Dual-carbon and chalcogenide engineered Ni₃Se₄/delaminated V₂C MXene /reduced graphene oxide composite as the bifunctional electrode for asymmetric supercapacitor and hydrogen evolution applications

Abstract

The rational design of multifunctional materials with superior electrochemical properties is vital for advancing energy storage and conversion technologies. Here, a ternary Ni₃Se₄/V₂C/reduced graphene oxide (rGO) composite was developed, combining dual-carbon components with chalcogenides for supercapacitors and hydrogen evolution applications. For comparison, Ni₃Se₄, Ni₃Se₄/V₂C, and Ni₃Se₄/rGO materials were also synthesized, and their electrochemical features were examined. In Ni₃Se₄/V₂C/rGO, the Ni₃Se₄ exhibits a nanorod–nanoparticle morphology, while V₂C nanosheets and rGO nanoflakes provide a conductive and robust framework. The Ni₃Se₄/V₂C/rGO yields a specific capacity of 186 mA h g⁻¹ (1031 F g⁻¹) at 1 A g⁻¹, with 67% rate capability at 30 A g⁻¹, and outstanding cycling stability of 96% after 5000 cycles at 10 A g⁻¹. The Ni₃Se₄/V₂C/rGO//activated carbon device delivers 72.4 Wh kg⁻¹ and 16 kW kg⁻¹ of energy and power densities, respectively. It shows 90% capacity retention after 10 000 cycles and 97% coulombic efficiency at 10 A g⁻¹. The Ni₃Se₄/V₂C/rGO exhibits excellent HER activity, providing an overpotential of 118.3 mV at 10 mA cm⁻², with a Tafel slope of 112 mV dec⁻¹, abundant active sites, and stability over 24 h. These results establish Ni₃Se₄/V₂C/rGO as a notable bifunctional component for integrated energy storage and energy conversion applications.

---

1. Introduction

Rapid global economic growth has led to an excessive reliance on fossil fuels, resulting in a sharp rise in CO₂ emissions and severe environmental consequences. Since the supply of fossil fuels is finite and their continued use poses long-term environmental and economic risks, a greater emphasis is now being placed on developing and adopting green, renewable energy resources to satisfy growing power needs.^1,2 Due to their intermittent nature, renewable energy sources have not yet been able to replace conventional sources completely. Therefore, a multifunctional energy system is the best way to address this concerning issue, as it can be used for both integrated energy storage and conversion. In this context, extensive research has been devoted to advancing electrochemical energy storage devices to ensure an uninterrupted energy supply and compensate for the irregularity of renewable energy sources.³ Energy conversion has also garnered significant attention, with a particular emphasis on creating green fuels. In this regard, the hydrogen evolution reaction (HER) has emerged as a fascinating area of study.⁴ Consequently, the focus has recently switched to creating bifunctional materials that are suitable for energy storage and conversion technologies.

Among energy storage devices, supercapacitors are considered significant candidates due to their higher safety, higher energy density, greater stability, and greater durability compared to conventional electrolytic capacitors.^5,6 There are two primary categories of supercapacitors: (i) electrical double-layer capacitors (EDLC), which rely on ion adsorption–desorption, and (ii) pseudocapacitors, which utilize redox reactions. EDLCs offer high rate capability and power density properties, but suffer from limited energy density.^7,8 In contrast, pseudocapacitors provide comparatively higher energy density with adequate rate capability and power density.⁹ Thus, exploration of various pseudocapacitor electrode materials is significant for the progression of next-generation supercapacitor energy storage devices. On the other hand, initiating hydrogen evolution requires an efficient catalyst to overcome the activation energy barrier, which in turn directly influences the overall energy efficiency of the process. To consider an efficient electrocatalyst for HER, a material would exhibit a low overpotential, fast electron transfer kinetics reflected by a low Tafel slope, and outstanding long-term stability.¹⁰ The platinum (Pt)-based materials are widely recognized as renowned electrocatalysts for the HER process, owing to their exceptionally low overpotential and minimal Tafel slope properties. However, these materials are unfeasible for real-world applications due to their high cost.¹¹ Therefore, numerous studies focused on developing alternative low-cost materials as effective electrocatalysts for HER application.

Transition metal chalcogenides (TMCs) have recently been explored for supercapacitor and HER applications owing to their unique physicochemical characteristics. Their excellent chemical stability is one of their main advantages, making them suitable for long-term electrochemical activities and resistant to hostile reaction conditions. Furthermore, a variety of oxidation states of TMCs provide a wide range of redox processes, thus increasing their catalytic and energy storage properties. Predominantly, nickel-based chalcogenides show great promise because of their rapid faradaic reaction and large theoretical capacitance, enabling efficient electrochemical characteristics. On the other hand, selenium (Se) interacts with transition metals more strongly than sulfur (S) due to its lower electronegativity. The valence electron configuration of nickel (3d⁸4s²), along with the electronegativity difference between Ni and Se, plays a crucial role in giving rise to the diverse structural and compositional forms of nickel-based selenides. Also, a larger atomic radius property of Se would lead to more frequent atom-to-atom collisions and thus accelerate chemical reactions.^12,13 Due to the battery-type energy storage mechanism, nickel selenides often experience volumetric expansion during stability analysis, which hinders their cyclic stability.¹⁴ Selenium enhances the conductivity of chalcogenides compared to their sulfide counterparts; however, it remains lower than that of MXenes or conductive carbons, which can limit the performance of the electrodes.¹⁵ To overcome these limitations, various strategies have been explored to enhance electrochemical properties using various materials. These approaches include nano-structuring,¹⁶ crystal structure modification,¹⁷ compositional engineering,¹⁸ hollow architecture design,¹⁹ and integration with conductive carbon materials.²⁰ Collectively, these strategies intend to enhance the total electrochemical performance and stability of materials for energy storage and conversion technologies. Within these approaches, carbon wrapping or coating has proven to be particularly effective due to the high electronic conductivity, thermal and chemical stability, and excellent heat transmission characteristics of carbon materials.²¹ Carbon coatings have several advantages, including stabilizing interfacial reactions, mitigating contact resistance, supplying flexibility properties and mechanical strength, and preventing heat raising in the electrochemical cell.²² Due to these benefits, it has been demonstrated that using carbon-based materials in composite structures with nickel selenides significantly improves electrode performance. Nevertheless, research is currently ongoing to determine the best carbon coating design that effectively improves all the required characteristics. During single carbon coating, a key limitation is associated with point–contact interactions, where irregular and limited electron paths cause poor electrical connectivity between the materials, thereby reducing the overall conductivity of the electrode. In an effort to circumvent these limitations, double-layer carbon coatings have been anticipated, where an internal carbon layer enhances conductivity and suppresses electrolyte/electrode side reactions, while an outer layer strengthens interparticle connections and provides mechanical stability.^23,24 For dual-carbon strategies, MXene and reduced graphene oxide are considered promising materials for effectively overcoming these challenges.

MXenes are 2D transition metals, nitrides, carbides, and carbonitrides that exhibit OH, F, O, and Cl as surface terminations.²⁵ MXenes have been extensively explored for energy storage applications, serving both as active materials and as conductive frameworks in supercapacitors. Their intrinsic metallic conductivity facilitates rapid electron transport, while the abundance of surface terminations and large electrochemically active surface area provides numerous active sites for charge storage.²⁶ Among the MXene family, two-dimensional (2D) V₂C has emerged as a notable candidate because of low density, high electrical conductivity (∼3300 S cm⁻¹), and favourable mechanical and electronic properties. Furthermore, its surface terminations can facilitate pseudocapacitive behavior, distinguishing V₂C as an attractive alternative to heavier or less conductive MXenes for energy storage technologies. In addition, the presence of transition metal centres contributes to high redox activity, further enhancing their capacitive performance.²⁷ In dual-carbon strategies, hybridizing rGO with MXene provides high surface area (2630 m² g⁻¹), chemical stability, abundant functional groups for anchoring active materials, improved electron transport, and mechanical support.²⁸ These properties enhance both supercapacitor performance and hydrogen evolution reaction properties efficiently.

In light of these considerations, dual-carbon architecture was employed to coat the Ni₃Se₄ material. The 2D V₂C MXene and rGO were strategically selected as the carbon-coating components on Ni₃Se₄ material, due to their favourable properties for both supercapacitor performance and HER activity. The Ni₃Se₄, Ni₃Se₄/V₂C, Ni₃Se₄/rGO, and Ni₃Se₄/V₂C/rGO materials were prepared using the hydrothermal method. Resulting from the unique morphological, high specific surface area, and synergistic properties, the Ni₃Se₄/V₂C/rGO composite delivers the specific capacity of 186 mA h g⁻¹ (1031 F g⁻¹) and the high rate capability of 67% at 30 A g⁻¹ with a 96% capacity retention after 5000 GCD cycles at 10 A g⁻¹. The ASC device provides maximum energy and power density values of 72.4 Wh kg⁻¹ and 16 kW kg⁻¹, respectively, with 90% capacity retention and 97% coulombic efficiency after 10 000 cycles at 10 A g⁻¹. In the HER applications, the Ni₃Se₄/V₂C/rGO delivers an overpotential of 118.3 mV at 10 mA cm⁻², with a Tafel slope of 112 mV dec⁻¹ and enhanced stability over 24 h. Overall, the Ni₃Se₄/V₂C/rGO composite demonstrates superior electrochemical performance, combining high capacity, excellent cycling stability, and efficient HER activity, thereby establishing it as a promising multifunctional candidate for energy conversion and storage devices.

2. Synthesis process

2.1 Synthesis of delaminated V₂C MXene

The material section is provided in SI Section 1. The delaminated V₂C MXene was derived through a two-step method consisting of the MAX phase etching, followed by a delamination step. Briefly, 1 g of MAXene (V₂AlC) powder was mixed with 8 mL of HCl and 12 mL of HF mixture. The resultant mixture was heated to 40 °C for 72 h in an oil bath to facilitate the Al etching process. Following the etching process, the reaction mixture was mixed with DI water to dilute the acids, and it was centrifuged for 10 minutes at 4000 rpm. After decanting the supernatant, the reaction product was collected. To minimize residual etching acids, the product was redispersed in DI water and subjected to an additional centrifugation step. The centrifugation and dispersion procedures continued when the supernatant attained a pH above 5.5. After that, the material was thoroughly washed with ethanol solvent to eliminate impurities and by-products, followed by centrifugation to separate the etched V₂C from the ethanol solvent. The final product was vacuum-dried at 60 °C for 24 h. For the delamination process, 0.5 g of etched V₂C was spread in 30 mL of DI water, using sonication for 30 minutes. After that, 10 mL of tetramethylammonium hydroxide (TMAOH, 40%) and 0.5 g of ascorbic acid were added and stirred for 30 minutes. Then, the material was treated with the hydrothermal method at 140 °C for 24 h. After attaining room temperature, it was centrifuged at 4500 rpm for 15 minutes to eliminate trace species. After centrifugation, the material was mixed with 120 mL of water using an ultrasonicator and then freeze-dried for 72 h.

2.2 Preparation of Ni₃Se₄/V₂C/rGO composite

The GO was synthesized by chemically oxidizing pristine graphite with KMnO₄ in H₂SO₄ using a modified Hummers' method.²⁹ For the preparation of Ni₃Se₄/V₂C/rGO composite, 0.06 M (1 g) of nickel chloride hexahydrate (NiCl₂·6H₂O) was mixed with 60 mL of DI water, and then 0.08 M (1.06 g) of sodium selenate was added. Then, hydrazine monohydrate (10 mL) was dropped into the precursor solution to reduce selenium and graphene oxide. Then, delaminated V₂C and graphene oxide were added. After 30 minutes of stirring, the resulting precursor material was hydrothermally treated at 180 °C for 24 hours. When the hydrothermal set-up reached room temperature, the precipitate was centrifuged several times at 7000 rpm (10 minutes) using water and ethanol solvents. Then, the solvents were expelled utilizing a vacuum oven at 60 °C for 24 h to attain the Ni₃Se₄/V₂C/rGO composite. The same procedure was followed to prepare Ni₃Se₄, Ni₃Se₄/V₂C, and Ni₃Se₄/rGO materials. Elaborately, the Ni₃Se₄ was prepared without delaminated V₂C and rGO materials. Furthermore, the Ni₃Se₄/V₂C and Ni₃Se₄/rGO composites were prepared without rGO and V₂C material, respectively. The weight percentages of all composites are provided in Table S1. Additionally, material characterizations, electrode fabrication procedure for supercapacitor and HER applications, along with the corresponding formulas, are provided in Sections 2–5 of the SI.

3. Results and discussion

3.1 Physical characterizations

The phase characteristics of all materials are evaluated using X-ray diffraction (XRD). The XRD analysis of vanadium aluminium carbide (V₂AlC, MAX phase) was analyzed in the 2θ = 5°–80° range, as shown in Fig. S1. There are noticeable powder XRD patterns in the V₂AlC MAX phase. The V₂AlC powder exhibits a signal at the 2θ values of 13.5°, 35.6°, 41.3°, 55.6°, and 63.9°, which corresponds to (002), (100), (103), (106), and (110) crystalline planes of V₂AlC MAX phase, which are indexed with JCPDS File No. 29-0101.³⁰ Following the selective etching and delamination processes using TMAOH, the successful formation of V₂C MXene was confirmed by the appearance of a diffraction peak at 2θ = 7.7°, corresponding to the (002) plane of the V₂C MXene. This indicates an expanded interlayer spacing compared to the MAX phase, and the introduction of –O, –OH, –F groups during the etching process. The XRD patterns of Ni₃Se₄, Ni₃Se₄/V₂C, Ni₃Se₄/rGO, and Ni₃Se₄/V₂C/rGO materials are shown in Fig. 1(a).

Fig. 1 (a) XRD analysis of Ni₃Se₄, Ni₃Se₄/V₂C, Ni₃Se₄/rGO and Ni₃Se₄/V₂C/rGO materials. (b) Ni 2p XPS spectra of Ni₃Se₄ material, (c)Ni 2p of Ni₃Se₄/V₂C/rGO material, (d) Se 3d of Ni₃Se₄ material, and (e) Se 3d of Ni₃Se₄/V₂C/rGO material. (f) V 2p, (g) C 1s and (h) O 1s spectra of the Ni₃Se₄/V₂C/rGO material. (i) BET pore size distribution (inset) analysis of all materials.

The Ni₃Se₄ shows the diffraction patterns at 2θ values of 16.6°,33.3°, 33.6°, 45.1°, 45.3°, 50.3°, 50.8°, 60.7°, 61.7°, and 69.9°, which are corresponds to (−201), (−312), (−402), (−514), (−604), (−515), (310), (−716), (−422) and (−624) crystal planes of monoclinic Ni₃Se₄ phase, which are more consistent with the JCPDS card No.-01-089-2020.³¹ These outcomes verify that Ni₃Se₄ was successfully prepared using hydrothermal technique. It is interesting to note that the same XRD patterns corresponding to the Ni₃Se₄ phase were observed in Ni₃Se₄/V₂C, Ni₃Se₄/rGO and Ni₃Se₄/V₂C/rGO materials as well. The XRD analysis of Ni₃Se₄/V₂C composite exhibited a broad peak in the 2θ range of 6–10°, along with a prominent peak at 41.3°, which corresponds to the (002) and (103) planes of the V₂C MXene. The XRD analysis of Ni₃Se₄/rGO shows a peak at 26.3° corresponding to the (002) plane, which arises from the formation of rGO. The use of hydrazine hydrate during synthesis efficiently removes oxygen-containing functional groups and generates the rGO.³² The XRD analysis of the Ni₃Se₄/V₂C/rGO reveals diffraction patterns corresponding to Ni₃Se₄, V₂C, and rGO, indicating the formation of a composite structure. The broad peak from 6 to 10° can be attributed to the overlapping contributions of both V₂C MXene and rGO, rather than a single-phase feature. Specifically, the (002) plane of V₂C typically appears at low diffraction angles due to its layered structure, while rGO also exhibits a broad (103) reflection associated with disordered graphitic layers. Upon hybridization, the interaction between V₂C nanosheets and rGO nanoflakes leads to peak broadening and merging, resulting in a combined diffraction feature in the 6–10° region. After the incorporation of V₂C and rGO into Ni₃Se₄ (forming Ni₃Se₄/V₂C, Ni₃Se₄/rGO, and Ni₃Se₄/V₂C/rGO), no significant changes in the diffraction peaks were observed. Owing to the small fraction (10 wt%) of V₂C and rGO, the crystal structure of Ni₃Se₄ remains unchanged.

Raman spectra of all materials are shown in Fig. S2. The Raman spectra of Ni₃Se₄ exhibit six peaks, corresponding to characteristic peaks of nickel selenide materials. The peaks appearing at 137 and 201 cm⁻¹ can be assigned to the Se–Se pair vibrations and stretching modes. Furthermore, the peak at 282 cm⁻¹ proves the Ni–Se bond in Ni₃Se₄ material. The peak at 401 cm⁻¹ is due to the Ni–O peaks, which resulted from the surface oxidation of the Ni₃Se₄ substance. Spectral bands appearing around 514 cm⁻¹ and 678 cm⁻¹ arise from the longitudinal optical (LO) one-phonon and two-phonon modes, respectively.^33–37 The Raman spectra of the Ni₃Se₄/V₂C composite displays the characteristic vibrational bands of both Ni₃Se₄ and V₂C MXene, clearly evidencing the coexistence of the two phases. The peaks at 137 and 195 cm⁻¹ are related to Ni₃Se₄ material, whereas the peaks appearing at 98, 245, 475, 1360, and 1582 cm⁻¹ are the characteristic peaks of V₂C MXene. The peaks at 245 and 475 cm⁻¹ are owing to the E_g mode of V₂C and the E_g mode of V₂C(OH)₂. The peaks visible at 1360 and 1582 cm⁻¹ are assigned to D and G bands of V₂C MXene.^38–40 The Raman spectra of the Ni₃Se₄/rGO material display four peaks at 201, 272, 409, and 671 cm⁻¹, which are related to Ni₃Se₄ peaks, whereas the peaks at 103, 1342, and 1550 cm⁻¹ are due to the rGO peak, respectively. The peak at 103 cm⁻¹ is due to the layer-breathing mode associated with the stacking of rGO layers. The peaks at 1342 and 1550 cm⁻¹ are attributed to the D and G bands of the rGO material.⁴¹ The Raman spectrum of the Ni₃Se₄/V₂C/rGO composite exhibits characteristic vibrational bands corresponding to Ni₃Se_4, V₂C MXene, and rGO materials. The simultaneous presence of these distinct peaks confirms the successful integration of all three components into a single composite, indicating that the Ni₃Se₄ is well anchored on the V₂C layers while being effectively combined with rGO sheets. These results validate the formation of the targeted Ni₃Se₄/V₂C/rGO composite heterostructure during the synthesis process.

The X-ray photoelectron spectroscopy (XPS) was employed to evaluate the surface valence states of the Ni₃Se₄ and Ni₃Se₄/V₂C/rGO materials. The survey spectrum of Ni₃Se₄ (Fig. S3) exhibits characteristic peaks corresponding to Ni 2p, Se 3d, O 1s, and C 1s, while the Ni₃Se₄/V₂C/rGO (Fig. S3) composite displays additional peaks for V 2p along with Ni 2p, Se 3d, O 1s, and C 1s. This clearly confirms the successful incorporation of all constituent materials within the composite. The deconvoluted Ni 2p spectra (Fig. 1(b)) of Ni₃Se₄ reveal the peaks at 870.4 and 853.1 eV, which are due to Ni 2p_1/2 and Ni 2p_3/2 states of the Ni²⁺. Furthermore, the peaks at 873.6 and 855.7 eV represent Ni 2p_1/2 and Ni 2p_3/2 states of Ni³⁺ in Ni₃Se₄.^42,43 Two satellite peaks observed at 879.4 and 860.8 eV are attributed to the oxides existing on the surface of nickel Ni₃Se₄ material.^44,45 In the Ni 2p spectra (Fig. 1(c)) of Ni₃Se₄/V₂C/rGO composite, the Ni²⁺ oxidation state is identified by characteristics at 870.7 and 853.4 eV, while the additional Ni³⁺ peaks appear at 873.7 and 855.7 eV, accompanied by two distinct satellite peaks visible at 880.1 and 861.2 eV. The negligible shifts (∼0.2–0.3 eV) occur between the Ni 2p spectra of Ni₃Se₄ and Ni₃Se₄/V₂C/rGO composite confirm that the Ni oxidation states (Ni²⁺ and Ni³⁺) in the composite remain essentially the same as in pure Ni₃Se_4. The Se 3d spectra (Fig. 1(d)) of the Ni₃Se₄ reveal the deconvoluted peaks at 54.6 and 53.8 eV, which are due to the Se 3d_3/2 and Se 3d_5/2, respectively. The broad peak observed at 58.8 eV is ascribed to SeO_x species, originating from the surface oxidation of selenium in the Ni₃Se₄ material.^46,47 The deconvoluted Se 3d spectra (Fig. 1(e)) Ni₃Se₄/V₂C/rGO composite displays the Se 3d_3/2 and Se 3d_5/2 states at 54.7 and 53.8 eV, whereas the SeO_x peak is visible at 58.8 eV. The V 2p spectra (Fig. 1(f)) of the Ni₃Se₄/V₂C/rGO composite shows two peaks at 524.3 and 517.1 eV, demonstrating the V p_3/2 and V p_1/2 states of the V material. The two peaks were deconvoluted into six peaks in the range from 513 to 528 eV, which is due to the presence of V³⁺ (522.9 and 516.3 eV), V⁴⁺ (523.8 and 517.1 eV), and V⁵⁺ (524.9 and 517.7 eV) oxidation states.^48,49 The coexistence of V³⁺, V⁴⁺, and V⁵⁺ states indicates a defect-rich and partially oxidized surface, which enhances electrical conductivity and catalytic activity. The mixed valence states facilitate electron hopping and provide abundant active sites, thereby improving interfacial charge-transfer kinetics and HER performance, as widely reported for MXene-based systems.⁵⁰ These results confirm the successful incorporation of V₂C within the Ni₃Se₄/V₂C/rGO material. The C 1s spectra (Fig. 1(g)) of the Ni₃Se₄/V₂C/rGO composite demonstrate that the peaks at 288.3, 286.1, 285.4 and 284.5 eV, which are attributed to the COOH, C=O, C–O and C=C/C–C groups of rGO, respectively.⁵¹ The O 1s spectra (Fig. 1(h)) of the Ni₃Se₄/V₂C/rGO composite show four deconvoluted peaks at 533.7, 532.6, 531.1 and 530.1 eV, which are related to –C–O–C, –OH, (C–O) OH and C=O groups in rGO, respectively.⁵² The XPS analysis confirms the successful integration of Ni₃Se₄, V₂C MXene, and rGO in the Ni₃Se₄/V₂C/rGO composite.

To further understand the surface properties of the substances, the Brunauer–Emmett–Teller (BET) adsorption isotherm and Barrett–Joyner–Halenda (BJH) pore volume analyses were also demonstrated, as illustrated in Fig. 1(i) and its inset. The N₂ adsorption–desorption isotherms of all materials reveal a typical IV curve along with H1-type hysteresis loops, and the hysteresis loops can be detected in the range of 0.84–1.0 P/P₀. It demonstrates a typical mesoporous nature, along with aggregated particles and unrestricted monolayer-multilayer adsorption. The specific surface area, pore volume, and BJH pore size distribution values of all materials are provided in Table S2. Among all the investigated materials, the Ni₃Se₄/V₂C/rGO composite exhibited the highest surface area properties, including specific surface area, pore volume, and BJH pore size distribution values of 17.1879 m² g⁻¹, 0.0212 cm³ g⁻¹, and 48 nm, respectively. The significantly enhanced surface area properties can be attributed to the incorporation of 2D V₂C MXene and rGO into the Ni₃Se₄ matrix, which provides abundant active sites and facilitates efficient electrolyte ion diffusion for both supercapacitor and HER applications.

The morphological studies were conducted using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. S4(a) shows the SEM image of V₂AlC MAX phase, and Fig. S4(b) shows the delaminated 2D V₂C MXene, respectively. The SEM images of V₂C clearly show a few-layered, ultrathin nanosheets with a characteristic crumpled and wrinkled shape. Notably, individual single-layer nanosheets can also be distinctly observed.

The SEM images (Fig. 2(a–c)) of Ni₃Se₄ reveal a densely aggregated morphology, where the individual nanoparticles are interconnected and clustered together to form larger agglomerates. The SEM images (Fig. 2(d–f)) of the Ni₃Se₄/V₂C composite demonstrate that the Ni₃Se₄ materials are anchored onto the surface of the V₂C sheets. The 2D layered morphology of V₂C provides a conductive and high-surface-area platform, enabling the Ni₃Se₄ particles to nucleate and grow directly on its surface. Fig. 2(g–i) presents the SEM images of the Ni₃Se₄/rGO material, establishing the simultaneous formation of both Ni₃Se₄ and rGO within the hybrid structure. The Ni₃Se₄ nanoparticles are well-distributed across the nanoflake architecture of rGO, while the rGO sheets act as a scaffold to accommodate the growth of Ni₃Se₄ material. Fig. 2(j–l) presents the SEM images of the Ni₃Se₄/V₂C/rGO composite, clearly revealing the coexistence of Ni₃Se₄ nanostructures, V₂C nanosheets, and rGO nanoflakes. The SEM images reveal that Ni₃Se₄ exhibits a distinctive mixed morphology, comprising both nanorods and nanoparticles. Meanwhile, the layered V₂C nanosheets and wrinkled rGO nanoflakes offer a conductive and mechanically robust framework, ensuring uniform dispersion of Ni₃Se₄ and preventing agglomeration. This synergistic design is predicted to enhance the electrochemical behavior and prolong the operational lifespan of the material. For comparison, commercially available activated carbon was used as the negative electrode material, and its morphology was analyzed by SEM (Fig. S4(c and d)). The porous and interconnected structure facilitates efficient ion transport and electric double-layer charge storage, thereby contributing to the enhanced power performance of the asymmetric device.

Fig. 2 SEM analysis of (a–c) Ni₃Se₄, (d–f) Ni₃Se₄/V₂C, (g–i) Ni₃Se₄/rGO and (j–l) Ni₃Se₄/V₂C/rGO materials.

The TEM analysis of the Ni₃Se₄/V₂C/rGO composite (Fig. 3(a–c)) reveals the presence of V₂C nanosheets, rGO nanoflakes, and Ni₃Se₄ material with mixed morphologies. The Ni₃Se₄ nanorods display an average diameter of 7 ± 2 nm with lengths extending to several nanometers, while the Ni₃Se₄ nanoparticles exhibit an average size of 25 ± 2 nm. These observations are in relation to the SEM analysis (Fig. 2(j–l)). The fringes, line lattice fringes, and Fast Fourier Transform (FFT) analysis (Fig. (d–f) and S5(a and b)) reveal interplanar spacings of 0.247 and 0.123 nm, which correspond to (101) and (200) planes of V₂C, respectively.

Fig. 3 TEM images of Ni₃Se₄/V₂C/rGO composite. (a–c) Morphological images, (d) fringes, (e and f) line lattice fringes, fringes and masking FFT images, (g) SAED pattern and (h–m) elemental mapping analysis.

Additionally, the interplanar spacings of 0.268 and 0.202 are attributed to the (−312) and (−515) planes of Ni₃Se₄ material, respectively. The selected electron area diffraction (SAED) pattern (Fig. 3(g)) confirms the presence of (−312) and (−402) planes of Ni₃Se₄. The observed lattice fringes and SAED results are consistent with the previously discussed XRD analysis (Fig. 1(a)). The elemental mapping analysis (Fig. 3(h–m)) clearly displays distinct signals for Ni, Se, V, O, and C, thereby demonstrating the effective development of the Ni₃Se₄/V₂C/rGO composite material. The uniform distribution of Ni and Se elements indicates the homogeneous presence of the Ni₃Se₄ phase, while the V signal confirms the incorporation of V₂C MXene sheets. Additionally, the O and C signals are ascribed to the oxygen-containing functional groups and carbon framework of rGO material. This comprehensive elemental mapping strongly supports the synergistic integration of all three components in the designed composite material.

Drawing on physicochemical analyses and literature precedents, the formation mechanism of the Ni₃Se₄/V₂C/rGO composite can be outlined as follows: at first, the V₂AlC MAX phase was etched and delaminated to get the few-layer V₂C material. During the etching step, HF selectively removes the Al layers from V₂AlC, while HCl serves as a proton donor that accelerates the dissolution of Al-containing intermediates. The elimination of Al exposes the underlying V–C layers, which become terminated with –F, –OH, and –O groups. The synthesized V₂C MXene exhibits a multilayered architecture stabilized by hydrogen bonds and van der Waals forces. In the subsequent delamination process using TMAOH, OH⁻ ions intercalate between the V₂C layers and partially replace –F and –Cl terminations. At the same time, tetramethylammonium cations (TMA⁺) insert between the negatively charged V₂C sheets, expanding the interlayer spacing and weakening inter-flake attractions. Sonication further facilitates this process, enabling efficient delamination of the MXene into few-layer nanosheets.^53–55 The formation mechanism of Ni₃Se₄ (ref. 56–60) is as follows: in this reaction, Na₂SeO₃ was selected instead of elemental Se powder since it dissolves readily in water and can be rapidly reduced by hydrazine hydrate upon heating. This process generates highly reactive, ultrafine Se species that are more easily converted into Se²⁻ ions than the less reactive commercial Se powder. In the first step of the reaction, the Na₂SeO₃ dissociates in aqueous solution to yield SeO₃²⁻ ions, which are subsequently reduced to Se²⁻ by hydrazine monohydrate under hydrothermal conditions. This reduction process of hydrazine monohydrate generates a highly reactive Se²⁻ ion that subsequently participates in the formation of nickel selenide. It is explained using eqn (1)–(3):

Na₂SeO₃(s) + H₂O → 2Na⁺(aq) + SeO₃²⁻ (aqueous) (1)

N₂H₄ → N₂ + 4H⁺ + 4e⁻ (2)

SeO₃²⁻ + 6H⁺ + 6e⁻ → Se²⁻ + 3H₂O (3)

The available Se²⁻ reacts with Ni²⁺ ions and forms Ni₃Se₄ during the hydrothermal process at 180 °C for 24 h, as displayed in eqn (4)

3Ni²⁺ + 4Se²⁻ → Ni₃Se₄ (4)

The Ni₃Se₄/V₂C/rGO composite is formed via an in situ growth and self-assembly process.

Initially, the V₂C MXene sheets provide a highly conductive and functionalized substrate with abundant surface groups (–OH, –O, –F). Graphene oxide (GO) nanosheets are then introduced, which interact with the V₂C layers and form a well-dispersed dual-carbon material. Simultaneously, the graphene oxide was reduced to rGO in the presence of hydrazine hydrate. When Ni²⁺ ions and Se²⁻ ions are added, the Ni₃Se₄ nucleates preferentially on the surfaces of V₂C layers and rGO flakes. The strong interfacial interactions between Ni₃Se₄ and the carbon-based support prevent agglomeration, ensuring the uniform distribution of Ni₃Se₄ nanostructures and forming the Ni₃Se₄/V₂C/rGO composite. Fig. 4 shows the schematic representation for the Ni₃Se₄/V₂C/rGO composite formation process.

Fig. 4 Schematic representation for the synthesis of Ni₃Se₄/V₂C/rGO composite.

3.2 Supercapacitor studies

To evaluate the charge storage characteristics of the Ni₃Se₄, Ni₃Se₄/V₂C, Ni₃Se₄/rGO and Ni₃Se₄/V₂C/rGO electrodes, the cyclic voltammetry (CV) and galvanostatic charging/discharging (GCD) studies were done in aqueous 2 M KOH electrolyte. A platinum foil electrode functioned as the counter electrode, and an Ag/AgCl electrode served as the reference. The electrodes were stimulated by performing 50 CV cycles at a sweep rate of 50 mV s⁻¹ before electrochemical analysis. The CV profiles were recorded within −0.1 to 0.6 V at varying sweep rates of 5–100 mV s⁻¹ (Fig. 5a–d). A wider potential window, such as 0.7 V, reflects the ability of the electrode materials to sustain stable redox activity over an extended voltage range, indicating high energy density properties.

Fig. 5 CV analysis of (a) Ni₃Se₄, (b) Ni₃Se₄/V₂C, (c) Ni₃Se₄/rGO and (d) Ni₃Se₄/V₂C/rGO electrodes. GCD analysis of (e) Ni₃Se₄, (f) Ni₃Se₄/V₂C, (g) Ni₃Se₄/rGO and (h) Ni₃Se₄/V₂C/rGO electrodes. (i) Stability studies of all electrodes.

Notable faradaic redox peaks are observed in all CV curves, signifying reversible electrochemical activity and strong battery-type charge storage in the electrodes. Furthermore, the absence of a rectangular shape in the CV curves signifies that the charge storage mechanism is administered by faradaic redox processes, which arise from the reversible transformation of Ni²⁺ to Ni³⁺ and vice versa. Eqn (5) explains the redox process:⁶¹

Ni₃Se₄ + OH⁻ ⇌ Ni₃Se₄OH + e⁻ (5)

The higher scan rates cause minor shifts of the redox peaks in both directions, reflecting the effects of electrode polarization and inherent resistance.⁶² CV analysis clearly shows that V₂C and rGO remain uninvolved in the redox processes of the Ni₃Se₄/V₂C, Ni₃Se₄/rGO, and Ni₃Se₄/V₂C/rGO electrodes. However, the higher current density observed in Ni₃Se₄/V₂C/rGO compared to the pure Ni₃Se₄ electrode indicates that their primary role is to improve the electrical conductivity features of the composite electrodes. The integral area under the CV curves gradually expands when the sweeping rate rises from 5 to 100 mV s⁻¹. This pattern points to fewer hindrances to ion diffusion and charge transfer. The CV graphs showed a distinctive and steady form profile at high sweep speeds. This improvement is evidence for the exceptional reversibility and strong rate capability of electrode materials.⁶³

The electrochemical properties are further evaluated using the GCD analysis, as shown in Fig. 5(e–h). The GCD analysis was demonstrated using the potential limits from −0.1 to 0.55 V with an input current density from 1 to 30 A g⁻¹. The GCD curves show a distinct non-linear profile rather than the ideal triangular shape associated with conventional electric double-layer capacitors. This non-linear behavior reflects faradaic redox reactions, which contribute to a battery-like charge storage mechanism. At the beginning of the discharge process, all electrode materials show a reduced IR drop, indicating their low charge transfer resistance. Among all electrodes, the Ni₃Se₄/V₂C/rGO electrodes reveal longer discharge time, demonstrating high electrochemical supercapacitor properties. The specific capacity (mA h g⁻¹) and specific capacitance (F g⁻¹) values were calculated from the GCD curves using eqn S4 and S6, respectively. The Ni₃Se₄, Ni₃Se₄/V₂C, Ni₃Se₄/rGO, and Ni₃Se₄/V₂C/rGO electrodes provide the specific capacity of 99 (549 F g⁻¹), 132 (730 F g⁻¹), 164 (908 F g⁻¹), and 186 mA h g⁻¹ (1031 F g⁻¹), respectively, at a current density of 1 A g⁻¹. Among all the electrodes, the Ni₃Se₄/V₂C/rGO composite exhibits the highest specific capacity, surpassing previously reported values, as presented in Table S3. Notably, the Ni₃Se₄/V₂C/rGO composite demonstrates a 1.88-fold enhancement in specific capacity compared to pristine Ni₃Se₄. The rate capability of all electrode materials was calculated using the specific capacity vs. current density graph, as shown in Fig. S6. The pristine Ni₃Se₄ electrode retained only 12% of its initial capacity, indicating poor rate performance. In contrast, the Ni₃Se₄/V₂C composite exhibited a significantly improved retention of 31%, while the Ni₃Se₄/rGO electrode delivered an even higher retention of 46%. Notably, the Ni₃Se₄/V₂C/rGO composite electrode demonstrated the best performance, retaining 67% of its capacity under the same conditions. Cyclic stability is another crucial parameter for assessing the supercapacitor properties of electrode materials, particularly in practical and real-time applications. At a current density of 10 A g⁻¹, 5000 continuous GCD cycles were used to assess the cyclic stability of each electrode, as shown in Fig. 5(i). The pristine Ni₃Se₄ electrode endures 89% of its initial capacity, reflecting moderate stability. The incorporation of V₂C MXene significantly enhanced stability, with the Ni₃Se₄/V₂C electrode maintaining 92% of its initial capacity. Similarly, the Ni₃Se₄/rGO composite showed improved durability, achieving 93% retention. Remarkably, the Ni₃Se₄/V₂C/rGO composite exhibited the highest stability, retaining 96% of its capacity even after prolonged cycling. Comparatively, the Ni₃Se₄/V₂C/rGO electrode material demonstrated high specific capacity, rate capability, and long cyclic stability than Ni₃Se₄, Ni₃Se₄/V₂C, and Ni₃Se₄/rGO electrodes, due to the following favorable properties: based on the dual-carbon design, when Ni₃Se₄, V₂C, and rGO are integrated into a single composite, their properties work synergistically, thereby enhancing the overall supercapacitive performance. Specifically, Ni₃Se₄ offers abundant redox-active sites, which deliver a high specific capacity; however, it suffers from limited conductivity and stability when used alone. The integration of V₂C MXene improves the electrical conductivity. It provides a mechanically robust 2D framework that can effectively shield the volume enlargement/contraction of Ni₃Se₄ during charge–discharge processes, thus improving cycling stability. On the other hand, rGO introduces a high surface area feature, oxygen-rich functional moieties, and layered channels that promote uniform dispersion of active materials, fast ion diffusion, and additional pseudocapacitive contribution. The high surface area of the Ni₃Se₄/V₂C/rGO composite, as confirmed by BET analysis, significantly contributes to its superior electrochemical performance. Greater surface area offers additional accessible active sites, enhances electrolyte accessibility, and shortens ion diffusion paths, all of which facilitate faster charge storage and improved utilization of the active material. Therefore, such features result in higher specific capacity, enhanced rate capability, and better long-term stability.

Both electric double-layer capacitance (EDLC) and faradaic charge storage mechanisms coexist in a supercapacitor; the dominance of one over the other can vary depending on the choice of electrode material and electrolyte. Dunn's method is used to calculate the ratios of capacitive/diffusion-controlled contributions depending on the redox current of the CV curve at a specific voltage (i(V)) using eqn (6) and (7):^64,65

i = av^b, (6)

log(i) = log(a) + b log(v). (7)

in these equations, a and b are both modifiable parameters.

The value of b provides insight into the energy storage method. When the value is close to 1, the surface-capacitive process is primarily responsible for energy storage. Moreover, a b-value of 0.5 validates the diffusion capacitive mechanism in electrode materials. The Ni₃Se₄, Ni₃Se₄/V₂C, Ni₃Se₄/rGO, and Ni₃Se₄/V₂C/rGO electrodes (Fig. 6(a) and S7(a–c)) exhibit b-values between 0.4 and 0.6, which signifies that charge storage predominantly occurs through a diffusion-controlled mechanism. Eqn (8) was utilized to enumerate the surface and diffusion-controlled processes.^64,65

i(V) = k₁v + k₂v^1/2. (8)

Fig. 6 Logarithmic plot of (a) Ni₃Se₄. Surface and diffusive contribution (at 10 mV s⁻¹) of (b) Ni₃Se₄, (c) Ni₃Se₄/V₂C, (d) Ni₃Se₄/rGO and (e) Ni₃Se₄/V₂C/rGO electrodes. Total surface and diffusive contribution at all sweep rates for (f) Ni₃Se₄, (g) Ni₃Se₄/V₂C, (h) Ni₃Se₄/rGO and (i) Ni₃Se₄/V₂C/rGO electrodes.

Both k₁ and k₂ are constants in this equation. The k₁v demonstrates that the surface-controlled contribution process, whereas k₂v^1/2 indicates the diffusion-controlled contribution process. Fig. 6(b–e) shows the surface and diffusion-controlled contributions of Ni₃Se₄, Ni₃Se₄/V₂C, Ni₃Se₄/rGO, and Ni₃Se₄/V₂C/rGO electrodes at a sweep rate of 10 mV s⁻¹, which establishes the surface contributions of 17%, 9%, 19% and 21%, whereas the diffusion contributions of 83%, 91%, 81% and 79% respectively. Among all the materials, Ni₃Se₄/V₂C exhibits the highest diffusion-controlled behavior, as both Ni₃Se₄ (ref. 66) and V₂C (ref. 67) inherently undergo intercalation/deintercalation processes when used as supercapacitor electrodes. Combining these two materials into a composite further enhances the diffusion-controlled charge storage due to the synergistic interaction between their redox-active sites. Fig. 6(f–i) illustrates the diffusion- and surface-controlled phenomena for all materials at all scan rates. The contribution of the surface-controlled process increases with higher sweep rates because, at faster scans, ions have less period to enter the bulk of the electrode, causing charge storage to be increasingly dominated by surface or near-surface processes.

Electrochemical tests of the asymmetric supercapacitor (ASC) device were carried out to evaluate the practical applicability of Ni₃Se₄/V₂C/rGO composite. The split cell set-up was employed to fabricate the ASC device in a 2 M KOH electrolyte, as shown in Fig. 7(a). The activated carbon (AC) and Ni₃Se₄/V₂C/rGO electrodes served as anode and cathode, respectively. The potential window of the ASC was assessed via CV tests on the AC and Ni₃Se₄/V₂C/rGO electrodes in a three-electrode system at a scan rate of 50 mV s⁻¹ (Fig. 7(b)).

Fig. 7 (a) Schematic representation of AC//Ni₃Se₄/V₂C/rGO device, (b) CV analysis of AC and Ni₃Se₄/V₂C/rGO electrodes in 2 M KOH at 50 mV s⁻¹, (c) CV analysis with different cell voltages at 10 mV s⁻¹, (d) CV, (e) GCD, (f) rate capability, (g) Ragone plot, (h) Nyquist plot with equivalent circuit (inset) and (i) cyclic stability studies with LED illumination at the initial time and after five minutes (inset).

The AC electrode functions within −1.2 to 0 V, in contrast to the Ni₃Se₄/V₂C/rGO electrode, which exhibits a potential range of 0 to 0.6 V. The CV curves of the AC electrode at different scan rates (Fig. S8(a)) exhibit a quasi-rectangular shape without noticeable redox peaks, indicating a dominant EDLC behavior. Consistently, the GCD profiles (Fig. S8(b)) display symmetric triangular shapes, confirming a reversible capacitive response. The AC electrode achieves a specific capacitance of 126 F g⁻¹ at 1 A g⁻¹, demonstrating its suitability as a negative electrode material. The optimal mass ratio of AC and Ni₃Se₄/V₂C/rGO was calculated to be 0.33 using eqn (S1)–(S3). This ratio was selected to balance the charge storage between the two electrodes, thereby enabling the assembly of an efficient and stable ASC (AC//Ni₃Se₄/V₂C/rGO) device. The CV curves (Fig. 7(c)) were recorded at 10 mV s⁻¹ over a cell voltage range of 1.3–1.8 V to determine the optimal operating voltage. When the cell voltage was raised to 1.6 V, no noticeable polarization was observed, indicating stable electrochemical properties within this range. However, upon extending to 1.7 V, minor polarization began to appear, suggesting the onset of side reactions. This polarization became more pronounced at 1.8 V, indicating that further increasing the cell voltage leads to greater electrochemical instability and possible decomposition processes. Consequently, the cell voltage was fixed to be 1.6 V for the present AC//Ni₃Se₄/V₂C/rGO device. The CV profiles of the device are obtained at sweep rates ranging from 5 to 100 mV s⁻¹ and a potential window of 0.0–1.6 V, and are illustrated in Fig. 7(d). The charge storage mechanism of the ASC device occurs from a combination of EDLC and pseudocapacitance processes. The EDLC nature results from the usage of AC in the negative electrode. The well-retained shape of the CV curves, featuring distinct redox peaks from the Ni₃Se₄/V₂C/rGO positive electrode and minimal distortion at 100 mV s⁻¹, confirms the good reversible and stable electrochemical properties of the device.

The GCD of the AC//Ni₃Se₄/V2C/rGO device is shown in Fig. 7(e), recorded from 1 to 20 A g⁻¹ at a cell voltage of 0–1.6 V. The redox activity of Ni₃Se₄ manifests as plateaus in the discharge curves, which correlate closely with the CV observations. The device delivers a specific capacity of 91 mA h g⁻¹ (204 F g⁻¹) at 1 A g⁻¹. As the current density increases, the specific capacity values are reduced, as shown in the rate capability plot (Fig. 7(f)). This is because the electrolyte ions experience a short time to penetrate the deeper active regions of the electrode material.⁶⁸ The ASC device delivers a remarkable energy density of 72.4 Wh kg⁻¹ and power density of 800 W kg⁻¹ at 1 A g⁻¹ (Fig. 7(g)). At a current density of 20 A g⁻¹, the device provides energy and power densities of 17.7 Wh kg⁻¹ and 16 kW kg⁻¹, respectively, which outperform most previously reported nickel selenides and their composite-based positive electrodes in similar asymmetric supercapacitor configurations. For instance, NiSe_2−x@C//AC (28.02 Wh kg⁻¹ and 773.78 W kg⁻¹),⁶⁹ Ni₃Se₂//AC (23.3 Wh kg⁻¹ and 398.1 W kg⁻¹),⁷⁰ Ni₆Co₆Se@NiTe//AC (67.7 Wh kg⁻¹ and 724.8 W kg⁻¹),⁷¹, (NiCo)Se₂/MoSe₂//AC (55.99 Wh kg⁻¹ and 193.74 W kg⁻¹),⁷² CuCoP@Ni₃Se₂/carbon cloth (CC)//chestnut shells-derived porous carbon (CSC) (62.2 Wh kg⁻¹ and 793 W kg⁻¹),⁷³ CNT@NiSe₂//AC (33.6 Wh kg⁻¹ and 800 W kg⁻¹),⁷⁴ three dimensional graphene (3DG)/(Co,Ni)Se₂/carbon nanowire(CNW)//AC (28 Wh kg⁻¹ and 800 W kg⁻¹),⁷⁵ carbon-coated bimetallic selenide nano-composites (NiCoSe₄@NC)//AC (31.62 Wh kg⁻¹ and 425 W kg⁻¹),⁷⁶ and MXene (Ti₃C₂Tx)/graphene/CoNiSe (MG-CoNiSe)//nitrogen and oxygen doped hierarchical porous carbon (HPC-4) (34.7 Wh kg⁻¹ and 266.7 W kg⁻¹).⁷⁷ One of the primary techniques for analyzing the resistance and capacitive behavior of supercapacitors is electrochemical impedance spectroscopy (EIS). Fig. 7(h) shows the Nyquist plots of the AC//Ni₃Se₄/V₂C/rGO ASC device in the frequency range from 0.01 Hz to 10⁵ Hz at open-circuit potential. The inset of Fig. 7(h) presents the equivalent circuit used to fit the impedance data, where R_s represents the solution resistance, including electrolyte resistance, intrinsic electrode resistance, and contact resistance. The Nyquist plot exhibits a depressed semicircle in the high-to-medium frequency region, which is attributed to the charge-transfer resistance (R_ct) and interfacial capacitance (C_dl). The intercept at the real axis in the high-frequency region represents the solution resistance (R_s). In contrast, the low-frequency region shows a nearly vertical line, indicating efficient ion diffusion and ideal capacitive behavior of the device.⁷⁸ The AC//Ni₃Se₄/V₂C/rGO ASC device exhibits R_s and R_ct values of 0.45 Ω and 1.21 Ω, respectively. The relatively low values of R_s and R_ct indicate improved electrical conductivity and fast charge-transfer kinetics, contributing to the enhanced electrochemical performance of the ASC device.^79,80 The AC//Ni₃Se₄/V₂C/rGO ASC device exhibits outstanding stability (90%) after 10 000 cycles at a current density of 10 A g-1, as shown in Fig. 7(i). At initial cycles, the ASC device exhibits a coulombic efficiency of 98% (eqn (S9)), indicating minimal energy loss during the electrochemical process. The device stabilizes and achieves a slightly higher efficiency of 99% at the middle cycles (cycles from 1100th to 2100th), and retains 97% of coulombic efficiency after 10 000 cycles, demonstrating excellent reversibility of the charge storage mechanism and high electrochemical stability. For a practical demonstration, an LED was successfully powered by the AC//Ni₃Se₄/V₂C/rGO ASC split-cell device, as illustrated in the inset Fig. 7(i). The device was connected to the LED, and it charged up to 1.6 V. Upon a single charging process, the device was able to illuminate the LED for more than five minutes. Such performance showcases the device's capability to maintain power delivery, reinforcing its applicability in practical energy storage systems.

The post-cycling stability analysis of the Ni₃Se₄/V₂C/rGO electrode was done, as illustrated in Fig. 8. The XRD patterns (Fig. S9 and 8(a)) display three distinct peaks corresponding to Ni-foam, which originate from the Ni-foam substrate used as the current collector during electrode fabrication. Additionally, diffraction peaks corresponding to the Ni₃Se₄ phase confirm the retention of the active electrode material after the prolonged electrochemical process.

Fig. 8 Post-studies after split cell cyclic stability studies. (a) XRD analysis, (b–e) SEM images and (f–k) elemental mapping analysis of Ni₃Se₄/V₂C/rGO/Ni foam electrode.

Interestingly, a strong diffraction peak is observed at 2θ = 29.3°, which is indexed to the (101) plane of SeO_2, consistent with the JCPDS card No. 06-0362.⁸¹ The emergence of this SeO₂ phase indicates that a portion of selenium present in the composite undergoes surface oxidation during electrochemical cycling.⁸² The SEM images presented in Fig. 8(b) and its inset reveal that the electrode materials remain firmly anchored to the Ni-foam substrate even after prolonged electrochemical cycling. Furthermore, the high-magnification SEM images (Fig. 8(c and d)) clearly confirm the presence of the Ni₃Se₄, V₂C sheets and rGO layers, which designates the robustness of the electrode architecture. After cycling stability analysis, the V₂C MXene exhibits crumbled outer surfaces, while the rGO nanosheets display roughened morphology. The crumbling of MXene arises from repeated ion intercalation/deintercalation and partial surface oxidation, which induce structural stress and collapse of the layered framework. In contrast, the roughened surface of rGO results from continuous ion adsorption/desorption and wrinkling effects during prolonged cycling. The elemental mapping analysis (Fig. 8(f–k)) reveals an even distribution of Ni, K, Se, V, C, and O across the electrode surface. The presence of K indicates residual KOH within the porous framework, suggesting good electrolyte penetration during the electrochemical process.

3.3 HER applications

The electrocatalytic hydrogen evolution activity of all electrode materials was evaluated in a 1 M KOH electrolyte. The materials-loaded Ni-foam was used as the electrode for HER analysis. The HER performance of the Ni₃Se₄ Ni₃Se₄/V₂C, Ni₃Se₄/rGO, and Ni₃Se₄/V₂C/rGO electrodes was assessed by linear sweep voltammetry (LSV) at 10 mV s⁻¹ of scan rate, as shown in Fig. 9(a). To evaluate the reaction kinetics mechanism of the electrocatalytic process, Tafel slopes (Fig. 9(b)) were derived from the polarization curves by plotting current density against overpotential. A lower Tafel slope indicates a reduced kinetic barrier, reflecting the enhanced catalytic efficiency of the electrode. As expected, the Ni₃Se₄/V₂C/rGO electrode demonstrates good activity and delivers an overpotential of 118.3 mV to generate a cathodic current density of 10 mA cm⁻² with a minimal Tafel slope of 112 mV dec⁻¹. These results are competitive with the benchmark Pt/C electrode, which delivers a superior performance with an overpotential of 51 mV and a Tafel slope of 87 mV dec⁻¹ at the same current density of 10 mA cm⁻². In contrast, Ni₃Se_4, Ni₃Se₄/V₂C, and Ni₃Se₄/rGO electrodes demonstrate the lower HER performance, with comparably higher overpotential values of 136.4 mV (120 mV dec⁻¹), 141 mV (129 mV dec⁻¹), and 143 mV (134 mV dec⁻¹), respectively. The Tafel slope values indicate that the HER process on the electrode surface follows a multi-step reaction pathway, as outlined using eqn (9)–(11):

H₂O + e⁻ → H* + OH⁻/Volmer (9)

H₂O + e⁻ + H* → H₂ + OH⁻/Heyrovsky (10)

H* + H* → H₂/Tafel (11)

Fig. 9 (a) LSV polarization curves, (b) Tafel plot, (c) comparison of overpotential and Tafel slopes. (d) Nyquist plots and (e) electrochemically active surface area analysis of all electrodes. (f) Stability test for the Ni₃Se₄/V₂C/rGO electrode.

The overpotential and Tafel slope values of all electrodes are displayed in Fig. 9 (c). The Tafel slope value of Ni₃Se₄/V₂C/rGO electrode is 112 mV dec⁻¹, which suggests Heyrovsky–Volmer is the rate-determining step during the hydrogen evolution reaction in alkaline media.⁸³ Furthermore, the electrocatalytic reaction kinetics at the electrode–electrolyte interface during the HER process were investigated using EIS, as shown in Fig. 9(d). The Ni₃Se₄/V₂C/rGO (9.35 Ω) and Pt/C (6.15 Ω) electrodes exhibit lower R_ct values related to the Ni₃Se₄ (13.02 Ω), Ni₃Se₄/V₂C (19.21 Ω), and Ni₃Se₄/rGO (9.74 Ω) electrocatalysts. The low R_ct value can be ascribed to the rapid electron transport across the electrode–electrolyte interface, which minimizes energy loss during charge transfer. The Rs values of Ni₃Se₄, Ni₃Se₄/V₂C, Ni₃Se₄/rGO, Ni₃Se₄/V₂C/rGO, and Pt/C are 2.34 Ω, 2.18 Ω, 1.98 Ω, 1.65 Ω, and 1.58 Ω, respectively. The relatively high R_ct of pristine Ni₃Se₄ reflects its limited conductivity and sluggish charge-transfer kinetics. Interestingly, the incorporation of V₂C increases the R_ct, likely due to restacking effects and poor interfacial contact, which hinder efficient electron transport.⁸⁴ In contrast, the Ni₃Se₄/rGO composite shows a lower R_ct owing to the improved conductivity and dispersion provided by the rGO network.⁸⁵ Notably, the ternary Ni₃Se₄/V₂C/rGO composite exhibits the lowest R_ct among the non-noble systems, indicating a strong synergistic effect, where rGO enhances interfacial contact and prevents V₂C restacking, while V₂C facilitates rapid electron transport. As a result, the ternary composite achieves superior charge-transfer kinetics compared to both binary and pristine systems. In addition, the specific surface area (SSA, cm²) and double-layer capacitance (C_dl, mF cm⁻²) were evaluated to estimate the electrochemically active surface area (ECSA), as illustrated in Fig. 9(e) The ECSA was determined by CV analysis, conducted at scan rates ranging from 20 to 100 mV s⁻¹ within the potential window of 0.17–0.27 V versus RHE, where the non-faradaic region is highlighted in Fig. S10(a–d). The elevated C_dl value of 7.11 mF cm⁻² for the Ni₃Se₄/V₂C/rGO electrode reflects a greater availability of electrochemically active sites for hydrogen evolution. Also, it is closely associated with improved interfacial electronic interactions in the ternary composite, where the integration of conductive V₂C and rGO promotes efficient electron delocalization and facilitates rapid charge transfer at the electrode–electrolyte interface.^86,87 In contrast, the Ni₃Se₄ (5.69 mF cm⁻²), Ni₃Se₄/V₂C (4.0 mF cm⁻²), and Ni₃Se₄/rGO (4.40 mF cm⁻²) electrodes exhibit lower C_dl values, reflecting fewer active sites and, consequently, reduced HER performance. Moreover, the specific surface area of Ni₃Se₄ Ni₃Se₄/V₂C, Ni₃Se₄/rGO, and Ni₃Se₄/V₂C/rGO electrodes is 71.12, 51.12, 55.0, and 88.75 cm², respectively. The greater SSA specified greatly increased electrochemically active sites and boosts the electron/proton transport capability in the alkaline electrolyte.⁸⁸ To investigate the stability of the Ni₃Se₄/V₂C/rGO electrode using chronoamperometry at a constant cathode potential of −0.1 V (vs. RHE) for 24 hours in 1.0 M KOH alkaline medium (Fig. 9 (f)). It sustains the current density without deterioration, representing the good stability of the electrocatalyst. Additionally, the HER LSV polarization curves (inset of Fig. 9(f)) of the before and after CA test demonstrated the stability of the newly constructed Ni₃Se₄/V₂C/rGO electrode. The HER properties are higher when compared to previous literature based on Ni₃Se₄/V₂C/rGO and related materials, as shown in Table S4. The post-cycling XRD patterns (Fig. S11(a and b)) confirm the retention of Ni₃Se₄ along with Ni-foam peaks, while the additional peak at 2θ = 29.3° corresponds to SeO₂, indicating partial selenium oxidation during cycling. SEM images (Fig. S11(c–f)) show that the Ni₃Se₄, V₂C sheets, and rGO layers remain well anchored to the Ni-foam, highlighting the robustness of the electrode architecture. Elemental mapping (Fig. S11(g–l)) further designates the even spreading of Ni, K, Se, V, C, and O, with K suggesting residual KOH penetration in the porous framework. The superior bifunctional performance of the Ni₃Se₄/V₂C/rGO composite in both supercapacitor and HER applications can be attributed to a combination of optimized morphology, enhanced interfacial electronic interactions, and improved electrochemical kinetics. SEM analysis shows that pristine Ni₃Se₄ exhibits a densely aggregated morphology, limiting active site accessibility, whereas the incorporation of V₂C and rGO improves dispersion by anchoring Ni₃Se₄ onto conductive 2D frameworks. Notably, the ternary composite displays a well-integrated hierarchical structure, where Ni₃Se₄ nanostructures are uniformly distributed over V₂C nanosheets and rGO nanoflakes, effectively preventing agglomeration and creating continuous electron transport pathways. As a result, the Ni₃Se₄/V₂C/rGO electrode exhibits a reduced R_ct and lower R_s, indicating enhanced charge-transfer kinetics at the electrode–electrolyte interface. Furthermore, the higher C_dl and SSA confirm an increased ECSA, providing abundant active sites. The strong interfacial coupling among Ni₃Se₄, V₂C, and rGO facilitates efficient electron delocalization and rapid charge transfer. These combined effects promote both efficient charge storage through enhanced ion adsorption and improved HER activity via accelerated reaction kinetics. Overall, the synergistic integration of structural optimization, interfacial electronic interactions, and charge-transfer enhancement enables the Ni₃Se₄/V₂C/rGO composite to function as an effective bifunctional platform for energy storage and hydrogen evolution applications.

4. Conclusion

The Ni₃Se₄/V₂C/rGO composite demonstrated markedly superior electrochemical performance compared to the binary systems, combining high capacity, excellent rate capability, and long-term cycling stability in supercapacitors with efficient HER activity, low overpotential, and sustained durability. These findings emphasize the synergistic effect of integrating Ni₃Se₄ mixed morphologies with conductive V₂C MXene nanosheets and rGO layers, where the dual-carbon design provides enhanced conductivity, structural stability, and ion-accessible channels, thereby ensuring outstanding electrochemical performance. Further improvements may be achieved through heteroatom doping, defect engineering, or constructing advanced heterostructures, supported by theoretical modelling to better understand charge-storage and catalytic mechanisms. On the application side, emphasis on scalable fabrication, electrode architecture optimization, and integration into flexible or hybrid energy devices could accelerate the transition of this ternary system toward practical technologies. Overall, this work not only validates Ni₃Se₄/V₂C/rGO as a promising bifunctional electrode but also offers a viable pathway for designing next-generation multifunctional materials for sustainable energy storage and conversion.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: materials, characterization, electrode preparation for supercapacitors and HER applications, XRD spectra, Raman, Survey spectra, BET results, SEM analysis, TEM analysis, rate capability graph, logarithmic plots comparison tables for supercapacitors and HER applications, ECSA analysis and post-stability analysis. See DOI: https://doi.org/10.1039/d6ta01599a.

Acknowledgements

The work was supported by the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (Grant No. RS-2023-00217581; RS-2024-00345983), Republic of Korea. This research was supported by the Korea Basic Science Institute (National Research Facilities and Equipment Center) grant funded by the Ministry of Education (No. RS-2025-02317677).

References

1. J. Yesuraj, J. Kim, R. Yang and K. Kim, Adv. Compos. Hybrid Mater., 2024, 7(2) DOI:10.1007/s42114-024-00881-y.
2. C. Zhong, Y. Deng, W. Hu, J. Qiao, L. Zhang and J. Zhang, Chem. Soc. Rev., 2015, 44, 7484–7539.
3. Y. Zhang, Q. Zhou, J. Zhu, Q. Yan, S. X. Dou and W. Sun, Adv. Funct. Mater., 2017, 27, 1702317.
4. M. Duraisamy, V. Chinnuswamy, P. Sengodu, S. Veeman and S. Sreekantan, Energy Fuels, 2023, 37, 19915–19924.
5. A. Padhy, S. Lakshmy, B. Chakraborty and J. N. Behera, Sustainable Energy Fuels, 2023, 7, 5271–5282.
6. T. T. Mishra, M. Chakraborty, J. N. Behera and D. Roy, Energy Fuels, 2024, 38, 9186–9217.
7. P. Veerakumar, A. Sangili, S. Manavalan, P. Thanasekaran and K.-C. Lin, Ind. Eng. Chem. Res., 2020, 59, 6347–6374.
8. Y. Dong, J. Zhu, Q. Li, S. Zhang, H. Song and D. Jia, J. Mater. Chem. A, 2020, 8, 21930–21946.
9. M. Salanne, B. Rotenberg, K. Naoi, K. Kaneko, P.-L. Taberna, C. P. Grey, B. Dunn and P. Simon, Nat. Energy, 2016, 1, 16070.
10. K. Jiang, B. Liu, M. Luo, S. Ning, M. Peng, Y. Zhao, Y.-R. Lu, T.-S. Chan, F. M. F. de Groot and Y. Tan, Nat. Commun., 2019, 10, 1743.
11. M. K. Singh, S. Krishnan, K. Singh and D. K. Rai, Energy Fuels, 2024, 38, 12098–12110.
12. S. Jiang, R. Zhang, M. Pang, Z. Song, Z. Wu, Y. Jiao and J. Zhao, ACS Appl. Nano Mater., 2024, 7, 23443–23453.
13. G. Liu, K. Yan, F. Zhou, Y. Wang, C. Zhuang, C. Wang and D. Tian, Electrochim. Acta, 2023, 464, 142922.
14. L.-J. Xu, X.-J. Wang, G.-Y. Tang, B.-C. Zhu, J.-G. Yu, L.-Y. Zhang and T. Liu, Rare Met., 2025, 44, 185–194.
15. C. (John) Zhang, Y. Ma, X. Zhang, S. Abdolhosseinzadeh, H. Sheng, W. Lan, A. Pakdel, J. Heier and F. Nüesch, Energy Environ. Mater., 2020, 3, 29–55.
16. L. Mi, Q. Ding, H. Sun, W. Chen, Y. Zhang, C. Liu, H. Hou, Z. Zheng and C. Shen, CrystEngComm, 2013, 15, 2624.
17. X. Lou, C. Lin, Q. Luo, J. Zhao, B. Wang, J. Li, Q. Shao, X. Guo, N. Wang and Z. Guo, ChemElectroChem, 2017, 4, 3171–3180.
18. S. Jiang, R. Zhang, M. Pang, Z. Song, Z. Wu, Y. Jiao and J. Zhao, ACS Appl. Nano Mater., 2024, 7, 23443–23453.
19. G. T. Chavan, R. U. Amate, H. Lee, A. Syed, A. H. Bahkali, A. M. Elgorban and C.-W. Jeon, J. Energy Storage, 2023, 61, 106757.
20. B. Kirubasankar, V. Murugadoss, J. Lin, T. Ding, M. Dong, H. Liu, J. Zhang, T. Li, N. Wang, Z. Guo and S. Angaiah, Nanoscale, 2018, 10, 20414–20425.
21. D. S. Su and G. Centi, J. Energy Chem., 2013, 22, 151–173.
22. H. Hao, R. Tan, C. Ye and C. T. J. Low, Carbon Energy, 2024, 6(12), e604.
23. J. Yesuraj, P. Naveenkumar, M. Maniyazagan, H.-W. Yang, S.-J. Kim and K. Kim, J. Mater. Chem. A, 2025, 13, 21830–21846.
24. P. Li, K. Zhang and J. H. Park, J. Mater. Chem. A, 2018, 6, 1900–1914.
25. Y.-J. Kim, S. J. Kim, D. Seo, Y. Chae, M. Anayee, Y. Lee, Y. Gogotsi, C. W. Ahn and H.-T. Jung, Chem. Mater., 2021, 33, 6346–6355.
26. X. Li, Z. Huang, C. E. Shuck, G. Liang, Y. Gogotsi and C. Zhi, Nat. Rev. Chem., 2022, 6, 389–404.
27. R. Venkatkarthick, N. Rodthongkum, X. Zhang, S. Wang, P. Pattananuwat, Y. Zhao, R. Liu and J. Qin, ACS Appl. Energy Mater., 2020, 3, 4677–4689.
28. S. Korkmaz and İ. A. Kariper, J. Energy Storage, 2020, 27, 101038.
29. N. Morimoto, T. Kubo and Y. Nishina, Sci. Rep., 2016, 6, 21715.
30. Y. Jiang, M. Tian, H. Wang, C. Wei, Z. Sun, M. H. Rummeli, P. Strasser, J. Sun and R. Yang, ACS Nano, 2021, 15, 19640–19650.
31. S. Anantharaj, J. Kennedy and S. Kundu, ACS Appl. Mater. Interfaces, 2017, 9, 8714–8728.
32. C. K. Chua and M. Pumera, Chem. Commun., 2016, 52, 72–75.
33. R. Que, G. Ji, D. Liu, M. Li and S. Liu, ChemNanoMat, 2019, 5, 814–819.
34. D. Song, H. Wang, X. Wang, B. Yu and Y. Chen, Electrochim. Acta, 2017, 254, 230–237.
35. Y. Feng, S. Wang, H. Wang, Y. Zhong and Y. Hu, J. Mater. Sci., 2020, 55, 13927–13937.
36. S. Kukunuri, M. R. Krishnan and S. Sampath, Phys. Chem. Chem. Phys., 2015, 17, 23448–23459.
37. A. Y. Faid, A. O. Barnett, F. Seland and S. Sunde, Electrochim. Acta, 2020, 361, 137040.
38. H. Ahmad, M. J. M. Makhfuz, N. Yusoff and H. S. Albaqawi, Phys. Scr., 2024, 99, 015505.
39. M. Wu, B. Wang, Q. Hu, L. Wang and A. Zhou, Materials, 2018, 11, 2112.
40. H. Ahmad, M. J. M. Makhfuz, N. Yusoff and H. S. Albaqawi, Phys. Scr., 2024, 99, 015505.
41. J.-B. Wu, M.-L. Lin, X. Cong, H.-N. Liu and P.-H. Tan, Chem. Soc. Rev., 2018, 47, 1822–1873.
42. H. Zhang, S. Zheng, J. Tang, R. Chen, J. Yang, W. Tong and J. Guo, New J. Chem., 2023, 47, 11675–11684.
43. B. Jiang, Y. Liu, J. Zhang, Y. Wang, X. Zhang, R. Zhang, L.-L. Huang and D. Zhang, RSC Adv., 2022, 12, 1471–1478.
44. W. Wei, L. Mi, Y. Gao, Z. Zheng, W. Chen and X. Guan, Chem. Mater., 2014, 26, 3418–3426.
45. H. Jiang, Z. Wang, Q. Yang, L. Tan, L. Dong and M. Dong, Nanomicro Lett., 2019, 11, 31.
46. Y. Gu, L.-Q. Fan, J.-L. Huang, C.-L. Geng, J.-M. Lin, M.-L. Huang, Y.-F. Huang and J.-H. Wu, J. Power Sources, 2019, 425, 60–68.
47. L. Meng, Y. Wu, T. Zhang, H. Tang, Y. Tian, Y. Yuan, Q. Zhang, Y. Zeng and J. Lu, J. Mater. Sci., 2019, 54, 571–581.
48. Y. Liu, Y. Jiang, Z. Hu, J. Peng, W. Lai, D. Wu, S. Zuo, J. Zhang, B. Chen, Z. Dai, Y. Yang, Y. Huang, W. Zhang, W. Zhao, W. Zhang, L. Wang and S. Chou, Adv. Funct. Mater., 2021, 31, 2008033.
49. F. N. M. Azlan, M. A. A. M. Abdah, Y. S. Tan, M. N. Mustafa, R. Walvekar and M. Khalid, J. Energy Storage, 2023, 72, 108620.
50. C. Zhang, Z. Hai Wu, Z. Qing Yang, Y. Xin Yu and Y. Yang, Chem. Eng. J., 2025, 511, 162146.
51. D. R. Dreyer, S. Park, C. W. Bielawski and R. S. Ruoff, Chem. Soc. Rev., 2010, 39, 228–240.
52. H.-J. Lee, J. S. Kim, K. Y. Lee, K. H. Park, J.-S. Bae, M. Mubarak and H. Lee, Sci. Rep., 2019, 9, 557.
53. O. Mashtalir, M. Naguib, V. N. Mochalin, Y. Dall'Agnese, M. Heon, M. W. Barsoum and Y. Gogotsi, Nat. Commun., 2013, 4, 1716.
54. K. Montazeri, H. Badr, K. Ngo, K. Sudhakar, T. Elmelegy, J. Uzarski, V. Natu and M. W. Barsoum, J. Phys. Chem. C, 2023, 127, 10391–10397.
55. F. Han, S. Luo, L. Xie, J. Zhu, W. Wei, X. Chen, F. Liu, W. Chen, J. Zhao, L. Dong, K. Yu, X. Zeng, F. Rao, L. Wang and Y. Huang, ACS Appl. Mater. Interfaces, 2019, 11, 8443–8452.
56. A. Sobhani and M. Salavati-Niasari, Mater. Res. Bull., 2012, 47, 1905–1911.
57. A. Sobhani, F. Davar and M. Salavati-Niasari, Appl. Surf. Sci., 2011, 257, 7982–7987.
58. Q. Peng, Y. Dong, Z. Deng and Y. Li, Inorg. Chem., 2002, 41, 5249–5254.
59. Q. Peng, Y. Dong, Z. Deng, H. Kou, S. Gao and Y. Li, J. Phys. Chem. B, 2002, 106, 9261–9265.
60. A. Sobhani and M. Salavati-Niasari, Superlattices Microstruct., 2014, 65, 79–90.
61. X. Liu, D. Zhang, Y. Ma, G. Li, X. Yuan, Y. Huang, G. Liu, M. Guo and W. Zheng, ACS Appl. Nano Mater., 2024, 7, 8880–8889.
62. Q.-Q. Liu, K.-F. Yue, X.-J. Weng and Y.-Y. Wang, CrystEngComm, 2019, 21, 6186–6195.
63. P. Naveenkumar, J. Yesuraj, M. Maniyazagan, N. Kang, H.-W. Yang, K. Kim and S.-J. Kim, Chem. Eng. J., 2024, 488, 150937.
64. G. Z. Chen, Prog. Nat. Sci.:Mater. Int., 2021, 31, 792–800.
65. J. Liu, J. Wang, C. Xu, H. Jiang, C. Li, L. Zhang, J. Lin and Z. X. Shen, Adv. Sci., 2018, 5(1), 1700322.
66. S. M. Bekhit, S. G. Mohamed, I. M. Ghayad, M. G. Fayed, W. Metwally, R. Abdel-Karim and S. M. El-Raghy, J. Energy Storage, 2022, 53, 105215.
67. B. Wu, M. Li, V. Mazánek, Z. Liao, Y. Ying, F. M. Oliveira, L. Dekanovsky, L. Jan, G. Hou, N. Antonatos, Q. Wei, M. Li, B. Pal, J. He, D. Koňáková, E. Vejmělková and Z. Sofer, Small Methods, 2024, 8(9), 2301461.
68. P. Naveenkumar, J. Yesuraj, M. Maniyazagan, H.-W. Yang, K. Kim and S.-J. Kim, J. Alloys Compd., 2025, 1014, 178737.
69. H. Liu, C. Liu, G. Zhao, Y. Wei, Y. Gao, S. Jia, Z. Yu, Y. Jiang, Y. Zhang, G. Shi and G. Wang, J. Mater. Sci., 2025, 60, 15113–15126.
70. S. Jiang, J. Wu, B. Ye, Y. Fan, J. Ge, Q. Guo and M. Huang, J. Mater. Sci.: Mater. Electron., 2018, 29, 4649–4657.
71. D. Khalafallah, W. Huang, M. Zhi and Z. Hong, Energy Environ. Mater., 2024, 7(1), e12528.
72. W. Sun, Z. Liu, D. Liu, B. Zhang, Y. Li, C. Wang, X. Liu, X. Wang, X.-Z. Song and Z. Tan, J. Mater. Chem. A, 2025, 13, 22792–22803.
73. Y. Zhang, C. Du, M. Xie, J. Chen and L. Wan, Appl. Surf. Sci., 2025, 697, 163058.
74. Y.-Z. Su, M. Mathankumar, W.-Y. Lee, P. Hasin, B. Subramanian, C.-K. Hsieh and J.-Y. Lin, J. Energy Storage, 2024, 89, 111728.
75. L. Zhou, G. Feng, X. Li, F. Jiang, H. Li, Y. Liu, Q. Yu, H. Cao, Y. Xu and Y. Zhu, Colloids Surf., A, 2024, 698, 134595.
76. C. Liu, G. Zhao, Y. Wei, Y. Gao, X. Li, T. Wang, H. Liu, Z. Yu, G. Shi and G. Wang, Diamond Relat. Mater., 2025, 152, 111913.
77. J. Chen, T. Song, M. Sun, S. Qi, C. Chen, Y. Zhao and X. Wu, J. Alloys Compd., 2024, 1009, 176786.
78. S. Feliu, Metals, 2020, 10, 775.
79. T. Wu, J. Li, L. Hou, C. Yuan, L. Yang and X. Zhang, Electrochim. Acta, 2012, 81, 172–178.
80. Z. Fan, J. Yan, T. Wei, L. Zhi, G. Ning, T. Li and F. Wei, Adv. Funct. Mater., 2011, 21, 2366–2375.
81. A. M. El Sayed and T. I. Alanazi, Sci. Rep., 2024, 14, 3398.
82. P. J. J. Sagayaraj, K. Oyama, N. Okibe, A. Sengeni, H. Kim and K. Sekar, J. Mater. Chem. A, 2025, 13, 29461–29470.
83. X. Wu, T. Dhanasekaran, W. Han, Z. Dan, Y. Li, W. Guiling and Z. Jing, J. Mater. Chem. C, 2025, 13, 19734–19748.
84. Y. Wu, J. Li, G. Sui, D. Chai, Y. Li, D. Guo, D. Chu and K. Liang, Carbon Energy, 2024, 6(10), e583,  DOI:10.1002/cey2.583.
85. F. Iskandar, U. Hikmah, E. Stavila and A. H. Aimon, RSC Adv., 2017, 7, 52391–52397.
86. S. Zhou, X. Yang, W. Pei, N. Liu and J. Zhao, Nanoscale, 2018, 10, 10876–10883.
87. L. Xu, T. Wu, P. R. C. Kent and D. Jiang, Phys. Rev. Mater., 2021, 5, 054007.
88. Y. Wang, L. Chen, H. Zhang, M. Humayun, J. Duan, X. Xu, Y. Fu, M. Bououdina and C. Wang, Green Chem., 2023, 25, 8181–8195.

---

Footnote

† These authors equally contributed to this work.

---