INTRODUCTION
A fascinating class of nanomaterials with unique properties that arise due to their small size and high surface area-to-volume ratio. Nanoclusters are made up of a limited number of geometrically organized metal atoms, typically consisting of a few hundred atoms. These metal nanoclusters lie at the molecular level and differ from their bulk materials resulting in unique electrical, optical, magnetic, and catalytic properties. Metal nanoclusters are tiny clusters made up of metal atoms that possess quantum confinement effects [1–3]. The quantum confinement effect brings distinct properties to MNCs compared with their corresponding bulk nanomaterials. Metal-based nanomaterials such as Co, Ru, Ni, etc., with various structures, have been extensively studied for their energy storage behavior [4–8]. The 2D structures of nanomaterials possess high surface area which is accountable for their capacitance behavior. The capacitance performance of 1D smaller nanoclusters is not been studied systematically.
Nickel oxide (NiO) finds great significance and applications in diversified fields [9–11]. Nickel oxide (NiO) is prominently utilized for the design of supercapacitors, due to its good pseudocapacitive gusted [12], practical vacuity, environmentally benign nature, and lower cost [13–15]. with their high power-supplying capabilities for short-term and long-cycling performance, electrochemical capacitors, also called supercapacitors, which are used as secondary power sources and bias for the enhancement of cycle life in electrical vehicles and other mobile products, are drawing attention in recent times [16]. NiO has high resistivity, which is a serious disfigurement for supercapacitors. It's pivotal to enhance the electrode conductivity to enrich the energy and power density of electrodes. Still, the specific surface area of the NiO is in general not high enough for high capacitance [17–19]. The specific capacitance of an oxide material increases especially with an increase in its surface area because a large surface area is associated with further faradic active spots and in turn higher pseudo-capacitance [20–23]. The morphology of NiO has been greatly important for its electrochemical performance as morphology is nearly related to the surface area [24].
Nanostructures with large surface areas have been synthesized via surfactant-template, sol-gel, anodization, and hard template styles. Wang et al. reported the hallow NiO nanostructures synthesized through calcination for supercapacitor applications with the capacitance of 1200 F/g at a current density of 1 Ag–1 [17]. Xiao et al. reported the synthesis of hierarchically assembled NiO nanosheets through a hydrothermal route, which shows a capacitance of 81.67 F/g with a current density of 0.5Ag–1 [18]. Jahromi et al., describe the capacitance based on the particle sizes of NiO nanoparticles. The lower-size NiO nanoparticles with the size of 8 nm show a higher capacitance of 549 F/g at a current density of 1 A/g. The higher porous channel in 8 nm facilitates the electrolyte diffusion. NiO nanoparticles of 12 and 22 nm show comparatively lower capacitance [25].
External factors such as temperature, pH, solvents, and metal ions influence the bonding of NCs that form self-assembled structures and enrich the properties of nanoclusters. The solvents influence the aggregation of NCs to alter their properties. These factors also influence the supercapacitor nature of NiNCs. The polar protic and aprotic solvents and their mixture regulate the assembly of NCs thereby altering their porosity, which derives the supercapacitance nature of NCs [26]. The influence of binary and ternary solvents in the assembly of Ni nanoclusters and their supercapacitance behaviour was systematically examined in this work.
In the present work, DFT frontier orbital analysis of Ni nanoclusters (NiNCs) revealed strong electron delocalization across Ni, S, and O atoms, confirming effective ligand-to-metal charge transfer with HOMO (0.23306 a.u.) and LUMO (0.18493 a.u) energies, in the experimental analysis we utilizing a ternary solvent system, comprising methanol, water, and acetone was utilized, and the thiol-protected NiNCs were achieved. The sizes of NiO@Ni(II)SR-NCs obtained with methanol-water, acetone-water, and methanol-water-acetone were characterized using high-resolution transmission electron microscopy (HRTEM) studies. The smallest NiNCs found in binary systems and ternary systems were comprehensively characterized. Here, it is described how thiol-protected NiNCs grow in the presence of methanol, acetone, and methanol-water-acetone. As achieved NiNCs in ternary (methanol-acetone-water), and binary (methanol-water, acetone-water) systems were evaluated for supercapacitance studies. The mechanistic pathway for the varying supercapacitance of NiNCs in binary and ternary systems is discussed in this work.
EXPERIMENTAL METHODOLOGY
Chemicals and Reagents
Nickel(II)nitrate hexahydrate (99%, SRL chemicals extra pure), Sodium-3-mercapto-1-propane sulfonate (MPS) (80%, Fluka analytical), Sodium hydroxide pellets (98%, SRL Chemicals), Methanol (99.8%, MembaChem Industries Pvt. Ltd), Acetone (Sisco research laboratory Pvt. Ltd), Double distilled water (DDW) have been used for this work.
Instrumentation
UV-Vis spectrometer: The UV-Vis spectra of the isolated NiNCs in different solvent ratios of the samples were recorded with a fiber optic spectrometer with deuterium tungsten lamp (Hitachi 300), in the range from 200-1000 nm. Particle size analyzer (PSA): PSA measurements of the isolated NiNCs were recorded using the HORIBA Z-100 instrument. Thermal gravimetric analysis: The stability of the isolated NiO nanoclusters was observed by NET ZSCH-TGI. Atomic force microscopy: AFM were NiNCs recorded using Agilent topographical images in 2D and 3D structure and surface roughness measured. High-resolution tunning electron microscopy: JEOL Japan, JEM-2100 Plus.
Synthesis of NiNCs:
The NiNCs synthesis procedure was reported in our earlier literature [27]. As achieved, 5 mg of NiNCs sample (1:4) metal-to-ligand ratio was transferred to a sterilized 10 mL vial. To this sample, binary solvents of methanol-water, acetone-water, acetone-methanol system, and ternary system methanol-acetone-water were dissolved individually. Further, the isolated samples were taken in different ratios and centrifuged, then the isolated sample was dried using rotavapor. Finally, the powder was collected for material characterization.
Working Electrode Preparation for Supercapacitor Studies
The working electrode was constructed by grinding the sample with acetylene black and polytetrafluoroethylene (PTFE) in an 8:1:1 ratio, coating it with on Nickel foil, and drying it at room temperature. The weight of active electrode material ranged from 1.2 to 2.5 mg. The supercapacitor behaviour of NiNCs nanocomposites on Ni foil was investigated utilizing CV, galvanostatic charge-discharge(GCD), and electrochemical impedance spectroscopy(EIS) in 0.1 M KOH as an electrolyte. The GCD tests were carried out in a potential range of 0 to 0.55 V. The specific capacitances (Cs) were determined using the discharge time. The following formula is used to calculate specific capacitance.
where I is the applied current and Δt is the discharge duration in sec.
RESULTS AND DISCUSSION
The molecular orbital representation of the Ni–MPS complex Fig. 1 reveals the spatial distribution of electron density in the frontier orbitals. The red and green lobes correspond to the positive and negative phases of the wavefunction, respectively. Notably, strong orbital contributions are observed around the sulfur (–SH) and oxygen (–COOH) donor sites, confirming their active role in coordination with the nickel center. This highlights the electron-donating capacity of MPS, which stabilizes the Ni nanocluster surface by forming robust Ni–S and Ni–O linkages. The orbital delocalization across the ligand backbone indicates effective charge transfer interactions, which can enhance the electronic interaction between the metal core and the stabilizing ligand. Such orbital overlap is crucial in tuning the electron donating properties of Ni nanoclusters. Furthermore, the presence of extended density around Ni suggests moderate electrophilicity, consistent with the DFT-derived frontier orbital energies. Together, these insights demonstrate that MPS not only provides steric stabilization but also actively modulates the electronic environment of Ni nanoclusters respectively.
Fig. 2 Frontier molecular orbital plots of the Ni–MPS complex obtained from DFT calculations (B3LYP/3-21G) Fig. 2A. The highest occupied molecular orbital (HOMO, 0.23306 eV) shows strong electron density localized over the Ni center, with significant delocalization onto the coordinating sulfur and oxygen atoms of the MPS ligand, confirming the electron-donating ability of MPS. This orbital overlap around the sulfur (-SH) and oxygen (-COOH) donor sites confirm their active role in coordination with the nickel center and highlights the robust electron-donating capacity of the MPS ligand Fig. 2B. The lowest unoccupied molecular orbital (LUMO, 0.18493 eV) exhibits charge distribution across both the Ni core and ligand framework, suggesting efficient ligand-to-metal charge transfer. The small HOMO–LUMO energy gap highlights the favourable electronic transitions and chemical stability of the Ni–MPS complex. These results demonstrate that MPS not only stabilizes Ni nanoclusters through Ni–S and Ni–O coordination but also tunes their electronic structure, thereby enhancing potential applications in energy storage. Based on these results, we proceeded with experimental studies, and the findings are presented below.
As prepared crude NiNCs using a 1:4 metal-to-ligand ratio resulted in a maximum of ~512 F/g at 1 A/g as reported in our earlier work [27]. These results motivated us to work further to isolate smaller nickel-based nanoclusters using solvent fractional techniques. To investigate the performance of individual NiNCs towards supercapacitor activity. From the literature reports, it is obvious that the crystal structure, size, morphology, and ligand-building strength in NiNCs change with the different solvent mediums of varying dipole moments and their interaction in nanoclusters [27]. As a follow-up, we tried to improve the specific capacitance by varying the solvent systems with a metal-to-ligand ratio of 1:4. The growth mechanism of NiNCs and its supercapacitor activity for binary and ternary solvents systems were investigated.
As prepared NiNCs in the binary and ternary mixture are discussed in detail in the experimental section as shown in Scheme 1. The schematic images confirm that synthesized NiNCs showed varying colors and solubility in different ratios of binary and ternary solvent medium. Further, the isolated NiNCs were characterized using UV-visible spectroscopy. Fig. SI. 1A The image shows isolated NiNCs in different ratios of methanol and water. Fig. SI. 1B shows UV-visible absorption spectra of the isolated NiNCs in different ratios of acetone and water systems. Here, we observed that the UV-Visible spectrum of core isolated NiNCs gives three absorption bands at 340, 417, and 553 nm, the 340 nm corresponding to transition, 417 nm is πdx2-y2 ← pп (charge transfer), and 553 nm d-d transition of isolated NiNCs which is consistent with our previous reported.
Fig. 3 shows the spectrum of isolated NiNCs in different ratios of methanol and water. As the water moiety increases, the smallest clusters are isolated from the crude mixture, proportionally the larger NiNCs tend to isolate in high concentrations of methanol moiety. i.e., at 80% of methanol there clusters result in low absorption as compared to 30% of methanol owing to the binary mixture of methanol: water system which gives rise to the bands at 260, 340, 417, and 553 nm, corresponding to MPS ligand charge transfer peak, σ transition, charge transfer, and metal charge transfer respectively. On knowing the impact of binary solvent systems i.e., acetone: water systems, we tried to investigate the behaviour of these NiNCs in a ternary system of methanol: acetone: water medium. Fig. SI. 2A spectrum shows the images of isolated NiNCs in ternary solvent systems with different ratios of acetone-methanol-water (A: M: W). Fig. SI. 2B shows the corresponding UV-visible spectral of giving rise to a similar absorption band at 260 nm, 340 nm, 417 nm, and 553 nm. In comparison, the ternary solvent systems increased the hydrophobic nanoclusters in the medium where these NiNCs are present. As the second nonpolar solvent either acetone or methanol, the system loses its hydrophilic microstructure from water molecules which stabilize NiNCs much better than the other two acetone and methanol microstructure thereby resulting in better absorption peak plateau in binary solvents involving water microstructure, among them methanol: water system seems that good choice of medium.
The NiNCs that resulted in highly intense absorption bands were further examined for morphological studies. The scanning electron microscopy analysis (SEM) confirms that isolated NiNCs. Wherein the NiNCs methanol: water system resulted in rose petals morphology as shown in Fig. SI. 3A, aggregated spherical morphology NiNCs acetone: water system as shown in Fig. SI. 3B and Fig. SI. 3C shows the SEM image of NiNCs in methanol, acetone, and water, the aggregation spherical morphology of isolated NiNCs. At larger sizes, NiNCs exhibit varying morphology for different solvent systems of acetone: water, methanol: water, and methanol: acetone: water. The morphological changes of nanoclusters resulted in the binding characteristic of surface capping ligands with varying dipole moments of solvent systems.
The HRTEM analysis confirmed the ultrasmall size of the NiNCs in the binary solvents of methanol-water (M: W) shows cubic particle aggregates in the size range of 1-9 nm for M: W systems as shown in Fig. 4A. The SAED pattern of isolated NiNCs as shown in Fig. 4B suggest a highly crystalline nature of cubic morphology with miller indices of 050, 080, 100, 002, 111, 200 and 220 planes of NiO species in both NiNCs which are in agreement with the earlier reports [27]. Fig. 4C shows the spherical morphology of isolated NiNCs in the A: W system in the size range of 1-5 nm. Fig. 4D shows the SAED pattern of the A: W system, it confirms the semi-crystalline nature. Fig. 4E shows the HRTEM image of a ternary solvent system i.e. methanol: water: acetone, it shows that polydisperse particles of varying shapes like cubic, spherical, and nanoflakes, etc., which fall in the size range of 1-20 nm. Fig. 4F shows the SAED pattern of nanoclusters in a ternary solvent system confirming the high crystalline nature. The overall HRTEM indicates that the methanol solvent-based isolated NiNCs show a higher crystallinity and different morphology, in other studies, acetone-based isolated NiNCs confirm the uniform morphology and size.
As prepared NiNCs were analyzed by the DLS technique, Interestingly, Fig. SI. 4A shows the particle size of crude NiNCs shows the particle size of isolated NiNCs within the size range of 20-90 nm. Fig. SI. 4B) acetone: methanol: water, Fig. SI. 4C) methanol: water, and Fig. SI. 4D) acetone: water medium were within the range 1-20, 4-8 nm, and 1-3 nm respectively. In the overall DLS studies, it concluded that the isolated NiNCs give the lowest particle size and higher mono-dispersity lesser aggregation in acetone: water medium as compared to methanol: water and acetone: methanol: water medium. The surface topography and morphology of isolated NiNCs were examined using advanced imaging techniques like atomic force microscopy (AFM) studies. Fig. SI. 5 shows the AFM images and histogram diagram of isolated NiNCs in [A & B] methanol and water, [C & D] acetone and water, and [E & F] methanol, acetone, and water system respectively. Fig. SI. 6 confirms that the 3D images of isolated NiNCs in (A) methanol: water, (B) acetone: water, and (C) acetone: methanol: water respectively. The overall AFM images confirm that NiNCs having spherical morphology and aggregation occurred in all isolated samples. The 3D images confirm that the isolated NiNCs acetone and water isolated sample shows smooth and uniform surfaces compared to other binary (methanol: water) and ternary (Acetone: methanol: water) solvents. It is obvious that isolated NiNCs in methanol: water medium is more aggregated as compared to acetone and water systems. We could observe sharp conical surfaces in isolated NiNCs in acetone: water medium which suggests staple modification over core NiNCs, which is in contact with our earlier report. In ternary solvent (A:M: W) medium, the staples motif of NiNCs over core NiO nanoclusters are more dominant as the organic microstructure is richer than the aqueous microstructure.
The thermal stability of isolated NiNCs was evaluated using thermogravimetric analysis (TGA). Fig. SI. 7 shows that TGA spectra of isolated NiNCs, the sample has thermal stable mass retain (40.7%) after 700°C. Before applying the temperature, 100% sample was loaded as the temperature was slowly increased from room temperature to high temperature the mass loss occurred from 25°C to 255°C, 13% mass loss, and further the temperature was raised from 255°C to 304°C resulted in 5% mass loss, 304°C to 413°C is 27% weight loss, 413°C to 569 is 9.4% losses and further increases the temperature up to 700°C the mass loss was 4.9% above 700°C the NiNCs sample retain 40.7% of the mass of isolated NiNCs. TGA analysis confirms that the isolated NiNCs show a robust nature and the differential thermal analysis (DTA) spectra here we observed that 25 to 268°C have an exothermic reaction and further the temperature increases it’s changes to the endothermic mode due to their mass loss.
Electrochemical Analysis
The electrochemical behavior for synthesized NiNCs was performed using 0.1 M KOH electrolyte medium, here we performed cyclic voltammetry (CV) analysis of isolated NiNCs. Fig. 5 illustrates the compressive results of the electrochemical performance of isolated NiNCs in binary and ternary solvents medium at the fixed scan rate of 50 mV/s respectively. Fig. 5A shows the CV response for the stable isolated NiNCs in acetone and water system at the ratio of 50:50 percentage, 5B shows the CV response for isolated NiNCs in methanol and water system for 50:50 percentage ratio, 5C shows the CV response for isolated NiNCs in ternary solvents for 30:35:35 percentage ratio respectively. As seen in Fig. 5, the CV curves for the isolated NiNCs show the redox behaviour with the cathodic and anodic peaks at ~0.15 V and ~0.28 V respectively, for A: W ~0.09 V and ~0.2 V respectively for M: W binary solvent systems for the ternary solvents (A: M: W) isolated NiNCs gives a high cathodic peak and anodic peak current at ~0.12 V and ~0.31 V respectively. On comparing the particle size and structure of isolated NiNCs from binary (M: W and A: W) and ternary (A: M: W) solvent systems. The electrochemical behaviour of the NiO@Ni(II)SR nanoclusters, as illustrated in the cyclic voltammetry (CV) profiles in Fig. 5, is dominated by reversible faradaic redox transitions. The distinct cathodic and anodic peaks observed at potentials between ~0.09 V and ~0.31 V (vs. Ag/AgCl) correspond to the Ni2+/Ni3+redox couple. This mechanism typically involves the following reaction in the alkaline (0.1 M KOH) electrolyte:
The presence of the thiol-protected staples and the ultra-small particle size (1–9 nm) significantly enhances this process by providing a higher density of surface-active sites for OH-ion adsorption and insertion. The effective ligand-to-metal charge transfer, confirmed by DFT analysis, further facilitates rapid electron transport during these transitions, resulting in the high specific capacitive nature.
The cyclic voltammetry (CV) profiles of NiNCs in acetone:water systems with varying solvent ratios (30–80%) at different scan rates (5–100 mV/s) demonstrate characteristic pseudocapacitive behavior as shown in Fig. 6A–F ((A) 30%, B) 40%, C) 50%, D) 60%, E) 70% and F) 80% respectively). The nearly rectangular CV shapes with redox humps indicate the coexistence of electric double-layer capacitance (EDLC) and faradaic charge storage. At lower scan rates (5–25 mV/s), distinct redox peaks are observed, confirming fast and reversible faradaic reactions associated with Ni2+/Ni3+ redox transitions. With increasing scan rate (50–100 mV/s), the CV curves maintain their shape with slight distortion, suggesting good rate capability and efficient ion diffusion at the electrode–electrolyte interface.
The variation in solvent composition strongly affects the current response, reflecting differences in cluster size, surface area, and electrolyte accessibility. Notably, NiNCs synthesized at 40% acetone: 60% water exhibited the highest current density and better-defined CV responses, correlating with superior capacitance, similarly the Fig. 7 shows the CV responses isolated NiNCs in ternary solvent resulting in different scan rates. Fig. 7A shows the CV response for NiNCs isolated in methanol, acetone, and water system at the ratio of 35:35:30 percentage Fig. 7B confirms the stability of replicated peaks at the same positions of continuous 1000 cycles at a fixed scan rate of 100 mV/s. Fig. 7C shows the CV response for NiNCs isolated in methanol, acetone, and water system for a 30:30:40 percentage ratio, Fig. 7D confirms the stability of replicated peaks at the same positions of continuous 1000 cycles at a fixed scan rate of 100mV/s. Fig. 7E shows the CV response for NiNCs isolated in ternary solvents for a 25:25:50 percentage ratio and Fig. 7F confirms the stability of replicated peaks at the same positions of continuous 1000 cycles at fixed scan rates of 100 mV/s respectively. Fig. SI. 8 shows the effect of scan rate responses for isolated NiNCs in Fig. SI. 8A 30% of acetone and 70% of water, Fig. SI. 8B 40% of acetone and 60% of water, Fig. SI. 8C 50% of acetone and 50% of water, Fig. SI. 8D 60% of acetone and 40% of water, Fig. SI. 8E 70% of acetone and 30% of water and Fig. SI. 8F 80% of acetone and 20% of water respectively.
The overall specific capacitance measured for the different scan rates like 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 mV and the corresponding solvent medium is shown in the Table 1. In Table 1 suggests that a 40:60 percentage ratio (Methanol: Water) gives a high specific capacitance value of ~796 F/g–1 compared to other nanoclusters. Similarly, the specific capacitance of NiNCs from the different ratios of acetone and water-isolated are shown in Table SI 1. Further, the CV responses and the overall specific capacitance of isolated NiNCs obtained from the different ratios of acetone, methanol, and water are shown in Table SI 2 respectively.
Charge-discharge analysis:
To further understand the electrochemical super capacitive behaviour of isolated NiNCs obtained from the binary and ternary solvent medium, were employed for the potentiometric method used to find the specific capacitance of isolated NiNCs binary and ternary solvents are investigated. The discharge curves of NiNCs electrode material isolated in methanol: water medium on the variation of current densities from 1 Ag–1 to 20 Ag–1is shown in Fig. 8. In the discharge curve, the NiNCs exhibit high specific capacitance of 801 Fg–1 at 1 Ag–1. The specific capacitance responses for the NiNCs for varying current density exhibits reliable specific capacitance of 801 Fg–1, 752 Fg–1, 496 Fg–1, 368 Fg–1, 298 Fg–1, and 250 Fg–1 at the different current density of 1 Ag–1, 2 Ag–1, 3 Ag–1, 4 Ag–1, 5 Ag–1 and 10 Ag–1 respectively. The high specific capacitance achieved for NiNCs may be owing to the insertion of ions at the time of redox potential between the working electrode and electrolytes. The stability for specific capacitance in NiNCs was further confirmed by continuous 5000 cycles as shown in Fig. 8B. The charge and discharge for continuous 5000 cyclic retention of almost 95% of the capacitance at a current density range of 20 Ag–1 as shown in Fig. 8C. The specific capacitance that was measured methanol: water systems in discharge analysis as compared in Table 2. The same methodology has been followed by acetone: water and ternary solvents effects, the comparison of different % of acetone: water system confirmed by Table SI 3. The mixture of ternary solvent isolated NiNCs specific capacitance was mentioned in Table SI 4.
The above findings on the effect of synthesis solvent medium for NiNCs formation and their electrochemical characterization were presented in the form of a three-three-solvent system phase diagram. This phase diagram, confirms that at which % ratio of solvent produced the high specific capacitance. The microstructure of solvent molecule presence towards high specific capacitance values. Thereby, we have to find the fine-tuned material for device-fabricated product development in the near future. The entire supercapacitance responses of NiNCs for binary systems (methanol: water and acetone: water) and ternary systems (methanol: acetone: water) were presented in a three-component phase diagram system as shown in Fig. 9.
The exceptionally high specific capacitance observed in the synthesized NiNC electrode materials reaching up to ~804 F/g for the 40:60 methanol-water system can be attributed to the synergistic effect of several structural and electronic factors. First, the ultrasmall particle size (1–5 nm in acetone-water and 1–9 nm in methanol-water) provides a massive electrochemically active surface area. This maximizes the number of faradaic active sites available for redox reactions. Second, the DFT analysis confirms strong electron delocalization across the Ni, S, and O atoms, which facilitates efficient ligand-to-metal charge transfer. This electronic environment lowers the charge transfer resistance at the electrode-electrolyte interface. Finally, the pseudocapacitive behaviour is driven by the reversible Ni2+/Ni3+ redox transitions, where the porous nature of the nanocluster aggregates facilitates rapid electrolyte ion diffusion. The stability provided by the MPS thiol “staples” ensures that these clusters maintain their structural integrity over thousands of cycles, resulting in the high cyclic retention of 95%.
The Nyquist impedance spectrum of the Ni nanoclusters (NiNCs) electrode displays a distinctive pattern, characterized by a minor intercept on the real axis in the high-frequency domain, succeeded by an inclined line in the low-frequency domain. This behaviour can be accurately represented by an equivalent circuit model comprising solution resistance (Rs) and charge transfer resistance (Rct). The Rs and Rct values can be evaluated to be 0.7 and 49 ohm respectively. The elevated frequency intercept on the Z′ axis correlates with a comparatively low Rs value, signifying negligible electrolyte resistance and favourable ionic conductivity at the electrolyte–electrode interface. The lack of a clearly defined semicircle or its minimal diameter indicates a low Rct, signifying rapid charge transfer kinetics at the NiNCs/electrolyte interface, attributable to the nanoclusters smaller size, extensive surface area, and many active sites. The low Rs and Rct values, along with advantageous diffusion characteristics, indicate that the NiNCs electrode exhibits superior electrical conductivity and alters electron/ion transport properties, rendering it highly appropriate for electrochemical sensing, energy storage, and catalytic applications.
CONCLUSIONS
The supercapacitance activity of NiNCs was systematically investigated in binary and ternary solvent systems. HRTEM confirmed that solvent polarity strongly influences nucleation and growth, yielding particle sizes of 1–9 nm (methanol:water), 1–5 nm (acetone:water), and 1–20 nm (methanol:acetone:water). Electrochemical studies revealed tunable capacitance with solvent composition, achieving 804 F/g (methanol: water, 40:60), 745 F/g (acetone:water, 60:40), and 778 F/g (methanol:acetone:water) at 1 A/g. Phase diagram analysis further demonstrated the role of solvent environment in nanocluster stabilization and energy storage behavior. Complementary DFT frontier orbital analysis showed strong electron delocalization across Ni, S, and O atoms with a narrow HOMO–LUMO gap, confirming effective ligand-to-metal charge transfer and favorable electronic transitions. Together, the experimental and theoretical results establish that solvent-directed synthesis enables control over NiNCs size, structure, and electrochemical performance, while electronic insights from DFT highlight the fundamental role of metal–ligand interactions in governing their stability and energy storage characteristics.
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