Engineered Cu₀.₅Ni₀.₅Al₂O₄/GCN Spinel Nanostructures for Dual-Functional Energy Storage and Electrocatalytic Water Splitting

Abstract

The rapid growth in population and industrialization have significantly increased global energy demand, placing immense pressure on finite and environmentally harmful conventional fossil fuel-based energy sources. In this context, the development of hybrid electrocatalysts presents a crucial solution for energy conversion and storage, addressing environmental challenges while meeting rising energy needs. In this study, the fabrication of a novel bifunctional catalyst, copper nickel aluminum spinel (Cu_0.5Ni_0.5Al₂O₄) supported on graphitic carbon nitride (GCN), using a solid-state synthesis process is reported. Because of its effective interface design and spinel cubic structure, the Cu_0.5Ni_0.5Al₂O₄/GCN nanocomposite, as synthesized, performs exceptionally well in electrochemical energy conversion, such as the oxygen evolution reaction (OER), the hydrogen evolution reaction (HER), and energy storage. In particular, compared to noble metals, Pt/C- and IrO₂-based water-splitting cells require higher voltages (1.70 V), while for the Cu_0.5Ni_0.5Al₂O₄/GCN nanocomposite, a voltage of 1.49 V is sufficient to generate a current density of 10 mA cm⁻² in an alkaline solution. When used as supercapacitor electrode materials, Cu_0.5Ni_0.5Al₂O₄/GCN nanocomposites show a specific capacitance of 1290 F g⁻¹ at a current density of 1 A g⁻¹ and maintain a specific capacitance of 609 F g⁻¹ even at a higher current density of 5 A g⁻¹, suggesting exceptional rate performance and charge storage capacity. The electrode’s exceptional capacitive properties were further confirmed through the determination of the roughness factor (Rf), which represents surface heterogeneity and active area enhancement, with a value of 345.5. These distinctive characteristics render the Cu_0.5Ni_0.5Al₂O₄/GCN composite a compelling alternative to fossil fuels in the ongoing quest for a viable replacement. Undoubtedly, the creation of the Cu_0.5Ni_0.5Al₂O₄/GCN composite represents a significant breakthrough in addressing the energy crisis and environmental concerns. Owing to its unique composition and electrocatalytic characteristics, it is considered a feasible choice in the pursuit of ecologically sustainable alternatives to fossil fuels.

1. Introduction

The worldwide problem regarding global energy demand is considerably exacerbated by the widespread usage of non-renewable energy sources like fossil fuels, particularly CO₂, which significantly contributes to global warming and air pollution and is produced by carbon-based fuel reduction [1,2,3]. The increasing depletion of fossil fuels necessitates the exploration of alternative fuel sources that are environmentally friendly and sustainable. Current fossil fuel-based energy systems, which are not only limited but are also significant contributors to greenhouse gas emissions and climate change, are under tremendous strain due to the rising global energy demand brought on by fast growth in industrialization and the population. The International Energy Agency (IEA) predicts that by 2040, the world’s energy consumption will have increased by around 25%, with developing nations accounting for a sizable amount of this demand. The need to switch to ecologically friendly and sustainable energy sources has become more urgent as a result [4].

Among these types of energy sources, natural resources like solar energy, wind power, and water electrolysis present compelling opportunities for producing green fuels [5]. Supercapacitors have demonstrated superiority over other renewable energy storage technologies due to their quick charge/discharge, strong cycling stability, minimal environmental impact, and excellent fire safety, all of which have significant potential to promote water electrolysis. While integrated configurations involving water-splitting systems powered by renewable energy-powered energy storage systems (such as zinc–air, lithium-ion and sodium-ion batteries) have been reported, fewer integrated configurations involving a two-electrode electrolyzer powered by supercapacitors and charged by a solar cell have been documented. Conversely, as the electrocatalyst’s activity in the cathodic hydrogen evolution process (HER) and the anodic oxygen evolution reaction (OER) is a major determinant of overall water-splitting efficiency, it is desirable to rationally design specific electrocatalysts. Pt for the HER and IrO₂/RuO₂ for the OER are examples of noble metal-based electrocatalysts that have been studied to date and have demonstrated excellent reaction kinetics [6,7]. However, these electrocatalysts’ low durability and high costs limit their practical use. Thus, there is interest in creating bifunctional electrocatalysts for the HER and OER that are highly effective, long-lasting, and reasonably priced. Bifunctional electrocatalysts can further save production costs by avoiding complex electrode manufacturing and simplifying water-splitting equipment. Designing a multipurpose electrode material that can electrocatalyze the HER and OER at the same time and offer supercapacitors a large charge storage capacity is therefore essential.

Through effective water splitting, water electrocatalysis is essential to the production of clean fuels like hydrogen and oxygen. Fujishima and Honda carried out groundbreaking work on TiO₂-based water-splitting devices and demonstrated the potential for converting electricity into usable energy [6,7]. However, in the OER a critical step in this process is inherently slow due to the complexity of breaking O-H bonds and forming O-O bonds, which requires a high overpotential and limits the efficiency of TiO₂-based systems [8].

The challenge of developing stable, cost-effective, and efficient catalysts for water splitting, despite extensive research, remains significant. Nanodevice performance is often hindered by inefficient ion and electron transfer processes [9,10]. While RuO₂, Pt, and IrO₂ nanocatalysts exhibit promising properties, including low Tafel slopes and high current densities, their scarcity and high cost limit their widespread application in the OER and HER [11,12,13,14,15]. Consequently, the persistent need exists for the development of active, non-precious metal-based catalysts. Researchers are exploring inexpensive, abundant earth metals capable of achieving high energy yields at lower potentials to address this bottleneck. Additionally, building green energy storage devices, like capacitors, lithium-ion batteries, and metal-ion batteries, is crucial to meet growing global energy demand [16,17].

Furthermore, energy storage system supercapacitors (SCs) have garnered devotion due to their exceptional characteristics, such as low internal resistance, extended life cycles (>105 cycles), rapid charge and discharge rates, and high-power density (10 kW kg⁻¹). These features make them favorable over traditional batteries and dielectric capacitors, leading to their application in mobile phones, laptops, electric vehicles, memory cards, and flashlights [18,19]. SCs are categorized into electrochemical double-layer capacitors (EDLCs storing charge at the electrode–electrolyte interface) and pseudocapacitors using reversible faradic processes [20,21,22]. However, in order to integrate SCs into everyday power supply systems, they need to be further developed to give high power and energy density [23].

The electrochemical performance of SCs is primarily influenced by the electrode material [24,25,26]. Nanostructured materials’ enormous surface area improves electrode performance by increasing the number of catalytic sites accessible for electrochemical processes [27,28]. However, challenges such as the limited stability of transition metal sulfides, poor electrical conductivity of metal oxides, and suboptimal performance of carbon-based materials hinder their effectiveness in fuel cells, batteries, and SCs [29,30,31,32]. Single transition metals often exhibit slow kinetics and limited electroactive sites, further restricting their electrochemical properties. Recent studies suggest that defect engineering and hybrid structures can address these limitations by enhancing material performance [33,34,35]. For example, Zhang and colleagues demonstrated that cobalt-deficient Co₃-xO₄ crystals improve water-splitting activities by facilitating water adsorption and activation at the catalyst surface [36]. Similarly, Tahir et al. developed a superior electrocatalyst using NiO/Co₃O₄ nanoparticles supported on nitrogen-doped carbon, enhancing OER and HER performance [37].

SCs and EWS electrode materials include metallic nitrides (Ni₃N, MoN, FeN), phosphides (NiP, CoP), and non-noble metal alloys (NiCo, FeNi, NiMn) [38,39,40,41]. Binary metal hybrid nanostructures, in particular, offer improved performance due to the synergistic interactions of distinct electronic structures, which enhance bond rearrangement and catalytic activity. For instance, chlorine-doped carbonated cobalt hydroxide nanowires demonstrate enhanced pseudocapacitance and energy density through deep faradic redox reactions involving the entire nanowire structure [42]. Copper electrodes are also highly regarded for SCs due to their abundant availability and excellent charge storage capacity [43].

Our research introduces a novel bifunctional catalyst Cu_0.5Ni_0.5Al₂O₄ supported on graphitic carbon nitride (GCN) for energy conversion and storage. This hybrid material has outstanding bifunctional performance, with a Csp of 1290 F g⁻¹ at 1 A g⁻¹ and catalytic activity for the OER (10 mA cm⁻² at 254 mV) and HER (101 mV). These results underscore the potential of Cu_0.5Ni_0.5Al₂O₄ as a dual-purpose nanocatalyst for commercial applications in EWS and SCs. Our findings highlight the importance of hybrid composite engineering and theoretical approaches to further optimize catalytic performance, bridging the gap between experimental challenges and practical applications.

2. Experimental Section

2.1. Reagents

All the purchased chemicals were used without further purity. Potassium hydroxide (KOH) 98%, (nickel (II) acetate) 99%, acetate di-hydrate, copper (I) chloride 99%, and aluminum oxide (Al₂O₃) 98% were acquired from Sigma-Aldrich Chemie GmbH, Steinheim, Germany Solvent 95% ethanol (C₂H₅OH) was bought from Merck KGaA, Darmstadt, Germany.

2.2. Fabrication of Spinel Cu_0.5Ni_0.5Al₂O₄

A solid-state approach for fabricating Cu_0.5Ni_0.5Al₂O₄ requires combining stoichiometric quantities of CuCl₂, (nickel (II) acetate), and Al₂O₃ powders, followed by contiunous grinding to achieve uniformity. The powders are subsequently subjected to calcination at temperatures going from 800 °C to 900 °C in order to facilitate a reaction and eliminate any volatile compounds. Following cooling and regrinding, the calcined material is subjected to high-temperature sintering at temperatures varying from 1200 to 1400 °C for a duration of 12–24 h in order to fully develop the spinel structure. The product is cooled to ambient temperature and analyzed using X-ray diffraction (XRD) and other methods to verify the growth and quality of the Cu_0.5Ni_0.5Al₂O₄ compound.

2.3. Fabrication of GCN

Graphitic carbon nitride (GCN) is commonly synthesized via a thermal polymerization method using thiourea. Thiourea, serving as both a nitrogen and carbon source, undergoes high-temperature heating to produce GCN. Specifically, thiourea is heated in a muffle furnace at 500–600 °C for 2–4 h under ambient air conditions. During heating, thiourea decomposes and polymerizes to form GCN. The resulting yellow powder is then allowed to cool spontaneously to ambient temperature. This technique produces GCN with a stratified crystal structure, making it suitable for various applications such as overall water splitting and energy storage.

2.4. Synthesis of Spinel Cu_0.5Ni_0.5Al₂O₄/GCN Composite

To synthesize a composite via sonication, first disperse the required amounts (1:1) of spinel and GCN powders in water to establish a homogenous suspension. This combination is then sonicated for about 1 h in an ultrasonic bath to break up agglomerates and improve material interaction. The suspension is dried by evaporating the solvent under reduced pressure or heating at a low temperature to a solid composite powder. The synthesis scheme of the spinel Cu_0.5Ni_0.5Al₂O₄/GCN composite is shown in Figure 1.

2.5. Electrode Preparation

In order to create a working electrode, this work employed drop casting to deposit the newly synthesized electrocatalyst onto nickel foam (NF). To do this, NF was cut into 1 cm by 1 cm pieces, which were then washed for 5 min each in acetone, deionized (DI) water, and ethanol. The NF pieces were dried for 4 h at 70 °C after washing. To prepare a homogeneous catalyst ink without binding agents for electrochemical experiments, 200 μL of deionized water was added dropwise to 5 mg of the synthesized electrocatalyst powder. After coating the freshly cleaned NF surface with the catalyst ink, it was allowed to air-dry before being used for general water splitting and supercapacitor (SCs) applications.

2.6. Examinations of Electrochemistry

The electrochemical workstation Corrtest Bi-Potentiostat C-2350 from Wuhan Corrtest Instruments Corp., Ltd., Wuhan, China, was used for all electrochemical investigations, including chronoamperometry (CA), cyclic voltammetry (CV), electrical impedance spectroscopy (EIS), and galvanostatic charge/discharge (GCD), and were performed with an electrolyte of a 2 M KOH aqueous solution. Electrochemical performances were tested using a standard three-electrode setup, consisting of Pyrex glass covered in Teflon, Pt wire (counter electrode), Ag/AgCl (reference electrode), and NF plated with synthetic material. Linear sweep voltammetry (LSV) and CV electrochemical data were developed on a scan rate of 5 mV s⁻¹. To ensure accuracy and reproducibility, these tests were repeated after each performance at room temperature. In this instance, the working electrode 0.5 cm² geometrical surface area was taken into calculation of all current densities (j).

The potential ranges for Cu_0.5Ni_0.5Al₂O₄/GCN, Cu_0.5Ni_0.5Al₂O₄, and GCN were discovered to be between −0.6 and 0.6 V; Equation (1) was used to determine the specific capacitance (Csp) of the spinel, GCN, and spinel/GCN composite.

$$Cs={\displaystyle \frac{IdE}{2(m\times S\times \Delta V)}}$$

(1)

In this case, dV denotes the potential change derived from the redox peaks generated by CV, and I indicates the current. In contrast, CV polarization measurements use m, S, and ∆V to represent the loaded mass, scan rate, and applied potential window (−0.6–0.6 V).

Equations (2)–(4) were utilized to determine the values of the Csp, energy density (Ed), and power density (Pd) for each produced electrode based on GCD analysis [44].

$$Cs={\displaystyle \frac{I\times \Delta t}{m\times \Delta V}}$$

(2)

$$Ed={\displaystyle \frac{C_{sp}\times {\Delta V}^2}{7.2}}$$

(3)

$$P_d={\displaystyle \frac{E\times 3600}{\Delta t}}$$

(4)

The potential window variation is represented by delta V, and the mass of the loaded material is indicated by m, the constant discharging current by (I), and the discharging duration by delta t.

The charge transfer resistance (Rct) and solution resistance (Rs) between the electrode and electrolyte phases were evaluated at frequencies ranging from 0.1 to 1 MHz using EIS. The Nyquist plot display was performed in an open circuit with the data and was carried out at an amplitude of 10 mV and potential of 0.55 V. ZView software (version 4.1b) was used for curve fitting in the analysis of Nyquist plots utilizing equivalent circuit modeling.

Next, using Equation (5), the reference electrode potential of Ag/AgCl was converted to the RHE.

$$E_{RHE}=E_{Ag/AgCl}+E+0.059\times pH$$

(5)

Here, the thermodynamic potential (E) in a 1 M KOH solution with a silver wire dip is represented by E_Ag/AgCl.

The electrocatalysts’ kinetic efficiency and catalytic efficiency were examined using CV Tafel graphs. Equation (6) was used to graph a steady-state polarization curve in the linear zone to create these plots [45].

$$\eta =a+\left(2.303\times R\times {\displaystyle \frac{T}{\alpha nF}}\right)\times \mathit{log}j$$

(6)

where α is the charge transfer coefficient, n is the number of electrons transported during a redox reaction, j is the current density in milliampere-seconds (mA cm⁻²), F represents Faraday constant = 96,485, η is the overpotential, and 2.303/αnF is the slope value.

This allowed for determination of the value of C_dl. The evaluation of the ECSA was conducted by dividing the obtained capacitance of the flat electrodes (C_dl) by the Csp, which was derived from the previously published data (0.04 μF cm⁻²) [46], as shown in Equations (7) and (8).

$$C_{dl}={\displaystyle \frac{slope}{2}}$$

(7)

$$ECSA={\displaystyle \frac{C_{dl}}{C_{sp}}}$$

(8)

One crucial factor in assessing the OER activity of a catalyst is its calculable turnover frequency (TOF), which is calculated using Equation (9). Under defined reaction circumstances, the TOF is applied to quantify the specific activity of a catalytic center for HER and OER reactions.

$$TOF={\displaystyle \frac{{j\times N}_A}{4}}$$

(9)

In this example, N_A is the Avogadro number, n is the quantity of active catalysts in moles, and F is the Faraday constant. The single conventional techniques for verifying the long-term stability and wear resistance of the Cu_0.5Ni_0.5Al₂O₄/GCN composite comprise chronopotentiometry. With the stability of Cu_0.5Ni_0.5Al₂O₄/GCN using the chronoamperometry approach, the electrode material was evaluated at a fixed voltage of 0.75 V.

2.7. Characterization (Physical)

The crystallographic microstructure of the nanocomposite Cu_0.5Ni_0.5Al₂O₄/GCN, spinel Cu_0.5Ni_0.5Al₂O₄, and GCN generated by reduction was investigated employing an advanced D8 Bruker diffractometer with a Cu-kα monochromatic radiation source for powder X-ray diffraction from Bruker AXS GmbH, Germany. Ten milliamperes of current and 30 kV of voltage were employed for the trials. Using a scanning electron microscope (SEM), FEI Nova NanoSEM-450 from FEI, USA, was utilized to study the microstructure and shape of the final products. The composition of the generated electrode materials was studied utilizing energy-dispersive X-ray spectroscopy (EDX) utilizing a JSM-5910 energy-dispersive spectrometer from JEOL Ltd., Japan.

3. Results and Discussions

3.1. X-Ray Diffraction Analysis

XRD examination was applied to determine the crystal structure of spinel Cu_0.5Ni_0.5Al₂O₄, GCN, and Cu_0.5Ni_0.5Al₂O₄/GCN composites. Figure 2a illustrates a diffractogram spanning a range of 2θ from 10° to 80°. The observed XRD peaks at 2θ (degree) 20.06°, 32.13°, 35.21°, 36.85°, 38.39°, 42.91°, 48.49°, 57.89°, 62.43°, 65.95°, 67.85°, 69.28°, 74.38°, and 78.69° correspond to crystal planes (111), (220), (311), (222), (400), (331), (422), (511), and (440), (531), (442), (620), and (622) of spinel Cu_0.5Ni_0.5Al₂O₄. These crystal planes align with the cubic structure of Cu_0.5Ni_0.5Al₂O₄, with standard lattice parameters of a = b = c = 8.0590 Å. This diffracted pattern is in accordance with JCPDS ref. # 01-078-1603. For GCN, the peak observed at an angle of 26.28° attributed to the diffraction plane labeled as (002) indicates an interlayer spacing along the c-axis of 3.372 Å. The determination of interlayer spacing was performed using Bragg law, applied to the (002) peak. The X-ray diffraction pattern of Cu_0.5Ni_0.5Al₂O₄/GCN composites exhibits identical distinct peaks as those observed in Cu_0.5Ni_0.5Al₂O₄, with slight displacement in the peak positions, validating the successful manufacturing of the nanocomposite. Scherrer expression (Equation (10) was employed to determine the crystallite sizes of Cu_0.5Ni_0.5Al₂O₄/GCN composites, GCN, and spinel Cu_0.5Ni_0.5Al₂O₄ samples, resulting in values of 17.58 nm, 17.2 nm, and 18.21 nm, respectively.

$$D={\displaystyle \frac{k\lambda }{\beta Cos\theta }}$$

(10)

All the crystallographic structural details determined are provided in

Table 1. Structural parameters calculated via XRD pattern.

Samples	(D) nm	(1/D2) (10−15)	($\mathsf{\epsilon }$) 10−3	(L.S) 10−3
Cu0.5Ni0.5Al2O4/GCN	17.58	32.34	2.13	1.84
GCN	17.2	33.47	0.20	5.07
Cu0.5Ni0.5Al2O4	18.21	30.16	2.22	0.38

. Cu_0.5Ni_0.5Al₂O₄/GCN composites exhibit the smallest crystallite size of 17.58 nm, the highest dislocation density of 32.34 × 10^–15, a moderate micro-strain of 2.13 × 10⁻³, and a relatively high lattice strain of 1.84 × 10⁻³. These characteristics indicate the highly defective structure of Cu_0.5Ni_0.5Al₂O₄/GCN composites, resulting in increased active sites, facilitating enhanced electrochemical reactions. The dislocation density (δ) and micro-strain (ε) were inferred from XRD data. Equation (11) was used to calculate the dislocation density (δ) [47].

$$\delta ={\displaystyle \frac{1}{D^2}}$$

(11)

where D is the size of the crystallite, and the line dislocation can be theoretically defined as

$$\delta ={\displaystyle \frac{1}{D}}$$

(12)

In order to calculate the lattice strain (LS) and micro-strain (ε), Equations (13) and (14) were used [47] as follows:

$$\epsilon ={\displaystyle \frac{\beta Cos\theta }{4}}$$

(13)

$$LS={\displaystyle \frac{\beta }{4tan\theta }}$$

(14)

Using Williamson–Hall (W-H) techniques, the micro-strain of the produced materials was also determined using the following expression (Equation (15)).

$$\beta cos\theta =ϵ\left(4sin\theta \right)+k\lambda /D$$

(15)

where D is the crystallite size, λ is the X-ray wavelength, θ is the Bragg angle, k is the forming factor, while ɑ is the micro-strain. Plotting the W-H graph for Cu_0.5Ni_0.5Al₂O₄/GCN and Cu_0.5Ni_0.5Al₂O₄ over βcosθ vs. 4sinθ for the substantial peaks and values of the positive micro-strain comes out to be 4.16 × 10⁻³ and 1.13 × 10⁻³, respectively, and it is displayed in Figure 2b,c.

The 3-D simulated illustrations of spinel Cu_0.5Ni_0.5Al₂O₄ are created by means of the structural characteristics obtained from XRD. Three-dimensional representations produced by the diamond software (version 4.6) are shown in Figure 2e. According to the diagram, the crystal structure space group is Fd-3m (227). Site preferences for Cu, Ni, Al, and O are evident in the observed event. The 3-D picture of Cu_0.5Ni_0.5Al₂O₄ with polyhedral view is demonstrated in Figure 2f.

The FTIR spectra of Cu_0.5Ni_0.5Al₂O₄/GCN are shown in Figure 2d. A prominent peak may be seen at 810 cm⁻¹. However, an alternative theory linked this peak to the collective out-of-plane wagging mode of N and C atoms in heptazine. The in-plane stretching modes of rings, which include common aromatic ring vibrations at 1423 cm⁻¹ (s-triazine ring), 1643 cm⁻¹ (C=N stretch), and 1348 cm⁻¹ (C=N stretch), are responsible for strong bands at 1248 cm⁻¹. Peaks at 3193 cm⁻¹, representing uncondensed NH and NH₂ groups, may overlap with the (H-bonded) OH stretch when adsorbed water is present [48]. The v₃ vibration of ${CO}_3^{2-}$ was identified as the cause of the broad band about 1399 cm⁻¹. Peaks smaller than 1000 cm⁻¹, as depicted in Figure 2d (embedded graph), within the 400–750 cm⁻¹ range are attributable to vibrations of M–O (M = Ni, Al, and Cu) [49].

Using an SEM technique, morphological characterization of the manufactured materials was completed. The Cu_0.5Ni_0.5Al₂O₄/GCN micrograph, as seen in Figure 3a, depicts irregular agglomerated nanoparticles spread on regular agglomerated Cu_0.5Ni_0.5Al₂O₄. On the other hand, Cu_0.5Ni_0.5Al₂O₄/GCN in Figure 3b displays a zoom view of Figure 3a at 20 µm focuses. The morphology of the nanocomposite is represented by the irregularly shaped Cu_0.5Ni_0.5Al₂O₄/GCN particles scattered throughout the Cu_0.5Ni_0.5Al₂O₄/GCN surface. The rough surface serves as a source of additional active sites for intermediate diffusion. They expose the electrolyte to catalytically active binding sites, significantly enhance water splitting by facilitating more efficient electron and ion transport across the electrode–electrolyte interface, and act as a mechanism for energy storage. EDS was applied to evaluate the elemental composition of the nanocomposite, as exhibited in Figure 3c. Cu, Ni, Al, N, and O are the only elements visible in the composite peaks on the EDX spectrum. The absence of any additional elements indicates the material’s exceptional purity. This demonstrates that the nanocomposite was successfully synthesized without any impurity.

3.2. Electrocatalyst for SCs

All electrochemical measurements were conducted under identical conditions to facilitate a comparison with Cu_0.5Ni_0.5Al₂O₄/GCN, GCN, and Cu_0.5Ni_0.5Al₂O₄, as presented in

Table 2. Summary of the electrochemically extracted values for all the investigated electrocatalysts.

Samples	Scan Rate (mV/s)	Csp (F/g)	Rct (Ω)	Rs (Ω)
Cu0.5Ni0.5Al2O4/GCN	5	487	1.27	1.88
GCN	5	326	4.49	2.40
Cu0.5Ni0.5Al2O4	5	375	4.65	2.45

. Figure 4a–c display polarization curves from CV methods at a scan rate of 5–60 mV s⁻¹ used to analyze the catalyst intrinsic characteristics on a potential window (−0.6–0.6 V) vs. Ag/AgCl as the reference electrode. The redox potential of Cu_0.5Ni_0.5Al₂O₄/GCN may be ascribed to the cooperative and synergistic impact of Cu, Ni, and Al ions. In Cu_0.5Ni_0.5Al₂O₄/GCN, a roughly linear relationship is shown between the scan speed and discharge current density, suggesting a quick charge–discharge process. The Csp value of pure Cu_0.5Ni_0.5Al₂O₄/GCN was 487 F g⁻¹ at 5 mV s⁻¹, but it gradually fell to 325 F g⁻¹ at 60 mV s⁻¹, as can be observed in the inset of Figure 4d. On other hand, when the scan rate was raised from 5 to 60 mV s⁻¹, the Csp values of the spinel Cu_0.5Ni_0.5Al₂O₄/GCN nanocomposite electrode material and GCN fell. The Csp value of GCN decreased from 326 to 285 F g⁻¹, while the Csp value of Cu_0.5Ni_0.5Al₂O₄ decreased from 375 to 249 F g⁻¹; the Cu_0.5Ni_0.5Al₂O₄/GCN composite electrode exhibited a maximum Csp value that is equal or higher than that of the conventional spinel metal-oxide composite. Increased scanning rates decrease the critical crystal point because of the polarization effects and ohmic resistance, which hinder the movement of electrolyte ions and redox kinetics. In addition to that, the comparative CV of the Cu_0.5Ni_0.5Al₂O₄/GCN composite, GCN, and Cu_0.5Ni_0.5Al₂O₄ electrode material at 10 mV/s is also displayed in Figure S1.

In a chronoamperometric stability test, the synthesized materials Cu₀.₅Ni₀.₅Al₂O₄/GCN, GCN, and Cu₀.₅Ni₀.₅Al₂O₄ were exposed to a constant applied potential, and the current density was measured for ninety hours. The stability of the Cu_0.5Ni_0.5Al₂O₄/GCN electrode material was evaluated by means of the chronoamperometry method at a constant voltage of 0.75 V. Furthermore, we evaluated the fabricated supercapacitor’s stability using GCD cycles at a current density of 1 Ag⁻¹; as can be seen in Figure S3, the Cu_0.5Ni_0.5Al₂O₄/GCN electrode material has good electrochemical stability because, prior to 5000 cycles, the capacitance decreased somewhat but still reached 80%.

The findings are shown in Figure 4e, whereby the Cu_0.5Ni_0.5Al₂O₄/GCN composite maintains a constant current density of 0.21 A cm⁻², GCN shows Cd of 0.19 mA cm⁻², spinel Cu_0.5Ni_0.5Al₂O₄ shows Cd of 0.23 mA cm⁻², and all the synthesized material shows stability for 90 h at these Cds. The decrease in Cd at the electrode contact caused by electrolyte ion coagulation indicates that the composite material is very stable electrochemically.

In Figure 4f, the Nyquist plots for each composite material are shown. This study demonstrates that the Cu_0.5Ni_0.5Al₂O₄/GCN composite exhibits the smallest semicircular diameter when compared to spinel Cu_0.5Ni_0.5Al₂O₄ and GCN. The radius of the semicircle, which is frequently closely proportional to the Rs, indicates a greater electron transfer rate across the contact. The Rct metrics of the electrocatalysts is demonstrated, namely the Cu_0.5Ni_0.5Al₂O₄/GCN composite. Owing to its increased conductivity, with a solution resistance (Rs) of 1.88 Ω and a charge transfer resistance (Rct) of 1.27 Ω, the Cu₀.₅Ni₀.₅Al₂O₄/GCN electrode shows much lower impedance values, suggesting enhanced ionic conductivity and quicker charge transfer kinetics. In contrast, the virgin Cu₀.₅Ni₀.₅Al₂O₄ exhibits an Rs of 2.45 Ω and Rct of 4.65 Ω, whereas GCN alone shows higher Rs and Rct values of 2.40 Ω and 4.49 Ω, respectively. These results demonstrate that the electrochemical performance of the electrode is much improved by the integration of GCN with the spinel, which makes it a viable option for energy conversion and storage applications.

Figure 5a–c shows the recorded GCD curves at CDs of 1–5 A g⁻¹ between the potential ranges of 0–0.5 V. The Cu_0.5Ni_0.5Al₂O₄/GCN composite possesses a particularly symmetrical and triangular structure in comparison to both spinel Cu_0.5Ni_0.5Al₂O₄ and GCN. These improvements not only significantly increase the conductivity but also demonstrate the hybrid functions as a reversible ideal capacitor. The prolonged period of discharge exhibited by Cu_0.5Ni_0.5Al₂O₄/GCN validates its exceptional specific capacitance. The enhanced surface electrode structure facilitates the migration of electrolyte ions, and the presence of Cu_0.5Ni_0.5Al₂O₄ spinel distributed over GCN may enhance electrical conductivity. The Csp of the Cu_0.5Ni_0.5Al₂O₄/GCN composite is 1290 F g⁻¹ at a Cd of 1 A g⁻¹, as depicted in Figure 4d. Spinel Cu_0.5Ni_0.5Al₂O₄ has a specific capacitance (Csp) value of 1133 F g⁻¹, whereas GCN has 1202 F g⁻¹. One proposed approach to enhance the efficiency of the diffusion process is the generation of active sites. A hypothesis posits that the incorporation of spinel Cu_0.5Ni_0.5Al₂O₄ into a GCN would facilitate diffusion and decrease the energy required for defect formation due to the present active sites. The experimental findings indicated that the energy density of the Cu_0.5Ni_0.5Al₂O₄/GCN composite was measured to be 28.07 Wh/kg. Additionally, the power density of Cu_0.5Ni_0.5Al₂O₄/GCN was found to be 205.19 W/kg, while the Pd and Ed of GCN were 154 W/kg and 17.22 Wh/kg, respectively, and for Cu_0.5Ni_0.5Al₂O₄ the Pd and Ed were 155.24 W/kg and 11.58 Wh/kg as demonstrated in Figure 5e. These energy and power density values were evaluated using Equations (3) and (4). The findings of this study indicate that the incorporation of a quantity of spinel Cu_0.5Ni_0.5Al₂O₄ into the GCN electrode significantly enhanced its porosity.

An essential parameter for the computation of the electrochemical surface area (ECSA) is established in Figure 6a–c. Therefore, the non-faradic region was chosen from the CV curve, namely within a potential window range of 0.50–0.75 V compared to the reference hydrogen electrode (RHE). Electrochemical CV was used to determine the restricted potential range at 10, 20, 30, 40, 50, and 60 millivolts per second. The cathodic and anodic currents were calculated by studying these graphs in order to identify the variation in CD (Δj). Plotting values of Δj against scan rate speeds using the linear fit approach yields a straight line. The resulting slope is divided in half to yield the Cdl. The ECSA CV curves were formed by dividing the Cdl value, which was 0.040 mF cm⁻² for the flat electrode [50,51], by the Csp value, as shown in Figure 6d–f.

Table 3. A list of the values provided for the Cdl, Rf, and ECSA for the nanocomposites that were synthesized using Cu_0.5Ni_0.5Al₂O₄/GCN, GCN, and Cu_0.5Ni_0.5Al₂O₄.

Synthesized Material	ECSA (cm2)	(Rf)	Cdl (mF)
Cu0.5Ni0.5Al2O4/GCN	172.75	345.5	6.91
GCN	150	300	6.7
Cu0.5Ni0.5Al2O4	141.5	283	5.66

displays the ECSA and R_f values that were calculated.

The following formulas (Equations (16) and (17)) are used to calculate the ECSA and roughness factor R_f.

$$ECSA={\displaystyle \frac{C_{dl}}{C_{sp\left(nickelfoam\right)}}}$$

(16)

$$R_f={\displaystyle \frac{ECSA}{Geometricsurfaceareaofnickelfoam}}$$

(17)

3.3. SC Performance in Two-Electrode System

The Cu_0.5Ni_0.5Al₂O₄/GCN nanocomposite was tested using a two-electrode setup, with a carbon rod serving as the counter electrode. Figure 7a displays the CV curves for the Cu_0.5Ni_0.5Al₂O₄/GCN composite at scan rates from 5 to 60 mV s⁻¹. The curves, corresponding to specific capacitance (C_sp) values of 303, 285, 264, 241, 228, 218, and 209 F g⁻¹, provide significant information. At a current density of 1 A g⁻¹, the C_sp was determined from the GCD curves shown in Figure 7b. For the Cu_0.5Ni_0.5Al₂O₄/GCN composite, the Rct was 4.53 Ω, as indicated by the EIS results in Figure 7c. The deliberate enhancement of the charge transfer conductivity achieved by merging GCN with the spinel Cu_0.5Ni_0.5Al₂O₄ lattice leads to a decrease in Rct. The complex features of the material, including its behavior during the redox process, become more pronounced as the scan rate increases. The calculated C_sp value reached 804 F g⁻¹, as described in Figure 7d. Furthermore, the device exhibits an energy density of 18.79 W kg⁻¹ and a power density of 207.27 Wh kg⁻¹, as shown in Figure 7e. The ECSA plot is presented in Figure 7g, which corresponds to the non-faradaic region, i.e., the potential window spanning from 0.50 to 0.75 V, based on the CV curve. Figure 7h illustrates a double-layer capacitance (Cdl) of 6.44 mF g⁻¹ for the Cu_0.5Ni_0.5Al₂O₄₄/GCN composite electrode, corresponding to an ECSA of 161 cm², consistent with the results obtained from a three-electrode system.

3.4. OER Analysis

The kinetics, stability, and performance of electrocatalysts for the OER were studied using various electrochemical methods. Figure 8a,b illustrate the CV and LSV curves. Figure 8a shows the CV polarization; at a benchmark current density of 10 mA cm⁻², the Cu₀.₅Ni₀.₅Al₂O₄/GCN nanocomposite exhibited significantly improved OER performance, with an overpotential of 254 mV and a lower onset potential of 1.49 V. These results indicate an enhanced intrinsic electrocatalytic activity of the Cu_0.5Ni_0.5Al₂O₄/GCN composite. Moreover, as shown in Figure 8c, the Cu_0.5Ni_0.5Al₂O₄/GCN electrode displayed the lowest Tafel slope (67.8 mV dec⁻¹), suggesting faster kinetics in the four-electron transfer process. Furthermore, a CV comparison between bare nickel foam and the Cu_0.5Ni_0.5Al₂O₄/GCN composite is presented in Figure S2.

The introduction of Cu_0.5Ni_0.5Al₂O₄ into the GCN matrix significantly enhanced the electrochemical performance. The modification of the nanocomposite’s surface area, through its interaction with GCN, improves permeability and increases the number of electrocatalytic binding sites. An examination of the turnover frequency (TOF) was undertaken to evaluate the inherent catalytic capability of the material. The TOF of Cu_0.5Ni_0.5Al₂O₄GCN was determined to be 1.81 s⁻¹, indicating that the incorporation of Cu_0.5Ni_0.5Al₂O₄ effectively enhances the intrinsic catalytic activity of GCN.

The electrochemical route for the Cu_0.5Ni_0.5Al₂O₄/GCN nanocomposite is suggested by the following equations (Equations (18)–(21)).

$$M+{OH}^{-}\to M-{OH}^{-}+e^{-}$$

(18)

The interaction between the spinel Cu_0.5Ni_0.5Al₂O₄ nanoparticles and hydroxide ions (OH^–) triggers the process of surface oxidation. This mechanism involves the accumulation of OH^– ions on the readily available binding sites of reactive copper (Cu), nickel (Ni), and aluminum (Al) sites within the Cu_0.5Ni_0.5Al₂O₄ structure, leading to the formation of metal hydroxide species (M⁻ OH^–).

$$M+{OH}^{-}+{OH}^{-}\to M-O+H^{+}+e^{-}$$

(19)

Following the production of M−OH^–, the second phase involves interaction with additional hydroxide (OH^–) ions. This interaction results in the release of protons (H⁺) and electrons (e⁻), thereby facilitating the formation of metal oxide species (M−O). The phenomenon of electron discharge is a key component of surface redox reactions.

$$M-O+{OH}^{-}\to M-OOH+e^{-}$$

(20)

The nucleophilic attack of M−O may be amplified by their conjunction with another electron. The formation of M−OOH, a critical stage in the catalytic cycle, happens after this reaction. This process leads to the accumulation of additional active sites within the Cu_0.5Ni_0.5Al₂O₄/GCN composite matrix.

$$M-OOH+{OH}^{-}\to M-O_2+H^{+}+e^{-}$$

(21)

The interaction of M−OOH with OH^– is the final phase. Protons (H⁺), molecular oxygen (O₂), and one additional electron are emitted following this contact. Significantly, the completion of the OER is proven by the development of O₂. This extensive mechanistic work stresses the complicated but well-organized sequence of chemical processes that the Cu_0.5Ni_0.5Al₂O₄/GCN nanocomposite coordinates, demonstrating its catalytic potential to increase the efficient OER. An important scientific instrument for properly assessing the rate at which charges travel between an electrode and an electrolyte is EIS. It was observed that the Cu_0.5Ni_0.5Al₂O₄/GCN composite exhibits a charge transfer resistance (Rct) of roughly 2.91 Ω. At the electrode–electrolyte interface, the Cu_0.5Ni_0.5Al₂O₄/GCN composite exhibited effective and rapid proton and electron transport during the (OER) process. The electrical efficiency of Cu_0.5Ni_0.5Al₂O₄/GCN, as indicated by a Tafel slope of 67.8 mV dec⁻¹, was further improved by EIS analysis, therefore validating its effectiveness as a catalyst for advancement of the OER. Through the use of chronoamperometry analysis, the electrode stability was evaluated. To evaluate the generated material long-term endurance against Ag/AgCl, a chronoamperometry test was performed at voltages up to 0.75 V. Figure 8e shows the results of the chronoamperometric study for Cu_0.5Ni_0.5Al₂O₄/GCN, which demonstrate the electrode material prolonged endurance over a 90 h period. Overall, the performance of the fabricated material and longevity were amazing. Due to its exceptional stability, it grew good electrode materials, particularly for the OER.

The exact quantity of the conveniently available electrocatalyst necessary for electrochemical activity is established by the ECSA approach. When the electrocatalyst includes more active and functional sites, it performs optimally and has greater ECSA values, which were determined by looking at CV graphs generated in the non-faradaic spectrum area of Figure 8f at different scan speeds (10–60 mVs⁻¹). Figure 8g depicts the experimental data, which demonstrate that the Cu_0.5Ni_0.5Al₂O₄/GCN composite exhibited a Cdl of 18 mF cm⁻².

3.5. HER Analysis

The current work studied the electrocatalytic activity of the Cu_0.5Ni_0.5Al₂O₄/GCN composite for the HER. Relevant LSV diagrams are provided in Figure 9a. The Cu_0.5Ni_0.5Al₂O₄/GCN composite demonstrated considerable HER activity, requiring an overpotential of 101 mV to obtain a current density of 10 mA cm⁻². The Tafel slope in Figure 9b demonstrates the activity of the Cu_0.5Ni_0.5Al₂O₄/GCN composite, with a measured value of 136.18 mV dec⁻¹. These results indicate a relatively fast electrochemical reaction. In alkaline solutions the HER begins with hydrogen dissociation from water, resulting in an adsorbed proton on the electrode surface, known as the Volmer step. This phase requires a Tafel slope value greater than 90 mV dec⁻¹. On the electrode surface, the adsorbed hydrogen reacts with water and electrons to produce a H₂ molecule during the Heyrovsky step. The device displays the Tafel slope in millivolts per decimal point. Recombination of the adsorbed protons on the electrode surface during the Tafel step typically requires an average Tafel slope of 30 mV dec⁻¹. This work highlights the importance of the Volmer step in determining the overall electrocatalytic efficiency of the material.

$$H_{2}O+M+e^{-}\to M-H^{*}+{OH}^{-}\left(\text{Volmer}\right)$$

(22)

$$M+H^{*}+H_{2}O+e^{-}\to M-{OH}^{-}+H_2\left(\text{Heyrovsky}\right)$$

(23)

The intrinsic catalytic capability of a material is measured using a turnover frequency (TOF) study. The TOF value of Cu_0.5Ni_0.5Al₂O₄/GCN was found to be 2.99 s⁻¹. The foregoing conclusions were further verified by empirical data on electronic conductivities. This shows that the synthesized composite is a potent electrocatalyst for the HER. The conductivity and electrochemical properties of samples were evaluated using EIS, as shown in Figure 9c. The Rct values of Cu_0.5Ni_0.5Al₂O₄/GCN was 178.36 Ω. This indicates that the combination of Cu_0.5Ni_0.5Al₂O₄ with GCN increases conductivity and promotes interfacial electron transport in the Cu_0.5Ni_0.5Al₂O₄/GCN composite. Consequently, the catalyst displays enhanced conductivity and rapid charge transfer capabilities, thereby improving reaction kinetics.

Consideration of catalyst durability is crucial for practical applications. Therefore, the results of prolonged stability tests carried out under a continuous applied voltage were analyzed (Figure 9d). Having demonstrated long-term stability for 90 h, Cu_0.5Ni_0.5Al₂O₄/GCN is confirmed to be an outstanding electrocatalyst suited for practical applications.

4. Conclusions

In conclusion, a spinel/GCN-based composite material was synthesized using the solid-state method, offering a feasible alternative to fossil fuels. Cu_0.5Ni_0.5Al₂O₄ was added to GCN to improve its surface area and significantly enhance its electrocatalytic activity.

Electrochemical tests demonstrate that the Cu_0.5Ni_0.5Al₂O₄/GCN nanocomposite achieved an overpotential of η₁₀ = 101 mV for the HER and η₁₀ = 254 mV for the OER. The hybrid nanocomposite exhibited a large ECSA of 172.75 cm² and favorable Tafel slopes of 67.8 mV dec⁻¹ (OER) and 136.18 mV dec⁻¹ (HER), highlighting its efficient catalytic properties. In an energy storage application, Cu_0.5Ni_0.5Al₂O₄/GCN nanocomposite showed a specific capacitance (Csp) of 1290 F g⁻¹ with an excellent energy and power density of 28.07 Wh/kg and 205.19 W/kg, respectively, obtained through GCD at a current density of 1 Ag⁻¹ in a three-electrode configuration. Moreover, EIS analysis showed exceptionally lower charge transfer resistance (Rct) of 1.27 Ω and smaller solution resistance of (Rs) of 1.88 Ω. In addition to that, a two-electrode system with a carbon rod as the counter electrode was used to assess the Cu₀.₅Ni₀.₅Al₂O₄/GCN nanocomposite’s electrochemical performance. The results showed higher specific capacitance (Csp) values of 303 F g⁻¹ obtained by CV in 2.0 M KOH at scan speeds of 5 mV s⁻¹. A high Csp of 804 F g⁻¹ with a power density of 207.27 Wh kg⁻¹ and an energy density of 18.79 W kg⁻¹ was found using GCD experiments at 1 A g⁻¹. The device showed promise for high-performance energy storage applications. Further demonstrating the electrode material’s exceptional capacitive behavior was the ECSA, which was determined from the non-faradaic part of the CV curve and was 161 cm² with a corresponding double-layer capacitance (Cdl) of 6.44 mF g⁻¹. The synergistic interaction between GCN and the spinel Cu₀.₅Ni₀.₅Al₂O₄ resulted in a low charge transfer resistance (Rct) of 4.53 Ω, according to the EIS measurement.