Tailoring supercapacitance and water splitting performance of nickel sulfides (NiS and Ni₃S₂): a comparative study of colloidal and solventless synthesis

Abstract

To overcome the potential issue of active site blockage by surfactants in colloidal synthesis, alternative synthetic approaches must be explored. In this study, we investigated both solvent-free and colloidal thermolysis routes to synthesize nickel sulfides (NiS and Ni₃S₂) using sulfur-based Ni complexes, [Ni(S₂CO(C₂H₅))₂] (Ni-Xan) and [Ni(S₂CN(C₂H₅)₂)₂] (Ni-DTC) as precursors. The solvent-free decomposition of these complexes produced ligand-free NiS and Ni₃S₂ in the absence or presence of triphenylphosphine (TPP), respectively. In contrast, colloidal thermolysis in oleylamine (OLA) led to phase-selective nickel sulfide formation (NiS and Ni₃S₂), with TPP facilitating desulfurization. The electrochemical performance of the synthesized materials was evaluated in water splitting and supercapacitance applications. Among the tested materials, NiS synthesized from Ni-Xan in OLA exhibited the highest specific capacitance (809.2 F g⁻¹ at 1 A g⁻¹) and energy density (34.9 Wh kg⁻¹), while NiS derived from Ni-DTC in OLA achieved the highest power density (281.7 Wh kg⁻¹). Additionally, the Ni₃S₂ electrode obtained via the colloidal route demonstrated superior HER performance, requiring only 197 mV (Tafel slope: 159 mV dec⁻¹) to reach a current density of 10 mA cm⁻². These findings underscore that simply eliminating surfactants and adopting a solvent-free method is not inherently sufficient to achieve high electrochemical performance. This study provides insights into the limitations of solvent-free synthesis and outlines potential prerequisites that may guide future optimization for improved electrochemical performance.

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Introduction

Energy conversion and storage systems, including water splitting and supercapacitors, are central to the global transition away from fossil fuels. Though fossil fuels have played a crucial role in the industrial development of the planet, their continued usage is associated with environmental challenges such as climate change and pollution.^1,2 Moreover, carbon-based energy reserves have continually been depleting with use, posing threats to our energy security.³ Hydrogen has been identified as an ideal substitute for traditional fossil energy sources due to its zero carbon emissions and high energy density.^4–6 The generation of hydrogen/oxygen (hydrogen/oxygen evolution reaction) from water splitting is however restricted by the use of noble metals which improve the reaction kinetics of these thermodynamically challenging processes.^7,8 The current focus is therefore on the preparation of cheap and readily available alternatives to the exotic metal catalysts for the generation of hydrogen.

Transition metal sulfides continually attract attention as potential substitutes for exotic metals in energy conversion and storage technologies. Nickel sulfides have probably drawn more exploration than other sulfides in sustainable energy technologies.⁹ It has been prepared in different compositions, NiS, NiS₂, Ni₃S₂ and N₉iS₈.¹⁰ NiS which has been more documented amongst the nickel sulfide system probably due to thermodynamic stability¹¹ has been deployed to improve the reaction kinetics of water splitting and in energy storage systems.⁹ For example, Zhang et al.¹² recently fabricated NiS deploying grapefruit peel and Ni(NO₃)·6H₂O as S and Ni precursors, respectively which was applied as an efficient catalyst in HER and OER. We also have reported NiS obtained from metal–organic compounds towards improving the water-splitting reaction kinetics.^10,13,14 The relatively scarcer Ni₃S₂ phase is an interesting catalyst due to its electronic structure. It has a rhombohedral structure with tetrahedral nickel atoms giving room for Ni–Ni interactions.¹⁵ These interactions give Ni₃S₂ its metallic behavior which makes it an excellent electrical conductor applicable in catalysis.¹⁶ The Zhong group¹⁷ for example formed Ni₃S₂ using Ni foam and Na₂S·9H₂O which demonstrated high OER activity. Also, nickel sulfides such as Ni₃S₂/NiS¹⁸ and NiS¹⁹ have been employed as electrode materials in supercapacitor fabrication. Nickel sulfides’ continued appeal to the research community stems from their abundance in earth reserves,²⁰ electrical conductivity¹⁷ and versatility in morphology and composition.¹⁰

The desire for phase-pure nickel sulfides, considering the multiplicity of phases in the sulfide system, is weighed down by the difficult synthetic protocols. Traditionally, sulfides of nickel have been prepared by high-temperature decomposition of dual or multiple precursors in the absence of any solvents. For instance, Kosmac and colleagues²¹ formed a mixture of nickel sulfide phases by the mechanical alloying of Ni and S powders in a glove box at high temperatures over an extended period. Other solventless synthetic routes like thermal diffusion,²² solid-to-solid calcination²³ and melt decompositions^10,24 have also been deployed to fabricate nickel sulfides. These solvent-free routes result not only in poor control of morphology, composition, size and purity of products but also impose high reaction temperatures and time. The use of multiple precursors also makes the synthetic process cumbersome. The colloidal synthetic pathway involving the use of solvents generally yields well-defined particles in terms of dimension, composition and purity.²⁵ For example, different researchers have employed Ni sources like Ni(NO₃)₂·6H₂O, nickel acetate tetrahydrate, Ni foam, Ni foil and S providers like thiourea, Na₂S·9H₂O and 2-mercaptoethanol to fabricate dimensionally controlled and pure Ni₃S₂ and NiS exercising colloidal methods like hydrothermal^17,26,27 and solvothermal.^28–30 Also, other routes like microwave-assisted decompositions,³¹ electrodeposition³² and product precipitation³³ have been deployed in the preparation of desired nickel sulfides similarly employing dual or multiple precursors.

The single-source precursor route primarily involving metal–organic frameworks to the formation of nickel sulfides bypasses the challenges associated with the use of multiple precursors. This straightforward and facile route to the fabrication of metal sulfides has been deployed in the synthesis of Ni₃S₂ and NiS. For instance, we have utilized sulfur-based coordination complexes like dithiocarbamates,^34,35 xanthates^24,36 and dithiophosphonates^10,13,14 to form or deposit nickel sulfides. The use of single-source starting materials or any of the other routes to the formation of nickel sulfides is, however, largely inflexible to the formation of more than one pure phase.

Herein, we utilized xanthate, [Ni(S₂CO(C₂H₅))₂] (Ni-Xan) and dithiocarbamate, [Ni(S₂CN(C₂H₅)₂)₂] (Ni-DTC) complexes to prepare different phases of nickel sulfide (NiS and Ni₃S₂) from a single precursor via a solvent-free and colloidal route. In the solvent-free route, the complexes were pyrolyzed under an inert atmosphere, whereas in colloidal synthesis, oleylamine (OLA) was used as a solvent and capping agent. The study investigates how the choice of starting precursors and the synthetic routes influence the charge storage and water-splitting performance of nickel sulfide phases. This study highlights critical considerations for designing suitable ligands and suggests potential prerequisites that may result in enhancing the performance of materials synthesized via solvent-free routes.

Experimental

Materials

Potassium ethyl xanthogente (96%), sodium diethyldithiocarbamate trihydrate, triphenylphosphine (TPP), nickel chloride hexahydrate, deionized water, chloroform (99.8%), methanol (99.8%) and acetone (99.5%) were purchased from Sigma-Aldrich, Durban, South Africa and used without further purification.

Preparation of nickel complexes

The synthesis of nickel xanthate [Ni{S₂CO(C₂H₅)}₂] and nickel dithiocarbamate [Ni{S₂CN(C₂H₅)₂}₂] complexes was carried out following previously reported procedures.^14,36 These complexes were confirmed by elemental analysis. [Ni{S₂CO(C₂H₅)}₂]: calc. C, 23.91%; H, 3.35%; S, 42.51%; Ni, 19.49%. Found: C, 23.81%; H, 3.20%; S, 42.17%; Ni, 20.1%. [Ni{S₂CN(C₂H₅)₂}₂]: calc. C, 33.62%; H, 6.21%; N, 7.84%; S, 35.90%; Ni, 16.43%. Found: C, 33.73%; H, 6.11%; N, 7.91%; S, 35.79%; Ni, 16.48%.

Preparation of nickel sulfides (NiS and Ni₃S₂) via a solvent-free route

To prepare NiS, 0.2 g of Ni-Xan or Ni-DTC was placed in a ceramic boat and transferred to a quartz tube, which was heated to 250 °C in a furnace under N₂ for an hour. The product was obtained as a black powder and used without any further treatment. The preparation of Ni₃S₂ was similar to that of NiS except that a composite of Ni-Xan (0.10 g, 0.33 mmol) with TPP (0.35 g, 1.32 mmol) or Ni-DTC (0.10 g, 0.28 mmol) with TPP (0.29 g, 1.12 mmol) (i.e. complex to TPP ratio of 1 : 4 in both cases) was ground before the pyrolysis reactions.

Preparation of nickel sulfides (NiS and Ni₃S₂) in oleylamine

For ligand-capped nickel sulfides, 0.30 g of Ni-Xan or Ni-DTC in olelylamine (5 mL) was placed in a 3-neck flask under N₂ at room temperature and heated slowly to 250 °C and kept at this temperature for 1 hour. The mixture was then allowed to cool to room temperature. The product was isolated as a black powder after two methanol/chloroform (10 mL; 5 : 1) and two acetone (10 mL) washings to yield NiS. The fabrication of Ni₃S₂ was similar to NiS above except for the homogenization by grinding of Ni-Xan (0.10 g, 0.33 mmol) and TPP (0.17 g, 0.66 mmol) or Ni-DTC (0.10 g, 0.28 mmol) and TPP (0.22 g, 0.84 mmol) before thermolysis in oleylamine.

Instrumentation

Elemental analysis of complexes Ni-Xan and Ni-DTC was obtained from a PerkinElmer CHN analyzer (2400 series II). Thermogravimetric analysis (TGA) was done using a Mettler-Toledo TGA/DSC instrument. The TGA analysis was done in an inert atmosphere (N₂) at a heating rate of 10 °C per minute. Powder X-ray diffraction patterns of the fabricated nickel sulfides were acquired from a Bruker D8 Diffractometer with a CuKα radiation source. The patterns obtained were compared to standard diffraction patterns from the International Centre for Diffraction Data (ICDD). Transmission electron microscopy (TEM) images of the formed materials were acquired from a JEOL TEM instrument (1400). Sample preparation for TEM involved dropping the diluted solution of the desired material onto Formvar-coated grids. The coated grids were left to dry at room temperature and then viewed for TEM at 120 kV. TEM images were analyzed via iTEM software. Scanning electron microscopy (SEM) investigations were carried out on Philips XL30 FEG-SEM while those for energy-dispersive X-ray spectroscopy (EDX) were performed with the same instrument that was equipped with a DX4 detector. X-ray photoelectron spectroscopy (XPS) was carried out to examine the surface chemistry via a Thermo ESCAlab 250 Xi spectrometer equipped with a monochromatic AI kα X-ray source.

Electrochemical studies

Electrochemical investigations of the formed nickel sulfides were performed using a Versastat 4-500 workstation from Applied Research, USA which deployed a three-electrode system. Nickel foams (MTI Corporation, USA, 99.99% purity) were employed as electrodes’ substrates. Sample preparation for these examinations was carried out by forming pastes of the nickel sulfides using (80 wt%), polyvinylidene difluoride (PVDF, 10 wt%), and acetylene black (10 wt%), which was prepared using N-methyl pyrrolidinone (NMP). The as-prepared paste was then applied to nickel foams which were pre-cleaned and weighed, and used as working electrodes. Platinum wire and saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. Supercapacitance investigations of synthesized materials were performed using a 3 M KOH electrolyte. Charge storage capacity was determined using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) at varying scan rates and current densities. The electrocatalytic properties of fabricated electrodes were analyzed by CV and linear sweep voltammetry (LSV).

Results and discussion

Synthetic procedures for the synthesis of the sulfur-based complexes; xanthates and dithiocarbamates, are well documented.^14,36 The preparation of Ni-Xan and Ni-DTC followed these standard protocols. The thermal stability and decomposition profile of the complexes, TPP and the composite of the complexes with TPP are shown in Fig. S1.^13,14,36 In brief, Ni-Xan and Ni-DTC show moderate thermal stability with decomposition between 150 to 230 °C resulting in NiS residues. TPP decomposes cleanly without any residue and the composites Ni-Xan/TPP and Ni-DTC/TPP decompose at lower temperatures in comparison to their relative single parent complexes.

The solvent-less pyrolysis of Ni-Xan/TPP and Ni-DTC/TPP in an inert atmosphere (N₂) at 250 °C for an hour yielded hexagonal NiS. The formation of phase-pure NiS (ICDD# 01-075-0613) for Ni-Xan (NiS-3) and Ni-DTC (NiS-4) was confirmed by the powder XRD as matched to the standard reference patterns [Fig. 1(i)]. The facile formation of phase-pure NiS from these precursors is anticipated since nickel is directly bonded to the sulfur in the complexes and their thermal decomposition has been reported to lead to volatile components that are easily removable.^36,37 The solvent-free preparation of NiS from Ni-Xan and Ni-DTC is not only straightforward but also scalable. We also explored the fabrication of nickel sulfides from both complexes mediated by TPP via the solvent-free pyrolysis route. This procedure requires the initial homogenization of the desired amounts of TPP with Ni-Xan or Ni-DTC before the pyrolysis reactions. The decomposition of the composite of Ni-Xan with TPP (1 : 4) at 250 °C for 1 hour under an N₂ atmosphere afforded phase-pure nickel sulfide which matched well to heazlewoodite Ni₃S₂ (Ni₃S₂-2) of the rhombohedral crystal system as per the ICDD# 01-085-0775 [Fig. 1(iii)]. Similarly, the pyrolysis of Ni-DTC with TPP (1 : 4) under the same conditions afforded phase-pure heazlewoodite Ni₃S₂ (ICDD# 01-085-0775) which will be referred to as Ni₃S₂-4. It is noteworthy that the stoichiometry of the metal complex precursor to TPP in these decomposition reactions is crucial for the successful fabrication of Ni₃S₂. For instance, the pyrolysis of a 1 : 3 composite of Ni-DTC with TPP yielded a mixture of nickel sulfides identified as rhombohedral Ni₃S₂ (ICDD# 01-073-0698) and orthorhombic Ni₉S₈ (ICDD# 01-078-1886) as shown in Fig. S2. Though the Ni₃S₂ material is highly desired due to its excellent catalytic activity,^38,39 synthetic protocols have been largely limited to the colloidal routes.^15,40 In addition, dual or multiple precursors have dominated the colloidal pathway to Ni₃S₂ fabrication hence their formation procedures are usually complex and time-consuming.⁴⁰ Therefore, the use of cheap, readily available, and chemically stable TPP to mediate the solvent-free formation of Ni₃S₂ from single-source precursors offers a significant advantage over conventional methods.

Fig. 1 p-XRD patterns of the decomposition of Ni complexes (a) Ni-Xan or (b) Ni-DTC via the (i) solvent-less and (ii) colloidal routes. p-XRD patterns of the decomposition of the composites (a) Ni-Xan/triphenylphosphine or (b) Ni-DTC/triphenylphosphine via the (iii) solventless and (iv) colloidal routes.

Similar reactions were also carried out via a colloidal method to gain further insight into the role of TPP in the formation of nickel sulfides. The decomposition of Ni-Xan (NiS-1) or Ni-DTC (NiS-2) in oleylamine (OLA) by the heat-up protocol at 250 °C for 1 hour formed nickel sulfide which matched well with hexagonal NiS (ICDD# 01-075-0613), as shown in Fig. 1(ii).

Table 1 summarizes the precursors, experimental conditions, phases obtained, and codes used in this study as well as the average crystallite sizes of the synthesized catalysts.

Table 1 Summary of precursors, synthetic route & conditions and phases obtained

Complexes/composites	Synthetic route	Reaction conditions	Phase formed	Code	Average crystallite size (nm)
[Ni{S2CO(C2H5)}2] (Ni-Xan)	Colloidal	250 °C, in oleylamine, 1 h	NiS	NiS-1	33 ± 5
[Ni{S2CN(C2H5)2}2] (Ni-DTC)	Colloidal	250 °C, in oleylamine, 1 h	NiS	NiS-2	22.5 ± 4
[Ni{S2CO(C2H5)}2] (Ni-Xan)	Solventless	250 °C, 1 h	NiS	NiS-3	23 ± 7
[Ni{S2CN(C2H5)2}2] (Ni-DTC)	Solventless	250 °C, 1 h	NiS	NiS-4	49 ± 3
[Ni{S2CO(C2H5)}2] (Ni-Xan)/triphenylphosphine (TPP)	Colloidal	250 °C, in oleylamine, 1 h	Ni3S2	Ni3S2-1	12 ± 8
[Ni{S2CO(C2H5)}2] (Ni-Xan)/triphenylphosphine (TPP)	Solventless	250 °C, 1 h	Ni3S2	Ni3S2-2	16 ± 2
[Ni{S2CN(C2H5)2}2] (Ni-DTC)/triphenylphosphine (TPP)	Colloidal	250 °C, in oleylamine, 1 h	Ni3S2	Ni3S2-3	26 ± 14
[Ni{S2CN(C2H5)2}2] (Ni-DTC)/triphenylphosphine (TPP)	Solventless	250 °C, 1 h	Ni3S2	Ni3S2-4	28 ± 8

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There are limited reports on the fabrication of NiS in OLA via the heat-up method employing single-source precursors and mostly involving the use of a combination of solvents or additives.^41–44 The preparation of NiS in OLA, as presented, is facile and straightforward. The thermolysis of composites of Ni-Xan or Ni-DTC with TPP in OLA, formed Ni₃S₂ just like the solvent-less pyrolysis reactions. The synthetic protocol for these reactions entailed the initial homogenization of the required complex and the desired amount of TPP by grinding before decomposition in OLA by the heat-up method at 250 °C for 1 hour under N₂. The thermolysis of the composite of Ni-Xan and TPP in 1 : 2 stoichiometry resulted in Ni₃S₂ (Ni₃S₂-1) of the cubic crystal system as per (ICDD# 01-076-1813) as shown in Fig. 1(iv). Similarly, the decomposition of Ni-DTC/TPP (1 : 3) in OLA at 250 °C resulted in nickel sulfide, which matched well with Ni₃S₂ (ICDD# 01-076-1813), referred to as Ni₃S₂-3, as shown in Table 1. The stoichiometry of the complex to TPP is critical in the formation of Ni₃S₂ in the colloidal route just like in the solvent-free pyrolysis path stated above. For example, the thermolysis of Ni-DTC/TPP (1 : 2) in OLA at 250 °C yielded a mixture of nickel sulfides that matched rhombohedral NiS (ICDD# 01-086-2280) and the non-stoichiometric hexagonal Ni_0.96S (ICDD# 00-050-1791) as shown in Fig. S3. It is worth noting that the colloidal synthesis of Ni₃S₂ is often carried out using dual or multiple precursors and typically relies on the time-intensive hydrothermal route.⁴⁰ In our previous work, we reported the use of single-source precursors in colloidal synthesis with trioctylphosphine (TOP) to obtain the Ni₃S₂ phase and proposed the role of TOP in achieving a sulfur-deficient composition.^35,45 The use of single-source precursors and the heat-up method in preparing Ni₃S₂ simplifies most documented protocols.

The role of TPP in the formation of Ni₃S₂ by colloidal as well as solvent-free routes employing sulfur-based coordination complexes is interesting and needs further probing. Trialkylphosphines have been used as desulfurization agents as well as a source of phosphorus for metal phosphide formation. For example, colloidal or solvent-less thermolysis of various chalcogen-based metal–organic precursors in the presence of trialkyl/arylphosphines at high temperatures resulted in the formation of metal phosphide nanomaterials.^10,13,14,46 In addition, they have been used for the desulfurization of metal chalcogenides to yield metallic (Ag, Sb, Bi) or bi-/intermetallic (AgBi, Ag₃Sb) compounds.^47–49 TPP therefore mediates the formation of the non-stoichiometric Ni₃S₂ in solvent-free and colloidal routes by partial desulfurization. The partial desulfurization by TPP also explains the formation of the sulfur-rich Ni_0.96S phase for the lower TPP composite amount (Ni-DTC/TPP; 1 : 2) compared to the sulfur-deficient Ni₃S₂ (Ni-DTC/TPP; 1 : 3) in the colloidal decompositions of Ni-DTC/TPP composite (Fig. 1 & S3). The mediating role of trialkylphosphines in the formation of functional materials can therefore be explored to prepare desirable but scarcely available metal chalcogenide analogues.

The microstructure of the formed NiS and the TPP-assisted fabricated Ni₃S₂ via the solvent-less and colloidal routes was examined by employing scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). The SEM images of the nickel sulfides indicate agglomerated particles without any defined shape at such low magnification (Fig. 2 & S4). However, Ni₃S₂ particles obtained in OLA (Fig. 2c) and the NiS and Ni₃S₂ formed from the solvent-less pyrolysis of complex 2 (Fig. S4b and d) showed sheet-like morphologies.

Fig. 2 SEM images of NiS obtained from the decomposition of (a) Ni-Xan and (b) Ni-DTC and Ni₃S₂ obtained from the decomposition of (c) Ni-Xan/TPP and (d) Ni-DTC/TPP all in OLA.

TEM analysis of nickel sulfides provided further insight into these materials’ microstructure. The TEM images (Fig. 3 & S5) of the prepared sulfides confirm the agglomeration and compact arrangement of the particles. It also revealed that these materials have a non-uniform morphology consisting of spherical and elongated-shaped particles. The elemental mapping images of the fabricated sulfides indicate exclusively the presence of Ni and S which were well distributed as shown in Fig. 2 & S3. Also, the experimental atomic percentage composition (EDX) of nickel sulfides agreed with the theoretical values (Table S1).

Fig. 3 TEM images of NiS obtained from the decomposition of (a) Ni-Xan and (b) Ni-DTC and Ni₃S₂ obtained from the decomposition of (c) Ni-Xan/TPP and (d) Ni-DTC/TPP, all in OLA and their (e–h) respective SAED and (i–l) HRTEM images.

The crystallinity of the formed materials was examined by SAED and HRTEM analysis (Fig. 3 & S5). SAED images show well-defined spots that match well with the respective XRD and ICDD standard patterns, indicative of good crystallinity. The SAED results were affirmed by the clear lattice fringes of the formed sulfides that were in agreement with the standard patterns, as shown in the HRTEM results. We also calculated the crystallite size of our formed sulfides from our XRD results via the Debye–Scherrer equation (; L = sulfide's average crystallite size, λ = X-ray wavelength, β = full width at half maximum, and θ = Bragg's angle) to check the effect of the synthetic route on particle size as given in Table 1. NiS particles from the colloidal route were relatively smaller and less agglomerated than those from the solventless method, due to better morphology control and surface capping by the solvent (Fig. 3 & S4). Likewise, pure NiS made without TPP was larger than TPP-containing Ni₃S₂, indicating TPP's role as a capping agent that limits particle growth.

X-ray photoelectron spectroscopy (XPS) spectra of the formed and transformed sulfides were employed to examine the surface chemistry of our materials (Fig. 4 & S6). The survey spectrum of the colloidally formed NiS (NiS-1) indicated a composition of Ni, S, C, O and N. Apart from the expected Ni, S and residual C, O and N were attributed to surface oxidation and the capping amine group (OLA), respectively. The deconvoluted Ni 2p peaks at 855.2 and 873.2 eV (Fig. 4b) are assigned to 2P_3/2 and 2p_1/2, respectively, characteristic of Ni²⁺ of the sulfide.^50,51 Additional Ni 2p peaks at 852.5 and 869.4 eV are assigned to Ni⁺ and metallic Ni⁰ while the satellite peak at 860.8 and shakeup peak at 878.8 eV are typical of nickel.⁵⁰ The S 2p region showed two peaks (Fig. 4c), including 160.8 (2p_3/2) and 162.2 eV (2p_1/2) attributed to S of the sulfide. Other regions with peaks at 398.8, 284.4 and 530.7 eV (Fig. 4d & S6Ib and c) are assigned to N, C and O, respectively. The colloidally transformed sulfide (Ni₃S₂-2) had similar XPS results as expected (Fig. 4e-I & S6Id–f) with the presence of the P 2p region, the major exception (Fig. 4i). The peaks at 2p_3/2 (129.2) and 2p_1/2 (132.7 eV) are typical of metal phosphides and are due to residual amorphous nickel phosphide formed in the transformation of NiS to Ni₃S₂.⁵² Also, all the deconvoluted XPS regions of Ni₃S₂-2 had slightly higher binding energies compared to NiS-1, indicative of phase transformation.

Fig. 4 XPS spectra of formed sulfides. Survey spectrum (a) and Ni 2p (b), S 2p (c) N 1s (d) regions of NiS-1. Survey spectrum (e) and Ni 2p (f), S 2p (g) N 1s (h), P 2p (i) regions of Ni₃S₂-1.

The XPS analysis of the solventless formed sulfides (Fig. S6II) is similar to the ones from the colloidal route except for the absence of the N 1s region since no solvents were employed. The minor P 2p signal observed in the XPS spectrum, in all the transformed sulfides, i.e. Ni₃S₂ as given in Fig. 4 & S6, may originate from residual TPP or surface-bound phosphorus-containing intermediate species formed during the phase transformation.

Electrochemical properties of formed nickel sulfides for supercapacitor applications

Supercapacitance studies were conducted to evaluate the electrochemical performance of the as-synthesized materials. The results suggest that the obtained materials are promising candidates to bridge the function of dielectric capacitors with high power density.⁵³ The electrochemical activity, capacitive performance and feasibility of charge transfer were comprehensively investigated via cyclic voltammograms (CV) in 3 M KOH in various cycles at different scan rates ranging from 2 mV s⁻¹ to 300 mV s⁻¹ in a potential window of 0.0–0.6 V as shown in Fig. 5 & S7. Henceforth, the NiS formed from the decomposition of complexes Ni-Xan and Ni-DTC in OLA will be referred to as NiS-1 and NiS-2 while the ones obtained from the solvent-less pyrolysis of these complexes will be denoted as NiS-3 and NiS-4, respectively. Similarly, Ni₃S₂ fabricated from the composites of Ni-Xan and Ni-DTC with TPP in the colloidal route will herewith be known as Ni₃S₂-1 and Ni₃S₂-2 while those obtained via the solvent-free method will be Ni₃S₂-3 and Ni₃S₂-4, respectively (Table 1). The CV results show a set of non-rectangular-shaped redox peaks suggestive of the pseudocapacitive behavior of synthesized materials.⁵⁴ The shape of the CV curves for all the electrodes was maintained at low and high scan rates, resembling a quasi-reversible reaction due to multiple valence states of the Ni such as Ni⁰/Ni²⁺, Ni⁰/Ni³⁺ and Ni²⁺/Ni³⁺ indicative of its fast charge–discharge ability. The deduced faradaic redox reaction for the NiS and Ni₃S₂ materials are shown in eqn (1) and (2).^55,56

NiS + OH⁻ ↔ NiSOH + e⁻ (1)

Ni₃S₂ + 3OH⁻ ↔ Ni₃S₂(OH)₃ + 3e⁻ (2)

Fig. 5 (a–c) CV voltammograms for the NiS electrodes and (d–f) for the Ni₃S₂ samples. (g) CV comparison for the NiS electrodes and (h) for the Ni₃S₂ ones all at a scan rate of 10 mV s⁻¹. Current density vs. scan rate for (i) anodic, (j) cathodic regime for the NiS materials and (k) anodic and (l) cathodic regime for the Ni₃S₂ electrodes.

To gain further insight into the charge transfer kinetics of these materials, a comparison of the CV curves for the NiS and Ni₃S₂ samples at a scan rate of 10 mV s⁻¹ was delineated as shown in Fig. 5g and h. These comparative curves clearly show that among the NiS and Ni₃S₂ samples, NiS-1, NiS-2 and Ni₃S₂-3 have more enclosed areas which are a pointer to their superior electrochemical kinetics. Also, the anodic potential (E_a) and cathodic potential (E_c) of the samples were noted where NiS-1 and NiS-2 had lower differences between E_a and E_c compared to Ni₃S₂-1 and Ni₃S₂-2, all formed via the colloidal route. On the other hand, the samples obtained via the solvent-free method showed a different result. Here, Ni₃S₂-3 and Ni₃S₂-4 showed higher peak potential differences than NiS-3 and NiS-4. It is important to note that the lower potential difference between the peaks shows better electronic transmission movement and ion fusibility due to low charge transfer resistance. NiS-2 and Ni₃S₂-3 offered the least ion resistance among the electrodes and hence amplified ion mobility suggestive of better charge storage capacity among all the electrodes. It is also important to note that a lower potential difference means faster redox processes which also enrich the power density and energy density of a material.

The anodic and cathodic peak current density of the electrodes was plotted against the scan rate as shown in Fig. 5i–l to probe into the charge storage capacity of nickel sulfides. No linear correlation between the anodic and cathodic peak current density as a function of scan rate is observed for all the electrodes indicating that charge storage has pseudocapacitance characteristics and not an absorption mechanism which is in agreement with the CV results. The charge storage mechanism for all the electrodes was probed further using the Sevcik equation.⁵⁷ The linear plots of the current density vs. the scan rates for all the electrodes indicated a diffusion-assisted mechanism of the electrochemical oxidation and reduction processes as shown in Fig. 6a–d. The linear fitting of these plots resulted in the highest anodic slope of 4.81 for NiS-2 (Fig. 6a) among all the NiS samples whereas for the Ni₃S₂ electrodes, Ni₃S₂-3 had the highest slope of 3.03 (Fig. 6b). These results suggest that NiS-2 and Ni₃S₂-3 electrodes with faster oxidation processes have the best storage capacity with the highest rate of permeation of ions compared to their regression rate. Furthermore, cathodic intercepts were also plotted as shown in Fig. 6c and d with NiS-2 and Ni₃S₂-3 similarly having the better reduction processes (i.e. the slopes and intercepts). NiS-2 (prepared by the colloidal route) and Ni₃S₂-3 (obtained by the solvent-less protocol) therefore show the best charge transfer rate and storage capability potentials compared to all the electrodes.

Fig. 6 Current density vs. square root of scan rate at the anodic regime for (a) the NiS and (b) the Ni₃S₂ electrodes. The corresponding cathodic regime for (c) the NiS and (d) the Ni₃S₂ electrodes. Specific capacitance obtained from the CV for (e) the NiS and (f) the Ni₃S₂ electrodes. GCD curves for the electrodes (g–l).

The specific capacitance of the samples was calculated using the CV curves employing eqn (3) and (4):⁵⁸

where I (V) denotes current, Δv is the potential window, C_p is the calculated gravimetric specific capacitance (F g⁻¹) and A (cm²) is the area under the CV curve. K (V s⁻¹) is the scan rate of the CV and a (cm²) is the area of the sample on Ni foam. The calculated capacitance at various scan rates is shown in Fig. 6e and f. As expected NiS-2 and Ni₃S₂-3 had the highest calculated specific capacitance of 1900 and 1400 F g⁻¹, respectively. These results compare well to the capacitance of recently formed nickel sulfides. For instance, Ni_0.5Co_0.5S (1512 F g⁻¹),⁵⁹ NiS–Sn (732 F g⁻¹)⁶⁰ and Ni₃S₂ (610 F g⁻¹)⁶¹ had comparative or lower capacitance at 1 mV s⁻¹, 1 A g⁻¹ and 0.5 A g⁻¹ respective current densities (Table S2). Furthermore, the galvanostatic charge–discharge (GCD) curves were employed to check the charging and discharging behavior of the as-synthesized materials for supercapacitor application at a potential window of 0.0–0.6 V for NiS electrodes and 0.0–0.57 V for the Ni₃S₂ ones at varying current densities in a 3 M KOH solution (Fig. 6g–l & S8). The non-linear GCD curves observed for all the materials indicate their redox nature which agrees with the CV results. Generally, discharge time increases with decreasing current density suggestive of improved storage capacity even at lower current densities. NiS-2 (prepared by the colloidal route) and Ni₃S₂-3 (formed by the solvent-less protocol) by the GCD curves had the best charge–discharge time which aligns with the CV results.

The specific capacitance (C_s) of the electrodes was also calculated by the following eqn (5) using GCD measurements.

C_s = I*Δt/(m*ΔV) (5)

Herein, C_s is the specific capacitance (F g⁻¹), I is the current (A), Δt is the discharge time (s), m is the mass of the active material (g) and ΔV is the potential window. From the calculated results, the specific capacitance vs. current density plots for all the electrodes were done and are shown in Fig. S9a and b. NiS-2 showed the highest specific capacitance of 809.2 F g⁻¹ compared to NiS-1 & NiS-3 (with an average of 600 F g⁻¹) and NiS-4 (50 F g⁻¹) at 1 A g⁻¹ of current density. For the Ni₃S₂ electrodes, the ones prepared by the solvent-less route (Ni₃S₂-3 & Ni₃S₂-4) had the best specific capacitance (about 500 F g⁻¹) in comparison to the samples formed by the colloidal method (Ni₃S₂-1 & Ni₃S₂-2) with 300 F g⁻¹ at the current density of 1 A g⁻¹. Therefore, the specific capacitance of the NiS samples formed by the colloidal method and the Ni₃S₂ ones fabricated by the solvent-free protocol was better than the respective other electrodes. The energy storage capacity of synthesized materials compares well with other recently documented Ni-based electroactive materials, as shown in Table S2.

Since the energy and power density of a capacitor are crucial to estimating its practical application, they were calculated using eqn (6) and (7):⁶²

E = 0.5CV² (Wh kg⁻¹) (6)

P = 3.6 × E/Δt (kW kg⁻¹) (7)

where C is the specific capacitance (F g⁻¹), V is the voltage window, and Δt is the discharge time obtained from GCD. The energy and power density of NiS-1, NiS-2, NiS-3, and NiS-4 were calculated as 25.5, 34.9, 25.7 and 2.3 Wh kg⁻¹, and 281.7, 278.2, 268.1 and 277.6 kW kg⁻¹, respectively. On the other hand, the energy and power density of Ni₃S₂-1, Ni₃S₂-2, Ni₃S₂-3, and Ni₃S₂-4 are 11.4, 11.7, 20.3, and 18.7 Wh kg⁻¹, and 261.5, 267.3, 267.3 and 266.4 kW kg⁻¹, respectively. The energy density of the NiS electrodes was better than that of the Ni₃S₂ ones with the NiS-2 (prepared by the colloidal route) having the best result which agrees with the capacitance performance. On the other hand, among the Ni₃S₂ electrodes, Ni₃S₂-3, obtained via the solvent-free method, as expected, showed the best energy density values. Also, all the nickel sulfide electrodes (both NiS and Ni₃S₂) had similar power density results as shown in Fig. S9c and d.

The electrochemical stability of an energy storage device is another key pointer towards its practical application and was evaluated through the cycling processes. The coulombic efficiency of the two best-performing electrodes was evaluated in terms of energy storage i.e. NiS-2 and Ni₃S₂-3 as shown in Fig. S9e and f. NiS-2 and Ni₃S₂-3 remarkably showed 100% coulombic efficiency over 5000 cycles indicative of cycling stability. NiS-2 and Ni₃S₂-3 electrodes also showed exceptional capacitance retention of 91.3% and 80.2%, respectively over 5000 cycles of testing. Therefore, energy storage testing parameters like CV, electrochemical kinetics, and capacitance derived using CV area and that obtained by GCD evaluations collectively indicate that colloidally prepared NiS and solvent-free formed Ni₃S₂ with NiS-2 and Ni₃S₂-3 materials showed the best performance among all the electrodes. This then means that the high carbon content of the solventless prepared NiS limited its energy storage performance more than the OLA capping agent blocking the active sites. Contrarily, OLA's adsorption on Ni₃S₂ had less of a negative impact on its storage functionality compared to the carbon content.

Electrocatalytic properties of formed nickel sulfides for water-splitting applications

We also investigated the electrocatalytic characteristics of fabricated nickel sulfides in water splitting. Though water splitting holds a lot of promise in hydrogen generation, these reactions are constrained in high thermodynamics, necessitating the use of catalysts.⁶³ The HER performance of the materials was evaluated in a three-electrode system in 1 M KOH via linear sweep voltammetry (LSV) and Tafel slopes (Fig. 7). NiS-1, NiS-2, NiS-3 and NiS-4 electrodes needed 225, 233, 247 and 240 mV to reach 10 mA cm⁻² current density alongside Tafel slopes of 158, 147, 152 and 147 mV dec⁻¹, respectively. NiS-1 was therefore the best catalyst among the NiS materials in reducing the constraining overpotential, while NiS-2 and NiS-4 had the fastest reaction kinetics. Similarly, Ni₃S₂-1, Ni₃S₂-2, Ni₃S₂-3, and Ni₃S₂-4 required overpotentials and Tafel slopes of 230, 197, 240 & 245 mV and 156, 159, 160 & 152 mV dec⁻¹, respectively, to attain the same current density. Pt/C, the state-of-the-art HER catalyst, had the best performance as expected with 31 mV at 10 mA cm⁻². The Ni₃S₂-2 material was therefore the best performing in HER among all electrodes with Ni₃S₂-4 having the fastest reaction kinetics within the Ni₃S₂ electrodes. Ni₃S₂ is structurally made up of Ni₃-atoms connected by Ni–Ni bonds, providing good electrical conductivity.¹⁵ Moreover, the incorporation of phosphorus into Ni₃S₂ has been shown to enhance its catalytic performance.⁶⁴ These could explain the good performance of the Ni₃S₂-2 sample in HER. The HER performance of nickel sulfide materials compares well with or outperforms similar electrodes documented in the literature (Table S3).

Fig. 7 HER polarization curves for (a) NiS, (e) Ni₃S₂ and their corresponding Tafel slopes (b) and (f). OER polarization curves for (c) NiS, (g) Ni₃S₂ and their corresponding Tafel slopes (d) and (h).

The performance of all materials towards OER was also investigated in an alkaline environment. Rare earth and exotic elemental oxides like IrO₂ and RuO₂ are the benchmark catalysts for OER, which restricts the practical application of these technologies. Readily available substitutes like nickel sulfides are therefore highly sought after. The OER performance of the fabricated electrodes was examined by polarization curves, as shown in Fig. 7. To attain 10 mA cm⁻² current density, Ni₃S₂-1, Ni₃S₂-2, Ni₃S₂-3 and Ni₃S₂-4 required 275, 298, 272, and 256 mV with Tafel slopes of 73, 66, 88 and 71 mV dec⁻¹, respectively. Also, the NiS samples needed overpotentials/Tafel slopes of 257/81 (NiS-1), 257/72 (NiS-2), 267/90 (NiS-3) and 290/45 (NiS-4) to reach the same current density. RuO₂, the benchmark OER catalyst, demanded 333 mV to reach 10 mA cm⁻². Ni₃S₂-4, NiS-1, NiS-2, and Ni₃S₂-4, NiS-4 are, therefore, better OER catalysts in terms of overpotential lowering and intrinsic reaction kinetics (Tafel slopes), respectively. Sulfides with relatively larger crystallite sizes tended to show better water splitting activity. For example, the sample with the highest HER performance and the one with the most favorable HER kinetics both fell into the group with larger particle sizes. Notably, surface morphology did not show a clear correlation with performance, as samples with distinct morphologies displayed comparable reaction kinetics. These findings suggest that intrinsic factors, such as crystallinity, conductivity, phase composition, or defect chemistry, may play a more decisive role in determining electrochemical behavior than particle size alone. The OER performance of synthesized nickel sulfides compares well with recently reported similar materials (Table S4). The HER and OER results indicate that the synthetic route had no major influence on the sulfide's performance.

The ion diffusion kinetics of an electrode sheds light on its charge/mass transfer processes, which are crucial to its practical application. We, therefore, investigated this property by electrochemical impedance spectroscopy (EIS) measurements. Fig. 8 gives the EIS measurements as Nyquist plots with linear behavior at low frequency and semicircle shape at higher frequency in congruence with other reports.⁶⁵ The charge transfer resistance of electroactive species is a function of the semicircle's diameter, with resistance increasing with an increase in diameter.^8,66 Ni₃S₄-4 showed the lowest charge transfer resistance of 0.6 Ω cm⁻² among all the samples, which correlates with the OER results. Apart from the charge transfer resistance, the stability and ruggedness of an electrode are crucial to its long-term application. The durability of the materials was assessed by employing the polarization curves before and after 1000 CV cycles and long-term chronoamperometry (CA) curves for 24 h. Fig. S10 and S11 give the HER and OER stability polarization curves of all the electrodes. These curves for the 1^st and 1000^th cycles matched well indicative of the synthesized materials’ ruggedness and stability. CA curves for all the nickel sulfides were also employed to probe further into their durability and are given in Fig. S12. All the sulfides showed good stability of current density over an extended period of 24 h with NiS-1, NiS-4 and Ni₃S₄-4 having the best results. The EIS and CA results are suggestive of the feasibility of synthesized nickel sulfides for practical application. The fabricated nickel sulfides have therefore been shown as suitable electrode materials applicable in lowering the constraining energy thermodynamics in HER and OER.

Fig. 8 EIS curves of the NiS and Ni₃S₂ electrodes.

Conclusion

Similar phases (NiS and Ni₃S₂) can be prepared via different synthetic routes (colloidal and solvent-less thermolysis) by optimizing the reaction conditions for the thermolysis of metal–organic precursors. The presence of triphenylphosphine (TPP) resulted in the formation of a sulfur-deficient phase (Ni₃S₂), regardless of the precursor and the synthetic route used. The performance of fabricated nickel sulfide electrodes was tested for application in energy storage and energy generation technologies. The Ni₃S₂ prepared by the colloidal route had the best HER performance with an overpotential of 197 mV and a Tafel slope of 159 mV dec⁻¹, to attain 10 mA cm⁻² of current density in an alkaline medium. On the contrary, the same phase obtained by the solvent-less route showed the best OER activity. Perhaps the residual carbon from organic moieties on the surface of solventless pyrolyzed Ni₃S₂ negatively affects HER more than OER. Likewise, supercapacitance is also affected by the residual surface carbon, and the best supercapacitance performance was shown by the NiS phase obtained by the decomposition of Ni-Xan in oleylamine. These results indicate careful designing of metal precursors, i.e. precursors that produce volatile by-products or very little residual carbon upon thermolysis. Designing suitable precursors will be a step forward in avoiding surfactants and enhancing the electrochemical performance of electrocatalysts.

Author contributions

G. E. Ayom: conceptualization, data curation, formal analysis, investigation, methodology, writing – original draft; M. D. Khan: validation, visualization, writing – review; R. Srivastava: data curation, writing – original; W. Lin: data curation, writing – original; R. K. Gupta: supervision, writing – review; N. Revaprasadu: funding acquisition, writing – review, supervision.

Conflicts of interest

There are no conflicts to declare.

Data availability

The authors declare that the data supporting the findings of this study are available within this article and the SI. TGA of complexes and complexes/TPP composites and their solventless decomposition p-XRD patterns. SEM/TEM images of solventless formed sulfides. EDX compositions of formed sulfides. XPS of prepared sulfides. Electrocatalytic/electrochemical characterization curves of sulfides and comparison tables of HER/OER and supercapacitance of the sulfides with other reported sulfides. See DOI: https://doi.org/10.1039/d5dt00703h.

Acknowledgements

This work is based on the research supported in part by the National Research Foundation of South Africa and Sasol Ltd (Ref Number SASPD22080147035).

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