Plasma-modified flower-like CuFe–Ni₃S₂/Nickel foam heterostructure as a robust bifunctional electrocatalyst for high-efficiency overall water splitting

2.1. Sample preparation

All chemical reagents were of analytical grade and used as received: copper nitrate trihydrate (Cu(NO₃)₂·3H₂O, 99%, Nantong Feiyu Biological Co., Ltd.), iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O, 98%, Xilong Scientific Co., Ltd.), urea (CH₄N₂O, 99%, Jiangsu Qiangsheng Functional Chemical Co., Ltd.), sodium sulfide nonahydrate (Na₂S·9H₂O, 98%) and NF (99%) from Shanghai Aladdin Chemical Technology Co., Ltd. Deionized water (18.2 MΩ·cm) was employed for all experimental procedures to ensure reagent purity. To accommodate the reactor’s diameter of ∼3 cm, the NF was sectioned into 3 cm × 3 cm pieces to ensure complete solution contact during reactions. Prior to use, the NF substrates were ultrasonically cleaned in 3 M hydrochloric acid for 30 min to remove surface oxides and impurities, followed by thorough rinsing with deionized water and overnight drying.

The CuFe precursor was synthesized on the pretreated NF substrate through a hydrothermal method. Specifically, 0.11 g of Cu(NO₃)₂·3H₂O and 0.43 g of Fe(NO₃)₃·9H₂O were dissolved as metal sources along with 0.36 g of urea in 40 mL deionized water (18.2 MΩ·cm) under vigorous stirring to obtain a homogeneous solution. The NF substrate immersed in this solution was subjected to hydrothermal treatment at 120°C for 5 h in a sealed autoclave. After natural cooling to room temperature, the resulting sample was thoroughly rinsed to remove residual reactants and dried at 60°C for 12 h to obtain the CuFe precursor/NF composite.

Subsequently, a 0.1 M aqueous solution of Na₂S·9H₂O was prepared and transferred into the autoclave containing the precursor. The hydrothermal reaction was conducted at 160°C for 8 h, yielding black-colored CuFe-Ni₃S₂/NF nanosheets. The product was carefully retrieved using Teflon-coated forceps, followed by sequential washing with deionized water and ethanol (three cycles each), and finally dried at 70°C for 12 h. The optimal-performing CuFe-Ni₃S₂/NF sample was selected for plasma modification based on preliminary characterization. The square specimen (3 × 3 cm²) was positioned in a quartz tube reactor (DBD-100B, Nanjing Suman Electronics Co., ltd., China) and subjected to DBD plasma generator treatment (CTP-2000 K, Nanjing Suman Electronics Co., ltd., China) under atmospheric conditions. The plasma treatment involves exposing the catalyst precursor to air plasma, in which highly energetic ions and radicals physically etch the surface to create defects while simultaneously functionalizing it with oxygen-containing groups. This dual effect increases both active site density and hydrophilicity, crucial for electrocatalytic reactions. The plasma’s unique advantages of being rapid, energy-efficient, and avoiding high-temperature agglomeration were highlighted [21]. This process generated reactive oxygen and nitrogen species that functionalized the material surface, producing the modified PA@CuFe-Ni₃S₂/NF composite. Comparative characterization was then performed to evaluate the morphological and physicochemical property evolution.

For comparative analysis, a control sample of Ni₃S₂/NF was synthesized under identical conditions. The preparation procedure involved immersing the pretreated NF substrate in a 0.2 M aqueous solution of sodium sulfide nonahydrate (Na₂S·9H₂O), followed by hydrothermal treatment at 160°C for 8 h. After natural cooling to room temperature, the product was purified through sequential washing with deionized water and ethanol, and subsequently dried at 70°C for 12 h. The catalytic performance of this reference Ni₃S₂/NF was systematically compared with the CuFe-doped Ni₃S₂/NF nanosheets to evaluate the doping effects.

2.2. Material characterization

The crystalline phase composition of the synthesized materials was characterized by X-ray diffraction (XRD, Bruker D8 Advance) with Cu-Kα radiation. Diffraction patterns were collected in the 2θ range of 20-80° with a scanning rate of 5°/min. Field emission scanning electron microscopy (FESEM, Zeiss Gemini SEM 300), transmission electron microscopy (TEM, Tecnai G2 F20), and energy dispersive spectrometer (EDS) element mapping were used to examine the morphology and microstructure of the materials. Surface chemical states were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha) using monochromatic Al-Kα radiation. The acquired spectra were deconvoluted using Gaussian-Lorentzian fitting to determine the valence states of constituent elements.

2.3. Electrochemical measurements

The electrochemical performance of all samples was systematically evaluated in a three-electrode configuration using 1 M KOH electrolyte on a CHI 660E electrochemical workstation manufactured by Shanghai Chenhua Instrument Co., Ltd. The as-prepared PA@CuFe-Ni₃S₂/NF nanosheets functioned as the working electrode, with a graphite rod counter electrode and Ag/AgCl reference electrode completing the cell assembly. Prior to measurements, the catalysts were activated through 20 cyclic voltammetry (CV) cycles at 100 mV s⁻¹ between 0-0.6 V vs. reversible hydrogen electrode (RHE). HER/OER were characterized by linear sweep voltammetry (LSV) at 5 mV s⁻¹ with 85% iR compensation. Kinetic analysis was performed using Tafel plots derived from the polarization curves. Electrochemical impedance spectroscopy (EIS) measurements were conducted from 10 kHz to 0.01 Hz with a 10 mV AC perturbation to determine charge transfer resistance (Rct). The electrochemically active surface areas (ECSA) was quantified via double-layer capacitance (Cdl) measurements, obtained from CV scans at varying rates in the non-Faradaic region. Durability was assessed through extended chronoamperometry (≥24 h) and accelerated stability testing (1000 CV cycles). For overall water splitting evaluation, a two-electrode configuration was employed where PA@CuFe-Ni₃S₂/NF served simultaneously as both anode and cathode. All reported potentials were converted to the RHE scale using the Nernst equation.

3.1. Synthesis and morphology characterization of PA@CuFe-Ni₃S₂/NF

Figure 1 presents a schematic illustration of the three-step synthesis procedure for PA@CuFe-Ni₃S₂/NF. The catalyst precursor is modified by the air plasma treatment through synergistic physical and chemical effects, such as highly energetic ions and radicals simultaneously etch the surface to create defects, while concurrently generating sulfur vacancies for electronic structure optimization. This multifunctional modification significantly boosts both active site density and interfacial properties critical for electrocatalytic performance [22]. This preparation method demonstrated both simplicity and cost-effectiveness while maintaining high efficiency. The initial step involved the hydrothermal deposition of CuFe precursors onto NF, with the characteristic yellow coloration arising from the presence of iron nitrate nonahydrate. Subsequently, the CuFe precursor underwent a sulfurization process mediated by a sulfur source, yielding CuFe-Ni₃S₂/NF nanosheets. During this stage, the NF served dual functions as both a nickel source and structural support, with the product exhibiting a distinct black coloration. The final step entailed plasma modification under ambient atmospheric conditions, which induced a morphological transformation from nanosheet arrays to robust flower-like architectures, thereby significantly improving the catalytic stability.

Close

Figure 1.

Schematic illustration of the preparation of the plasma-modified CuFe-Ni₃S₂/NF flower-like nanosheet arrays.

Export to PPT

It is well established that the catalytic performance of HER and OER is strongly influenced by the surface morphology of the catalyst. To investigate this relationship, the morphology of PA@CuFe–Ni₃S₂/NF was systematically analyzed and compared with control samples using scanning electron microscope (SEM). As shown in Figure 2(a), pristine NF displayed a smooth, interconnected network structure devoid of secondary micro/nanostructures, providing an ideal substrate for subsequent material synthesis. In contrast, the Ni₃S₂/NF reference sample (Figure 2b), obtained via direct sulfidation of NF, exhibited a flower-like microarchitecture composed of aggregated nanosheets grown on the NF framework. Upon incorporation of Cu and Fe, the morphology of CuFe-Ni₃S₂/NF underwent a distinct transformation, evolving into vertically aligned nanosheets that uniformly encapsulate the porous NF scaffold (Figures 2c,d). This well-defined nanostructure not only maximized catalyst-electrolyte contact but also ensured efficient mass transport, owing to its highly accessible open framework. These findings confirmed that elemental doping effectively modulated the catalyst’s morphological properties. Further morphological evolution was observed following plasma modification. As illustrated in Figures 2(e,f), the initial nanosheets reorganized into an interconnected 3D flower-like superstructure. The flower-like nanosheet architecture synergistically enhances both HER and OER by providing high surface area and edge sites for reaction while facilitating rapid bubble release during the water splitting reaction. Importantly, the robust integration of this 3D nanosheet assembly with the NF backbone ensured exceptional structural stability under prolonged catalytic operation. To confirm the successful synthesis of PA@CuFe-Ni₃S₂/NF, energy dispersive x-ray spectroscopy (EDX) was performed on the flower-like nanosheets (Figure 2g). The results confirmed the homogeneous distribution of Cu, Ni, Fe, O, and S, consistent with the intended composition. Collectively, the SEM and EDX analyses demonstrated that both elemental doping and plasma modification induced significant morphological restructuring, which was expected to contribute to enhanced electrocatalytic performance.

Close

Figure 2.

SEM images of (a) NF, (b) Ni₃S₂/NF, (c-d) CuFe-Ni₃S₂/NF, (e-f) PA@CuFe-Ni₃S₂/NF and SEM-EDX spectrum of PA@CuFe-Ni₃S₂/NF nanosheet arrays, (g) SEM-EDX spectrum of PA@CuFe-Ni₃S₂/NF nanosheet arrays.

Export to PPT

To elucidate the morphological and structural properties of the plasma-modified samples, the PA@CuFe–Ni₃S₂/NF catalyst was subjected to ultrasonic treatment to detach it from the NF substrate, followed by dispersion in ethanol for TEM analysis. Upon initial observation of the low-resolution transmission images (Figure 3a), it was further confirmed that PA@CuFe–Ni₃S₂/NF exhibited overlapping nanosheet structures. As is well-established in crystallography, lattice spacing served as a direct indicator of crystallographic plane orientation. High-resolution TEM (HRTEM) analysis of the flower-like nanosheets (Figure 3b) exhibited well-defined lattice fringes with a measured interplanar distance of 0.165 nm. This value corresponded to the (122) crystallographic plane of PA@CuFe–Ni₃S₂/NF, as verified by comparison with standard reference data (JCPDS 44-1418) and further supported by fast Fourier transform (FFT) analysis. To comprehensively characterize the elemental composition and spatial distribution within the catalyst, TEM-EDX spectroscopy was employed in conjunction with high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The TEM-EDX spectra (Figure 3c) conclusively demonstrated the coexistence of all expected constituent elements in the PA@CuFe-Ni₃S₂/NF composite. More significantly, the elemental mapping analysis (Figures 3d-h) revealed a homogeneous distribution of these components throughout the flower-like nanosheet architecture.

Close

Figure 3.

(a) TEM image of PA@CuFe-Ni₃S₂/NF nanosheet. (b) The high-resolution TEM image of PA@CuFe-Ni₃S₂/NF nanosheet. The blue rectangle shows the enlarged images of the selected region. (c) TEM-EDX spectrum. (d-h) The HADDF image and EDX mapping of Cu, Fe, Ni and S from PA@CuFe-Ni₃S₂/NF nanowire.

Export to PPT

XRD analysis was conducted to investigate the crystallographic properties of the synthesized catalysts across a 2θ range of 20°-80° (Figure 4a). The diffraction pattern of pristine NF exhibited characteristic peaks at 44.6° (111), 52.1° (200), and 76.5° (220), consistent with the face-centered cubic structure of metallic nickel (JCPDS 04-0850) [23-25]. The Ni₃S₂/NF sample demonstrated additional distinct diffraction peaks at 21.9° (101), 31.3° (110), 37.8° (003), 49.8° (113), and 55.3° (300), which were indexed to the hexagonal phase of Ni₃S₂ (JCPDS 44-1418) [26,27], confirming successful sulfidation of the NF substrate. Notably, the XRD pattern of CuFe-Ni₃S₂/NF showed no significant shift in peak positions compared to Ni₃S₂/NF, suggesting preservation of the host lattice structure after CuFe doping. The absence of distinct Cu or Fe-related diffraction peaks implies that these dopants either exist in amorphous phases or are incorporated substitutionally within the Ni₃S₂ lattice, as supported by complementary characterization results. Remarkably, plasma treatment yielded PA@CuFe-Ni₃S₂/NF with identical peak positions to its precursor, confirming structural integrity preservation during plasma modification. The observed enhancement in diffraction peak intensities indicates improved crystallinity, likely resulting from plasma-induced atomic rearrangement and defect healing [28,29]. This crystallinity improvement may contribute to enhanced catalytic performance by facilitating charge transport within the material. The enhancement occurs through multiple mechanisms as follows: the localized heating is generated by the plasma-mediated energy transfer at the catalyst surface, promoting atomic rearrangement and reducing amorphous regions, while air plasmas can introduce beneficial dopants or vacancies that facilitate lattice reorganization [30,31].

Close

Figure 4.

(a) XRD patterns of NF, Ni₃S₂/NF, CuFe-Ni₃S₂/NF, and PA@CuFe-Ni₃S₂/NF. (b) XPS survey spectra of PA@CuFe-Ni₃S₂/NF, XPS spectra of (c) Cu 2p, (d) Fe 2p, (e) Ni 2p, (f) S 2p regions.

Export to PPT

XPS analysis confirmed the elemental composition and chemical states of PA@CuFe-Ni₃S₂/NF (Figure 4b). The survey spectrum revealed characteristic signals corresponding to Cu, Ni, Fe, S, and O, along with adventitious carbon arising from atmospheric exposure during sample handling. The detected carbon and oxygen species were attributed to surface adsorption within the 1-2 nm probing depth of XPS analysis. All spectra were calibrated using the C 1s reference at 284.8 eV prior to detailed peak fitting. High-resolution XPS analysis provided detailed chemical state information for each element. The Cu 2p spectrum (Figure 4c) displayed two spin-orbit doublets at binding energies of 932.1/952.1 eV and 934.4/954.7 eV, corresponding to Cu⁺ and Cu²⁺ species, respectively. The characteristic 20 eV separation between Cu 2p₃_/₂ and Cu 2p₁_/₂ peaks, along with prominent satellite features, confirmed Cu²⁺ as the dominant oxidation state [32]. Similarly, the Fe 2p spectrum (Figure 4d) exhibited peaks at 711.6 eV (Fe 2p₃/₂) and 725.4 eV (Fe 2p₁/₂) with associated satellite structures, indicative of Fe²⁺ species [33]. The Ni 2p spectrum (Figure 4e) showed characteristic doublets at 855.9 eV (Ni 2p₃_/₂) and 873.6 eV (Ni 2p₁_/₂), consistent with Ni²⁺ in the Ni₃S₂ matrix [34,35]. The S 2p spectrum (Figure 4f) revealed two components at 161.8 eV (S 2p₃_/₂) and 162.9 eV (S 2p₁_/₂), characteristic of S²⁻ in metal sulfides [36-38]. An additional peak at 168.6 eV was assigned to oxidized sulfur species, likely resulting from surface sulfate or sulfite formation upon air exposure [39,40]. These comprehensive XPS results, combined with previous structural characterization, conclusively demonstrated the successful synthesis of PA@CuFe-Ni₃S₂/NF with its unique 3D flower-like architecture grown on NF.

3.2. Hydrogen evolution reaction

The electrocatalytic HER performance of PA@CuFe–Ni₃S₂/NF flower-like nanosheets was systematically evaluated in a standard three-electrode configuration. Before electrochemical measurements, all catalysts were activated through 20 cycles of CV at 100 mV s⁻¹ to achieve stable surface states. As depicted in Figure 5(a), the LSV curves obtained at 5 mV s⁻¹ revealed that the plasma-modified CuFe-Ni₃S₂/NF nanosheets exhibited superior HER activity, as evidenced by their lower onset potential and steeper current increase. Quantitative comparison of catalytic performance was conducted based on the overpotentials derived from LSV measurements. Figure 5(b) demonstrated that bare NF showed negligible catalytic activity, requiring an overpotential of 287 mV to achieve 10 mA cm⁻². Sulfidation treatment significantly enhanced the performance, with Ni₃S₂/NF exhibiting a reduced overpotential of 196 mV. Remarkably, both CuFe-Ni₃S₂/NF and PA@CuFe-Ni₃S₂/NF catalysts demonstrated substantially improved catalytic efficiency, achieving current densities of 10 mA cm⁻² at overpotentials of only 155 and 125 mV, respectively. These results unambiguously confirmed the synergistic effects of bimetallic doping and plasma modification in optimizing the electrocatalytic properties. The incorporation of Cu and Fe species effectively modulated the electronic structure of Ni₃S₂, thereby optimizing hydrogen adsorption energetics, while the plasma treatment created abundant phase boundaries that facilitated charge transfer processes. Consequently, the PA@CuFe-Ni₃S₂/NF catalyst demonstrated competitive performance compared with recently reported transition metal-based HER electrocatalysts (Figure 5c). To elucidate the reaction kinetics of the catalysts, Tafel slopes were calculated from the corresponding polarization curves. As shown in Figure 5(d), PA@CuFe-Ni₃S₂/NF exhibited a Tafel slope of 118 mV dec⁻¹, while CuFe-Ni₃S₂/NF displayed a slightly higher value of 134.5 mV dec⁻¹. Both values were significantly lower than those of bare NF and Ni₃S₂/NF, indicating enhanced HER kinetics.

Close

Figure 5.

HER performance of the prepared materials in 1.0 M KOH solution. (a) polarization curves with 85% iR compensation. (b) Comparison of overpotentials for various catalysts at current densities of 10 mA cm⁻² and (c) Comparison of overpotential with other recently reported efficient electrocatalysts (d) the Tafel plots.

Export to PPT

The Tafel slope of PA@CuFe-Ni₃S₂/NF suggested that the HER process followed the Volmer-Heyrovsky mechanism, with hydrogen desorption as the rate-determining step. The 3D hierarchical architecture of PA@CuFe-Ni₃S₂/NF not only provided abundant active sites but also facilitated mass and charge transfer, thereby accelerating the reaction kinetics [41]. These results demonstrated that the PA@CuFe-Ni₃S₂/NF catalyst possesses exceptional electrocatalytic activity, making it a highly promising candidate for HER applications.

CV measurements were systematically performed within non-Faradaic potential windows at varying scan rates to evaluate the electrochemical Cdl, which serves as a quantitative indicator of the ECSA. As illustrated in Figure 6(a), the linear slope of current density versus scan rate plots yielded the Cdl values for each catalyst. Comparative analysis revealed that bare NF and Ni₃S₂/NF exhibited minimal Cdl values, whereas CuFe-Ni₃S₂/NF demonstrated a substantial 68.3 mF cm⁻² enhancement relative to Ni₃S₂/NF (10.7 mF cm⁻²). Subsequent plasma modification further increased the Cdl by an additional 35.7 mF cm⁻².

Close

Figure 6.

(a) Estimation of C_dl by plotting the current density variation (Δj = (ja-jc)/2). (b) Nyquist plots of various catalysts. (c) HER polarization curves of PA@CuFe-Ni₃S₂/NF before and after 1000 CV tests. (d) Chronoamperometric plot of PA@CuFe-Ni₃S₂/NF material for 20 h.

Export to PPT

Remarkably, PA@CuFe-Ni₃S₂/NF displayed a 41-fold greater Cdl value compared to NF, representing the most significant ECSA among all tested catalysts. This dramatic improvement can be primarily ascribed to the synergistic effects of heteroatom doping and plasma treatment, which collectively optimized the catalyst’s surface properties. The exceptional performance was further supported by the unique three-dimensional flower-like nanosheet architecture, which provided abundant exposed active sites and facilitated efficient charge/mass transport.

Figure 6(b) shows the EIS of all samples, from which the HER kinetic information of each electrode was derived. The interfacial transfer resistance (Rct) of each catalyst was read from the figure. The interfacial Rct, extracted from the semicircle diameter in the high-frequency region, revealed significant differences among the catalysts. Notably, PA@CuFe-Ni₃S₂/NF exhibited the lowest Rct value (7.1 Ω), demonstrating superior charge transfer characteristics compared to CuFe-Ni₃S₂/NF (14.4 Ω), Ni₃S₂/NF (23.4 Ω), and bare NF (63.8 Ω). This systematic reduction in charge transfer resistance, consistent with the performance trends observed in other electrochemical characterizations, provided compelling evidence that the plasma-assisted modification strategy effectively enhanced both interfacial electron transfer kinetics and bulk charge transport properties [21,22]. The remarkably low Rct value of PA@CuFe-Ni₃S₂/NF suggested that the unique flower-like nanosheet architecture, combined with the plasma-induced surface modification, created highly efficient pathways for electron conduction during the HER process.

The PA@CuFe-Ni₃S₂/NF flower-like nanosheets demonstrated exceptional electrocatalytic performance, exhibiting not only outstanding HER activity but also remarkable long-term stability and corrosion resistance. This superior durability was systematically verified through two key electrochemical assessments. First, as depicted in Figure 6(c), the LSV curves obtained before and after 1000 accelerated CV cycles revealed negligible degradation in catalytic activity. Second, chronoamperometric measurements conducted at a constant current density of 10 mA cm⁻² (Figure 6d) demonstrated excellent operational stability, with minimal current fluctuations over a continuous 20 h testing period. Notably, our PA@CuFe-Ni₃S₂/NF electrocatalysts exhibited excellent stability comparable to other transition metal sulfide-based electrocatalysts, including MoS₂/NiFe-LDH (10 h) [42], Co₄S₃@CoMoS₃-P (24 h) [43], and Fe-CoS₂ (25 h) [44]. The outstanding stability can be primarily ascribed to the unique three-dimensional flower-like nanosheet architecture of PA@CuFe-Ni₃S₂/NF. This structural configuration ensures both strong contact between PA@CuFe-Ni₃S₂ and NF, and is favorable for gas release, which could result in the exceptional stability of PA@CuFe-Ni₃S₂F and NF.

3.3. Oxygen evolution reaction

In addition to its exceptional HER activity, the OER performance of PA@CuFe–Ni₃S₂/NF flower-like nanosheets was systematically evaluated in alkaline media to assess its potential as a bifunctional electrocatalyst. As illustrated in Figure 7(a), the OER polarization curves exhibited distinct electrochemical oxidation peaks between 1.3-1.5 V vs. RHE, corresponding to the electrochemical reconstruction process that generates highly active species for enhanced OER activity at elevated current densities [45,46].

Close

Figure 7.

OER performance of the prepared materials in 1.0 M KOH solution. (a) polarization curves with 85% iR compensation. (b) Comparison of overpotentials for various catalysts at current densities of 10 mA cm⁻² and (c) Comparison of overpotential with other recently reported efficient electrocatalysts. (d) the Tafel plots.

Export to PPT

Quantitative analysis of the overpotentials (Figure 7b) revealed a clear performance hierarchy. Bare NF exhibited the poorest activity (419 mV), while Ni₃S₂/NF showed moderate improvement (334 mV). Notably, CuFe-Ni₃S₂/NF demonstrated substantially enhanced performance (192 mV), underscoring the efficacy of interfacial engineering. Most impressively, the plasma-modified PA@CuFe-Ni₃S₂/NF achieved an exceptional overpotential of merely 147 mV, representing state-of-the-art activity among non-precious metal catalysts.

Comparative benchmarking with reported Ni- and Fe-based catalysts (Figure 7c) confirmed the superior performance of our material, attributable to the synergistic effects of heteroatom doping and plasma-induced surface modification. Further kinetic analysis through Tafel slope measurements (Figure 7d) revealed that PA@CuFe-Ni₃S₂/NF exhibited the most favorable reaction kinetics (45.6 mV dec⁻¹), suggesting the rate-determining step involves optimal balancing between OH* adsorption and O–O bond formation at the active sites. In contrast, the higher Tafel slopes observed for NF, Ni₃S₂/NF, and CuFe-Ni₃S₂/NF (Figure 7d) correlate with their inferior OER activities. The comprehensive electrochemical characterization unambiguously demonstrated that the unique structural and electronic properties of PA@CuFe-Ni₃S₂/NF flower-like nanosheets endow them with exceptional bifunctional catalytic performance, making them highly promising for practical water electrolysis applications.

ECSA serves as a critical parameter for evaluating catalytic activity, with larger ECSA values correlating directly with enhanced catalytic performance. Through CV measurements conducted in non-Faradaic potential regions, we determined the Cdl as a quantitative measure of ECSA. As depicted in Figure 8(a), it can be observed that the Cdl values of NF, Ni₃S₂/NF, CuFe-Ni₃S₂/NF, and PA@CuFe-Ni₃S₂/NF are 3.8 mF cm^-2, 11.9 mF cm^-2, 16.2 mF cm^-2, and 37.7 mF cm^-2, respectively. PA@CuFe–Ni₃S₂/NF exhibited a remarkable Cdl value of 37.7 mF cm⁻², representing a tenfold increase compared to pristine NF and superior performance relative to all other samples. This substantial enhancement in ECSA can be attributed to the plasma-induced surface modification, which effectively increases accessible active sites and facilitates mass transport.

Close

Figure 8.

(a) Estimation of C_dl by plotting the current density variation (Δj = (ja-jc)/2). (b-c) EIS of various catalysts.

Export to PPT

Complementary EIS analyses (Figures 8b and c) provided further insight into the charge transfer kinetics. The semicircle radii in Nyquist plots directly correlate with Rct, where smaller values indicate more efficient charge transfer and consequently improved OER kinetics. Notably, bare NF displayed the highest Rct (∼120 Ω), reflecting its limited catalytic activity. The formation of Ni₃S₂/NF significantly reduced the Rct to 13.8 Ω, demonstrating the beneficial role of transition metal sulfides in enhancing conductivity. Further improvement was achieved through Cu/Fe co-doping in CuFe-Ni₃S₂/NF (Rct = 2.8 Ω), highlighting the synergistic effect of heteroatom incorporation on electron transfer efficiency. Most significantly, the plasma-modified PA@CuFe-Ni₃S₂/NF catalyst exhibited the lowest Rct value of 1.9 Ω, conclusively demonstrating that plasma treatment optimally enhances charge transfer kinetics. This improvement, coupled with the substantial ECSA increase, accounts for the exceptional OER performance observed in our study. The combined ECSA and EIS analyses provide compelling evidence that both surface area expansion and charge transfer optimization contribute synergistically to the outstanding electrocatalytic activity of PA@CuFe-Ni₃S₂/NF.

The long-term operational stability, a critical performance metric for practical catalyst applications, was systematically evaluated under identical testing conditions. Mirroring the HER assessment protocol, the durability of PA@CuFe-Ni₃S₂/NF flower-like nanosheets was examined through comparative analysis of polarization curves before and after accelerated degradation testing (Figure 9a). Remarkably, the post-cycling polarization curves demonstrated merely a marginal current density attenuation within the operational potential window of 1.3-1.5 V versus RHE.

Close

Figure 9.

(a) OER polarization curves of PA@CuFe-Ni₃S₂/NF before and after 1000 CV tests. (b) Chronoamperometric plot of PA@CuFe-Ni₃S₂/NF material for 20 h.

Export to PPT

Complementary chronoamperometric measurements conducted at a constant applied potential provided further validation of the catalyst’s robust stability. As illustrated in Figure 9(b), the current density remained exceptionally stable at 10 mA cm⁻² throughout the 20-h continuous operation, exhibiting only a minimal deviation of +0.9 mA cm⁻² by the test conclusion. This outstanding stability metric significantly surpasses the performance benchmarks reported for most transition metal-based electrocatalysts. The enhanced stability can be mechanistically explained by the synergistic effects arising from three key structural features. The 3D hierarchical architecture ensures structural integrity by effectively mitigating electrochemical dissolution, while the robust interfacial bonding between active nanosheets and nickel foam substrate prevents material delamination. Furthermore, plasma-induced surface modification significantly improves corrosion resistance. These combined structural advantages enable the PA@CuFe-Ni₃S₂/NF catalyst to maintain exceptional durability during extended electrocatalytic operation.

3.4. Overall water splitting

The symmetric electrolysis system demonstrates exceptional practical potential by simplifying device architecture, reducing manufacturing costs, and avoiding incompatible interfaces between different catalysts. The demonstrated catalytic efficacy of PA@CuFe-Ni₃S₂/NF flower-like nanosheets in both hydrogen and oxygen evolution half-reactions prompted further investigation of their bifunctional performance in overall water splitting. Employing identical geometric areas (1 cm²) for both cathode and anode in alkaline media, the integrated system exhibited exceptional catalytic activity. LSV analysis (Figure 10a) revealed that the PA@CuFe-Ni₃S₂/NF||PA@CuFe-Ni₃S₂/NF configuration achieved superior current density across the applied potential window compared to control systems.

Close

Figure 10.

(a) The polarization curves of all catalysts for overall water splitting in 1 M KOH with a scan rate of 5 mV S^-1, (b) The voltage required for all catalysts to drive the 10 mA cm^-2 current density, (c) Chronoamperometric plot recorded at PA@CuFe-Ni₃S₂/NF ||PA@CuFe-Ni₃S₂/NF under the fixed potential.

Export to PPT

At the benchmark current density of 10 mA cm⁻², the bifunctional electrocatalyst required only 1.575 V, significantly lower than the potentials needed by CuFe-Ni₃S₂/NF (1.682 V), Ni₃S₂/NF (1.862 V), and bare NF (1.922 V) systems (Figure 10b). This performance advantage, corresponding to a 5.5-18.1% reduction in required potential, underscores the synergistic effects of the unique three-dimensional flower-like architecture and plasma-induced surface modification. The catalyst’s exceptional activity was further highlighted by its superior performance relative to recently reported transition metal-based bifunctional electrocatalysts, including NiFeMo/NF (1.62 V) [47], CoFe-P/NF (1.58 V) [48], and other advanced systems such as Co₂P-1/Ni₂P-1@NF (1.63 V) [49], MoS₂/NiCo₂O₄/NF (1.62 V) [50], and WP₂ NW/NF (1.65 V) [51]. The outstanding water-splitting efficiency can be attributed to the combined effects of optimized electronic structure through Cu/Fe co-doping, enhanced active site exposure from the hierarchical morphology, and improved charge transfer kinetics resulting from plasma treatment. These structural and electronic advantages collectively contribute to the remarkable catalytic performance, establishing PA@CuFe-Ni₃S₂/NF as one of the most efficient non-precious metal-based bifunctional electrocatalysts for alkaline water electrolysis.

The electrochemical stability of PA@CuFe-Ni₃S₂/NF was systematically assessed through chronoamperometric measurements under continuous operation. As shown in Figure 10(c), the time-dependent current response exhibited exceptional stability, with minimal deviation over an extended 20 h testing period, demonstrating the catalyst’s robustness under operational conditions. This outstanding stability, combined with its superior catalytic performance, confirms the successful development of a practical bifunctional electrocatalyst. The remarkable durability and activity can be attributed to the synergistic effects of the 3D flower-like architecture, which provides structural stability, along with the optimized electronic structure resulting from Cu/Fe co-doping and plasma-induced surface modification. These findings underscore the effectiveness of this combined strategy in designing efficient and durable electrocatalysts for sustainable energy conversion applications.