Ni Foam as a Self-Standing Electrode

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Energy, Environmental, and Catalysis Applications

Heteromorphic NiCo2S4/Ni3S2/Ni Foam as a Self-Standing Electrode for Hydrogen Evolution Reaction in Alkaline Solution Hui Liu, Xiao Ma, Yuan Rao, Yang Liu, Jialiang Liu, Luyang Wang, and Mingbo Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00296 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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ACS Applied Materials & Interfaces

Heteromorphic NiCo2S4/Ni3S2/Ni Foam as a Self-Standing Electrode for Hydrogen Evolution Reaction in Alkaline Solution Hui Liu,†,# Xiao Ma,†,# Yuan Rao,†,‡ Yang Liu,† Jialiang Liu,† Luyang Wang,† Mingbo Wu*,†

†

State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China

University of Petroleum (East China), Qingdao 266580, China

‡

Institute of Fundamental and Frontier Sciences, University of Electronic Science and

Technology of China, Chengdu 610054, China

#

H. L. and X. M. contributed equally to this work.

ACS Paragon Plus Environment

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ABSTRACT: Considerable works have been devoted on developing high-efficiency non-platinum electrocatalysts for hydrogen evolution reaction (HER). Herein, 3D heteromorphic NiCo2S4/Ni3S2 nanosheets network has been constructed on Ni foam (denoted as NiCo2S4/Ni3S2/NF) serving as a self-standing electrocatalyst through directly thermal sulfurization of a single-source NiCo-layered double hydroxide precursor. The resultant NiCo2S4/Ni3S2/NF electrode exhibits outstanding electrocatalytic HER performance with an extremely low onset overpotential of 15 mV and long-term durability in alkaline solution. Such enhanced HER performance can be credited to: (1) the massive exposed active sites provided by mixed transition metal chalcogenides (NiCo2S4 and Ni3S2), (2) the strong interfacial interaction at NiCo2S4/Ni3S2 heterojunction interfaces with the strengthened H binding, and (3) the porous highly-conductive Ni foam substrate with accelerated electron transfer. This work opens up a new direction to fabricate effective and non-noble-metal electrodes for water splitting and hydrogen generation.

KEYWORDS: layered double hydroxide, transition metal chalcogenides, self-standing, hydrogen evolution reaction, nickel


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1. INTRODUCTION The abuse of fossil fuels has resulted in series of thorny issues such as global warming, ecosystem destroying and energy crisis. Thus, searching for environmentally friendly and sustainable energy substitutes is extremely urgent.1,2 With the zero carbon footprint and high energy density output during burning process, hydrogen has been regarded as a renewable resource to replace the increasingly depleted fossil fuels.3-5 It is known that electrochemical water splitting is widely recognized as a promising strategy to generate H2 from intermittent renewable energy resources to radically realize zero pollution,6,7 and two reactions as hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are involved.8,9 To obtain considerable reaction rates, it is necessary to minimize the overpotentials of these two reactions by design desirable electrocatalysts sanely. Despite noble-metal based materials, especially Pt, exhibit excellent catalytic activity for HER while the scarcity and inferior durability greatly restrict their large-scale application.10,11 Therefore, it is crucial to construct highly efficient and robust non-noble-metal electrocatalysts as promising alternatives for HER. Besides, since only a few OER catalysts can perform well in acid electrolyte, overall water splitting is generally carried out in alkaline or neutral solution in practical application. In this regard, non-noble-metal electrocatalysts with excellent HER catalytic activity in alkaline solution is highly needed. Recently, transition metal chalcogenides (TMCs), such as W–S,12,13 Mo–S,14-18 Mo–Se,19 Ni–S,20-23 Ni–Co–Se,24 and Ni–Co–S,25-27 have been widely reported. As a typical TMCs, Ni–S materials with different stoichiometries like Ni3S2,21-22,28 NiS2,22,29 NiS,22,28 and morphologies


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including nanoparticles,22,28 nanorods,20 nanowires,30 and nanosheets,21,23,29 have been demonstrated as effective HER catalysts. Nevertheless, their HER performance still needs to be further improved compared with the noble metal catalysts. In response, hybridization of mixed TMCs,

such

as

MoS2/Ni3S2

heterostructures,31

MoS2-Ni3S2

heteronanorods,32

and

NixCo3-xS4/Ni3S2 nanosheets,33 has been proved to be a feasible approach to enhance HER performance based on the strong synergistic effect between different TMCs. However, it still remains a challenge to construct a subtly integrated architecture via a facile and simple process, in which structural characteristics and electrocatalytic property of each phase can be well investigated. Layered double hydroxides (LDHs) are a kind of ionic lamellar compounds containing divalent M2+ (such as Mg2+, Co2+, Ni2+) and trivalent M3+ (such as Al3+, Fe3+) metal cations.34-36 Previous studies have shown that LDH could serve as varied precursors to prepare multifarious mixed metal oxide nanocomposite by direct thermal decomposition for magnetic application,37 photocatalytic reaction,38 electrocatalytic reaction,39 or lithium-ion batteries.40,41 For instance, Zhao et al. developed a NiFe2O4/NiO nanocomposite via calcining the NiFe-LDH precursor and the concentration of NiFe2O4 can be regulated through tunig the Ni/Fe molar ratio of the LDH precursor.37 Li et al. presented the fabrication of CoO/CoFe2O4 nanocomposites via calcining CoFe-LDH for lithium-ion batteries.41 Inspired by these, thermal sulfurization of LDH precursor could potentially lead to the formation of novel mixed metal chalcogenides for electrocatalytic applications, though limited work has been conducted in this field so far. Furthermore, we have


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reported that in-situ growth hydrotalcite-like of Ni(OH)2 nanosheets onto Ni foam (NF) could afford electrodes with largely active areas and fast electron diffusion abilities.42 With a 3D macroporous metallic skeleton, Ni foam can serve as a desirable raw material to homogeneously in situ construct various nanostructures on the surface, along with the excellent conductivity, giving enhanced HER performance. Herein, we developed a facile strategy to fabricate a 3D heteromorphic NiCo2S4 and Ni3S2 nanosheets network on Ni foam (denoted as NiCo2S4/Ni3S2/NF) as self-standing cathode for HER in alkaline solution. NiCo-layered double hydroxide (NiCo-LDH) were chosen as the precursor of NiCo2S4/Ni3S2 due to their high abundance and good conductivity. As expected, the optimal NiCo2S4/Ni3S2/NF electrode exhibits greatly catalytic activity and stability, which outperform most of those reported non-noble-metal based HER catalysts.

2. EXPERIMENTAL SECTION 2.1. Synthesis of NiCo-LDH/NF A tailored Ni foam (1 mm in thickness, 1 cm × 4 cm) was pretreated with acetone, ethanol, 3 M HCl solution and water (each for 10 min), respectively. Typically, 6 mmol Ni(NO3)2·6H2O (≥98%), 2 mmol Co(NO3)2·6H2O (≥98%) and 10 mmol urea were dissolved in 30 mL deionized water, followed by stirring for 30 min to get a clear pink solution. The as-cleaned Ni foam was then immersed into the solution and transferred into a Teflon-lined stainless autoclave and reacted at 180 °C for 12 h with the assistance of rotating oven. After the reaction, the resulting


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material was ultrasonically cleaned with deionized water several times and dried at 60°C in vacuum, giving the NiCo-LDH/NF precursor. 2.2. Synthesis of NiCo2S4/Ni3S2/NF The as-made NiCo-LDH/NF and 1.0 g sulfur powder were closely placed at the downstream and upstream side of a tube furnace, respectively. After flushing with N2 gas for 30 min, the sample was heated at 400 °C with a heating rate of 5 °C min-1 and kept for 1 h. After the reaction, the final NiCo2S4/Ni3S2/NF was obtained. In addition, the other four NiCo2S4/Ni3S2/NF samples were synthesized by regulating the amounts of sulfur powder (0.5 g and 1.5 g) and the sulfurization time (0.5 h and 2 h), respectively. For comparison, the single component transition metal chalcogenides Ni3S2/NF and NiCo2S4/NF were also synthesized. Ni3S2/NF was obtained through a similar process without adding Co(NO3)2·6H2O. The fabrication of NiCo2S4/NF was similar to NiCo2S4/Ni3S2/NF, except for changing the amount of Ni(NO3)2·6H2O to 1 mmol. 2.3. Characterization and Instrumentations The phase formation of obtained samples were investigated by X-ray diffraction (XRD) using a PANalytical X-ray diffractometer with Cu-Kα radiation (k=0.15406 nm). The morphologies and structure of electrocatalysts were characterized using a scanning electron microscopy (SEM, ZEISS MERLIN) with energy dispersive X-ray spectroscopy (EDX) and a JEM2100F transmission electron microscopy (TEM, JEOL, Japan). The X-ray photoelectron spectra (XPS) were obtained using a Thermol scientific Escalab 250Xi spectrometer with Al-Kα


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radiation. 2.4. Electrochemical Measurements All the HER catalytic measurements were performed on a CHI760E electrochemical workstation

(Shanghai

Chenhua)

in

a

standard

three-electrode

system

by

using

NiCo2S4/Ni3S2/NF, NiCo2S4/NF, Ni3S2/NF, NiCo-LDH/NF and bare Ni foam as the working electrode, carbon rod and Ag/AgCl electrode as the counter and reference electrode, respectively. For all electrochemical test, the electrolyte (1.0 M KOH) was purged with high-purity argon. All linear sweep voltammetry (LSV) curves were recorded at a scan rate of 5 mV s-1. Electrochemical impedance spectroscopy (EIS) was recorded over a frequency range from 105 to 0.01 Hz at an overpotential of 200 mV. Cyclic voltammograms (CV) were tested with different scan rates (20, 40, 60, 80, 100 mV s-1). The stability tests were performed using chronoamperometry measurements and also conducted by CV from 0 to -0.6 V (vs Ag/AgCl) for 5000 cycles, and the LSV curves were recorded. In addition, all potentials reported were not iR-corrected and referenced to reversible hydrogen electrode (RHE) according to the formula E(RHE) = E(Ag/AgCl) + 0.197 + 0.059 × pH. The current densities (j) were calculated by geometric immersed area of electrode in the electrolyte (1 cm × 1 cm). 3. RESULTS AND DISCUSSION Scheme 1. Schematic illustration of 3D NiCo2S4/Ni3S2/NF.


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The synthesis of 3D heteromorphic NiCo2S4/Ni3S2/NF network is illustrated schematically in Scheme 1. First, a series of vertically aligned NiCo-LDH nanosheets was in-situ grown on Ni foam via a hydrothermal procedure. Figure 1A exhibits the SEM image of bare Ni foam, which has many macropores with diameter around 100-500 µm. Owing to the porous structure, Ni foam is an excellent material to offer plenty of space for ions penetration and hydrogen transport;43 meanwhile, catalysts can anchor on the interior surface of Ni substrate to serve as self-standing electrodes directly. SEM images of NiCo-LDH nanosheets on Ni foam (denoted as NiCo-LDH/NF) in Figure 1B-C display that the previous smooth Ni foam surface is totally covered by NiCo-LDH nanosheets with a length of about 1-2 µm. And they are vertically aligned and interconnected with each other on the skeletons of Ni foam (Figure 1C). Figure 1D-E show the morphology of the interconnected nanosheets was well preserved after the sulfurization treatment of NiCo-LDH/NF, but the resulting nanosheets exhibited a rough surface. We further confirmed that the resulting nanosheets belong to NiCo2S4 and Ni3S2 by the X-ray diffraction (XRD) pattern. As displayed in Figure 1F, the well-defined diffraction peaks at 27.1°, 31.5°, 38.1°, 47.7°, 50.6° and 54.9° can be assigned to (220), (311), (400), (422), (511) and (440) planes of the cubic NiCo2S4 phase (JCPDS card No. 43-1477), and the peaks at 21.7°, 31.1°,


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37.8°, 49.7°, 50.2°, 55.1° and 55.3° corresponded to (101), (110), (003), (113), (211), (122) and (300) planes of rhombohedral Ni3S2 (JCPDS card No. 71-1682), respectively. In addition, the two sharp peaks at 44.4° and 51.8° belong to the metallic Ni foam (JCPDS card No. 03-1043). The XRD pattern of NiCo-LDH/NF (Figure S1) exhibits characteristic (003), (006), (009), and (015) peaks, which can be assigned to a typical NiCo-LDH phase (JCPDS card No. 33-0429).44-46 The above results indicated the successful fabrication of NiCo2S4/Ni3S2/NF after thermal sulfurization of NiCo-LDH/NF, and the Ni foam substrate turned to black (Figure S2). For comparison, NiCo2S4/NF and Ni3S2/NF were synthesized by a similar procedure to NiCo2S4/Ni3S2/NF (see Experimental Section). And XRD patterns were characterized to demonstrate the formation of NiCo2S4/NF and Ni3S2/NF, respectively (Figure S3a and S3b). Instead of the aligned nanosheets architecture as NiCo2S4/Ni3S2/NF shows, NiCo2S4 and Ni3S2 present nanoflower and nanosphere morphology (Figure S3c and S3d), respectively.


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Figure 1. (A) SEM image of bare Ni foam. (B, C) SEM images of NiCo-LDH/NF. (D, E) SEM images, (F) XRD pattern and (G) EDX elemental mapping of NiCo2S4/Ni3S2/NF. Furthermore, the energy-dispersive X-ray (EDX) spectra and the corresponding elemental mapping further confirm the element composition of NiCo2S4/Ni3S2/NF. As shown in Figure S4, the EDX spectrum displays the chemical composition of Ni, Co, and S elements and the Ni/Co molar ratio is calculated to be 16.7 (inset of Figure S4), indicating that Ni3S2 is the key component in NiCo2S4/Ni3S2. The EDX elemental mapping images (Figure 1G) show the uniformly distribution of Ni, Co and S, revealing the even integration of NiCo2S4 and Ni3S2. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images of one nanosheet were further employed to study the microstructures of the resultant NiCo2S4/Ni3S2/NF (Figure 2A-C). The highly crystalline of


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NiCo2S4/Ni3S2 nanoparticles was first confirmed through TEM observation (Figure 2B). In the HRTEM image in Figure 2C, the two apparent lattice spacings of 0.23 and 0.28 nm are ascribed to the (400) and (311) planes of NiCo2S4, and the lattice spacing of 0.40 nm correspond to the (101) plane of Ni3S2. In addition, an interfacial heterostructure can be evidently observed between the NiCo2S4 and Ni3S2 nanodomains, as displayed by dashed lines. This kind of interfacial nanodomains was also observed in other multi-component materials, such as biactive NiFe2O4/NiO nano-composite calcined from NiFe-LDH and Co2SnO4/Co3O4/Al2O3/C obtained from CoAlSn-LDH.37,47 The formation of interfacial nanodomains can be attributed to the topotactic transformation of LDH precursor as well as the uniformly ordered arrangement of metal cations.46,48 To certify this assumption, the NiCo-LDH powder (without Ni foam) was synthesized and thermal sulfurized at 350°C for 1 h, which is similar to NiCo2S4/Ni3S2/NF. The XRD pattern in Figure S5A shows that the resultant product consists of NiCo2S4 and Ni3S2 after sulfurization treatment, which indicates Ni3S2 still can be formed in the absence of Ni foam. The SEM image (Figure S5B) further reveals the formation of the NiCo2S4 and Ni3S2 microspheres with diameter around 50-150 nm. From the previous studies,33 the heterojunction interfaces in NiCo2S4/Ni3S2 is expected to contain some highly electrocatalytic active sites.

Figure 2. (A, B) TEM and (C) HRTEM images of NiCo2S4/Ni3S2/NF. Furthermore, X-ray photoelectron spectroscopy (XPS) was employed to analysis the surface state of NiCo2S4/Ni3S2/NF. Figure 3A demonstrates the coexistence of Ni, Co and S elements while the signal of O element almost disappears, compared with that of NiCo-LDH/NF (Figure


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S6). In the high-resolution Ni 2p spectrum (Figure 3B), Ni atoms in 2p3/2 and 2p1/2 spin-orbital splitting photo-electrons are located at 855.6 and 873.0 eV, which are due to Ni2+ and Ni3+ species, respectively. The two satellite peaks reveal that the majority of Ni existing in the final product is Ni2+ cation,49 which is in accordance with NiCo2S4. Similarly, two appreciable peaks observed at 781.5 and 797.3 eV in the high-resolution Co 2p spectrum (Figure 3C) can be ascribed to the Co 2p3/2 and Co 2p1/2, respectively. The spin-energy separation of 15.8 eV demonstrates the coexistence of Co2+ and Co3+. Moreover, the weak satellite peaks prove that the majority of Co atoms exist as Co3+ state.50 As for the XPS spectra of S 2p (Figure 3D), the peaks centered at 161.8 and 162.7 eV are corresponding to S 2p3/2 and S 2p1/2, which suggest the presences of S2- in a low coordination state at the surface and metal-sulfur bonds, respectively.51,52 The peak at 168.7 eV is well matched to the highly oxidized state S4+ at the edge of NiCo2S4 and Ni3S2.


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Figure 3. (A) XPS survey spectrum and the high-resolution XPS spectra of (B) Ni 2p, (C) Co 2p, and (D) S 2p of NiCo2S4/Ni3S2/NF. To investigate the electrocatalytic HER activity, the NiCo2S4/Ni3S2/NF electrode was tested in 1.0 M KOH solution. NiCo-LDH/NF, NiCo2S4/NF, Ni3S2/NF and bare Ni foam electrodes were also investigated for comparison. Figure 4A shows the polarization curves of all these electrodes obtained by linear sweep voltammetry (LSV) at a scan rate of 5 mV s-1 without iR correction. Especially, NiCo2S4/Ni3S2/NF displays best HER catalytic activity with the lowest onset overpotential (ηonset = 15 mV). The overpotentials required for current density of 10 mV cm-2 is as low as 119 mV (η10), superior to those of NiCo-LDH/NF (231 mV), NiCo2S4/NF (137 mV), Ni3S2/NF (271 mV) and bare Ni Foam (301 mV). Moreover, the NiCo2S4/Ni3S2/NF


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electrode delivers the largest current density of 600 mA cm-2 at the overpotential of 600 mV among all the samples. Accordingly, Figure 4B displays the Tafel plots of the samples derived from the polarization curves. It can be seen that NiCo2S4/Ni3S2/NF delivers a Tafel slope of 105.2 mV dec-1, which is smaller than those of NiCo-LDH/NF (142.3 mV dec-1), NiCo2S4/NF (147.3 mV dec-1), Ni3S2/NF (156.5 mV dec-1), and bare Ni foam (196.5 mV dec-1). As previously reported,53 the mechanism of HER occurred in alkaline conditions can be summarized as the following major reactions: M + H2O + e- = M-Hads +OH- (Volmer reaction, 118 mV dec-1) M-Hads + H2O + e- = M + H2 + OH- (Heyrovsky reaction, 40 mV dec-1) 2M-Hads = 2M + H2 (Tafel reaction, 30 mV dec-1) Here, M represents for metal atoms. The relatively small Tafel slope of NiCo2S4/Ni3S2/NF (105.2 mV dec-1) implies a more efficient Volmer step and an increased hydrogen generation rate,54 which can be further confirmed by XPS. As shown in Figure S7, the peaks of Ni 2p and Co 2p in NiCo2S4/Ni3S2/NF heterostructures were blue-shifted in comparison to those of NiCo2S4/NF and Ni3S2/NF, indicating that the charge is rearranged between NiCo2S4 and Ni3S2.33 It can be speculated that the reduced electron density can provide plenty of unfilled d orbitals to bind with the hydrogen atom,32 thus lead to more efficient catalytic activity for HER at the NiCo2S4/Ni3S2 heterojunction interfaces. Electrochemical impedance spectroscopy (EIS) measurement was employed to further explore the electrocatalytic HER kinetics. As shown in Figure 4C, NiCo2S4/Ni3S2/NF delivers a


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charge transfer resistance (Rct) of only 2 Ω, which is lower than those of NiCo-LDH/NF (6 Ω), NiCo2S4/NF (4 Ω), Ni3S2/NF (21 Ω) and bare Ni foam (34 Ω). This suggests that the synergistic effect between the unique heteromorphic NiCo2S4 and Ni3S2 nanostructures make positive contribution to the rapid electron transport for HER. To assess the electrochemical active surface area (ECSA) of the samples described above, the geometric double layer capacitance (Cdl) was measured, which is linearly proportional to ECSA. The Cdl is estimated by cyclic voltammograms (CVs) with various scan rates (Figure S8A-E). As shown in Figure S8F, the Cdl of NiCo2S4/Ni3S2/NF (113.5 mF cm-2) is much higher than that of NiCo-LDH/NF (84.6 mF cm-2), NiCo2S4/NF (60.8 mF cm-2), Ni3S2/NF (34.8 mF cm-2) and bare Ni foam (3.07 mF cm-2). The high Cdl value implies the the increased ECSA, which is attributed to the enhanced anion exchangeability between the electrolyte and catalytic active sites. In return, the massive exposed active sites provided by mixed transition metal chalcogenides resulted in the significant improvement of catalytic performance. As the long-term stability is vital to the final application of NiCo2S4/Ni3S2/NF in HER,55 the stability of NiCo2S4/Ni3S2/NF was evaluated using long-term electrolysis. As shown in Figure 4D, a higher current density of ~35 mA cm-2 was maintained after 24 h, which can be further observed in the LSV curves of NiCo2S4/Ni3S2/NF after 0 h, 3 h, and 24 h, respectively (Figure S9A). Furthermore, The LSV curves of NiCo2S4/Ni3S2/NF before and after 5000 CV cycles are given in Figure S9B. The increase of current density is mainly caused by the mutual effect between NiCo2S4/Ni3S2 heterostructure and metallic Ni produced by the cathodic reduction of


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high valence Ni in NiCo2S4/Ni3S2 nanosheets, as evidenced by the high-resolution XPS spectrum of Ni 2p. As shown in Figure S10A, the peak intensity of metallic Ni56 at 852.2 eV increased and red-shifted, suggesting the improvement of electron transfer rate for NiCo2S4/Ni3S2/NF. Moreover, a strong electrostatic absorption effect presented in the empty d orbitals of Ni and the H2O molecule can accelerate the H2 generation rate in alkaline solution (Volmer reaction: M + H2O + e- = M-Hads + OH-), which contributes to the improved HER activity.57 Therefore, it will be more effective to produce H2 with abundant NiCo2S4/Ni3S2-Ni component in NiCo2S4/Ni3S2/NF. In addition, the phase and morphology of NiCo2S4/Ni3S2/NF were well maintained with no obvious changes in XRD pattern (Figure S11A), SEM image (Figure S11B) and XPS spectra (Figure S10), indicating NiCo2S4/Ni3S2/NF has a good HER stability.


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Figure 4. (A) Polarization curves and (B) the corresponding Tafel slopes of NiCo2S4/Ni3S2/NF, NiCo-LDH/NF, NiCo2S4/NF, Ni3S2/NF and bare Ni foam at a scan rate of 5 mV s-1 without IR correction. (C) Nyquist plots of NiCo2S4/Ni3S2/NF, NiCo-LDH/NF, NiCo2S4/NF, Ni3S2/NF and Ni foam at an overpotential of 200 mV. (D) Chronoamperometry curve of NiCo2S4/Ni3S2/NF at a static overpotential of 180 mV. To better understand the catalytic activity of heteromorphic NiCo2S4/Ni3S2/NF, comparison experiments were carried out by regulating the amounts of S powder and the sulfurization time, respectively (see Experimental Section). From Figure 5A and 5B, it is clearly observed that all NiCo2S4/Ni3S2/NF electrodes deliver typical onset overpotentials of 15 to 65 mV, which were measured using the tangent method.58 Furthermore, NiCo2S4/Ni3S2/NF obtained by heating for 1 h with 1.0 g S powder shows superior HER catalytic activity with a fairly smaller η100 (overpotential at 100 mA cm-2) of 270 mV, which can be attributed to the phase transformation,


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as revealed by the XRD pattern (Figure 1F and Figure S12). As shown in Figure S12, LDH was not completely transformed into NiCo2S4 and Ni3S2 due to the insufficient sulfur source as well as reaction time. Nevertheless, excess amount of sulfur resulted in the formation of NiS (JCPDS card No. 01-1286) owing to the further sulfurization of Ni foam,59 which lead to a broken substrate (Figure S13A). Analogously, when the sulfurization time extended to 2 h, the uniformly aligned nanosheets structures are destroyed due to the excessive growth of rhombohedral Ni3S2 phase (Figure S12 and S13B). All these results from morphological and compositional characterization manifest that the overall heterogeneous architecture of NiCo2S4/Ni3S2/NF exhibits high-efficiency HER performance.

Figure 5. (A) Polarization curves of NiCo2S4/Ni3S2/NF using different amounts of S powder: 0.5 g, 1.0 g, 1.5 g. (B) Polarization curves of NiCo2S4/Ni3S2/NF obtained at different sulfurization time: 0.5 h, 1 h, 2 h. According to all the results above, NiCo2S4/Ni3S2/NF displays excellent HER performance which can be ascribed to the following reasons: (1) Owing to the high electrocatalytic properties of transition metal chalcogenides (TMCs), the heteromorphic mixed NiCo2S4 and Ni3S2 phases provide massive active sites for HER. (2) The strong interfacial interaction in the NiCo2S4/Ni3S2


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heterojunction structure which are derived from the topotactic transformation of LDH precursor, plays crucial roles to enhance binding with H atom and then generate H2 bubbles. (3) The NiCo2S4/Ni3S2/NF electrode is self-standing and the electrical resistance can be minimized without any insulating chemical binder. Moreover, the strong adhesion of NiCo2S4/Ni3S2 grown directly on Ni foam benefits their mechanical and catalytic stabilities. 4. CONCLUSIONS In summary, a self-standing, noble-metal-free NiCo2S4/Ni3S2/NF electrode was successfully constructed via a facile hydrothermal and thermal sulfurization process. The NiCo2S4/Ni3S2/NF as a HER electrocatalyst exhibited better electrocatalytic performance than the most reported non-noble metal catalysts (Table S1). The superior HER catalytic activity of NiCo2S4/Ni3S2/NF can be ascribed to the massive active sites of mixed metal chalcogenides system, the favored binding with H atom in heteromorphic NiCo2S4/Ni3S2 interfaces, and the facilitated electron transport of conductive Ni foam substrate. This work provides a new insight for developing TMCs electrocatalysts to achieve higher HER activity in alkaline electrolyte, and the fabrication method can also be extended to prepare other hybrid and electrochemical devices with heteromorphic structure. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. XRD pattern of NiCo-LDH/NF (Figure S1), Photograph of bare Ni foam and NiCo2S4/Ni3S2/NF


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(Figure S2), XRD patterns and SEM images of NiCo2S4/NF and Ni3S2/NF(Figure S3), EDX spectrum of NiCo2S4/Ni3S2/NF (Figure S4), XRD pattern and SEM image of NiCo2S4/Ni3S2 synthesized by NiCo-LDH powder (Figure S5), XPS survey spectrum of NiCo-LDH/NF (Figure S6), High-resolution XPS spectra of Ni 2p, Co 2p and S 2p for NiCo2S4/Ni3S2/NF, NiCo2S4/NF, Ni3S2/NF (Figure S7), Cyclic voltammetry curves and Cdl of bare Ni foam, Ni3S2/NF, NiCo-LDH/NF, NiCo2S4/NF and NiCo2S4/Ni3S2/NF (Figure S8), Polarization curves of NiCo2S4/Ni3S2/NF after 24 h chronoamperometry measurements and 5000 CV cycles (Figure S9), High-resolution XPS spectra of Ni 2p, Co 2p and S 2p of NiCo2S4/Ni3S2/NF before and after 24 h constant HER (Figure S10), XRD pattern and SEM image of NiCo2S4/Ni3S2/NF after HER experiments for 24 h (Figure S11), XRD pattern a of NiCo2S4/Ni3S2/NF with different amounts of S powder and sulfurization time (Figure S12), SEM image of NiCo2S4/Ni3S2/NF prepared at 1.5g S powder and heated for 2 h (Figure S13), and comparison of the HER performance of NiCo2S4/Ni3S2/NF with other non-noble catalyst reported recently (Table S1). (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

ORCID Hui Liu: 0000-0001-7815-4200 Mingbo Wu: 0000-0003-0048-778X


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Author Contributions #

H. L. and X. M. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 51572296, U1662113, 51372277); the Fundamental Research Fund for the Central Universities (15CX08005A, 17CX06029).

REFERENCES (1) Chow, J.; Kopp R. J.; Portney P. R. Energy Resources and Global Development. Science 2003, 302, 1528-1531. (2) Faber, M. S.; Jin, S. Earth-abundant Inorganic Electrocatalysts and Their Nanostructures for Energy Conversion Applications. Energy Environ. Sci. 2014, 7, 3519-3542. (3) Turner, J. A. Sustainable Hydrogen Production. Science 2004, 305, 972-974. (4) Chen, J.; Xia, G. L.; Guo, Z. P.; Huang, Z. G.; Liu, H. K.; Yu, X. B. Porous Ni Nanofibers with Enhanced Catalytic Effect on the Hydrogen Storage Performance of MgH2. J. Mater. Chem. A 2015, 3, 15843-15848. (5) Shi, Y.; Zhang, B. Recent Advances in Transition Metal Phosphide Nanomaterials: Synthesis and Applications in Hydrogen Evolution Reaction. Chem. Soc. Rev. 2016, 45, 1529-1541.


21


Page 22 of 31

(6) Gong, M.; Zhou, W.; Tsai, M. C.; Zhou, J. G.; Guan, M. Y.; Lin, M. C.; Zhang, B.; Hu, Y. F.; Wang, D.Y.; Yang, J., Pennycook S. J.; Hwang B. J.; Dai H. J. Nanoscale Nickel Oxide/Nickel Heterostructures for Active Hydrogen Evolution Electrocatalysis. Nat. Commun. 2014, 5, 4695-4700. (7) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of Electrocatalysts for Oxygen- and Hydrogen- Involving Energy Conversion Reactions. Chem. Soc. Rev. 2015, 44, 2060-2086. (8) Yagi, M.; Tomita, E.; Sakita, S.; Kuwabara, T.; Nagai, K. Self-Assembly of Active IrO2 Colloid Catalyst on an ITO Electrode for Efficient Electrochemical Water Oxidation. J. Phys. Chem. B 2005, 109, 21489-21491. (9) Rausch, B.; Symes, M. D.; Chisholm, G.; Cronin, L. Decoupled Catalytic Hydrogen Evolution from a Molecular Metal Oxide Redox Mediator in Water Splitting. Science 2014, 345, 1326-1330. (10) Fang, B. Z.; Kim, J. H.; Yu, J. S. Colloid-imprinted Carbon with Superb Nanostructure as an Efficient Cathode Electrocatalyst Support in Proton Exchange Membrane Fuel Cell. Electrochem. Commun. 2008, 10, 659-662. (11) Zou, X. X.; Zhang, Y. Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev. 2015, 44, 5148-5180. (12) Cheng, L.; Huang, W. J.; Gong, Q. F.; Liu, C. H.; Liu, Z.; Li, Y. G.; Dai, H. J. Ultrathin WS2 Nanoflakes as a High-Performance Electrocatalyst for the Hydrogen Evolution Reaction. Angew. Chem., Int. Ed. 2014, 53, 7860-7863.


22

Page 23 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


(13) Wang, F. M.; He, P.; Li, Y. C.; Shifa, T. A.; Deng, Y.; Liu, K. L.; Wang, Q. S.; Wang, F.; Wen, Y.; Wang, Z. X.; Zhan, X. Y.; Sun, L. F.; He, J. Interface Engineered WxC@WS2 Nanostructure for Enhanced Hydrogen Evolution Catalysis. Adv. Funct. Mater. 2017, 27, 1605802-1605808. (14) Geng, X. M.; Wu, W.; Li, N.; Sun, W. W.; Armstrong, J.; Al-hilo, A.; Brozak, M.; Cui, J. B.; Chen, T. P. Three-Dimensional Structures of MoS2 Nanosheets with Ultrahigh Hydrogen Evolution Reaction in Water Reduction. Adv. Funct. Mater. 2014, 24, 6123-6129. (15) Li, H.; Tsai, C.; Koh, A. L.; Cai, L.; Contryman, A. W.; Fragapane, A. H.; Zhao, J. H.; Han, H. S.; Manoharan, H. C.; Abild-Pedersen, F.; Norskov J. K.; Zheng X. L. Activating and Optimizing MoS2 Basal Planes for Hydrogen Evolution through the Formation of Strained Sulphur Vacancies. Nat. Mater. 2016, 15, 48-53. (16) Pramoda, K.; Gupta, U.; Chhetri, M.; Bandyopadhyay, A.; Pati, S. K.; Rao, C. N. R. Nanocomposites of C3N4 with Layers of MoS2 and Nitrogenated RGO, Obtained by Covalent Cross-Linking: Synthesis, Characterization, and HER Activity. ACS Appl. Mater. Interfaces 2017, 9, 10664-10672. (17) Yang L. J; Zhou W. J.; Lu J.; Hou, D. M.; Ke, Y. T.; Li, G. Q.; Tang, Z. H.; Kang, X. W.; Chen, S. W. Hierarchical Spheres Constructed by Defect-Rich MoS2/Carbon Nanosheets for Efficient Electrocatalytic Hydrogen Evolution. Nano Energy 2016, 22, 490-498. (18) Zhao L. L.; Jia J; Yang Z. Y.; Yu, J. Y.; Wang, A. L.; Sang, Y. H.; Zhou, W. J.; Liu, H. One-Step Synthesis of CdS Nanoparticles/MoS2 Nanosheets Heterostructure on Porous Molybdenum Sheet for Enhanced Photocatalytic H2 Evolution. Appl. Catal. B: Environ. 2017,


23


Page 24 of 31

210, 290-296. (19) Yin, Y.; Zhang, Y. M.; Gao, T. L.; Yao, T.; Zhang, X. H.; Han, J. C.; Wang, X. J.; Zhang, Z. H.; Xu, P.; Zhang, P.; Cao, X. Z.; Song, B.; Jin, S. Synergistic Phase and Disorder Engineering in 1T-MoSe2 Nanosheets for Enhanced Hydrogen-Evolution Reaction. Adv. Mater. 2017, 29, 1700311-1700318. (20)Ouyang, C. B.; Wang, X.; Wang, C.; Zhang, X. X.; Wu, J. H.; Ma, Z. L.; Dou, S.; Wang, S. Y. Hierarchically Porous Ni3S2 Nanorod Array Foam as Highly Efficient Electrocatalyst for Hydrogen Evolution Reaction and Oxygen Evolution Reaction. Electrochim. Acta 2015, 174, 297-301. (21) Feng, L. L.; Yu, G. T.; Wu, Y. Y.; Li, G. D.; Li, H.; Sun, Y. H.; Asefa, T.; Chen, W.; Zou, X. X. High-Index Faceted Ni3S2 Nanosheet Arrays as Highly Active and Ultrastable Electrocatalysts for Water Splitting. J. Am. Chem. Soc. 2015, 137, 14023-14026. (22) Jiang, N.; Tang, Q.; Sheng, M. L.; You, B.; Jiang, D. E.; Sun, Y. J. Nickel Sulfides for Electrocatalytic Hydrogen Evolution under Alkaline Conditions: A Case Study of Crystalline NiS, NiS2, and Ni3S2 Nanoparticles. Catal. Sci. Technol. 2016, 6, 1077-1084. (23) Yu, J.; Ma, F. X.; Du, Y.; Wang, P. P.; Xu, C. Y.; Zhen, L. In Situ Growth of Sn-Doped Ni3S2 Nanosheets on Ni Foam as High-Performance Electrocatalyst for Hydrogen Evolution Reaction. ChemElectroChem 2017, 4, 594-600. (24) Liu, B.; Zhao, Y. F.; Peng, H. Q.; Zhang, Z. Y.; Sit, C. K.; Yuen, M. F.; Zhang, T. R.; Lee, C. S.; Zhang, W. J. Nickel-Cobalt Diselenide 3D Mesoporous Nanosheet Networks Supported on Ni


24

Page 25 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


Foam: An All-pH Highly Efficient Integrated Electrocatalyst for Hydrogen Evolution. Adv. Mater. 2017, 29, 1606521-1606528. (25) Liu, D. N.; Lu, Q.; Luo, Y. L.; Sun, X. P.; Asiri, A. M. NiCo2S4 Nanowires Array as an Efficient Bifunctional Electrocatalyst for Full Water Splitting with Superior Activity. Nanoscale 2015, 7, 15122-15126. (26) Sivanantham, A.; Ganesan, P.; Shanmugam, S. Hierarchical NiCo2S4 Nanowire Arrays Supported on Ni Foam: An Efficient and Durable Bifunctional Electrocatalyst for Oxygen and Hydrogen Evolution Reactions. Adv. Funct. Mater. 2016, 26, 4661-4672. (27) Liu, J.; Wang, J. S.; Zhang, B.; Ruan, Y. J.; Lv, L.; Ji, X.; Xu, K.; Miao, L.; Jiang, J. J. Hierarchical NiCo2S4@NiFe LDH Heterostructures Supported on Nickel Foam for Enhanced Overall-Water-Splitting Activity. ACS Appl. Mater. Interfaces 2017, 9, 15364-15372. (28) Chung, D. Y.; Han, J. W.; Lim, D. H.; Jo, J. H.; Yoo, S. J.; Lee, H.; Sung, Y. E. Structure Dependent Active Sites of NixSy as Electrocatalysts for Hydrogen Evolution Reaction. Nanoscale 2015, 7, 5157-5163. (29) Tang, C.; Pu, Z. H.; Liu, Q.; Asiri, A. M.; Sun, X. P. NiS2 Nanosheets Array Grown on Carbon Cloth as an Efficient 3D Hydrogen Evolution Cathode. Electrochim. Acta 2015, 153, 508-514. (30) Qu, Y. J.; Yang, M. Y.; Chai, J. W.; Tang, Z.; Shao, M. M.; Kwok, C. T.; Yang, M.; Wang, Z. Y.; Chua, D.; Wang, S. J.; Lu, Z. G.; Pan, H. Facile Synthesis of Vanadium-Doped Ni3S2 Nanowire Arrays as Active Electrocatalyst for Hydrogen Evolution Reaction. ACS Appl. Mater.


25


Page 26 of 31

Interfaces 2017, 9, 5959-5967. (31) Zhang, J.; Wang, T.; Pohl, D.; Rellinghaus, B.; Dong, R. H.; Liu, S. H.; Zhuang, X. D.; Feng, X. L. Interface Engineering of MoS2/Ni3S2 Heterostructures for Highly Enhanced Electrochemical Overall-Water-Splitting Activity. Angew. Chem., Int. Ed. 2016, 55, 6702-6707. (32) Yang, Y. Q.; Zhang, K.; Lin, H. L.; Li, X.; Chan, H. C.; Yang, L. C.; Gao, Q. S. MoS2–Ni3S2 Heteronanorods as Efficient and Stable Bifunctional Electrocatalysts for Overall Water Splitting. ACS Catal. 2017, 7, 2357-2366. (33) Wu, Y. Y.; Liu, Y. P., Li, G. D.; Zou, X.; Lian, X. R.; Wang, D. J.; Sun, L.; Asefa, T.; Zou, X. X. Efficient Electrocatalysis of Overall Water Splitting by Ultrasmall NixCo3-xS4 Coupled Ni3S2 Nanosheet Arrays. Nano Energy 2017, 35, 161-170. (34) Wang, Q.; O Hare, D. Recent Advances in the Synthesis and Application of Layered Double Hydroxide (LDH) Nanosheets. Chem. Rev. 2012, 112, 4124-4155. (35) Han, X. T.; Yu, C.; Yang, J.; Zhao, C. T.; Huang, H. W.; Liu, Z. B.; Ajayan, P. A.; Qiu, J. S. Mass and Charge Transfer Coenhanced Oxygen Evolution Behaviors in CoFe-Layered Double Hydroxide Assembled on Graphene. Adv. Mater. Interfaces 2016, 3, 1500782-1500789. (36) Qian, L.; Lu, Z. Y.; Xu, T. H.; Wu, X. C.; Tian, Y.; Li, Y. P.; Huo, Z. Y.; Sun, X. M.; Duan, X. Trinary Layered Double Hydroxides as High-Performance Bifunctional Materials for Oxygen Electrocatalysis. Adv. Energy Mater. 2015, 5, 1500245-1500250. (37) Zhao, X. F.; Xu, S. L.; Wang, L. Y.; Duan, X.; Zhang, F. Z. Exchange-biased NiFe2O4/NiO Nanocomposites Derived from NiFe-Layered Double Hydroxides as a Single Precursor. Nano


26

Page 27 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


Res. 2010, 3, 200-210. (38) Zhao, X. F.; Wang, L.; Xu, X.; Lei, X. D.; Xu, S. L.; Zhang, F. Z. Fabrication and Photocatalytic Properties of Novel ZnO/ZnAl2O4 Nanocomposite with ZnAl2O4 Dispersed inside ZnO Network. AIChE J. 2012, 58, 573-582. (39) Wang, D. D.; Chen, X.; Evans, D. G.; Yang, W. S. Well-dispersed Co3O4/Co2MnO4 Nanocomposites as a Synergistic Bifunctional Catalyst for Oxygen Reduction and Oxygen Evolution Reactions. Nanoscale 2013, 5, 5312-5315. (40) Liu, J. P.; Li, Y. Y.; Huang, X. T.; Li, G. Y.; Li, Z. K. Layered Double Hydroxide Nano- and Microstructures Grown Directly on Metal Substrates and their Calcined Products for Application as Li-Ion Battery Electrodes. Adv. Funct. Mater. 2008, 18, 1448-1458. (41) Li, M. X.; Yin, Y. X.; Li, C. J.; Zhang, F. Z.; Wan, L. J.; Xu, S. L.; Evans, D. G. Well-dispersed

Bi-component-Active

CoO/CoFe2O4

Nanocomposites

with

Tunable

Performances as Anode Materials for Lithium-Ion Batteries. Chem. Commun. 2012, 48, 410-412. (42) Rao, Y.; Wang, Y.; Ning, H.; Li, P.; Wu, M. B. Hydrotalcite-like Ni(OH)2 Nanosheets in Situ Grown on Nickel Foam for Overall Water Splitting. ACS Appl. Mater. Interfaces 2016, 8, 33601-33607. (43) Lu, X. Y.; Zhao, C. Electrodeposition of Hierarchically Structured Three-Dimensional Nickel–Iron Electrodes for Efficient Oxygen Evolution at High Current Densities. Nat. Commun. 2015, 6, 6616-6622. (44) Chen, H.; Hu, L. F.; Chen, M.; Yan, Y.; Wu, L. M. Nickel-Cobalt Layered Double


27


Page 28 of 31

Hydroxide Nanosheets for High-performance Supercapacitor Electrode Materials. Adv. Funct. Mater. 2014, 24, 934-942. (45) Ma, L. B.; Hu, Y.; Chen, R. P.; Zhu, G. Y.; Chen, T.; Lv, H. L.; Wang, Y. R.; Liang, J.; Liu, H. X.; Yan, C. Z.; Zhu, H. F.; Tie, Z. X.; Jin, Z.; Liu, J. Self-assembled Ultrathin NiCo2S4 Nanoflakes Grown on Ni Foam as High-performance Flexible Electrodes for Hydrogen Evolution Reaction in Alkaline Solution. Nano Energy 2016, 24, 139-147. (46) Bai, D. X.; Wang, F.; Lv, J. M.; Zhang, F. Z.; Xu, S. L. Triple-Confined Well-Dispersed Biactive NiCo2S4/Ni0.96S on Graphene Aerogel for High-Efficiency Lithium Storage. ACS Appl. Mater. Interfaces 2016, 8, 32853-32861. (47) Wu, B. B.; Zhang S. L.; Yao, F.; Zhang, F. Z.; Xu, S. L. Synergistic Lithium Storage of a Multi-Component Co2SnO4/Co3O4/Al2O3/C Composite from a Single-Source Precursor. RSC Adv. 2015, 5, 69932-69938. (48) Sideris, P. J.; Nielsen, U. G.; Gan, Z.; Grey, C. P. Mg/Al Ordering in Layered Double Hydroxides Revealed by Multinuclear NMR Spectroscopy. Science 2008, 321, 113-117. (49) Xiong, X. H.; Waller, G.; Ding, D.; Chen, D. C.; Rainwater, B.; Zhao, B.; Wang, Z. X.; Liu, M. L. Controlled Synthesis of NiCo2S4 Nanostructured Arrays on Carbon Fiber Paper for High-Performance Pseudocapacitors. Nano Energy 2015, 16, 71-80. (50) Kong, W.; Lu, C. C.; Zhang, W.; Pub, J.; Wang, Z. H. Homogeneous Core-Shell NiCo2S4 Nanostructures Supported on Nickel Foam for Supercapacitors. J.Mater. Chem. A 2015, 3, 12452-12460.


28

Page 29 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


(51) Liu, Q.; Jin, J. T.; Zhang, J. Y. NiCo2S4@graphene as a Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions. ACS Appl. Mater. Interfaces 2013, 5, 5002-5008. (52) Shen, L. F.; Yu, L.; Wu, H. B.; Yu, X. Y.; Zhang, X. G.; Lou, X. W. Formation of Nickel Cobalt Sulfide Ball-In-Ball Hollow Spheres with Enhanced Electrochemical Pseudocapacitive Properties. Nat. Commun. 2015, 6, 6694-6701. (53) Krstajić, N.; Popović, M.; Grgur, B.; Vojnović, M.; Šepa, D. On the Kinetics of the Hydrogen Evolution Reaction on Nickel in Alkaline Solution. J. Electroanal.Chem. 2001, 512, 16-26. (54) Fan, X. J.; Peng, Z. W.; Ye, R. Q.; Zhou, H. Q.; Guo, X. M3C (M: Fe, Co, Ni) Nanocrystals Encased in Graphene Nanoribbons: An Active and Stable Bifunctional Electrocatalyst for Oxygen Reduction and Hydrogen Evolution Reactions. ACS Nano 2015, 9, 7407-7418. (55) Xiao, C. L.; Li, Y. B.; Lu, X. Y.; Zhao, C. Bifunctional Porous NiFe/NiCo2O4/Ni Foam Electrodes with Triple Hierarchy and Double Synergies for Efficient Whole Cell Water Splitting. Adv. Funct. Mater. 2016, 26, 3515-3523. (56) Rao, Y.; Ning, H.; Ma, X.; Liu, Y.; Wang Y.; Liu, H.; Liu, J. L.; Zhao, Q. S.; Wu, M. B. Template-free Synthesis of Coral-like Nitrogen-doped Carbon Dots/Ni3S2/Ni Foam Composites as Highly Efficient Electrodes for Water Splitting. Carbon 2018, 129, 335-341. (57) Wang, L.; Lin, C.; Huang, D.K.; Chen, J.M.; Jiang, L.; Wang, M.K.; Chi, L. F.; Shi, L.; Jin, J. Optimizing the Volmer Step by Single-Layer Nickel Hydroxide Nanosheets in Hydrogen Evolution Reaction of Platinum. ACS Catal. 2015, 5, 3801-3806.


29


Page 30 of 31

(58) Miao J. W.; Xiao F. X.; Yang H. B.; Khoo, S. Y.; Chen, J. Z.; Fan, Z. X.; Hsu, Y. Y.; Chen, H. M.; Zhang, H.; Liu, B. Hierarchical Ni-Mo-S Nanosheets on Carbon Fiber Cloth: A Flexible Electrode for Efficient Hydrogen Generation in Neutral Electrolyte. Sci. Adv. 2015, 1, e1500259-1500272. (59) Zhu, W. X.; Yue, X. Y.; Zhang, W. T.; Yu, S. X.; Zhang, Y. H.; Wang, J.; Wang, J. L. Nickel Sulfide Microsphere Film on Ni Foam as an Efficient Bifunctional Electrocatalyst for Overall Water Splitting. Chem. Commun. 2016, 52, 1486-1489.


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Ni Foam as a Self-Standing Electrode

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