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Feb 18, 2014 - small secondary nanoneedles grown on primary rhombus- shaped pillar arrays. The nanostructures have large roughness factor, high porosi...
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Hierarchical ZnxCo3−xO4 Nanoarrays with High Activity for Electrocatalytic Oxygen Evolution Xijun Liu, Zheng Chang, Liang Luo, Tianhao Xu, Xiaodong Lei, Junfeng Liu,* and Xiaoming Sun* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: The design and fabrication of efficient and inexpensive electrodes for use in the oxygen evolution reaction (OER) is essential for energy-conversion technologies. In this study, high OER performance is achieved using novel hierarchical ZnxCo3−xO4 nanostructures constructed with small secondary nanoneedles grown on primary rhombusshaped pillar arrays. The nanostructures have large roughness factor, high porosity, and high active-site density. Only a small overpotential of ∼0.32 V is needed for a current density of 10 mA/cm2 with a Tafel slope of 51 mV/decade. The nanostructures are also found to perform significantly better than pure Co3O4 and a commercial Ir/C catalyst and to perform similarly to the best OER catalysts that have been reported for alkaline media. These merits combined with the satisfactory stability of the nanostructures indicate that they are promising electrodes for water oxidation. als.9,11 For example, a previous study has shown that NixCo3−xO4 nanowire arrays exhibit better electrochemical performance than their nanoparticle counterpart.9 Other researchers have reported that the electrochemical performance of Co-based catalysts could be markedly improved by constructing hierarchical architectures with larger surface area, better permeability, and higher porosity.23−26 However, further optimization of the surface structure of electrodes to improve their performance remains a challenge. In the present study, we developed hierarchical ZnxCo3−xO4 nanoarrays (i.e., primary rhombus-shaped pillar arrays with secondary nanoneedles grown on them). The nanoarrays, which showed high electrocatalytic performance for OER, were prepared by the co-deposition of Zn and Co precursors on Ti foil followed by thermal transformation to spinel ZnxCo3−xO4. The as-prepared arrays exhibited higher OER catalytic activity and better stability in alkaline media than an industrial Ir/C catalyst as well as comparable performance to previously reported high-performing Co-based OER catalysts. This substantially enhanced performance was attributed to the unique hierarchical nanostructures that provided a large roughness factor (Rf), high porosity, increased surface area, enhanced chemical accessibility, and high active-site density. The tight binding of the 3D porous architecture to the conductive substrates ensured their long-term stability and provided highways for both electrolyte ions and electrons.27−29

1. INTRODUCTION The oxygen evolution reaction (OER) is involved in many important electrochemical processes, such as water splitting for hydrogen production. The development of an effective electrode with low overpotential and long-term stability for OER is constantly being pursued.1,2 Iridium (Ir) oxides are considered to be the best OER catalysts, but their scarcity and high cost limit their wide application.3−5 Accordingly, extensive research efforts have been devoted to the development of alternative OER electrocatalysts based on transition-metal elements that are inexpensive, sufficiently active, and stable upon prolonged exposure under oxidizing conditions.6−8 Co3O4 and its substituted cobaltites have been demonstrated to be promising electroactive materials because of their good catalytic activity and corrosion stability for electrochemical OER in alkaline media.9−15 The structures of electrodes strongly affect their performance. Previous studies have shown that Co-based spinel oxides for OER are usually in the form of thin films or particle agglomerates bound together by polymers.12,14−17 Considering such structures, electrocatalytic activity is significantly limited by the small surface area and ineffective electron-mass transfer. Recently, well-aligned nanoarrays with active components directly grown on current collectors have been found to have intrinsic advantages, such as good electrical conductivity, low diffusion resistance to ionic species, and ease of electrolyte penetration in pseudocapacitor electrodes and Li-ion batteries.18−22 These nanoarrays also enable the fabrication of novel OER catalysts with enhanced reactivity and stability by providing pathways for both electrolyte ions and electrons, thereby realizing maximum utilization of electroactive materi© 2014 American Chemical Society

Received: December 13, 2013 Revised: February 13, 2014 Published: February 18, 2014 1889

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These results demonstrate the advantages of nanoarrays for constructing gas-evolution electrodes.

2. EXPERIMENTAL SECTION 2.1. Synthesis of ZnxCo3‑xO4 Nanoarrays on Ti Foil. All chemicals were analytical grade, purchased from Beijing Chemical Reagents Company, and used without further purification. In a typical synthesis, Zn(NO3)2·6H2O and Co(NO3)2·6H2O at an appropriate molar ratio, x (x = 0, 1:4, 1:3, and 1:2), were mixed in 50 mL of distilled water at room temperature. Then, 10 mmol of NH4F and 12.5 mmol of CO(NH2)2 were added into the mixture under vigorous stirring. The as-formed homogeneous solution was then transferred into a Teflon-lined stainless steel autoclave. Titanium substrates were carefully washed with a 1 M HCl solution and then rinsed with absolute ethanol and distilled water. After immersing a washed titanium substrate in the homogeneous solution, the autoclave was sealed, maintained at 120 °C for 10 h, and allowed to cool to room temperature naturally. The substrate was washed several times with distilled water, dried at 80 °C for 12 h, and calcined in air at 250 °C for 3 h. 2.2. Materials Characterization. Powder X-ray diffraction (XRD) was performed on a Shimadzu XRD-6000 diffractometer with Cu Kα radiation (λ = 1.54056 Å). The size and morphology of assynthesized samples were monitored using a scanning electron microscopy (SEM) system (Zeiss Supra 55). The structure and composition of the products were characterized using a high-resolution transmission electron microscopy (HRTEM) system (JEM 2100) equipped with an energy-dispersive X-ray spectrometry (EDS) instrument and an X-ray photoelectron spectroscopy (XPS) system (ESCALAB 250). The Brunauer−Emmett−Teller (BET) surface area of the catalysts was measured by N2 adsorption using the single-point method. Pore-size distribution was determined by the Barrett− Joyner−Halenda model on the desorption branch. Inductively coupled plasma−optical emission spectroscopy (ICP−OES) analysis was performed on a PerkinElmer Optima 3000 instrument. 2.3. Electrochemical Measurements. ZnxCo3−xO4 grown on Ti foil was directly used as the anode for electrochemical characterization. Electrochemical experiments were carried out in a three-electrode glass cell in a O2-saturated 1 M KOH solution using a carbon counter electrode and an Hg/HgO (1 M KOH) reference electrode with CHI 660D to collect data. Steady-state OER polarization curves were obtained at a scan rate of 0.5 mV/s. For cyclic voltammograms, working electrodes were cycled at least 100 times before data were collected between −0.1 and 0.65 V at a scan rate of 5 mV s−1. The electrochemical surface area (Rf) of the arrays was measured from double-layer charging curves using cyclic voltammetry within a small potential range (0.200 to 0.250 V) versus Hg/HgO. Double-layer capacitance values were determined from the slope of the capacitive current versus the scan rate and divided by the value of 60 mF cm−2 to obtain the Rf. Electrochemical impedance spectroscopy (EIS) measurements were performed by applying an ac voltage with 5 mV amplitude within the frequency range of 0.01 to 100 kHz. All experiments were conducted at room temperature (25 °C). The mass loadings of the active materials were approximately 1 mg/cm2, which was obtained by calculating the increase in mass of the Ti foil. The results are shown in Table S1.

Figure 1. SEM images of ZnxCo3−xO4 arrays: (A) pure Co3O4, (B) Zn/Co = 1:4, (C) Zn/Co = 1:3, and (D) Zn/Co = 1:2. The inset shows high-magnification SEM images taken from the as-obtained ZnxCo3−xO4 samples.

was doped into Co3O4, the morphologies markedly changed to hierarchical structures. Roughly rhombus-shaped ZnxCo3−xO4 pillar arrays with a fluffy surface were obtained as the ratio of starting Zn(NO3)2 to Co(NO3)2 increased to 1:4 (the product was denoted as ZnxCo3−xO4-1:4; Figure 1B). Each pillar showed an average length of 15 μm and an edge length of 1.3 μm, with small nanoneedles grown on the sides and ends of the pillars. The average length of the nanoneedles was estimated to be ∼2 μm, and the diameter of the bottom of the nanoneedle was around 100 nm, which gradually decreased to several nanometers at the tips. When the atomic ratio of Zn to Co in the starting solution was further increased to 1:3 (denoted ZnxCo3−xO4-1:3), the hierarchical structure became more regular (i.e., well-aligned nanoneedles epitaxilly grew on the primary rhombus-shaped pillars; Figure 1C). The highmagnification SEM image (inset of Figure 1C) showed that the pillar had a clear-cut rhombic contour with an average edge length of ∼2 μm and a sharp angle of ∼50°. However, when the ratio of Zn to Co was larger than 1:2, a remarkable transformation from hierarchical arrays to 1D pillars was observed, as shown in Figure 1D. Figure 2 shows the powder XRD patterns of the ZnxCo3−xO4 samples. The diffraction peaks of the Co3O4 nanoarrays were consistent with the standard pattern of cubic spinel Co3O4 (JCPDS 09-0418). The sharp XRD peaks of Co3O4 sample suggested high crystallinity and large grain size. For the Zndoped samples, the peaks slightly shifted to small-angle reflections, and no additional diffraction peak emerged regardless of the variation in the Zn/Co ratio. This observation suggested that isomorphous replacement occurred (i.e., the incorporation of Zn2+ into the Co3O4 lattice formed a solid solution of ZnxCo3−xO4 when x was less than 1.0).30,31 The crystallographic properties of ZnxCo3−xO4 samples are summarized in Table 1. The values of the lattice parameter a0 were found to increase with increased Zn doping, as indicated by the position of the (311) peaks. With increased Zn content in the oxides, the peaks broadened, and their intensity

3. RESULTS AND DISCUSSION Hierarchical ZnxCo3−xO4 nanoarrays were fabricated by codepositing mixed-metal (Zn and Co) nitrites followed by calcination in air. Different amounts of Zn(NO3)2 and Co(NO3)2 were added to the growth solution to tune the Zn-doping concentrations, and the results showed that the Zndoping degree significantly changed the morphology of the final products. Figure 1 shows that for pure a Co3O4 sample long nanorods with sharp tips were densely grown on the entire surface of the Ti foil. The average diameter and length were about 400 nm and 10 μm, respectively (Figure 1A). After Zn2+ 1890

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Table S2. The ratio of Co3+/Co2+ was found to increase because of the incorporation of Zn 2+ . The real Zn concentration in the prepared electrodes was also examined by ICP−OES analysis, and the findings agree well with the rawmaterial ratios of the samples (Table 1). The TEM image in Figure 4A shows an individual ZnxCo3−xO4-1:3 nanoneedle taken from the rhombus-shaped

Figure 2. XRD patterns of (a) Co3O4, (b) ZnxCo3−xO4-1:4, (c) ZnxCo3−xO4-1:3, and (d) ZnxCo3−xO4-1:2 (# symbol indicates peaks originating from the Ti foil substrate).

Table 1. Lattice Parameters, Crystallite Sizes, Zn/Co Ratios (Examined by ICP-OES Analysis), Specific Surface Area, and Roughness Factors of ZnxCo3−xO4 sample

a0 (nm)

crystallite size (nm)

Co3O4 ZnxCo3−xO4-1:4 ZnxCo3−xO4-1:3 ZnxCo3−xO4-1:2

0.80857 0.80872 0.80915 0.81123

26.6 14.2 11.8 12.4

Zn/Co ratio

specific surface area (m2/g)

Rf

0.23 0.31 0.48

36.6 66.7 78.5 52.3

1292 2063 2512 1956

decreasing when x < 1:3, indicating the decreased grain size. The crystallite size along the [311] directions of ZnxCo3−xO4 gradually decreased from 26.6 to 11.8 nm. In addition, the Co3O4 sample showed clearly enhanced (220) and (440) diffraction, indicating the oriented growth of [110]. Moreover, the Zn-doped samples showed XRD patterns with relative peak intensities similar to the standard one, consistent with the formation of hierarchical structures that have a relatively random orientation. XPS analysis was performed to investigate the chemical binding states of ZnxCo3−xO4-1:3. Figure 3 shows that the Co

Figure 4. (A) TEM image and corresponding FFT pattern (inset), (B, C) HRTEM images, and (D) nitrogen adsorption−desorption isotherms and pore-size distribution curves (inset) of the ZnxCo3−xO4-1:3 sample.

pillar. The nanoneedle was porous with a rough surface, as commonly seen in oxides obtained from hydrate precursors.28 The pores were believed to be generated in the calcination process because of CO2 release. The corresponding fast-Fourier transformation (FFT) patterns (inset of Figure 4A) showed rings with several obviously brighter dots, indicating their polycrystalline characteristics with a certain orientation inherited from the precursors. The HRTEM image (Figure 4B) provided more detailed structural information on the zinc− cobaltite spinel nanoneedles. The grain size of these crystalline ZnxCo3−xO4 nanoparticles was about 5 to 15 nm, in accordance with the size calculated from the XRD pattern (11.8 nm). The lattice fringes showed a lattice spacing of 0.23 nm (Figure 4C), corresponding to the {222} planes of cubic ZnxCo3−xO4. EDS mapping (Figure S2) confirmed the uniform distribution of Zn, Co, and O within the ZnxCo3−xO4 nanoneedles. The mesoporous feature could also be verified by BET measurements. From the N2 adsorption−desorption isotherm shown in Figure 4D, a distinct hysteresis loop was observed with typical type IV sorption behavior, indicating the existence of a typical mesoporous microstructure. The pore-size distribution was mainly centered at ∼3 and 6 nm, consistent with the HRTEM observation (inset of Figure 4D). Consequently, the mesoporous structure gave rise to a relatively high specific surface area of 78.5 m2 g−1 (Table 1), which is highly desirable for electrochemical reactions. A two-step process for the formation mechanism of 3D architectures of the ZnxCo3−xO4 hierarchical structures on Ti foil was proposed. First, Zn(OH)2 is easier to precipitate because of its lower solubility in aqueous solution, which led to hydroxide growth on Ti foil both to and from the “truck” of the

Figure 3. XPS spectra of ZnxCo3−xO4-1:3: (A) Co 2p and (B) Zn 2p.

2p spectrum had two main 2p3/2 and 2p1/2 spin−orbit lines at 780.2 and 794.7 eV, respectively. Two major peaks at 1045.1 and 1021.6 eV were ascribed to Zn 2p3/2 and 2p1/2 of Zn2+, respectively.32 Compared with pure Co3O4, a negative shift of Co 2p was observed for Zn-doped samples, which was probably due to the incorporation of Zn2+ into Co3O4 (Figure S1, Supporting Information). As the Zn-doping level increased, the shift became more evident, and the ZnxCo3−xO4-1:3 sample gave rise to a maximum shift of 0.7 eV. This shift was considered to be valuable for the enhancement of the OER activity of the catalyst, as demonstrated by Yu and Lin.8,33 Integration of the peaks of Co3+ and Co2+ was performed to determine the Co3+/Co2+ ratio,34 and the results are shown in 1891

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cathodic peak (III/IV at 1.35−1.55 V) were observed for Co3O4 electrode, whereas Zn-doped samples showed much stronger Co3+/Co4+ redox peaks (Figure 5A). The variation in peak currents for Co3+/Co4+ indicated the different electrocatalytic activities of ZnxCo3−xO4 electrodes for OER. Notably, the anodic peak was not clearly defined because it coincided with the rapid rise in OER current. Therefore, we used the reduction peak to quantify the redox characteristics of ZnxCo3−xO4 samples.42 The area of the reduction peak of Co4+/Co3+ for ZnxCo3−xO4-1:3 was approximately seven times higher than that for pure Co3O4, which indicated a significantly larger active surface area for electrocatalytic oxygen evolution. The peak potential positions of this redox couple also changed with x. The results showed that the anodic peak of Co3+/Co4+ for ZnxCo3−xO4-1:3 shifted to a more negative potential, which is commonly considered to have a positive effect on promoting the oxidation of water by lowering the overpotential.15 Linear-sweep measurements further confirmed the enhancement of OER electrocatalytic activity with Zn substitution by showing the different overpotentials required above the standard reaction potential to oxidize water at a desired current density (Figure 5B). The overpotential for a current density of 10 mA/cm2 decreased from ∼0.44 V of Co3O4 to ∼0.32 V (∼1.55 V vs RHE) of ZnxCo3−xO4-1:3. Such performance was comparable to that of the best Co-based oxides OER catalysts that have been reported (Table 2). The Tafel behavior,

hierarchical structure. The consequent precipitation of Co(OH)2 led to the formation of “branches” of the complex. The shape evolution of Zn-doped Co3O4 is shown in Figure S3. Meanwhile, ligand addition (F− or the NH3 from the hydrolysis−precipitation of urea) may also play a role in the formation of hierarchical structures by interfering in the precipitation process to some extent. Supersaturation is known to be effectively reduced by coordinating with F− or NH3 in solution, which drastically decreases the concentration of free ions and interferes in the precipitation/growth behavior.35,36 EDS analysis of the precursors (data not shown), which showed the Co-rich surface composition of the branches, partially supports our assumption. The diffusion and homogenization of Co and Zn cations occurred during the subsequent calcination step and finally resulted in phase-pure ZnxCo3−xO4. The ZnxCo3−xO4 arrays were directly used as electrodes for OER without a binder involved.30 Electrochemical results were acquired in a standard three-electrode cell consisting of a carbon counter electrode, an Hg/HgO (1 M KOH) reference electrode, and a 1 M aqueous KOH solution as the electrolyte. Figure 5A shows the cyclic voltammetry (CV) patterns of

Table 2. Comparsion of OER Activities for Various Transition-Metal Oxides type of material nanostructured Mn(III) oxidea Mn3O4/CoSe2a Co3O4/N-rmGOb Co-oxidec NiFe(OH)2d Ni−Fe oxidese ZnxCo3−xO4-3:1 RP arrays

Figure 5. (A) Cyclic voltammograms of ZnxCo3−xO4 electrodes recorded at a potential sweep rate of 5 mV/s. (B, C) State polarization curves recorded at a scan rate of 0.5 mV/s and their corresponding Tafel plots (a, Ir/C; b, Co3O4; c, ZnxCo3−xO4-1:4; d, ZnxCo3−xO4-1:3; and e, ZnxCo3−xO4-1:2). (D) Chronoamperometric responses (percentage of retained current vs operation time) of ZnxCo3−xO41:3 and Ir/C electrodes kept at 1.63 V vs RHE in 1 M KOH solution.

overpotential (V) at I = 10 mA/cm2

catalyst loading (mg/cm2)

ref

0.54

NA

6

0.45 0.31 0.57 (5 mA/cm2) 0.27 (0.5 A/cm2, 80 °C) 0.25 (0.4 A/cm2, 40 °C) 0.32

∼0.2 ∼1 NA NA ∼0.25 ∼1

8 16 17 45 46 this work

a

Glassy carbon, 0.1 M KOH, 1600 rpm. bNi foam, 1.0 M KOH. Indium tin oxide, 0.1 M potassium phosphate. dStainless steel mesh, 4.0 M KOH. eNi foam, 1.0 M KOH.

c

especially the Tafel slope, is an important kinetic parameter that can reveal changes in the apparent OER mechanism. Specifically, the rate-determining step (rds) for a specific electrode is normally believed to correspond to its Tafel slope for OER 43,44 because the adsorption strength of the intermediates greatly depends on the type, composition, and physical properties of the oxide electrodes,45,46 which essentially indicate the rds and controls the Tafel slope to a large extent.47 Approximately 60 mV/decade Tafel slopes were observed for the Co3O4 and ZnxCo3−xO4-1:2 arrays, demonstrating that the catalysts exhibited similar rds’s for the OER (Figure 5C). This value indicated that the formation and adsorption of the first intermediate (MOH, where M is the active site) was the rds.48 This finding can be ascribed to the intermediate MOH, which is first formed as an unstable species that rearranges through a surface reaction (spillover).49 However, the measured Tafel slope for ZnxCo3−xO4-1:3 slightly decreased to 51 mV/decade and approached that of the Ir/C (∼43 mV/decade),50 which indicated an OER mechanism

Co3O4 and ZnxCo3−xO4 samples obtained in 1 M KOH within the potential range of 0.85 to 1.6 V (vs RHE). The electrodes were initially stabilized within the same potential range for 100 cycles at a rate of 50 mV/s before recording the final voltammetric curves. The anodic (positive current density) and cathodic (negative current density) peaks caused by the oxidation and reduction processes, respectively, at various potentials in the CV patterns correspond to the formation of cobalt oxide phases with different oxidation states.37 The pair of redox peaks I/II can be assigned to the Co2+/Co3+ redox couple (∼1.1−1.2 V), whereas another pair of redox peaks, III/IV (∼1.35−1.55 V), can be attributed to those of Co3+/Co4+.38−40 Oxidation currents at higher potentials were due to the evolution of oxygen, 4OH− → O2 + H2O + 4e−. However, the CV patterns varied from sample-to-sample and strongly depended on the morphology and surface properties.38,41 Only a small shoulder anodic peak and a corresponding 1892

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properties of ZnxCo3−xO4, it exhibited the following five advantages. (1) The ZnxCo3−xO4-1:3 electrode had a high active surface area that can be attributed to the following two reasons. First, by doping Zn into Co3O4, the obtained ZnxCo3−xO4 arrays had a pillar−nanoneedle hierarchical structure with a large specific surface area (Table 1) that consequently enhanced the utilization of electroactive materials. This finding was further confirmed by the Rf of the materials (Figure S8). As shown in Table 1, ZnxCo3−xO4-1:3 had a larger Rf because of its higher surface area and pore volume. The large surface area provided greater accessible surface for OH− ions for more redox reactions compared with pure Co3O4. In addition, a series of control experiments with samples of Ni- or Cu-doped cobaltites were conducted. The SEM image and OER results are shown in Figures S9 and S10. Figure S9 shows that Ni- or Cu-doped cobaltites had morphologies similar to those of pure Co3O4; only the doping of Zn ions into cobaltite resulted in a 3D hierarchical structure with a larger surface area. The OER tests confirmed that the Zn-doped sample exhibited higher OER activity than the Ni- or Cu-doped samples (Figure S10). Second, Zn doping also contributed to the high active surface area by increasing active sites (Co3+) on the pillar array surface. Co3+ cations are known to be the active centers in cobalt-based anode materials for OER and to form Co4+ at potentials prior to oxygen evolution. Co4+ species are believed to enhance the electrophilicity of the adsorbed O and thus facilitate the formation of O−OH by nucleophilic attack, thereby promoting the deprotonation of OOH species through the electron-withdrawing inductive effect and producing O2.9,10,56 Regarding zinc cobaltite, the incorporation of foreign Zn2+ into the Co3O4 lattice resulted in changes in the chemical environment of Co2+ and Co3+. More Zn2+ ions occupying tetrahedral sites corresponded to increased distance of Co3+−O bonds, in accordance with the increase in d values as revealed by the shifting of the XRD peaks to smaller angles.32,57,58 As previously reported, the transformation of Co3+/Co4+ could be as follows: CoOOH + OH− → CoO2 + H2O + e−.40,59 The change in the distance of Co3+−O by Zn doping can enable the anions (OH−) to adsorb onto the Co cations and thus facilitate the formation of Co4+ species. Therefore, a high peak current density for the Co3+/Co4+ redox reaction of ZnxCo3−xO4-1:3 indicated significantly larger active surface areas for OER (Figure 5A). Consequently, Zn doping improved the catalytic activity of Co3O4 in OER, with ZnxCo3−xO4-1:3 exhibiting the highest catalytic activity. Furthemore, the number of oxygenevolution sites per unit of true surface area was calculated by measuring and integrating the anodic peak area of Co4+/Co3+ (Qpa), NAQpa/RfF for different ZnxCo3−xO4 electrodes (NA is Avogadro’s constant and F is Faraday’s constant) (Figure 6B). As expected, the ZnxCo3−xO4-1:3 pillar array electrode had the highest number of active sites (Co4+), which was almost 7.3 times larger than those of pure Co3O4, in good agreement with previous reports.14,60,61 This result was also in accordance with the variation in current density at 1.65 V vs RHE (Figure 6B). (2) The 3D nanostructures provided hierarchical porous channels that are valuable to the fast penetration of electrolyte ions (OH−) because of structural interconnectivity.21 The high electrochemical performance of ZnxCo3−xO4 electrodes was confirmed by EIS measurements (Figure S11). The EIS data were fitted by an equivalent circuit consisting of an internal resistance (Rs), a charge-transfer resistance (Rct), a pseudocapacitive element (Cp), and a constant-phase element (CPE), as shown in the inset of Figure S11. The Rct of ZnxCo3−xO4-1:3

involving a pre-equilibrium consisting of a one-electron electrochemical step and a possible chemical step followed by a one-electron electrochemical rds: MOH + OH− → MO + H2O + e− (oxidation of CoOOH to CoO2).39,51 These results suggested that ZnxCo3−xO4-1:3 had an obvious change in OER mechanism that can be attributed to the unique architecture and isomorphous replacement of Co2+ with Zn2+. We further compared the ZnxCo3−xO4-1:3 electrocatalyst with a commercial Ir/C (20 wt % Ir on a Vulcan XC-72R with the same ∼1.0 mg/cm2 loading coated on Ti foil). As shown in Figure 5B, the Ir/C catalyst afforded a slower increase in OER current density. The currents of the ZnxCo3−xO4-1:3 nanoarray and Ir/C at 1.57 V were 16.85 and 1.78 mA/cm2, respectively, which was 9.5 times the O2 productivity of ZnxCo3−xO4-1:3 and higher than that of the Ir/C catalyst at such voltage. These ZnxCo3−xO4-1:3 pillar arrays also demonstrated OER activity that was higher than that of Mn oxides6,52 and close to that of Ni−Fe catalysts.53−55 Thus, these arrays are some of the most promising nonprecious catalyst candidates for OER (Table 2). The real loading and specific activity of the electrocatalysts are essential to their practical application. To understand the relationship between electrode loading and its activity further, we performed a series of time-dependent experiments, and the results are shown in Figure S4. At the initial stage, the electroactivity of the electrode increased with increased electrode loading. However, when the loading amount was >1 mg/cm2 with a prolonged reaction time, the activity for OER decreased because of the increased charge-transfer resistance of a thicker film. The mass activities and specific activities based on the electrode loading, surface area, and Rf of ZnxCo3−xO4 electrodes compared with previously reported values are summarized in Table S2, which confirmed that ZnxCo3−xO4-1:3 exhibited superior OER activity. Furthermore, stability tests showed that the ZnxCo3−xO4-1:3 pillar array electrode was inherently stable during OER (Figure 5D) in alkaline media. The OER current slightly decreased after 1000 cycles (Figure S5). By contrast, the Ir/C catalysts showed drastic fluctuations during the test, possibly because of the slow desporption rate of O2 from the surface active site. Further characterization of the ZnxCo3−xO4-1:3 electrode by XRD revealed identical results before and after 1000 cycling tests. Moreover, slight morphology changes were observed in the SEM image of the sample after 1000 cycling tests, confirming its high structural stability. EDS analysis also demonstrated that Zn remained in Co3O4 and that the Zn/Co ratio was close to 1:3 (Figure S7). All of the above results confirmed that the designed hierarchical ZnxCo3−xO4 pillar arrays enabled the performance of the active materials to be maximized for OER (Figure 6A). Given the unique structural features and modified intrinsic

Figure 6. (A) Illustration of electron transfer for OER on a ZnxCo3−xO4 pillar array surface. (B) Variation in the current density and active sites with respect to different ratios of Zn to Co. 1893

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(1.3 Ω) was smaller than that of ZnxCo3−xO4-1:4 (2.7 Ω), ZnxCo3−xO4-1:2 (3.1 Ω), and Co3O4 (4.5 Ω). Thus, the ZnxCo3−xO4-1:3 electrode possessed a high charge-transfer rate between the electrolyte and the active material. These findings agree well with the CV and linear-sweep results. In addition, internal resistances of about 0.1 and 0.3 Ω can be estimated from the intercept on the real axis within the high-frequency range for the Zn-doped samples and pure Co3O4, respectively. Thus, the Zn-doped samples had much better utilization of electrons during the electrochemical process. (3) Electron transport was significantly enhanced by the direct growth of active materials on conductive substrate (Ti foils) and the incorporation of foreign Zn2+ into the Co3O4 lattlice, which produced more polarons in the materials.62 (4) The separation and immobilization of pillar arrays onto the substrate was good, which avoided the use of polymer binder and conductive additives and substantially reduced the dead volume in the electrode materials. This finding cannot be observed in most Co-based electrode systems in which nanosized active sites are largely hindered by numerous unwanted interfaces.9,12 (5) Such architectures directly grown on substrate ensured good mechanical adhesion and promoted structural stability. To reiterate, the extremely high performance, including the high reactivity, high current density, small overpotential, and longterm stability, was attributed to the optimized chemical environments for Co ions and construction of hierarchical pillar arrays. These features resulted in the nanoarray electrodes being powerful electrocatalysts for water oxidation.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC, the Program for New Century Excellent Talents in Universities, Beijing Nova Program (Z121103002512023), the 973 Program (2011CBA00503), the 863 Program (2012AA03A609), and the Fundamental Research Funds for the Central Universities.

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4. CONCLUSIONS An easy, cost-effective, and potentially scalable method for fabricating novel hierarchical ZnxCo3−xO4 arrays on a conductive substrate for high-performance OER catalysts is reported. The ZnxCo3−xO4-1:3 pillar arrays exhibited outstanding OER electrochemical activity with a small overpotential of ∼0.32 V at a current density of 10 mA/cm2, which was more active than Ir/C in alkaline media and comparable to the best OER catalysts reported. Moreover, a small Tafel slope of 51 mV/decade was observed for the ZnxCo3−xO4-1:3 electrode. We believe that the unique architecture and its corresponding modified physical properties (large Rf, high porosity, and high active-site density) are responsible for its superior electrochemical performance. Furthermore, accelerated tests of the ZnxCo3−xO4-1:3 pillar arrays catalyst revealed satisfactory stability. These results showed that effective OER electrocatalysts can be fabricated with properly designed electrodes having advanced structures, thereby providing new opportunities for preparing nonprecious metal oxides with high activity for water electrolysis.

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ASSOCIATED CONTENT

S Supporting Information *

EDS mapping results and electrochemical tests of ZnxCo3−xO4, comparisons of the mass activity and specific activity of ZnxCo3−xO4, and SEM image and XRD pattern of ZnxCo3−xO41:3 after 1000 cycling tests. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.L.). *E-mail: [email protected] (X.S.). 1894

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