Engineering Ni(OH)2 Nanosheet on CoMoO4 Nanoplate Array as

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Engineering Ni(OH)2 Nanosheet on CoMoO4 Nanoplate Array as Efficient Electrocatalyst for Oxygen Evolution Reaction Yan Xu, Linjie Xie, Di Li, Rong Yang, Deli Jiang, and Min Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02663 • Publication Date (Web): 21 Oct 2018 Downloaded from http://pubs.acs.org on October 22, 2018

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ACS Sustainable Chemistry & Engineering

Engineering Ni(OH)2 Nanosheet on CoMoO4 Nanoplate Array as Efficient Electrocatalyst for Oxygen Evolution Reaction Yan Xua , Linjie Xiea , Di Lib , Rong Yanga , Deli Jiang a,*, Min Chen a,* a

School of Chemistry and Chemical Engineering, Jiangsu University, Xuefu Road 301, Zhenjiang 212013, China b

Institute for Energy Research, Jiangsu University, Xuefu Road 301, Zhenjiang 212013, China Corresponding author: Deli Jiang, Min Chen E-mail address: [email protected]; [email protected]

ABSTRACT The exploration of earth-abundant and high-efficiency electrocatalysts for water oxidation is of great significance for sustainable energy conversion. Co-Mo based bimetallic oxides are considered as promising candidates for oxygen evolution reaction (OER) due to their high intrinsic activity and low cost. In this work, we reported an interfacial engineering design of CoMoO 4 nanoplate arrays wrapped with Ni(OH)2 nanosheets supported on Ni foam for high-performance OER. Remarkably, benefiting from the hierarchical heterostructure arrays with strong interfacial interaction, enlarged surface areas and more active sites, the optimized CoMoO 4-Ni(OH)2 electrocatalyst manifests the excellent OER catalytic performance with a low overpotential of 349 mV at the current density of 100 mA cm-2 , a long-term stability and high (nearly 100%) Faradic efficiency, which is superior to those of most previously reported Co-Mo based bimetallic oxides electrocatalysts. Therefore, this interfacial engineering demonstrates an effective strategy for designing and fabricating high performance electrocatalysts for OER reaction. KEYWORDS 1

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CoMoO4 , Ni(OH)2 , nanoarrays, interfacial engineering, oxygen evolution reaction INTRODUCTION Increasingly growing energy consumption and severe environmental pollution have aroused wide public concern over the recent years, reflecting a burning issue to explore alternative and sustainable energy conversion and storage systems to deal with the current crisis. 1,2 As an effective means of generating molecular oxygen through the process of electrochemical water oxidation, oxygen evolution reaction (OER) plays a crucial role in many fields associated with energy conversion and storage, such as water splitting and metal–air batteries.3-6 However, its complicated four-electron transfer process leads to the sluggish kinetics and requirement of high overpotential, ultimately restricting the overall efficiency of energy conversion. 7-9 As a consequence, it is of utmost urgency to develop OER electrocatalysts with low-overpotential, high-efficiency and long-term stability to accelerate the reacting process of OER.10-13 IrO2 and RuO 2 have been considered as the benchmark of OER electrocatalysts, whereas the constraints of resource scarcity and high cost severely prohibit their widespread applications, further stimulating the search for earth-abundant and low-cost transition- metal-based OER catalysts to replace these noble- metal oxides.14-20 In recent years, tremendous research efforts have been devoted to bimetallic oxides for their superior catalytic characteristics when compared to corresponding single-component oxides.21,22 So far, transition- metal molybdates have drawn much attention for their relatively high electrochemical activities due to the rich polymorphism and high conductivity of metal molybdates.23 Among them, bimetallic CoMoO4 is considered to be an impressive OER catalyst due to its high intrinsic activity stemmed from the synergetic effect between the excellent redox behavior of Co and high electrical 2


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conductivity of Mo.24,25 Although some strategies such as nanostructure control and heterostructure construction have been made to improve the OER catalytic activity of CoMoO4 to some extent, the limited active sites and relatively poor charge transport efficiency still impede the catalytic activity of these CoMoO 4 -based electrocatalysts.26-28 Recently, direct growth of bimetallic metal oxides nanoarrays on the current collector to form three-dimensional (3D) electrodes has been proven to be an effective way to expose more active sites and facilitate charge transfer in the interconnecting networks.29-31 Furthermore, the integration of interface engineering and the 3D architecture design could further boost the OER performance because of the strong coupling and synergistic effects between individual components on the current collector.32-34 In spite of these efforts, the utilization of interface engineering and architecture design to improve the OER performance of CoMoO4 electrocatalyst has not been achieved yet. Herein, we set out to design and construct an novel integrated 3D architecture by engineering Ni(OH)2 nanosheet on CoMoO4 nanoplates arrays supported on Ni foam (CoMoO 4 -Ni(OH)2 /NF). Such CoMoO 4 -Ni(OH)2 /NF hierarchical structure have several advantages: (i) The Ni(OH)2 nanosheets electrodeposited onto CoMoO 4 greatly enlarge the surface area and afford more active sites. (ii) The strong electronic interactions between CoMoO 4 and Ni(OH)2 would facilitate the charge transfer and lead to improved OER performance. (iii) The unique hierarchical structure provides enhanced surface area for diffusion of ions and water molecules. (iv) The highly conductive NF with 3D open structure could enhance the conductivity and facilitates the rapid release of oxygen gas bubbles. The resulting CoMoO4 -Ni(OH)2 /NF heterostructure arrays manifest the excellent OER catalytic performance with a low overpotential of 349 mV at the current density of 100 mA cm-2 , a long-term stability and high (nearly 100%) Faradic efficiencies, which is 3



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superior to the most previously reported molybdates and Co-containing oxides. This work provides a rational design of heterostructures as efficient electrocatalyst by combination of interface engineering and heterostructure construction. EXPERIMENTAL Materials. Cobalt

(II)

nitrate

hexahydrate

(Co(NO 3 )2 ·6H2 O),

sodium

molybdate

dihydrate

(Na2 MoO4 ·2H2 O), Nickel (II) nitrate hexahydrate (Ni(NO 3 )2 ·6H2 O), potassium hydroxide (KOH), hydrochloric acid (HCl), absolute ethanol and acetone were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Iridium powder (Ir/C, 5 wt%) was a commercial product obtained from Aladdin (Shanghai, China). Nafion solution (5 wt%) was bought from Dupont China Holding Co., Ltd (Tianjin, China). NF with a thickness of 1.6 mm and 120 ppi (pore per square inch) was purchased from Jia Shide Foam Metal Co., Ltd (Suzhou, China). All the chemicals and reagents were of analytical grade and used as received without further purification. Deionized water was used throughout the experiments. Synthesis of CoMoO4 NF. Prior to the synthesis, NF substrate was soaked in 3 M HCl for 30 minutes to remove the oxide layer of surface, and then rinsed with ethanol, acetone and deionized water by ultrasonication for 30 min each, and dried at 60 °C overnight. CoMoO4 was prepared by a facile hydrothermal reaction, according to the previously reported method. 27 Typically, 0.363 g of Co(NO 3 )2 ·6H2 O and 0.302 g of Na2 MoO4 ·2H2 O were mixed in 40 mL deionized water by vigorous stirring for 20 minute. The NF (1×3 cm) was placed standing against the wall of a 50 mL Teflon- lined stainless steel autoclave. The obtained solution was transferred into the autoclave to immerse NF and heated at 180 °C for 12 h. 4


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When the autoclave was cooled down to room temperature, NF coated with purple product was washed by deionized water and ethanol for 3 times and dried in vacuum at 50 °C overnight. Then, NF with the as- grown hydrate precursors was annealed in N 2 atmosphere at 300 °C for 1 h with a ramp rate of 1°C·min-1 . Synthesis of CoMoO4 -Ni(OH)2 /NF Electrocatalyst. Ni(OH)2 nanosheets were loaded on the surface of CoMoO4 by the method of potentiostatic cathodic electrodeposition, which was performed using the CoMoO4 NF (1×1 cm) as the working electrode, a platinum plate as the counter electrode, and Ag/AgCl as the reference electrode, respectively. The electrolyte is 0.1 M Ni(NO 3 )2 ·6H2 O aqueous solution. Ni(OH)2 ultrathin nanosheets were deposited on CoMoO4 nanoplates by the constant potential of -0.9 V for different time. The obtained CoMoO 4 -Ni(OH)2 electrocatalyst on NF was rinsed several times with deionized water and ethanol, and dried in vacuum at 50 °C overnight. Synthesis of Commercial Electrode. The CoMoO4 -Ni(OH)2 electrodes were directly used as the anodes for electrochemical characterizations. While the commercial electrocatalyst Ir/C was loaded onto NF for comparison as the following the steps: 10 mg Ir/C (5 wt%) was dissolved in the mixed solution containing 0.7 mL ethanol, 0.3 mL deionized water and 40 μL of Nafion solution (5 wt%), with the sonication treatment for about 30 minutes to form a homogeneous dispersion, and then the electrocatalyst ink was uniformly coated onto NF (1×1 cm), and dried in vacuum at 50 °C for 10 h.35 Structural Characterization. The as-prepared CoMoO4 -Ni(OH)2 were characterized by X-ray diffraction (XRD, Cu Kα radiation; λ = 1.5406 Å) with a Bruker D8 Advance X-ray diffractometer in the range of 10 to 80° 5



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2θ at a scan rate of 2°/min. X-ray photoelectron spectroscopy (XPS, ESCA PHI500 spectrometer) was operated to analyze the surface of samples with an Al Kα radiator. The morphology and structure were characterized using scanning electron microscopy (SEM, S-4800 II FESEM), transmission electron microscopy (TEM, Tecnai 12), and high-resolution TEM (HRTEM, Tecnai G2 F30 S-Twin TEM) with an acceleration voltage of 15 kV, 120 kV and 300kV, respectively. The specific surface area and porous structures were characterized by the TriStar II 3020-BET/BJH Surface Area (BET). Electrochemical Measurements. All electrochemical measurements were performed using a CHI-760E Electrochemical Workstation within a standard three-electrode setup consisting of working electrode (the prepared electrocatalyst), counter electrode (a platinum wire), and reference electrode (Ag/AgCl electrode). Electrochemical performances, including cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and amperometric i-t curve (I-t) were tested in 1 M KOH aqueous solution (pH = 13.6). All the measured potentials were calibrated to the reversible hydrogen electrode (RHE) scale based on the equation: ERHE = EAg/AgCl + 0.197 V + 0.059 × pH.

(1)

The working electrode was firstly activated by CVs at a scan rate of 10 mV s -1 until reaching a steady-state situation. LSV curves were obtained at a sweep rate of 5 mV s -1 from 0 to 0.8 V with 90% iR-correction. EIS measurement was carried out using AC impedance spectroscopy over a frequency range from 0.01 to 10 4 Hz. The electrochemical active surface area (ECSA) was measured from double- layer capacitance (C dl), which was obtained from CVs in a small potential range (1.02 ~ 1.09 V vs RHE) with the scan rates of 20, 40, 60, 80, 100, 120, 140, and 160 mV s -1 . 6


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The Cdl was obtained by plotting the ΔJ = (Ja - Jc) corresponding to midpoint of the potential against the scan rate. The linear slope is twice of the C dl.36 The Turnover Frequency (TOF) values was calculated according to the equation: TOF = (j ×A)/(4 × F × m)

(2)

Where j is the current density at the overpotential of 300 mV, A is the surface area of the electrode (1 cm-2 ), F is the Faraday constant (96485 C/mol), and m is the number of moles of the active materials that deposited onto NF. The long-term durability measurements were carried out using the chronoamperometric and CV methods. The fresh KOH solution was added into electrolytic cell to compensate the consumption of electrolytes during the durability measurements. RESULTS AND DISCUSSION The schematic diagram in Figure 1 describes the synthesis process of CoMoO4 -Ni(OH)2 architecture. The pretreated conductive NF substrate takes the advantages of high surface area and porous structure, which is favorable for the fast electron transfer and rapid release of oxygen gas bubbles.37-39 The CoMoO4 precursor are grown in-situ on NF by means of a facile hydrothermal method, in which cobalt nitrate reacts with sodium molybdate in aqueous solution. After calcination, CoMoO4 precursor is dehydrated into CoMoO4 . The interaction between CoMoO4 nanoplates and NF results in a strong bonding force between active species and substrate. 40 Subsequently, Ni(OH)2 nanosheets were coated directly on the surface of CoMoO4 via electrodeposition, leading to formation of hierarchical CoMoO4 -Ni(OH)2 nanoarrays. The morphologies and microstructures of the as-prepared bare CoMoO4 and CoMoO4 -Ni(OH)2 nanoarrays were investigated by the detailed microscopic characterizations and the relevant results are shown in Figure 2 and Figure S1 (Supporting Information). The SEM in Figure 2a clearly shows 7



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that CoMoO 4 nanoplates with smooth surface and thickness of around 128 nm grow uniformly and vertically on the surface of NF to form well-defined nanoarrays with an open network structure (Figure S1). Of note, this interconnected nanoplate arrays could serve as the ideal conductive scaffolds for the subsequent growth of the Ni(OH)2 nanosheets and be highly accessible for the electrolyte when used as the working electrode. After electrodeposition, the interconnected structures of bare CoMoO 4 nanoarrays were well retained and the loading amount of Ni(OH)2 nanosheets can be well controlled by changing the deposition time (Figure S2). At short deposition time (200 s), some tiny Ni(OH)2 nanoparticles were deposited on CoMoO4 nanoplate surface owing to the inadequate growth of Ni(OH)2 . The longer deposition time (i.e., 400 s, 600 s, and 800 s) results in the uniform assembly of Ni(OH)2 nanosheets on CoMoO 4 surface, forming well-defined crumpled nanoplates heterostructures with a dense Ni(OH)2 nanosheet layer (Figure 2b and Figure S2). Such well-defined heterostructures were further demonstrated by TEM images (Figure 2c and d). Compared to the bare CoMoO4 nanoplate, the outer layer of CoMoO 4 -Ni(OH)2 -400s become rougher and thicker compared to bare CoMoO 4 nanoplate, indicating that Ni(OH)2 nanosheets were successfully coated on its surface. The HRTEM image in Figure 2e provides more interfacial information about the heterostructures in detail. It can be found that two distinct lattice fringes with spacing of 0.275 nm and 0.230 nm appear clearly, which correspond to the (-131) plane of CoMoO4 arrays and (101) plane of Ni(OH)2 , respectively. The HAADF-STEM image (Figure 2f) and corresponding EDS elemental mapping images (Figure 2g–j) suggest that cobalt, molybdenum, nickel and oxygen elements are uniformly distributed throughout the whole CoMoO 4 -Ni(OH)2 -400s, further confirming that Ni(OH)2 was successfully loaded on the surface of CoMoO 4 nanoplates. 8


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The phase and composition of CoMoO4 -Ni(OH)2 as well as the pristine CoMoO 4 were described by the XRD analyses, as shown in Figure 3. On account of the strong diffraction intensity of NF substrate, the peak signals assigned to the electrocatalyst materials appear to be relatively weak. For the CoMoO 4 -Ni(OH)2 and bare CoMoO4 , the relatively weak diffraction peaks at 2θ values of 13.6°, 24.8°, 28.4°, 39.5°, 59.3°can be ascribed to the (001), (021), (002), (040) and (024) lattice planes of monoclinic CoMoO 4 (JCPDS No. 21-0868), respectively.27 The characteristic diffraction peaks at 34.1° and 62.2° in the CoMoO4 -Ni(OH)2 nanoarrays correspond well to the (012) and (110) lattice planes of Ni(OH)2 (JCPDS No. 38-0715),41 indicating the coexistence of CoMoO4 and Ni(OH)2 on NF. In order to investigate the oxidation states and chemical composition of as-synthesized CoMoO4 and CoMoO4-Ni(OH)2 -400s, XPS analyses were performed. As shown in Figure 4a, the presence of additional peaks at around 646.8, 856.2, and 873.8 eV in the survey spectrum of CoMoO4-Ni(OH)2 -400s indicates the successful combination of Ni element. The Ni 2p core-level spectra (Figure 4b) CoMoO 4 -Ni(OH)2 -400s includes two spin-orbit peaks located at 856.3 and 873.9 eV, which could be attributed to Ni2+.42 Compared with bare Ni(OH)2 , the Ni 2p spectrum has a slightly positive shift towards higher binding energy, revealing that there is a strong interfacial interaction

between

CoMoO4

and

Ni(OH)2 .

For

Co

2p

spectrum

(Figure

4c)

of

CoMoO4-Ni(OH)2 -400s, two major peaks centered at 781.6 and 797.5 eV, are typically ascribed to the Co 2p3/2 and Co 2p1/2 .43 It is worth noting that, compared to those of bare CoMoO4 , the binding energies of Co 2p3/2 and Co 2p1/2 have a positive shift of 0.4 eV. This shift suggests the presence of strongly interfacial electronic interactions between Ni(OH)2 and CoMoO4 , indicative of the achievement of coupled interfaces. The Mo 3d core-level peak (Figure 4d) of pure CoMoO4 appears 9



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at 231.8 and 234.9 eV, affected by the interfacial interactions between Ni(OH)2 and CoMoO4 as well, the Mo 3d spectrum of CoMoO4 -Ni(OH)2 -400s has positively shifted 0.3 eV to 232.1 and 235.2 eV.44 The O 1s spectrum (Figure 4e) for CoMoO4 -Ni(OH)2 -400s composite can be deconvoluted into two segments. The binding energy value at 530.5 eV is ascribed to the lattice oxygen in CoMoO 4 , while the binding energy at 532.4 eV is assigned to the oxygen in hydroxide ions, in agreement with Ni(OH)2 .45 In comparison, primal CoMoO4 of the O 1s spectrum only shows one peak at 530.5 eV, which corresponds to the oxygen ions in oxides. 46 These spectroscopic results confirm that the Ni(OH)2 is combined with CoMoO4 in the resultant heterostructures. The porosity and specific surface area of CoMoO4 and CoMoO4 -Ni(OH)2 -400s were characterized through the N 2 adsorption-desorption isotherms method. As can be seen from Figure 4f, there is a typical type IV isotherm, featuring a H3-type hysteresis loop on the basis of the IUPAC classification, which indicates the presence of mesoporous structure of the composites. It can be found that the specific surface area of bare CoMoO4 was 1.65 m2 g-1 , which is smaller than that of CoMoO 4 -Ni(OH)2 -400s (4.39 m2 g-1 ). The improvement of specific surface area is of great benefit to provide more catalytic active cites and possibly absorb more active species and reactants on the surface, which could improve the catalytic activity.47 The OER catalytic performances of CoMoO4 -Ni(OH)2 nanoarrays were evaluated through a series of tests conducted in 1 M KOH solution by using a three-electrode configuration. The pure NF, pristine CoMoO4 and NF modified with commercial Ir/C were also tested for comparison. Figure 5a displays the polarization curves with 90%- iR correction. The oxidation peaks at 1.38 V (vs RHE) are attributed to the oxidation of Ni2+ to Ni3+, which was needed for the high activity in the water oxidation reaction.48 The corresponding overpotentials at the current density of 100 mA 10


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cm-2 (η100 ) were compared in Figure 5b. It is obvious that the CoMoO4 -Ni(OH)2 heterostructures exhibit good performances for OER with lower overpotentials than those of bare CoMoO4 (η100 = 413 mV), Ni(OH)2 (η100 = 427 mV), and even the Ir/C (η100 =405 mV). It can be seen that the optimal CoMoO4 -Ni(OH)2 -400s only demands 349 mV to reach the current density of 100 mA cm-2 . It is worth noting that this overpotential is much lower than most of other previously reported molybdates and Co-containing oxides OER electrocatalysts (Table S3, Supporting Information). To offers insight into the OER reaction kinetics, Tafel slopes derived from the preceding polarization curves were shown in Figure 5c, which were calculated by the equation: η = b log j + a

(3)

where η represents overpotential, b refers to Ta fel slope, and j is on behalf of the current density.49 Remarkably, CoMoO4 -Ni(OH)2 -400s exhibits a smaller Tafel slope of 67.6 mV dec -1 , as compared to commercial Ir/C loaded onto NF (89.4 mV dec -1 ), pure CoMoO 4 (91.7 mV dec-1 ), pure Ni(OH)2

(111.2

mV

CoMoO4-Ni(OH)2 -200s

dec-1 ) (74.7

and mV

other dec-1 ),

CoMoO4 -Ni(OH)2

composites

CoMoO4 -Ni(OH)2 -600s

(70.7

mV

including dec-1 ),

CoMoO4-Ni(OH)2 -800s (80.9 mV dec-1 ). Such low Tafel slope indicates the efficient kinetics of OER reaction, which is due to the favo rable electron transport in the CoMoO4 -Ni(OH)2 heterostructures. In quest of the OER kinetics of CoMoO4 -Ni(OH)2 nanoarrays, EIS measurement was performed. By fitting the EIS spectra with a simplified Randles equivalent circuit, the charge transfer resistance (Rct ) was obtained from the Nyquist plots. 50 It is obvious in Figure 5d that CoMoO4-Ni(OH)2 -400s shows a much smaller Rct (1.43 Ω) than that of CoMoO4 (2.64 Ω), CoMoO4-Ni(OH)2 -200s (2.36 Ω), CoMoO4-Ni(OH)2 -600s (2.28 Ω) and CoMoO 4-Ni(OH)2 -800s (2.18 Ω) (Table S1, Supporting Information), indicating the fastest charge-transfer process in the 11



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heterostructures, confirming the abovementioned relatively rapid OER kinetics. In order to further evaluate the accessibility of active sites, the electrochemically active surface areas (ECSAs) of the materials were estimated on the basis of the electrochemical double-layer capacitance (C dl), which were tested in the form of CV from 1.02 to 1.09 V vs RHE at the scan rates ranging from 20 to 160 mV s-1 (Figure S3). As can be seen from Figure 5e, CoMoO4 -Ni(OH)2 -400s exhibits the highest C dl value (5.06 mF cm-2 ) among the samples, signifying to possess the largest ECSA, which is almost 3 folds than that of pure CoMoO 4 , approximately 1.6 folds than that of CoMoO4-Ni(OH)2 -600s and about 2 times bigger than that of the other two CoMoO4 -Ni(OH)2 composites. The increased ECSA of CoMoO4 -Ni(OH)2 -400s reflects the more exposure of active sites, mainly owing to the uniformly assembled nanoarrays with larger exposed surface area. The outer Ni(OH)2 nanosheet arrays have not yet been formed completely in the CoMoO 4 -Ni(OH)2 -200s, resulting in the small ECSA value,

which leads to a low OER activity. In the

CoMoO4-Ni(OH)2 -400s catalyst, Ni(OH)2 nanosheets were uniformly coated on the surface of CoMoO 4 nanoarrays, resulting in good synergistic effect between the CoMoO4 and Ni(OH)2 , contributing to the optimization of OER performance. However, in the CoMoO4-Ni(OH)2 -600s and CoMoO4-Ni(OH)2 -800s catalysts, excessive Ni(OH)2 nanosheets deposited onto CoMoO 4 nanoplate surface will hinder the accessibility of OH- in the electrolyte to some active sites on the electrode surface to some extent, thus lowering the catalytic performance.51 It can be induced that optimal interface control is pivotal to achieve the maximum enhancement of OER catalytic activity. It is unlikely that such a small variation in ECSA fully accounts for the dramatic difference in performance.52 To further assess the intrinsic activity, we normalized the current density to ECSA and found that CoMoO4 -Ni(OH)2 -400s still shows a larger current density than those of other 12


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catalysts at the same overpotentials (Figure 5f). This result indicates that CoMoO4 -Ni(OH)2 -400s is intrinsically more active than bare CoMoO4 and other CoMoO4 -Ni(OH)2 composites. It is crucial for catalysts to possess outstanding stability and durability for practical application. The I-t curve in Figure 6a shows that CoMoO4 -Ni(OH)2 -400s exhibits the excellent stability compared with the original current density after continuous oxygen evolution of 100 h. In addition, the long-term durability of the catalyst is further confirmed by the polarization curve (Figure 6b) after I-t test. Even after 1000 CV cycles, the polarization curve of CoMoO4 -Ni(OH)2 -400s approximates to the initial one on the condition of neglecting the offset of the nickel peak, indicating the excellent stability and durability in alkaline so lution, which provides more evidence for CoMoO 4 -Ni(OH)2 -400s to be a promising catalyst in the field of water oxidation. In order to further investigate the inherent catalytic activity of CoMoO4 -Ni(OH)2 composites towards OER, the mass activity, specific activity, and turnover frequency (TOF) were also taken into consideration, which are the basic but principal factors that influence the catalytic performance.53 The values of mass activity (A g-1 ), specific activity (mA cm-2 ) and TOF (s-1 ) are calculated from the catalysts with the loading density m (3.6 mg cm-2 ) and the current density j (mA cm-2 ) at η = 300 mV, following the formulas of Mass activity = j/m, Specific Activity = j/(10 × m × SBET ) and TOF = (j × A)/(4 × F × m) which has mentioned above, respectively. 54 It can be found that the mass activity and specific activity of CoMoO4 -Ni(OH)2 -400s catalyst are 11.14 A g-1 and 0.25 mA cm-2 , superior to bare CoMoO4 and other CoMoO4 -Ni(OH)2 heterostructures (Table S2 and Figure S4). Furthermore, CoMoO4 -Ni(OH)2 -400s shows the highest TOF value of 0.0063 s-1 among all the samples, which is about 1.58, 1.34, 2.33 and 5.25 times higher than that of CoMoO4-Ni(OH)2 -200s, CoMoO4-Ni(OH)2 -600s, CoMoO4-Ni(OH)2 -800s and bare CoMoO4 , 13



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respectively, highlighting its excellent performance for OER catalysis. Faradaic efficiency (FE) is another significant parameter to evaluate OER activity of catalysts by comparing the O 2 quantity of experimentally evolved with theoretically calculated.55 The amount of O 2 evolved during the electrolysis is presented in Figure S5. It is worth noting that the CoMoO4 -Ni(OH)2 exhibited nearly 100% Faradaic efficiency, suggesting the high energy-conversion rate of CoMoO4 -Ni(OH)2 -400s catalyst on water oxidation. The catalysts after OER tests were further carried out by XPS measurement to identify the phase transition during the OER process. Figure S6a displays the Ni 2p XPS spectra in CoMoO4-Ni(OH)2 -400s after OER, in which the peaks at 856.8 and 873.9 eV are indexed to Ni2+ and the peaks at 855.6 and 872.4 eV are ascribed to Ni3+, indicating Ni3+ oxidation state (NiOOH) has appeared during OER.56 The Co spectrum (Figure S6b) of CoMoO4 -Ni(OH)2 -400s after OER are fitted with two different peaks, where the peaks at 782.0 and 798.4 eV are indexed to Co 2+ and the peaks at 780.6 and 796.9 eV are ascribed to Co 3+, revealing that Co2+ in CoMoO4 -Ni(OH)2 -400s has been partly oxidized into oxyhydroxides phase, which is also the real active species for the OER. As can be seen in Figure S6c, the Mo 3d spectra of CoMoO4 -Ni(OH)2 -400s after OER has negatively shifted 0.3 eV to 231.8 and 234.9 eV compared with that before OER owing to the interfacial interactions during OER. The peak analysis of O1s spectrum for CoMoO4 -Ni(OH)2 -400s after OER (Figure S6d) shows three components at 529.9, 530.8, and 531.7 eV, which corresponds to metal–oxygen bond, hydroxide species and adsorbed H2 O respectively, which confirms the existence

of nickel and

cobalt

with

different

oxidation

states

as

compared

with

CoMoO4-Ni(OH)2 -400s before OER.57,58 Compared with the XPS spectrums before and after OER, it can be clearly found that the Ni 2p peaks shift positively, while Co 2p shift negatively after OER, 14


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indicating electron transfers from Ni sites to Co sites during the OER process, resulting in the oxidation of Ni2+ and Co2+, possibly creating a synergistic effect to lower the energy barrier for the OER, which contributes greatly to the high electrocatalytic activity. 12,59 Notably, the outstanding OER performance of the prepared CoMoO4 -Ni(OH)2 is comparable and even superior to previously reported molybdates and Co-containing OER electrocatalysts (Table S3). This outstanding catalytic performance can be mainly ascribed to the following reasons: (i) the ultrathin Ni(OH)2 nanoplates homogeneously distributed on the surface of CoMoO 4 could enlarge the specific surface area and endow the high-density active sites; (ii) The strong electronic interactions between CoMoO 4 and Ni(OH)2 would facilitate the charge transfer, leading to enhancement of OER performance; (iii) The unique hierarchical structure provides enhanced surface area for diffusion of ions and water molecules; (iv) The highly conductive NF not only offers a 3D backbone to support the CoMoO4 -Ni(OH)2 nanoarrays, but also enhances the conductivity and facilitates charge transfer and the rapid release of oxygen gas bubbles. By integrating these aforementioned advantages, the as-prepared CoMoO4 -Ni(OH)2 nanoarrays exhibit impressive OER performance with superior activity. CONCLUSIONS In summary, a novel integrated 3D architecture constructed by engineering Ni(OH)2 nanosheet on CoMoO 4 nanoplates arrays supported on NF is reported. The optimized CoMoO4 -Ni(OH)2 electrocatalyst exhibits a superior OER catalytic activity with a low overpotential of 349 mV at the current density of 100 mA cm-2 , as well as small Tafel slope and enhanced turnover frequency. The remarkable electrocatalytic performance can be mainly ascribed to the hierarchical heterostructure arrays with strong interfacial interaction, enlarged surface areas, more active site and 3D porous 15



Page 16 of 31

structure. The outstanding OER performance make this CoMoO 4 -Ni(OH)2 heterostructures as promising candidate for water-splitting, and we anticipate that our work could provide new insights into the rational design of advanced electrocatalysts.

ASSOCIATED CONTENT Supporting Information. SEM images of bare CoMoO4 and CoMoO4 -Ni(OH)2 samples; CV curves; Nitrogen sorption isotherm and specific activity calculations normalized to the BET surface area; The amount of oxygen

theoretically

calculated

and

experimentally

measured

versus

time

for

CoMoO4-Ni(OH)2 -400s; High-resolution XPS spectra of CoMoO4 -Ni(OH)2 -400s before and after OER measurements; The simulated series resistance (Rs) and charge transfer resistance (Rct ) based on the Nyquist plots in the catalysts; Comparison of OER activity about CoMoO4 -Ni(OH)2 composites catalysts; Comparison of the electrocatalytic activity of CoMoO 4 to recently reported Co-based OER electrocatalysts in the base electrolyte. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]

NOTES The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the financial supports of National Nature Science Foundation of China 16


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(No. 21606111, 21878130, and 21576121), Natural Science Foundation of Jiangsu Province (BK20150482) and Key Research Plan of Zhenjiang City (GY2015031 and GY2015044).

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24


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Calcination

Hydrothermal

bare NF

substrate

CoMoO4 precursor

precursor

Electrodeposition

CoMoO4/NF

CoMoO4

CoMoO4-Ni(OH)2/NF

Ni(OH)2

Figure 1 Schematic illustrating for the synthesis of the CoMoO4 -Ni(OH)2 /NF nanoarrays.

25



(a)

(b)

200 nm

d(CoMoO4)=128 nm

(c)

Page 26 of 31

d(Ni(OH)2)=16 nm

200 nm

d(400s)=161 nm

(e)

(d)

0.230nm (101)

0.275nm (-131) Ni(OH)2

100 nm

50 nm

1 nm

CoMoO4

(f)

(g)

(h)

(i)

(j)

100 nm

Co-K

Mo-K

Ni-K

O-K

Figure 2 SEM images of (a) CoMoO4 and (b) CoMoO4 -Ni(OH)2 -400s; (c) TEM image of CoMoO4 ; (d-f) TEM, HRTEM and HAADF-STEM images of CoMoO4 -Ni(OH)2 -400s; (g-j) the corresponding EDS elemental mapping images of Co, Mo, Ni, and O for CoMoO4 -Ni(OH)2 -400s nanoarray.

26


▲

（040）

★ ★

★

（012）

（021）（002）

（001）

CoMoO4 PDF#21-0868 Ni(OH)2 PDF#38-0715

★

◆ ◆

★CoMoO4 ▲ Ni(OH)2 ◆Ni foam ◆

★▲

e

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


（024）（110）

Page 27 of 31

d c b a

10

20

30

40 50 60 2 Theta (degree)

70

80

Figure 3 XRD patterns of the as-prepared samples grown on NF: (a) CoMoO 4 , (b) CoMoO4-Ni(OH)2 -200s,

(c)

CoMoO4 -Ni(OH)2 -400s,

(d)

CoMoO 4-Ni(OH)2 -600s,

(e)

CoMoO4 -Ni(OH)2 -800s.

27


Ni 2p

(b) CoMoO -Ni(OH) -400s 4

2

856.3 Ni 2p3/2

Intensity (a.u.)

Co 2p

Co LMM

Ni LMM

Mo 3p

Mo 3s

Mo 3d C 1s

Mo 4p Co 3p Co 3s

Intensity (a.u.)

CoMoO4-Ni(OH)2-400s

O 1s

(a)

Sat.

Page 28 of 31

Ni 2p

873.9 Ni 2p1/2

Sat.

Ni(OH)2 856.2 Ni 2p3/2

Sat.

873.8 Ni 2p1/2

Sat.

CoMoO4 400 600 Binding Energy (eV)

CoMoO4-Ni(OH)2-400s

Intensity (a.u.)

781.6 Co 2p3/2

Sat.

775

780

785 790 795 800 Binding Energy (eV)

Co 2p

805

810

CoMoO4 530.5

528

530 532 534 Binding Energy (eV)

860 865 870 875 Binding Energy (eV)

CoMoO4-Ni(OH)2-400s

Sat.

532.4

530.5

(d)

855

232.1 Mo 3d5/2

O 1s

CoMoO4-Ni(OH)2-400s

850

1000

797.1 Co 2p1/2 Sat.

CoMoO4

770

(e)

797.5 Co 2p1/2

Sat.

781.2 Co 2p3/2

800

Intensity (a.u.)

(c)

200

536

CoMoO4

228

885

Mo 3d

235.2 Mo 3d3/2

234.9 Mo 3d3/2

231.8 Mo 3d5/2

230

880

232 234 236 Binding Energy (eV)

238

240

(f)8 Volume adsorbed (cm3 STP g-1)

0

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Co 2s O KLL


6

CoMoO4 CoMoO4-Ni(OH)2-400s

4

2

0 0.0

0.2

0.4 0.6 0.8 Relative pressure (P/P0)

1.0

Figure 4 (a) XPS survey spectrum of the as-synthesized CoMoO 4 and CoMoO4 -Ni(OH)2 -400s; High-resolution XPS spectra of (b) Ni 2p, (c) Co 2p, (d) Mo 3d and (e) O 1s region; (f) Nitrogen sorption isotherm of CoMoO4 and CoMoO4 -Ni(OH)2 -400s samples.

28


Page 29 of 31

250

(b) 450

NF CoMoO4

200

CoMoO4-Ni(OH)2-200s CoMoO4-Ni(OH)2-400s CoMoO4-Ni(OH)2-600s CoMoO4-Ni(OH)2-800s

100

393

400

Ni(OH)2-400s

150

427 413

η100 (mV vs. RHE)

Ir/C

50

374

368

300

250

0 1.2

(d) 2.5 111.2 mV/dec

0.5

2.0

-Z'' （ohm）

91.7 mV/dec 80.9 mV/dec 0.4

CoMoO4

89.4 mV/dec

Ni(OH)2-400s

74.7 mV/dec

CoMoO4-Ni(OH)2-200s CoMoO4-Ni(OH)2-400s

0.3

CoMoO4-Ni(OH)2-600s CoMoO4-Ni(OH)2-800s

0.2 0.8

(e)

Ir/C 1.0

1.2

1.4 1.6 1.8 log j (mA cm-2)

0.0 1.5

(f) 50

0.8

CoMoO4-Ni(OH)2-400s

5.06 mF/cm2

CoMoO4-Ni(OH)2-600s

2

3.13 mF/cm

CoMoO4-Ni(OH)2-800s

0.6

2.63 mF/cm2 0.4

2.56 mF/cm

2

1.72 mF/cm2

0.2

J (mA per ECSA cm-2)

CoMoO4-Ni(OH)2-200s

C Ir/

0s 80

0s 60

0s 40

20

0s

)

2

H i(O

CPE

CoMoO4-Ni(OH)2-400s

Rs Rct

CoMoO4-Ni(OH)2-800s

1.0

67.6 mV/dec

CoMoO4

1.0

1.5

0.5

2.2

CoMoO4 CoMoO4-Ni(OH)2-200s CoMoO4-Ni(OH)2-600s

70.7 mV/dec

2.0

N

oM oO

1.8

4

200 1.4 1.6 Potential(V) vs RHE

(c) Overpotential (V)

405

349

350

C

Current Density (mA cm-2)

(a)

Δj (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 30

2.0

2.5

3.0 3.5 Z' （ohm）

4.0

4.5

5.0

CoMoO4 CoMoO4-Ni(OH)2-200s CoMoO4-Ni(OH)2-400s CoMoO4-Ni(OH)2-600s CoMoO4-Ni(OH)2-800s

20 10 0

0.0 0

40

80 120 Scan Rate (mV/s)

160

1.2

1.4 1.6 Potential(V) vs RHE

1.8

Figure 5 OER performance of pure NF, NF modified with Ir/C and a series of CoMoO4 -Ni(OH)2 catalysts: (a) Polarization curves; (b) Comparison of the overpotential at the current density of 100 mA cm-2 ; (c) Corresponding Tafel plots derived from (a); (d) Nyquist plots of CoMoO4 -Ni(OH)2 samples with the fitting curves; (e) Capacitive J (ΔJ = Ja - Jc) vs scan rate of as- made samples and the corresponding linear slopes; (f) ECSA normalized polarization curves.

29



(a) 60

CoMoO4-Ni(OH)2-400s

(b) 250 Current Density (mA cm-2)

Current Density (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

55

50

45

Page 30 of 31

Initial After 1000 CVs

200 150

100 50 0

40 0

20

40 60 Time (h)

80

100

1.2

1.3

1.4 1.5 1.6 Potential(V) vs RHE

1.7

1.8

Figure 6 (a) Chronoamperometric curves (I-t) of CoMoO4 -Ni(OH)2 -400s at the constant overpotential of 320 mV; (b) Polarization curves of CoMoO4 -Ni(OH)2 -400s before and after 1000 cycles for the durability test.

30


Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphical Abstract

The Ni(OH)2 nanosheets were synergistically coupled with CoMoO4 nanoplate arrays to form heterostructures with excellent OER catalytic performance and long-term stability.

31


Engineering Ni(OH)2 Nanosheet on CoMoO4 Nanoplate Array as

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