Electrocatalytic Water Splitting through the Ni x S y Self-Grown Superstructures Obtained via a Wet Chemical Sulfurization Process.

We report water-splitting application of chemically stable self-grown nickel sulfide (Ni x S y ) electrocatalysts of different nanostructures including rods, flakes, buds, petals, etc., synthesized by a hydrothermal method on a three-dimensional Ni foam (NiF) in the presence of different sulfur-ion precursors, e.g., thioacetamide, sodium thiosulfate, thiourea, and sodium sulfide. The S2- ions are produced after decomposition from respective sulfur precursors, which, in general, react with oxidized Ni2+ ions from the NiF at optimized temperatures and pressures, forming the Ni x S y superstructures. These Ni x S y electrocatalysts are initially screened for their structure, morphology, phase purity, porosity, and binding energy by means of various sophisticated instrumentation technologies. The as-obtained Ni x S y electrocatalyst from sodium thiosulfate endows an overpotential of 200 mV. The oxygen evolution overpotential results of Ni x S y electrocatalysts are comparable or superior to those reported previously for other self-grown Ni x S y superstructure morphologies.

Introduction

The oxygen evolution reaction (OER) for water splitting is a top agenda in electrocatalytic energy storage applications.[1−7] From the cost and availability points of view, ruthenium oxide and platinum electrode materials are envisaged rarely for OER applications in the past; therefore, the research activities are being focused on developing OER electrode materials, such as Co3S4, NiS, MnS, Bi2S3, CuS, etc.[8−15] Among them, nickel sulfide (NiS), with various phases α-NiS, β-NiS, NiS2, Ni3S2, Ni3S4, Ni7S6, Ni9S8, etc., has been used, in the past, for electrocatalytic applications.[16−21] For the case of water splitting, an electrocatalyzer assembly consisting of a cathode (platinum–carbon) for the hydrogen evolution reaction with an anode (ruthenium/iridium oxide) for the OER with 1.23 V overpotential as a benchmarking value is used for fixing electrode materials of choice.[7] To date, very few reports highlight the use of self-grown NiS superstructure electrocatalysts in OER studies. For example, Zhang et al. obtained an overpotential of 317 mV for a Ni3S2 electrocatalyst fabricated by a hydrothermal method.[22] Ghim et al. reported an overpotential of 340 mV for Ni3S2 at 20 mA cm–2.[23] Chaudhari et al. noted moderate catalytic activity and stability of a hydrothermally grown Ni3S2 electrocatalyst with an overpotential of ∼310 mV.[24] Wang et al. claimed an overpotential of 335 mV for Ni3S2 at 50 mA cm–2 in 1 M KOH.[25] Most of the published articles include a common feature, i.e., the use of only one type of Ni3S2 morphology, at one time, that includes nanorods, nanoflakes, nanowires, nanosheets, etc. In this study, for self-grown NiS superstructures with different morphologies for electrocatalytic energy storage applications, we report a very convenient and scalable self-grown chemical synthesis approach to obtain the NiS superstructure electrodes of different morphologies. After structure, morphology, phase purity, and binding energy measurements, they are envisaged in water-splitting applications where a low OER overpotential and a long-term chemical stability are evidenced.

Experimental Section

Experimental Procedure

The experimental procedure has been reported in detail in our previous work.[21] In brief, NiF in the presence of different sulfur source precursors, i.e., sodium thiosulfate (STS), thioacetamide (TAA), thiourea (TU), sodium sulfide (SS) in 3, 0.45, 6, and 3 g, respectively, was added sequentially in 50 mL of deionized water as a solvent. The above-prepared sulfurized solutions were poured in a 50 mL capacity stainless steel autoclave and heated at 120 °C for 4 h; during the heating treatment, each sulfur precursor dissociated to sulfur ions (S2–) (see the detailed chemical reaction in the Supporting Information S1 (SI), which successively could react with nickel ions from NiF, resulting in the formation of the NiS superstructure (Figure ). As-prepared self-grown electrocatalysts were labeled as (a) NiF (for comparison), (b) NiS–STS, (c) NiS–TAA, (d) NiS–TU, and (e) NiS–SS. All electrocatalyst electrodes were characterized for their structure, surface morphology, and binding energy by means of different characterization tools, as reported previously.[21]

Figure 1

Schematic presentation of in situ, hydrothermally grown NiS superstructures with an actual hydrothermal unit.

OER Confirmation

The OER measurements for the NiS electrocatalysts were carried out using a three-electrode system (in addition to the use of NiF as a reference) in the presence of a Hg/HgO reference electrode and a platinum counter electrode. Before undertaking experiments, a nitrogen purge was operated for 10 min to normalize the system with respect to external as well as internal oxygen.

Formulas

Formulas used for estimating the three-electrode electrocalatalytic energy parameters are provided in the SI S2.

Results and Discussion

The surface morphologies of the pristine NiF and self-grown NiS superstructure electrodes are displayed in field emission scanning electron microscopy (FE-SEM) images shown in Figure a–e. The surface of NiF obviously changed after hydrothermal sulfurization. We selected a single-branch NiF and self-grown NiS for analysis (Figure a,a1)) under low and high magnifications. As can be seen, the FE-SEM image of pristine NiF reflected an uneven, continuous, and smooth surface of Ni oval-shaped grain boundaries and NiS superstructure electrodes of different morphologies, suggesting a role of precursor solution in the growth process. Figure b,b1 confirms the surface of the NiS–STS electrode as volcano-type nanorods of 800 (±100) nm heights. The diameter of these nanorods, separated from one another with open air voids of 200 (±50) nm, dramatically reduced from the top to bottom in the range of 60 (±20) nm. The NiS–TAA electrode surfaces (Figure c,c1), consisting of well-grown and uniformly distributed nanoflakes with 5 (±1) nm widths and 100 (±30) nm sized pores, are interlocked into one another. The morphology of NiS–TU (Figure d,d1) presents a budlike structure with a diameter of 900 (±300) nm and separation spacing of 1200 (±500) nm (Figure e,e1).1

Figure 2

FE-SEM images of NiF (a, a1) and self-grown NiS [NiS–STS (b, b1), NiS–TAA (c, c1), NiS–TU (d, d1), and NiS–SS (e, e1)] at different magnifications.

In the end, the NiS–SS electrode shows less number of 3 (±2) nm width nanopetals. In a nutshell, after sulfurizing, NiF with different sulfur precursors can be evaluated in different NiS morphologies, which play an important role in easy electrolyte ion transformation into an interior part of the electrode material when employed in electrochemical energy storage devices.

Furthermore, the presence of the Ni and S elements on the NiF, NiS–STS, NiS–TAA, NiS–TU, and NiS–SS surfaces is confirmed by energy dispersive X-ray (EDX) elemental mapping analysis (Figure S1). From the results obtained, it is confirmed that Ni and S followed a uniform distribution over the NiF surface with an expected atomic percentage; Ni varies from 100 to 43%, whereas S increases from 0 to 40%, suggesting the successful incorporation of S2– into NiF as NiS. With different methods, trial-and-error experiments have been carried out to optimize the condition of the sulfur source, by unoptimizing sulfur precursor weights, with failure results in terms of not-well-sulfurized FE-SEM images shown in Figure S2. This result confirms that the sulfur precursor solution source was allowing NiF to withstand without any breakage during an optimization of the sulfurization process.

The phases present in NiS were confirmed by X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET), transmission electron microscopy (TEM), and selected area electron diffraction (SEAD) measurements, as given in Figure (A, B, C–E, F–I, J–K, and L, respectively). NiS was polycrystalline in nature as the XRD patterns showed reflection peaks with moderate intensities (Figure A). The two strong intensity peaks (marked as “Δ”) at 44.4 and 51.7° were due to (111) and (200) reflections, respectively, of NiF (JCPDS no. 04-0850).[28] Four new peaks reflected at 18.71° (110), 32.34° (330), 40.74° (021), and 52.01° (401) were attributed to NiS (denoted “”); three peaks at 31.29° (200), 35.93° (210), and 59.85° (321) were attributed to NiS2 (denoted “⧫”); and rest five peaks at 21.29° (101), 30.68° (110), 49.04° (113), 50.43° (210), and 55.33° (112) were attributed to Ni3S2 (denoted “*”). All reflected peaks evidenced the presence of three different phases in NiF after sulfurization.[17−21] Raman modes were generated in the NiS superstructure. The reflected Raman peaks were at 142, 246, 298, and 373 cm–1 for NiS; 346, 462, 487, and 557 cm–1 for NiS2; and 185, 280, and 633 cm–1 for Ni3S2.[21,26−38] A single Raman scattering peak was reflected in NiF due to its metallic character (Figure B,a). The presence of numerous peaks in the broad range of 900–1200 cm–1 indicated the existence of sulfate (S2–) ions. An XPS survey scan was carried out to identify the surface valance states; the Ni 2p and S 2p spectra for the NiS–STS electrode are shown in Figure C–E (the XPS spectra for the other electrodes are given in Figure S3). Occurrence of Ni and S elements was clarified from the survey spectrum. The binding energy positions for Ni 2p3/2 and 2p1/2 were at 855 and 874 eV (shown in Figure D). A pair of satellite peaks at 860 and 879 eV was also recognized. The peaks at 162.3 and 169 eV in the high-resolution (HR) XPS spectrum for S 2p (Figure E) were ascribed to 2p1/2 and SO42–, which indicate the presence of the S–S band in all of the NiS electrodes (see Figure S3 for more details for other electrodes). The nitrogen adsorption–desorption isotherm and the pore size distribution (inset) graphs for the NiS–STS electrode are shown in Figure F. The obtained specific surface area and pore size distribution values were 62 m2 g–1 and 7.4 nm, respectively. The adsorption isotherm in the range of 0.4–0.9 with a slope value ca. 0.4 was allotted to capillary condensation, which is a typical feature of mesoporous materials.[19,21] The obtained surface area and pore size distribution values for other NiS electrodes were smaller (38–47 m2 g–1) and are given in Figure G–I. The HR-TEM images (Figure J) of the optimized NiS–STS electrode, i.e., the formation of volcano-type nanorods, which is well consistent with the FE-SEM images shown in Figure b,b1. The HR-TEM scan, as shown in Figure K, revealed interplanar lattice fringe separation distances of 0.29 and 0.28 nm for the (111) and (200) planes of NiS2 and Ni3S2, respectively. The irregular compact lattice fringes (dotted circle shown in Figure K) could be due to an amorphous NiS, suggesting the existence of NiS2, Ni3S2, and NiS separately in NiS. Interestingly, the SAED image recorded for the NiS–STS electrode in Figure L showed bright and circular concentric rings, confirming that it is nanocrystalline in nature.

Figure 3

(A, B) XRD and Raman spectra for NiF (a), NiS–STS (b), NiS–TAA (c), NiS–TU (d), and NiS–SS (e) electrodes. (C–E) XPS, Ni 2p, and S 2p spectra of NiS–STS. The BET adsorption isotherm (with inset showing pore size distribution) of NiS–STS (F) and rest of the electrodes (G–I). (J) TEM, (K) HR-TEM, and (L) SEAD pattern of the NiS–STS electrode.

The NiF and in situ grown NiS electrocatalysts in comparison with RuO2 were envisaged for OER water catalysis activity, as shown in Figure A, wherein NiF/RuO2 showed a very low/high OER activity, whereas NiS–STS after sulfurization showed enhanced electrocatalytic activity at a lower potential of 200 mV relative to the other NiS and RuO2 electrodes (210, 220, 230, and 138 mV), indicating an improvement in electrocatalytic activity for OER after sulfurization. An oxidation peak noted could be due to the surface reaction of Ni2+ with active Ni3+ species in NiS.[41] The inset in Figure A shows an actual photograph of electrocatalytic OER testing, with marked evolution of oxygen bubbles, which strongly supported that NiS is used for OER activity.

Figure 4

(A, B) Polarization and Tafel plots for (a) NiF, (b) NiS–STS, (c) NiS–TAA, (d) NiS–TU, and (e) NiS–SS electrocatalysts for the OER, measured at a scan rate of 10 mV s–1 (the inset shows an actual photograph of the three-electrode configuration system used for OER study). (C) Summary of reported overpotential vs Tafel slope values in relation to the NiS–STS electrocatalyst. (D) Schematic representation of the possible reasons to obtain an enhanced electrocatalytic performance of the electrocatalyst for water splitting.

Figure B shows the Tafel plots for the corresponding polarization curves, whose slopes provide information that is favorable for OER activity on the surfaces of the NiS electrocatalysts.[39−41] With an enhanced OER rate, the slope of the Tafel plots is reduced from 165 to 138 mV dec–1 for the NiS electrocatalyst (see detailed information about formulas used for calculation in the Supporting Information S2 (SI)). The different NiS phases, three-dimensional (3D) structures, and high surface areas with a minimum series resistance increased the water splitting rate for better OER activity. The numerical values obtained for the overpotential and Tafel slope of the NiS–STS electrode were in close agreement to those reported earlier[22−25,42−45] (Figure C). Figure S4A shows the Nyquist plots obtained for the self-grown and NiF electrodes. The value for the charge-transfer resistance (in Ω) obtained for NiS–STS was ≈0.40 (±0.10), lower than that for NiS–TAA (≈0.90 (±0.10)), NiS–TU (≈1.40 (±0.20)), NiS–SS (2.10 (±0.10)), and NiF (≈3.15 (±0.10)) electrodes. The series resistance values for the NiF and NiS electrodes were 3.5, 0.5, 1.4, 1.9, and 1.5 Ω, respectively, meaning a smaller series resistance for the NiS–STS electrode compared to that for other electrodes. The cyclability of the NiS–STS electrocatalyst for OER was further examined by successive CV plots for 1000 cycles (Figure S4B and its inset show polarization curves after 1st and 1000th cycles). A small reduction in the OER activity was corroborated from the NiS–STS CV curves after 1000th polarization curves. Considering the excellent OER catalytic ability for the 1000 polarization curves, the FE-SEM images of the NiS–STS electrode (Figure S4C,D) showed a change in morphology from volcano-type nanorods to aggregated nanoflakes with more crystallinity, one of the common practices in electrochemical measurements. The above result and discussion indicates that NiS–STS acts as an excellent catalyst for OER and there are possible reasons (see Figure D) that after sulfurization of NiF its surface area increases with more active sites, wherein OH– intercalation and electron transportation at lower potentials become much easier. The XRD and XPS analyses after the stability study are also shown in Figure S5a,b–d. The XRD pattern of the NiS–STS electrode shows a strong peak of Ni metal with a suppressed NiS peak position due to the formation of an amorphous phase, where, as shown in the XPS spectrum, peak positions of Ni and S shifted toward a lower wavelength side with decreased peak intensity; both elements were decreasing the presence in the NiS–STS electrode except the O peak.

Conclusions

Self-grown 3D NiS superstructures comprising NiS, NiS2, and Ni3S2 crystal structures with different morphologies and high surface areas have been successfully prepared using a hydrothermal method. The structural, morphological, and electrochemical properties of NiS confirm an excellent catalysis performance as compared to that of the pristine NiF. On a similar line, efforts for self-growing NiSe and NiTe together with MoS, Fe (iron), V (vanadate), and their hybrid superstructures with varying morphologies for better and sustainable electrochemical and electrocatalytic energy storage performance are underway.[46−49]