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CuCoOx/FeOOH Core-shell Nanowires as an Efficient Bifunctional Oxygen Evolution and Reduction Catalyst Min Kuang, Qihao Wang, Hongtao Ge, Peng Han, Zhengxiang Gu, Abdullah M. Al-Enizi, and Gengfeng Zheng ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00835 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 27, 2017

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ACS Energy Letters

CuCoOx/FeOOH Core-shell Nanowires as an Efficient Bifunctional Oxygen Evolution and Reduction Catalyst Min Kuang,1 Qihao Wang,1 Hongtao Ge,1 Peng Han,1 Zhengxiang Gu,1 Abdullah M. Al-Enizi2 and Gengfeng Zheng1,* 1

Laboratory of Advanced Materials, Department of Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai 200433, China

2

Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia

Abstract: The rational design of active and durable reversible oxygen electrocatalysts plays a key role in renewable energy conversion and storage. Here we developed a copper, cobalt-based oxide/iron hydroxide hybrid nanowire arrays (CuCoOx/FeOOH), via a three-step growth-annealing-conversion approach. These hybrid nanowires offer a large surface area for electrocatalytic sites, abundant pores for fast electrolyte access, efficient charge transfer, and strong coupling between CuCoOx and FeOOH components. Attributed to these features, the CuCoOx/FeOOH nanowires exhibit excellent bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) activities, including low overpotentials, high diffusion-limited current densities, and outstanding stabilities. Using the CuCoOx/FeOOH electrocatalyst as the oxygen electrode, a rechargeable zinc (Zn)-air battery was fabricated to exhibit a small charge/discharge overpotential (0.75 V at 10 mA·cm-2) and a long-term cycling stability (150 cycles), thus suggesting new bifunctional electrocatalysts for energy conversion and storage applications. 1

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Electrocatalysis involved oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) represents the cores of a variety of renewable energy conversion and storage technologies, including fuel cells, rechargeable metal-air batteries and water spitting devices,1-3 with key foci on improving the sluggish oxygen electrochemical kinetics, electrode structure stability, and utilizing nonprecious metal-based catalysts.4-6 The recent efforts in exploring new bifunctional oxygen electrocatalysts for both OER and ORR have been further sparking the capability of regenerative metal-air batteries (such as Zn-air batteries and Li-air batteries).7-10 Both theoretical calculations and experiments have suggested that composite materials with multiple components, such as metals,11,

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oxides/hydroxides,13-15 chalcogenides,16 heteroatom-doped carbon nanomaterials,17-19 or their composites,20-23 are needed to achieve optimal catalytic activities for these two reversible reactions of OER and ORR. In spite of many exciting progresses to date, the realization of optimal bifunctional electrocatalysts with structural and activity merits remains a significant challenge, due to the large free energy difference of these two reversible reactions.24,

25

Ideally, all the active

components need to be directly exposed to the electrolytes to present their electrocatalytic functions, while they also need to be in nanoscale proximity to achieve cooperative effects. For instance, Guo et al.26 reported the fabrication of high-performance bifunctional oxygen-involving electrocatalysts in the form of high-energy interfacial structures, which was prepared by chemical coupling of active CoO nanoclusters and high-index Mn3O4 nano-octahedrons. Li et al.9 synthesized the pomegranate-like electrocatalysts based on Co3O4 embedded in a nitrogen-doped carbon framework, which exhibited bifunctional 3


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catalytic activities towards OER and ORR. Thus, the controlled integration of both OER and ORR active sites into a hierarchical architecture with synergetic electrocatalytic activity is an effective means to promote active functions towards both oxygen evolution and reduction. In this work, we developed an interlaced CuCoOx/FeOOH nanowire array structure as a highly efficient OER/ORR bifunctional electrocatalyst, in which CuCoOx and FeOOH function as the ORR and OER catalytic sites, respectively. Key to the design of this composite is the controlled annealing of pre-grown CuCoOx nanowires to partially form Cu2O domains, followed by etching of those Cu2O domains and deposition of FeOOH. Due to

the

high

diffusion

capability

of

Cu

ions

at

elevated

temperatures,

this

growth-annealing-conversion approach not only forms abundant pores inside the CuCoOx nanowires and large surface area (116 m2·g-1), but allows achieving close proximity and strong interaction between FeOOH and CuCoOx phases for synergetic, bifunctional catalytic activities. The CuCoOx/FeOOH nanowire arrays not only demonstrate excellent OER performances with a low onset of 1.46 V vs. reversible hydrogen electrode (RHE), high current density property (309 mA·cm-2 at potential of 1.55 V vs. RHE) and outstanding durability (only ~ 0.89% loss after 30 h), but also efficiently drive ORR with a high diffusion-limited current density of 5.3 mA·cm-2. More importantly, the CuCoOx/FeOOH nanowire electrocatalyst exhibits a metric OER/ORR potential difference of 0.72 V in 1 M KOH, featuring the most active OER/ORR electrocatalysts reported to date. As a result, Zn-air batteries fabricated with these CuCoOx/FeOOH oxygen electrodes exhibit excellent performance and long-term durability. The CuCoOx/FeOOH hybrid was fabricated by a growth-annealing-conversion process 4


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(Figure 1). First, well-aligned CuCoOx nanowires were hydrothermally grown on Ni foam, followed by thermal annealing to convert into oxides (Experimental Procedures).27 The as-made CuCoOx NWs possess a high aspect ratio with diameters ranging between 30 – 80 nm and lengths of around 5 – 10 µm (Figure S1). XRD analysis confirms that the grown CuCoOx NWs have mixed components of Cu2O, CoO and CuCo2O4 (Figure S2). Afterwards, these CuCoOx NWs were used as a scaffold for the controlled deposition of FeOOH nanoflakes (designated as CuCoOx/FeOOH) via a growth-annealing-conversion method at room temperature (Experimental Procedures). In this step, the Cu2O phase in CuCoOx NWs was gradually etched and released OH- ions near the etching interface, and FeOOH nanoflakes were simultaneously grown over the entire nanowire interface. The XRD pattern of the obtained CuCoOx/FeOOH (scraped from Ni foam) is displayed in Figure S2. The broad diffraction peak at ~ 25o is assigned to the (310) plane of FeOOH. The diffraction peaks at ~ 36.6o, 44.4o, and 65.3o are indexed to the (311), (400) and (440) planes of spinel CuCo2O4, and the diffraction peaks at ~ 42.5o, 61.8o, 73.8o are assigned to (200), (220) and (311) planes of CoO, respectively, demonstrating the as-prepared Cu2O phases are converted into FeOOH by. Scanning electron microscopy (SEM) images show that the obtained CuCoOx/FeOOH has a nanowire array structure covered with nanoflakes (Figure 2a). Transmission electron microscopy (TEM) images (Figure 2b, c) further confirm the core-shell structure of the CuCoOx/FeOOH hybrid, with outer FeOOH shells consisting of 10 – 100 nm-thick nanoflakes. In addition, abundant pores exist in the inner CuCuOx nanowires. High-resolution TEM images of the interface between CuCuOx nanowires and FeOOH nanoflakes 5


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demonstrate the polycrystalline nature of both core and shell structures (Figure 2d). The interplanar spacing of the shell is 0.257 nm, corresponding to the (211) lattice fringe of FeOOH.28 The interplanar spacings of the core are 0.232 and 0.215 nm, in accord to the (222) and (200) lattice spacings of CuCo2O4 and CoO, respectively.29,

30

Energy dispersive

spectroscopy (EDS) element mapping carried out on a single CuCoOx/FeOOH nanowire (Figure S3) shows that both Cu and Co elements are homogeneously distributed at the core of the hierarchical structure, while Fe is uniformly distributed over the shell. For comparison, FeOOH was also directly grown on Ni foam and on CuCoOx nanowires by electrochemical deposition, which are designated as FeOOH and CuCoOx@FeOOH, respectively (Figure S4,

S5). N2 sorption measurements were carried out at 77 K to investigate the porosity properties of CuCoOx/FeOOH and pure CuCoOx (Figure S6). Both of the adsorption/desorption isotherms show a typical type-IV shape with H3 hysteresis loop, suggesting the characteristics of mesoporous materials.31,

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These CuCoOx/FeOOH present a larger

Brunauer-Emment-Teller (BET) surface area of 116 m2·g-1 and higher pore volume of 0.421 cm3·g-1 than those of the original CuCoOx template (68 m2·g-1, 0.236 cm3·g-1). These high surface area and pore volume are ascribed to the controlled annealing and subsequent solution-phase conversion and regrowth processes, with the removal of Cu2O phase and pore formation. The hierarchical porosity of CuCoOx/FeOOH with high surface area and pore volume is favorable for the accessibility of electrolyte and offers more active sites for oxygen-involving reactions.33, 34 The surface electronic structure of the as-prepared CuCoOx/FeOOH, CuCoOx@FeOOH, 6


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and CuCoOx were further studied by X-ray photoelectron spectroscopy (XPS). The survey XPS spectra (Figure S7) demonstrate that the Cu, Co and O elements exist in all three samples, and Fe only exists in CuCoOx/FeOOH and CuCoOx@FeOOH. For Fe 2p region in CuCoOx/FeOOH, all the deconvoluted Fe 2p3/2 spectra display a featured peak of Fe 2p3/2 at ~ 710 eV associated with a shake-up satellite at ~ 718.6 eV (Figure S8), confirming the existence of Fe3+.35-37 The Fe 2p peaks in CuCoOx/FeOOH show a positive shift of ~ 0.62 eV compared to those of CuCoO@FeOOH (Figure 2e), suggesting strong electronic interaction between FeOOH and CuCoOx.38 Similarly, compared to those of CuCoOx@FeOOH and pure CuCoOx, the Co 2p and Cu 2p spectra of CuCoOx/FeOOH positively shift ~ 0.62 and ~ 0.56 eV, respectively, further indicating strong electron interaction between the FeOOH and CuCoOx phases in CuCoOx/FeOOH (Figure 2f, g). Moreover, the molar ratio of Cu+/Cu2+ for CuCoOx/FeOOH is ~ 0.49, which is lower than that of CuCoOx@FeOOH (~ 0.87) and CuCoOx (~ 1.01), further suggesting that as-formed Cu2O phases are removed during the cation-exchange process (Figure S9). Interestingly, the O 1s XPS spectrum of CuCoOx/FeOOH

exhibits

very

different

featured

peak

compared

to

those

of

CuCoOx@FeOOH and CuCoOx (Figure S10). Taken together, it can be confirmed that strong electronic interactions exist between the FeOOH and CuCoOx phases in CuCoOx/FeOOH. The electrocatalytic activities of the CuCoOx/FeOOH catalysts and other controlled samples were studied in 1 M KOH solution (pH ~ 14) in a standard three-electrode system (Experimental Procedures). As shown in Figure 3a, the anodic current of the CuCoOx/FeOOH catalyst recorded by linear sweep voltammetry (LSV) demonstrates superior OER electrocatalytic activity with a small onset of 1.45 V, compared to 7


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CuCoOx@FeOOH (1.50 V) as well as its individual component (i.e., 1.51 V for CuCoOx and 1.49 V for FeOOH). The pure Ni foam exhibits negligible OER electrocatalytic activity. The remarkable activity of CuCoOx/FeOOH is further exhibited by its significantly higher current density than other samples over the whole potential range tested. For example, at 1.55 V vs. RHE, the current density of the CuCoOx/FeOOH reaches 309 mA·cm-2, which is much larger than that of CuCoOx@FeOOH (113 mA·cm-2), CuCoOx (44 mA·cm-2) and FeOOH (143 mA·cm-2), respectively. The Tafel slope of CuCoOx/FeOOH is 63 mV·dec-1 (Figure 3b), which is much smaller than that of CuCoOx@FeOOH (68 mV·dec-1), CuCoOx (82 mV·dec-1), and FeOOH (70 mV·dec-1), further confirming the favorable kinetics towards OER. The more conclusive comparison with other recently reported OER catalysts is displayed in Table S1. The OER measurement was also carried out on the rotating disk electrode (RDE) and compared with other catalysts including IrO2 with the same mass loading of ~ 0.2 mg·cm-2. The CuCoOx/FeOOH catalyst also presents the highest OER catalytic activity (Figure S11). The excellent OER catalytic activity of CuCoOx/FeOOH hybrids can be mainly attributed to the large exposed surface active sites and efficient mass transport, as well as synergistic interplay between the CuCoOx core and FeOOH shells. In addition to the electrocatalytic activity, the high durability of catalysts also plays a key role in the energy conversion and storage systems.6, 39 Both the chronoamperometry and LSV plots of CuCoOx/FeOOH (Figure S12a) demonstrates a high stability in alkaline solution, exhibiting negligible current degradation of only 0.89% at 1.48 V and a high current density of ~30 mA·cm-2 for 30 h. The Nyquist plots of CuCoOx/FeOOH before and after 30 h of catalytic reaction at potential of 1.48 V indicate almost the same intrinsic resistance and 8


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ACS Energy Letters

charge-transfer resistance (Figure S12b and Figure S13), further confirming the high stability of CuCoOx/FeOOH. To investigate the bifunctionality of the CuCoOx/FeOOH catalyst, the ORR activities of these samples were investigated and compared by LSV polarization curves in 1 M KOH solution (Figure 3c). The CuCoOx/FeOOH displays a superior electrocatalytic activity toward ORR with an onset potential of 0.89 V and a half-wave potential (E1/2) of 0.78 V vs. RHE, much more positive than those of CuCoOx@FeOOH (onset potential: 0.77 V, E1/2: 0.73 V) and pure CuCoOx (onset potential: 0.78 V, E1/2: 0.74 V). Furthermore, as compared to commercial Pt/C (20% Pt), CuCoOx/FeOOH exhibits a comparable diffusion-limited current density (~ 5.3 mA·cm-2), suggesting the advantage of its electronic interaction and porosity architecture. The LSV curves of CuCoOx/FeOOH carried out with an RDE indicate that the onset potential is constant under various rotation rates, while the diffusion-limited current density increases with the rotation rate, owing to the enhanced mass transport at larger rates (Figure 3d).20,

40, 41

The linearity of the Koutecky-Levich (K-L) plots and the near

coincidence of the fitting lines (Figure 3d inset) indicate a first-order reaction kinetics towards the concentration of dissolved oxygen and similar electron transfer number (n) in ORR at various potentials.42, 43 The n value was calculated to be 3.87 – 3.92 at the potentials ranging from 0.2 to 0.6 V for CuCoOx/FeOOH, suggesting a close-to-four-electron transfer pathway for ORR,44 similar to that of commercial Pt/C catalyst (Figure S14). In contrast, the

n values were measured as 3.46 – 3.59 and 3.22 – 3.43 for pure CuCoOx and CuCoOx@FeOOH, respectively (Figure S15, S16), implying the coexistence of both twoand four-electron pathways for ORR. Furthermore, the kinetic-limiting density of 9


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CuCoOx/FeOOH was calculated to be 16.8 mA·cm-2 at 0.5 V vs. RHE under rotation rate of 1600 rmp, which is higher than that of the pure CuCoOx (11.7 mA·cm-2) and CuCoOx@FeOOH (10.1 mA·cm-2). The excellent ORR electrocatalytic performance of CuCoOx/FeOOH highlights the key role of unique porosity architecture and strong synergetic interaction between the hydroxides and oxides. Furthermore, the methanol crossover test of CuCoOx/FeOOH in ORR was evaluated by chronoamperometric measurement in O2-saturated 1 M KOH with a constant voltage of 0.6 V vs. RHE (Figure S17). After the injection of methanol, no noticeable change in the ORR current density was observed for CuCoOx/FeOOH, while the Pt/C electrocatalyst showed a significant decreased current density, suggesting that the suitability of the CuCoOx/FeOOH catalyst in the methanol fuel cell applications. In addition, CuCoOx/FeOOH also exhibits excellent long-term stability (Figure S18). After 8 h of continuous test, the Pt/C catalyst suffered from a rapid decrease (~ 36.7%) of current density, but the CuCoOx/FeOOH catalyst retained around 77.9% of the initial current density. The overall oxygen electrode activities can be evaluated by the value of ∆E,45 which is defined as the potential difference between Ej=10 and E1/2, namely ∆E = Ej=10 – E1/2. Lower ∆E value indicates better bifunctional catalytic activity of an electrocatalyst.12, 46 As displayed in

Figure 3e and f, CuCoOx/FeOOH exhibits the smallest ∆E of 0.72 V, much lower than that of pure CuCoOx (∆E = 0.81 V), CuCoOx@FeOOH (∆E = 0.80 V), and even comparable to the state-of-the-art noble metals electrocatalysts for ORR and OER, respectively (Pt/C for ORR and IrO2 for OER; ∆E = 0.69 V). Furthermore, the electrochemically active surface areas (ECSA) of CuCoOx/FeOOH, CuCoOx, CuCoOx@FeOOH and IrO2 were measured and 10


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compared using their double-layer capacitances (Cdl, Figure S19–22, Experimental Procedures). The CuCoOx/FeOOH shows the highest Cdl (16 mF·cm-2) compared to CuCoOx (13 mF·cm-2), CuCoOx@FeOOH (8 mF·cm-2) and IrO2 (12 mF·cm-2), demonstrating the increase of active sites of CuCoOx/FeOOH. A detailed comparison of various bifunctional catalysts for OER and ORR reported recently is summarized in Table S2, verifying the excellent catalytic activity of the CuCoOx/FeOOH.47, 48 Considering the excellent bifunctional electrocatalytic activity of CuCoOx/FeOOH for ORR and OER, the possibility of using CuCoOx/FeOOH catalysts for practical energy devices was investigated. A primary Zn-air battery was first built, in which a Zn foil was used as the anode and the CuCoOx/FeOOH as the air-cathode. The discharge polarization and power density curves of the primary Zn-air battery are exhibited (Figure 4a). At a voltage of 1.0 V, the Zn-air battery with CuCoOx/FeOOH electrocatalyst demonstrates a high current density of 106 mA·cm-2, with the maximum power density of 158 mW·cm-2, which is close to the same battery structure using noble-metal-based Pt/C and IrO2 catalysts. Importantly, the battery with CuCoOx/FeOOH also exhibited impressive operation stability (Figure 4b). When galvanostatically discharged at 10 mA·cm−2 for 20 h, the working voltages were stabilized at ~ 1.27 V with almost no drop. With the same configuration as that of the primary counterpart, a rechargeable Zn-air battery was further demonstrated (Figure 4c). The discharge and charge polarization curves for Zn-air batteries with CuCoOx/FeOOH as the bifunctional catalyst was demonstrated (Figure 4d). Compared to the battery constructed with Pt/C and IrO2 catalysts, a significantly better charging performance and a comparable discharging performance were observed for the battery using CuCoOx/FeOOH. When cycled at a current density of 10 11


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mA·cm-2, the battery with the CuCoOx/FeOOH electrode exhibited initial charge and discharge potentials of 1.96 and 1.21 V, respectively, demonstrating a small overpotential of 0.75 V (Figure 4e). After 150 cycles, the battery with the CuCoOx/FeOOH electrode exhibited a small increased overpotential of 0.20 V, whereas the battery with Pt/C and IrO2 showed a higher increased overpotential (0.23 V) under the same condition. The comparison of our battery with other recently reported rechargeable Zn–air batteries was summarized in

Table S3, indicating the superior performances of this CuCoOx/FeOOH bifunctional electrocatalyst. In summary, we have developed a CuCoOx/FeOOH nanowire composite by a three-step growth-annealing-conversion process, which exhibits efficient bifunctional OER/ORR electrocatalytic activities for air-electrodes in primary or rechargeable Zn-air batteries. The outstanding bifunctional OER and ORR activities of the CuCoOx/FeOOH catalyst are attributed to the following compositional and architectural aspects. First, the FeOOH and CuCuOx phases exhibit effective electrocatalytic activity for oxygen-involving reactions. Second, the porous core/shell architecture with a large surface area presents highly exposed surface active sites, as well as facilitates mass transport and intimate contact between electrolyte and active materials. Third, the homogeneous incorporation of FeOOH on CuCoOx effectively tunes the electronic structure, thereby enhancing the synergistic interplay between the CuCoOx core and FeOOH shells, which also facilitates the electrocatalytic performance of the material. The present findings are expected to open up new opportunities for the development of effective electrocatalysts in a facile and viable strategy.

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Associated Content Supporting Information Experimental details and data. Figures S1-S22 and Table S1-S3.

Author Information Corresponding Author *E-mail: [email protected]

ORCID Gengfeng Zheng: 0000-0002-1803-6955

Author Contributions M.K. and Q.W. contributed equally to this work.

Notes The authors declare no competing financial interest.

Acknowledgements We thank the following funding agencies for supporting this work: the National Key Research and Development Program of China (2017YFA0206901, 2017YFA0206900), the Natural Science Foundation of China (21473038, 21773036), the Science and Technology Commission of Shanghai Municipality (17JC1402000), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, and the Collaborative Innovation Center of Chemistry for Energy Materials (2011-iChem). The

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authors extend their appreciation to the International Scientific Partnership Program ISPP at King Saud University for funding this research work through ISPP #0017.

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Bifunctional Oxygen Electrode. Angew. Chem. Int. Ed. 2016, 55, 4087-4091. (13) Ling, T.; Yan, D. Y.; Jiao, Y.; Wang, H.; Zheng, Y.; Zheng, X.; Mao, J.; Du, X. W.; Hu, Z.; Jaroniec, M.; Qiao, S. Z. Engineering surface atomic structure of single-crystal cobalt (II) oxide nanorods for superior electrocatalysis. Nat. Commun. 2016, 7, 12876. (14) Qian, L.; Lu, Z.; Xu, T.; Wu, X.; Tian, Y.; Li, Y.; Huo, Z.; Sun, X.; Duan, X. Trinary Layered Double Hydroxides as High-Performance Bifunctional Materials for Oxygen Electrocatalysis. Adv. Energy Mater. 2015, 5, 1500245-1500251. (15) Sa, Y. J.; Kwon, K.; Cheon, J. Y.; Kleitz, F.; Joo, S. H. Ordered mesoporous Co3O4 spinels as stable, bifunctional, noble metal-free oxygen electrocatalysts. J. Mater. Chem. A 2013, 1, 9992-10001. (16) Shen, M.; Ruan, C.; Chen, Y.; Jiang, C.; Ai, K.; Lu, L. Covalent entrapment of cobalt-iron sulfides in N-doped mesoporous carbon: extraordinary bifunctional electrocatalysts for oxygen reduction and evolution reactions. ACS Appl. Mater. Interfaces 2015, 7, 1207-1218. (17) Tang, C.; Wang, H. F.; Chen, X.; Li, B. Q.; Hou, T. Z.; Zhang, B.; Zhang, Q.; Titirici, M. M.; Wei, F. Topological Defects in Metal-Free Nanocarbon for Oxygen Electrocatalysis. Adv. Mater. 2016, 28, 6845-6851. (18) El-Sawy, A. M.; Mosa, I. M.; Su, D.; Guild, C. J.; Khalid, S.; Joesten, R.; Rusling, J. F.; Suib, S. L. Controlling the Active Sites of Sulfur-Doped Carbon Nanotube-Graphene Nanolobes for Highly Efficient Oxygen Evolution and Reduction Catalysis. Adv. Energy Mater. 2016, 6, 1501966. (19) Qu, K.; Zheng, Y.; Dai, S.; Qiao, S. Z. Graphene oxide-polydopamine derived N, S-codoped carbon nanosheets as superior bifunctional electrocatalysts for oxygen reduction and evolution. Nano Energy 2016, 19, 373-381. (20) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat. Mater. 2011, 10, 780-786. (21) Liu, Y.; Jiang, H.; Zhu, Y.; Yang, X.; Li, C. Transition metals (Fe, Co, and Ni) encapsulated in nitrogen-doped carbon nanotubes as bi-functional catalysts for oxygen electrode reactions. J. Mater. Chem. A 2016, 4, 1694-1701. (22) Xia, B. Y.; Yan, Y.; Li, N.; Wu, H. B.; Lou, X. W. D.; Wang, X. A metal–organic framework-derived bifunctional oxygen electrocatalyst. Nat. Energy 2016, 1, 15006-150013. (23) Dresp, S.; Luo, F.; Schmack, R.; Kühl, S.; Gliech, M.; Strasser, P. An efficient bifunctional two-component catalyst for oxygen reduction and oxygen evolution in reversible fuel cells, electrolyzers and rechargeable air electrodes. Energy Environ. Sci. 2016, 9, 2020-2024. (24) Park, S.; Shao, Y.; Liu, J.; Wang, Y. Oxygen electrocatalysts for water electrolyzers and reversible fuel cells: status and perspective. Energy Environ. Sci. 2012, 5, 9331-9344. (25) Kuang, M.; Zheng, G. Nanostructured Bifunctional Redox Electrocatalysts. Small 2016, 12, 5656-5675. (26) Guo, C.; Zheng, Y.; Ran, J.; Xie, F.; Jaroniec, M.; Qiao, S. Z. Engineering High-Energy Interfacial Structures for High-Performance Oxygen-Involving Electrocatalysis. Angew. Chem. Int. Ed. 2017, 56, 1-6. (27) Kuang, M.; Han, P.; Wang, Q.; Li, J.; Zheng, G. CuCo Hybrid Oxides as Bifunctional Electrocatalyst for Efficient Water Splitting. Adv. Funct. Mater. 2016, 26, 8555-8561. (28) Feng, J. X.; Xu, H.; Dong, Y. T.; Ye, S. H.; Tong, Y. X.; Li, G. R. FeOOH/Co/FeOOH Hybrid Nanotube Arrays as High-Performance Electrocatalysts for the Oxygen Evolution Reaction. Angew. Chem. In.t Ed. 2016, 55, 3694-3698. (29) Ning, R.; Tian, J.; Asiri, A. M.; Qusti, A. H.; Al-Youbi, A. O.; Sun, X. P. Spinel CuCo2O4 nanoparticles supported on N-doped reduced graphene oxide: a highly active and stable hybrid electrocatalyst for the oxygen reduction reaction. Langmuir 2013, 29, 13146-13151. 15


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(30) Li, K.; Zhang, J.; Wu, R.; Yu, Y.; Zhang, B. Anchoring CoO Domains on CoSe2 Nanobelts as Bifunctional Electrocatalysts for Overall Water Splitting in Neutral Media. Adv. Sci. 2016, 3, 1-7. (31) Zhang, G.; Lou, X. W. General solution growth of mesoporous NiCo2O4 nanosheets on various conductive substrates as high-performance electrodes for supercapacitors. Adv. Mater. 2013, 25, 976-979. (32) Wang, Y.; Jiang, K.; Zhang, H.; Zhou, T.; Wang, J.; Wei, W.; Yang, Z.; Sun, X.; Cai, W. B.; Zheng, G. Bio-Inspired Leaf-Mimicking Nanosheet/Nanotube Heterostructure as a Highly Efficient Oxygen Evolution Catalyst. Adv. Sci. 2015, 2, 1500003. (33) Zhu, Q. L.; Xia, W.; Akita, T.; Zou, R.; Xu, Q. Metal-Organic Framework-Derived Honeycomb-Like Open Porous Nanostructures as Precious-Metal-Free Catalysts for Highly Efficient Oxygen Electroreduction. Adv. Mater. 2016, 28, 6391-6398. (34) Woojeong Bak, H. S. K., Hyungphil Chun and Won Cheol Yoo. Facile synthesis of metal/metal oxide nanoparticles inside a nanoporous carbon matrix (M/MO@C) through the morphology-preserved transformation of metal–organic framework. Chem. Commun. 2015, 51, 7238-7241. (35) Fan, K.; Ji, Y.; Zou, H.; Zhang, J.; Zhu, B.; Chen, H.; Daniel, Q.; Luo, Y.; Yu, J.; Sun, L. Hollow Iron-Vanadium Composite Spheres: A Highly Efficient Iron-Based Water Oxidation Electrocatalyst without the Need for Nickel or Cobalt. Angew. Chem. Int. Ed. 2017, 56, 3289-3293. (36) Feng, J. X.; Ye, S. H.; Xu, H.; Tong, Y. X.; Li, G. R. Design and Synthesis of FeOOH/CeO2 Heterolayered Nanotube Electrocatalysts for the Oxygen Evolution Reaction. Adv. Mater. 2016, 28, 4698-4703. (37) Zhang, Y.; Jia, G.; Wang, H.; Ouyang, B.; Rawat, R. S.; Fan, H. J. Ultrathin CNTs@ FeOOH nanoflake core/shell networks as efficient electrocatalysts for the oxygen evolution reaction. Mater. Chem. Front. 2017, 1, 709-715. (38) Feng, J. X.; Xu, H.; Dong, Y. T.; Ye, S. H.; Tong, Y. X.; Li, G. R. FeOOH/Co/FeOOH Hybrid Nanotube Arrays as High-Performance Electrocatalysts for the Oxygen Evolution Reaction. Angew. Chem. Int. Ed. 2016, 55, 3694-3698. (39) Ji, X.; Hao, S.; Qu, F.; Liu, J.; Du, G.; Asiri, A. M.; Chen, L.; Sun, X. Core–shell CoFe2O4@ Co-Fe-Bi nanoarray: a surface-amorphization water oxidation catalyst operating at near-neutral pH. Nanoscale 2017, 9, 7714-7718. (40) Gorlin, Y.; Jaramillo, T. F. A bifunctional nonprecious metal catalyst for oxygen reduction and water oxidation. J. Am. Chem. Soc. 2010, 132, 13612-13614. (41) Liu, Y.; Chen, F.; Ye, W.; Zeng, M.; Han, N.; Zhao, F.; Wang, X.; Li, Y. High-Performance Oxygen Reduction Electrocatalyst Derived from Polydopamine and Cobalt Supported on Carbon Nanotubes for Metal-Air Batteries. Adv. Funct. Mater. 2017, 27, 1606034. (42) Zeng, M.; Liu, Y.; Zhao, F.; Nie, K.; Han, N.; Wang, X.; Huang, W.; Song, X.; Zhong, J.; Li, Y. Metallic Cobalt Nanoparticles Encapsulated in Nitrogen-Enriched Graphene Shells: Its Bifunctional Electrocatalysis and Application in Zinc-Air Batteries. Adv. Funct. Mater. 2016, 26, 4397-4404. (43) Tian, G.-L.; Zhang, Q.; Zhang, B.; Jin, Y.-G.; Huang, J.-Q.; Su, D. S.; Wei, F. Toward Full Exposure of “Active Sites”: Nanocarbon Electrocatalyst with Surface Enriched Nitrogen for Superior Oxygen Reduction and Evolution Reactivity. Adv. Funct. Mater. 2014, 24, 5956-5961. (44) Jiang, G.; Zhu, H.; Zhang, X.; Shen, B.; Wu, L.; Zhang, S.; Lu, G.; Wu, Z.; Sun, S. Core/shell face-centered tetragonal FePd/Pd nanoparticles as an efficient non-Pt catalyst for the oxygen reduction reaction. ACS nano 2015, 9, 11014-11022. (45) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of electrocatalysts for oxygen-and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. 2015, 44, 2060-2086. (46) Fu, G.; Chen, Y.; Cui, Z.; Li, Y.; Zhou, W.; Xin, S.; Tang, Y.; Goodenough, J. B. Novel Hydrogel-Derived 16


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ACS Energy Letters

Bifunctional Oxygen Electrocatalyst for Rechargeable Air Cathodes. Nano Lett. 2016, 16, 6516-6522. (47) Song, F.; Hu, X. Ultrathin cobalt-manganese layered double hydroxide is an efficient oxygen evolution catalyst. J. Am. Chem. Soc. 2014, 136, 16481-16484. (48) McCrory, C. C.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction. J. Am. Chem. Soc. 2013, 135, 16977-16987.

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Figures

Figure 1. Schematic illustration showing the fabrication of the CuCoOx/FeOOH hybrid nanowire electrocatalysts, including the growth, annealing and conversion synthesis process.

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ACS Energy Letters

Figure 2. Characterization of the CuCoOx/FeOOH, CuCoOx@FeOOH and CuCoOx Arrays. (a) SEM image of CuCoOx/FeOOH grown on Ni foam. (b and c) TEM images, (d) HRTEM image of CuCoOx/FeOOH. (e) XPS spectra of Fe 2p for CuCoOx/FeOOH (red curve) and CuCoOx@FeOOH (blue curve), (f, g) XPS spectra of (f) Co 2p and (g) Cu 2p for CuCoOx/FeOOH (red curves), CuCoOx@FeOOH (blue curves) and CuCoOx (black curves), respectively. Sat. represents satellite peaks.

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Figure 3. (a) LSV curves of various samples for OER in 1 M KOH. (b) The corresponding Tafel curves. (c) ORR polarization curves of CuCoOx/FeOOH, CuCoOx@FeOOH, CuCoOx, Pt/C and glass carbon (GC) in O2-saturated 1 M KOH solution at a sweep rate of 10 mV·s-1 and electrode rotation speed of 1600 rmp. (d) LSV curves of CuCoOx/FeOOH at various rotation rates. Inset: the corresponding K-L plots at different potentials. (e) LSV curves of both OER and ORR for CuCoOx/FeOOH, CuCoOx@FeOOH, CuCoOx, IrO2 and Pt/C 20


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ACS Energy Letters

catalysts in O2-saturated 1 M KOH solution. (f) Values of ∆E for these catalysts (∆E = Ej10 – E1/2). The catalyst loadings in all cases were 0.2 mg·cm-2.

Figure 4. (a) Polarization and power density curves of the primary Zn-air batteries with CuCoOx/FeOOH (red curves) and mixed Pt/C + IrO2 (black curves). (b) Galvanostatic discharge curves of CuCoOx/FeOOH with a current density of 10 mA·cm-2. (c) Schematic representation of the rechargeable Zn-air battery. (d) Charge and discharge polarization curves of CuCoOx/FeOOH and mixed Pt/C + IrO2. (e) Galvanostatic discharge–charge cycling curves at 10 mA·cm-2 of rechargeable Zn-air batteries of CuCoOx/FeOOH and mixed 21


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Pt/C + IrO2.

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