nickel sulfide heterostructure directly grown on nickel

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 27712−27722

Cobalt Sulfide/Nickel Sulfide Heterostructure Directly Grown on Nickel Foam: An Efficient and Durable Electrocatalyst for Overall Water Splitting Application Subhasis Shit,† Suman Chhetri,†,‡ Wooree Jang,§ Naresh C. Murmu,†,‡ Hyeyoung Koo,§ Pranab Samanta,†,‡ and Tapas Kuila*,†,‡

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†

Surface Engineering & Tribology Division, Council of Scientific and Industrial Research-Central Mechanical Engineering Research Institute, Durgapur 713209, India ‡ Academy of Scientific and Innovative Research (AcSIR), CSIR-CMERI Campus, Durgapur 713209, India § Soft Innovative Materials Research Centre, Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), Jeonbuk, Seoul 565905, South Korea S Supporting Information *

ABSTRACT: Fabrication of high-performance noble-metal-free bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in water is a promising strategy toward future carbon-neutral economy. Herein, a one-pot hydrothermal synthesis of cobalt sulfide/nickel sulfide heterostructure supported by nickel foam (CoSx/Ni3S2@ NF) was performed. The Ni foam acted as the three-dimensional conducting substrate as well as the source of nickel for Ni3S2. The formation of CoSx/ Ni3S2@NF was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy. The formation of CoSx/Ni3S2@NF facilitated easy charge transport and showed synergistic electrocatalytic effect toward HER, OER, and overall water splitting in alkaline medium. Remarkably, CoSx/Ni3S2@NF showed catalytic activity comparable with that of benchmarking electrocatalysts Pt/C and RuO2. For CoSx/Ni3S2@NF, overpotentials of 204 and 280 mV were required to achieve current densities of 10 and 20 mA cm−2 for HER and OER, respectively, in 1.0 M KOH solution. A two-electrode system was formulated for overall water splitting reaction, which showed current densities of 10 and 50 mA cm−2 at 1.572 and 1.684 V, respectively. The prepared catalyst exhibited excellent durability in HER and OER catalyzing conditions and also in overall water splitting operation. Therefore, CoSx/ Ni3S2@NF could be a promising noble-metal-free electrocatalyst for overall water splitting application. KEYWORDS: hydrogen evolution reaction, oxygen evolution reaction, bifunctional electrocatalyst, overall water splitting, heterostructure Ir-based oxides.6,7 However, their low abundance, high cost, and short-term durability impede their large-scale application as electrocatalysts.8,9 In addition, the efficiency of overall water splitting depends on both HER and OER rate kinetics. Therefore, the development of a material which can concurrently catalyze both the HER and OER in the same electrolytic medium could be advantageous, which might further simplify the development process of an electrolyzer device.10−13 Over the last few years, extensive research is being carried out to develop earth-abundant, noble-metal-free bifunctional electrocatalysts. Several materials such as transition-metal chalcogenides, pnictides, borides, and so forth were developed

1. INTRODUCTION The harness and storage of renewable energy are challenging yet promising approach to confront with the two major problems of contemporary timeenergy depletion and environmental degradation. Central to this challenge, hydrogen has a great merit to be used as a clean energy carrier because it has a high energy density of 283 kJ mol−1 and produces only water (H2O) as a product after combustion.1 Therefore, electrochemical water splitting using energy derived from sources such as solar, wind, and so forth to produce hydrogen at the cathode and oxygen at the anode shows great commitment.2,3 However, both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have sluggish kinetics; thus, employment of an effective catalyst is necessary to narrow the gap between theoretically calculated and practically required potential.4,5 The state-of-the-art catalysts used for HER are Pt-based materials and for OER are Ru- and © 2018 American Chemical Society

Received: March 14, 2018 Accepted: July 25, 2018 Published: July 25, 2018 27712

DOI: 10.1021/acsami.8b04223 ACS Appl. Mater. Interfaces 2018, 10, 27712−27722

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ACS Applied Materials & Interfaces as electrocatalysts for the water splitting reaction.14−22 However, these types of materials suffer from low electrical conductivity and less specific surface area. Direct growing of nanostructured catalyst materials on conducting substrates such as carbon cloth, metal foam, metal mesh, metal foil, and so forth have proven effective in overcoming those lacunae.23−27 To the present, nickel sulfide has been extensively studied and been proposed to be active toward electrocatalytic water splitting in the alkaline medium,12,16,28 but its catalytic activity toward HER is not up to the mark.11,13 In order to augment the catalytic activity, Ni3S2 has been hybridized with a highly efficient electrocatalyst, such as MoS2, which exhibited improved catalytic activity.11,29,30 Although MoS2 has good catalytic activity toward HER in the acidic medium, its activity is not up to the mark in the alkaline medium.31 Prior studies revealed that sulfides of cobalt, such as Co9S8, CoS2, CoS, and so forth, have better catalytic activity toward both HER and OER in the alkaline medium and have excellent chemical stability and electrical conductivity.32−35 Furthermore, previous studies revealed that the free energy of adsorption of H- and O-containing species decreased with the incorporation of Co atoms on the edge sites of other transition-metal chalcogenides. Moreover, the charge transfer within the electrocatalyst material via S-linkages as well as between the electrocatalyst interface with H- and O-containing species increased synergistically. As a consequence, the HER and OER catalyzing activity increased significantly.36−38 Therefore, integration of cobalt sulfide with nickel sulfide may be promising toward the electrocatalytic water splitting application. Herein, cobalt sulfide-anchored nickel sulfide on nickel foam (CoSx/Ni3S2@NF) was fabricated for the first time via a onestep hydrothermal process, using NF itself as the nickel precursor. The prepared CoSx/Ni3S2@NF electrode showed excellent performance as an electrocatalyst toward overall water splitting. It exhibited comparable catalytic activity with that of Pt- and Ru-based electrocatalysts used for HER and OER. Overpotentials of 204 and 280 mV were required to achieve current densities of 10 and 20 mA cm−2 for HER and OER, respectively, in 1.0 M KOH solution. In order to ensure its overall water splitting competency, a two-electrode system was constructed using CoSx/Ni3S2@NF as both cathode and anode in 1.0 M KOH solution.

rinsed thoroughly with distilled water. Thereafter, it was dried inside a vacuum oven at 60 °C for 12 h. The change in mass was determined with the help of a high-precision balance, and the average mass loading was ∼2.83 mg cm−2. Two other materials were prepared according to the similar procedure where CoCl2·H2O was taken at an amount of 0.05 and 0.2 mmol, which were denoted as C0.05NS and C0.2NS, respectively. For comparison, Ni3S2@NF was prepared according to the same procedure as mentioned earlier without using the cobalt precursor. CoSx was also prepared following the same procedure without putting NF in the hydrothermal reactor. After the preparation of CoSx, a catalyst ink was prepared and loaded on to the pretreated NF to fabricate CoSx@NF. 2.2.2. Fabrication of Pt/C@NF and RuO2@NF. About 71 mg of Pt/ C (10 wt % Pt content) and RuO2 was taken in two different sample tubes each containing ∼5 mL of N,N-dimethylformamide. As a binder material, ∼7 mg of polyvinylidene fluoride was added to each dispersion. The catalyst inks were then sonicated for ∼30 min. After that, the calculated amount of catalyst inks was deposited on two precleaned NF pieces. The loading of the catalysts was ∼2.83 mg cm−2 in each case. 2.3. Physicochemical Characterizations. Field emission scanning electron microscopy (FE-SEM) and energy-dispersive Xray spectroscopy (EDXS) assisted elemental mapping were performed using Sigma HD, Carl Zeiss, Germany. High-resolution transmission electron microscopy (HR-TEM) images, selective area electron diffraction (SAED) pattern, and EDXS spectra were recorded at 200 kV using a JEM 2100 microscope, JEOL, Japan. The powder Xray diffraction (PXRD) patterns were recorded using Cu Kα radiation (λ = 0.15418 nm) by D2 PHASER, Bruker, Germany. X-ray photoelectron spectroscopy (XPS) was carried out using Thermal Scientific equipped with a monochromatic Al Kα X-ray source (1486.6 eV). 2.4. Electrochemical Measurements. All the electrochemical measurements were carried out using the PARSTAT 4000 (Princeton Applied Research, USA) electrochemical workstation. About 1.0 M KOH solution was used as the electrolyte throughout the experiments. In the three-electrode electrochemical system, catalyst-loaded NFs, Pt wire, and Ag/AgCl/saturated KCl were used as working, counter, and reference electrodes, respectively. The potentials recorded were converted into reversible hydrogen electrode (RHE) scale according to the following equation:

E RHE = EAg/AgCl/Sat.KCl + 0.059pH + 0.197

(1)

The current densities reported in this current work were normalized as per the geometric area.39 Prior to recording the polarization curves, for HER and OER, several voltammetric cycles were performed at a scan rate of 100 mV s−1. The linear sweep voltammetry (LSV) scans were performed at a scan rate of 5 mV s−1 to minimize the double-layer charging current. All the above data were 100% iR corrected to eliminate the influence of uncompensated resistance. The Tafel plot was derived from the LSV curves, and the analysis was carried out according to the following equation:

2. EXPERIMENTAL SECTION 2.1. Materials. CoCl2·6H2O, thiourea, KOH pellets, and HCl (∼35%) were purchased from Merck Specialties Pvt. Ltd. (Mumbai), India. NF was bought from Shanghai Winfay New Material Co. Ltd., China. Absolute ethanol (99.9%) was obtained from Honyon International Inc., China. Pt on activated carbon (10 wt %) and RuO2 were purchased from Sigma-Aldrich. The mentioned chemicals were of analytical grade and thus used without further purification unless specifically mentioned. 2.2. Preparation of the Electrocatalyst. 2.2.1. Synthesis of CoSx/Ni3S2@NF, Ni3S2@NF, and CoSx@NF. NF was cut into three pieces with dimensions 2 × 3 cm2. These pieces were then cleaned sequentially with HCl solution (3 M), ethanol, and distilled water by water bath sonication to remove oxide layers. At first, 0.1 mmol of CoCl2·6H2O and 1 mmol of thiourea were dissolved in 40 mL of distilled water separately. Then, one piece of cleaned NF, Co(II) solution, and thiourea solution were transferred into a 100 mL Teflonlined stainless steel autoclave reactor. The autoclave reactor was locked properly and placed inside a preheated oven at 120 °C for 12 h. After that, the autoclave reactor was cooled down to room temperature and the prepared CoSx/Ni3S2@NF was taken out and

η = a + b log j

(2)

where η is the overpotential, j is the current density, and b is the Tafel slope. η was calculated according to the following equations: For HER, η = 0 − E RHE

(3)

For OER, η = E RHE − 1.229

(4)

The electrochemically active surface area (ECSA) and the roughness factor (RF) are calculated from the electrochemical double-layer capacitance (Cdl) value following the equations: ECSA = 27713

Cdl 40 μF cm−2 per cm 2ECSA

(5)


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ACS Applied Materials & Interfaces RF =

ECSA geometric area of the electrode

formed a highly ordered scaffold with a size in the submicron level. It has been found that such a type of porous structure is advantageous for HER and OER, as it renders easy access to the active surface area to reactive substrates.43 Figure 1b,c shows the micrograph of CoSx/Ni3S2@NF. It is discernible from the micrograph that the cuboid shaped CoSx has vertically protruded from the porous Ni3S2 scaffold. It is likely that spatially well connected Ni3S2 assisted the homogeneous growth of CoSx particles. The thickness of protruded CoSx is in the range of submicron. The firm embedment of cuboidshaped CoSx (Figure 1d) on a porous Ni3S2 base might have facilitated a better charge transfer among them, leading to effective catalytic performance. The FE-SEM image suggested that the growth of the cuboid-shaped CoSx initiated on the porous scaffold of Ni3S2. It was observed that the porous network of Ni3S2 conserved even after the growth of cuboid CoSx particles, which must have played a crucial role in electrolyte diffusion and gas release. FE-SEM images of C0.05NS showed crumpled sheet-like structures with a porous network of Ni3S2@NF (as shown in Figure S1a). In the case of C0.2NS, it was seen that the cuboidshaped CoSx completely covered the porous network of Ni3S2@NF (as shown in Figure S1b). As the exposed network of Ni3S2@NF was almost blocked, the access of ions to Ni sites became difficult. For efficient catalysis, easy movement of ions toward the catalytic sites is pertinent; thus, it well corroborates with the decreased performance of the catalyst toward water splitting. The gray-colored NF turned black when CoSx/Ni3S2@NF was formed after the hydrothermal reaction, whereas Ni3S2@ NF appeared brownish in color (Figure S2). The corresponding elemental mapping for CoSx/Ni3S2@NF sample was carried out, and it is presented in Figure 2. The elemental

(6)

The capacitance of atomically smooth planar surface in 1.0 M KOH electrolyte ranges from 20 to 60 μF cm−2. For calculation, the areanormalized specific capacitance was taken to be 40 μF cm−2.39,40 To obtain Cdl, cyclic voltammetry (CV) scans were performed between a narrow potential window, with different scan rates (ν). Then, Δjν−1/2 were plotted against ν1/2 to obtain the Cdl value. Here, halves of the difference between the anodic and cathodic current densities (ja and jc) are denoted as Δj, that is Δj =

ja − jc 2

(7)

In order to test the durability of the electrode, 500 CV scans at a scan rate of 100 mV s−1 were performed between both 0.00 to −0.35 V and 1.23 to 1.58 V potential regions. Long-term stability in operational condition was verified using chronoamperometric studies applying constant potentials after dipping the fabricated electrode deep inside the electrolyte. Electrochemical impedance spectroscopy (EIS) was conducted in the potentiostatic mode between frequency ranging from 100 kHz to 100 mHz applying a sinusoidal alternating current (ac) potential of 10 mV and dc biases. The obtained Nyquist plots were fitted with the Randles equivalent circuit using ZView software (Scribner Associates Inc., USA). The overall water splitting performances of the materials were tested in the two-electrode system using symmetric electrodes as the cathode and anode in 1.0 M KOH solution.

3. RESULTS AND DISCUSSION 3.1. Physicochemical Analysis. In order to understand how the sulfides of Ni and Co are attached onto the surface of the NF, the FE-SEM image was analyzed. The threedimensional structured porous NF was extensively used as a substrate to fabricate electrocatalysts for HER and OER in the alkaline electrolyte medium.41,42 One of the advantages of using the NF as a catalyst template is that the prospective catalyst can be directly grown and anchored on its surface without the use of a binder for immobilizing the catalyst. The current study is an effort to fabricate an effective catalyst by growing and anchoring sulfides of Ni and Co on the NF surface. In order to have a holistic knowledge of morphology and homogeneity of the prepared heterostructure, first, the FESEM image of Ni3S2 was analyzed and is presented in Figure 1. On the sulfurization of NF, a uniform interconnected network of Ni3S2 was formed (Figure 1a). The vertically grown Ni3S2

Figure 2. Elemental mapping images of CoSx/Ni3S2@NF: (a) Ni (green), (b) Co (white), and (c) S (red) and (d) merged mappings of Ni, Co, and S.

mapping images of Ni, Co, and S are presented as points with green (Figure 2a), white (Figure 2b), and red (Figure 2c) colors, respectively. The point mappings of all the individual elements were merged and presented in a single twodimensional image as shown in Figure 2d. Therefore, the mapping results confirmed that the constituting elements Ni, Co, and S were homogeneously distributed throughout the CoSx/Ni3S2@NF sample.

Figure 1. (a) FE-SEM images of (a) Ni3S2@NF, (b,c) CoSx/Ni3S2@ NF, and (d) cuboid-shaped CoSx formed in CoSx/Ni3S2@NF. 27714


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observed (R1).44 Furthermore, the interplanar distances of (200) and (111) planes of CoS2 were found to be 0.28 and 0.32 nm, respectively, as indicated by R2 and R3 in Figure 3.45 Both CoS and CoS2 were observed to be present in the surface region. The (101) and (110) planes of Ni3S2 were found to be present with interplanar distances of 0.40 and 0.28 nm, respectively (R4).46,47 The angle between these (110) and (101) planes was measured to be ∼60°. Along with the abovementioned lattice fringes, some surplus fringes with different spacing were also observed, which could be due to the formation of a small amount of amorphous NiSx. The SAED pattern image of CoSx/Ni3S2@NF (Figure 3c,d) was found to contain fewer bright spots, which suggested polycrystallinity or poor crystallinity. This might be due to the presence of two kinds of moieties. Moreover, the EDXS spectrum (Figure S3) confirmed the presence of Ni, Co, and S in CoSx/Ni3S2@NF. After scratching off some materials from the electrodes, the PXRD patterns (shown in Figure 4a) had been recorded and investigated. The peaks at 2θ values of 45, 52.4, and 77° appeared because of the presence of metallic nickel (JCPDS no. 65-2865).30 The peaks located at 2θ ≈ 22.5, 31.8, 38.45, 50.53, and 56° could be indexed to the (101), (110), (003), (113), and (122) planes of Ni3S2 (JCPDS no. 44-1418), respectively.46 Thus, the formation of Ni3S2 was confirmed in both the cases. Along with the above-mentioned peaks, some less prominent peaks also appeared in the PXRD spectrum of the cobalt-containing electrocatalyst. The peaks located at 2θ ≈ 30.5, 34.56, 47.79, and 73.62° appeared because of the diffraction from the (100), (101), (102), and (202) planes of the hexagonal CoS phase (JCPDS no. 75-0605), respectively.48 Two other small peaks at 2θ ≈ 32.88 and 40.35° were

To further study the structural and morphological features, the HR-TEM images as depicted in Figure 3a,b were

Figure 3. (a,b) HR-TEM images and (c,d) SAED patterns of CoSx/ Ni3S2@NF.

examined. The distances between the lattice fringes were calculated from the fast Fourier transformation at various regions (indicated as R1, R2, R3, and R4). The presence of CoS(101) planes with a lattice spacing of 0.25 nm were

Figure 4. (a) PXRD patterns of CoSx/Ni3S2@NF, CoSx, and Ni3S2@NF [“β” denotes Ni3S2 peaks (JCPDS no. 44-1418), “γ” denotes CoS peaks (JCPDS no. 75-0605), and “ψ” denotes CoS2 peaks (JCPDS no. 41-1471)] and high-resolution XPS profiles of (b) Ni 2p, (c) Co 2p, and (d) S 2p of CoSx/Ni3S2@NF. 27715


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Figure 5. (a) HER polarization curves of CoSx/Ni3S2@NF, Ni3S2@NF, CoSx@NF, bare NF, and Pt/C@NF in 1.0 M KOH (with iR correction); (b) corresponding Tafel plots; and (c) Nyquist plots of CoSx/Ni3S2@NF, CoSx@NF, Ni3S2@NF, and bare NF at an overpotential of 200 mV.

confirmed good electronic interactions between CoSx and Ni3S2. 3.2. Electrochemical Hydrogen Evolution Activity. The electrocatalytic activity of CoSx/Ni3S2@NF toward HER was studied in 1.0 M KOH solution in a three-electrode setup. For comparison, a similar experiment was carried out for Pt/ C@NF, CoSx@NF, Ni3S2@NF, and bare NF. Figure 5a depicts the obtained polarization curves for CoSx/Ni3S2@NF, Ni3S2@NF, CoSx@NF, bare NF, and Pt/C@NF. As expected, Pt/C@NF showed a superior catalytic activity toward HER, which exhibited a benchmarking current density of 10 mA cm−2 at an overpotential of 139 mV. Nevertheless, a slight decrease in the catalytic activity of Pt/C was observed, which might be due to the low Pt percentage and use of a polymeric binder (see Figures S5 and S6). CoSx/Ni3S2@NF achieved a current density of 10 mA cm−2 at an overpotential (η10) of 204 mV, which is better than or comparable with some of the recently reported non-noble metal-based electrocatalysts (as shown in Table S2). CoSx@NF and Ni3S2@NF achieved a current density of 10 mA cm−2 at overpotentials of 240 and 269 mV, respectively, which indicated that the catalytic performance augmented after impregnation of CoSx in Ni3S2 scaffold. Bare NF exhibited the least activity toward HER, as it achieved 10 mA cm−2 at an overpotential of 288 mV. Ni3S2@ NF achieved j = 100 mA cm−2 at an overpotential of 449 mV, whereas CoSx/Ni3S2@NF achieved j = 100 and 200 mA cm−2 at overpotentials of 340 and 383 mV, respectively. CoSx@NF achieved j = 100 and 200 mA cm−2 at overpotentials of 396 and 472 mV, respectively. The catalytic activity of CoSx/ Ni3S2@NF surpassed the activity of Pt/C@NF beyond an overpotential of 317 mV. The Tafel slope for CoSx/Ni3S2@NF derived from the Tafel plot (Figure 5b) was found to be of 133.32 mV dec−1, which is less than that of Ni3S2@NF (slope = 150.18 mV dec−1) and CoSx@NF (136.40 mV dec−1). All these observations established the fact that CoSx/Ni3S2@NF

identified as Bragg diffraction peaks of (200) and (211) planes of CoS 2 (JCPDS no. 41-1471), respectively. 44 These observations confirmed the formation of amorphous CoSx on Ni3S2. When the PXRD pattern was recorded in the case of prepared CoSx, the peaks related to (111), (200), and (211) Bragg planes of CoS2 were observed.45 Along with them, a peak with lesser intensity was observed, which was indexed to the (101) plane of CoS.48 XPS was used to characterize the valance state of the constituting elements in CoSx/Ni3S2@NF. The deconvoluted spectra of Ni 2p showed two strong peaks at 856.32 and 873.88 eV, which were indexed to the Ni 2p3/2 and Ni 2p1/2 peaks, respectively (Figure 4b). These peaks emerged because of the presence of Ni2+ and Ni3+ in Ni3S2.30 Along with these, two satellite peaks (indicated as “Sat.”) also appeared at 861.59 and 879.91 eV. The 853 eV peak was identified to be the characteristic peak of Ni3S2.46,49 For comparison, the Ni 2p spectra of Ni3S2@NF (see Figure S4a) were investigated, and it was observed that all the peaks in the Ni 2p spectra of CoSx/ Ni3S2@NF were blue shifted, which indicated charge redistribution between the interfaces of CoSx and Ni3S2.11,29 In the deconvoluted XPS spectra of Co 2p, two strong peaks were observed at 781.79 and 797.44 eV, which could be attributed to the Co 2p3/2 and Co 2p1/2 peaks (Figure 4c). In addition, two satellite peaks (indicated as “Sat.”) were also observed at 803.45 and 786.07 eV. Both Ni and Co XPS spectra confirmed the presence of multiple oxidation states. The peak at 168.2 eV in the S 2p spectrum (Figure 4d) could be due to the presence of S atoms being at a higher oxidation state.11 Oxidation was unavoidable in the case of hydrothermal reaction; thus, the prepared electrocatalyst could have been dominated by sulfite (SO32−).50 This peak was also blueshifted in comparison to the peak observed in the S 2p spectra of Ni3S2@NF (see Figure S4b). All the above observations 27716


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Figure 6. (a) HER polarization curves for CoSx/Ni3S2@NF before and after 500 CV cycles at 100 mV s−1 scan rate and (b) chronoamperometric curve for CoSx/Ni3S2@NF recorded at an overpotential of 300 mV for ∼10 h and 350 mV for another ∼10 h (no iR correction).

Figure 7. (a) OER polarization curves of CoSx/Ni3S2@NF, Ni3S2@NF, CoSx@NF, bare NF, and RuO2@NF in 1.0 M KOH (with iR correction), (b) corresponding Tafel plots, (c) Nyquist plots of CoSx/Ni3S2@NF, Ni3S2@NF, CoSx@NF, and bare NF at an overpotential of 300 mV, and (d) Δjν−1/2 vs ν1/2 plots of CoSx/Ni3S2@NF, Ni3S2@NF, CoSx@NF, and bare NF.

equivalent to an overpotential of 200 mV for HER, and the corresponding Nyquist plot is shown in Figure 5c. The presence of a capacitive semicircle in the Nyquist plots confirmed that the electrocatalysis process is dynamically controlled and can be characterized by a single time constant. This observation was in close agreement with the fact that the HER process is controlled by a charge-transfer step.52 The charge-transfer resistance (RCT) of CoSx/Ni3S2@NF was found to be ∼29.2 Ω, whereas ∼98.59 and ∼53.81 Ω RCT were recorded for Ni3S2@NF and CoSx@NF, respectively (Figure S7). It can be anticipated that because of nanointerface formation, the charge transfer between Co and Ni was facilitated via an S-bridge, which in turn facilitated the charge transfer between the electrocatalyst and the electrolyte. This finding was in accordance with the inference obtained from the j0 value calculation. The HER catalysis activity was studied for C0.05NS and C0.2NS as well and was compared with CoSx/Ni3S2@NF, CoSx@NF, and Ni3S2@NF (see Figure S8 for details).

performed better as an electrocatalyst for HER than Ni3S2@ NF and CoSx@NF. It is well-known that in alkaline medium, HER proceeds through water dissociation and adsorption of H (Volmer step) and then desorption of Hads either chemically (Tafel) or electrochemically (Heyrovsky).51 From the Tafel slope, it is possible to predict the rate-determining step. In the present study, the Volmer step was predicted to be the ratedetermining step, that is H 2O + e− → Hads + OH−

(8)

The exchange current density (j0) values of the materials were also obtained from the Tafel plot. The j0 values of CoSx/ Ni3S2@NF, CoSx@NF, and Ni3S2@NF were found to be 0.287, 0.177, and 0.151 mA cm−2, respectively. The obtained value suggested that in an equilibrium condition, the intrinsic charge-transfer efficiency of CoSx/Ni3S2@NF is better than that of both CoSx@NF and Ni3S2@NF electrodes. For further insights into the HER kinetics, EIS studies of the electrocatalysts had been performed by applying a dc potential 27717


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better than both Ni3S2@NF and CoSx@NF, as an OER electrocatalyst. For a better understanding of the OER catalysis, the Nyquist plots were obtained (Figure 7c) from the EIS study, which was performed by applying a bias equivalent to an overpotential of 300 mV. The RCT for OER in the case of CoSx/Ni3S2@NF, Ni3S2@NF, and CoSx@NF was found to be 19.3, 35.2, and 82 Ω, respectively (Figure S9). Thus, on the basis of electrochemical performances, it can be concluded that CoSx/Ni3S2@NF showed better OER catalyzing activity than Ni3S2@NF and CoSx@NF. Furthermore, from the Nyquist plot, “true” capacitance values were calculated for CoSx/Ni3S2@NF and Ni3S2@NF (see Table S5). The capacitance value for CoSx/Ni3S2@NF was found to be ∼2.78 times to that of Ni3S2@NF. In the case of CoSx@NF, the calculated capacitance value was too low compared to CoSx/Ni3S2@NF. Quantification of active site strategy is very useful to comprehend the factors responsible for improved catalytic performance. For all Ni-based OER catalysts, it is believed that the Ni(III) centers formed after oxidation of Ni(II) are the potential active sites.56,57 Therefore, the amount of charge (Q) involved in the oxidation of Ni(II) can be estimated by integrating the oxidation peak in the oxidation wave and dividing by scan rate (ν)

The durability of CoSx/Ni3S2@NF as the HER electrocatalyst was tested by performing 500 CV cycles with a scan rate of 100 mV s−1 between 0 and −0.35 V (vs RHE). After the 500th cycle, an LSV scan was performed, and the obtained polarization curve was compared with the initial polarization curve (Figure 6a). A negligible change in current density was observed. To further confirm the durability, chronoamperometry was performed at an overpotential of 300 mV for ∼10 h and then again at an overpotential of 350 mV for another ∼10 h. Interestingly, it was found that the catalyst retained ∼92.8% current density (no iR correction) even after ∼20 h long performance (Figure 6b). Both studies confirmed the longterm performance ability of the prepared material as the HER electrocatalyst. 3.3. Electrochemical Oxygen Evolution Activity. The catalytic activity toward OER was studied with three-electrode setup by performing an LSV scan for RuO2@NF, CoSx/ Ni3S2@NF, Ni3S2@NF, CoSx@NF, and bare NF in 1.0 M KOH solution. The polarization curves (Figure 7a) of both CoSx/Ni3S2@NF and Ni3S2@NF exhibited an oxidation peak at ∼1.34 V (vs RHE), which is due to the oxidation of Ni(II) to form Ni(III).28,53 In order to avoid the influence of oxidation peak toward the current density (j), an overpotential (η) required to achieve a j value of 20 mA cm−2 was considered. It was observed that for CoSx/Ni3S2@NF, an overpotential of 280 mV was required to achieve the j value of 20 mA cm−2 (η20). The efficiency and viability of the prepared catalyst toward OER were compared with the prior published studies, and it was seen that its activity is comparable with or even better than some of the recently reported electrocatalysts in the alkaline medium (as shown in Table S4). When the same test was performed with Ni3S2@NF and CoSx@NF, the required overpotentials to achieve a current density of 20 mA cm−2 were 347 and 353, respectively. This suggested that the impregnation of CoSx is advantageous for OER catalysis too. For comparison, the catalytic activity of RuO2@NF was investigated, which showed a superior activity till 387 mV overpotential, but beyond that potential, its performance was inferior to CoSx/Ni3S2@NF. The overpotential required to achieve a current density of 20 mA cm−2 for both RuO2@NF and CoSx/Ni3S2@NF was almost the same, but RuO2@NF achieved a current density of 50 mA cm−2 at a lower overpotential of 14 mV than that of CoSx/Ni3S2@NF. For CoSx/Ni3S2@NF, the η100 and η200 values were 373 and 414 mV, respectively, whereas, for Ni3S2@NF, the η100 value was 448 mV. For CoSx@NF, the η100 and η200 values were 440 and 509 mV, respectively. Bare NF required an overpotential of 448 mV to achieve j = 20 mA cm−2, indicating its poor activity toward OER. RuO2@NF achieved current densities of 100 and 200 mA cm−2 at overpotentials of 371 and 432 mV, respectively. To have more understanding on the OER kinetics in the case of the prepared electrocatalyst, a Tafel plot (Figure 7b) was obtained and examined. Lowest and highest Tafel values were recorded for RuO2@NF (92.7 mV dec−1) and bare NF (166.48 mV dec−1), respectively. The calculated Tafel slope values for CoSx/Ni3S2@NF, Ni3S2@NF, and CoSx@NF were 105.4, 113.4, and 133 mV dec−1, respectively. From the Tafel slope values, the predicted rate-determining step for OER was the first electron transfer step as indicated below: M + OH− → MOH + e−

Q=

1 ν

∫E

E2

i(E) dE

1

(10)

As one electron is involved in this oxidation process, the number of Ni(III) centers can be estimated from Q, which in turn gives the number of active sites, if assumed that all Ni(III) centers are catalytically active. Therefore, Number of active sites =

Q 1.60217662 × 10−19 coulombs (11)

The number of catalytically active sites for CoSx/Ni3S2@NF and Ni3S2@NF was found to be 1.993 × 1018 and 0.683 × 1018 atoms cm−2, respectively (Figure S10). The number of active sites became ∼2.92 times greater after incorporation of CoSx and as a consequence showed a better OER catalysis. This inference showed close agreement with the previous findings obtained from the Nyquist plot. In the case of CoSx@NF, the Ni(II) oxidation peak was absent as the Ni site was not an OER catalysis active site. Thus, active site quantification as discussed above could not be done in the case of CoSx@NF. To further confirm the number of active sites, another measuring approach was also used. The ECSA and RF values were calculated from the Cdl. It is a common practice to plot Δj against ν for obtaining the Cdl value. However, because of the presence of Faradaic influence in current density, linear variation deviates to some extent.58 As the Faradaic and capacitive currents vary linearly with ν1/2 and ν, respectively, ́ et al. proposed a procedure to deconvolute the Faradaic Diaz component.59,60 From the following equation, it can be understood that plotting Δjν−1/2 against ν1/2 could furnish the absolute Cdl value Δjν−1/2 = K Cν1/2 + KF

(12)

where KC and KF are double-layer charging component and Faradaic component, respectively. For each catalyst, the CV scans with different scan rates (ν) of 20, 50, 80, 100, 120, 150, and 180 mV s−1 were performed between the potential range

(9)

where the catalytically active site is abbreviated as M.54,55 A smaller Tafel slope value signified that CoSx/Ni3S2@NF acted 27718


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Figure 8. (a) OER polarization curves for CoSx/Ni3S2@NF before and after 500 CV cycles at a scan rate of 100 mV s−1 and (b) chronoamperometric curve for CoSx/Ni3S2@NF recorded at an overpotential of 350 mV for ∼10 h and 370 mV for another ∼10 h (no iR correction).

Figure 9. (a) Polarization curves of CoSx/Ni3S2@NF||CoSx/Ni3S2@NF, Ni3S2@NF||Ni3S2@NF, CoSx@NF||CoSx@NF, bare NF||bare NF, and RuO2@NF||Pt/C@NF in 1.0 M KOH (without iR correction); (b) chronoamperometric curve for CoSx/Ni3S2@NF||CoSx/Ni3S2@NF recorded at 1.9 V potential (no iR correction); and (c) polarization curves of CoSx/Ni3S2@NF||CoSx/Ni3S2@NF in 1.0 M KOH solution before and after ∼30 h long operation.

The durability of CoSx/Ni3S2@NF under the OER catalyzing condition was tested by performing 500 CV cycles from 1.23 to 1.58 V at a scan rate of 100 mV s−1. Only a small change in the current density and shifting of oxidation potential toward a lower value by ∼10 mV were observed when the initial and final polarization curves were compared and matched (Figure 8a). After performing the chronoamperometric test at an overpotential of 350 mV for ∼10 h and again at 370 mV for the next ∼10 h, the catalyst found to retain ∼90.8% current density (no iR correction, Figure 8b). The FESEM images of CoSx/Ni3S2@NF were recorded after OER catalysis operation (see Figure S13), and it was found that the morphology of the material was retained with very minute changes. The EDXS analysis confirmed the presence of Ni, Co, and S in the CoSx/Ni3S2@NF sample after the OER study.

of 1.02−1.17 V (vs RHE) (see Figure S11 for cyclic voltammograms). In each case, the Δj values at the potential of 1.07 V for different ν had been calculated, and Δjν−1/2 was plotted against ν1/2 (Figure 7d). From the slope, the derived Cdl value for CoSx/Ni3S2@NF was 8.68 mF cm−2, which was found to be ∼2.35 times higher than that of Ni3S2@NF (3.69 mF cm−2). In the case of CoSx@NF, the Cdl value was found to be 6.12 mF cm−2. From the Cdl values, the ECSA and RF were calculated for each material (Table S6). The results achieved from both the approaches indicated greater active sites in CoSx/Ni3S2@NF than in Ni3S2@NF and CoSx@NF, suggesting its superior electrocatalytic activity toward OER. The polarization curves of OER were recorded for C0.05NS and C0.2NS and compared with CoSx/Ni3S2@NF, CoSx@NF, and Ni3S2@NF (see Figure S12 for details). 27719


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ACS Applied Materials & Interfaces The above studies proved that CoSx/Ni3S2@NF could be operated as the OER electrocatalyst for a long period. 3.4. Overall Water Splitting Activity. As the prepared catalyst exhibited good electrocatalytic activity toward both HER and OER, a two-electrode system was constructed using CoSx/Ni3S2@NF as both anode and cathode to explore its overall water splitting competency. The constructed twoelectrode electrolyzer required a potential of 1.572 and 1.863 V to acquire current densities of 10 and 50 mA cm−2, respectively (Figure 9a). The obtained values suggested that the bifunctional electrocatalyst CoSx/Ni3S2@NF showed comparable water splitting potentiality with respect to the recently reported electrocatalysts, as summarized in Table S7. For comparison, a two-electrode system consisting of Pt/C@NF and RuO2@NF was fabricated, which required 1.573 V to achieve a current density of 10 mA cm−2. Remarkably, the reference electrolyzer exhibited water splitting activity almost comparable to that of CoSx/Ni3S2@NF||CoSx/Ni3S2@NF until 1.789 V potential. When the similar study was carried out for Ni3S2@NF, CoSx@ NF, and bare NF, it was found that the potentials required to achieve a current density of 10 mA cm−2 were 1.798, 1.778, and 1.904 V, respectively. Long-term operational durability was investigated by performing chronoamperometry at 1.9 V for ∼30 h using CoSx/Ni3S2@NF as both cathode and anode in 1.0 M KOH solution. As depicted in Figure 9b, the electrocatalyst retained ∼86.60% of its current density after ∼30 h long operation. Further, to confirm the stability, LSV was performed, and the obtained curve was compared with the initially obtained polarization curve (Figure 9c). The final polarization curve was slightly changed with a visible oxidation peak located at ∼1.4 V. The oxidation peak might appear because of the oxidation of Ni sites after the longtime operation. The final polarization curve showed that the electrocatalyst required 144 mV more overpotential than it initially required to achieve a current density of 10 mA cm−2. These observations confirmed the long-term operational efficiency of the electrocatalyst for overall water splitting.

Ni3S2@NF a promising earth-abundant noble-metal-free bifunctional electrocatalyst for overall water splitting application in the alkaline medium.

4. CONCLUSIONS In summary, a one-pot hydrothermal synthetic procedure to fabricate a binder-free CoSx/Ni3S2@NF electrode was presented. Herein, the Ni foam not only acted as the conducting material but also acted as the nickel source. The porous structure of CoSx/Ni3S2@NF helped in the better distribution of active sites and adsorptions of H- and Ocontaining intermediates at the active sites. The prepared electrocatalyst showed an excellent activity because of the (a) elimination of the binder on electrode fabrication, (b) better charge-transfer efficiency because of Co−S−Ni moiety formation, (c) increment in the number of active sites, and (d) decrease in H- and O-containing species adsorption-free energy at the active sites. In 1.0 M KOH solution, the prepared catalyst required an overpotential of 204 mV to achieve a current density of 10 mA cm−2 in the case of HER, and in the case of OER, it required an overpotential of 280 mV to achieve a current density of 20 mA cm−2. Furthermore, 1.572 and 1.684 V were required to gain current densities of 10 and 50 mA cm−2 in the case of overall water splitting operation, which is comparable to the activity of some recently reported electrocatalysts. Along with efficiency, the catalyst showed good durability in operating conditions. The good catalytic activity, stability, and simple fabrication procedure make CoSx/

Author Contributions

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b04223. Digital photographs of bare NF, Ni3S2@NF, and CoSx/ Ni3S2@NF; FE-SEM images of C0.05NS and C0.2NS; EDXS spectrum of CoSx/Ni3S2@NF obtained from HRTEM; high-resolution XPS profiles of Ni 2p and S 2p of Ni3S2@NF; polarization curves and corresponding Tafel plots for Pt/C@NF and Pt wire; polarization curves and corresponding Tafel plots for Pt/C@NF with and without using a binder; comparison of recently reported HER catalytic activity of Pt/C in 1.0 M KOH solution; comparison of recently reported HER, OER, and bifunctional electrocatalysts; Nyquist plots for bare NF, Ni3S2@NF, CoSx@NF, and CoSx/Ni3S2@NF in the case of HER and OER along with circuit modules used to fit the curves; HER and OER polarization curves for C0.05NS and C0.2NS, cyclic voltammograms for Ni3S2@NF and CoSx/Ni3S2@NF along with integrated oxidation peaks; cyclic voltammograms at different scan rates for bare NF, Ni3S2@NF, and CoSx/Ni3S2@NF; FE-SEM images of CoSx/Ni3S2@NF after OER catalysis performance; and XPS survey spectra of CoSx/Ni3S2@ NF (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-9647205077. Fax: 91343-2548204. ORCID

Tapas Kuila: 0000-0003-0976-3285 This manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are thankful to the Director of CSIR-CMERI. The authors also acknowledge the Department of Science and Technology, New Delhi, India, for the financial support vide project numberGAP215312.

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REFERENCES

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nickel sulfide heterostructure directly grown on nickel

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