Ni

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Letter

Improving Electrocatalysts for Oxygen Evolution Using NixFe3xO4/Ni Hybrid Nanostructures Formed by Solvothermal Synthesis Jinzhen Huang, Jiecai Han, Ran Wang, Yuanyuan Zhang, Xianjie Wang, Xinghong Zhang, Zhihua Zhang, Yumin Zhang, Bo Song, and Song Jin ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00888 • Publication Date (Web): 22 Jun 2018 Downloaded from http://pubs.acs.org on June 23, 2018

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ACS Energy Letters

Improving Electrocatalysts for Oxygen Evolution Using NixFe3-xO4/Ni Hybrid Nanostructures Formed by Solvothermal Synthesis Jinzhen Huang,1 Jiecai Han,1 Ran Wang,1 Yuanyuan Zhang,2 Xianjie Wang,2 Xinghong Zhang,*,1 Zhihua Zhang,3 Yumin Zhang,1 Bo Song*,1,2,4,5 and Song Jin*,6 1

Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150001,

China 2

Department of Physics, Harbin Institute of Technology, Harbin 150001, China

3

School of Materials Science and Engineering, Dalian Jiaotong University, Dalian 116028, China

4

Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology,

Harbin 150001, China; 5

School of Advanced Study, Taizhou University, Taizhou, 317000, China

6

Department of Chemistry, University of Wisconsin–Madison, 1101 University Avenue,

Madison, Wisconsin 53706, USA ABSTRACT: Spinel-type oxides have been found to be active electrocatalysts for OER. However, their semiconductor character severely limits their catalytic performance. Herein, we reported a facile solvothermal pathway for the synthesis of spinel-type NixFe3−xO4 oxides/Ni metal nanocomposites. The good electrical contact between metal and semiconductor oxide interface and well-tuned compositions of NixFe3-xO4 spinel oxides are crucial to achieve better OER performance. Specifically, the NixFe3−xO4/Ni nanocomposite sample prepared from a metal

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precursor ratio of y = 0.15 [y = Fe/(Fe + Ni)] that results in an x value of about 0.36 exhibits catalytic activity with an overpotential of 225 mV to achieve electrocatalytic current density of j = 10 mA cm−2 and a Tafel slope of 44 mV dec−1 in alkaline electrolyte. This study not only provides new perspectives to designing nanocomposite catalysts for OER, but also opens a promising avenue for further enhancing electrocatalytic performance via interface and composition engineering.

Table of Contents Graphic

The increase in global energy demands, depletion of conventional fossil fuels, and the negative impacts of burning fossil fuels on environment promote the development of clean and sustainable alternative energy sources and technologies.1-3 Hydrogen is regarded as a future promising renewable energy resource when it can be produced efficiently and inexpensively.4 Solar or electricity-driven water splitting is an efficient, cost-effective technique to produce hydrogen fuel on large scale.5-10 However, the current bottleneck to further improve the watersplitting technique is still the oxygen evolution reaction process. Even the noble metal oxides


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ACS Energy Letters

(for instance, rutile-type IrO2 and RuO2) or Ir-/Ru-based multimetallic compounds11-14 still require large overpotentials (η) to overcome the kinetic barriers in OER process that involves four sequential proton-coupled electron transfer and oxygen–oxygen bond formation.11,

15, 16

Moreover, the scarcity and high cost of the noble metal-based electrocatalysts severely limit their large-scale applications in energy conversion. Significant research efforts have been devoted to exploring earth-abundant first-row (3d) transition metal oxides (TMO)/hydroxides compounds, including the spinel-type oxides,17-21 perovskite-type oxides,22-26 and (layered double-) hydroxides,15,

27-37

as efficient catalysts for

OER. However, finding the ideal high performance OER catalysts is challenging, because the metal oxides must possess high catalytic activity, excellent electrical conductivity and chemical stability. Since most metal oxides are insulators or wide bandgap semiconductors thus electrically not very conducting, the efficient production of oxygen at the electrode level could be severely hampered. Besides, the electron transfer within the film and the mass transport of the ions in and the O2 out of the catalyst layer is also an important issue that should be considered in the catalysts since higher electrical conductivity can effectively improve the OER activity, which could be achieved by micro- and nano-scale engineering of the film architecture to facilitate the electron and mass transfer, and then guarantee the accessibility of active sites during the OER.38, 39

Specifically, a wide variety of spinel-type metal oxides, such as CoFe2O4, NiFe2O4, NiCo2O4,

have been directly grown on various three-dimensional (3D) porous conductive substrates (for example, nickel foam, iron foam, carbon cloth, etc.) to overcome the issues related to conductivity.40-43 However, utilization of such porous substrates poses limitations, including: (i) the potential interfacial resistance between the metal oxides and the substrates caused by the oxidation of substrate surface during the growth process, hinders further improvement of the


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electrochemical reaction kinetics; and (ii) the lack of reliable procedures for determining the high specific surface area, and then, establishing structure–performance relationships between the atomic-level surface structures and the catalytic activity. Although graphene44-46 and carbon nanotubes47, 48 have been applied in nanocomposites to improve the catalytic performance of spinel oxides, the coupling of the spinel-type oxides with metal nanoparticles (NPs) at the nanoscale level could be a more promising strategy to address the conductivity issues. In fact, it has been demonstrated that the interactions between metal and oxides can enhance the OER activity in nanocomposites, such as Au@MxOy (M = Ni, Co, Fe and CoFe),49 Au@Co3O4,50 CoO/Co51 and CuCoOx/CuCo.52 However, to date, the approach for maximizing the intrinsic activity and active sites of spinel-type oxides to achieve better OER performances without the support of conductive substrates has still challenged the researchers.

Figure 1. Synthesis procedure for the NixFe3−xO4/Ni spinel oxides/metal nanocomposites for different metal precursor ratios (y) of 0, 0.05, 0.15, 0.30, 0.50, 0.75 and 1.


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ACS Energy Letters

In this study, simultaneous optimization of the electrical conductivity and chemical compositions (and thus electronic structure) of spinel oxide OER catalyst was achieved via the preparation of NixFe3−xO4/Ni hybrid nanostructures made by a solvothermal synthesis strategy to enable significantly enhanced OER performance. The optimal NixFe3−xO4/Ni nanocomposites deposited on a glassy carbon electrode (GCE) exhibited excellent OER electrocatalytic performance, with an overpotential (η) of 225 mV vs. reversible hydrogen electrode (RHE) to achieve the electrocatalytic current density of j = 10 mA cm−2 and a Tafel slope of 44 mV dec−1 in 1 M KOH. This overall apparent catalytic performance of the electrode is the best among the spinel oxides, and at least comparable or even better than those best metal oxides-based OER catalysts in alkaline electrolyte as collected on GCE [without electrochemical active surface area (ECSA)-normalization]. This study not only demonstrates a new approach to enhance the OER electrocatalytic performance of spinel-type OER catalysts, which enables us to achieve superior OER performance via simple solvothermal reaction, but also paves the way for further design of nanocomposites electrocatalysts for OER and other applications via interface engineering.


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Figure 2. SEM images of NixFe3−xO4/Ni nanocomposites, A) y = 0, B) y = 0.15, and C) y = 1. XRD patterns of the samples, D) y = 0, E) y = 0.15, and F) y = 1 before (black curves) and after (red curves) solvothermal treatment. The synthesis procedure for NixFe3−xO4/Ni nanocomposites is illustrated in Figure 1. The typical procedure is a two-step reaction as follows: 1) the reduction of metal chlorides such as NiCl2⋅6H2O and FeCl3⋅6H2O using sodium borohydride (NaBH4) as a reductant; and 2) solvothermal treatment using alcohol as the solvent at 200 °C. A series of samples was synthesized by tuning the iron (Fe) molar ratio y [y = Fe/(Fe + Ni)] in the precursors. Scanning electron microscopy (SEM) images reveal that both the intermediates and the final products with various y value are NPs (Figures 2A–C, and Figures S1 and S2). The X-ray diffraction (XRD) patterns clearly show that when y is 0 (i.e. no iron precursor is added), the intermediate is X-ray amorphous (Figure 2D). X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) elemental mapping characterizations (Figures S3 and S4A) further demonstrate the as-obtained X-ray amorphous intermediate is nickel boride.53 After the solvothermal reaction in alcohol, only three peaks located at 44.5°, 51.8°, and 76.3° can be observed (Figure 2D), corresponding well to the (111), (200), and (220) reflections of cubic(c)-Ni (JCPDS#04-0850), indicating a transformation from amorphous nickel boride to crystalline Ni NPs. This is attributed to the reduction of the metal borides to metal by alcohol during the solvothermal treatment.54 When y is 1 (i.e. only iron precursor is used), the XRD peaks located at 30.1°, 35.4°, 43.0°, 56.9°, and 62.5°, respectively, assigned to the (220), (331), (400), (511), and (440) lattice planes of cubic magnetite [Fe3O4 (FeFe2O4), JCPDS#190629],44 are observed in both the intermediate and the final product (Figure 2F). When y


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ACS Energy Letters

increases to 0.15 (i.e. both nickel and iron precursors are used and the molar ratio of iron is 0.15), both the XRD pattern (Figure 2E) and elemental mapping (Figure S4B) reveal the intermediate is composed of both nickel iron layered double hydroxides (NiFe-LDH) (JCPDS#49-0188) and amorphous nickel boride because of the coprecipitation process of Ni2+ and Fe3+ anions with the assistance of NaOH, which is common method to prepare LDH.30 Then, after the solvothermal treatment, two phases of c-Ni and NiFe2O4 (JCPDS#54-0964) were detected in the samples, suggesting a successful transformation from the intermediate of NiFe-LDH/NizB to the final composites of NiFe2O4/Ni. The XRD patterns of the samples with other Fe molar ratios are presented in Figure S5. Interestingly, no apparent XRD peaks for NiFe2O4 can be observed in the sample with y = 0.05, probably due to its low content. As y increases, the intensity of the characteristic XRD peaks for c-Ni gradually decreases, until it completely disappears when y = 0.75. Note that, the chemical composition of the spinel oxides might be the mixed spinel phases of NixFe3−xO4, or more accurately expressed as NixFe1−xFe2O4. This is very difficult to differentiate with only XRD analysis due to the very close lattice constants between the NiFe2O4 and FeFe2O4 phases (Table S1), but will only be better differentiated using XPS, as discussed later. So, the final products are denoted as NixFe3−xO4/Ni in the following discussions.


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Figure 3. Low-magnification TEM images of NixFe3−xO4/Ni nanocomposites, A) y = 0, B) y = 0.15, and C) y = 1 (insets show the corresponding particle size distribution histograms of the samples). D) High resolution TEM image of NixFe3−xO4/Ni (y = 0.15) [insets, the corresponding FFT of different selected areas (SA)], E) SAED pattern of NixFe3−xO4/Ni (y = 0.15). F) TEM image, and G) elemental mapping (Ni, Fe, O, and Fe + O) of NixFe3−xO4/Ni (y = 0.15). Transmission electron microscopy (TEM) images (Figure 3, Figures S6 and S7) reveal that all samples exhibit typical NP morphology with a narrow size distribution after solvothermal treatment (insets in Figures 3A–C and Figures S7 A1–D1). The increase in the y value leads to a slight increase in the average size of NPs from 14.0 to 18.1 nm. Highresolution TEM (HRTEM) image of the NixFe3−xO4/Ni sample with y = 0.15 (Figure 3D) shows that the as-synthesized NPs are tightly integrated with each other and the interface


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ACS Energy Letters

between Ni and NiFe2O4 NPs can be observed distinctly. Clearly visible diffraction spots in the corresponding fast Fourier transform (FFT) images of three representative selected areas (SAs) reveal the single-crystal nature of at least parts of the NPs (insets in Figure 3D). The lattice spacings in SA1 were measured to be ~ 0.18 nm, corresponding to the (200) planes of cNi (JCPDS#04-0850). The lattice spacings in SA2 and SA3 were measured to be ~ 0.29 and 0.16 nm, well assigned to the (220) and (511) planes of NiFe2O4 (JCPDS#54-0964), respectively. This convincingly shows the coexistence of c-Ni and NiFe2O4 phases in the samples. The presence of many brighter diffraction spots within the diffraction rings in the selected area electron diffraction (SAED) patterns (Figure 3E and Figures S7A3–D3) indicates the polycrystalline characteristics of the NixFe3−xO4/Ni nanostructures. Only one set of diffraction rings, which can be ascribed to the (111), (200), (220), (311), and (331) planes of cNi, is observed in the case of NixFe3−xO4/Ni (y = 0.15), probably due to the low y value. Similar results were also observed for y = 0 and y = 0.05 (Figures S6B and S7A3). HRTEM images of other NixFe3−xO4/Ni samples (y = 0.30 and 0.50) exhibit the lattice spacings of 0.20 and 0.25 nm assigned to the (111) planes of c-Ni and (311) planes of NiFe2O4 or FeFe2O4, respectively (Figures S7B2 and C2). Consistent with the HRTEM results, two sets of diffraction rings, one ascribed to c-Ni and the other corresponding to NiFe2O4 or FeFe2O4, are seen in SAEDs patterns of the two samples (Figures S7B3 and C3). Only one set of diffraction rings, corresponding to the (220), (331), (400), (511), and (440) planes of NiFe2O4 or FeFe2O4, is observed for the samples with the y value greater than 0.75 (Figures S6C, D and Figures S7 D2–D3). This result is in good agreement with the aforementioned XRD results. The elemental mappings of NixFe3−xO4/Ni (y = 0.15) (Figures 3F–G) reveal homogeneous distribution of Fe


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and O with an enhanced distribution of Ni in the center, further demonstrating that NiFe2O4 is well mixed with metallic Ni in the nanocomposites.

Figure 4. Representative XPS spectra of A) Ni 2p, B) Fe 2p, and C) O 1s for selected samples. D) Three-dimensional (3D) column figure of Ni0 content (red), Fe2+/Fe3+ (green) ratio and lattice O content (blue) for all the samples. To further understand the surface chemical compositions of these NixFe3−xO4/Ni samples, Raman and XPS measurements were performed (Figure S8-9) and the corresponding chemical compositions and valence states are summarized in Table S2. Notably, no signal was observed for the B 1s (Figure S9B), indicating the complete transformation from nickel boride to


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ACS Energy Letters

metallic Ni. High-resolution XPS spectra of Ni 2p (Figure 4A and Figure S10A) show two dominant peaks located at 855 and 872.5 eV due to the spin-orbit coupling, with the associated broad satellite peaks at ~ 860 and 880 eV (denoted as Sat.), respectively. These can be assigned to Ni 2p3/2 and Ni 2p1/2 core level signals. Furthermore, the Ni 2p3/2 spectra could be fitted into two peaks at the binding energies of ~ 852.5, and 855.8 eV, attributed to Ni0 and Ni2+, respectively.55 The ratios of different Ni species are summarized in Table S3. Clearly, the percentage of Ni0 species decreases with the increase of y value, due to the decrease of c-Ni content. The Fe 2p spectra show that the Fe 2p3/2 and Fe 2p1/2 core level signals are located at 710 and 723.5 eV, respectively, with doublet separation of ~ 13.5 eV. The Fe 2p3/2 core level signal can be further deconvoluted into Fe2+ 2p3/2 (709.5 eV) and Fe3+ 2p3/2 (710.9 eV) signals (Figure 4B and Figure S10B).56 The ratio of Fe2+/Fe3+in various NixFe3−xO4/Ni samples increases from 0.28 to 0.48 (Table S4), and is close to 0.5 in FeFe2O4. Similarly, the deconvoluted O 1s spectra (Figure 4C and Figure S10C) consisted of four peaks located at ~ 529.8, 530.8, 531.5, and 533 eV, ascribed to lattice oxygen, defect sites with low coordination (labeled as O1), hydroxyl group (labeled as O2), and absorbed water molecule (labeled as O3), respectively.24 An enhancement in lattice oxygen species (0.16 to 0.67) can be observed with the increase of y, further demonstrating the increase content of spinel oxide in the final products (Table S5). In order to provide a more visual representation of the composition changes in NixFe3−xO4/Ni nanocomposites, the contents of Ni0, the Fe2+/Fe3+ ratio and the lattice oxygen are presented in Figure 4D. In addition, the range of x value corresponding to the molar ratio of y in each NixFe3−xO4/Ni [or better expressed as NixFe(II)1−xFe(III)2O4/Ni] nanocomposite determined based on the ratio of Fe2+/Fe3+ were shown in Table S6.


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Figure 5. Electrochemical properties of the various NixFe3−xO4/Ni samples. A) Polarization curves (iR-corrected) for OER (inset is the corresponding polarization curves in the potential window of 1.3 ~ 1.5 V vs. RHE), B) The corresponding Tafel plots, C) Plots showing the


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ACS Energy Letters

dependence of current density on the scan rate for the extraction of double layer capacitance (Cdl), and D) LSV curves showing the current density normalized by ECSA (jECSA-normalized) for samples (y = 0, 0.05, 0.15 and 0.30), E) Nyquist plots of all the samples, F) OER stability test for NixFe3−xO4/Ni (y = 0.15) (insets, the corresponding polarization curves with iR-correction before and after the stability test). To evaluate the electrocatalytic activities of the NixFe3−xO4/Ni samples for water oxidation, we performed electrochemical measurements of the as-synthesized nanostructures drop-casted on GCE with a constant catalysts loading of ~ 0.50 ± 0.05 mg cm−2 in 1 M aqueous KOH solution. The key electrochemical results for various samples with different y values are also summarized in Table 1. Before OER, there are anodic waves ascribed to the oxidation of Ni2+ to Ni3+ in the samples when y < 0.75 (inset in Figure 5A), which shift positively with the increase of y due to the strong interaction between Ni and Fe, indicating the decrease in an average oxidation state of Ni.34, 57 The iR-corrected polarization curves (Figure 5A) show that the catalysts composed of only pure Ni (y = 0) or FeFe2O4 (y = 1) NPs exhibit limited performance toward OER, requiring an overpotential (η10) of 300 and 313 mV, respectively, to achieve the geometric current density of 10 mA cm−2. In comparison, NixFe3−xO4/Ni with y = 0.05, 0.15, 0.30, 0.50, and 0.75 can successfully achieve a geometric catalytic current density of 10 mA cm−2 at η of 261, 225, 235, 270, and 299 mV (vs. RHE after iR correction), respectively. The extrapolation of the linear region of η vs. log j (Figure 5B) reveals the Tafel slopes of 62, 65, 44, 58, 60, 56, and 64 mV dec−1 (after iR correction) for y = 0, 0.05, 0.15 0.3, 0.5, 0.75 and 1, respectively. Among the measured samples, the best OER catalytic performance was by the NixFe3−xO4/Ni nanocomposite with y = 0.15 (which corresponds to an x value of about 0.36), which shows a η10 of 225 mV and the smallest Tafel


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slope of 44 mV dec−1. This performance is the best among spinel oxide OER, and comparable or better than previously reported best TMO-based OER catalysts measured on GCE or even 3D conducting substrates (without ECSA-normalization, see comparison in Table 1 and Table S7). Table 1. Summary of the electrochemical data derived from measurements collected on GCE in alkaline electrolyte in comparison with other reported highly active OER catalysts. OER

NixFe3−−xO4/Ni y=0 y = 0.05 y = 0.15 y = 0.30 y = 0.50 y = 0.75 y=1 G-FeCoW15 58

CoNiOx/rGO

59

Rct

Rf

7.10 6.30 5.49 6.40 6.73 6.90 6.56

43.20 15.88 4.99 9.47 12.23 102.80 322.56

2.90 5.40 -

37

-

-

8.1

-

280

42

21.2

-

-

-

-

-

-

7.4

-

223

246

46

97

257

30.1

42

55

224

39

-

4.45

4.29

1.02

340 ± 20

-

-

-

-

-

FeaCo1−−aOx/N-rGO

Ni–Fe–O Nanowire NiFe-(b)

EIS

8.05 5.22 4.10 2.57 0.25 0.17 0.16

TSOER 62 65 44 58 60 56 64

60

Mn3O4@MnxCo3−−xO4

7

Cdl

Rs

η10 300 261 225 235 270 299 313

η10: overpotential at the current density of 10 mA cm−2 during OER (mA cm−2). TSOER: Tafel slope in OER (mV dec−1). Cdl: double layer capacitance (mF cm−2). Rs, Rct, and Rf: electrolyte resistivity, electron transfer resistivity, and interfacial resistivity (Ω) fitted from EIS analysis, respectively. Furthermore, ECSA was estimated by double layer capacitance (Cdl) extracted from the cyclic voltammetry cycles in non-Faradaic region with a potential window of 0.85–0.95 V vs. RHE (Figure S11). Figure 5C shows that the values of Cdl are 8.05, 5.22, 4.10, 2.57, 0.25, 0.17, and 0.16 mF cm−2 for NixFe3−xO4/Ni (y = 0, 0.05, 0.15, 0.30, 0.50, 0.75, and 1) nanocomposites, respectively. The variation trend of Cdl can be ascribed to the different


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ACS Energy Letters

double-layer capacitance behaviors of Ni (y = 0, 8.05 mF cm-2) and spinel oxide (y = 1, 0.16 mF cm-2), which decreases with the increase of y value (i.e. the increase content of NixFe3−xO4 in the nanocomposites) since the contribution from NixFe3−xO4 to Cdl is limited in comparison to metallic Ni in low potential region. Here, the samples were divided into two groups according to their main component: for the first group, metallic Ni is the main component (y = 0, 0.05, 0.15 and 0.30); for the second group, the spinel oxides are the main component (y = 0.50, 0.75 and 1), respectively. The ECSA-normalized LSV curves were then obtained to further highlight the intrinsic catalytic activity (Figure 5D and S12). Since Cdl is only a reliable indicator of ECSA when the catalyst has good electrical conductivity, the ECSA-normalized current density(jECSA-normalized) obtained in the first group is more reliable.38 It can be clearly seen that, after normalization with Cdl, the optimized NixFe3−xO4/Ni sample with y = 0.15 displays the best intrinsic activity among the NixFe3-xO4/Ni nanocomposites. To better understand the electrode kinetics during OER, electrochemical impedance spectroscopy (EIS) was measured (Figure 5E). Interestingly, the Nyquist plots for both y = 0.30 and y = 0.50 nanocomposites display a "double-semicircle" feature (inset in Figure 5D) that can be fitted using an equivalent Voigt circuit (Figure S13B), which yielded not only the charge transfer resistivity (Rct), but also the interfacial resistivity (Rf). Nonetheless, other samples display only one typical semicircle that can be fitted with an equivalent Randles circuit, which is typical for OER catalysis (Figure S13A).39 The extracted values of Rct was 43.2, 15.88, 4.99, 9.47, 12.23, 102.80, and 322.56 Ω for NixFe3−xO4/Ni with y = 0, 0.05, 0.15, 0.30, 0.50, 0.75, and 1 nanocomposites, and Rf of 2.9 and 5.4 Ω for NixFe3−xO4/Ni with y = 0.30 and 0.50 nanocomposites, respectively. The values of Rs, Rct, and Rf are also listed in Table 1. Herein, the NixFe3−xO4/Ni sample with y = 0.15 displays the best electron transfer


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kinetics during OER with no obvious ohmic drop caused by the interfacial resistivity Rf; its Rct value is 2 ~ 60 times smaller than those of the other samples. This could be attributed to the suitable electronic structure for OER facilitated by the composition engineering of spinel oxides in NixFe3−xO4/Ni.51, 61 In general, the solvothermal reaction temperature is crucial to the OER performance of the composite samples. Using NixFe3−xO4/Ni (y = 0.15) as an example, the influence of the solvothermal reaction temperature was investigated. The solvothermal temperature of 200 °C used during synthesis leads to the best electrocatalytic performance (Figure S14). The durability of the catalysts is critical for their practical applications. Therefore, the amperometric i–t curves with a current density of 20 mA cm−2 was recorded for NixFe3−xO4/Ni (y = 0.15) (Figure 5F). After 10 h, the current density for OER was 17.48 mA cm−2, indicating the attenuations of 12.6%, which is attributed to the mechanical loss and thus loss of active sites during drastic gas evolution process. The TEM images (Figure S15) and XPS spectra (Figure S16) of the catalyst after the OER stability test both confirm that metallic Ni NPs (as well as the NixFe3−xO4 NPs) still exist, even though the intensity for Ni0(Ni 2p) decreased slightly in comparison to that before the OER test, which is likely due to the formation of amorphous layer of metal oxyhydroxide on the surface. Note that, some research work recently revealed that a reversible structural transformation can occur on the surface of spinel oxides and the produced metal oxyhydroxide on the surface may contribute to the OER catalytic activity,62-66 which has prompted the researchers to further deeply consider the intrinsic OER mechanisms. In the current study, a thin amorphous layer on the NixFe3−xO4/Ni NPs was detected by HRTEM after OER test (Figure S15), which might be defect-rich mixture of NiFebased oxyhydroxides and oxides.65, 67, 68 This amorphous layer is similar to that observed in


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ACS Energy Letters

perovskite oxides and other transition-metal-based materials,69-71 which is in situ derived from the crystalline phase in the core during the OER to protect the further oxidation of metal Ni and could further facilitate the oxygen production at a lower overpotential. To further understand the enhanced electrocatalysis in the NixFe3−xO4/Ni nanocomposites, the role of the semiconductor/metal interface should be discussed. As a typical p-type semiconductor, NiFe2O4 has a direct band gap of 1.55 eV and work function of 5.9 eV.72-74 In contrast, the work function for Ni metal is 4.60 eV, indicating that Ni has higher Fermi energy. Thus, the electrons in Ni can easily pass through the NixFe3−xO4/Ni interface into NixFe3−xO4 because of the good electrical contact and increase the Fermi level in NixFe3−xO4 until the work function equilibrium is achieved (illustrated in Figure 6A).75 We believe that the atomic-level interface and the good electrical contact between Ni and NixFe3−xO4 NPs play a significant role in enhancing the catalytic activity of these NixFe3−xO4/Ni nanocomposites. However, further increasing the content of NixFe3−xO4 will reduce the catalytic performance due to the ohmic drop caused by the thick oxide layer around metal Ni. To verify this hypothesis, the OER performance of the samples with y = 0, 0.15 and 1 before and after the solvothermal treatment is compared (Figure 6B). Distinctly, when y = 0, the transformation from amorphous nickel boride to metallic Ni during solvothermal treatment only results in a small reduction of ~ 13 mV in η10 (Table S8). When y = 1, no obvious change in performance is observed after solvothermal treatment, since the intermediate is purely Fe3O4. Interestingly, the transformation from NiFe-LDH/NizB to NixFe3−xO4/Ni for y = 0.15 leads to a significant reduction of 48 mV in η10, indicating the significant contribution of the interface contact between Ni and NixFe3−xO4 NPs in these composite samples toward enhanced catalytic activity.


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Figure 6. A) The schematic illustration showing the mechanism of electron transfer at the NixFe3−xO4/Ni semiconductor-metal interface. B) Comparison of the OER performance of NixFe3−xO4/Ni (y = 0, 0.15 and 1) samples before (solid) and after (dash) ethanol thermal treatment (iR-corrected). In summary, a novel strategy for the synthesis of NixFe3−xO4/Ni nanocomposites via a solvothermal reaction was developed for the first time to prepare high performance nanocomposite OER catalysts. By tuning the molar ratio of metal (Fe and Ni) precursors used during synthesis, the composition of the spinel oxide (thus its electronic structure) and the metal-metal oxide interface contact in the composites were synergistically modulated. These factors resulted in enhancement of the overall catalytic performance. Among all of the samples, the NixFe3−xO4/Ni nanocomposite sample made with y = 0.15 (an x value of about 0.36) displayed the highest electrocatalytic performance toward OER, requiring an overpotential of only 225 mV to achieve the geometric current density of 10 mA cm−2 with a Tafel slope of 44 mV dec−1. This optimized catalytic performance toward OER is the best


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ACS Energy Letters

among spinel oxides to our knowledge, and at least comparable or even better than the best previously reported metal oxides-based OER catalysts as collected on GCE in alkaline electrolyte (without ECSA-normalization). The excellent catalytic properties, combined with the facile and scalable solvothermal synthesis, make these NixFe3−xO4/Ni nanocomposites very promising earth-abundant alternatives for noble metal-based OER electrocatalysts. This study also provides new insights into the design and construction of novel nanocomposite electrocatalysts for oxygen evolution or other catalytic applications via interface and composition engineering. ASSOCIATED CONTENT Supporting Information Experimental section, details about the additional SEM images, TEM images, EDS, Raman, XRD results, and electrochemical performances of relevant samples is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; (X.Z.) [email protected] (B. S.); [email protected] (S. J.) ORICD Bo Song: 0000-0003-2000-5071 Song Jin: 0000-0001-8693-7010 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS


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This work is supported by the Major State Basic Search Program (No. 2014CB46505), National Natural Science Foundation of China (Grant Nos. 51372056, 51472064, 51672057, 51722205), National Science Fund for Distinguished Young Scholars (Grant No.51525201), Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 11421091),

International

Science

&

Technology

Cooperation

Program

of

China

(2012DFR50020), the Fundamental Research Funds for the Central Universities (Grant No. HIT.BRETIV.201801), the Natural Science Foundation of Heilongjiang Province (Grant No. E2018032), and the Program for New Century Excellent Talents in University (NCET-13-0174). S.J. acknowledges support by NSF Grant DMR-1508558.

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