Continuous Network of Phase-Tuned Nickel Sulfide Nanostructures for

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Continuous Network of Phase-Tuned Nickel Sulfide Nanostructures for Electrocatalytic Water Splitting Anand P. Tiwari, Yeoheung Yoon, Travis G. Novak, Ki-Seok An, and Seokwoo Jeon ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00985 • Publication Date (Web): 01 Aug 2019 Downloaded from pubs.acs.org on August 1, 2019

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ACS Applied Nano Materials

Continuous Network of Phase-Tuned Nickel Sulfide Nanostructures for Electrocatalytic Water Splitting Anand P. Tiwari,¶‡ Yeoheung Yoon,§‡ Travis G. Novak,¶‡ Ki-Seok An,§* and Seokwoo Jeon¶* ¶

Department of Materials Science and Engineering, KAIST Institute for the Nanocentury,

Advanced Battery Center, KAIST, Daejeon 34141, Republic of Korea § Thin

Film Materials Research Center, Korea Research Institute of Chemical Technology,

Yuseong, Post Office Box 107, Daejeon 34114, Republic of Korea. ‡ Equally

contributed

* To whom all correspondence should be addressed: [email protected]

KEYWORDS Phase tuned, continuous networked nanostructures, nickel sulfide, bifunctional electrocatalyst, water splitting

ACS Paragon Plus Environment

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ABSTRACT To date, nanostructures of 3d-group transition metal (i.e. Fe, Co, Ni etc.) derivatives show the highest electrocatalytic performance among non-noble metal electrocatalysts for water splitting in acidic electrolyte. However, poor electrochemical conductivity (~10-4 S/cm) of nanostructures restricts practical application for overall electrocatalytic activity. Herein, continuously networked nanostructures of phase-tuned nickel sulfide foams for efficient water splitting electrocatalysts in both acidic and alkaline electrolytes is reported. Since continuously networked nanostructures of nickel sulfide foams possess an integral structure, they exhibit high electrochemical conductivity (~1 S/cm), which eases adsorption/desorption of H+ and OH- ions for efficient overall water splitting. By tuning the stoichiometry of sulfur, four different phases of continuously networked nanostructures of nickel sulfides (αNiS, βNiS, Ni3S2, and Ni7S6) foams are formed by facile phase transformation of nickel. Among them, the Ni7S6-foam (Ni7S6-F) possesses superior electrocatalytic activity with extremely low overpotential of 70 mV (for hydrogen evolution reaction) and 1.37 V (for oxygen evolution reaction) at 10 mA/cm2 in acidic and alkaline medium, respectively, which is close to noble metal based electrocatalysts. As a result, this work demonstrates a facile synthesis route to optimize nickel sulfide electrocatalysts through phasetuning and continuous networking for overall water splitting and would be applicable on other nanostructured electrocatalysts to improve their electrocatalytic activity for practical applications in future energy devices. 1. INTRODUCTION Hydrogen (H2) is a promising alternative energy source because it possesses the highest energy density among all fuels.1 Since water is an abundant and renewable resource on earth, H2 produced from water splitting is considered as the most auspicious green and renewable energy carrier to


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reduce our dependence on limited fossil fuels in the near future.2,3 Electrochemical or photoelectrochemical water splitting is one of the most appealing and environmentally friendly approaches for providing high efficiency and large scale H2 production among the existing methods.4,5 Scalable water splitting requires efficient and stable electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER).6 Currently, the state-of-the-art electrocatalyst for OER is ruthenium oxide (RuO2), a noble metal derivative, while for HER it is platinum (Pt), a noble metal.7,8 However, the low elemental abundance and high cost of noble metals and their derivatives restrict large-scale hydrogen production and applications. In this regard, extensive research efforts have been focused on various substitutes to replace noble metal electrocatalysts while maintaining high HER and OER electrocatalytic activity. Numerous earthabundant compounds such as transition-metal dichalcogenides (MoS2, WS2, TaS2, etc.), transition metal carbides (WC, Mo2C, etc.), and transition metal phosphides (WP, MoP, etc.) for efficient water splitting electrocatalysts9-17 have been studied; however, most of them possess inferior efficiency to noble metals because of insufficient reactive sites, and poor electrical conductivity. First row transition metal chalcogenides (CoS2, CoSe2, FeS2 etc.) have been frequently investigated as a promising low-cost material with high electrocatalytic performances.18-25 However, the majority of these catalysts are in the form of different nano-architectures such as nano rods, nanosheets, and nanoparticles, which have poor conductivity and are difficult to use in real applications.10,11,26-32 In this respect, the fabrication of the nanostructures (nano rods, nanosheets, etc.) on high surface area and highly conductive materials such as carbon cloth, carbon fiber, carbon nano tubes, has been studied.33,34 These nanostructured electrocatalytic materials have been grown via hydrothermal or solvothermal methods on highly conductive and high surface area materials,35 but are limited due to the loading amount restriction and lack of catalytic active


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surface control. In addition, surface tension differences of carbon and active materials restricts stability of the structures.36 Nickel (Ni) foam is a promising alternative nanostructure template for electrocatalysts because of its low price, high conductivity, and rich macroporosity.37,38 Still, the non-homogeneous configuration of nanostructures in Ni-foam restricts the stability towards electrocatalytic activity. The more important issue is that metal chalcogenides, metal carbides, and metal phosphides are not chemically stable during OER activity even if they are grown on highly conductive materials.39 Recent works have been demonstrated that the pyrite type transition metal chalcogenides grown on modified Ni-foam can overcome this oxidation issue of active materials in alkaline solutions for OER40,41 but these require additional research regarding structural and compositional changes for active sites to ensure stable OER and HER. Here, we propose a simple strategy to utilize low cost, highly conductive, and porous NixSy continuously networked foam nanostructures to make a highly active water splitting electrocatalyst in both acidic and alkaline media. The successful phase transformation of Ni-foam to different phases of continuously networked nanostructures of nickel sulfide foams promises efficient electrocatalysts towards overall water splitting. First, the Ni-foam (Ni-F) surface is modified by acetic acid, and then four different phases of continuously networked nanostructures of nickel sulfide foam are synthesized by tuning the sulfur amount, including αNiS-F, βNiS-F, Ni3S2-F, and Ni7S6-F. These continuously networked nanostructures of nickel sulfide foams possess exceptional characteristics, such as good electrochemical conductivity (~1 S/cm), high porosity (∼95% porous), and sharp surfaces with highly efficient catalytic active sites. Nickel sulfide foams can be applied directly into the electrolyte without an additional binder at relatively low current density (10 mA/cm2), and the solid contact with other electrodes solves the contact resistance issue between catalyst and electrode, which ensures fast exchange of the ions that is the key factor


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contributing to enhance the electrocatalytic performances. Moreover, by phase tuning the nickel sulfide foam, we have found that nickel rich Ni7S6-foam (Ni7S6-F) shows excellent electrocatalytic activity for water splitting in both acidic and alkaline electrolytes. Ni7S6-F possesses very low overpotentials (in acidic electrolyte 70 mV vs RHE at 10 mA/cm2 with Tafel slope of 63 mV/dec for HER, and in alkaline electrolyte 1.37 V vs RHE at 10 mA/cm2 with Tafel slope of 119 mV/dec for OER), which are very close to commercial Pt/C (in acidic electrolyte with overset potentials = 50 mV vs NHE at 10 mA/cm2 for HER) and RuO2 (in alkaline electrolyte with overpotentials = 1.41 V vs RHE at 10 mA/cm2 for OER) electrocatalysts under the same testing conditions. Our new strategy demonstrates electrocatalysts for efficient overall water splitting using low cost, highly active, and continuously networked nanostructures of nickel sulfide foams. 2. EXPERIMENTAL METHODS 2.1. Material Synthesis. First, the surface of Ni foams (1 cm2) are modified by immersing in acetic acid and PVP solution for 15 min. The resulting Ni foam is dried at room temperature. For preparation of nickel sulfide foams, the modified Ni-foam is annealed in an Ar atmosphere at different temperature 500 0C, 6000C, 700 0C with 1 gram and 2 gram of sulfur for 30 min. We have synthesized four different phases αNiS-F, βNiS-F, Ni3S2-F, and Ni7S6-F of nickel sulfide by changing sulfur amount and temperature. 2.2. Chemical and Microstructural Characterizations. The structures, morphology and, chemical compositions of as-synthesized nickel sulfide foams are investigated by a transmission electron microscope (TEM) (KARA, Titan cubed G2 60-300 TEM), a field-emission scanning electron microscope (FESEM) (FBI company, Magellen400), a powder X-ray diffractometer (XRD) (Cu kα1 radiation, (Rigaku, SmarLab)), and X-ray photospectroscopy (Thermo VG Scientific, K-alpha). The Raman analysis of the samples has been done with laser 532 nm.


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2.3. Electrochemical Characterizations. Electrocatalytic investigation are done with VersaSTATE 3 (principle applied research) electrochemical workstation. The electrochemical performance is carried with a three-electrode systems (working electrode is the active material, the reference electrode is Ag/AgCl in 3M KCl, and the counter electrode is a Pt mesh). The active materials (nickel sulfide foams) are directly connected as working electrode without adding any binder. The HER activity are measured in N2-saturated 0.5 M H2SO4 electrolyte. However, OER activity are measured in O2-saturated 0.1 M KOH electrolyte. All of the potentials are calibrated to a RHE, ERHE = EAg/AgCl + 0.196 + 0.0591pH V. 3. RESULTS AND DISCUSSION 2.1. Characterization of the morphology and structure To achieve highly efficient continuously networked nanostructures of nickel sulfide based electrocatalysts, we have employed a surface modification and sulfurization technique over commercially available Ni-foam, as shown in Figure 1. Surface modification with polyvinylpyrrolidone (PVP) in acetic acid (HAc) is first used to remove oxides and activate the surface, followed by annealing in a sulfur atmosphere to create high porous and continuous NixSy foams. By varying the sulfur amount and temperature in this reaction, different phases on NixSy foams can be synthesized, as confirmed by the high resolution transmission electron microscope (HRTEM) images in Figure 1. αNiS (Figure 1b) adopts a hexagonal nickel arsenide structure in which each Ni atom is surrounded octahedrally by six S atoms packed fairly closely by two other Ni atoms, wherein d-spacing is 2.9 Å. Increasing the temperature of the reaction leads to the formation of millerite βNiS (Figure 1c), which processes identical stoichiometry, but crystallizes in a rhombohedral structure with d-spacing of 4.8 Å. Further increasing the reaction temperature yields Ni-rich heazlewoodite Ni3S2 (Figure 1e), with a rhombohedral crystal structure in which each nickel atom is found at a pseudo-tetrahedral site in an approximately body centered cubic


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sulfur lattice and d-spacing of 2.8 Å. Finally, the orthorhombic Ni7S6-F phase (Figure 1d), with dspacing of 5.8 Å, is synthesized by increasing the sulfur loading.

Figure 1. Fabrication of continuously networked nanostructures of phase tuned nickel sulfide. (a) Schematic illustration for the synthesis of nickel sulfide foams with real images of nickel (Ni) foams, surface modified Ni-foams, and as synthesized nickel sulfide foam, (scale bar for real image is 0.5 cm). High-resolution transmission electron microscope (HRTEM) images with diffraction patterns and crystal structures of (b) αNiS-F, (c) βNiS-F, (d) Ni7S6-F, and (e) Ni3S2-F samples. In addition tuning the phase, the synthesis conditions also greatly affected the continuously networked nanostructure foam morphology, as confirmed by field emission scanning electron microscopy (FESEM). Figure 2 reveals that Ni3S2-F has the comparatively least sharp continuously networked structure among as synthesized foams, while Ni7S6-F is composed of highly sharp structures. Moreover, Ni7S6-F has the densest structure among as-synthesized nickel sulfide foams. It can be seen from edges of structures that Ni7S6-F has well-ordered aligned


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nanostructures along edges, however, Ni3S2-F has smooth structures at edges. In addition, energy dispersive x-ray spectroscopy (EDX) analysis indicates that S and Ni are homogeneously dispersed in all continuously networked nanostructures foams (shown in supporting information Figure S1). We also note that the highly porous structure is fully preserved after sulfurization, with no poreclogging or structural degradation observed (SEM of Ni-foam is shown in supporting information Figure S2).

Figure 2. Surface morphology of continuously networked nanostructures of nickel sulfide foams. Scanning electron microscope (SEM) images of (a) Ni3S2-F (b) βNiS-F, (c) αNiS-F, and (d) Ni7S6-F, samples (scale bar 500 μm). Magnified SEM images are shown vertically down side for all as-synthesized nickel sulfide foams to reveal the sharpness of the foams (scale bar 250 μm). Highly magnified SEM are shown vertically down to magnified SEM to reveal the continuously networked nanostructures of the foams (scale bar 20 μm). To determine the purity and exact crystal structures of as synthesized continuously networked nanostructures of nickel sulfide foams, X-ray diffraction (XRD) analysis has been conducted (shown in Figure 3 a). We have indexed all obtained XRD spectra with the standard XRD patterns of nickel sulfide such as; αNiS (JCPDF: 65-3419), βNiS (JCPDF: 65-3686), Ni3S2 (JCPDF: 44-


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1418) and Ni7S6 (JCPDF: 14-364). As expected, as-synthesized continuously networked nanostructure foams are composed of different nickel sulfide phases with the peaks in a one-toone correspondence with the standard XRD patterns, confirming the observations by HRTEM (Figure 1). It is worth noting that nearly no XRD peaks are observed corresponding to metallic Ni, confirming the very high purity of the as-synthesized samples and nearly complete conversion of the Ni foam to porous different phases of nickel sulfide foams. Moreover, to ensure the complete conversion of Ni-foam, we have crushed the nickel sulfide foams in the powder form and checked the XRD of the samples. We have not observed any Ni peaks in the XRD of the different nickel sulfide, which confirms complete conversion of the Ni-foam to the different nickel sulfide foams (shown in supporting information Figure S3). Raman spectroscopy is employed to further confirm the phases of continuously networked nanostructures of nickel sulfide foams (shown in supporting information Figure S4). Two prominent Raman peaks are clearly detected with frequencies located at around 220 cm-1, and 290 cm-1 for αNiS-F, and βNiS-F samples, which is in agreement with the literature data.42 However, shift is observed in αNiS Raman peaks that might be occurred be due to phase differences between βNiS and αNiS since Raman vibrational modes mightily depend on the vibration of the crystal lattice. The Ni3S2-F sample shows four prominent peaks at 224 cm-1, 250 cm-1, 304 cm-1, and 325 cm-1, suggesting high purity of the as-grown sample.43 Moreover, the Ni7S6-F sample contains four peaks with a shift towards low frequency, confirming the phase difference between Ni3S2-F and Ni7S6-F foams. To determine the elemental composition X-ray photoelectron microscopy (XPS) is employed (shown in Figure 3 b-e). The XPS results show clearly the main peaks of S and Ni elements for all as-synthesized nickel sulfide foams. The XPS peaks at 161.59 eV and 162.65 eV appear in all samples, corresponding to the S 2p3/2 and S 2p1/2 states of S, respectively. The peaks at 852.80 eV and 855.60 eV indicate the different oxidation


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states of 2p3/2 of Ni, however, peaks at 870.24 eV and 873.66 eV show the different oxidation states of 2p1/2 of Ni, which are consistent with previously reported nickel sulfide chemical structures.44-46 Electronic states of S suggest that residual S is not remaining on the surface, confirming the formation of different phases of nickel sulfide without any impurities. Elemental compositional analysis of αNiS-F, βNiS-F, Ni3S2-F, and Ni7S6-F shows that all the as-synthesized foams are in the appropriate stoichiometric ratio (shown in supporting information Table S1). Similar to the XRD and Raman spectroscopy results, XPS results show that we have successfully synthesized different phases of continuously networked nanostructures of nickel sulfide foams without any discernible impurities or additional phases.

Figure 3. Compositional and chemical analysis of continuously networked nickel sulfide foams. (a) X-ray diffraction patterns of as-synthesized nickel sulfide foams, and X-ray photoelectron spectroscopy spectra of (b) βNiS-F, (c) αNiS-F, (d) Ni3S2-F, and (e) Ni7S6-F samples (Sat. stands for satellite peak in b-e). 2.2. Electrocatalytic performance The water splitting performances of the as-prepared continuously networked nanostructures of nickel sulfide foams are evaluated as electrocatalysts in both acidic and alkaline media using a


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three-electrode setup (for details, please see experimental section). Since nickel sulfide foams are self-supported, it is not necessary to use a binder for electrode fabrication, and the catalyst can be directly connected to the working electrode for electrochemical tests. Firstly, water splitting in alkaline electrolytes, called OER, is tested in an O2-saturated 0.1 M KOH electrolyte. For comparison, the as-synthesized nickel sulfide foams, including αNiS-F, βNiS-F, Ni3S2-F, Ni7S6-F and commercial RuO2, are measured under the same cell configurations (for details please see experimental section). The OER activities of different as-synthesized foams are evaluated by comparing the polarization curves obtained with linear sweep voltammetry (LSV) at scan rate of 10 mV/s. All potentials are referenced to the reversible hydrogen electrode (RHE). The normal method to know a good electrocatalyst is to compare the overpotentials, which is the additional potential vs. RHE (defined as 0.0 V) for achieving a current density of 10 mAcm-2.47 Figure 4 (a) shows the LSV curves of the αNiS-F, βNiS-F, Ni3S2-F, Ni7S6-F in an O2-saturated 0.1 M KOH electrolyte. It can be seen from LSV that Ni7S6-F catalysts possess lowest overpotential (1.37 V vs RHE) among as-synthesized nickel sulfide foams, even lower than that of the commercial RuO2 (1.41 V vs RHE), to obtain 10 mAcm-2 current density. Furthermore, overpotentials for αNiS-F, βNiS-F, and Ni3S2-F samples are 1.42 V, 1.43 V, and 1.47 V respectively to obtain 10 mAcm-2 current density. Moreover, the electrocatalytic activity of Ni7S6-F for OER is the highest among reported transition metal chalcogenides.48 The autoxidation peaks on Ni7S6-F and Ni3S2-F samples at 1.20 V and 1.30 V are observed prior to the OER, while these peaks are absent in the case of αNiS-F and βNiS-F samples, which is consistent with the fact that the redox charge in the oxidation process become significantly larger in the Ni-rich phases of nickel sulfide.49,50 The highest electrocatalytic activity of Ni7S6-F can be attributed to the highly dense and sharp nanostructures, which allow easy diffusion of the electrolyte and fast movement of electrons. To investigate the


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kinetics of electrocatalytic activity, the Tafel slope is generally used. A small value of the Tafel slope implies fast electrocatalytic activity with an incremental overpotential. Tafel slopes are derived from the polarization curve obtained in alkaline electrolytes to understand the kinetics of electrocatalytic activity (shown in Figure 4 b). As predicted, Ni7S6-F sample has the smallest (119 mV/dec) Tafel slope for OER among all as-synthesized nickel sulfide foams and is very close to commercially available RuO2 (82 mV/dec). However, the Tafel slopes for αNiS-F, βNiS-F, and Ni3S2-F samples are 125 mV/dec, 151 mV/dec, and 159 mV/dec, respectively, for OER activity. Furthermore, to confirm the lowest Tafel values of Ni7S6-F, we have plotted Tafel plot derived from slow scan rate polarization curve (shown in supporting information Figure S5). It is noted that despite of the lower overpotential of Ni7S6-F compared to RuO2, the Tafel slope of Ni7S6-F is higher than RuO2, which might be caused by the difference in resistance of the materials and the charge transfer on the surface, both of which are known to influence the Tafel slope. Water splitting in an acidic medium, called HER, are tested in an N2-saturated 0.5 M H2SO4 electrolyte with a three-electrode setup. Figure 4 (c) shows the LSV curves of the αNiS-F, βNiSF, Ni3S2-F, Ni7S6-F in N2-saturated 0.5 M H2SO4 electrolyte, measured at scan rate of 10 mV/s. Similar to OER, it can be seen in Figure 4 (c) that the Ni7S6-F sample has lowest overpotential 70 mV to obtain of 10 mAcm-2 current density, which is very close to a commercially available Pt/C electrode (50 mV) under the same conditions. However, the overpotential of αNiS-F, βNiS-F, and Ni3S2-F samples at 10 mAcm-2 are 100 mV, 140 mV and 170 mV respectively. The lowest overpotential to reach 10 mAcm-2 of the Ni7S6-F sample can be attributed to the highly dense, sharp features, which expose many active catalytic sites, ensure fast exchange of the protons for hydrogen adsorption, and desorption. In addition, electrocatalytic activity of Ni7S6-F is the highest


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among those for the first-row transition metal chalcogenides (e.g. CoSe2, NiSe2, FeS2, etc.) in terms of overpotentials at 10 mAcm-2 for HER.51-54

Figure 4: Electrocatalytic activity of as synthesized nickel sulfide foams for water splitting. (a) LSV for as-synthesized samples in 0.1 M KOH electrolyte, (b) Tafel plots for as-synthesized samples in 0.1 M KOH electrolyte, (c) Linear sweep voltammetry (LSV) for as-synthesized samples in 0.5 M H2SO4 electrolyte, (d) Tafel plots for as-synthesized samples in 0.5 M H2SO4 electrolyte, and plots to determine Cdl for as synthesized samples in (e) 0.1M KOH, (f) 0.5 M H2SO4.


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The Tafel plots in an acidic electrolyte of as-synthesized nickel sulfide foams are obtained from the polarization curves (Figure 4 c) (shown in Figure 4 d). The Tafel slopes of the Ni7S6-F sample for HER is the smallest (63 mV/dec) among all as-synthesized nickel sulfide foams and very close to commercial available Pt/C (33 mV/dec). However, the Tafel slopes for αNiS-F, βNiSF, and Ni3S2-F samples are 89 mV/dec, 92 mV/dec, and 104 mV/dec, respectively, for HER activity. The lower Tafel slope of the Ni7S6-F sample indicates excellent intrinsic catalytic activity, and shows the pronounced effect of phase-tuning on HER performance. Furthermore, the double-layer capacitances (Cdl) are derived from the cyclic voltammetry (CV) to evaluate the electrochemical surface area (ECSA) of as-synthesized continuously networked nanostructures of nickel sulfide foams in acidic and alkaline electrolytes (shown in Figure 4 e and f). Cyclic voltammetry of as-synthesized nickel sulfide foams are shown in supporting information Figure S6 and S7. The ECSA is proportional to the Cdl, a larger Cdl represents a higher surface area of the electrocatalysts. The Ni7S6-F sample shows the highest double-layer capacitances in both acidic and alkaline electrolytes (37.0 mF cm-2 in acidic and 52.2 mF cm-2 in alkaline electrolyte) among as-synthesized nickel sulfide foams. The remarkable Cdl of Ni7S6-F sample may be caused by sharp active edges, which increase electrochemical active surface area. To confirm the intrinsic electrocatalytic activity of as-synthesized continuously networked nanostructures of nickel sulfide foams, we have normalized the polarization curves with electrochemical active surface area (ECSA) for HER (shown in supporting information Figure S8). It is revealed from the normalized polarization curves that Ni7S6-F possesses highest electrochemical activity among as-synthesized foams, which is consistence with Figure 4 (a). The electrochemical active areas of different assynthesized nickel sulfide foams in acidic and alkaline electrolyte along with their HER and OER activities are listed in Tables 1 and 2.


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Table 1: Hydrogen Evolution Reaction (HER) activity comparisons for all samples Samples

Overpotentials (mV) at 10 mA cm-2

Tafel slope (mV dec-1)

Double layer capacitance Cdl (mF cm-2)

ToF @ 80 mV (H2 S-1)

Ni7S6-F

70

63

37

0.60

αNiS-F

100

89

19

0.09

βNiS-F

140

92

6

0.08

Ni3S2-F

170

104

3

0.01

Table 2: Oxygen Evolution Reaction (OER) activity comparisons for all samples Samples

Overpotentials (V) at 10 mA cm-2

Tafel slope (mV dec-1)

Double layer capacitance Cdl (mF cm-2)

ToF @ 140 mV (O2 S-1)

Ni7S6-F

1.37

119

52.2

0.70

αNiS-F

1.42

125

12.3

0.30

βNiS-F

1.43

151

3.2

0.10

Ni3S2-F

1.47

159

3.0

0.06

To gain further evidence of excellent electrochemical catalytic activities of continuously networked nanostructures of nickel sulfide foams, we calculated the overall turnover frequency (TOF) to quantify each of the active sites in both acidic and alkaline electrolytes (details of the calculation are written in supporting information). The TOFs for H2 and O2 of different assynthesized nickel sulfide foams are shown in Figure 5 ((a) and (b)) and listed in Tables 1 and 2. The Ni7S6-F sample shows the highest TOF for H2 at an overpotential of -0.08 V vs RHE (0.60 s1),

which is the highest among as-synthesized nickel sulfide foams and comparable to the

commercial available Pt/C (0.90 s-1) electrodes.55 Moreover, the TOF of Ni7S6-F for O2 is 0.70 s-1 at 1.40 V vs RHE, which is higher than commercially available RuO2 (0.49 s-1) electrodes, confirming the important role of continuously networked sharp structures of nickel sulfide with


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proper chemical compositions promoting the intrinsic catalytic activity. Based on these results, the excellent HER and OER activity of the nickel sulfide foams especially Ni7S6-F sample can be attributed to the synergistic effects of the abundant highly dense sharp active sites, and high intrinsic catalytic activity with integral structures, which allows easy adsorption and desorption of H+ and OH- ions for the excellent electrocatalytic activities. Furthermore, electrochemical impedance spectroscopy (EIS) is performed to confirm the fast reaction kinetics (fast movement of electrons) of Ni7S6-F sample for water splitting. We have plotted Nyquist plots of as-synthesized nickel sulfide foams at the overpotential of 150 mV (shown in supporting information Figure S9). The EIS data reveals that the Ni7S6-F has the lowest charge transfer resistance (Rct, 0.75 Ω) among the as-synthesized nickel sulfide foams, suggesting faster electron transfer for water splitting. The chemical stability of the electrocatalysts in both acidic and alkaline electrolytes is a significant factor for efficient water splitting process via HER and OER activities. Although the foams are brittle because of the complete conversion to the sulfide phase, they do not suffer from the chemical degradation that is common among non-precious electrocatalysts. The durability of nickel sulfide foams is investigated by chronoamperometry (j ~ t). This chronoamperometry process is performed at a constant potential of -100 mV vs. RHE in 0.5 M H2SO4 to know the stability of nickel sulfide foams in an acidic electrolyte (shown in Figure 5 c). It can be clearly seen that the Ni7S6-F sample shows a constant current for the continuous electrolysis process, revealing excellent stability in the acidic electrolyte. Moreover, after 1000 cycles of HER activity, the LSV is measured for Ni7S6-F sample, which shows similar overpotentials as the initial sample (shown in inset of Figure 5 c), confirming that Ni7S6-F sample is stable in the acidic electrolyte for HER activities. After 1000 cycles of electrocatalytic activity at current density (10 mA/cm2) for HER, XPS, XRD and SEM are performed to know the chemical of Ni7S6-F sample (shown in


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supporting information Figure S10). It is revealed from XPS and SEM measurements that the Ni7S6-F sample is stable and robust in the acidic environment, with XPS results confirming no significant change in the electronic and chemical states of Ni and S. The phase stability is also confirmed by XRD analysis (supporting information Figure S10 c), where the major peaks corresponding to Ni7S6 are clearly still present after the electrochemical tests. Some slight differences in XRD could be attributable to electrolyte diffusion in the material. Similar to the chemical stability test in acidic electrolyte, chronoamperometry test is conducted in 0.1 M KOH at 1.35 V vs RHE for Ni7S6-F sample (shown in Figure 5 d). It can be seen from chronoamperometry in the alkaline electrolyte that current density is constant for continuous electrolysis process. There is no change in overpotential for OER after 1000 cycles, indicating the robustness of Ni7S6-F sample for OER activity (shown in inset of Figure 5 d). In addition, SEM, XPS and XRD measurements reveal no change in electronic and chemical state and structure of the Ni7S6-F sample after continuous electrolysis in the alkaline electrolyte (shown in supporting Information Figure S11), indicating significant chemical and phase stability the material for OER activity. The XPS analysis of the Ni7S6-F sample after continuous electrocatalytic activity in alkaline electrolyte shows the partial oxidation of the surface of the nickel sulfide foams, however nickel sulfide is prominently observed in the samples, which is consistent with LSV results (shown in supporting information Figure S12). The good bifunctional performance towards HER and OER encouraged us to construct a complete water-splitting cell by employing Ni7S6-F as both anode and cathode to investigate its overall water splitting performance. It is revealed that the Ni7S6-F sample shows good HER activity in alkaline electrolyte as well as good OER activity in acidic electrolyte (shown in supporting information Figure S 13 (a) and (b)). For full cell, Ni7S6-F sample exhibits high activity


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toward full water splitting with the required cell voltages of -154 mV to obtain 10 mA/cm2 in acidic electrolyte (0.5 M H2SO4). However, in 0.1 M KOH aqueous solution, Ni7S6-F requires cell voltages of 1.51 V to obtain 10 mA/cm2, which is lower than those values reported with nonprecious metal bifunctional electrocatalysts toward full water splitting.56,57 The full cell performances are shown in supporting information S13 (c) and (d).

Figure 5. Active sites calculation and chemical stability of continuously networked nickel sulfide foams. Turn over frequency (TOFs) per surface site over as-synthesized samples (a) for H2, (b) for O2, and chronoamperometry measurement (j ~ t) of Ni7S6-F sample in (c) 0.5 M H2SO4, (d) 0.1M KOH. Inset of (c) and (d) are showing LSV curves of Ni7S6-F after 1000 cycles in 0.5 M H2SO4 and 0.1 M KOH electrolytes respectively. 3. Conclusions In summary, we have synthesized highly porous and continuously networked nanostructures of nickel sulfide foams via simple surface modifications and direct sulfurization of low cost and highly conductive Ni foam. The as-synthesized continuously networked nanostructures of nickel


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sulfide foams have sharp surfaces with high catalytic active edges and high electrochemical conductivity. By tuning the sulfur amount and temperature, we have synthesized different phases of nickel sulfide foams, such as αNiS-F, βNiS-F, Ni3S2-F, and Ni7S6-F. We have found that nickelrich Ni7S6-F sample shows excellent electrocatalytic activity for water splitting in both alkaline (overpotentials = 1.37 V vs RHE at 10 mA/cm2 with Tafel slope of 119 mV/dec for OER), and acidic (overpotentials = 70 mV vs NHE at 10 mA/cm2 with Tafel slope of 63 mV/dec for HER) electrolytes, which is better than most of the reported results on well-studied catalysts, and nearly as good as the noble metal catalysts. The highest electrocatalytic activity is attributed to synergetic effects of the sharp surfaces with highly rich active sites and continuous nanostructures that create easily accessible electrochemical surfaces and easy diffusion of electrolytes for H+ and OH- ion adsorption/desorption on the surface of electrocatalysts for HER and OER activities. This synthesis route could be applied to various nickel structures supported by other conductive materials to improve the mechanical stability, and the overall phase-tuning strategy outlined here could greatly influence future metal sulfide bifunctional electrocatalyst research in general. ASSOCIATED CONTENT Supporting Information. Further information regarding characterization tools, supporting table and figures is available. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Prof. Seokwoo Jeon, Email: [email protected] * Dr. Ki-Seok An, Email: [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) under grant no. NRF-2017R1D1A1B03032791, Nano-Material Technology Development Program through the NRF funded by the Ministry of Science, ICT and Future Planning grant no. NRF2017M3A7B4041987, and Multi-Ministry Collaborative R&D Program through the NRF funded by KNPA, MSIT, MOTIE, ME, NFA (NRF-2017M3D9A1073858). REFERENCES (1) Liu, C.; Tang, J.; Chen, H. M.; Liu, B.; Yang, P. A fully integrated nanosystem of semiconductor nanowires for direct solar water splitting. Nano Lett. 2013, 13 (6), 2989-2992. (2) Dresselhaus, M. S.; Thomas, I. L. Alternative energy technologies. Nature 2001, 414, 332337. (3) Li, Y.; Zhang, H.; Jiang, M.; Kuang, Y.; Wang, H.; Sun, X. Amorphous Co–Mo–S ultrathin films with low-temperature sulfurization as high-performance electrocatalysts for the hydrogen evolution reaction. J. Mater. Chem. A 2016, 4 (36), 13731-13735. (4) Tiwari, A. P.; Novak, T. G.; Bu, X.; Ho, J.; Jeon, S. Layered ternary and quaternary transition metal chalcogenide based catalysts for water splitting. Catalysts 2018, 8 (11), 551-580. (5) Chu, S.; Cui, Y.; Liu, N. The path towards sustainable energy. Nat. Mater. 2017, 16, 16-22. (6) Seh, Z.W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I. B.; Norskov, J. K.; Jaramillo, T. F. Combining theory and experiment in electrocatalysis: Insights into materials design. Science 2017, 355 (6321), 146-158.


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SYNOPSIS (TOC)


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Continuous Network of Phase-Tuned Nickel Sulfide Nanostructures for

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