Hierarchically Structured NiFeOx/CuO ... - Wiley Online Library

DOI: 10.1002/cctc.201701441

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Hierarchically Structured NiFeOx/CuO Nanosheets/ Nanowires as an Efficient Electrocatalyst for the Oxygen Evolution Reaction Steffen Czioska, Jianying Wang, Shangshang Zuo, Xue Teng, and Zuofeng Chen*[a] and FeIII. The presence of concentrated carbonate anions allows FeIII to be dissolved in a basic solution by complex ion formation. This strategy therefore avoids the destruction of CuO nanowires caused by the usual cathodic procedure in acidic solutions. At a planar copper foil, a small Tafel slope of 36 mV dec1 was obtained in 1 m KOH and a current density of 100 mA cm2 was reached at a very low overpotential of 300 mV from the Tafel plot. Electrolysis experiments showed high stability of the catalyst with nearly no loss in efficiency and morphology change after a prolonged period. The same catalyst could be established on a three-dimensional copper foam.

It is desirable to fabricate hierarchically structured NiFe-based materials to further boost the performance of these state-ofthe-art electrocatalysts for the oxygen evolution reaction (OER). Most of the NiFeOx catalysts in the form of layered double hydroxides feature a nanosheet structure, which limits their spatial extension at the electrode substrate. Herein, we report the fabrication of unique NiFeOx/CuO nanosheets/nanowires and their use as efficient electrocatalysts for the OER. The surface-bound Cu(OH)2 nanowires were grown in situ on a copper substrate by a simple solution-based method, which were then converted to CuO nanowires by calcination in air. The coating of NiFeOx nanosheets was achieved by anodic codeposition in concentrated carbonate solution containing NiII

Introduction Fossil fuels such as coal and oil account for most of the energy consumed by mankind today. With rising concerns about the energy crisis and environmental pollution, the use of clean and renewable alternative energy sources to substitute these fossil fuels is attracting more and more attention. One of the most promising clean energy sources is hydrogen produced by water splitting. The water splitting process consists of two half-reactions, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). In comparison with the HER,[1] the OER is energetically more demanding and embodies the bottleneck of water splitting. The OER consists of four-electron and four-proton transfer concomitant with O=O double bond formation, which is more complex and kinetically sluggish than the HER.[2] Although significant progress has been made, there remains an ongoing need to develop affordable electrocatalysts to efficiently speed up the OER. For instance, IrO2 and RuO2 exhibit excellent OER performance,[3] but their high cost and nonabundance hinder their large-scale application. Therefore, one focus lies on non-noble metal-based materials, mainly the 3d

first-row transition metal oxides, hydroxides, or oxyhydroxides.[4] Among these materials, electrocatalysts consisting of a combination of nickel and iron have been demonstrated to catalyze the OER with high efficiency by taking advantage of synergistic metal–metal interactions.[4c–h] Copper as another 3d transition metal element was also found recently to form OER electrocatalysts. However, the performances reached by copper-based materials were comparably low.[5] Besides the nature of the electrocatalyst materials, the electrode structure and morphology play a key role in determining the electrochemically active surface area (ECSA) and therefore the catalyst performance.[6] In earlier reports, bimetallic oxide catalysts, such as CoFeOx,[7a] CoNiOx,[7b] and NiFeOx[4c–h] were typically in the form of layered double hydroxides (LDHs), which tend to assemble into two-dimensional nanosheets. It is challenging to construct one-dimensional nanostructured materials, in particular, NiFeOx for the OER, which should maximize the specific surface area of the electrocatalyst at a given electrode substrate, and therefore further boost the catalyst performance. By contrast, there are a large number of reports regarding the synthesis or fabrication of various Cu-based nanowire materials for various applications.[8] For example, metallic Cu nanowires have been synthesized by a one-pot hydrothermal reaction,[8a] and Cu(OH)2 nanowires have been reported to grow from the copper surface either by surface reaction in a chemical bath or anodic treatment, which might be further converted to various copper-based materials.[5a, 8b–d] Herein, we report the fabrication of NiFeOx/CuO nanosheets/ nanowires and their use for efficient electrocatalytic water

[a] S. Czioska, J. Wang, S. Zuo, X. Teng, Prof. Z. Chen Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering Tongji University Shanghai 200092 (P.R. China) E-mail: [email protected] Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/ cctc.201701441.

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Full Papers oxidation. The nanowires consisting of copper hydroxide were grown from a copper substrate in a chemical bath and then converted to copper oxide nanowires by calcination in air. Subsequently, the NiFeOx nanosheets catalyst was anodically codeposited onto the CuO nanowires in concentrated carbonate solution containing NiII and FeIII, a method generally employed to in situ deposit individual Co, Ni, or Cu oxides onto electrodes in buffer solutions for the OER.[9] In this procedure, high concentrations of carbonate anions are essential in the complex ion formation to avoid precipitation of FeIII (Fe(OH)3, Ksp = 2.6 1039). Owing to the poor solubility of FeIII in basic solution, the cathodic co-deposition was usually performed in acidic solutions for the preparation of NiFe-based catalysts,[4c–e] which in this case may destruct CuO nanowires by dissolution and reduction. In addition, the direct growth of nanowires (NWs) from the underlying conductive substrate and in situ electrodeposition of NiFeOx catalysts can eliminate the use of any conductive agents and binders, which ensures good electrical contact between the electrocatalyst and the conductive substrate. The as-prepared nanosheets/nanowires at a planar copper foil exhibit a low Tafel slope of 36 mV dec1 and a low overpotential of 300 mV to reach a current density of 100 mA cm2 from the Tafel plot. Long-term electrolysis revealed the high stability of the catalyst performance with nearly no change in structure and morphology of the electrode. If established on a three-dimensional copper foam as the electrode substrate, the electrocatalyst requires an overpotential of only 265 mV for a current density of 100 mA cm2, which is among the best NiFeOx catalysts reported to date. The fabrication of this hierarchically structured nanosheets/ nanowires electrode to maximize the NiFeOx catalyst efficiency is appealing and will contribute to the future development of this class of OER catalysts.

Scheme 1. Fabrication process together with photographs of (A) Cu foil, (B) Cu(OH)2 NWs/Cu, (C) CuO NWs/Cu, and (D) NiFeOx@CuO NWs/Cu to show the color change of the electrode after each step.

Results and Discussion Preparation and characterization of electrocatalyst The fabrication process of the NiFeOx@CuO NWs/Cu electrode is illustrated in Scheme 1. Firstly, copper hydroxide nanowires were developed on a copper foil in an aqueous solution containing NaOH and (NH4)2S2O8 for 25 min.[11] The general reaction is depicted in Equation (1) and the reaction mechanism for the formation of Cu(OH)2 nanowires is discussed in detail elsewhere.[11] In an alkaline solution, ammonium sulfate is reductively decomposed on the copper surface with concomitant formation of a blue layer of Cu(OH)2 nanowires. As shown by SEM images in Figure 1 A,B, the nanowires were uniformly grown on the copper surface. Cu þ 4 OH þðNH4 Þ2 S2 O8 ! CuðOHÞ2 þ 2 SO4 2 þ 2 NH3 " þ2 H2 O

Figure 1. SEM images of (A) Cu foil, (B) Cu(OH)2 NWs/Cu, (C) CuO NWs/Cu, and (D) NiFeOx@CuO NWs/Cu. (E) and (F) High-magnification SEM images of CuO NWs/Cu and NiFeOx@CuO NWs/Cu, respectively; (G) and (I) are their corresponding TEM images.

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Cu(OH)2 nanowires did not prove stable during prolonged anodic NiFeOx deposition, which were therefore converted to CuO nanowires by calcination in air prior to the coating. The annealing treatment is expected to improve the mechanical

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strength or stability of the electrode. Although the color of the copper foil changed from light blue to brown after 2


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Full Papers dehydration, the nanowires are retained perfectly, as shown in Figure 1 C,E. The CuO nanowires were then coated with NiFeOx by anodic co-deposition in concentrated carbonate solution containing 1 mm NiII and 1 mm FeIII.[12] The CuO NWs were highly stable during the electrodeposition by holding the potential at 1.74 V versus reversible hydrogen electrode (RHE) for 60 min. As can be seen in Figure 1 D,F, the arranged structure of the nanowires was unchanged, whereas the diameter of the nanowires increased and the surface became rough owing to the coating of NiFeOx nanosheets. The TEM images in Figure 1 G,H confirm the formation of the expected core–shell nanowire structure. The diameter of uncoated CuO nanowires is around 200 nm and the coating layer has a thickness of around 50 nm, giving the core–shell nanowires a diameter of around 300 nm. The electrodes were characterized by XRD, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Figure 2 A shows the XRD patterns of the Cu foil, Cu(OH)2 NWs/Cu, CuO NWs/Cu, and NiFeOx@CuO NWs/Cu. In all samples, the three main peaks at 43.38, 50.48, and 74.08 are attributed to (111), (2 0 0), and (2 2 0) of the Cu foil substrate (JCPDS No. 03-1005),

respectively. The XRD pattern of Cu(OH)2 NWs/Cu shows a set of diffraction peaks at 16.78, 23.88, 34.08, 35.88, 38.28, 39.88, and 53.28, corresponding to (0 2 0), (0 2 1), (0 0 2), (111), (0 2 2), (1 3 0), and (1 5 0) of Cu(OH)2 (JCPDS No. 13-0420). By contrast, the XRD pattern of CuO NWs/Cu exhibits three diffraction peaks at 35.78, 39.08, and 49.28 for (111), (111), and (2 0 2) of CuO (JCPDS No. 01-1117) with no evidence for the presence of Cu(OH)2 residues. These results confirm the complete conversion of Cu(OH)2 NWs into CuO NWs after calcination. With NiFeOx coated onto CuO NWs/Cu, there is no observable change in XRD pattern, indicating the amorphous nature of the NiFeOx coating. The electrodes were also investigated by Raman spectroscopy, as shown in Figure 2 B. The copper foil substrate exhibits no Raman signals as metallic copper is not responsive toward Raman scattering. For the Cu(OH)2 NWs/Cu electrode, the Raman bands observed at 300 and 488 cm1 are consistent with the presence of Cu(OH)2.[13] For this sample, the Raman spectrum is complicated by the bands at 275, 320, and 615 cm1 for the Ag, Bg1, and Bg2 mode symmetries of CuO, which are exclusively observed at the CuO NWs/Cu electrode. The presence of the CuO bands at the Cu(OH)2 NWs/ Cu electrode could be rationalized by the metastability of Cu(OH)2 with respect to dehydration by laser power during the Raman measurement, which was also observed for other hydroxide materials.[14] On the other hand, the absence of any Raman bands of Cu(OH)2 at the CuO NWs/Cu electrode indicates the complete conversion of Cu(OH)2 NWs into CuO NWs after calcination, consistent with the XRD results. In contrast to the XRD result, Ni and Fe at the NiFeOx@CuO NWs/Cu electrode are observable by Raman spectroscopy. The peaks at approximately 460 and 526 cm1 are attributed to the stretching NiO(H) bond and structural defects of Ni(OH)2.[15] The band located between 650–710 cm1 can be assigned to either g-FeOOH or Fe(OH)3.[16] We performed XPS on the four electrodes to further characterize their compositional changes during the fabrication process. The survey XPS are shown in Figure S1 (in the Supporting Information). In Figure 2 C, the Cu 2p XPS of copper foil exhibits peaks at 932.0 and 951.8 eV, consistent with the 2p3/2 and 2p1/2 binding energies of Cu0 metal. The small rounded peaks at 943.6 and 962.4 eV can be attributed to CuO/Cu(OH)2, which arises from the surface oxidation of Cu foil. These intense satellite peaks are characteristic of the Cu 2p XPS of Cu(OH)2 NWs/Cu and CuO NWs/Cu electrodes. The Cu 2p XPS of the two nanowires electrodes are similar with the exception of the peaks shifted toward lower binding energies from Cu(OH)2 to CuO. For the NiFeOx@CuO NWs/Cu electrode, the intensity of Cu 2p XPS signals is greatly decreased because of the overlaying of NiFeOx. The Ni 2p XPS in Figure 2 D displays two Figure 2. (A) XRD pattern, (B) Raman spectra, (C–E) high-resolution XPS of Cu 2p, Ni 2p, dominant peaks at 854.9 and 872.6 eV accompanied and Fe 2p, and (F–I) EDX spectra of copper foil (black), Cu(OH)2 NWs/Cu (red), CuO NWs/ by two less intense satellite peaks, consistent with Cu (green), and NiFeOx@CuO NWs/Cu (blue). ChemCatChem 2018, 10, 1 – 8

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Full Papers the presence of trivalent Ni, presumably NiOOH.[17] In the Fe 2p XPS in Figure 2 E, two dominant peaks are observed at binding energies of 710.8 and 725.2 eV, consistent with the presence of trivalent Fe.[18] The elemental composition was also investigated by energydispersive X-ray spectroscopy (EDX). As shown in Figure 2 F–I, the only elements observed at the Cu(OH)2 NWs/Cu and CuO NWs/Cu electrodes are copper and oxygen. As expected, the oxygen content at the latter electrode is significantly lower than that at the former one. At the NiFeOx@CuO NWs/Cu electrode, the EDX analysis reveals a molar ratio of nearly 1:1 for Ni (8.36 %) and Fe (9.67 %) (Table S1 in the Supporting Information). The earlier studies also revealed a similar optimized ratio to achieve the optimal performance of the FeNiOx catalysts.[12, 16a]

The Tafel plot in Figure 3 B of NiFeOx@CuO NWs/Cu reveals a Tafel slope of 36 mV dec1 for the electrode, which lies within the range of reported values (30–50 mV dec1) of well-known NiFeOx catalysts.[19] From the Tafel plot, the NiFeOx@CuO NWs/Cu electrode exhibits an impressive low overpotential of 300 mV to reach a current density of 100 mA for the OER, making it among the best NiFeOx catalysts supported at planar electrode substrates (Table S2 in the Supporting Information). Additional electrocatalytic data for NiFeOx@CuO NWs/Cu fabricated under various conditions for NiFeOx deposition are presented and discussed in Figures S2–S4 (in the Supporting Information). To further investigate the high surface area of CuO NWs/Cu, ECSA measurements of CuO/Cu and CuO NWs/Cu were taken (Figures S5 and S6 in the Supporting Information). The CuO NWs exhibit a much higher active surface area, which explains the high performance of NiFeOx@CuO NWs. The electrode exhibited high stability with nearly no decrease in current density during long-term electrolysis in 1 m KOH solution, as shown in Figure 3 C. After 15 h of electrolysis at an overpotential of 350 mV, no change in electrode structure and surface morphology was observable as seen in Figure 3 E,F. To further examine the electrode stability at different applied potentials, a multi-step chronocoulometry experiment was conducted. The controlled potential was increased from h = 250 to 500 mV with an increment of 25 mV per 600 s, and the corresponding change of current density was recorded. As shown in Figure 3 D, the response currents remained constant at each step, indicating the high stability of the electrode within a wide range of applied potentials. In addition, the fast response of currents to potential ramps indicated efficient charge transfer and mass diffusion across the nanostructured

Electrocatalysis The OER activity of the NiFeOx@CuO NWs/Cu electrode was investigated in 1 m KOH (pH 13.6). For reference, the performance of Cu(OH)2 NWs/Cu, CuO NWs/Cu, and NiFeOx coated on an untreated copper foil (NiFeOx@Cu) are also presented. As shown in Figure 3 A, the NiFeOx@CuO NWs/Cu electrode exhibits much higher performance than all the other reference electrodes, which is attributable to the high ECSA of the core–shell nanowires combined with the high catalytic activity of NiFeOx. The high ECSA of the NiFeOx@CuO NWs/Cu electrode is evident from the oxidation wave of NiII to NiIII, which is approximately 25 times larger than that of the unsophisticated NiFeOx@Cu electrode.

Figure 3. (A) Polarization curves of Cu(OH)2 NWs/Cu (black), CuO NWs/Cu (red), NiFeOx@Cu (green), and NiFeOx@CuO NWs/Cu (blue) of the same geometric surface area in 1 m KOH at 20 mV s1 and (B) the corresponding Tafel plots. (C) CPE of NiFeOx@CuO NWs/Cu at an overpotential of 350 mV. (D) Multi-potential process obtained at NiFeOx@CuO NWs/Cu. (E,F) SEM images of NiFeOx@CuO NWs/Cu after 15 h electrolysis.

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Figure 4. SEM images of Cu foam, that is, CF (A), Cu(OH)2 NWs/CF (B), CuO NWs/CF (C), and NiFeOx@CuO NWs/CF before (D,F) and after (E,G) long-term electrolysis. (H) Polarization curves of Cu(OH)2 NWs/CF (black), CuO NWs/CF (red), NiFeOx/CF (green), and NiFeOx@CuO NWs/CF (blue) in 1 m KOH at 20 mV s1. (I) Tafel plot of NiFeOx@CuO NWs/CF. (J) CPE of NiFeOx@CuO NWs/CF at an overpotential of 300 mV.

123 mA cm2 at an overpotential of 300 mV (Figure 4 J). The SEM images in Figure 4 E,G show that the electrode structure and surface morphology was well-retained during electrolysis, accounting for the high stability of the catalyst performance. Being able to establish the catalyst on a 2D as well on a 3D electrode opens a wide range of commercial applications.

electrode. The current curve became fluctuated at higher potentials owing to the disturbance of vigorous bubbles. At an overpotential of 500 mV, a current density of up to 230 mA cm2 was achieved. Extension to a three-dimensional copper foam We extended the strategy for the fabrication of the core–shell nanowires to a three-dimensional copper foam as the electrode substrate. The SEM images in Figure 4 A–D,F confirm the formation of the core–shell nanowires on the copper foam (NiFeOx@CuO NWs/Cu foam). As expected, the hierarchically structured 3D electrode leads to greatly improved catalytic performance in comparison with the reference electrodes (Figure 4 H). The catalyst also showed a low Tafel slope of 35 mV dec1 and an extremely low overpotential of 265 mV is required for a current density of 100 mA cm2 (Figure 4 I). As listed in Table S3 (in the Supporting Information), these values are among the best reported for 3D electrode substrates. The constant potential electrolysis (CPE) experiment shows a sustained current density of ChemCatChem 2018, 10, 1 – 8

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Conclusions We demonstrated the fabrication of hierarchically structured nanosheets/nanowires electrocatalysts based on earth-abundant 3d first-row transition metals—copper, iron, and nickel and their use for efficient electrocatalytic water oxidation. Ni and Fe were anodically co-deposited onto CuO nanowires, which were conveniently prepared by a simple solution-based method followed by a calcination procedure in air. The high specific surface area provided by CuO nanowires and the coated NiFeOx nanosheets, combined with the excellent catalytic properties of the NiFeOx led to efficient electrocatalytic water oxidation. To achieve a current density of 100 mA cm2, the as-constructed electrodes with planar copper foil and 5


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Full Papers were reported versus RHE with iR compensation, whereas all potentials in constant potential electrolysis (CPE) were reported versus RHE without iR compensation. All electrochemical experiments were performed at 22 2 8C.

three-dimensional copper foam require low overpotentials of only 300 mV and 265 mV, respectively. These electrodes can deliver sustained high current density without modification of the electrode structure and surface morphology. The use of inexpensive first-row metals, the fast and scalable preparation method, the unique hierarchical structure of nanosheets/nanowires, and the high catalytic performance with high catalyst stability are remarkable. In addition, the preparation of the catalyst by anodic deposition in alkaline solution makes it possible for incorporating the preparation in the OER process. This study may therefore lead to more convenient and competitive routes for water splitting.

Preparation of hierarchically structured electrode First, Cu(OH)2 nanowires were developed on a copper foil according to a method reported earlier.[10] Briefly, an aqueous solution (40 mL) containing 2 m NaOH and 0.1 m (NH4)2S2O8 was added into a 100 mL glass beaker. A piece of 4 cm 4 cm copper foil was then immersed in the solution, leaning on the beaker wall. After 25 min in the chemical bath, a layered film of Cu(OH)2 nanowires in light blue was formed on the surface of the copper foil (Cu(OH)2 NWs/ Cu). The foil was then rinsed by using ultrapure water and ethanol and dried in air. The conversion of Cu(OH)2 nanowires into CuO nanowires was achieved by calcination in air at 200 8C for 3 h with a ramping rate of 5 8C min1. The resulting electrode was designated as CuO NWs/Cu.

Experimental Section Chemicals Nickel(II) nitrate (Ni(NO3)2·6 H2O, 98 %), iron(III) sulfate (Fe2(SO4)3, 99 %), sodium carbonate (Na2CO3, > 99.8 %), sodium bicarbonate (NaHCO3, 99.3 %), potassium hydroxide (KOH, 99 %), sodium hydroxide (NaOH, 99 %), and ammonium persulfate ((NH4)2S2O8, 99 %) were obtained from Fisher Scientific. Copper foil and copper foam (1.6 mm thickness) were obtained from Delta Technologies, Ltd. All reagents were analytical grade and used as received. All electrolyte solutions were prepared by using Milli-Q ultrapure water (> 18 MW cm) unless stated otherwise. The concentrated carbonate solution of pH 10.8 was prepared by mixing Na2CO3 and NaHCO3 (c(CO32) + c(HCO3) = 2 m; molar ratio Na2CO3/NaHCO3 = 9:1). The pH for 1 m KOH is 13.6 according to literature reports.[9]

The CuO nanowires were coated by a layer of NiFeOx nanosheets by anodic co-deposition of Ni and Fe in concentrated carbonate solution,[11] forming the NiFeOx@CuO nanowires. Briefly, the copper foil with CuO nanowires was covered by scotch tape to define a geometric surface area of 0.25 cm2 on the front of the electrode for electrochemical measurements. To obtain a pure copper contact, one end of the electrode was scratched by a knife to remove the CuO nanowires layer. The NiFeOx catalyst was deposited onto the CuO nanowires by electrolysis at 1.74 V versus RHE for 1 h in 2 m Na2CO3 (pH 10.8) containing 1 mm NiII and 1 mm FeIII. The resulting electrode, designated as NiFeOx@CuO NWs/Cu, was then rinsed with water and ethanol and dried in air. For comparison, a pure copper foil was also coated with nickel iron by the same procedure to obtain the NiFeOx/Cu electrode.

Apparatus Scanning electron microscope (SEM) images and energy dispersive X-ray analysis (EDX) data were obtained with a Hitachi S-4800 (Hitachi, Japan) equipped with a Horiba EDX system (X-max, silicon drift X-Ray detector). After anodic electrodeposition, the NiFeOxcoated electrode was rinsed with deionized water and ethanol and dried in air before being loaded into the instrument. Transmission electron microscopy (TEM) images were obtained by using Tecnai G2 F20 S-Twin. The catalyst sample was removed from the electrode by sonication in absolute ethanol, and a drop of the mixture was dried on a micro grid copper network for analysis.

For preparation of the hierarchically structured three-dimensional electrode, a piece of copper foam was used as the electrode substrate and a similar procedure as for NiFeOx@CuO NWs/Cu was applied to develop NiFeOx@CuO nanowires on the three-dimensional copper foam. The resulting electrode is designated as NiFeOx@CuO NWs/CF.

Tafel plot The current–potential data of the electrodes were obtained by linear sweep voltammetry (LSV) at a very slow scan rate (0.1 mV s1). The Tafel slope was obtained from the LSV plot by using a linear fit applied to points in the Tafel region. The solution resistance measured prior to the data collection (using iR test function) was used to correct the Tafel plot for iR drop.

Powder X-ray diffraction (XRD) was measured by using a Bruker Focus D8 with ceramic monochromatized CuKa radiation of 1.54178 , operating at 40 kV and 40 mA. The scanning rate was 58 min1 in 2q and the scanning range was 15–808. Raman spectroscopy was conducted with a confocal microscope laser Raman spectrometer of Rainshaw invia. X-ray photoelectron spectroscopy (XPS) for elemental analysis was conducted with a Kratos Axis Ultra DLD X-ray Photoelectron Spectrometer using 60 W monochromated MgKa radiation as the X-ray source for excitation. The carbon 1s peak (284.6 eV) was used for internal calibration. Electrochemical measurements were performed with a CHI 660E electrochemical workstation (Chenhua Corp., Shanghai, China). The three-electrode system consisted of a working electrode, a platinum plate counter electrode, and a saturated calomel reference electrode (SCE, 0.244 V vs. NHE, the normal hydrogen electrode). Prior to each measurement, the platinum plate counter electrode was routinely treated by soaking in 1 m hydrochloric acid to remove any deposited Ni or Fe and afterwards rinsed with ultrapure water. Unless stated otherwise, all potentials in linear sweep voltammetry (LSV)

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Acknowledgments We thank the National Natural Science Foundation of China (21573160, 21405114), the Fundamental Research Funds for the Central Universities, and the Science & Technology Commission of Shanghai Municipality (14DZ2261100) for support.

Conflict of interest The authors declare no conflict of interest. 6



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Manuscript received: September 5, 2017 Revised manuscript received: September 30, 2017 Accepted manuscript online: && &&, 0000 Version of record online: && &&, 0000

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FULL PAPERS S. Czioska, J. Wang, S. Zuo, X. Teng, Z. Chen*

NiFe-based electrocatalysts: CuO nanowires were developed by a solution-based method on copper foil or copper foam and an anodic deposition method in basic carbonate buffer solution was utilized for preparation of hierarchically structured NiFeOx/CuO nanosheets/nanowires for high-performance oxygen evolution reactions (OER).

&& – && Hierarchically Structured NiFeOx/CuO Nanosheets/Nanowires as an Efficient Electrocatalyst for the Oxygen Evolution Reaction

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