3D Self-Supported Porous NiO@NiMoO4 Core–Shell Nanosheets for

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3D Self-Supported Porous NiO@NiMoO4 Core−Shell Nanosheets for Highly Efficient Oxygen Evolution Reaction Dandan Jia,† Hongyi Gao,*,† Liwen Xing,† Xiao Chen,§ Wenjun Dong,† Xiubing Huang,† and Ge Wang*,† †

Beijing Advanced Innovation Center for Materials Genome Engineering, Beijing Key Laboratory of Function Materials for Molecule & Structure Construction, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China § Institute of Advanced Materials, Beijing Normal University, Beijing 100875, People’s Republic of China Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF SUSSEX on 05/09/19. For personal use only.

S Supporting Information *

ABSTRACT: Novel 3D self-supported porous NiO@NiMoO4 core−shell nanosheets are grown on nickel foam through a facile stepwise hydrothermal method. Ultrathin NiO nanosheets on the nickel foam cross-linked to each other are used as the core, and tiny NiMoO4 nanosheets are further engineered to be immobilized uniformly on the NiO nanosheets to form the shell. This step-by-step construction of the architecture composed of ultrathin primary and secondary nanosheets efficiently avoids the agglomeration problems of individual ultrathin nanosheets. The ingenious architecture possesses the advantages of numerous diffusion channels for electrolyte ions, ideal pathways for electrons, and a large interfacial area for electrochemical reaction. The introduction of the NiMoO4 secondary nanosheets on the NiO primary nanosheets not only endows the heterostructure with high electrical conductivity and a large active area but also promotes an increase in oxygen vacancy content, which favors the improvement of electrocatalytic properties for the oxygen evolution reaction. The Tafel plot for the NiO@NiMoO4 core−shell architecture is as low as 32 mV dec−1, and the overpotential needed to reach 10 mA·cm−2 for NiO@NiMoO4 nanosheets is only 0.28 V.

1. INTRODUCTION Over the past few years, energy conversion and storage have become extremely important topics due to the gradual depletion of traditional energy sources and ever-increasing environmental pollution.1−4 Electrocatalytic water splitting as an efficient energy conversion pathway has attracted extensive attention.5 Considerable efforts have been made to develop oxygen evolution reaction (OER) electrocatalysts with low overpotential and high activity to enhance the efficiency of water splitting.6,7 Traditionally, noble metals (Ir, Ru) and their oxides are regarded as the best OER electrocatalysts, but their expensive nature and toxicity significantly limit their widespread applications.8,9 First-row transition-metal (Co, Ni, Fe, Mn) oxides have been extensively investigated as competent OER electrocatalysts due to their easy availability, low price, good activity, and stability.10,11 Among them, ultrathin two-dimensional (2D) nanosheets of transition-metal oxides have attracted considerable research interest due to the great advantage of large surface area and excellent electron transfer ability.12 However, the easy restructuring of 2D nanosheets during the electrochemical reaction has triggered many problems and hampered their application.13,14 The assembly of 2D nanosheets with other nanostructures, such as one-dimensional nanoarrays or nano© XXXX American Chemical Society

belts, 2D nanosheets, or three-dimensional networks, to construct rational configurations can anchor nanosheets effectively and prevent their restacking.15−17 Construction of intriguing heteroarchitectures of transition metal oxides (TMO) nanosheets with binary transition metal oxides (BTMO) nanosheets could boost the electrochemical performance. Liu et al. fabricated CoMoO4@MnO2 core−shell nanosheet arrays by a mild hydrothermal method on nickel foam, which exhibits superior cycling stability and a high specific capacitance.18 Zhang et al. constructed layered CuCo2O4@MnO2 nanosheet core−shell arrays (CuCo2O4@ MnO2) on the nickel foam, which also showed good reversibility features and excellent rate capability.19 The greatly improved electrochemical performance was attributed to the hierarchical nanostructure assembled from the tiny nanosheets, as well as the synergistic contribution from the TMO and BTMO. TMO@BTMO with well-designed architectures have proven to be promising pseudocapacitive materials. However, to the best of our knowledge, they are rarely applied as OER electrocatalysts. It is highly desirable to develop advanced integrated TMO@BTMO electrode materials with wellReceived: January 17, 2019

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DOI: 10.1021/acs.inorgchem.9b00162 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Schematic Diagram of the Production for NiO@NiMoO4 Nanosheets

diffraction (XRD) pattern of α-Ni(OH)2 on nickel foam is shown in Figure S1. The nickel foam was taken out and rinsed with water and ethanol after natural cooling of the autoclave. The porous NiO nanosheets grown on nickel foam were obtained after being dried overnight and calcined at 350 °C for 2 h. Preparation of NiO@NiMoO4 Nanosheets Grown on Nickel Foam. A homogeneous solution consisting of 0.197 mmol of Ni(NO3)2·6H2O and 0.125 mmol of NaMoO4·2H2O in 15 mL of H2O was poured into a 25 mL Teflon-lined stainless steel autoclave. After the addition of the above Ni(OH)2-grown nickel foam, the autoclave was maintained for 6 h at 110 °C. The nickel foam was dried at 50 °C overnight after rinsing with water and ethanol. NiO@ NiMoO4 nanosheets grown on nickel foam were obtained after being calcined at 350 °C for 2 h. Preparation of NiMoO4 Grown on Nickel Foam. The synthesis method for NiMoO4 grown on nickel foam is similar to that of the NiO@NiMoO4 nanosheets grown on nickel foam, with the addition of a clean nickel foam. 2.2. Materials Characterization. The morphologies of the assynthesized products were carried out by means of scanning electron microscopy (SEM, ZEISS SUPRA55). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were carried out using a TEI Tecnai F20. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Escalab 250Xi X-ray photoelectron spectrometer with Al Kα radiation. The phase structural characteristic was examined by XRD (M21X, Cu Kα radiation: λ = 0.154 nm). 2.3. Electrochemical Measurements. The electrochemical tests were obtained using a PARSTAT MC-1000 electrochemical workstation. The prepared NiO@NiMoO4 nanosheets with 1.4 mg cm−2, NiO nanosheets with 0.3 mg cm−2, and NiMoO4 with 1.0 mg cm−2 on the nickel foam were directly used as electrodes. The Hg/HgO electrode and platinum sheet were used as the reference electrode and the counter electrode, respectively. OER tests were performed in an O2−-purged aqueous solution of 1 M KOH in polypropylene beakers. All linear sweep voltammetry (LSV) curves were tested at a scanning rate of 5 mV s−1. All polarization potentials reported here are iRcompensated and converted to the reversible hydrogen electrode (RHE), and current densities are per geometric area. Chronopotentiometry was measured at 10 mA cm−2 for 12 h. The uncompensated solution resistance (Ru) was determined between the frequency range 0.01 Hz to 100 kHz with a 5 mV amplitude.

designed architectures for OER and investigate their electrochemical properties. Herein, we report a facile stepwise hydrothermal method to construct a novel 3D self-supported NiO@NiMoO4 core−shell architecture grown on nickel foam. The synthetic strategy of the NiO@NiMoO4 nanosheets as an efficient electrocatalyst can be described in Scheme 1. Ultrathin Ni(OH)2 nanosheets grown on nickel foam were first obtained using as the core through a simple solvothermal route. The ethylene glycol is a stabilizer to promote the nucleation and growth of the Ni(OH) 2 nanosheet structure.20 The nickel ions and molybdate ions were adsorbed on Ni(OH)2 nanosheets and further grew into tiny NiMoO4·xH2O nanosheets uniformly along Ni(OH)2 nanosheets after hydrothermal reaction. The NiO@NiMoO4 nanosheets were finally obtained after being calcined. This step-by-step strategy to anchor tiny secondary nanosheets on ultrathin primary nanosheets and construct a 3D self-supported core/shell heterostructure efficiently avoids the agglomeration problems of individual ultrathin nanosheets, together with combining the merits of both NiO and NiMoO4. The introduction of the NiMoO4 secondary nanosheets on the NiO primary nanosheets not only endows the heterostructure with high electrical conductivity and a large active area but also promotes an increase in oxygen vacancy content, which favors the improvement for OER catalytic performance. The ingenious architecture possesses the advantages of numerous diffusion channels for electrolyte ions, ideal pathways for electrons, and a large interfacial area for electrochemical catalysis. The Tafel plot for NiO@NiMoO4 core−shell architecture is as low as 32 mV dec−1, and the overpotential needed to reach 10 mA·cm−2 for NiO@NiMoO4 nanosheets is only 0.28 V. Our work not only suggests that NiO@NiMoO4 is a promising electrocatalyst with high performance but also provides guidance for the rational design of advanced integrated electrode materials with excellent stability, high porosity, and good electrical conductivity for OER.

2. EXPERIMENTAL SECTION 2.1. Synthesis of NiO@NiMoO4 Nanosheets Grown on Nickel Foam. All the reagents were analytical grade and used without further purification. The final product was prepared via a mild two-step hydrothermal reaction. Preparation of NiO nanosheets grown on nickel foam. In a typical synthesis, nickel foam was cleaned (1 cm × 1 cm) with acetone, diluted hydrochloric acid (2 M), water, and ethanol in turn under assistance of ultrasonication for 10 min. The treated nickel foam was transferred to a 25 mL Teflon-lined autoclave after being dried in a vacuum oven overnight. A 0.935 mmol amount of Ni(NO3)2·H2O and 3.75 mmol of CO(NH2)2 were dissolved in a 15 mL transparent solution containing water and ethylene glycol (EG) (the volume proportion of EG to water was 7) with constant magnetic stirring. The solution was transferred to the 25 mL autoclave after forming a homogeneous solution. Then the autoclave was heated for 4 h at 120 °C, and Ni(OH)2 grown on nickel foam was obtained. The X-ray

3. RESULTS AND DISCUSSION 3.1. Materials Characterization. The morphologies of the as-synthesized products were studied using SEM, as shown in Figure 1. The nickel foam substrate shows a flat and clean surface topography after acid treatment (Figure S2). The surface of nickel foam becomes rougher after the growth of nickel oxide (Figure 1a). It can be seen that nickel oxide ultrathin nanosheets are distributed uniformly on the surface of the nickel foam, which intertwine with each other to form a network structure (Figure 1b). The precursor nickel hydroxide nanosheets grown on nickel foam serve as the precursor and backbone for the hydrothermal growth of the NiMoO4 nanosheets in Figure S3(a). The morphology of nickel oxide is consistent with that of its precursor nickel hydroxide, both of B

DOI: 10.1021/acs.inorgchem.9b00162 Inorg. Chem. XXXX, XXX, XXX−XXX

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NaMoO4·2H2O) grew along the surface of nickel hydroxide nanosheets in the nucleation stage. The next stage was the growth of nickel molybdate nanosheets attributed to the “oriented attachment” and “self-assembly” processes.21,22 Figure 2 displays the chemical composition and crystal structures of as-synthesized nanosheets on nickel foam by the

Figure 2. XRD pattern of (a) NiO, (b) NiMoO4, and (c) NiO@ NiMoO4.

XRD pattern. Three high-intensity peaks located at 44.3°, 51.7°, and 76.3° can be indexed to the (111), (200), and (220) planes of nickel foam in all the products.23,24 The three reflection peaks located at 37.2°, 43.2°, and 62.9° are in agreement with the standard patterns of NiO, which can be indexed to its (111), (200), and (220) plane in Figure 2a,c.25,26 The patterns of NiMoO4 in Figure 2b are consistent with the standard patterns of NiMoO4 (JCPDS card: no. 45-0142), and the peaks at 27.4° and 33.7° correspond to (11−2) and (22− 2) planes of NiMoO4, respectively. After the growth of the NiMoO4, the NiO@NiMoO4 nanosheets in Figure 2c contain the peaks of NiMoO4 and NiO, indicating that NiMoO4 nanosheets grow on the nanosheets of NiO successfully. In addition, the diffraction peaks of NiO belonging to the NiO@ NiMoO4 are broader compared with that of the NiO alone on nickel foam, probably due to the winding of NiMoO4 nanosheets on the surface of the NiO nanosheets and the covering up of NiO by the NiMoO4 diffraction peaks.27 The weaker signal of NiMoO4 is due to the ultrathin features of the NiMoO4 nanosheets and the poor crystallinity. More details of the morphology and structure for the asobtained samples scraped off of the nickel foam are further characterized by TEM and HRTEM. The TEM image of NiO nanosheets is shown in Figure 3a. Figure 3b shows that the NiMoO4 products are sphere structures of 2 μm in diameter, which are composed of agglomerated nanosheets. The sheetlike structures with ultrathin features are clearly observed for NiO and NiO@NiMoO4 as shown in Figure 3c. It should be noted that some smaller nanosheets are covered by NiO nanosheets (Figure 3c) compared to the bare NiO nanosheets (Figure 3a), which correspond to NiMoO4 and are in good agreement with the SEM results. The lattice fringes of Figure 3d show an interplanar spacing of 0.21 nm, corresponding to the (200) plane of NiO, which is in agreement with the XRD results.28 Meanwhile, the ultrathin characteristics of the nanosheets are further demonstrated with the occurrence of the dark region, which can be regarded as the edge of an individual nanosheet.29 The thickness of the dark region is about 5.83 nm, indicating the characteristics of the few-atomscale arrangement. As shown in Figure 3e, the distinct lattice fringes of 0.33 nm correspond to the (11−2) plane of

Figure 1. SEM images of the products at different magnifications grown on nickel foam: (a, b) NiO nanosheets; (c, d) NiMoO4 microspheres; (e, f) NiO@NiMoO4 nanosheets (the inset is a higher magnification image).

which are cross-linked nanosheet structures. However, when nickel molybdate was directly grown on the nickel foam, the formed nickel molybdate nanosheets tended to agglomerate into microspheres larger than 1 μm in diameter (Figure 1c,d). When nickel molybdate grows on the nickel hydroxide nanosheet skeleton, the tiny NiMoO4 nanosheets are wrapped around the nickel hydroxide nanosheets uniformly and interconnect with each other, as shown in Figure S3(b). The final product NiO@NiMoO4 after calcination in Figure 1e,f is almost unchanged compared with the precursor. It can be seen that the morphology of the NiO nanosheets is well retained in view of the meandering path of NiMoO4 nanosheets. The large pores of the cross-linked nickel oxide nanosheets at the yellow dotted lines and the smaller holes between the nickel molybdate nanosheets at the red dotted lines form a highly porous structure. The inset of Figure 1f shows a higher magnification of the nanosheets. This superimposed ultrathin nanosheet structure avoids the problem of easy agglomeration for individual ultrathin nanosheets. Simultaneously, the loose porous nanostructure leads to a great increase of the active area and makes the electrolyte able to contact more active sites effortlessly. In order to explore the growth mechanism of the core−shell structure, samples obtained at different reaction times were observed. Only some nanoparticles grew on nickel hydroxide nanosheets at the beginning of the reaction (1 h). These nanoparticles converted into small nanosheets when the reaction time was 2 h (Figure S4a). The nickel molybdate nanosheets were further formed into cross-linked nanosheets as the reaction time increased (Figure S4b). The nickel hydroxide nanosheets grown on nickel foam served as the backbone for the hydrothermal growth of the NiMoO4 nanosheets (Figure S4c,d). The nickel molybdate nucleation centers derived from the raw materials (Ni(NO3)2·6H2O and C

DOI: 10.1021/acs.inorgchem.9b00162 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. TEM and HRTEM images of (a, d) NiO; (b, e) NiMoO4; and (c, f) NiO@NiMoO4.

Figure 4. XPS spectra of Ni 2p, Mo 3d, and O 1s.

NiMoO4.30 The HRTEM of NiO@NiMoO4 (Figure 3f) shows interplanar spacings of 0.21 and 0.26 nm, which correspond to (200) planes of NiO and (22−2) planes of NiMoO4, respectively.31,32 XPS measurements were carried out to further study the electronic structure and composition of the products. In Figure 4a, the intense peaks around 855.5 eV (satellite: 862 eV) and 873.1 eV (satellite: 880 eV) belonging to the Ni 2p3/2 and Ni 2p1/2 level are separated by 17.6 eV, which indicates the form of Ni in the products is Ni2+.33 The appearance of the peak at 853 eV for NiO and its disappearance for NiMoO4 and NiO@ NiMoO4 implicate the changes in the environment around the nickel atoms and can be explained by the nonlocal screening model.34,35 The peak at 852 eV of NiMoO4 belongs to nickel metal, a result of the exposure of nickel foam caused by the agglomeration of nickel molybdate nanosheets. The curveresolved data for the Ni 2p3/2 region with four peaks indicate

similar chemical environments and oxidation states for Ni2+ in Figure S5. Figure 4b,c show the XPS spectrum of the Mo 3d region for NiMoO4 and NiO@NiMoO4, which proved the successful coating of NiMoO4 on the NiO nanosheets. The two peaks around 232.1 and 235.2 eV corresponding to Mo 3d5/2 and Mo 3d3/2 are separated by 3.1 eV, which is a signature of the Mo6+ oxidation state.36 The XPS spectra of Mo 3d for the core−shell structure shifts slightly to lower binding energy compared with that of the pure nickel molybdate in Figure S6, demonstrating the interaction between NiO and NiMoO4 nanosheets. This close interfacial contact of NiO and NiMoO4 nanosheets facilitates charge transfer and promotes synergistic effects of the components, which can result in enhanced electrocatalytic activity.37,38 The curveresolved data of O 1s spectra are shown in Figure 4d,e,f to explain the contributions of three oxygen-containing species. The dominant peaks of O 1s between 529 and 531 eV (O1) D

DOI: 10.1021/acs.inorgchem.9b00162 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Electrocatalytic OER performance test of the products in 1 M KOH at room temperature: (a) linear sweep voltammograms (LSVs) with a scanning rate of 5 mV s−1; (b) Tafel plots; (c) chronopotentiometric curves at 10 mA cm−2 for 12 h; (d) typical cyclic voltammetry curves of NiO@NiMoO4 nanosheets with different scanning rates (the scanning rates are 5, 10, 15, 20, 25, 30, 35, 40, 45, and 50 mV s−1, respectively); (e) scanning rate dependence of current densities obtained at 1.08 V vs RHE; (f) electrochemical impedance spectroscopy.

alone after the test had been greatly damaged due to the agglomeration of NiO nanosheets and the detachment of NiMoO4 from the nickel foam. The introduced NiMoO4 nanosheets prevented the agglomeration of nickel oxide nanosheets during the electrochemical process and combined with nickel oxide closely on the nickel foam, which enhanced the structural stability. The XPS spectra of NiO@NiMoO4 before and after the stability test were studied and are shown in Figure S9. There is no significant change in the XPS spectra of the NiO@NiMoO4 before and after the test. According to recent studies, NiMoO4 and NiO are easily oxidized to NiOOH under oxygen evolution conditions.42,43 In this paper, this phenomenon has not been detected, which is due to the small amount or amorphous state of NiOOH.44,45 There are two well-known factors that affect electrocatalytic performance: electrochemical active surface area and conductivity. The electrochemical double-layer capacitance (Cdl) of all the products that reflect the electrochemical active surface area was investigated, as shown in Figure 5d,e. NiO@ NiMoO4 has a maximum Cdl of 10.17 mF cm−2, which indicates the largest active surface area of the hybrid oxide. The Cdl of NiO is 6.80 mF cm−2, which is higher than the 3.15 mF cm−2 of NiMoO4. Figure 5f shows the electrochemical impedance spectra of all the electrodes. The Nyquist plot shows that NiO@NiMoO4 has the smallest semicircle, which reflects the fastest charge transfer process between the electrode surface and electrolyte compared with the other electrodes. The larger semicircles in diameter for the NiO and NiMoO4 imply slower charge transfer efficiency. Both the conductivity and the active surface area have been increased after the coating of NiMoO4 nanosheets, which is beneficial to improve the electrochemical performance for OER. The demonstrated outstanding electrocatalytic performance for NiO@NiMoO4 nanosheets may first be ascribed to the poor crystallinity and ultrathin features, increasing the degree of structural disorder, which leads to the appearance for more active sites. The larger active surface area arose from the two kinds of ultrathin nanosheets, promoting the exposure of catalytic active sites. This novel self-supported core−shell

are resulting from the lattice oxygen in Figure 4d,e,f. The binding energy of O1 is slightly higher after loading the NiMoO4 nanosheets. The peak between 531 and 532 eV (O2) is generally related to oxygen vacancies.39,40 It can be seen that the peak area ratio (O2/O1) of NiO@NiMoO4 (0.81) is higher than that of NiO (0.53) and NiMoO4 (0.67). This means that nanosheet composites have the highest oxygen vacancy content. The peak at 532.7 eV can be attributed to bound waters of hydration at or near the surface.41 3.2. Electrocatalytic Performance for OER. The assynthesized NiO, NiO@NiMoO4, and NiMoO4 grown on nickel foam were used as the working electrode directly. As shown in Figure 5a, the overpotential needed to reach 10 mA cm−2 for NiO@NiMoO4 nanosheets is only 0.28 V. However, NiO and NiMoO4 need 0.34 V to achieve 10 mA cm−2. An obvious oxidation peak due to the transition of the oxidation state for nickel from Ni2+ to Ni3+ appears at 1.40 V for NiO@ NiMoO4. The significantly weakened oxidation peak for NiO and NiMoO4 can be attributed to their reduced electrochemical activity. NiO@NiMoO4 nanosheets exhibit the best catalytic activity, which leads to a huge difference in peak intensity. The Tafel slope for NiO@NiMoO4 is 32 mV dec−1 (Figure 5b), which is much lower than the 97 mV dec−1 for NiO and 44 mV dec−1 for NiMoO4. In addition, the pure nickel foam exhibits poor OER activity with a Tafel slope of 109 mV dec−1, as shown in Figure S7. The core−shell structure exhibits excellent electrochemical performance, with lower Tafel slope and overpotential compared with most of the other nickel-based oxides in Table S1. The stability of the products was further studied by chronopotentiometry at 10 mA cm−2 for 12 h, as shown in Figure 5c. The overpotential of NiO@NiMoO4 increased from 0.28 V to 0.30 V, an increase of only 0.02 V. However, the overpotentials for NiO and NiMoO4 increased by 0.04 and 0.03 V from 0.34 V, respectively. The SEM images (Figure S8) show that the morphology of the NiO@NiMoO4 nanosheets was almost maintained after the stability test, which indicates the excellent stability of the core−shell sheet structure. However, the morphologies of NiO and NiMoO4 grown E

DOI: 10.1021/acs.inorgchem.9b00162 Inorg. Chem. XXXX, XXX, XXX−XXX

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China (No. 2016 YFB0701100), and “13th 5-year-Plan” advanced research on equipment of the Equipment Development Department (6140A64020116QT02001) for financial support.

nanosheet with high porosity offers numerous diffusion channels between the electrolyte ions and the electrochemically active site and promotes the rapid release of bubbles generated during electrocatalytic processes. The effective assembly of NiO and NiMoO4 nanosheets prevents the aggregation of nanosheets as well as improves the conductivity during the electrochemical processes, which favors the improvement of the structural stability. In addition, after coating the secondary nanosheets, disorganized lattice coordination at the interface between NiO and NiMoO4 leads to a decrease in crystallinity (as shown by the XRD results) and the generation of more defects, which benefits the increase of the oxygen vacancy content. The higher content of oxygen vacancies for NiO@NiMoO4 nanosheets also helps to improve the electrochemical performance.39,46

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4. CONCLUSION A 3D porous NiO@NiMoO4 core−shell architecture grown on nickel foam was successfully synthesized with excellent electrocatalytic performance toward water oxidation. The intimate contact between NiO and NiMoO4 nanosheets prevented the agglomeration of the NiMoO4 nanosheets. The open space between the nanosheets allows for the easy diffusion of electrolyte into the interior of the electrode. Both the conductivity and the specific surface area increased after the coating of NiMoO4, which contributes to improving the OER catalytic performance. The introduction of more oxygen vacancies is also favorable for the enhancement of the catalytic performance. The Tafel plot for NiO@NiMoO4 nanosheets is 32 mV dec−1, which is much lower than NiO (97 mV dec−1) and NiMoO4 (44 mV dec−1). It requires 0.28 V of overpotential to deliver a current density of 10 mA cm−2. The overpotential increased by merely 0.02 V at 10 mA cm−2 after 12 h, demonstrating a suitable electrocatalyst for water oxidation with good operational stability.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00162. The XRD pattern of α-Ni(OH)2 on nickel foam, the morphology of nickel foam substrate after acid treatment, the XPS spectra of Ni 2p3/2, and the morphologies of the electrodes after the cycle (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Xiao Chen: 0000-0002-4421-6271 Wenjun Dong: 0000-0002-0888-0845 Xiubing Huang: 0000-0002-3779-0486 Ge Wang: 0000-0002-4069-4284 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank National Natural Science Foundation of China (51702013), National Key Research Development Program of F

DOI: 10.1021/acs.inorgchem.9b00162 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.9b00162 Inorg. Chem. XXXX, XXX, XXX−XXX