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Mar 17, 2017 - J. Y. Wang, L. L. Ji, S. S. Zuo, Prof. Z. F. Chen. School of Chemical Science and Engineering. Tongji University. Shanghai 200092, China.
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Hierarchically Structured 3D Integrated Electrodes by Galvanic Replacement Reaction for Highly Efficient Water Splitting Jianying Wang, Lvlv Ji, Shangshang Zuo, and Zuofeng Chen* side, with the same iron group elements, their phosphide compounds recently have been intensively investigated as new earthabundant electrocatalysts that can potentially replace Pt to catalyze the HER.[23–32] In both cases, the electrocatalyst materials were typically electrodeposited or prepared by solid-state reaction or hydrothermal reaction followed by cast as an extraneous layer onto electrode substrate surfaces using electrical insulating binding agents such as Nafion.[15,33,34] These fabrication procedures increase the cost and energy consumption for catalyst preparation and also decrease catalyst performance stability. The spontaneous galvanic replacement reaction (GRR) is a classical single-step reaction. Driven by the different electrochemical potentials between two substances, the GRR causes the deposition of noble elements and the dissolution of other elements.[35] The electroless nature of the GRR offers the significant advantages of simplicity and zero energy consumption. However, most studies have been performed with noble metals, such as Pt, Pd, Au, and Ag, as substitution substances for electrocatalyst fabrication.[36–39] The fabrication of the OER and HER electrocatalysts with first-row transition metals using this method is desirable but has not been explored, which necessitates judicious choice of catalytically synergistic components with appropriate electrochemical potential difference. In this study, we prepared the NiFe-based electrocatalysts for water splitting by means of the GRR as the main step for fabrication of both OER and HER electrocatalysts, which can simplify the fabrication procedure of electrolyzers, substantially lower the production costs, and improve the catalytic performance. The fabrication process and the utilization of the electrode materials for the overall water splitting are illustrated in Scheme 1. The GRR-facilitated electrode fabrication was accomplished by simply immersing a piece of 3D iron foam (IF) into a solution containing Ni(II) cations, which required no complex instrumentation, only a beaker. The NiFe integrated electrode can be used immediately for the OER; otherwise, it is simply pretreated by cyclic voltammetry (CV) to oxidize the surface Fe and Ni, which eliminates the occurrence of corrosive microcell reactions during long-term storage of the electrode. A layered NiFe-based film of uniform nanosheets was formed on the iron

A NiFe-based integrated electrode is fabricated by the spontaneous galvanic replacement reaction on an iron foam. Driven by the different electrochemical potentials between Ni and Fe, the dissolution of surface Fe occurs with electroless plating of Ni on iron foam with no need to access instrumentation and input energy. A facile cyclic voltammetry treatment is subsequently applied to convert the metallic NiFe to NiFeOx. A series of analytical methods indicates formation of a NiFeOx film of nanosheets on the iron foam surface. This hierarchically structured three dimensional electrode displays high activity and durability against water oxidation. In 1 m KOH, a current density of 1000 mA cm−2 is achieved at an overpotential of only 300 mV. This method is readily extended to fabricate CoFe or NiCoFe-based integrated electrodes for water oxidation. Phosphorization of the bimetallic oxide (NiFeOx) generates the bimetallic phosphide (NiFe-P), which can act as an excellent electrocatalyst for hydrogen production in 1 m KOH. An alkaline electrolyzer is constructed using NiFeOx and NiFe-P coated iron foams as anode and cathode, which can realize overall water splitting with a current density of 100 mA cm−2 at an overpotential of 630 mV.

1. Introduction Water splitting to produce hydrogen represents one of the most attractive ways to obtain clean and sustainable energy.[1–4] It is composed of two half cell reactions, i.e., the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) with the former providing the reductive equivalents and protons for the latter.[4–6] For both half reactions, catalysts are the key to expedite the kinetics and thus reduce the energy input for hydrogen production from electrolysis. To obtain more economical and efficient catalysts for practical applications, the catalysts should be made of inexpensive materials and prepared in a way that is as simple as possible.[7] Over the past decade, significant advances have been achieved in developing OER catalysts based on the first-row transition metals.[8–15] It has now been well-established that the combination of iron and nickel can significantly increase the OER activity by taking advantage of synergistic metal–metal interactions.[16–22] On the other J. Y. Wang, L. L. Ji, S. S. Zuo, Prof. Z. F. Chen School of Chemical Science and Engineering Tongji University Shanghai 200092, China E-mail: [email protected]

DOI: 10.1002/aenm.201700107

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Scheme 1.  Fabrication process of the NiFeOx/IF and NiFe-P/IF electrodes and their utilization for overall water splitting.

foam surface, which afforded a hierarchically structured 3D electrode. The integrated electrode displayed high structural strength, large specific surface area, and excellent water oxidation activity and durability. This electroless plating method has been extended to fabricate other bimetallic (i.e., CoFe) or even trimetallic (i.e., NiCoFe) integrated electrodes and their catalytic activity toward the OER was also tested. On the basis of the hierarchically structured 3D NiFeOx/IF electrode, we further developed a convenient and straightforward method to fabricate a bimetallic phosphide (i.e., NiFe-P/ IF) electrode for the HER, which was realized by a simple onestep phosphorization of the NiFeOx/IF electrode. The electrode material maintains the nanosheet-like morphology after phosphorization and exhibits excellent catalytic performance toward the HER in alkaline solution. With both NiFeOx and NiFe-P electrocatalysts available, we assembled an alkaline electrolyzer for overall water splitting which could deliver a current density of 100 mA cm−2 at an applied cell voltage of only 1.86 V.

the iron foam by formation of iron(III) hydroxide, as seen in Equation (1) and (2) (1) Ni (cathode ) O2 + 2H2O + 4e −  4OH− (2) To enable the long-term storage, the GRR NiFe/IF electrode was pretreated by CV to oxidize the surface Ni and Fe. Figure 1B shows the linear scan voltammetry (LSV) plots for the untreated and CV-treated GRR NiFe/IF electrodes in 1 m KOH solution. The NiFe/IF electrode without CV treatment exhibited an anodic current density of ≈15 mA cm−2 between 1.1 and 1.4 V versus RHE (reversible hydrogen electrode), resulting from oxidation of the metallic nickel and iron on the iron foam surface. In contrast, the electrodes with CV treatment of three

2. Fabrication of the NiFe-Integrated Electrode Iron foam is commercially available at low cost. In this study, the NiFe-integrated electrode was prepared by the GRR by simply immersing the iron foam in a NiSO4 aqueous solution for different times to regulate the surface Ni content and the activity toward the OER or HER; in the latter, the electrocatalyst was facilely fabricated by further phosphorization (discussed later). As shown in Figure 1A, the GRR NiFe/IF electrode is unstable during storage, and its color changed from grey/black to dark yellow after sitting in air for 24 h. Because of the different electrochemical potentials between Ni and Fe, numerous microcells form on the iron foam surface, with Fe as the anode and Ni as the cathode and adsorbed O2 and water as the electrolytes. As a result, electrocorrosion occurs, which will corrode 1700107  (2 of 8)

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Figure 1.  A) Images of the bare iron foam (upper), the freshly prepared GRR NiFe/IF electrode (middle), and the untreated or CV-treated GRR NiFe/IF electrodes after exposure to air for 24 h (lower). B) LSV plots of the untreated or CV-treated GRR NiFe/IF electrodes in 1 m KOH solution. Scan rate: 20 mV s−1. The GRR NiFe/IF electrode was fabricated by immersing in 50 × 10−3 m NiSO4 solution for 2 h.

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to five cycles exhibited only a small double-layered capacitance charging current in the same potential range, with no evidence of surface oxidation. As shown in Figure 1A, after CV treatment, the corrosion of the iron foam was essentially inhibited, with the electrode intact after exposure to air for a prolonged period of time, which was attributed to the formation of a layer of protective metal oxides on the iron foam surface. Therefore, prior to further measurements, the integrated electrode was subjected to five cycles of CV treatment, which converted the GRR NiFe/IF electrode to the GRR NiFeOx/IF electrode in the event that the electrode was not used immediately.

3. The OER Electrocatalysis Figure 2A shows CV plots of the bare iron foam, and the GRR NiFeOx/IF electrodes prepared with different immersion times for the OER in 1 m KOH solution. The bare iron foam electrode showed negligible catalytic current within the studied potential range. By contrast, the GRR NiFeOx/IF electrodes exhibited significantly enhanced catalytic currents dependent on the immersion time. In general, the catalytic activity of the electrode increased rapidly when the immersion time was initially increased, and it then gradually decreased by further prolonging the immersion time. With an optimized immersion time of 2 h, the catalytic current density was maximized. The CV of the GRR NiFeOx/IF electrode featured an apparent redox couple before the catalytic OER onset, consistent with conversion of incorporated divalent Ni to/from trivalent Ni. Figure S1 (Supporting Information) provides additional CV data of the GRR NiFeOx/IF electrode in KOH solutions of different concentrations. Figure S2 (Supporting Information) provides the electrochemical impedance spectroscopy (EIS) spectra of the bare iron

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Figure 2.  A) CV plots of an IF and NiFeOx/IF electrodes prepared by different immersion time at 20 mV s−1. B) Tafel plot of the GRR NiFeOx/ IF electrode. C) CPE of the GRR NiFeOx/IF electrode at 1.71 V versus RHE. D) Multicurrent process obtained with the GRR NiFeOx/IF electrode. (A,B) were obtained in 1 m KOH solution and (C,D) were obtained in 10 m KOH.

foam and the GRR NiFeOx/IF electrode in 1 m KOH and the GRR NiFeOx/IF electrode in KOH solutions of different concentrations, which reveal systematically variable charge transfer resistance in consistence with the results of CVs. Tafel measurements were conducted to evaluate the intrinsic catalytic activity of the GRR NiFeOx/IF electrode in the OER. According to the Tafel equation, the Tafel slope was obtained from the polarization curve (0.1 mV s−1) using a linear fit applied to points in the Tafel region. As shown in Figure 2B, the GRR NiFeOx/IF electrode displays a Tafel slope of 34 mV dec−1 in 1 m KOH. The Tafel slopes were nearly identical in KOH solutions of different concentrations (Figure S3, Supporting Information). From the Tafel plot, an appreciable catalytic current of 5 mA cm−2 was observed with an overpotential of 220 mV and a current density of 1000 mA cm−2 was achieved with an overpotential of only 300 mV in 1 m KOH. The Tafel slope was also obtained by EIS analysis, which eliminated the effects of some arbitrary factors, such as different choices of overpotential region and different means for iR-compensation. Figure S4 (Supporting Information) shows the Nyquist plots of the GRR NiFeOx/IF electrode at different overpotentials. By fitting the experimental data, a model of two time-constants can be used to describe the OER behavior. According to the Nyquist plots and the corresponding equivalent circuit model, the value of the charge transfer resistances (Rct) at different overpotentials (η) can be conveniently extracted. The plot of η versus logRct−1 shows a Tafel slope of 36 mV dec−1 for the GRR NiFeOx/IF electrode in 1 m KOH. This value was slightly higher than that obtained based on the polarization curve but is considered to be more comprehensive. Compared to previously reported 3D OER systems that have exclusively used nickel foams as the electrode substrates to maximize the catalytic current densities, the iron foam-based integrated electrode exhibits competitive or even superior catalytic activity under similar conditions (Table S1, Supporting Information). To test the stability of the NiFeOx/IF electrode under vigorous OER conditions, a controlled potential electrolysis (CPE) experiment was conducted at 1.71 V versus RHE in 10 m KOH solution, which serves as a harsher medium for the stability test. As shown in Figure 2C, a sustained current density of 1400 mA cm−2 was achieved during a prolonged electrolysis period of 10 h. Vigorous effervescence was observed at the electrode surface during electrolysis. Unlike electrodeposited or cast catalysts on supporting substrates, the consubstantial growth by the GRR in this study is believed to prevent the exfoliation of electrocatalysts, which becomes critical under violent O2 evolution conditions.[40] To further examine the electrode stability at different catalytic current densities, a multistep chronopotentiometry experiment was conducted in 10 m KOH solution. The current density was increased from 200 to 2400 mA cm−2 with an increment of 200 mA cm−2 per 600 s, and the corresponding changes of potential were recorded. As shown in Figure 2D, the response potentials remained constant at each step, indicating the high stability of the electrode within a wide range of current densities. In addition, the fast response of potentials to current ramps and the gradual decrease in the step height with increasing current density indicated efficient charge transfer and mass diffusion across the 3D electrode.

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Table 1.  Comparison of the NiFeOx/IF electrodes prepared by different immersion times in terms of nickel content, catalytic current, and Nibased TOF in 1 m KOH. Immersion time [h]

Ni content [μmol cm−2]

j [mA cm−2] at η = 300 mV

TOFNi [s−1] at η = 300 mV

0.25

0.75

270

0.94

0.5

0.88

490

1.45

1

1.3

880

1.76

2

2.3

1000

1.13

6

10.7

770

0.19

The quantity of deposited Ni atoms was determined by dissolving the catalyst film in 0.1 m HNO3 followed by analysis with inductively coupled plasma optical emission spectroscopy. By the working-curve method, Table 1 lists the nickel content (geometrically per square centimeter) deposited on the iron foam by different immersion times. By increasing the immersion time from 0.25 to 6 h, the surface Ni content changed from 0.75 to 10.7 μmol cm−2. Based on the number of deposited Ni atoms, the turnover frequency (TOF) of the NiFeOx film was calculated, which was also listed in Table 1. A higher Ni content will not only provide more catalytic active sites but also cause larger charge transfer resistance across the thicker active layer. Consequently, the GRR NiFeOx/IF electrode prepared by a 2 h immersion time exhibited the highest catalytic current density of 1000 mA cm−2 at η = 300 mV from the Tafel plot, while the electrode fabricated with 1 h immersion time exhibited the highest TOF of 1.76 s−1 at the same overpotential due to the presence of a smaller amount of Ni.

capacitance measurements in Figure S5 (Supporting Information), the GRR NiFeOx/IF electrode exhibited a high roughness factor of 230 in comparison with a planar electrode of NiFeOx. The hierarchically structured 3D electrode could also facilitate the dispersion of gas bubbles generated at high current density. Figure S6 (Supporting Information) provides additional SEM images of the GRR NiFeOx/IF electrodes prepared by different immersion times. In general, the increase in the immersion time (0–2 h) increased the formation of an active nanosheet-like film on the surface which is necessary for the catalytic water oxidation; however, a continuously increasing film thickness (6 h) may block rapid electron transfer from/to the underlying iron foam substrate, resulting in the decrease in catalytic current as shown in Figure 1A. The consubstantial growth of the NiFe-based layer by the GRR was further demonstrated by transmission electron microscopy (TEM) images. In Figure 3C,D, the TEM images

4. Characterization The morphology and structure of the GRR NiFeOx/IF electrode fabricated by 2 h immersion time were investigated by scanning electron microscopy (SEM). Figure 3A shows that the GRR NiFeOx/IF electrode retained the 3D interconnected macroscopic porous structure. The high-resolution SEM (HRSEM) image in Figure 3B shows that the surface of the iron foam was covered by layered, wellarranged nanosheets that were ≈60 nm in size. These nanosheets constituted a welldefined porous structure on the iron foam surface with pore sizes of 30–50 nm. The hierarchical coating of the porous nanosheet structure on the 3D iron foam maximized the specific surface area and thus provided abundant active sites for electrocatalysis on the electrode surface. As determined from the 1700107  (4 of 8)

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Figure 3.  A,B) SEM and HRSEM images of the NiFeOx/IF electrode obtained from immersing the IF electrode in 50 × 10−3 m NiSO4 solution for 2 h. C,D) TEM, E) HRTEM, and F) TEM–EDS mapping images of the NiFeOx nanosheets scrapped from the NiFeOx/IF electrode; the inset in (E) is the SAED pattern.

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vibrations in NiOOH.[18,42,43] The Raman bands of Fe oxyhydroxide or hydroxide species are generally overlapped with those of NiOOH, which could not be distinguished from the spectrum.[18] The oxidation states of the GRR NiFeOx/IF electrode were elucidated by X-ray photoelectron spectroscopy (XPS) measurements. The survey XPS spectrum in Figure 4C corroborates the coexistence of Fe, Ni, and O on the electrode surface with an atomic ratio of ≈3:1 for Ni:Fe, close to that obtained in the TEM–EDS analysis. Figure 4D–F shows the high-resolution XPS spectra of Fe 2p, Ni 2p, and O 1s, respectively. In the Fe 2p XPS spectrum, two dominant peaks were observed at binding energies of 710.8 and 725.2 eV, consistent with the presence of trivalent Fe.[16,20,44,45] The Ni 2p spectrum displayed two dominant peaks at 855 and 872.6 eV, accompanied by two less intense satellite peaks, consistent with the presence of trivalent Ni, presumably NiOOH in the GRR NiFeOx/IF electrode.[44] The O 1s spectrum showed three oxygen contributions, which were denoted as O1, O2, and O3. Specifically, the component O1 at 528.9 eV is typical of metal–oxygen bonds.[45,46] The component O2 at 530.3 eV is usually associated with oxygen in OH− groups as a result of either surface oxyhydroxide or the substitution of oxygen atoms at the surface by hydroxyl groups.[47,48] The component O3 at 531.6 eV was attributed to a higher number of defect sites with low oxygen coordination within the oxide crystal, to adsorbed oxygen, or hydroxide species at the surface.[49]

5. Extension to CoFe and NiCoFe Integrated Electrodes

Figure 4.  A) XRD patterns of the bare iron foam and the GRR NiFe/IF electrode. B) Raman spectra of the bare iron foam and the GRR NiFeOx/ IF electrode. C) The survey XPS spectrum and the high-resolution XPS spectra of D) Fe 2p, E) Ni 2p, and F) O 1s of the GRR NiFeOx/IF electrode.

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To demonstrate the versatility of the GRR as a way to make high-performance electrocatalysts based on the iron foam substrate, we also fabricated CoFeOx/IF and NiCoFeOx/IF electrodes and used them for the OER. As shown in Figure 5A,B, the iron foam electrode was also coated with nanosheets by immersing the iron foam in CoSO4 solution for 2 h. Figure 5C shows that Fe, Co, and O were distributed uniformly within the nanosheets. Figure 5D shows the electrocatalytic performance of the CoFeOx/IF electrode. The catalytic onset in CV occurred at 1.47 V versus RHE (η = 240 mV) in 1 m KOH. During electrolysis, a sustained catalytic current density of 280 mA cm−2 was achieved at 1.65 V versus RHE (η = 420 mV) in 1 m KOH solution. In Figure S8 (Supporting Information), Tafel slopes of 39 and 40 mV dec−1 were obtained in 1 m KOH solution from the polarization curve and EIS analysis, respectively. Similarly, the trimetallic-integrated NiCoFeOx/IF electrode was prepared by the GRR in a mixed solution of Ni(II) and Co(II). The relative amount of Ni and Co on the NiCoFeOx/IF electrode could be conveniently controlled by varying the molar ratio of Ni(II) and Co(II) in the GRR solution while fixing the total moles of Ni(II) and Co(II). In addition, the ratio of Ni and/ or Co to Fe in the trimetallic-integrated film could be conveniently controlled by different immersion times. The SEM images in Figure S9A,B (Supporting Information) show that the GRR NiCoFeOx/IF electrode prepared with 1:1 Ni(II):Co(II) and by 2 h immersion time exhibited similar morphology of nanosheet microstructure with those of bimetallic-integrated electrodes.

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of the material scraped from the GRR NiFeOx/IF electrode exhibits a clear nanosheet structure, consistent with the SEM images. The high-resolution TEM (HRTEM) image in Figure 3E demonstrates a fringe spacing of 0.25 nm in certain domains, which agrees well with the spacing of the (012) lattice plane of NiFe oxyhydroxide.[16,41] The selected-area electron diffraction (SAED) pattern of the nanosheets shows barely discernible diffraction rings (inset of Figure 3E), indicating the poor polycrystalline nature of this material. In Figure 3F, the TEM–energy dispersive X-ray spectroscopy (EDS) mapping image recorded from the nanosheet shows that the Fe, Ni, and O elements are distributed uniformly, indicating the consubstantial growth of NiFe oxyhydroxide. In Figure S7 (Supporting Information), the quantitative analysis by TEM–EDS reveals an atomic ratio of Ni:Fe of ≈2.5:1. The integrated electrode was further characterized by other analysis techniques. The X-ray diffraction (XRD) pattern in Figure 4A shows that the bare iron foam exhibits diffraction peaks of (110) and (200) (JCPDS No. 01-1262). At the GRR NiFe/IF electrode, these background signals were greatly decreased with the concomitant appearance of three diffraction peaks of (111), (200), and (220) (JCPDS No. 04-0850) for metallic Ni. Figure 4B shows Raman spectra of the bare iron foam and the GRR NiFeOx/IF electrodes. No Raman signals were detected for the bare iron foam. At the GRR NiFeOx/ IF electrode, the Raman spectrum exhibited a pair of broad peaks between 200 and 650 cm−1, which is attributed to NiO

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Figure 5.  A,B) SEM of different magnifications and C) SEM–EDX mapping images of the GRR CoFeOx/IF electrode. D) CV plots of the bare IF electrode and the GRR CoFeOx/IF electrode in 1 m KOH at 20 mV s−1. Inset: CPE of the GRR CoFeOx/IF electrode at 1.65 V versus RHE.

The SEM–EDX mapping images in Figure S9C (Supporting Information) demonstrate that the Ni, Co, and Fe elements were distributed uniformly within the nanosheets. Figure S10 (Supporting Information) shows CV plots of the NiCoFeOx/IF electrodes prepared with varying the molar ratio of Ni(II) and Co(II) by 2 h immersion time. In general, the oxidation of Ni2+ to Ni3+ prior to the catalytic onset was facilitated by increasing Co in the film; on the contrary, the corresponding electrocatalytic activity was slightly decreased. An earlier study on the simpler Ni-based electrocatalyst with incorporated La, Mn, Ce, and Ti has also revealed no clear correlation between the catalytic activity and the nominal Ni2+/3+ redox potential.[50] The observation in this study and elsewhere in the literature that the anodic prefeature of Ni is not directly correlated with the OER activity is mechanistically important and may have implications for future catalyst design.

at around 32.0°, and 49.3°, corresponding to (011) and (103) in the FeP phase (JCPDS No. 89-4863). The strong diffraction peaks located at around 2θ = 44.6° and 64.7° arise from the iron foam substrate. Figure S12 (Supporting Information) shows the high-resolution XPS spectra of Fe 2p, Ni 2p, and P 2p of the NiFe-P/IF electrode. In the Fe 2p XPS spectrum, two peaks are observed at ≈707.1 and 720.3 eV, which correspond to the binding energies of Fe in FeP.[23,51] The Ni 2p spectrum displays two peaks at 853.1 and 869.3 eV, consistent with the binding energies of Ni in Ni2P.[24,25] The other peaks in XPS spectra of Ni 2p and Fe 2p are associated with nickel and iron oxides/oxyhydroxides as observed in Figure 4D,E. In the P 2p XPS spectrum, the peaks at 129.4 and 134.0 eV are attributed to the phosphide signal and PO species, respectively.[23–26] We evaluated the HER activity of the NiFe-P/IF electrode in 1 m KOH solution. For comparison, the iron foam, the NiFe/ IF electrode, and the commercial 20% Pt/C electrocatalyst were also examined. Figure 6A shows the LSV curves of these electrode materials at a scan rate of 2 mV s−1. The iron foam and the NiFe/IF electrode exhibit poor activities toward the HER. After phosphorization, the NiFe-P/IF electrode displays remarkably enhanced catalytic performance, achieving a current density of 200 mA cm−2 at an overpotential of 220 mV. Figure 6B shows that the Tafel slope of NiFe-P/IF obtained from the polarization curve (0.1 mV s−1) is 78 mV dec−1. The NiFe-P/IF electrode also exhibits high durability during the HER. After being subjected to 3000 CVs between 0 and −0.6 V versus RHE in 1 m KOH solution, the NiFe-P/IF electrode experienced negligible catalytic degradation, as shown in Figure 6C. In the studied potential range, the oxidation of the metal phosphide was not expected to occur.[52] To further test its durability, a long-term CPE experiment was conducted at an overpotential of 200 mV. Figure 6D shows that a catalytic

6. The HER Electrocatalysis and the Overall Water Splitting Recently, the transition metal (such as Ni and Fe) phosphides were found to be active for the HER.[23–25] In this study, the bimetallic phosphide, i.e., NiFe-P coated on the surface of iron foam was facilely fabricated via a direct phosphorization process of NiFeOx (see details of the Experimental Section in the Supporting Information). Figure S11A (Supporting Information) shows the SEM image of the NiFe-P/IF electrode by phosphorization. The morphology of the electrode after phosphorization appears nearly unchanged with the exception that the surface of nanosheets becomes rougher. As depicted in Figure S11B (Supporting Information), the XRD pattern of NiFe-P/IF shows three diffraction peaks at around 40.7°, 47.4°, and 54.2°, corresponding to (111), (210), and (300) in the Ni2P phase (JCPDS No. 74-1385). In addition, two diffraction peaks were observed 1700107  (6 of 8)

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Figure 6.  A) LSVs of bare IF, NiFe/IF, NiFe-P/IF, and 20% Pt/C in 1 m KOH solution. Scan rate: 2 mV s−1. B) Tafel plots of NiFe-P/IF and 20% Pt/C. C) LSVs of NiFe-P/IF before and after potential sweeps (0 to –0.6 V vs RHE, 50 mV s−1) for 3000 cycles. D) CPE of NiFe-P/IF electrode at an overpotential of 200 mV for 20 h.

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current density of 50 mA cm−2 was obtained and sustained for at least 20 h. Compared to previously reported metal phosphide electrocatalysts which were typically electrodeposited or cast on the supportive electrodes, our iron foam-based integrated electrode exhibits competitive or superior catalytic performance under similar conditions (Table S2, Supporting Information). As mentioned above, the consubstantial growth strategy by the GRR is believed to ensure good electrical contact between the electrocatalyst and the conductive substrate and prevent the exfoliation of catalysts. With high-catalytic-performance NiFeOx/IF and NiFe-P/ IF available, we built an alkaline electrolyzer for overall water splitting using these electrode materials as anode and cathode, respectively. Figure 7A shows the LSV curve of the electrolyzer in 1 m KOH solution. An anodic wave was observed prior to the occurrence of water splitting, which was due to the oxidation of Ni(II) to high oxidation states. The catalytic onset (20 mA cm−2) appeared at an applied cell voltage of 1.6 V, corresponding to an overpotential of 370 mV for overall water splitting. This performance corresponds to an anodic overpotential of 240 mV to achieve an OER current density of 20 mA cm−2 at NiFeOx and a cathodic overpotential of 130 mV to achieve the same HER current density at NiFe-P. Vigorous gas bubbles were observed at both cathode and anode during the LSV measurement. The overall water splitting performance of the NiFeOx/ NiFe-P electrolyzer at various current densities from 20 to 100 mA cm−2 was investigated using chronopotentiometry (CP) for a period of 24 h, as shown in Figure 7B. At the beginning of CP test at 20 mA cm−2, the cell voltage increases initially and finally gets stabilized at 1.70 V. Upon increasing the current density to 30, 50, and 100 mA cm−2, the cell voltage increases accordingly and quickly becomes stable at 1.73, 1.78, and 1.86 V, respectively. When the current density comes back to 50 mA cm−2, the cell voltage restores to 1.79 V with only a very slight increase. These results demonstrate clearly the high performance of the NiFeOx/NiFe-P electrolyzer for the overall water splitting in alkaline solution.

7. Conclusions In summary, we demonstrated that the low-cost iron foam could be used as an ideal 3D electrode substrate for the fabrication of integrated bimetallic or even trimetallic electrocatalysts by the spontaneous galvanic replacement reaction. Owing

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8. Experimental Section All experimental details are included in the Supporting Information.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by the National Natural Science Foundation of China (21573160 and 21405114), the Recruitment Program of Global Youth Experts by China, and the Science & Technology Commission of Shanghai Municipality (14DZ2261100). Received: January 11, 2017 Revised: February 1, 2017 Published online: [1] T. R. Cook, D. K. Dogutan, S. Y. Reece, Y. Surendranath, T. S. Teets, D. G. Nocera, Chem. Rev. 2010, 110, 6474. [2] N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. USA 2006, 103, 15729. [3] M. S. Dresselhaus, I. L. Thomas, Nature 2001, 414, 332. [4] M. G. Walter, E. L. Warren, J. R. Mckone, S. W. Boettcher, Q. Mi, E. A. Santori, N. S. Lewis, Chem. Rev. 2010, 110, 6446. [5] M. W. Kanan, D. G. Nocera, Science 2008, 321, 1072. [6] J. Luo, J. H. Im, M. T. Mayer, M. Schreier, M. K. Nazeeruddin, N. G. Park, S. D. Tilley, H. J. Fan, M. Gratzel, Science 2014, 345, 1593. [7] J. R. Mckone, B. F. Sadtler, C. A. Werlang, N. S. Lewis, H. B. Gray, ACS Catal. 2013, 3, 166. [8] J. Du, Z. Chen, S. Ye, B. J. Wiley, T. J. Meyer, Angew. Chem., Int. Ed. 2015, 54, 2073. [9] Y. Wu, M. Chen, Y. Han, H. Luo, X. Su, M.-T. Zhang, X. Lin, J. Sun, L. Wang, L. Deng, W. Zhang, R. Cao, Angew. Chem., Int. Ed. 2015, 127, 4952.

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Figure 7.  A) LSV of the NiFeOx/NiFe-P electrolyzer. Scan rate: 2 mV s−1. B) Chronopotentiometric curve of the NiFeOx/NiFe-P electrolyzer recorded at different current densities.

to the noble nature of Ni and Co relative to Fe, the NiFe-, CoFe-, and NiCoFe-based electrodes were produced by simply immersing the iron foam in a solution of Ni(II), Co(II), or their mixture, respectively. A facile CV pretreatment was subsequently applied to oxidize the surface NiFe which eliminated the occurrence of corrosive microcell reactions between Ni and Fe domains and enabled the storage of the electrode in the air atmosphere. The integrated 3D electrode was hierarchically structured with a coated NiFeOx/CoFeOx/NiCoFeOx film of uniform nanosheets which were able to be converted to the corresponding phosphide coatings by phosphorization. In alkaline solution, the NiFeOx and NiFe-P electrodes exhibited high catalytic activity and durability toward the OER and HER, respectively, and an electrolyzer combining both electrode materials was constructed to realize efficient overall water splitting. The simplicity of the GRR, which requires no complex instrumentation or energy input, the low-cost of iron foam as 3D electrode substrates, the bifunctionality of NiFe-based materials with related fabrication procedure, and the high performance of the coatings with unique microstructures are appealing and may be of value for real applications in energy conversion and storage.

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