# Advanced NiCo2O4 /ZnO-CuO/NF composite for high-performance asymmetric supercapacitor and efficient oxygen evolution reaction applications

Muhammad Ali Bhatti, Shusheel Kumar, Aneela Tahira, Adeel Liaquat

Bhatti, Zaheer Ahmed Ujjan, Mansab Ali Jakhrani, Umair Aftab, Riyadh

Alshammari, Ayman Nafady, Elmuez Dawi, et al.

► To cite this version:

Muhammad Ali Bhatti, Shusheel Kumar, Aneela Tahira, Adeel Liaquat Bhatti, Zaheer Ahmed Ujjan, et al.. Advanced NiCo2O4 /ZnO-CuO/NF composite for high-performance asymmetric supercapacitor and efficient oxygen evolution reaction applications. Advanced Composites and Hybrid Materials, 2025, 8 (1), pp.148. 10.1007/s42114-025-01238-9 . hal-04966854

HAL Id: hal-04966854

https://hal.univ-lorraine.fr/hal-04966854v1

Submitted on 2 Mar 2025

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Distributed under a Creative Commons Attribution 4.0 International License
Advanced NiCo₂O₄/ZnO-CuO/NF composite for high performance asymmetric
supercapacitor and efficient oxygen evolution reaction applications

Muhammad Ali Bhatti<sup>a</sup>, Shusheel Kumar<sup>c</sup>, Aneela Tahira<sup>d</sup>, Adeel Liaquat Bhatti<sup>c</sup>, Zaheer Ahmed Ujjan<sup>c</sup>,
Mansab Ali Jakhrani<sup>c</sup>, Umair Aftab<sup>c</sup>, Riyadh H. Alshammari<sup>h</sup>, Ayman Nafady<sup>h</sup>, Elmuez Dawi<sup>i</sup>, Mélanie
Emo<sup>f</sup>, Brigitte Vigolo<sup>f</sup>, Antonia Infantes-Molina<sup>g</sup>, Zafar Hussain Ibupoto<sup>b*</sup>

<sup>a</sup>Centre for Environmental Sciences, University of Sindh Jamshoro, 76080, Sindh, Pakistan.

<sup>b</sup>Institute of Chemistry, University of Sindh Jamshoro, 76080, Sindh, Pakistan.

<sup>c</sup>Institute of Physics, University of Sindh Jamshoro, 76080, Sindh, Pakistan.

<sup>d</sup>Institute of Chemistry, Shah Abdul Latif University Khairpur Mirs, Sindh, Pakistan.

<sup>e</sup>Mehran University of Engineering and Technology, 7680 Jamshoro, Sindh, Pakistan.

<sup>f</sup>Université de Lorraine, CNRS, Institut Jean Lamour-IJL, F-54000 Nancy, France.

<sup>g</sup>Department of Inorganic Chemistry, Crystallography and Mineralogy, Unidad Asociada al ICP-CSIC,
Faculty of Sciences, University of Malaga, Campus de Teatinos, 29071, Malaga, Spain.

<sup>h</sup>Chemistry Department, College of Science, King Saud University, Riyadh, 11451, Saudi Arabia.

<sup>i</sup>College of Humanities and Sciences, department of Mathematics and Sciences, Ajman University, P.O.
Box 346, United Arab Emirates.

*Corresponding author(s): Zafar Hussain Ibupoto, PhD, Email: zaffar.ibhupoto@usindh.edu.pk

**Abstract**

It is imperative that composite systems with high performance, low cost, enhanced simplicity, and scalability be developed in order to convert and store energy. This, however, has been a challenging endeavor throughout the years. In this study, we present a cost-effective, efficient, scale-up-friendly, and environmentally friendly method of producing in-situ sandwich layers of ZnO-CuO composite between NiCo₂O₄ nanostructures and nickel foam using lemon peel extract (LPE) during hydrothermal processes. NiCo₂O₄/ZnO-CuO/nickel foam was analyzed using powder X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS). According to XRD and HRTEM studies of NiCo₂O₄/ZnO-CuO/nickel foam, NiCo₂O₄, ZnO, and CuO exhibit cubic, hexagonal, and monoclinic phases, respectively. With NiCo₂O₄/ZnO-CuO/nickel foam as the active anode electrode, an asymmetric supercapacitor has been developed in an alkaline solution of 3M KOH. At a low current density of 2 Ag⁻¹, the asymmetric supercapacitor exhibited a high specific capacitance of 3614.8 F g⁻¹, a power density of 1549.2 W kg⁻¹, and an energy density of 75.3 Wh
kg⁻¹. Upon repeatable 40000 galvanic charge-discharge cycles, the asymmetric device demonstrated a high specific capacitance retention percentage of approximately 100 percent to 95% and a columbic efficiency of 98%. Moreover, NiCo₂O₄/ZnO-CuO/nickel foam composite had a low overpotential of 210 mV at 40 mA cm⁻² and a Tafel slope of 70 mV dec⁻¹ for OER in 1 M KOH. During continuous OER measurements over a period of 40 hours, NiCo₂O₄/ZnO-CuO/nickel foam composites demonstrated high durability and stability. NiCo₂O₄/ZnO-CuO/nickel foam exhibits good electrochemical performance as a result of its synergetic effects, its high conductivity, its abundant exposed catalytic sites, its oxygen vacancies, and its high durability.

**Keywords:** NiCo₂O₄/ZnO-CuO/nickel foam; lemon peel extract; oxygen evolution reaction; asymmetric supercapacitor.
## Graphical abstract

The illustration for the synthesis of high performance in situ NiCo₂O₄/ZnO-CuO/nickel foam composite for OER and supercapacitor application.

<figure><img src="image_2.png" /></figure>

## 1. Introduction

As people face challenges with the environment and renewable energy today, it is crucial that we develop new methods and technologies for generating and storing renewable energy. Due to these factors, electrochemical energy conversion and storage systems, such as electrochemical water splitting and supercapacitors, are currently gaining increased attention [1-3]. Although extensive research has been conducted on the design of materials for the production of green hydrogen fuel, the performance of these materials is limited due to their poor intrinsic functional properties. As these catalytic materials are synthesized using complex methods, toxic chemicals are used during preparation, and stability issues exist, they are not sufficiently performing for practical applications or large-scale applications. Water splitting technology is a viable strategy
for developing green hydrogen fuels that has a number of potential benefits in terms of providing
a sustainable source of energy. Water splitting involves two half-cell processes: hydrogen
evolution reaction (HER) and oxygen evolution reaction (OER). There is a general recognition
that OER, in particular, poses the greatest challenge to the competitive use of water splitting as a
source of green hydrogen. It is currently known that a number of noble metal-based
electrocatalysts are most effective for water splitting, including those that are based on Ru, Ir,
and Pt [4,5]. In spite of this, their scarcity and high cost limit their widespread use. The field of
non-precious electrocatalyst fabrication is currently undergoing extensive research. A variety of
non-noble materials have been investigated for their ability to split water, including transition
metal oxides, metal chalcogenides, and phosphides [6-8]. During water splitting, these materials
inherit high overpotentials, which limit their commercial and practical applications [9].
Additionally, electrochemical energy storage systems, such as fuel cells, supercapacitors, and
rechargeable batteries, have been proposed as potential energy sources for smart devices and
electric vehicles [10]. Due to their long cycle stability and high energy density, the fabrication of
supercapacitors has received considerable attention [10]. As a result of their intriguing electronic
and structural properties, metal oxides have demonstrated improved and unique advantages for
supercapacitors [11]. Metal oxides are also capable of overcoming the unsuitability of carbon
materials and eliminating their limited cycling stability [10, 12-15]. Recently, many single metal
oxide electrode materials, including NiO, RuO₂, MnO₂, Co₃O₄ and SnO₂, have been explored for
use in electrochemical water splitting and supercapacitors [12, 16-21]. However, single metal
oxides are expensive, toxic, electrically in-conductible, and low in energy density, limiting their
use on a large scale. Furthermore, spinel transition metal oxides (Mn, Ni, etc.) have also shown
promising characteristics as electrode materials [22]. NiCo₂O₄ is one of the spinel metal oxides
that has demonstrated enhanced electrical conductivity, electrochemical stability, and catalytic
activity [23-26]. A significant enhancement of NiCo₂O₄'s catalytic activity has been attributed to
its remarkable shape orientation, which can lead to the preparation of morphologies [27]. In
some NiCo₂O₄ morphologies, ion transport channels are reduced and conductivity is increased,
resulting in a multitude of active edges at the interface that facilitate electron transport [28]. In
addition, a variety of conducting materials, including graphene, carbon nanotubes, conducting
polymers, are combined with metal oxides to provide efficient electrode materials [29-33]. It is
also possible to enhance electrochemical reactions by adding metallic foams, such as nickel or
copper [30, 31]. The advantage of composites of nanostructures is that they have a large surface area, a fast charge transport rate, and a high diffusion rate compared to their bulk counterparts [32, 33].

Single metal oxides and binary metal-metal oxide hybrid systems have already been compared in terms of electrochemical activity, catalytic performance, and rapid electron channels [34-1]. Although considerable progress has been made regarding the development of electrode materials based on binary metal oxides, these materials still lack sufficient intrinsic functional properties to be used on a large scale in practical applications. It is therefore imperative that new electrode configurations are developed in order to overcome the challenges in the field of electrode materials for the conversion and storage of energy.

Aside from selecting the appropriate electrode materials, developing a green manufacturing process is also of critical importance [42]. Citrus lemon peel aqueous extract contains a variety of phytochemicals, such as alkaloids, flavonoids, cardiac glycosides, steroids, phenols, amino acids, fixed oils, proteins, tannins, and carbohydrates. A biomass waste derived from lemon peel extract (LPE) is an excellent source of growth-directing molecules, reducing agents, and capping agents. The alkaloids and flavonoids serve as capping, reducing, stabilizing, and reducing agents. The purpose of this study was to develop a hybrid electrode material that is easy to scale up, eco-friendly, and efficient using a green mediated process. Moreover, through the modification of surface properties such as abundant catalytic sites, favourable morphology, and a tuneable interface chemistry, the green synthesis may accelerate the bifunctional properties of hybrid materials for the efficient implementation of OER processes and the enhanced performance of supercapacitors. Our proposal is therefore to use lemon peel extract to facilitate the green mediated synthesis of ZnO-CuO sandwich layers between NiCo₂O₄ and nickel foam substrates in order to develop high-performance OERs and asymmetric supercapacitors. It has not been possible to synthesize a composite of multi-metal oxides using lemon peel extract for use as electrochemical energy storage materials. In the synthesis of electrode materials, green-mediated approaches have proven to be versatile and capable of overcoming challenges associated with complex steps and chemical toxicity. Furthermore, the growth of multi-metal oxide composites on nickel foam provides abundant surface-active sites, high electron adsorption, robotic redox kinetics, increased charge transport, and a high degree of compatibility. These characteristics of
NiCo₂O₄/ZnO-CuO/nickel foam composite have been shown to be extremely reliable and stable in applications such as asymmetric supercapacitors and half-cell water splitting OER processes.

## 2. Materials and Methods

### 2.1. Chemical reagents

Sigma Aldrich of Karachi, Pakistan provided the chemicals used as received: cobalt chloride hexahydrate (CoCl₂·6H₂O, 98%), nickel chloride hexahydrate (NiCl₂·6H₂O, 99.9%), copper acetate dihydrate (C₆H₁₀CuO₆, 98%) zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O, 98%), nickel foam (NF), acetone (99.9%), ethanol (98%), hydro chloric acid (37%), and urea (CH₄ N₂ O, 99%). A lemon was purchased from a local market in Jamshoro, Sindh, Pakistan. In order to prepare lemon peel extract (LPE), 50 grams of lemon peel are added to 250 mL of deionized water and stirred for 24 hours at room temperature. Filtering the solution produced a faint green solution, and the filtrate was used to prepare the desired materials as LPE.

### 2.2. Synthesis of NiCo₂O₄/ZnO-CuO/NF nanostructures using lemon peel extract

A schematic representation of the synthesis process of multi-metal oxides on nickel foam electrode materials is shown in Scheme 1. Two glass beakers were filled with 200 mL of prepared LPE for a typical synthesis. For the preparation of ZnO-CuO/NF, copper acetate dihydrate (C₄H₁₀CuO₆·2H₂O) and zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) precursor were added to first LPE solution beaker to reach 0.1 M for each of them and the pH was adjusted at 10. In the next step, a nickel foam chip was added. In a second synthesis experiment to produce NiCo₂O₄/ZnO-CuO/NF, cobalt chloride hexahydrate (CoCl₂·6H₂O) (0.1 M), nickel chloride hexahydrate (NiCl₂·6H₂O) (0.015 M) and urea (0.1 M) precursors and ZnO-CuO/NF chips were added to the second beaker. A hydrothermal process was initiated by mixing the solutions and covering both beakers with aluminum foil. Afterwards, the beakers were heated at 95 °C for five hours in an electric oven. Afterward, the resultant nickel-cobalt hydroxide deposited ZnO-CuO/NF chips were collected by filtration and put into muffle furnace at 400 °C for 4 h. As part of the synthesis of NiCo₂O₄/NF, 200 mL of cobalt chloride hexahydrate at 0.1 M, nickel chloride hexahydrate at 0.015 M, urea at 0.1 M precursor solutions, along with nickel foam chips, were subjected to a hydrothermal reaction and thermal combustion under the same conditions described above. The proportion of 75% for NiCo₂O₄, and of 25% for ZnO-CuO were found to
be constant in the hybrid NiCo2O4/ZnO-CuO system. As a result, the three samples synthesized, ZnO-CuO/NF, NiCo2O4/NF, and NiCo2O4/ZnO-CuO/NF, were thoroughly characterized and used to investigate supercapacitors and OERs. The relative mass for the NiCo2O4, ZnO-CuO, and NiCo2O4/ZnO-CuO was found to be 0.15mg, 0.05mg and 0.2mg respectively. It's true the control on the mass is important in designing the asymmetric supercapacitor, hence we observed on our in situ growth of hybrid materials that a uniform layer of active material was deposited.

<figure><img src="image_3.png" /><figcaption>**Scheme 1:** Stepwise preparation of ZnO-CuO/NF, NiCo₂O₄/NF, NiCo₂O₄/ZnO-CuO/NF.</figcaption></figure>

## 2.3. Material characterization

Different physicochemical characterization techniques were used to ascertain the prepared hybrid materials. Prior to the analysis, the synthesized metal oxides have been removed from the NF.
The X-ray diffraction (XRD, Bruker D8-Phaser, CuKα₁, λ = 1.5406 Å) patterns were recorded within the scanning range of 10–80 degrees. A ZEISS Gemini SEM 500 equipment linked to a field emission gun at an accelerated voltage of 2 kV was used for the scanning electron microscopy (SEM) observations. Transmission electron microscopy (TEM) and high transmission electron microscopy (HRTEM) observations were performed on a JEOL JEM ARM200F Cold FEG instrument, equipped with a spherical aberration (Cs) probe corrector and working at 200 kV. For X-ray photoelectron spectroscopy, an ESCA 5701 unit, Physical Electronics (PHI), composed of a Hemispheric Energy Analyzer/Detector (SCA), LTD JPS 9030 was used. Fourier transform infrared spectroscopy (Tensor 27, Bruker Optics FT-IR, Karlsruhe, Germany) was used to analyze the functional groups of as-synthesized material.

## 2.4. Electrochemical measurements

A three-electrode cell setup was used to investigate the electrochemical behavior of the prepared samples. For the supercapacitor and half-cell OER applications in 3 M KOH and 1 M KOH aqueous solutions, cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) measurements were performed on CuO-ZnO/NF, NiCo₂O₄/NF and NiCo₂O₄/ZnO-CuO/NF electrodes. During the EIS, frequencies ranging from 100 kHz to 0.1 Hz were used, as well as an onset potential of 5 mV for the OER. Based on the GCD curves, the specific capacitance (C_s) of synthesized electrodes was calculated using the following equation [43]:

$$C_s = \frac{I \times \Delta t}{\Delta V \times \Delta m} \quad (1)$$

Where C_s is the specific capacitance (F g⁻¹), I is the current density (A), Δt is the discharge time (s), Δm is the mass of the electrode (g), and ΔV is the potential window (V).
**3. Results and discussion**

**3.1. Morphology, crystalline properties and chemical composition**

In Figure 1, different composite systems including ZnO-CuO, NiCo₂O₄, and NiCo₂O₄/ZnO-CuO
were examined in terms of their morphology using SEM. Figure 1a illustrates the flower-like
morphology of ZnO-CuO, with nanoflakes representing the petals. According to Figures 1b and
1c, higher magnification images reveal nanoflakes with lateral dimensions between 500
nanometers and 1µm. The thickness of nanoflakes of ZnO-CuO was noticed less than 20 nm as
shown in Figure 1c. As shown in Figures 1d, 1e, and 1f, NiCo₂O₄ grown on nickel exhibits radial
assemblies of platelets approximately 1µm in width and thickness of approximately 60 nm. ZnO-
CuO/ NiCo₂O₄ flakes synthesized on nickel foam appear as a flower-like structure (Figures 1g,
1h, and 1j). The NiCo₂O₄ flakes were exhibiting thickness of less than 20nm. As opposed to
ZnO-CuO, NiCo₂O₄ and the hybrid ZnO-CuO/ NiCo₂O₄ electrocatalyst, NiCo₂O₄ nanoflakes are
thicker than ZnO-CuO and ZnO-CuO/ NiCo₂O₄. The composite system has a greater surface area
than both ZnO-CuO and NiCo₂O₄ due to its thinner thickness than ZnO-CuO. The LPE provided
useful phytochemicals such as carbohydrates, alkaloids, and flavonoids, which act as structure-
directing, capping, stabilizing, and reducing agents. Thus, the shape orientation, particle size, and
surface chemical composition of hybrid materials were significantly altered.
<figure><img src="image_4.png" /><figcaption>**Figure 1:** SEM images at different magnifications of a), b) and c) ZnO-CuO, d) e) and f) NiCo₂O₄ and g) h) and i) NiCo₂O₄/ZnO-CuO grown on NF.</figcaption></figure>

Figure 2a illustrates the XRD reflection measurements for ZnO-CuO sandwich layers, pure NiCo₂O₄ and NiCo₂O₄/ZnO-CuO sandwich layers. In ZnO-CuO system, CuO diffraction patterns are attributed to monoclinic crystal phases of CuO with reflections at 32.13, 35.70, 38.82, 48.93, 53.19, 56.64, 58.46, 61.74, 65.72, 75.11° indexed to 112, 111, 200, -202, 020, 021, 202, -113, 022, and 222 miller indices, respectively (JCPDS card number 01-080-1916). It has been found that the ZnO reflections of the hexagonal phases 100, 002, 101, 102, 103, 200 and 112 can also be seen at their respective two theta angles as supported by (JCPDS card no. 01-075-0576). In accordance with the JCPDS card number 01-073-1702, the pure NiCo₂O₄ exhibited typical cubic phase reflections such as 220, 311, 222, 400, 331, 422, 511, 440, 422, 620, 533 and 622. In the ZnO-CuO/ NiCo₂O₄ sample, the ZnO patterns were observed at 31.99, 36.54, 47.79, 56.61, and 66.43°, corresponding to hexagonal wurtzite crystal systems for 100, 101, 102, 103, and 200 crystal planes, respectively (JCPDS card no: 01-075-0576). Also included in the NiCo₂O₄/ZnO-CuO composite were the CuO diffraction patterns 112, 111, 112,
020, 202, 113, and 312 with their specific two theta angles corresponding to monoclinic phase. A number of prominent diffraction patterns were assigned to the cubic crystal phase of NiCo₂O₄ at 18.75, 36.69, 38.39, 44.79, 49.21, 55.47, 59.17, 65.16, 72.98, 77.54, and 78.25° respectively for reflections 111, 311, 222, 400, 331, 422, 511, 440, 620, 533 and 622 (JCPDS card number 01-073-1702). It has been observed that NiCo₂O₄/ZnO-CuO composite exhibits many of the ZnO and CuO reflections as well as some of the reflections associated with NiCo₂O₄. During the fabrication of composite electrode materials, some of the reflections of ZnO and CuO were lost due to a possible thick coating of NiCo₂O₄.

Each composite system was also investigated by TEM. The related TEM images of pure NiCo₂O₄ are reported in Supplementary Figure (S1). The TEM study has revealed that NiCo₂O₄ sample presents a structure consisting of nanoparticles of size between 10 and 50 nm (Figures S1b and S1d). Selected area electron diffraction (SAED) and HRTEM observations were also performed Supplementary Figures (S1c-S1d) and demonstrated that these nanoparticles consist of the typical spinel cubic phase of NiCo₂O₄ (Fd-3m, a = 8.1 Å) which is fully supported by the XRD measurements. Additionally, TEM experiments were performed on the in-situ grown ZnO-CuO composite and the corresponding TEM images are shown in Supplementary Information, Figure S2. As already shown by SEM, ZnO-CuO sample is composed of thick nanoflakes, difficult to observe at higher magnifications by TEM (due to the thickness). Supplementary Figure (S2a-S2b). SAED observations have revealed that this material can be a mixture of the monoclinic phase of CuO (C12/c1, a = 4.7 Å, b = 3.4 Å, c = 5.1 Å, α = γ = 90°, β = 99.5°) and the hexagonal phase of ZnO (P6₃mc, a = b = 3.2 Å, c = 5.2 Å). Supplementary Figure (S2c). Finally, the NiCo₂O₄/ZnO-CuO composite was also observed by TEM, as depicted in Figure 2. NiCo₂O₄/ZnO-CuO composite presents a morphology of thinner nanoflakes (Figure 2b), which is in good agreement with SEM observations. These nanoflakes are composed of aggregated nanoparticles of less than 50 nm (Figures 2c and 2e). SAED (Figure 2d) and HRTEM (Figure 2e) observations of NiCo₂O₄/ZnO-CuO sample demonstrated the possible coexistence of the three different phases in the composite, i.e. the ZnO hexagonal phase (P6₃mc, a = b = 3.2 Å, c = 5.2 Å), the CuO monoclinic phase (C12/c1, a = 4.7 Å, b = 3.4 Å, c = 5.1 Å, α = γ = 90°, β = 99.5°) and the NiCo₂O₄ cubic phase (Fd-3m, a = 8.1 Å), confirming the successful formation of multi metal oxide composite system. From SAED and XRD, it was shown that a pure NiCo₂O₄/ZnO-CuO hybrid was successfully grown.
<figure><img src="image_5.png" /><figcaption>**Figure 2:** a) Structural characterization of ZnO-CuO (i), NiCo₂O₄ (ii), NiCo₂O₄/ZnO-CuO (iii) by XRD with monoclinic crystal phase of CuO (JCPDS card no: 01-080-1916) in red, hexagonal wurtzite phase of ZnO (JCPDS card no: 01-075-0576) in blue and cubic crystal phase of NiCo₂O₄ (JCPDS card no: 01-073-1702) in black. TEM images (b,c), SAED (d) and HRTEM (e) of NiCo₂O₄/ZnO-CuO grown on NF sample, with Fast Fourier transform (FFT) (insert of Figure e) of the yellow square area. For SAED,</figcaption></figure>
possible indexations for CuO monoclinic phase (in red), ZnO hexagonal phase (in blue) and NiCo₂O₄ cubic phase (in green).

Through XPS analysis, we examined the chemical and valence states of oxygen and metals within ZnO-CuO, NiCo₂O₄, and NiCo₂O₄/ZnO-CuO. First, in-situ ZnO-CuO composites have been studied and corresponding high resolution XPS spectra of Zn 2p and Cu 2p have been obtained from ZnO-CuO and NiCo₂O₄/ZnO-CuO composites, respectively (see Figures 3a and 3b). In the presence of NiCo₂O₄, Zn 2p3/2 has a binding energy of approximately 1021.6 eV, which is typical of ZnO [44]. It is worth noting that this energy does not change in the presence of NiCo₂O₄. As shown in Figure 3b, Cu 2p spectra for both ZnO-CuO and NiCo₂O₄/ZnO-CuO show a main peak centered between 932.8 and 933.5 eV, as reported when Cu⁺, Cu²⁺, and Cuº species coexist [45]. There is a well-defined shake up satellite in the case of ZnO-CuO/ NiCo₂O₄ hybrid, which suggests the presence of Cu²⁺ as in the case of CuO phase [45]. The absence of the Cu²⁺ satellite in ZnO-CuO samples can be attributed to its partial reduction to Cu⁺ or Cuº. This means that NiCo₂O₄ interacts with ZnO-CuO in order to stabilize Cu²⁺ species and prevent their reduction. In both NiCo₂O₄ and NiCo₂O₄/ZnO-CuO hybrid materials, it is evident that Ni 2p has four distinct contributions (Fig. 3c). The features centered at 854.0 and 855.8 eV are assigned to Ni³⁺ and Ni²⁺, respectively. The less pronounced peak at 857.3 eV is a consequence of Ni(OH)₂ sites due to water adsorption on the metal oxide surface, and a satellite peak is located at 861.0 eV [2, 46]. Co 2p XPS signal of both NiCo₂O₄ and NiCo₂O₄/ZnO-CuO reveal the Co²⁺ contribution noticeable at 781.2 eV and the Co³⁺ one centered at 779.2 eV (Figure 3d). There are two satellites peaks located at 783.6 and 787.6 eV [2, 28]. By comparing both samples, the proportion of Ni³⁺ and Co²⁺ species are lower for the hybrid material, corroborating the interaction with the ZnO-CuO composite.

The O 1s spectra of ZnO-CuO, NiCo₂O₄, NiCo₂O₄/ZnO-CuO hybrid have been decomposed with three contributions (Figure 3e). The peak at around 529 eV is assigned to the lattice oxygen (O₂⁻) connected to metals (O-Lat: O-Me, O Surf: O₂⁻ and O⁻O Chem_ OH⁻), the component at around 531 eV is attributed to oxygen vacancies at the surface with low oxygen coordinated sites (O-Surf). And the contribution at higher binding at around 532 eV is related oxygen in hydroxyl groups adsorbed on the surface of material or chemisorbed water molecules on the surface of
nanostructures (O-Chem) [46-50]. The study from XPS has revealed that the presence of NiCo₂O₄/ZnO-CuO maintains a high concentration of surface oxygen vacancies, as well as greater concentrations of Co³⁺, Ni²⁺ and Cu²⁺ on the surface.

<figure><img src="image_6.png" /><figcaption>**Figure 3:** Core level XPS spectra of ZnO-CuO, NiCo₂O₄, NiCo₂O₄/ZnO-CuO: a) Zn 2p, b) Cu 2p, c) Ni 2p, d) Co 2p and e) O 1s.</figcaption></figure>
Supplementary Figure (S3) illustrates the FTIR analysis of ZnO-CuO, NiCo₂O₄, and NiCo₂O₄/ZnO-CuO. Since the measurements were not conducted in an environment free from carbon dioxide, the peak at 2360 cm⁻¹ could be attributed to atmosphere carbon dioxide. Absorption bands found at 3300-3700 cm⁻¹ and 1651 cm⁻¹ are attributed to O-H stretching vibration and bending vibration caused by moisture absorbed on the surface of the samples [41]. Additionally, the C-H stretching vibrations observed at 1459 cm⁻¹ may have been caused by the residual ethanol used for washing the sample. Even though some peak modification is evident, it is probably due to NiCo₂O₄'s interaction with sandwich-layered ZnO-CuO, and in the grown NiCo₂O₄/ZnO-CuO hybrid, Co-O and Ni-O bonds are clearly seen at 581 cm⁻¹ and 648 cm⁻¹, respectively [37]. In NiCo₂O₄/ZnO-CuO hybrid electrode material, Cu-O chemical bond is observed at 580 cm⁻¹ [51], and Zn-O stretching vibration is observed at 553 cm⁻¹ [52]. Based on FTIR analysis, many of the variations of NiCo₂O₄, ZnO-CuO, and NiCo₂O₄/ZnO-CuO samples matched the reported works [53, 54], and the results were fully consistent with those obtained from XRD, XPS, SEM, and TEM/SAED, which provided well controlled metal oxide hybrids with nanostructures with chemical properties highly desirable for electrocatalysts.
## 3.2. Supercapacitor characterization of different NiCo₂O₄/NF, ZnO-CuO/NF, NiCo₂O₄/ZnO-CuO/NF composites

NiCo₂O₄/NF, ZnO-CuO/NF, NiCo₂O₄/ZnO-CuO/NF, and nickel foam were evaluated as electrodes for supercapacitors using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) in 3 M KOH electrolyte. The potential window used during CV measurements was considered that could clearly show the Faradic behavior of propsoed electrode material [55,56]. Figures 4a-d illustrate the CV curves for NiCo₂O₄/NF, ZnO-CuO/NF, NiCo₂O₄/ZnO-CuO/NF and nickel foam in the potential range between -0.1 and 0.6 V using different scan rates between 10 and 100 mV/s.

<figure><img src="image_7.png" /><figcaption>**Figure 4:** Cyclic voltammetry curves (CV), (a) bare nickel foam, (b) ZnO-CuO/NF, (c) NiCo₂O₄/NF, (d) NiCo₂O₄/ZnO-CuO/NF electrode materials at various scan rates using 3.0 M KOH aqueous solution</figcaption></figure>
Based on the CV curves, the redox peaks were well-resolved and indicated the typical Faradic reaction in the given potential window using various sweeping scan rates [57]. Increasing scan rate exhibited a progressive increase in the area of CV polarization curve, which indicated that the electrode material possessed the properties of rapid charge and discharge. It was evident from CV analysis that the NiCo₂O₄/ZnO-CuO/NF composite exhibited typical pseudocapacitive like charge storage characteristics at high scan rates. In addition, CV curves exhibited a slight shift in the redox region with increasing scan rate, which indicated a dominant contribution from surface active sites and a negligible contribution from inherent sites. Figure 4a-d describes the CV curves of bare nickel foam, ZnO-CuO composite/NF, NiCo₂O₄/NF and NiCo₂O₄/ZnO-CuO/NF electrode at different scan rate and it be seen that when the scan rate increases, the oxidation and reduction peaks are shifted to positive and negative potential respectively, with sudden large increase in peak current. The surge in the peak current has slightly influenced the polarization and inherent resistance of the electrode because the increasing scan rate allowed the insertion and extraction of the charge carriers could be raised in the electrolyte and on the surface of the electrode [58]. For the pure NiCo₂O₄/NF, and NiCo₂O₄/ZnO-CuO/NF electrodes a pair of redox peaks was observed through the CV polarization curves because of Ni²⁺/Ni⁺ and Co³⁺/Co⁴⁺ pair species [59]. Same way, a pair of redox peaks was noticed for the ZnO-CuO/NF composite and they emerged because of the direct growth of active material NiCo₂O₄/ZnO-CuO onto nickel foam, hence participated to improve the electrochemical properties due to increased surface area. Nanostructures deposited on porous nickel foam electrodes have revealed anodic and cathodic peaks as a result of their typical surface morphology. Due to the porous nature of the electrode decorated with the nanoflake morphology of NiCo₂O₄/ZnO-CuO/NF, a high number of electrolytic ions were able to be adsorbate. In addition to these features, the proposed electrode provides a large amount of surface area to allow frequent interaction with the electrolytic ions, thus facilitating the transport of charges. An electrochemical device has a plateaued CV curve that illustrates its favorable and enhanced charge and discharge characteristics [60]. The rapid redox process is possible because of the larger surface area of electrode hybrid NiCo₂O₄/ZnO-CuO/NF electrode. The CV curves reported in Figure 4a-d exhibited well redox features, describing their potential capacitive characteristics. Generally, Faradic reaction kinetics are related to electrode interactions with alkaline electrolytes, and could be represented as M-O/M-
O-OH (where M is Ni or Co) [25, 29, 31]. Moreover, the CV curves of all materials are associated with a pair redox peaks, especially highly visible on all curves of NiCo₂O₄/ZnO-CuO/NF electrode, they are mainly caused by the reversible redox reaction of Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺ in the presence of HO⁻ anions as illustrated below [61-64]:

$$NiCo_2O_4 + OH^- \leftrightarrow NiOOH + 2CoOOH + e^-$$

$$CoOOH + OH^- \leftrightarrow CoO_2 + H_2O + e^-$$

Using the slope of Iₚ vs v¹/² of graphs from CV curves as shown in Figure 5a-c for the ZnO-CuO/NF, NiCo₂O₄//NF and NiCo₂O₄/ZnO-CuO/NF, the diffusion coefficient of each material was calculated [28]. The corresponding diffusion coefficient values for NiCo₂O₄//NF, ZnO-CuO/NF and NiCo₂O₄/ZnO-CuO/NF were successively increasing with the scan rate as shown in Figure 5d-e, confirming that the hybrid material NiCo₂O₄/ZnO-CuO/NF has shown significant diffusion coefficient value, thus explaining the enhanced electrochemical activity of the NiCo₂O₄/ZnO-CuO/NF.

<figure><img src="image_8.png" /></figure>
**Figure 5:** (a-c) Plot of $I_p$ vs $v^{1/2}$ of ZnO-CuO/NF, NiCo₂O₄/NF, NiCo₂O₄/ZnO-CuO/NF for the calculation of diffusion coefficient values, (d, e) Corresponding estimated diffusion coefficient values

Further, GCD curves were measured at different current densities for the estimation of specific capacitance, energy density, and power density using three electrode configurations as shown in Figures 6a-d.

<figure><img src="image_9.png" /></figure>

**Figure 6:** Galvanic charge-discharge (GCD) curves (a) bare nickel foam, (b) ZnO-CuO/NF, (c) NiCo₂O₄/NF, (d) NiCo₂O₄/ZnO-CuO/NF electrode materials at various at current densities like 2, 2.5, 3, 3.5, 4, 4.5 Ag⁻¹ in 3.0M electrolytic solution of KOH, (e-f) Calculation of specific capacitance for different supercapacitor performance values of bare nickel foam, ZnO-CuO/NF, NiCo₂O₄/NF and NiCo₂O₄/ZnO-CuO/NF electrode materials at different current densities like 2, 2.5, 3, 3.5, 4, 4.5 Ag⁻¹ (g) columbic efficiency at 2 Ag⁻¹, (h) specific capacitance retention % at 2 Ag⁻¹, inset describes first and last 12 GCD cycles for brief view of negligible loss of columbic efficiency and capacitance retention %, revealing excellent recycle stability of electrode material.
Based on the GCD analysis, it was observed that a current density of 2 Ag⁻¹ resulted in a maximum specific capacitance as shown in Supplementary Table (S1). In the same manner as CV results (Figure 4), specific capacitance decreased with increasing scan rate, demonstrating excellent coherence between CV and GCD results. Figures 6a-d show the GCD curves recorded for NiCo₂O₄/NF, ZnO-CuO/NF, NiCo₂O₄/ZnO-CuO/NF, and nickel foam at 2, 2.5, 3, 3.5, 4 and 4.5 Ag⁻¹. A GCD curve has demonstrated that the NiCo₂O₄/ZnO-CuO/NF composite exhibits an enhanced discharge time. By in-situ growing various multi metal oxides onto nickel foam, the specific capacitance and charge-discharge time for NiCo₂O₄/ZnO-CuO/NF have increased to achieve the desired hybridization goal. Observations such as these are clearly evident in Figure 6d. The GCD profiles indicated the obvious plateau behavior, which indicates the Faradic characteristics of battery-like performance, which is fully in agreement with the CV analysis. It has been demonstrated that GCD curves have a symmetric appearance and a non-linear phenomenon, which indicates the existence of a strong battery-type redox mechanism, high reversibility, and almost negligible polarization [65, 66]. To evaluate the performance of supercapacitors, specific capacitance, energy density and power density are important parameters that were estimated using the relationships from the published work [67]. As shown in Figure 6c-d, a significant difference exists between NiCo₂O₄/NF and NiCo₂O₄/ZnO-CuO/NF in terms of GCD profiles. This may be due to the enhanced surface area, charge transport, and structural stability of NiCo₂O₄/ZnO-CuO/NF electrode materials. In addition, the synergetic effects of ZnO and CuO may contribute to the improvement of NiCo₂O₄/ZnO-CuO/NF electrochemical performance. Furthermore, multi-metal ions present in the electrode material offer variable oxidation states, improving the electronic environment and resulting in defects, which are responsible for increased specific capacitance through rapid ion diffusion [68, 69]. The corresponding values of specific capacitance, energy density and power density of NiCo₂O₄/NF, ZnO-CuO/NF, NiCo₂O₄/ZnO-CuO/NF and nickel foam at different energy densities are given in Figures 6e-f.

In the NiCo₂O₄/ZnO-CuO/NF composite, the maximum specific capacitance was 2639.80 F g⁻¹, the energy density was 57.79 Wh kg⁻¹, and the power density was 397.0 W kg⁻¹ at 2 Ag⁻¹. As shown by SEM and TEM results, the enhanced electrochemical performance towards supercapacitors can be attributed to the highly porous structure of the electrode. By using a nickel foam substrate, the electrode material was able to effectively adsorb a high density of
electrocatalytic ions. It was the sand that contained a layer of ZnO-CuO that provided the favorable environment for the deposition of NiCo₂O₄ in-situ. By using LPE, ZnO-CuO, and NiCo₂O₄ enriched the porosity of electrode, resulting in excellent performance of hybrid electrode material. In addition, the redox components Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺ improved swift ionic conduction in NiCo₂O₄/ZnO-CuO/NF due to the solid foundation of charge transport provided by ZnO-CuO/NF and nickel foam. Furthermore, the three-dimensional nickel foam network decorated with three-dimensional NiCo₂O₄/ZnO-CuO/NF nanoflakes enhanced ionic adsorption, reduced charge transfer resistance, and thus accelerated charge storage. As confirmed by CV and GCD results, the proposed study induced a dynamic role for ZnO-CuO/NF nanoflakes in enhancing the electrochemical charge storage capacity of NiCo₂O₄. As shown in Figures 6g-h, the NiCo₂O₄/ZnO-CuO/NF composite was evaluated for the 40000 repeatable GCD cycles. The NiCo₂O₄/ZnO-CuO/NF composite demonstrated a high specific capacitance retention of 100% - 99% and columbic efficiency of 99 % at 2 Ag⁻¹, as shown in Figures 6g-h. The use of 3D Ni foam for the deposition of NiCo₂O₄/ZnO-CuO materials could have electrochemical stability via maintaining the high specific capacitance of 99% after 40000 GCD cycles at 2 Ag⁻¹. This could attributed to the no use of binder and the 3D network of electrode mater that allowed the electrolytic ions to be in uniform and frequent interaction with the surface of electrode material, thereby without flocculation charge transport took place, hence the high specific capacitance was maintained and demonstrated by the hybrid electrode material. Based on these observations, the presented NiCo₂O₄/ZnO-CuO/NF composite could be utilized as an efficient electrode material for supercapacitor applications. Based on experimental findings of CV and GCD analysis, the proposed electrode material demonstrates pseudocapacitance dominance through enhanced electrochemical double layer capacitance (ELDC). Besides its superior performance, NiCo₂O₄/ZnO-CuO/NF is facile, scalable, low cost, ecofriendly, and polymer binder free electrode fabrication, enabling real-time application of the electrode configuration into practical energy storage devices.

### 3.3. Asymmetric device configuration of NiCo₂O₄/ZnO-CuO/NF and energy storage characterization

The device fabrication of NiCo₂O₄/ZnO-CuO/NF composite was designed for the demonstration of asymmetric supercapacitor (ASC) application using two electrode cells setup. The NiCo₂O₄/ZnO-CuO/NF composite was made as anode and the activated carbon was used as
cathode electrodes in 3 M KOH electrolytic solution. The activated carbon (AC) was used as a cathode. The representation of ASC device was shown as (NiCo₂O₄/ZnO-CuO/NF//ASC). The CV curves for AC as cathode electrode were measured in the potential window of -1.0 V to 0.1 V at different scan rates as shown in Supplementary Figure (S4). The CV curves of AC were described by negligible capacitance and were purely no-Faradic in nature. The CV curves of NiCo₂O₄/ZnO-CuO/NF anode electrode was measured using potential window from -0.1 V to 1.6V versus calomel electrode at various scan rates as shown in Figure 7a.

<figure><img src="image_10.png" /><figcaption>**Figure 7:** (a) Asymmetric supercapacitor device CV curves using NiCo₂O₄/ZnO-CuO/NF as anode electrode material and activated carbon as cathode material at various scan rates from 10 mV s⁻¹ to 110mV s⁻¹ in 3 M KOH aqueous solution, (b) GCD curves of asymmetric supercapacitor device configured with NiCo₂O₄/ZnO-CuO/NF as anode electrode material and activated carbon as cathode material at different current densities like 2, 2.5, 3, 3.5, 4, 4.5 Ag⁻¹ in 3 M KOH electrolytic solution, (c) columbic efficiency of ASC device, (d) specific capacitance retention % of ASC device.</figcaption></figure>

It could be found that the working potential window of ASC device from -0.1 to 1.6 V has shown obviously EDLC with high power density and battery type behaviour towards the realization of
high energy electrochemical device. The CV results of ASC demonstrated the distinctive
reversible Faradic properties. The mass loading of positive and negative electrode was estimated
using the reported mathematic relations [67, 70]. The maximum and overall loading mass of
positive electrode NiCo₂O₄/ZnO-CuO/NF to the negative electrode (AC) was found
approximately 0.2 mg.

The GCD curves of ASC at different current densities like 2, 2.5, 3, 3.5, 4 and 4.5 Ag⁻¹ are shown in Figure 7b. The measured GCD curves revealed the almost symmetric behaviour and CV and GCD analysis revealed an excellent electrochemical storage capability with obvious reversibility rate and stability of the ASC system. From GCD profiles, the specific capacitance, energy density and power density were calculated for ASC device using relations from reported work [70]. The ASC device exhibited a specific capacitance of 3614.769 Fg⁻¹ at current density of 2Ag⁻¹ with an energy density of 75.30 Wkg⁻¹ and power density of 1549.2 Wkg⁻¹ as shown in Figure 7c-d. Using NiCo₂O₄/ZnO-CuO/NF system, the energy storage is described by the pseudo capacitance mechanism as supported by CV and GCD analysis of supercapacitor application. While in ASC device, the configuration of two electrodes was proposed by the combination of non-Faradic of AC and Faradic mechanism of NiCo₂O₄. Moreover, the Faradic mechanism was also strengthened by the support of ZnO-CuO/NF. The aqueous solution of KOH electrolyte also favored the ionic diffusion at the interface of electrolyte and NiCo₂O₄/ZnO-CuO/NF.

The outstanding energy storage performance of proposed ASC device was compared with the
existing different NiCo₂O₄ electrode materials based ASC devices as given in Supplementary
Table (S1). Again in the ASC device fabrication, the NiCo₂O₄/ZnO-CuO/NF as an anode
electrode material has shown outperform functionality in terms of high specific capacitance,
power density, energy density, columbic efficiency, and specific capacitance retention compared
to the reported ASC devices based on NiCo₂O₄. To support the excellent electrochemical
performance of proposed ASC device for the long-term application, the repeatable 40000 GCD
cycles were measured as shown in Figure 7e-f. The ASC device retained the high specific
capacitance retention of 100 % - 95 % for 40000 GCD cycles at 2 Ag⁻¹, confirming the high
capacitance of the electrode material as shown in Figures 7e-f. Moreover, the 98 % columbic
efficiency was also estimated for the 40000 GCD cycles at 2 Ag⁻¹, verifying the outstanding
performance of ASC device. The robustness performance for maintaining high specific
capacitance is evidently proven from the stable micro/nanoflake architecture of the NiCo₂O₄/ZnO-CuO/NF electrode supported by high compatibility witnessed by the bio mimic effect of LPE. The overall performance of NiCo₂O₄/ZnO-CuO/NF electrode and ASC device in terms of rapid Faradic reaction kinetics, and high specific capacitance were observed. The use of ZnO-CuO/NF sandwich layer induced the highly unique nano environment as inner layer between the metallic nickel foam and NiCo₂O₄ nanostructures with excellent electrochemical contribution. Whereas the direct growth of bare NiCo₂O₄ could suffer with poor compatibility as previously reported by previous study [66], while the ZnO-CuO/NF might offer the better compatibility for NiCo₂O₄, thereby showed an enhanced cyclic stability, charge storage capability and fast charge transfer rate. The role of ZnO-CuO component was to offer the favorable surface for the deposition of NiCo₂O₄, and acted as co catalyst for promoting the catalytic properties NiCo₂O₄. Actually, the most catalytic material was NiCo₂O₄. The nickel foam was used working electrode due to its large surface area cowing to its porous structure for the deposition of significant amount of active electrode material

### 3.4. Oxygen evolution reaction performance of as prepared various NiCo₂O₄/NF, ZnO-CuO/NF, NiCo₂O₄/ZnO-CuO/NF systems

The OER activity of various electrocatalytic materials including NiCo₂O₄/NF, ZnO-CuO/NF, NiCo₂O₄/ZnO-CuO/NF and bare nickel foam was evaluated in 1 M KOH aqueous solution using LSV, and chronoamperometry. In order to stabilize the electrode material, CV cycles were conducted at a scanning rate of 5mV/s in 1M KOH aqueous solution prior to LSV tests. The LSV curves were measured at 5 mV/s as described materials, and the NiCo₂O₄/ZnO-CuO/NF shows better catalytic activity through a lowest overpotential of 300 mV against RHE at a 40 mAsm⁻² current density as shown in Figure 8a.
<figure><img src="image_11.png" /><figcaption>**Figure 8:** (a) Linear sweep voltammetry (LSV) curves of bare nickel foam, ZnO-CuO/NF, NiCo₂O₄/NF and NiCo₂O₄/ZnO-CuO/NF electrode materials at a scan rate of 5 mV·s⁻¹ in 1M KOH aqueous solution (b) Tafel analysis of bare nickel foam, ZnO-CuO/NF, NiCo₂O₄/NF and NiCo₂O₄/ZnO-CuO/NF electrode materials, (c) stability of NiCo₂O₄/ZnO-CuO/NF electrode material using LSV before and after durability test of 40 hrs, (d) chronopotentiometry test at 20 mA·cm⁻² for the illustration of durability for 40 hours after 40000 GCD cycles, (e) Nyquist plots of bare nickel foam, ZnO-CuO/NF, NiCo₂O₄/NF and NiCo₂O₄/ZnO-CuO/NF electrode materials using EIS measurement at 100kHz to 0.1 Hz, 5 mV amplitude and OER onset potential in 1 M KOH electrolytic solution.</figcaption></figure>

NiCo₂O₄/NF, ZnO-CuO/NF, and bare nickel foam had overpotential of 360 mV at 40 mA·cm⁻² and 420 mV at 30 mA·cm⁻², respectively. Nevertheless, the overpotential of bare nickel foam was estimated to be 430 mV at 32 mA·cm⁻². The overpotential of NiCo₂O₄/ZnO-CuO/NF electrode is either superior or equal to many recently reported electrocatalysts fabricated by hydrothermal, solvothermal and electrospinning techniques [71-74]. The NiCo₂O₄/ZnO-CuO/NF electrode showed the lower overpotential than many of the existing Cu and Co oxides based
electrocatalytic materials as given in Supplementary Table (S2). To have insightful understanding of NiCo₂O₄/ZnO-CuO/NF electrode, the Tafel analysis was carried out on LSV curves through the use of Tafel equation:

$$ \eta = a + b \log j_0, \quad (2) $$

Herein, the components like η as overpotential, j₀ as the current density, a is the intercept connected to exchange current density j₀, and b as Tafel slope. The Tafel values of NiCo₂O₄/ZnO-CuO/NF, ZnO-CuO/NF, NiCo₂O₄/NF, and bare nickel foam were estimated as 154 mVdec⁻¹, 188 mVdec⁻¹, 303 mVdec⁻¹, and 385 mVdec⁻¹ respectively. Among them, the NiCo₂O₄/ZnO-CuO/NF electrode possessed the lowest Tafel slope, suggesting the rapid OER kinetics as shown in Figure 8b. The Tafel slope of NiCo₂O₄/ZnO-CuO/NF electrode was found lower to similar to many of the already fabricated electrocatalysts such as CuCo₂O₄ (75.9 mV dec⁻¹) [75], CuCo₂O₄@carbon nanotubes (74 mV dec⁻¹) [76] and N-CuCo₂O₄@CNF-800 (77 mV dec⁻¹) [77] synthesized through various growth techniques including modified phase-transfer coprecipitation method, facile coaxial electrospinning and electrospinning. The comparison of Tafel analysis of NiCo₂O₄/ZnO-CuO/NF electrode is given in Supplementary Table (S2) with a Tafel slope of 154 mVdec⁻¹, indicating the involvement of adsorption of intermediate in the rate determining step for the proposed electrode material which is in good agreement with the proposed OER mechanism by Krasil' Shchikov [77]. The enhanced OER activity NiCo₂O₄/ZnO-CuO/NF could be con attributed from the porous morphology, abundant presence of Zn, Cu into the ZnO-CuO/NiCo₂O₄ through high density of surface oxygen vacancies, Co³⁺ and Ni²⁺ ions on the surface as shown by obtained SEM, and XPS information.

The LSV curves before and after the durability measurement were compared as shown in Figure 8c. It was seen that the NiCo₂O₄/ZnO-CuO/NF electrode has maintained the OER onset potential and almost same trend of current density generation was noticed. From OER analysis, it was noticed that all the samples exhibited certain oxidation reactions prior to OER onset potential. The oxidation peaks between 1.33 to 1.43 V against RHE could be attributed from oxidation of Ni/Co species [78-81]. These pre-oxidation peaks were attributing to Ni²⁺ to Ni³⁺ and Co²⁺ to Co³⁺ ions. The occurrence of pre-oxidation peaks suggested the possible Ni/Co(II)
transformation into active Ni/Co(III) species, consequently improved catalytic performance was
demonstrated [79-84].

Furthermore, to ensure the practical aspects of NiCo₂O₄/ZnO-CuO/NF electrode, after all the electrochemical measurements including 40000 GCD cycles for studying the cyclic stability of ASC device, the chronoamperometry was used for 40 hours at fixed overpotential as shown in Figure 8d. Despite the demonstration of 40000 GCD cycles on NiCo₂O₄/ZnO-CuO/NF electrode, there was negligible loss of current density at fixed overpotential, confirming the enhanced catalytic effectiveness and long term capability of the NiCo₂O₄/ZnO-CuO/NF electrode.

To get a deep insight on the robotic performance of NiCo₂O₄/ZnO-CuO/NF electrode, EIS experiments were performed. The electrochemical performance of ZnO-CuO/NiCo₂O₄ electrode through EIS was compared with the ZnO-CuO/NF, bare NiCo₂O₄/NF, and bare nickel foam using Nyquist plots as shown in Figure 8e. The NiCo₂O₄/ZnO-CuO/NF electrode experienced the lowest charge transfer resistance with a 1.73 Ohms, while ZnO-CuO/NF, bare NiCo₂O₄/NF, and nickel foam exhibited charge transfer resistance with values of 3.51 Ohms, 2.17 Ohms and 96.9 Ohms respectively. The lower real resistance/ charge transfer resistance of NiCo₂ O₄ /ZnO-CuO/NF compared to ZnO-CuO/NF electrode could be assigned to various factors such as morphology, size, porosity, high surface area, mixed oxidation of Co²⁺/Co³⁺ and Ni²⁺/Ni³⁺), high ion diffusion rate, and the synergistic interaction between NiCo₂ O₄ and ZnO-CuO composites. Importantly, the EIS study has revealed the lower diffusion resistance and rapid ionic diffusion kinetics of electrolyte ions using NiCo₂O₄/ZnO-CuO/NF electrode [78, 79].

To strengthen, the outstanding electrochemical performance of NiCo₂O₄/ZnO-CuO/NF electrode, CV analysis was performed with various sweeping scan rates for the illustration of non-faradic behavior in the potential window from 1.01 to 1.11 V against RHE for the estimation of double layer capacitance (Cdl) and electrochemical active surface area (ECSA). The corresponding CV curves of various materials such as NiCo₂O₄/ZnO-CuO/NF, ZnO-CuO/NF, NiCo₂O₄/NF and bare nickel foam at different scan rates are shown in Supplementary Figure (S5). The CV curves have almost shown the non-Faradic behavior. The Cdl was estimated from the linear fitting between difference of current density and scan rate. While the ECSA could be calculated through the relation ECSA= Cdl/Cs [69, 70]. The distinctive value of Cs= 0.04 mFcm⁻² was employed for the electrodes based on Cu and Co under alkaline electrolytic conditions [82-85]. The
corresponding C<sub>dl</sub> of NiCo<sub>2</sub>O<sub>4</sub>/ZnO-CuO/NF, ZnO-CuO/NF, NiCo<sub>2</sub>O<sub>4</sub>/NF and bare nickel foam is shown in Supplementary Figure (S5). Moreover, the ECSA and C<sub>dl</sub> of AC is also shown in Supplementary Figure (S6). The NiCo<sub>2</sub>O<sub>4</sub>/ZnO-CuO/NF electrode has possessed highest C<sub>dl</sub> value of 0.190 Fcm<sup>-2</sup> and likewise the ECSA, hence NiCo<sub>2</sub>O<sub>4</sub>/ZnO-CuO/NF has demonstrated an outperform electrochemical performance. The half-cell OER water splitting performance of NiCo<sub>2</sub>O<sub>4</sub>/ZnO-CuO/NF was compared with the published results on OER activity as given in Supplementary (Table 2). The stability of NiCo<sub>2</sub>O<sub>4</sub>/ZnO-CuO hybrid material was studied through measuring the XRD diffraction patterns as shown in Supplementary Figure (S7). It could be seen that the hybrid material maintained the typical XRD reflection of NiCo<sub>2</sub>O<sub>4</sub>, ZnO, and CuO and did not show any phase transformation, confirming the excellent stability of electrode material during electrochemical process.

The electrocatalyst based on NiCo₂O₄/ZnO-CuO/NF exhibited low over potential, Tafel slope, facile, inexpensive and ecofriendly for the fabrication of large scale electrodes for the practical water electrolyzers. A high porosity of NiCo₂O₄/ZnO-CuO/NF electrode via nanoflake shape orientation shows high exposure of surface active sites and enhance ionic transport, consequently superior electrochemical performance was demonstrated. The use of nickel foam facilitated the porous nanostructures through provision of zigzag structure which resulted the enhancement in active surface area. All the multi oxides onto NF were strongly interconnected and acted as single unit to strengthen the stability of electrode, electrical conductivity, fostering the ionic mass transport very efficiently. Importantly, the proposed NiCo₂O₄/ZnO-CuO/NF electrode was used without any polymer binder and conductive inclusions, thus significant loss of electrode dead volume could be achieved. Together with these advantages' aspects of NiCo₂O₄/ZnO-CuO/NF has shown outperform electrochemical functionality. For this reason, a brief illustration of NiCo₂O₄/ZnO-CuO/NF towards ASC and half-cell OER applications in KOH electrolytic solution is described in Scheme 2. However, the generalized OER mechanism in alkaline KOH aqueous solution is described through following chemical reactions. The NiCo₂O₄/ZnO-CuO/NF allowed the adsorption of hydroxide ions and thereby metal hydroxyl phase formation takes place and oxygen radicals are subsequently produced and the correspond interaction of oxygen radicals with hydroxyl ions resulted the formation of oxygen.

$$NiCo + OH^{-} \rightarrow NiCoOH^{-} + e^{-} \quad (3)$$
$$ \mathrm{NiCoOH}^* + \mathrm{OH}^- \rightarrow \mathrm{NiCoO}^* + \mathrm{H}_2\mathrm{O} \quad (4) $$

$$ \mathrm{NiCoO}^* + \mathrm{OH}^- \rightarrow \mathrm{NiCoOOH}^* + \mathrm{e}^- \quad (5) $$

$$ \mathrm{NiCoOOH}^* + \mathrm{OH}^- \rightarrow \mathrm{NiCo} + \mathrm{O}_2 + \mathrm{H}_2\mathrm{O} \quad (6) $$

<figure><img src="image_12.png" /><figcaption>**Scheme 2:** The illustration of OER and asymmetric supercapacitor mechanism in KOH using NiCo₂O₄/ZnO-CuO/NF electrode material.</figcaption></figure>

The proposed study describes the combined role of phytochemicals of LPE towards the in-situ growth of ZnO-CuO/NF and NiCo₂O₄//ZnO-CuO/NF composites and an enhanced electrochemical performance was observed. The role of each component towards improved electrochemical activity of NiCo₂O₄//ZnO-CuO/NF composites shows the limited of presented study because it was not possible for authors to investigate due to their limited laboratory facilities. The absence of experimental evidences for the understanding of growth mechanism of ZnO-CuO/NF and NiCo₂O₄//ZnO-CuO/NF composites and the electrochemical mechanism for the OER process are also the limiting aspects of presented study.
## 4. Conclusions

To summarize, ZnO-CuO/NF and NiCo₂O₄//ZnO-CuO/NF composites were grown in-situ using lemon peel extract (LPE). The LPE provided a natural source of growth directing agents as well as a high nucleation rate during the in-situ growth of ZnO-CuO/NF composites. In-situ growth of ZnO-CuO/NF composites was followed by deposition of NiCo₂O₄/Nanostructures using hydrothermal methods without the use of LPE. By morphologically analyzing ZnO-CuO/NF and NiCo₂O₄//ZnO-CuO/NF composites, we were able to identify each component and determine its orientation. In the XRD, HRTEM, SAED, and XPS analyses, it has been confirmed that NiCo₂O₄//ZnO-CuO/NF contains typical phases of ZnO, CuO, and NiCo₂O₄. ZnO-CuO/NF and NiCo₂O₄//ZnO-CuO/NF composites were used to study asymmetric supercapacitors and OERs. In NiCo₂O₄//ZnO-CuO/NF, the mediator role of ZnO-CuO/NF was found to be dominant due to the provision of multiple nanochannels for charge transport, high electrolytic ion adsorption, and favorable compatibility. The NiCo₂O₄//ZnO-CuO/NF electrodes used in the design of the ASC demonstrated high specific capacitances of 3614.769 F g⁻¹ high energy densities of 75.30 WhKg⁻¹, and high power densities of 1549.2 Wkg⁻¹ at 2 Ag⁻¹. During 40000 GCD cycles, a high specific capacitance retention of over 100 percent and a columbic efficiency of over 98 percent could be achieved. Several factors contributed to the high performance of NiCo₂O₄//ZnO-CuO/NF, including the microenvironment of nickel foam, the high mediating role of ZnO-CuO/NF, the enhanced electrical conductivity, the large surface active sites, and the rapid electrochemical reaction rates. Additionally, NiCo₂O₄//ZnO-CuO/NF electrodes were used for OER studies and exhibited the lowest overpotential of 210 mV at 40 mAcm⁻² and the highest Tafel slope of 70 mVdec⁻¹. Furthermore, the NiCo₂O₄/ZnO-CuO/NF electrode was highly durable for 40 hours and showed rapid charge transfer resistance. NiCo₂O₄/ZnO-CuO/NF electrodes were electrochemically demonstrated to provide a solid foundation for fabricating next-generation electrode materials using in-situ composite sandwich layers via green mediated synthesis.

## Acknowledgement
The authors would like to acknowledge the Higher Education Commission of Pakistan for its partial support of the project (NRPU/8350/8330). This publication is part of the R&D project PID2021-126235OB-C32 funded by MCIN/AEI/10.13039/501100011033/ and FEDER funds. Brigitte Vigolo and Mélanie Emo would like to thank the technical and scientific platform "Microscopies, Microprobes and Metallography (3 M)" (Institut Jean Lamour, IJL, Nancy, France) for access to TEM and SEM facilities. A.T and E.D would like to acknowledge the partial support of Ajman University, Internal Research Grant No. DRGS ref. 2024-IRG-HBS-1. The authors extend their sincere appreciation to the Researchers Supporting Project number (RSP2024R442), King Saud University, Riyadh, Saudi Arabia, for partial funding of this work.

**Funding**

Not Applicable

**Conflicts of Interest**

Authors have no conflict of interest in the presented research work.

**Data Availability Statement**

The authors declare that the data supporting the findings of this study are available within the paper.

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## Supplementary Information

### Advanced NiCo₂O₄/ZnO-CuO/NF composite for high performance asymmetric supercapacitor and efficient oxygen evolution reaction applications

Muhammad Ali Bhattiª, Shusheel Kumarᶜ, Aneela Tahiraᵈ, Adeel Liaquat Bhattiᶜ, Zaheer Ahmed Ujjanᶜ, Mansab Ali Jakhraniᶜ, Umair Aftabᵉ, Riyadh H. Alshammariʰ, Ayman Nafadyʰ, Elmuez Dawiᵢ, Mélanie Emoᵉ, Brigitte Vigoloᵉ, Antonia Infantes-Molinag, Zafar Hussain Ibupotoᵇ*

<footnote>ªCentre for Environmental Sciences, University of Sindh Jamshoro, 76080, Sindh, Pakistan.</footnote>

<footnote>ᵇInstitute of Chemistry, University of Sindh Jamshoro, 76080, Sindh, Pakistan.</footnote>

<footnote>ᶜInstitute of Physics, University of Sindh Jamshoro, 76080, Sindh, Pakistan.</footnote>

<footnote>ᵈInstitute of Chemistry, Shah Abdul Latif University Khairpur Mirs, Sindh, Pakistan.</footnote>

<footnote>ᵉMehran University of Engineering and Technology, 7680 Jamshoro, Sindh, Pakistan.</footnote>

<footnote>fUniversité de Lorraine, CNRS, Institut Jean Lamour-IJL, F-54000 Nancy, France.</footnote>

<footnote>gDepartment of Inorganic Chemistry, Crystallography and Mineralogy, Unidad Asociada al ICP-CSIC, Faculty of Sciences, University of Malaga, Campus de Teatinos, 29071, Malaga, Spain.</footnote>
hChemistry Department, College of Science, King Saud University, Riyadh, 11451, Saudi Arabia. iCollege of Humanities and Sciences, department of Mathematics and Sciences, Ajman University, P.O. Box 346, United Arab Emirates.

*Corresponding author(s): Zafar Hussain Ibupoto, PhD, Email: zaffar.ibhupoto@usindh.edu.pk

<figure><img src="image_13.png" /><figcaption>**Figure S1:** TEM images (a,b), SAED (c) and HRTEM (d) of NiCo₂O₄ sample with Fast Fourier transform (FFT) (insert of Figure d) of the yellow square area.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Figure S2:** TEM images (a,b) and SAED (c) of ZnO-CuO sample. For SAED, possible indexations for CuO monoclinic phase (in red) and ZnO hexagonal phase (in blue).</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Figure S3:** FTIR analysis of ZnO-CuO, NiCo₂O₄, NiCo₂O₄/ZnO-CuO.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Figure S4:** CV curves of activated carbon (AC) at various scan rates in 3.0M KOH electrolytic solution.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**Figure S5:** (a–f) CV curves of (a) bare nickel foam, (b) ZnO-CuO/NF, (c) NiCo₂O₄/NF, (d) NiCo₂O₄/ZnO-CuO/NF electrode materials at different scan rates from 10 mVs⁻¹ to 100 mVs⁻¹ in 1M</figcaption></figure>
KOH aqueous solution, (e) Estimated Cdl of bare nickel foam, ZnO-CuO/NF, NiCo₂O₄/NF, NiCo₂O₄/ZnO-CuO/NF electrode materials.

<figure><img src="image_18.png" /><figcaption>**Figure S6:** (a) CV curves of activated carbon (AC) at various scan rates (b) Estimated Cdl of activated carbon (AC).</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>**Figure S7:** XRD patterns of NiCo₂O₄/ZnO-CuO hybrid material after durability test of 40 hours.</figcaption></figure>
<table><caption>**Table (S1):** Comparative analysis of presented ASC based on NiCo₂O₄/ZnO-CuO/NF prepared with LPE with recently reported ASC results.</caption><thead><tr><th>Material</th><th>Specific Capacitance (F/g)</th><th>Current Density (A/g)</th><th>Energy Density (Wh kg<sup>-1</sup>)</th><th>Power Density (W kg<sup>-1</sup>)</th><th>Reference</th></tr></thead><tbody><tr><td>NiCo₂O₄ (OP-1)</td><td>1035 C g<sup>-1</sup></td><td>1 A g<sup>-1</sup></td><td>66.8 Wh kg<sup>-1</sup></td><td>900 W kg<sup>-1</sup></td><td>1</td></tr><tr><td>NiCo₂O₄@NiMoO₄ (5h)//AC</td><td>2806 F g<sup>-1</sup></td><td>5 A g<sup>-1</sup></td><td>64.2 Wh kg<sup>-1</sup></td><td>750 W kg<sup>-1</sup></td><td>2</td></tr><tr><td>NiO/MoO₃//AC</td><td>1516 F/g</td><td>1 A/g</td><td>50.13 Wh/kg</td><td>749.9 W/kg</td><td>3</td></tr><tr><td>NiMoO₄//AC</td><td>66 Fg<sup>-1</sup></td><td>1 Ag<sup>-1</sup></td><td>18 Wh kg<sup>-4</sup></td><td>704 W kg<sup>-1</sup></td><td>4</td></tr><tr><td>NiCo₂O₄/rGO//AC</td><td>1757 F g<sup>-1</sup></td><td>5 A g<sup>-1</sup></td><td>32.38 Wh kg<sup>-6</sup></td><td>797 W kg<sup>-1</sup></td><td>5</td></tr><tr><td>CdCO₃/CdO/Co₃O₄</td><td>84 F g<sup>-1</sup></td><td>1 mA cm<sup>-2</sup></td><td>26.3 Wh kg<sup>-1</sup></td><td>2290 W kg<sup>-1</sup></td><td>6</td></tr><tr><td>NiCo-NP</td><td>208.8 F g<sup>-1</sup></td><td>10 A g<sup>-1</sup></td><td>59.56 Wh kg<sup>-1</sup></td><td>3403 W kg<sup>-1</sup></td><td>7</td></tr><tr><td>NiO/GF</td><td>1225 F g<sup>-1</sup></td><td>2 A g<sup>-1</sup></td><td>32 Wh kg<sup>-1</sup></td><td>700 W kg<sup>-1</sup></td><td>8</td></tr><tr><td>NiO//AC</td><td>568.7 F g<sup>-1</sup></td><td>0.5 A g<sup>-1</sup></td><td>52.4 Wh kg<sup>-1</sup></td><td>800 W kg<sup>-1</sup></td><td>9</td></tr><tr><td>LWIP-1M</td><td>27.96 F g<sup>-1</sup></td><td>10 A g<sup>-1</sup></td><td>30.23 Wh kg<sup>-1</sup></td><td>31.82 Wh kg<sup>-1</sup></td><td>10</td></tr><tr><td>Co(OH)<sub>2</sub></td><td>400 F g<sup>-1</sup> to 1250 F g<sup>-1</sup></td><td>10 mA cm<sup>-2</sup></td><td>50 Wh kg<sup>-1</sup> to 156.25 Wh kg<sup>-1</sup></td><td>1.66 Wh kg<sup>-1</sup> to 5.20 Wh kg<sup>-1</sup></td><td>11</td></tr><tr><td>Cd(OH)<sub>2</sub></td><td>356 F g<sup>-1</sup></td><td>1 A g<sup>-1</sup></td><td>----------</td><td>----------</td><td>12</td></tr><tr><td>NiCo₂O₄/ZnO-CuO/NF</td><td>3614.769</td><td>2</td><td>75.30833</td><td>1549.2</td><td>Present work</td></tr></tbody></table>
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<table><caption>**Table (S2):** OER comparative study of NiCo₂O₄/ZnO-CuO/NF as an efficient electrocatalyst with published electrocatalysts performance.</caption><thead><tr><td>Catalyst</td><td>Over potential (mV) @ 10 mAcm<sup>-2</sup></td><td>Electrolyte</td><td>References</td></tr></thead><tbody><tr><td>NiS-rGo/NF</td><td>185</td><td>1.0 M KOH</td><td>[1]</td></tr><tr><td>GN@Ni<sub>3</sub>S<sub>2</sub></td><td>305</td><td>1.0 M KOH</td><td>[2]</td></tr><tr><td>MoS<sub>2</sub>/NiS/NF</td><td>216</td><td>1.0 M KOH</td><td>[3]</td></tr><tr><td>NiS/NiP</td><td>219</td><td>1.0 M KOH</td><td>[4]</td></tr></tbody></table>
<table><tr><td>NiS</td><td>354</td><td>1.0 M KOH</td><td>[5]</td></tr><tr><td>Ni@NF</td><td>285</td><td>1.0 M KOH</td><td>[6]</td></tr><tr><td>Mesoporous NiO</td><td>335</td><td>1.0 M KOH</td><td>[8]</td></tr><tr><td>NiO Film</td><td>360</td><td>1.0 M KOH</td><td>[9]</td></tr><tr><td>NiCo<sub>2</sub>O<sub>4</sub> nanostructures</td><td>260</td><td>1.0 M KOH</td><td>[10]</td></tr><tr><td>NiCo<sub>2</sub>O<sub>4</sub>/ZnO-CuO/NF</td><td>300@40mAcm<sup>-2</sup></td><td>1.0 M KOH</td><td>This work</td></tr></table>

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