NiCo2O4

Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 12214−12221

pubs.acs.org/journal/ascecg

Superior Oxygen Evolution Reaction Performance of Co3O4/NiCo2O4/ Ni Foam Composite with Hierarchical Structure Man Yang,† Wei Lu,† Renxi Jin,*,‡ Xian-Chun Liu,† Shuyan Song,§ and Yan Xing*,†

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†

Jilin Provincial Key Laboratory of Advanced Energy Materials, Department of Chemistry, Northeast Normal University, 5268 Renmin Street, Changchun 130024, P. R. China ‡ Department of Chemical and Biomolecular Engineering, University of Notre Dame, 54417 Leahy Drive, Notre Dame, Indiana 46556, United States § State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, P. R. China S Supporting Information *

ABSTRACT: In this work, ZIF-67 is self-grown on the NiCo layered double hydroxide (NiCo-LDH) nanosheets-modified Ni foam (NF) surface. Its derivative, the hierarchically structured Co3O 4/NiCo2O4/Ni foam (CO/NCO/NF) composite, is successfully fabricated by the oxidation of ZIF-67/NiCo-LDH/ NF, which exhibits excellent electrocatalytic performance. When assessed as an electrode for oxygen evolution reaction (OER), the CO/NCO/NF electrode shows low overpotentials of 320 and 407 mV at 10 and 50 mA cm−2, respectively, in 0.1 M KOH aqueous electrolyte and an excellent long-term electrochemical durability with 88% current preservation during a 60 h test. It is also found that the OER activities of CO/NCO/NF electrode are superior to those of NCO/NF and CO/NF electrodes, demonstrating that the strong interaction of the interconnected interface of Co3O4 nanocages (CO NCs) and NiCo2O4 nanosheets (NCO NSs) is advantageous to increase the number of Co3+, improve conductivity, enhance structural stability, facilitate electronic transfer, and reduce reversible ion accumulation at the interface. In addition, the hierarchical architecture of the composite is binder-free and good for enhancing mass transport, boosting the dissipation of gases, as well as exposing catalytic active sites in the process of water electrolysis. KEYWORDS: In situ growth, Hierarchical structure, Synergistic effect, Oxygen evolution reaction

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INTRODUCTION

However, single metal oxide Co3O4 or NiCo2O4 still exhibits insufficient electrocatalytic performance for OER to meet the practical requirements.8,11,18−20 Therefore, developing novel hybrid composite materials has attracted great attention due to their multiple oxidation states and structures and the synergistic effect of individual components. Up to now, most hybrid nanostructures, normally fabricated by a tedious procedure or a simple mechanical mixing approach, endow a weak coupling between individual components, restraining a possible synergistic effect, resulting in low electrochemical properties.21 Thus, it is imperative to explore a simple and effective fabrication approach for constructing novel composites with improved electrocatalytic performance for OER. Metal−organic frameworks (MOFs) are porous materials with uniform porosity, superior surface area, and high flexibility of the material composition.22−24 MOFs have recently been served as a family of precursors for the preparation of hollow porous metal oxides as water electrolyzers.24−27 However, free-

To reduce fossil fuel dependency and pollutant emission, water electrolysis has been considered as one of the most potential ways to generate renewable hydrogen fuel. However, targeted efficiency is still restricted by the sluggish kinetics of the oxygen evolution reaction (OER).1−3 Up to now, RuO2 and IrO2 are the best known catalysts for OER. Nevertheless, the high cost and scarcity largely hinder their practical application.4,5 Therefore, searching for low-cost and highly active OER electrocatalysts has become a vital issue for improving the efficiency of energy-generation technologies.6−9 Recently, the spinel-type Co-based oxides have been widely studied as a class of fascinating OER electrocatalysts in alkaline medium, due to their high abundance, complex compositions, and high electroactivity and stability.10−14 Among them, Co3O4 is regarded as a promising electrode material for OER with low overpotential and good chemical stability.15,16 Meanwhile, spinel bimetallic oxides NiCo2O4 have been reported to perform better OER activities compared with Co3O4, owing to the enhanced electronic conductivity and effective surface area.17 © 2019 American Chemical Society

Received: March 18, 2019 Revised: June 12, 2019 Published: June 15, 2019 12214

DOI: 10.1021/acssuschemeng.9b01535 ACS Sustainable Chem. Eng. 2019, 7, 12214−12221

ACS Sustainable Chemistry & Engineering

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RESULTS AND DISCUSSION The preparation route of CO/NCO/NF electrode is given in Scheme 1. First, the NiCo-LDH nanosheets were synthesized

standing MOFs are not stable and are subject to agglomeration, which greatly reduces the resulting electrochemical performances of the MOF derivatives.28 Layered double hydroxides (LDHs), as a family of inorganic lamellar compounds consisting of brucite-like layers of cations and an anionic exchangeable interlayer, have been demonstrated to provide the active sites for in situ nucleation and directed epitaxial growth of MOFs. 29,30 Also LDHs and their corresponding derivatives have displayed catalytic activity for OER.31−33 In this regard, using MOFs and LDHs hybrids as precursors to prepare integrated derivatives as OER electrocatalysts is expected to achieve outstanding performance. Recently, Tang et al. reported a Co3O4@N-doped carbon bifunctional catalyst for overall water splitting, constructed from a ZIF@LDH precursor on Ni foam.34 Moreover, NiCo alloy@C/NixCo1−xO/Ni foam hierarchical nanocomposite has been synthesized through an in situ growth of NiCo-MOF on the NiCo-LDH layer-modified Ni foam surface; the hybrid composites exhibit outstanding OER activity.35 However, there are very few reports on the fabrication of Co3O4/NiCo2O4/Ni foam composite used as an electrode material for OER. Herein, we have successfully prepared a hierarchically structured ZIF-67/NiCo-LDH/NF composite via a facile solvothermal synthesis of NiCo-LDH nanosheets on Ni foam followed by the in situ growth of ZIF-67. Its derivative, Co3O4/NiCo2O4/Ni foam (CO/NCO/NF) integrated electrode, was acquired through the subsequent calcination of ZIF67/NiCo-LDH/NF, which exhibited excellent electrocatalytic OER performances. The NiCo-LDH nanosheets not only were used as a support but also provided the active sites for the in situ nucleation and epitaxial growth of ZIF-67. ZIF-67 was strongly immobilized onto the surface of NiCo-LDH nanosheets, which guaranteed the structural transformation from ZIF-67 to Co3O4 nanocages with fine control over morphology and composition. Moreover, the Co3O4/NiCo2O4 composite directly grew on the surface of Ni foam to form the hierarchical structure, which could avoid the use of binder. The binder-free composite electrode not only improved the conductivity, exposed abundant active sites, and facilitated fast charge transport and mass transfer but also helped to buffer the volume expansion/contraction upon cycling. Consequently, the CO/NCO/NF electrode exhibited excellent OER activity and durability.

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Research Article

Scheme 1. Schematic Illustration of the Synthesis of CO/ NCO/NF Electrode

on the surface of Ni foam through a solvothermal method. Figure 1D gives the transmission electron microscopy (TEM) image of the NiCo-LDH nanosheets grown on the surface of NF. The open-up network structure of NiCo-LDH nanosheets can be clearly observed in the scanning electron microscopy (SEM) image (Figure 1G), in which numerous nanosheets are interconnected with each other. Subsequently, the activated NiCo-LDH/NF was immersed in a MeOH solution of Co(NO3)2·6H2O and 2-MeIM to obtain the ZIF-67/NiCoLDH/NF under solvothermal condition. In this step, wellcrystallized ZIF-67 in situ growth on the surface of NiCo-LDH nanosheets was observed (Figure 1H). Moreover, the TEM image (Figure 1E) indicates the integrated structure of ZIF67/NiCo-LDH/NF, in which the integration of ZIF-67 with LDH was disclosed. Lastly, ZIF-67/NiCo-LDH/NF as the precursor was treated by calcination in air to obtain hierarchically porous structured Co3O4/NiCo2O4/NF (CO/ NCO/NF) composite. The optical image of all the samples exhibits the color changes and high structural integrity during the preparation process (Figure S1). Figure 1A displays the Xray diffraction (XRD) pattern of the CO/NCO/NF composite; except the diffraction peaks of NF, the diffraction peaks of NCO NSs and CO NCs match well with the standard values of NCO (JCPDS no. 20-0781) and CO (JCPDS no. 431003), respectively. The morphology and microstructure of the CO/NCO/NF composite were characterized by using SEM and TEM. As given in Figure 1I, it is clear that CO/NCO/NF basically retains the overall morphology of ZIF-67/NiCoLDH/NF and the dodecahedron shaped ZIF-67 precursor is inherited by the Co3O4 derivation, except that the sizes become smaller due to shrinkage in the process of calcination. Strikingly, TEM images reveal that Co3O4 has a well-defined hollow structure (Figure 1F), and the surface is made up of small particles (Figure 1C). The HRTEM image is shown in Figure 2A; the lattice fringe with spacings of 0.24 and 0.47 nm is well in accordance with the (311) plane and the (111) plane of the NiCo2O4, respectively. The observed lattice spacings of 0.24 and 0.28 nm are consistent with the (311) and (220) planes of Co3O4, respectively. Furthermore, the energy-dispersive X-ray spectroscopy (EDS) spectrum (Figure 2B) displays that the CO/

EXPERIMENTAL SECTION

Preparation of NiCo-LDH/NF and ZIF-67/NiCo-LDH/NF. The detailed fabrication of NiCo-LDH/NF and ZIF-67/NiCo-LDH/NF is displayed in our Supporting Information. Fabrication of NiCo2O4/NF, Co3O4/NF, and Co3O4/NiCo2O4/NF Composite. The NiCo2O4/NF (NCO/NF) and Co3O4/NiCo2O4/ NF (CO/NCO/NF) composites were prepared through calcining the as-synthesized NiCo-LDH/NF and ZIF-67/NiCo-LDH/NF in air at 350 °C for 3 h, respectively. The Co3O4/NF (CO/NF) composite was obtained via the same steps as those of CO/NCO/NF composite without adding nickel nitrate. Preparation of NiCo2O4 Nanosheets, Co3O4 Nanocages, and Co3O4/NiCo2O4 Composite. The synthesis of NiCo2O4 nanosheets (NCO NSs), Co3O4 nanocages (CO NCs), and Co3O4/NiCo2O4 (CO/NCO) composite follows the same procedure as those of NCO/NF, CO/NF, and CO/NCO/NF composite in the absence of NF, respectively. 12215

DOI: 10.1021/acssuschemeng.9b01535 ACS Sustainable Chem. Eng. 2019, 7, 12214−12221

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ACS Sustainable Chemistry & Engineering

Figure 1. (A) XRD patterns of the NF, NiCo-LDH/NF, ZIF-67/NiCo-LDH/NF, and CO/NCO/NF composite, (B) SEM image of NF, (C) the high-magnification TEM image of CO/NCO/NF composite, and TEM and SEM images of (D, G) NiCo-LDH/NF, (E, H) ZIF-67/NiCo-LDH/ NF, and (F, I) CO/NCO/NF composite.

verify the formation of NCO/NF and CO/NF. The structure and morphology of NCO NSs, CO NCs, and CO/NCO composite without using NF as a substrate were also characterized by XRD and SEM (Figure S4). N2 adsorption−desorption isotherms and pore size distribution curves of NCO/NF, CO/NF, and CO/NCO/NF composite are shown in Figure 3. It can be seen from these

Figure 2. (A) HRTEM image, (B) EDS pattern, and (C) EDS element mappings of CO/NCO/NF composite. Figure 3. Nitrogen adsorption−desorption isotherms and corresponding pore size distribution curves (inset) of NCO/NF, CO/NF, and CO/NCO/NF composite.

NCO/NF composite is mainly composed of elemental O, Co, and Ni. EDS element mappings of CO/NCO/NF composite were also conducted to verify the elemental spatial distributions (Figure 2C), which clearly revealed that elements O, Co, and Ni were homogeneously distributed in the entire structure with some apparent enhancement in the regions of Co3O4 nanocages (elements O and Co) and NF (element Ni). These results further confirm the formation of the hierarchically structured CO/NCO/NF composite, which is in accordance with the XRD, TEM, and SEM observations. Additionally, the structure and morphology of NCO/NF and CO/NF were also characterized by XRD, SEM, and TEM (Figures S2 and S3). As given in Figure S2, XRD patterns of the annealed product of NiCo-LDH/NF and ZIF-67/NF

isotherms that NCO/NF, CO/NF, and CO/NCO/NF composite have typical type IV isotherms and H3 hysteresis loops (P/P0 > 0.4), suggesting that all the samples are mesoporous. The CO/NCO/NF composite possesses the largest Brunauer−Emmett−Teller (BET) specific surface area (63 m2 g−1), which is larger than those of NCO/NF (57 m2 g−1) and CO/NF (49 m2 g−1). The higher surface area of CO/ NCO/NF composite not only provides larger effective areas but also exposes more active sites, suggesting outstanding OER activity. 12216

DOI: 10.1021/acssuschemeng.9b01535 ACS Sustainable Chem. Eng. 2019, 7, 12214−12221

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ACS Sustainable Chemistry & Engineering

Figure 4. (A) Survey, (B) Co 2p, (C) Ni 2p, and (D) O 1s XPS spectra of CO/NCO/NF composite.

Figure 5. (A) Co 2p and (B) magnified Co 2p3/2 XPS spectra of NCO/NF, CO/NF, and CO/NCO/NF composite.

Table 1. Summary of the Electrocatalytic Performance of the As-Prepared Electrocatalysts in This Work catalyst

onset potential (V vs RHE)

ηa (V)

ηb (V)

Tafel slope (mV dec−1)

Cdl (mF cm−2)

TOF (× 10−2 s−1)

Rct (Ω)

RuO2 NF NCO/NF CO/NF CO/NCO/NF

1.39 1.59 1.52 1.54 1.47

0.16 0.36 0.29 0.31 0.24

0.28 0.44 0.37 0.41 0.32

68 162 115 135 84

12 28 19 34

0.69 1.19 1.00 2.87

0.30 0.22 0.26 0.19

Overpotential (η) at j = 0 mA cm−2. bη at j = 10 mA cm−2.

a

lower binding energy compared with that of NCO/NF and CO/NF in Figure 5. The variation of the binding energy of Co 2p3/2 demonstrates the electron transfer in CO/NCO/NF composite, caused by the strong interaction between NCO NSs and CO NCs. Therefore, constructing the hierarchically structural composites enables an electron coupling at the interface of each component, which is of benefit to accelerate electron transfer and enhance OER electrocatalytic performance.21 Moreover, the binding energy of Co3+ species is lower than that of Co2+, and lower binding energy has a stronger correlation to the CoOOH species.38 Thus, we also calculated the relative content of Co3+ in the NCO/NF, CO/NF, and CO/NCO/NF from XPS, respectively.20 As shown in Table S1, CO/NCO/NF composite has a Co3+ content (68.1%) that is larger than those of NCO/NF (59.7%) and CO/NF

X-ray photoelectron spectroscopy (XPS) analysis was utilized to study the surface electronic state and the composition of the as-obtained CO/NCO/NF composite (Figure 4). As given in Figure 4A, the survey spectrum of CO/ NCO/NF composite indicates the existence of Co, Ni, and O. By using a Gaussian fitting method, the high-resolution Co 2p and Ni 2p XPS spectra (Figure 4B and C) can be fitted into two spin−orbit doublets and two shakeup satellites (indicated as “Sat.”); the O 1s high-resolution spectrum in Figure 4D can be deconvolved into two peaks. These results indicate that the chemical composition of the CO/NCO/NF composite includes Co2+, Co3+, Ni2+, Ni3+, and O.36,37 To survey the benefit of composite, the magnified XPS spectra of Co 2p in NCO/NF, CO/NF, and CO/NCO/NF were compared. The Co 2p3/2 peak of CO/NCO/NF composite slightly shifts to a 12217

DOI: 10.1021/acssuschemeng.9b01535 ACS Sustainable Chem. Eng. 2019, 7, 12214−12221

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ACS Sustainable Chemistry & Engineering

Figure 6. (A) Linear scan voltammogram (LSV) plots and (B) Tafel plots of the RuO2, NF, NCO/NF, CO/NF, and CO/NCO/NF in 0.1 M KOH. (C) Differences in current density vs scan rate for estimation of Cdl, and (D) Nyquist plots of the NF, NCO/NF, CO/NF, and CO/NCO/ NF in 0.1 M KOH.

ization curves (Figure 6A). The Tafel plots of the as-fabricated catalysts matched well with the Tafel equation. As observed in Figure 6B, the Tafel slope of CO/NCO/NF electrode (84 mV dec−1) is lower than those of NF (162 mV dec−1), NCO/NF (115 mV dec−1), and CO/NF electrode (135 mV dec−1), indicating a more effective electron transfer, more favorable electrocatalytic reaction kinetics, and more catalytic activity toward OER for CO/NCO/NF composite. In contrast, neither NCO/NF nor CO/NF is a high-efficiency OER catalyst. Therefore, the synergy coupling of Co3O4 nanocages and NiCo2O4 nanosheets accounts for the high OER activity, similar to that in recent published works.34,35 It is known that the OER activity of the electrode relies enormously upon the electrochemically active surface area (ECSA). Thus, Cdl is calculated from the CV curve in the potential region of 0.00− 0.10 V without redox processes (Figure 6C and Figure S6).34,46,59 As shown in Table 1, it illustrates that the Cdl of NF, NCO/NF, CO/NF, and CO/NCO/NF electrodes are 12, 28, 19, and 34 mF cm−2, respectively, implying that the CO/ NCO/NF electrode possesses a much higher ECSA than other electrodes. This provides additional evidence for the improved electrocatalytic performance observed from the CO/NCO/NF electrode. To obtain further insight into the OER kinetics of electrode materials, turnover frequencies (TOFs) of the CO/ NCO/NF at a η of 350 mV were calculated. The CO/NCO/ NF electrode shows the largest TOF of 0.0287 s−1 (Table 1).60,61 We also carried out an electrochemical impedance spectroscopy (EIS) test to confirm the conductivity advantages of the CO/NCO/NF electrode. The charge-transfer resistance (Rct) across the electrolyte/electrode interface was acquired from the Nyquist plot. As given in Figure 6D, the Rct of the CO/NCO/NF electrode (0.19 Ω) is lower than those of NF (0.30 Ω), NCO/NF (0.22 Ω), and CO/NF electrodes (0.26 Ω), which suggests a better electrical conductivity and faster charge transport of the CO/NCO/NF electrode, leading to a superior electrocatalytic performance of OER. The OER

(57.6%). These results suggest that CO/NCO/NF composite with the largest Co3+ content is favorable for adsorbing anions (OH−), thereby increasing the amount of OOH species and accelerating the transformation of Co3+/Co4+.38−40 In addition, the presence of Co4+ also improves the electrophilicity of the adsorbed O and thus facilitates the forming of O−OH intermediates, accelerating the deprotonation of OOH species to generate O2 via the electron-withdrawing inductive effect (Figure S5).38,41,42 The OER activities of the as-synthesized samples were investigated in a 0.1 M KOH solution (pH = 13) using a threeelectrode cell, and their electrochemical performances are listed in Table 1. As given in Figure 6A, except that of RuO2 electrode (1.39 V vs RHE), the polarization curve of CO/ NCO/NF electrode displays an onset potential of 1.47 V vs RHE, which is lower than that of NF (1.59 V), NCO/NF (1.52 V), and CO/NF electrode (1.54 V). Remarkably, the overpotential of CO/NCO/NF electrode is only 320 mV at 10 mA cm−2, which is lower than those of NF (440 mV), NCO/ NF (370 mV), and CO/NF electrode (410 mV). The overpotential of CO/NCO/NF electrode at 10 mA cm−2 is lower than or comparable to those of previously reported Cobased OER electrode materials (Table S2), such as PNGNiCo2O4 (η5 = 373 mV), GO/MnO2/NCO/NF (η0 = 321 mV), Graphene-Co3O4 (η1.5 = 650 mV),43−45 Co3O4/ NiCo2O4/NF DSNCs (η10 = 340 mV),46 Co@NC/NF (η10 = 390 mV),47 NiCo−OH/NF (η10 = 404 mV), NiCo2O4 NA/ CC (η0 = 320 mV), NiCo2O4 nanoneedles (η10 = 565 mV), NiCo2O4/C (η5.1 = 650 mV),48−51 CFP/NCO/ Co0.57Ni0.43 (η10 = 340 mV),52 CoxNi1-x@CoyNi1-yO@C (η10 = 570 mV), Ni@CoO@CoNC (η10 = 309 mV), NiCo2O4 @Ni0.440Co LDH/NF (η10 = 242), NiCo2O4/NiORGO (η10 = 350 mV), NiCo@NiCoO2/C PMRAs (η10 = 340 mV), 50-CoxNi1-x@CoyNi1-yO@C (η10 = 570 mV),53−58 Moreover, the Tafel plots (Figure 6B) were calculated by their corresponding linear scan voltammogram (LSV) polar12218

DOI: 10.1021/acssuschemeng.9b01535 ACS Sustainable Chem. Eng. 2019, 7, 12214−12221

Research Article

ACS Sustainable Chemistry & Engineering activities of NCO NSs, CO NCs, and CO/NCO electrode without using NF as a substrate were also estimated by LSV, Tafel slope, Cdl, and EIS, and the data are listed in Table S3. As shown in Figure S7, all of these electrodes exhibit poor electrocatalytic performances for OER, implying the important role of high conductivity and rich macroporosity that originated from Ni foam. Other than high activity, the durability and long-term stability are also critical parameters that should be considered for the practical application of electrocatalysts. The stability of CO/NCO/NF electrode was surveyed by continuous cyclic voltammetry (CV) at a scan rate of 5 mV s−1. It can be seen (inset in Figure 7) that the LSV

nanosheets, the hierarchically structured CO/NCO/NF, and the high conductivity and rich macroporosity of Ni foam might be responsible for the distinct electrocatalysis performance. The remarkable features of CO/NCO/NF make it a promising candidate as electrode material for OER.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b01535.

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Experimental details, characterization techniques, and electrochemical data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Shuyan Song: 0000-0002-7758-752X Yan Xing: 0000-0001-6076-8941 Notes

The authors declare no competing financial interest.

Figure 7. Chronoamperometry curve of CO/NCO/NF under 10 mA cm−2, and its polarization curves (inset) before and after 1500 cycles at 5 mV s−1.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant nos. 21872023 and 21473027) and the Opening Fund of State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, CASs.

curve after 1500 cycles is almost the same as the first cycle, indicating the high stability of the CO/NCO/NF electrode. The CO/NCO/NF electrode also demonstrates an extraordinary long-term electrochemical durability with 88% and 91% current preservation for up to 60 h at 10 (Figure 7) and 50 mA cm−2 (Figure S8), respectively. After the durability and stability tests, the CO/NCO/NF electrode still retained its hierarchically structural feature (Figure S9), indicating the structural stability of the CO/NCO/NF electrocatalyst for OER. The high durability and stability of the CO/NCO/NF electrode can be ascribed to the in situ growth of ZIF-67 on NiCo-LDH nanosheets; meanwhile, the macroporosity of Ni foam also helps to buffer the volume expansion/contraction upon cycling.2,62 Thus, the high catalytic activity and stability of the CO/NCO/NF composite may benefit from the following facts. First, NiCo2O4 nanosheets act as a support for the stable existence of Co3O4 nanocages, shortening the electron-transport path and preventing the agglomeration of the Co3O4 nanocages. Second, the high specific surface area of CO/NCO/NF composite not only provides larger effective areas but also exposes more active sites. Third, the use of Ni foam is good for improving conductivity, enhancing mass transport, boosting the dissipation of gases, and buffering the volume expansion/contraction upon cycling.

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REFERENCES

(1) Wang, L. Y.; Gu, C. D.; Ge, X.; Zhang, J. L.; Zhu, H. Y.; Tu, J. P. Anchoring Ni2P Sheets on NiCo2O4 Nanocone Arrays as Optimized Bifunctional Electrocatalyst for Water Splitting. Adv. Mater. Interfaces 2017, 4 (20), 1700481. (2) Xiao, C. L.; Li, Y. B.; Lu, X. Y.; Zhao, C. Bifunctional Porous NiFe/NiCo2O4/Ni Foam Electrodeswith Triple Hierarchy and Double Synergies for Efficient Whole Cell Water Splitting. Adv. Funct. Mater. 2016, 26 (20), 3515−3523. (3) You, B.; Jiang, N.; Sheng, M. L.; Bhushan, M. W.; Sun, Y. J. Hierarchically Porous Urchin-Like Ni2P Superstructures Supported on Nickel Foam as Efficient Bifunctional Electrocatalysts for Overall Water Splitting. ACS Catal. 2016, 6 (2), 714−721. (4) Yagi, M.; Tomita, E.; Kuwabara, T. Remarkably High Activity of Electrodeposited IrO2 Film for Electrocatalytic Water Oxidation. J. Electroanal. Chem. 2005, 579 (1), 83−88. (5) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett. 2012, 3 (3), 399−404. (6) Chen, R.; Wang, H.-Y.; Miao, J. W.; Yang, H. B.; Liu, B. A Flexible High-Performance Oxygen Evolution Electrode with ThreeDimensional NiCo2O4 Core-Shell Nanowires. Nano Energy 2015, 11, 333−340. (7) Chen, D. J.; Chen, C.; Baiyee, Z. M.; Shao, Z. P.; Ciucci, F. Nonstoichiometric Oxides as Low-Cost and Highly-Efficient Oxygen Reduction/Evolution Catalysts for Low-Temperature Electrochemical Devices. Chem. Rev. 2015, 115 (18), 9869−9921. (8) Bao, J.; Zhang, X. D.; Fan, B.; Zhang, J. J.; Zhou, M.; Yang, W. L.; Hu, X.; Wang, H.; Pan, B. C.; Xie, Y. Ultrathin Spinel-Structured Nanosheets Rich in Oxygen Deficienciesfor Enhanced Electrocatalytic Water Oxidation. Angew. Chem., Int. Ed. 2015, 54 (25), 7399−7404.

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CONCLUSIONS We report a novel route to prepare ZIF-67/NiCo-LDH/Ni foam by a facile solvothermal fabrication of NiCo-LDH nanosheets on Ni foam followed by the in situ growth of ZIF67. After oxidation, the derived Co3O4/NiCo2O4/Ni foam (CO/NCO/NF) composite with a hierarchical architecture show greatly enhanced OER performances, including high activity, favorable kinetics, and good durability. It is proposed that the synergistic coupling of Co3O4 nanocages and NiCo2O4 12219

DOI: 10.1021/acssuschemeng.9b01535 ACS Sustainable Chem. Eng. 2019, 7, 12214−12221

Research Article

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DOI: 10.1021/acssuschemeng.9b01535 ACS Sustainable Chem. Eng. 2019, 7, 12214−12221

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DOI: 10.1021/acssuschemeng.9b01535 ACS Sustainable Chem. Eng. 2019, 7, 12214−12221