to the Fe3O4 thin film. The catalytic activity of the CoxFe3-xO4 film increases with the increase of the amount of Co-doping in the film, but is not significantly affected by the film thickness. The growth orientation of the CoxFe3-xO4 film also has impact on the catalytic activity of the OER. The CoxFe3-xO4 film electrodeposited at 0.88 VAg/AgCl shows the best OER catalytic activity (comparable to that of Co3O4) among all the CoxFe3-xO4 films studied in this work with an overpotential of about 0.42 V at a current density of 10 mA cm 2, a Tafel slope of about 53 mV dec 1, and an exchange current density of about 2.39  10 10 A cm 2. All of the CoxFe3-xO4 films show great stability under the conditions of the OER. The satisfactory catalytic activity and stability combined with the simplicity of fabrication make the electrodeposited CoxFe3-xO4 films promising catalysts for the OER. Moreover, the electrodeposition strategy in this study should also be applicable for fabricating thin films of other mixed metal oxides (e.g., Ni-Fe oxides, Ni-Co oxides, and Zn-Co oxides) to be used as the catalytic electrodes for the OER as well as other reactions (e.g., oxygen reduction reaction). Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 21303270), Scientific Research Foundation for the Returned Overseas Chinese Scholars, Ministry of Education of P. R. China (Grant No. 2013[1792]), and Hunan Provincial Natural Science Foundation of China (Grant No. 2015JJ3144). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.05.194.

[1] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Solar Water Splitting Cells, Chem. Rev. 110 (2010) 6446. [2] D.K. Zhong, D.R. Gamelin, Photoelectrochemical Water Oxidation by Cobalt Catalyst (Co-Pi)/a-Fe2O3 Composite Photoanodes: Oxygen Evolution and Resolution of a Kinetic Bottleneck, J. Am. Chem. Soc. 132 (2010) 4202. [3] J.K. Hurst, In Pursuit of Water Oxidation Catalysts for Solar Fuel Production, Science 328 (2010) 315. [4] I. Katsounaros, S. Cherevko, A.R. Zeradjanin, K.J. Mayrhofer, Oxygen Electrochemistry as A Cornerstone for Sustainable Energy Conversion, Angew. Chem. Int. Ed. 53 (2014) 102. [5] S. Trasatti, Electrocatalysis in the Anodic Evolution of Oxygen and Chlorine, Electrochim. Acta 29 (1984) 1503. [6] Y. Lee, J. Suntivich, K.J. May, E.E. Perry, Y. Shao-Horn, Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions, J. Phys. Chem. Lett. 3 (2012) 399. [7] L.L. Duan, F. Bozoglian, S. Mandal, B. Stewart, T. Privalov, A. Llobet, L.C. Sun, A Molecular Ruthenium Catalyst with Water-Oxidation Activity Comparable to That of Photosystem II, Nat. Chem. 4 (2012) 418. [8] K.A. Stoerzinger, L. Qiao, M.D. Biegalski, Y. Shao-Horn, Orientation-Dependent Oxygen Evolution Activities of Rutile IrO2 and RuO2, J. Phys. Chem. Lett. 5 (2014) 1636. [9] M. Huynh, D.K. Bediako, D.G. Nocera, A Functionally Stable Manganese Oxide Oxygen Evolution Catalyst in Acid, J. Am. Chem. Soc. 136 (2014) 6002. [10] Y. Qiu, S.F. Leung, Q. Zhang, B. Hua, Q. Lin, Z. Wei, K.H. Tsui, Y. Zhang, S. Yang, Z. Fan, Efficient Photoelectrochemical Water Splitting with Ultrathin Films of Hematite on Three-Dimensional Nanophotonic Structures, Nano Lett. 14 (2014) 2123. [11] W.D. Chemelewski, H.C. Lee, J.F. Lin, A.J. Bard, C.B. Mullins, Amorphous FeOOH Oxygen Evolution Reaction Catalyst for Photoelectrochemical Water Splitting, J. Am. Chem. Soc. 136 (2014) 2843. [12] M.W. Kanan, D.G. Nocera, In Situ Formation of An Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co2+, Science 321 (2008) 1072. [13] M. Gong, Y. Li, H. Wang, Y. Liang, J.Z. Wu, J. Zhou, J. Wang, T. Regier, F. Wei, H. Dai, An Advanced Ni-Fe Layered Double Hydroxide Electrocatalyst for Water Oxidation, J. Am. Chem. Soc. 135 (2013) 8452. [14] D.M. Robinson, Y.B. Go, M. Mui, G. Gardner, Z. Zhang, D. Mastrogiovanni, E. Garfunkel, J. Li, M. Greenblatt, G.C. Dismukes, Photochemical Water Oxidation by Crystalline Polymorphs of Manganese Oxides: Structural Requirements for Catalysis, J. Am. Chem. Soc. 135 (2013) 3494. [15] K.J. McDonald, K.S. Choi, Photodeposition of Co-Based Oxygen Evolution Catalysts on Alpha-Fe2O3 Photoanodes, Chem. Mater. 23 (2011) 1686. [16] X.H. Deng, H. Tuysuz, Cobalt-Oxide-Based Materials as Water Oxidation Catalyst: Recent Progress and Challenges, ACS Cataly. 4 (2014) 3701. [17] J.A. Koza, Z. He, A.S. Miller, J.A. Switzer, Electrodeposition of Crystalline Co3O4A Catalyst for the Oxygen Evolution Reaction, Chem. Mater. 24 (2012) 3567. [18] S.Q. Chen, Y.F. Zhao, B. Sun, Z.M. Ao, X.Q. Xie, Y.Y. Wei, G.X. Wang, MicrowaveAssisted Synthesis of Mesoporous Co3O4 Nanoflakes for Applications in Lithium Ion Batteries and Oxygen Evolution Reactions, ACS Appl. Mater. Interfaces 7 (2015) 3306. [19] T.W. Kim, M.A. Woo, M. Regis, K.S. Choi, Electrochemical Synthesis of Spinel Type ZnCo2O4 Electrodes for Use as Oxygen Evolution Reaction Catalysts, J. Phys. Chem. Lett. 5 (2014) 2370. [20] X.J. Liu, Z. Chang, L. Luo, T.H. Xu, X.D. Lei, J.F. Liu, X.M. Sun, Hierarchical ZnxCo3xO4 Nanoarrays with High Activity for Electrocatalytic Oxygen Evolution, Chem. Mater. 26 (2014) 1889. [21] S.K. Bikkarolla, P. Papakonstantinou, CuCo2O4 Nanoparticles on Nitrogenated Graphene as Highly Efficient Oxygen Evolution Catalyst, J. Power Sources 281 (2015) 243. [22] C.Z. Zhu, D. Wen, S. Leubner, M. Oschatz, W. Liu, M. Holzschuh, F. Simon, S. Kaskel, A. Eychmuller, A. Nickel Cobalt Oxide Hollow Nanosponges as Advanced Electrocatalysts for the Oxygen Evolution Reaction, Chem. Commun. 51 (2015) 7851. [23] D.M. Jang, I.H. Kwak, E.L. Kwon, C.S. Jung, H.S. Im, K. Park, J. Park, TransitionMetal Doping of Oxide Nanocrystals for Enhanced Catalytic Oxygen Evolution, J. Phys. Chem. C 119 (2015) 1921. [24] P.X. Li, R.G. Ma, Y. Zhou, Y.F. Chen, Z.Z. Zhou, G.H. Liu, Q. Liu, G.H. Peng, Z.H. Liang, J. Wang, In Situ Growth of Spinel CoFe2O4 Nanoparticles on Rod-Like Ordered Mesoporous Carbon for Bifunctional Electrocatalysis of Both Oxygen Reduction and Oxygen Evolution, J. Mater. Chem. A 3 (2015) 15598. [25] M.S. Burke, L.J. Enman, A.S. Batchellor, S. Zou, S.W. Boettcher, Oxygen Evolution Reaction Electrocatalysis on Transition Metal Oxides and (Oxy)hydroxides: Activity Trends and Design Principles, Chem. Mater. 27 (2015) 7549. [26] M.S. Burke, M.G. Kast, L. Trotochaud, A.M. Smith, S.W. Boettcher, Cobalt-Iron (Oxy)hydroxide Oxygen Evolution Electrocatalysts: The Role of Structure and Composition on Activity Stability, and Mechanism, J. Am. Chem. Soc. 137 (2015) 3638. [27] L. Trotochaud, J.K. Ranney, K.N. Williams, S.W. Boettcher, Solution-Cast Metal Oxide Thin Film Electrocatalysts for Oxygen Evolution Reaction, J. Am. Chem. Soc. 134 (2012) 17253. [28] M.S. Burke, S. Zou, L.J. Enman, J.E. Kellon, C.A. Gabor, E. Pledger, S.W. Boettcher, Revised Oxygen Evolution Reaction Activity Trends for First-Row Transition Metal (Oxy)hydroxides in Alkaline Media, J. Phys. Chem. Lett. 6 (2015) 3737.

S. Han et al. / Electrochimica Acta 210 (2016) 942–949 [29] J.S. Switzer, R.V. Gudavarthy, E.A. Kulp, G.J. Mu, Z. He, A.J. Wessel, Resistance Switching in Electrodeposited Magnetite Superlattices, J. Am. Chem. Soc. 132 (2010) 1258. [30] Z. He, J.A. Koza, G.J. Mu, A.S. Miller, E.W. Bohannan, J.A. Switzer, Electrodeposition of CoxFe3-xO4 Epitaxial Films and Superlattices, Chem. Mater. 25 (2013) 223. [31] H.M. Kothari, E.A. Kulp, S.J. Limmer, P. Poizot, E.W. Bohannan, J.A. Switzer, Electrochemical Deposition and Characterization of Fe3O4 Films Produced by the Reduction of Fe(III)-Triethanolamine, J. Mater. Res. 21 (2006) 293. [32] Z. He, R.V. Gudavarthy, J.A. Koza, J.A. Switzer, Room-Temperature Electrochemical Reduction of Epitaxial Magnetite Films to Epitaxial Iron Films, J. Am. Chem. Soc. 133 (2011) 12358. [33] J.A. Switzer, G. Hodes, Electrodeposition and Chemical Bath Deposition of Functional Nanomaterials, MRS Bull. 35 (2010) 743. [34] S. Zou, M.S. Burke, M.G. Kast, J. Fan, N. Danilovic, S.W. Boettcher, Fe (Oxy) hydroxide Oxygen Evolution Reaction Electrocatalysis: Intrinsic Activity and the Roles of Electrical Conductivity Substrate, and Dissolution, Chem. Mater. 27 (2015) 8011. [35] J.B. Gerken, J.G. McAlpin, J.Y.C. Chen, M.L. Rigsby, W.H. Casey, R.D. Britt, S.S. Stahl, Electrochemical Water Oxidation with Cobalt-Based Electrocatalysts from pH 0-14: The Thermodynamic Basis for Catalyst Structure Stability, and Activity, J. Am. Chem. Soc. 133 (2011) 14431.

949

[36] L. Addadi, M. Geva, Molecular Recognition at the Interface Between Crystals and Biology: Generation Manifestation and Detection of Chirality at Crystal Surfaces, CrystEngComm 5 (2003) 140. [37] E.A. Garcia, M.A. Romero, C. Gonzalez, E.C. Goldberg, Neutralization of Li+ Ions Scattered by the Cu (100) and (111) Surfaces: A Localized Picture of the AtomSurface Interaction, Surf. Sci. 603 (2009) 597. [38] D. Su, S. Dou, G. Wang, Single Crystalline Co3O4 Nanocrystals Exposed with Different Crystal Planes for Li-O2 Batteries, Sci. Rep. 4 (2014) 5767. [39] H.Q. Sun, H.M. Ang, M.O. Tade, S.B. Wang, Co3O4 Nanocrystals with Predominantly Exposed Facets: Synthesis Environmental and Energy Applications, J. Mater. Chem. A 1 (2013) 14427. [40] X. Xie, Y. Li, Z.-Q. Liu, M. Haruta, W. Shen, Low-Temperature Oxidation of CO Catalysed by Co3O4 Nanorods, Nature 458 (2009) 746. [41] L. Hu, Q. Peng, Y. Li, Selective Synthesis of Co3O4 Nanocrystal with Different Shape and Crystal Plane Effect on Catalytic Property for Methane Combustion, J. Am. Chem. Soc. 130 (2008) 16136. [42] C.Y. Ma, Z. Mu, J.J. Li, Y.G. Jin, J. Cheng, G.Q. Lu, Z.P. Hao, S.Z. Qiao, Mesoporous Co3O4 and Au/Co3O4 Catalysts for Low-Temperature Oxidation of Trace Ethylene, J. Am. Chem. Soc. 132 (2010) 2608. [43] X.Z. Wang, L. Ding, Z.B. Zhao, W.Y. Xu, B. Meng, J.S. Qiu, Novel Hydrodesulfurization Nano-Catalysts Derived from Co3O4 Nanocrystals with Different Shapes, Cataly. Today 175 (2011) 509.