Electrochemical Synthesis of Spinel Type ZnCo2O4 Electrodes for

Letter pubs.acs.org/JPCL

Electrochemical Synthesis of Spinel Type ZnCo2O4 Electrodes for Use as Oxygen Evolution Reaction Catalysts Tae Woo Kim,† Myong A Woo,† Morrisa Regis,‡ and Kyoung-Shin Choi*,† †

Department of Chemistry, University of WisconsinMadison, Madison, Wisconsin 53706, United States Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States

‡

S Supporting Information *

ABSTRACT: A new electrochemical synthesis route was developed to prepare spinel-type ZnCo2O4 and Co3O4 as high quality thin film-type electrodes for use as electrocatalysts for oxygen evolution reaction (OER). Whereas Co3O4 contains Co2+ in the tetrahedral sites and Co3+ in the octahedral sites in the spinel structure, ZnCo2O4 contains only Co3+ in the octahedral sites; Co2+ in the tetrahedral sites is replaced by Zn2+. Therefore, by comparing the catalytic properties of ZnCo2O4 and Co3O4 electrodes prepared with comparable surface morphologies and thicknesses, it was possible to examine whether Co2+ in Co3O4 is catalytically active for OER. The electrocatalytic properties of ZnCo2O4 and Co3O4 for OER in both 1 M KOH (pH 13.8) and 0.1 M phosphate buffer (pH 7) solutions were investigated and compared. The results suggest that the Co2+ in Co3O4 is not catalytically critical for OER and ZnCo2O4 can be a more economical and environmentally benign replacement for Co3O4 as an OER catalyst. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

T

ZnCo2O4 has a regular spinel structure where Zn2+ only replaces Co2+ in the tetrahedral (Td) sites in Co3O4, leaving the Co3+ content in the octahedral (Oh) sites unchanged. In NiCo2O4 and MnCo2O4, Ni and Mn are known to mainly occupy octahedral sites.16−18 Also, because Ni and Mn can be present with mixed oxidation states (i.e., 2+/3+ for Ni and 3+/ 4+ for Mn), incorporation of Ni and Mn into Co3O4 can result in a more complicated change in Co2+/Co3+ compositions.2,11,12 Therefore, the invariable oxidation state and coordination preference of Zn2+ in the ZnCo2O4 spinel structure can allow us to investigate whether the Co ions in the tetrahedral sites have a major catalytic role for the OER reaction by comparing the OER performances of Co3O4 and ZnCo2O4. If ZnCo2O4 shows comparable catalytic performances to Co3O4, ZnCo2O4 can be identified as a more economical and environmentally benign replacement for Co3O4 as an OER catalyst. In order to make meaningful comparisons between the OER performances of Co3O4 and ZnCo2O4 electrodes, it is critical that the two electrodes are prepared from the same or similar synthesis procedures that produce comparable surface morphologies and thicknesses so that any difference in OER performance can be solely assigned to the difference in their compositions (i.e., Co2+ versus Zn2+). The electrochemical synthesis conditions reported in this study produced ZnCo2O4 and Co3O4 electrodes that satisfy the aforementioned require-

he spinel-type cobalt oxide (Co3O4) exhibits various interesting electrochemical and catalytic properties and has been investigated for use as an anode for Li-ion batteries and as an electrocatalyst for water oxidation and oxygen reduction reactions.1−5 Recent efforts in the development of Co-based anodes for Li-ion batteries included partially replacing Co with more ecofriendly and cheaper alternative metals to form MCo2O4 (M = Zn, Cu, Ni, Mg, and Fe) without sacrificing its electrochemical performance.6−9 This replacement was motivated by the fact that cobalt is relatively toxic and expensive. It was also reported that Co3O4 substituted with Ni, Cu, and Mn show high activity and stability for the oxygen reduction reaction.10−13 These results encourage the development of facile synthesis methods to prepare MCo2O4-type electrodes to further examine their electrochemical and catalytic properties. In this study, we report electrochemical synthesis methods to produce uniform and adherent ZnCo2O4 and Co3O4 electrodes and their electrocatalytic properties for oxygen evolution reaction (OER). The present study has two main goals. One is to provide a simple synthesis procedure that can be used to prepare a variety of spinel type oxides as thin film type electrodes for use in electrochemical applications. The other is to compare the catalytic properties of ZnCo2O4 and Co3O4 electrodes for OER in order to determine for the first time whether Co2+ in the tetrahedral site in Co3O4 is catalytically critical for OER. Although spinel-type NiCo2O4 and MnCo2O4 were examined as OER catalysts previously, the study of ZnCo2O4 as an OER catalyst is scarce.11,13−15 The distinctive feature of ZnCo2O4 compared to NiCo2O4 and MnCo2O4 is that © XXXX American Chemical Society

Received: May 28, 2014 Accepted: June 19, 2014

2370

dx.doi.org/10.1021/jz501077u | J. Phys. Chem. Lett. 2014, 5, 2370−2374

The Journal of Physical Chemistry Letters

Letter

X-ray diffraction (XRD) patterns of the as-deposited and annealed Zn−Co−O electrodes before and after ZnO removal are shown in Figure 1a. The as-deposited film showed only

ment and their OER performances were examined in both strong alkaline and neutral solutions. The synthesis procedure for ZnCo2 O4 electrodes is illustrated in Scheme 1. In the first step, an electrochemical Scheme 1. Schematic Illustration for the Synthesis of ZnCo2O4 Using Electrochemical Codeposition of Co(OH)2 and ZnOa

a Photographs of actual electrodes corresponding to each figure are also shown.

codeposition of ZnO and Co(OH)2 was achieved by nitrate reduction at 60 °C in an undivided three electrode cell composed of a Pt working electrode, a Pt counter electrode, and a Ag/AgCl in 4 M KCl reference electrode.19 The Pt electrodes were prepared by e-beam, evaporating 20 nm of titanium followed by 100 nm of platinum on clean glass slides. The reduction of nitrate generates OH− at the working electrode and increases the local pH (eq 1), inducing precipitation of Zn2+ and Co2+ in the form of ZnO and Co(OH)2, respectively (eqs 2−3).20 A total of 50 mL of an aqueous solution containing Zn(NO3)2·6H2O (40 mM) and Co(NO3)2·6H2O (80 mM) was used as a plating solution. The deposition was carried out by applying −1.0 V against the Ag/ AgCl reference electrode and passing 0.27 C/cm2, which takes about 20−25 s, followed by a resting time of 10 s. This cycle was repeated for 5−25 times to optimize the thickness and the catalytic performance. The optimum film was prepared when the deposition/resting cycle was applied 10 times NO3− + H 2O + 2e− → NO2− + 2OH−

Figure 1. (a) X-ray diffraction patterns of (i) as-deposited and annealed Zn−Co−O film (ii) before and (iii) after ZnO removal. Peaks from the substrate and ZnO are indicated by * and •, respectively. The arrows and Miller indices represent peaks generated by ZnCo2O4. (b) Co 2p XPS of ZnCo2O4 (black) and Co3O4 (gray). The black and gray arrows show expected peak center positions for Co3+(Oh) and Co2+(Td) in the Co3O4 spinel structure, respectively.

crystalline ZnO peaks, suggesting that Co(OH)2 was present as an amorphous phase. Although not detected by XRD, the presence of Co(OH)2 was evident by the characteristic bright sky-blue hue of the as-deposited film. When heated at 550 °C, the XRD peaks of ZnO became sharper, as the annealing procedure increased, the crystallinity of the unreacted ZnO crystals. Also, a new set of peaks corresponding to ZnCo2O4 appeared (JCPDF No. 23-1390). The broad and weak peaks of ZnCo2O4 indicated the nanocrystalline nature of the ZnCo2O4 film. After soaking in NaOH solution, all the ZnO peaks disappeared, leaving a pure ZnCo2O4 phase (see Figure S1 in Supporting Information for a higher quality XRD pattern for the ZnCo2O4 film). Energy dispersive spectroscopy (EDS) confirmed the ratios of Zn/Co = 0.5, and the electron mapping analysis showed that Zn and Co ions were homogeneously distributed throughout the electrode (Supporting Information Figure S2). The crystallinity and purity of the Co3O4 electrodes were also confirmed by XRD (Supporting Information Figure S3). The absence of Co2+ in ZnCo2O4 was confirmed by comparing the Co 2p region of X-ray photoelectron spectroscopy (XPS) spectra with that of Co3O4 (Figure 1b). The Co 2p1/2 and Co 2p3/2 peaks of Co3O4 are broad because both Co2+ in the Td sites and Co3+ in the Oh sites contribute to these peaks.21,22 The Co 2p1/2 and Co 2p3/2 peaks from Co2+ in the Td site are centered around 796.8 and 780.9 eV, respectively, whereas the Co 2p1/2 and Co 2p3/2 peaks from Co3+ in the Oh

E0 = 0.01V (1)

Zn

2+

+ H 2O → ZnO + 2H

+

log[Zn 2 +] = 10.96 − 2(pH)

(2)

Co2 + + 2H 2O → Co(OH)2 + 2H+ log[Co2 +] = 12.60 − 2(pH)

(3)

The as-deposited film was heated at 550 °C for 1 h in air to form ZnCo2O4. The plating solution was formulated such that the Zn to Co ratio in the as-deposited film is larger than 0.5 so that excess ZnO was present to react with Co(OH)2 to form ZnCo2O4. After ZnCo2O4 was formed, the remaining ZnO was easily removed by soaking the electrode in 1 M NaOH solution at 60 °C for 30−40 min with gentle stirring. Having excess ZnO in the as-deposited film allowed for a more stoichiometric and reproducible formation of ZnCo2O4. For comparative study, Co3O4 electrodes were also prepared using the same procedure. The only difference was that the plating solution did not contain Zn2+ ions and, therefore, the as-deposited film contained pure Co(OH)2, which converted to Co3O4 during the annealing procedure. 2371

dx.doi.org/10.1021/jz501077u | J. Phys. Chem. Lett. 2014, 5, 2370−2374

The Journal of Physical Chemistry Letters

Letter

site are centered around 794.8 and 779.8 eV, respectively.22 The Co 2p1/2 and Co 2p3/2 peaks for ZnCo2O4 are much narrower, representing only Co3+ contributions, which confirms the absence of Co2+ ions in the ZnCo2O4 electrode. The Scanning Electron Microscopy (SEM) images of the asdeposited and annealed ZnCo2O4 films before and after ZnO removal are shown in Figure 2a−d. The as-deposited film was

Figure 2. SEM images of (a) as-deposited film, (b) ZnCo2O4 film before ZnO removal, and (c) ZnCo2O4 film after ZnO removal; (d) side view SEM image of ZnCo2O4 film; (e) top-view and (f) side-view SEM images of Co3O4.

Figure 3. (a) LSVs of ZnCo2O4 (black) and Co3O4 (gray) (scan rate, 0.5 mV/s). LSV of Pt used as the substrate is also shown for comparison (dashed). Tafel plots of (b) ZnCo2O4 and (c) Co3O4 and LSVs of (d) ZnCo2O4 and (e) Co3O4 comparing the initial and the 100th scans (scan rate, 1.0 mV/s). All results were obtained in 1 M KOH (pH 13.8) solution.

composed of wavy Co(OH)2 layers (Figure 2a). ZnO crystals were likely hidden between or under Co(OH)2 layers. After annealing, the layered morphology of Co(OH)2 disappeared because Co(OH)2 reacted with ZnO to form ZnCo2O4 (Figure 2b). Some unreacted ZnO crystals embedded throughout the film were also shown. After soaking in 1 M NaOH, the film no longer contained ZnO crystals (Figure 2c). The side-view SEM image clearly shows the uniformity and good adherence of the film throughout the electrode (Figure 2d). The thickness was estimated to be ca. 600 nm. The SEM images of Co3O4 films are also shown for comparison (Figure 2e−f). The Co3O4 electrode retains more of the plate-like features of Co(OH)2 and looks more crystalline. However, the overall surface features and thicknesses of the ZnCo2O4 and Co3O4 electrodes look comparable. The catalytic properties of Co3O4 and ZnCo2O4 for OER were first examined by measuring a linear sweep voltammogram (LSV) in 1 M KOH (pH 13.8) solution. The Co3O4 electrode achieved current densities of 10 mA/cm2 at 1.64 V vs RHE (η = 0.41 V) and 20 mA/cm2 at 1.71 V vs RHE (η = 0.48 V) (Figure 3a). The LSV of Co3O4 shown here is comparable to the best performances of Co3O4 reported to date,23 which include the performance of Co3O4 nanoparticles dispersed on a graphene surface and nanoporous Co3O4 films. Considering that much higher surface areas were available for these nanostructured

Co3O4 electrodes, the comparable performance of our Co3O4 electrode with a relatively compact thin film morphology confirms the capability of the simple electrodeposition method presented in this study to produce high quality Co3O4 electrodes. Compared to the Co3O4 electrode, the ZnCo2O4 electrode showed slightly better performance, achieving current densities of 10 mA/cm2 at 1.62 V vs RHE (η = 0.39 V) and 20 mA/cm2 at 1.68 V vs RHE (η = 0.45 V). Another interesting feature revealed in the comparative study of ZnCo2O4 and Co3O4 is that whereas Co3O4 shows a preoxidation peak before water oxidation onset, which is known to be due to oxidation of Co3+ to Co4+,4,24 the ZnCo2O4 electrode does not show any preoxidation peak (Figure 3a, inset). This result suggests that this peak is most likely associated with the oxidation of Co ions in Td sites. The Tafel slope of the Co3O4 electrode was estimated to be 54 mV/dec, which is in the low end of the range of Tafel slopes reported for Co3O4 (Figure 3b−c).3,23,24 The Tafel slope of ZnCo2O4 was slightly lower, reaching 46 mV/dec. The stability of ZnCo2O4 was tested by repeatedly scanning the potential from 1.0 to 1.9 V vs RHE for 100 cycles. Comparing the first and the 100th scans confirmed that ZnCo2O4 is stable in a 1 M KOH solution (Figure 3d). In comparison, Co3O4 shows a 2372

dx.doi.org/10.1021/jz501077u | J. Phys. Chem. Lett. 2014, 5, 2370−2374

The Journal of Physical Chemistry Letters

Letter

As expected, the OER performances of the Co3O4 and ZnCo2O4 electrodes in a pH 7 solution were not as good as those demonstrated in the 1 M KOH solution. However, their OER performances compare favorably with other best known OER catalysts tested in neutral media. For example, the Co-Pi catalyst was reported to generate 1 mA/cm2 at ∼1.89 vs RHE in 0.1 M phosphate solution (pH 7).25,26 As in the case of the 1 M KOH solution, the preoxidation peak of Co ions before O2 evolution was present only in the LSV of Co3O4, suggesting that this peak is associated with the oxidation of Co ions in the Td sites of the spinel structure (Figure 4a, inset). Some OER catalysts that perform well in an alkaline medium are not stable in neutral media. However, the ZnCo2O4 as well as the Co3O4 electrodes showed good stability in a pH 7 medium, which was confirmed by LSV, scanning from 1.2 to 2.0 V vs RHE for 100 cycles (Figure 4d−e). The average OER performance data of the Co3O4 and ZnCo2O4 electrodes with standard deviations are summarized in Tables S1−S2 in Supporting Information. An additional advantageous feature of ZnCo2O4 compared to Co3O4 as an OER catalyst was recognized when their absorption spectra were compared (Supporting Information, Figure S5). ZnCo 2 O 4 shows considerably lower light absorption in the visible and the near IR region than Co3O4. This suggests that if they are used as OER catalysts on the surface of semiconductor light absorbers for solar water splitting, ZnCo2O4 will interfere less with light absorption by the semiconductors. In summary, we produced highly catalytic ZnCo2O4 as uniform and adherent thin film electrodes using electrochemical codeposition of ZnO and Co(OH)2 followed by a mild annealing process. The method described here can be modified to produce various spinel type compounds as high quality electrodes. The resulting ZnCo2O4 electrodes show excellent and stable catalytic performance for OER in 1 M KOH solution, generating 10 mA/cm2 at 1.62 V vs RHE (η = 0.39 V) and 20 mA/cm2 at 1.68 V vs RHE (η = 0.45 V), which is slightly better than the performance of Co3O4 electrodes prepared from the same synthesis conditions with comparable film thicknesses and morphologies. ZnCo2O4 also showed comparable performance and stability to Co3O4 in a neutral medium. These results suggest that the Co ions in the Td sites in the Co3O4 spinel structure are not catalytically critical for OER reaction and ZnCo2O4 can be a more economical and environmentally benign replacement for Co3O4 as a OER catalyst. We expect that the OER catalytic properties of ZnCo2O4 may be further improved by optimization of composition (e.g., both the Zn2+ site and Co3+ site) and morphology.

slight current decrease during cycling, which is similar to the behavior of the Co3O4/graphene composite electrode recently reported (Figure 3e).23 However, for electrolysis performed by applying an overpotential to achieve 20 mA/cm2 (1.68 V vs RHE for ZnCo2O4 and 1.71 V vs RHE for Co3O4) for 5 h, both ZnCo2O4 and Co3O4 showed a stable current density (Supporting Information, Figure S4), indicating that the slight change in the LSV profile for the 100th cycle was not due to the electrochemical instability of the electrodes. The results shown in Figure 3 suggest that Co2+ in the Co3O4 spinel structure is not catalytically critical for OER and the replacement of Co2+ by Zn2+ in Co3O4 does not cause any negative effect on the catalytic performance or stability in a 1 M KOH solution. The OERs of Co3O4 and ZnCo2O4 were also examined in a 0.1 M phosphate buffer solution (pH 7). The two electrodes again show very comparable LSVs, but the ZnCo2O4 electrode shows a slightly better performance, generating 1 and 5 mA/ cm2 at 1.71 V vs RHE (η = 0.48 V) and 1.89 V vs RHE (η = 0.66 V), respectively (Figure 4a). The Tafel slopes for Co3O4 and ZnCo2O4 were estimated to be 76 and 85 mV/dec, respectively (Figure 4b−c).

■

ASSOCIATED CONTENT

S Supporting Information *

Experimental methods, EDS elemental mapping results for ZnCo2O4 electrode, XRD patterns and absorption spectra of ZnCo2O4 and Co3O4 electrodes, J−t plots for ZnCo2O4 and Co3O4 electrodes in 1 M KOH, and tables summarizing catalytic performances of ZnCo2O4 and Co3O4 electrodes with standard deviations. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 4. (a) LSVs of ZnCo2O4 (black) and Co3O4 (gray) (scan rate, 0.5 mV/s). LSV of Pt used as the substrate is also shown for comparison (dashed). Tafel plots of (b) ZnCo2O4 and (c) Co3O4 and LSVs of (d) ZnCo2O4 and (e) Co3O4 comparing the initial and the 100th scans (scan rate, 1.0 mV/s). All results were obtained in 0.1 M phosphate (pH 7).

■

AUTHOR INFORMATION

Corresponding Author

*K.-S. Choi. E-mail: [email protected]. 2373

dx.doi.org/10.1021/jz501077u | J. Phys. Chem. Lett. 2014, 5, 2370−2374

The Journal of Physical Chemistry Letters

Letter

Notes

(18) Marco, J. F.; Gancedo, J. R.; Gracia, M.; Gautier, J. L.; Rios, E.; Berry, F. J. Characterization of the Nickel Cobaltite, NiCo2O4, Prepared by Several Methods: An XRD, XANES, EXAFS, and XPS Study. J. Solid State Chem. 2000, 153, 74−81. (19) Therese, G. H. A.; Kamath, P. V. Electrochemical Synthesis of Metal Oxides and Hydroxides. Chem. Mater. 2000, 12, 1195−1204. (20) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions, 2nd ed.; National Association of Corrosion Engineers: Houston, TX, 1974. (21) Jaramillo, T. F.; Baeck, S.-H.; Kleiman-Shwarsctein, A.; Choi, K.S.; Stucky, G. D.; McFarland, E. W. Automated Electrochemical Synthesis and Photoelectrochemical Characterization of Zn1‑xCoxO Thin Films for Solar Hydrogen Production. J. Comb. Chem. 2005, 7, 264−271. (22) Chuang, T. J.; Brundle, C. R.; Rice, D. W. Interpretation of the X-ray Photoemission Spectra of Cobalt Oxides and Cobalt Oxide Surfaces. Surf. Sci. 1976, 59, 413−429. (23) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T. Z.; Dai, H. Co3O4 Nanocrystals on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 10, 780−786. (24) Koza, J. A.; He, Z.; Miller, A. S.; Switzer, J. A. Electrodeposition of Crystalline Co3O4−A Catalyst for the Oxygen Evolution Reaction. Chem. Mater. 2012, 24, 3567−3573. (25) Kanan, M. W.; Nocera, D. G. In Situ Formation of an OxygenEvolving Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321, 1072−1075. (26) Surendranath, Y.; Dincǎ, M.; Nocera, D. G. ElectrolyteDependent Electrosynthesis and Activity of Cobalt-Based Water Oxidation Catalysts. J. Am. Chem. Soc. 2009, 131, 2615−2620.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Center for Chemical Innovation of the National Science Foundation (POWERING THE PLANET, grant no. CHE-1305124).

■

REFERENCES

(1) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J.-M. Nano-Sized Transition-Metal Oxides as Negative-electrode Materials for Lithium-Ion Batteries. Nature 2000, 407, 496−499. (2) Wang, X.; Guan, H.; Chen, S.; Li, H.; Zhai, T.; Tang, D.; Bando, Y.; Golberg, D. Self-Stacked Co3O4 Nanosheets for High-performance Lithium Ion Batteries. Chem. Commun. 2011, 47, 12280−12282. (3) Hamdani, M.; Singh, R. N.; Chartier, P. Co3O4 and Co-Based Spinel Oxides Bifunctional Oxygen Electrodes. Int. J. Electrochem. Sci. 2010, 5, 556−577. (4) Chou, N. H.; Ross, P. N.; Bell, A. T.; Tilley, T. D. Comparison of Cobalt-based Nanoparticles as Electrocatalysts for Water Oxidation. ChemSusChem. 2011, 4, 1566−1569. (5) Tüysüz, H.; Hwang, Y. J.; Khan, S. B.; Asiri, A. M.; Yang, P. Mesoporous Co3O4 as an Electrocatalyst for Water Oxidation. Nano Res. 2013, 6, 47−54. (6) Sharma, Y.; Sharma, N.; Subba Rao, G. V.; Chowdari, B. V. R. Nanophase ZnCo2O4 as a High Performance Anode Material for LiIon Batteries. Adv. Funct. Mater. 2007, 17, 2855−2861. (7) Alcántara, R.; Jaraba, M.; Lavela, P.; Tirado, J. L. NiCo2O4 Spinel: First Report on a Transition Metal Oxide for the Negative Electrode of Sodium-Ion Batteries. Chem. Mater. 2002, 14, 2847−2848. (8) Li, J.; Xiong, S.; Li, X.; Qian, Y. A facile Route to Synthesize Multiporous MnCo2O4 and CoMn2O4 Spinel Quasi-Hollow Spheres With Improved Lithium Storage Properties. Nanoscale 2013, 5, 2045− 2054. (9) Sharma, Y.; Sharma, N.; Subba Rao, G. V.; Chowdari, B. V. R. Lithium Recycling Behaviour of Nano-Phase-CuCo2O4 as Anode for Lithium-Ion Batteries. J. Power Sources 2007, 173, 495−501. (10) Koninck, M. D.; Poirier, S.-C.; Marsan, B. CuxCo3−xO4 Used as Bifunctional Electrocatalyst II. Electrochemical Characterization for the Oxygen Reduction Reaction. J. Electrochem. Soc. 2007, 154, A381− A388. (11) Liang, Y.; Wang, H.; Zhou, J.; Li, Y.; Wang, J.; Regier, T.; Dai, H. Covalent Hybrid of Spinel Manganese−Cobalt Oxide and Graphene as Advanced Oxygen Reduction Electrocatalysts. J. Am. Chem. Soc. 2012, 134, 3517−3523. (12) Lee, D. U.; Kim, B. J.; Chen, Z. One-pot Synthesis of a Mesoporous NiCo2O4 Nanoplatelet and Graphene Hybrid and Its Oxygen Reduction and Evolution Activities as an Efficient BiFunctional Electrocatalyst. J. Mater. Chem. A 2013, 1, 4754−4762. (13) Jin, C.; Lu, F.; Cao, X.; Yang, Z.; Yang, R. Facile Synthesis and Excellent Electrochemical Properties of NiCo2O4 Spinel Nanowire Arrays as a Bifunctional Catalyst for the Oxygen Reduction and Evolution Reaction. J. Mater. Chem. A 2013, 1, 12170−12177. (14) Chi, Bo.; Li, J.; Yang, X.; Lin, H.; Wang, N. Electrophoretic Deposition of ZnCo2O4 Spinel and Its Electrocatalytic Properties for Oxygen Evolution Reaction. Electrochim. Acta 2005, 50, 2059−2064. (15) Liu, X.; Chang, Z.; Luo, L.; Xu, T.; Lei, X.; Liu, J.; Sun, X. Hierarchical ZnxCo3−xO4 Nanoarrays with High Activity for Electrocatalytic Oxygen Evolution. Chem. Mater. 2014, 26, 1889−1895. (16) Bordeneuve, H.; Guillemet-Fritsch, S.; Rousset, A.; Schuurman, S.; Poulain, V. Structure and Electrical Properties of Single-Phase Cobalt Manganese Oxide Spinels Mn3−xCoxO4 Sintered Classically and by Spark Plasma Sintering(SPS). J. Solid State Chem. 2009, 182, 396−401. (17) Knop, O.; Reid, K. I. G.; Sutarno; Nakagawa, Y. Chalkogenides of the Transition Elements. VI. X-Ray, Neutron, and Magnetic Investigation of the Spinels Co3O4, NiCo2O4, Co3S4, and NiCo2S4. Can. J. Chem. 1968, 46, 3463−3476. 2374

dx.doi.org/10.1021/jz501077u | J. Phys. Chem. Lett. 2014, 5, 2370−2374