Fabrication of CuO-Decorated Fe₂O₃ Nanoparticles as Efficient and Durable Electrocatalyst for Oxygen Evolution Reaction

Hira Khalid , Muhammad Asim ^* , Akbar Hussain , Tehmeena Maryum Butt , Sadia Kanwal , Naveed Kausar Janjua ^* 

1. Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan

* Correspondences: Muhammad Asim  and Naveed Kausar Janjua 

Academic Editor: Azam Akbari

Special Issue: Two-Dimensional Materials for Environmental Catalysis

Received: August 09, 2025 | Accepted: October 20, 2025 | Published: November 07, 2025

Catalysis Research 2025, Volume 5, Issue 4, doi:10.21926/cr.2504009

Recommended citation: Khalid H, Asim M, Hussain A, Butt TM, Kanwal S, Janjua NK. Fabrication of CuO-Decorated Fe₂O₃ Nanoparticles as Efficient and Durable Electrocatalyst for Oxygen Evolution Reaction. Catalysis Research 2025; 5(4): 009; doi:10.21926/cr.2504009.

© 2025 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

1. Introduction

In developing countries, the pursuit of economic sustainability, population growth, and better living standards has resulted in a rise in ecological disparities and environmental problems due to industrialization and the usage of fossil fuels [1]. Hydrogen gas is a viable alternative due to its higher energy density, accessibility, availability, and reactivity [2]. In this regard, electrochemical water splitting reactions have become essential processes to produce green hydrogen from renewable sources [3]. The electrochemical splitting of water consists of two half-reactions: oxygen evolution reaction (OER) occurs at the anode (2H₂O → 2H⁺ + O₂ + 4e^-), and hydrogen evolution reaction (HER) occurs at the cathode (2H⁺ + 2e^- → H₂) [4,5]. Anodic OER hinders the large-scale synthesis of green hydrogen due to its sluggish kinetics. To address this, electrode materials or electrocatalysts are utilized to attain lower overpotential. Although materials based on Ir and Ru are selective and practical, their limited availability prevents them from being widely used in the commercial sector for the affordable manufacture of hydrogen. Since transition metal-based materials, particularly transition metal oxides, are abundant and non-precious, they are being investigated as alternative electrocatalysts [6,7,8,9,10].

According to conventional research, heterostructures—which combine two or more metals to produce exceptional catalytic capabilities—often exhibit outstanding catalytic properties, while simple metal oxides frequently perform inadequately. Oxides of transition metals like iron, copper, cobalt, zinc, and manganese have become promising electrocatalysts for OER in alkaline media due to their high performance and good stability [11,12,13,14,15,16,17,18,19,20]. Among them, the iron oxide and copper oxide heterostructure is an efficient catalyst for OER due to its high surface-to-volume ratio, porous structure, and high stability. The semi-conducting properties of hematite are often beneficial in the conversion of solar energy, as a photocatalyst, and water splitting [21,22,23,24,25,26,27]. Hematite (α-Fe₂O₃) is an intriguing n-type semiconductor metal oxide due to its favorable optical band gap (about 2.1 eV), chemical stability, natural abundance, nontoxicity, and inexpensive cost. In contrast, cupric oxide (CuO), a p-type semiconductor with a low band gap of ~1.2 eV, offer promise performance in solar cells, batteries, supercapacitors, catalysis, and sensors [28,29,30,31].

Anza Farooq and co-workers synthesized CoFe₂O₄/Fe₂O₃ and C-CoFe₂O₄/Fe₂O₃ (carbon-doped) electrocatalysts through a facile calcination method for electrochemical water oxidation. The synthesized composite materials showed an overpotential of 260 mV and a Tafel slope of 183 mV dec^-1 [32]. P. Mohana et al. synthesized a non-noble CuO/NiO/rGO nanocomposite through a co-precipitation approach and applied it for the electrochemical OER process in alkaline media. As-synthesized nanocomposite maintained an overpotential of 200 mV at 10 mAcm^-2 Tafel and a Tafel slope of 165 mVdec^-1 [33]. Ye et al. tested various iron-copper oxides with different Cu/Fe ratios for OER in 1 M KOH. The oxide that demonstrated the best performance was 6CuO₂-Fe₂O₃, with an overpotential of 510 mV at 10 mAcm^-2. All the samples were stable in the reaction conditions. The performance was attributed to the coexistence of the metal oxides, but the overpotentials are still high, which shows that further improvements can be made [34].

In this contribution, we follow a rapid and cost-effective synthesis route for the fabrication of α-Fe₂O₃ and CuO@Fe₂O₃ (5-15% CuO). As-synthesized nano-composite materials confirmed the synergistic effect between CuO and Fe₂O₃ with enhanced oxygen evolution activity. The crystalline structure, morphological and textural properties were studied through different characterization techniques including XRD, FTIR, and SEM. Electrochemical activity of as-synthesized nanocomposites for OER was investigated through CV, LSV, EIS, and chronoamperometry. CuO@Fe₂O₃ (10% CuO) nanocomposite demonstrated outstanding OER activity with a low overpotential of 170 mV vs. NHE at 10 mAcm^-2 in alkaline media. Moreover, the composite electrocatalysts exhibited excellent electrocatalytic parameters such as slight Tafel slope (143 mVdec^-1), low charge transfer resistance (R_ct, 22.4 Ω), highest diffusion co-efficient (D°, 3.5 × 10^-8 cm²s^-1), mass transport co-efficient (m_t, 3.6 × 10^-4 cms^-1), heterogenous rate constant (k°, 4.3 × 10^-4 cms^-1) were observed for CuO@Fe₂O₃ with 10% of CuO contents [35]. This work could provide valuable insights into the design of coupled metal oxide advanced catalysts for OER and beyond.

2. Materials & Methods

2.1 Chemicals

Iron nitrate nonahydrate (Fe(NO₃)₃·9H₂O) (99.99%), Potassium hydroxide (KOH) (99%), Copper nitrate hexahydrate (Cu(NO₃)₂·6H₂O) (98%), Acetone (CH₃OCH₃) (98%), Potassium ferrocyanide trihydrate (K₄[Fe(CN)₆]·3H₂O) (98%), Potassium hydroxide (KOH), Potassium chloride (KCl) (98%), Aluminum oxide (Al₂O₃) (97%), Methanol (MeOH) (96%), Nafion (5 wt./V% solution) (C₇HF₁₃O₅C₂F₄), and Ethanol (C₂H₅OH) (97%) were acquired from Sigma Aldrich and used in synthesis and experimental process without further treatment.

2.2 Synthesis of α-Fe₂O₃ Nanoparticles

The precipitation method was employed for the synthesis of single-phase iron oxide (α-Fe₂O₃) nanoparticles by mixing 4.052 g of iron (III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O) in 100 ml of deionized water, and a 2 M aqueous solution of KOH base in 50 ml of deionized water was used to maintain the pH at 11 [36]. The solution was heated at 80°C under magnetic stirring at 400 rpm for 3 h until the rusty brown precipitates of Fe(OH)₃ began to form, which were then subjected to centrifugation at 6000 rpm and then dried at 80°C for 2 h. After that, the powdered hydroxide precursor was calcined for 4 h at 500°C. The powdered material was ground in the acetone and media, and then ball milling was performed for 2 h to get homogeneous nanoparticles.

2.3 Synthesis of CuO@Fe₂O₃ Nanoparticles

Five different wt% compositions (5, 7, 10, 12, and 15) of CuO supported on α-Fe₂O₃ were synthesized by using the wet impregnation method. For this purpose, a calculated amount of pre-dried support i.e., α-Fe₂O₃ was soaked in the measured volume of 1 M copper nitrate hexahydrate (Cu(NO₃)₂·6H₂O) solution for all the five compositions and left overnight. Subsequently, the materials were dried for 1 h in an oven, and then grinding and calcination took place at 500°C for 2 h. After that, acetone media was used to grind the final powder, dried, and then ball milling was performed for 2 h to get homogeneous nanoparticles.

2.4 Physical Characterizations

To study the crystallinity of nanoparticles powders XRD PANalytical X'PERT high score diffractometer was used, equipped with Cu-Kα radiations of 0.154 nm in the scan rate of 0.625 s at a scan range of 20-80° [37]. By using the Nicolet 5PC instrument, where all the samples were mixed with KBr powder, FTIR spectra were acquired for all the synthesized nanoparticles in the wavelength range of 400-4000 cm^-1. Morphology of all synthesized compositions was examined through scanning electron microscopy (SEM) by TESCAN, acquired from Germany.

2.5 Electrochemical Characterizations and Electrode Modification

The drop cast method was used for the modification of the working electrode [38], in which a cleaned GC electrode's surface was dampened with 2 μl of pure ethanol, and 0.1 mg of purely ground powder of electrocatalyst was drop cast on it by using 2 μl of 0.5% Nafion. The electrode was dried at 45°C in an oven for 30 min [16,17,39].

The electrochemical measurements were conducted using the Gamry-potentiostat interface 1000. Electrochemical characterization of metal oxides was done to study the conductive behavior of synthesized materials using three electrode systems: a glassy carbon electrode functions as the working electrode, a platinum wire acts as the counter electrode, and Ag/AgCl (3 M KCl) is used as the reference electrode. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were performed in alkaline medium, and the potential recorded against Ag/AgCl was converted to RHE using the equation (E_RHE = E_Ag/AgCl + 0.197 + 0.0591 * pH), and electrocatalytic parameters were calculated. Conductive nature and electron transfer behavior of all prepared electrocatalysts were studied by performing electrochemical impedance spectroscopy (EIS) in both AC and DC voltage. Double-layer capacitance (C_dl) was calculated by performing CV in the non-Faradic region. Chronoamperometric analysis was carried out for the stability test of modified electrodes.

3. Results & Discussion

3.1 Physical Characterizations

X-ray diffraction analysis was carried out to examine the crystallinity and phase purity of the prepared samples. The XRD pattern of the pure Fe₂O₃ is well matched with the reference pattern of the rhombohedral structure of hematite (JCPDS No. 00-024-0072). The XRD pattern for all doped compositions is shown in Figure 1(a), in which a peak at 38.5° confirms the successful incorporation of CuO nanoparticles on Fe₂O₃ support. It is evident that the peak intensity kept on increasing with the incorporation of different concentrations of CuO. The peaks indexed to CuO and α-Fe₂O₃ are sharp, which reveals crystallized behavior of the prepared samples.

Figure 1 (a) XRD patterns and (b) FTIR spectra of pure and doped compositions, (c & d) SEM micrographs of Fe₂O₃ and 10% CuO@Fe₂O₃ at 1 µm magnification scale, respectively, (e & f) EDS spectra of Fe₂O₃ and 10% CuO@Fe₂O₃, respectively.

Average crystallite size of synthesized materials was estimated by using Debye Scherer formula [40]:

\[ \mathrm{D_{av}~(nm)=\frac{k\lambda}{\beta cos\theta}} \tag{1} \]

Where, D_av is the average crystallite size, k is the shape factor, and β is the diffraction peak width (radians). The calculated crystallite size of the synthesized samples was 10-28 nm and is shown in Table 1.

Table 1 Crystallite size of all compositions.

FTIR was performed in the range of 400-4000 cm^-1 in order to examine the phase purity and successful fabrication of nanocomposites. Sharp bands are observed at wavenumber below 800 cm^-1 that correspond to characteristics CuO vibrational modes [41], confirming that CuO has been successfully incorporated into the Fe₂O₃ matrix. FTIR spectra of all loaded compositions are shown in Figure 1(b). SEM and EDS analysis were carried out to examine the surface morphology and phase purity of all samples. The SEM micrographs of Fe₂O₃ and 10% CuO@Fe₂O₃ at 1 μm revealed an agglomerated structure, as shown in Figure 1(c & d), respectively. EDS analysis results of wt% and atomic% are in good agreement with the stoichiometric amount taken for the synthesis of the nanocomposite material. Figure 1(e & f) represents the EDS spectra of Fe₂O₃ and 10% CuO@Fe₂O₃, respectively.

3.2 OER Response of Pure and Composite Nanomaterials (xCuO@Fe₂O₃)

3.2.1 CV and EIS Analysis in Potassium Ferricyanide Solution

Cyclic voltammetry was performed in a 5 mM solution of potassium ferricyanide with 3 M KCl electrolyte. The active surface was calculated from the current responses of the working electrode for the Fe²⁺/Fe³⁺ redox couple, which showed the single-electron transfer process. Figure 2(a) represents the CV profile of all the catalyst-modified electrodes, which demonstrates the change in current responses as the compositions of CuO varied. Randles-Sevick equation was used to calculate the active surface area of modified electrodes [16,42]:

\[ \mathrm{I_P}=2.69\times10^5\mathrm{n}^{3/2}\text{ A C D°}^{1/2}\upsilon^{1/2} \tag{2} \]

Where I_p represents the redox peak current of the analyte, v denotes the scan rate in Vs^-1, C is the concentration in mol cm^-3, D denotes the diffusion coefficient (0.76 × 10^-5 cm²s^-1), A is the surface area in cm², and n is the number of electrons transferred. The maximum current response was observed for the 10% CuO@Fe₂O₃, thus offering the highest active surface area of 0.041 cm², indicative of the enhanced redox potential of 10% copper-doped iron oxide.

Figure 2 (a) CV response of xCuO@Fe₂O₃ modified GC electrode in 5 mM K₃[Fe(CN)₆] + 3 M KCl at 0.1 V s^-1 in potential window range of -0.2 to 0.6 V, (b) Simulated EIS spectra of all compositions performed in 5 mM K₃[Fe(CN)₆]:3 M KCl, with its equivalent circuit model in the inset.

EIS was also performed for all the compositions in 5 mM K₃[Fe(CN)₆] and 3 M KCl to study the charge transfer parameters, and a Nyquist plot was obtained from the acquired data. At the high frequency domain, a Nyquist plot shows a distinct semicircle which is analogous to the electron transfer resistance (R_ct) and at lowest frequency an inclined part which is analogous to the diffusion process that defines Warburg resistance (R_w). The faster transfer of electrons at the electrode surface is shown by a small R_ct value, because of high conductivity. To calculate the apparent electron transfer rate constant, the following equation is used [43].

\[ \mathrm{k_{app}=RT/F^2R_{ct}C} \tag{3} \]

In this equation, R represents the gas constant (8.314 J mol^-2 K^-1), T denotes the absolute temperature, F is the Faraday constant, and C is the analyte's concentration (mol cm^-3). Figure 2(b) shows the EIS profiles along with model-fitted spectra of all compositions in 5 mM K₃[Fe(CN)₆] and 3 M KCl. Various EC parameters were acquired from EIS data by the model fitting of the observed spectra of all the compositions are shown in Table 2. 10% copper-doped iron oxide shows the highest k_app and the lowest R_ct values, which marks it for the fast electron transfer process as compared to other catalysts [44]. A small value of R_ct represents a fast electron transfer process at the electrode-electrolyte interface, rendered by the coherence between a smaller particle size and better conductivity.

Table 2 EIS parameters values for all the compositions.

3.2.2 Determination of Electrocatalytic Parameters (j, E_on, η, TS, R_s, and R_ct)

Current density is one of the most essential OER criteria for assessing any catalyst's performance. The catalyst becomes more active as its current density increases. To distinguish the response of bare glassy carbon electrode (GCE) and modified iron oxide in both 1 M KOH and 1 M MeOH systems at 0.1 V s^-1, a comparison of cyclic voltammograms recorded in the potential range of 0 to 1.5 V is shown in Figure 3(a). Methanol was used as a facilitator, and from the CV profile, it can be inferred that the current density was enhanced from 8 mA cm^-2 to 16 mA cm^-2 for iron oxide by the addition of methanol. Peak current densities were observed to be higher for the doped iron oxide as compared to pure iron oxide. CV responses of all compositions in 1 M KOH and 1 M KOH + 1 M methanol are shown in Figure 3(b & c), respectively. All doped iron oxide nanomaterials were responsive to water electrocatalysis and exhibited good OER performances. However, 10% CuO@Fe₂O₃ composition shows the highest current density, thus exhibiting better electrocatalytic composition than others. Current densities were estimated from the peak heights in the voltammograms collected in 1 M KOH and 1 M KOH/1 M methanol system, and it is also represented in graphical form in Figure 3(d). Due to the increase in oxidation rate of OH- ions to produce the oxygen in the OER process, there was a 3 to 4 order increment in the peak current densities in KOH+ methanol as compared to that in KOH only. This may have occurred because of salt-like species, MeO^-K⁺, produced due to OH^- and methanol, which facilitates the migration of OH- ions towards the electrode surface (KOH + MeOH → MeO^-K⁺ + H⁺ + OH^-). More OH^- ions move toward the electrode's surface, to get oxidized, and more oxygen is produced. The enhanced charge transfer kinetics of the OER process at the electrode surface render an increase in the current density.

Figure 3 (a) Comparison of voltammograms of bare GCE and modified GCE at 0.1 V s^-1 in 1 M KOH and 1 M KOH+ 1 M MeOH systems, (b & c) Comparison of voltammograms of all composition modified electrodes at 0.1 V s^-1 in 1 M KOH and 1 M KOH + 1 M MeOH, respectively, (d) graphical representation of current densities of all the compositions in 1 M KOH and 1 M KOH + 1 M MeOH, (e) comparison of LSV plot of 10% CuO@Fe₂O₃ at 0.1 V s^-1, and (f) Tafel slopes of all compositions.

The performance of a catalyst can also be examined by its peak current and onset potential. To determine the catalytic behavior of a material, the onset potential is a vital parameter [45]. The catalyst with the highest peak current and lowest onset potential value is the optimal one. Figure 3(e) shows peak current and onset potential of 10% CuO@Fe₂O₃ in both systems at 100 mV s^-1. Among all the electrocatalysts, 10% CuO@Fe₂O₃ showed low onset and overpotential values, which confirms that it is a powerful catalyst for the OER process.

To examine the mechanism and kinetics of the redox process, Tafel analysis was carried out, which is also an applicable criterion for the comparison of the catalytic activity of various catalysts [46]. The Tafel equation (η = a + b log(J/J˳)) was used to calculate the Tafel slopes for all the modified electrodes. Figure 3(f) represents the comparison of Tafel slopes of all the catalysts in 1 M KOH: 1 M MeOH. The lowest Tafel slope is observed for 10% CuO@Fe₂O_3, which indicates that current density increases with a smaller overpotential value, suggesting a faster reaction rate. All the above electrochemical parameters obtained for all compositions are summarized in Table 3.

Table 3 EC parameters of all the composition determined from CV, LSV and EIS.

Electrochemical impedance spectroscopy was carried out in 1 M KOH and 1 M KOH + 1 M MeOH, to study the charge transfer kinetics of OER process. Figure 4(a) shows the EIS spectra of 10% CuO@Fe₂O₃ at different voltage ranges (1.1-1.5 V), and the lowest charge transfer resistance was observed at 1.5 V. Figure 4(b) represents the comparison of EIS data of all compositions in 1 M KOH + 1 M MeOH. At a high frequency region (1 MHz), a complete semicircle is obtained, which demonstrates the charge transfer process. In contrast, at a low frequency region (0.1 Hz), an inclined part confirms the diffusion process. Table 3 shows the charge transfer resistance values of all the compositions, and 10% CuO@Fe₂O₃ showed the fastest electron transfer reaction with the minimum charge transfer resistance of 22.4 Ω in 1 M KOH: 1 M MeOH. Figure 4(c) represents the model-fitted Nyquist plot obtained for Fe₂O₃ in 1 M KOH + 1 M MeOH. At high frequency domain, Nyquist plot shows a distinct semicircle which is analogous to the electron transfer resistance (R_ct) and at lowest frequency an inclined part which is analogous to the diffusion process that defines Warburg resistance (R_w) [44]. The faster transfer of electrons at the electrode surface is shown by a small R_ct value, because of high conductivity.

Figure 4 (a) EIS spectra of 10% CuO@Fe₂O₃ at 1.1-1.5 V in 1 M KOH + 1 M MeOH, (b) comparison of EIS plots for all the compositions at 1.5 V in 1 M KOH + 1 M MeOH, (c) Model fitted EIS spectra of Fe₂O₃ in 1 M KOH + 1 M MeOH, and (d) Equivalent circuit model of data acquisition.

For the quantification of EIS data, equivalent circuit models are fitted for simulation of the EIS data as shown in Figure 4(d). Any modification on the electrode surface has no effect on solution resistance (R_s) and Warburg resistance (R_w), which show the properties of electrolyte solution and electrocatalyst-dependent diffusion in the solution, respectively.

3.2.3 Determination of D°, m_T, k°, C_dl, and ECSA

Cyclic voltammetry was performed for all compositions and Figure 5(a & c) represent the CV response of Fe₂O₃ and 10% CuO@Fe₂O₃, respectively at different scan rates in 1 M KOH + 1 M MeOH where the current density increases by increasing the scan rate. This study explores the impact of scan rate on diffusion kinetics. The water oxidation process being irreversible, the relevant Randles-Sevcik equation was employed to estimate the diffusion coefficient values [42].

\[ \mathrm{I_p=2.99\times10^5n^{3/2}\alpha^{1/2}~A~C~D°^{1/2}\upsilon^{1/2}} \tag{4} \]

Figure 5 Cyclic voltammograms of (a) Fe₂O₃ and (c) 10% CuO@Fe₂O₃ modified electrode in 1 M KOH + 1 M methanol recorded at 20-100 mVs^-1 scan rates, (b & d) Linear functional plots of I_p vs. ν^1/2 of Fe₂O₃ and 10% CuO@Fe₂O₃ respectively, (e & g) CV response for OER recorded in 0.25-2 M methanol and (f & h) their resultant linear plots of I_p vs. [MeOH] for Fe₂O₃ and 10% CuO@Fe₂O₃ respectively, (i & k) cyclic voltammetric responses recorded in non-faradic region and (j & l) the resultant linear plots of Fe₂O₃ and 10% CuO@Fe₂O₃, respectively for the determination of C_dl.

The linear functional plot of peak current versus square root of scan rate was used for the estimation of diffusion coefficient value and are given in Figure 5(b & d) for Fe₂O₃ and 10% CuO@Fe₂O₃. The oxidation process at the electrode-electrolyte interface was facilitated by all electrodes, which showed a similar trend of rising currents with the scan rate. The linear trends of Randles-Sevcik plots confirmed that the water oxidation is a diffusion-controlled reaction. The mass transport coefficient was also calculated by using the following equation [47].

\[ \mathrm{m_T=[D°/(RT/F\upsilon)]^{1/2}} \tag{5} \]

The reversibility of any chemical reaction can be identified by comparison of its diffusion coefficient values with the k° values. Slow and fast electrode kinetics are related to mass transport and for an irreversible system; k° ≤ 10^-5 is followed.

The effect of methanol concentration was studied by performing cyclic voltammetric measurements for all the modified electrodes. By changing the methanol concentration from 0.25 M to 2 M in a 1 M KOH solution at room temperature, the OER peak current was enhanced in all systems. Methanol concentration effects on OER responses of Fe₂O₃ and 10% CuO@Fe₂O₃ are represented in Figure 5(e & g), respectively.

The heterogeneous rate constant is a crucial factor for the estimation of the activity of synthesized materials, and it demonstrates how quickly the electrochemical reaction occurs. It also reflects the reversibility and irreversibility of response. The value of the heterogeneous rate constant was estimated by using the Reinmuth equation:

\[ \mathrm{I_p=0.227\,\,n\,\,FACk°} \tag{6} \]

Where F represents the Faraday's constant, n denotes the number of electrons, I_p represents the anodic peak current, A is the area of the electrode, C is the methanol concentration, and k° is the heterogeneous rate constant. A Reinmuth graph is plotted between I_p and methanol concentration, and from the slope value, k is estimated. Figure 5(f & h) show the plots of I_p vs. methanol concentration for Fe₂O₃ and 10% CuO@Fe₂O₃, respectively. D°, m_T and k° for all compositions are summarized in Table 4 where the highest values are obtained for 10% CuO@Fe₂O₃, which is an indication of the good potency of catalyzing the water oxidation process.

Table 4 Estimated kinetic parameters for water oxidation on CuO@Fe₂O₃ materials.

Double layer capacitance C_dl, was calculated by performing the cyclic voltammetry in non-faradic region at various scan rates (20-100 mV s^-1) in 1 M KOH + 1 M MeOH for Fe₂O₃ and 10% CuO@Fe₂O₃ are shown in Figure 5(i & k), while their linear plots are presented in Figure 5(j & k), respectively [48]. Double-layer capacitance was calculated from the slope of linear plots and ECSA was calculated by using the equation (ECSA = C_dl/C_sp) where C_sp denotes the general specific capacitance for metal oxide electrodes in alkaline solution (0.040 mF cm^-2) [49]. For catalytic activity, another parameter is used, which is called the roughness factor (R_F). It is calculated by dividing the ECSA by the geometric surface area of the glassy carbon electrode (0.0707 cm²). Electrochemical parameters obtained for all compositions are shown in Table 4.

3.2.4 Effect of Temperature on OER Response and Stability Test

Cyclic voltammograms were recorded in the temperature range of 298 to 323 K with a 5°C interval to study the temperature effect on the OER kinetics. The peak current was observed to increase slightly with temperature. As the temperature rises, more molecules become mobile, which in turn increases the oxidation process, thereby enhancing the OER kinetics. Figure 6(a) represents the cyclic voltammograms for the temperature effect of OER on CuFe-10.

Figure 6 (a) CV responses of CuFe-10 at different temperature range in 1 M KOH + 1 M MeOH, (b) and (c) Functional plots of CuFe-10 for determination of ∆H and ∆S values in 1 M KOH and 1 M KOH + 1 M MeOH respectively, (d) Chronoamperometry stability test performed for all compositions for 3100 s, and (e) Stability test performed for CuFe-10 for 24 hours.

Electrocatalytic stability of catalysts can be studied by using an effective technique called chronoamperometry [50]. Figure 6(d) represents the current-time response for all prepared samples in 1 M KOH + 1 M methanol for 3100 s. The scan shows no significant change in current; however, a slight fluctuation in the scan could be due to oxygen bubble formation on the electrode surface. This conforms to good stability and long-term application of the synthesized compositions.

To calculate the thermodynamic parameters, Marcus equation was used:

\[ \mathrm{k°=Z_{het}\exp[-\Delta G/RT]} \tag{7} \]

Where, Z_het is the collision frequency, and its value can be calculated at different temperature with the help of following relation:

\[ \mathrm{Z}_{\mathrm{het}}=(\mathrm{R}\mathrm{T}/2\pi\mathrm{M})^{1/2} \tag{8} \]

Where, M stands for the analyte's molar mass.

Marcus equation can be written in the more simplified form in the following way:

\[ \ln(\mathrm{k}°/\mathrm{Z}_{\mathrm{het}})=-\Delta\mathrm{H}°/\mathrm{RT}+\Delta\mathrm{S}°/\mathrm{R} \tag{9} \]

Figure 6(b & c) shows the linear functional plots of ln (k°/Z_het) vs. 1/T in 1 M KOH and 1 M KOH + 1 M methanol, respectively and ΔS and ΔH values were calculated from the intercept and slope of linear plots. ΔH and ΔS, calculated from the above plots, are tabulated in Table 5.

\[ \Delta\mathrm{G}°=\Delta\mathrm{H}°-\mathrm{T}\Delta\mathrm{S}° \tag{10} \]

Table 5 Thermodynamic parameters for OER on CuFe-10 modified GC electrode.

The positive value of ∆G indicates the reaction is non-spontaneous and energy-driven at low temperatures in the 298 to 323 K range. The thermodynamic parameters are the indicators of the compatibility between the material surface properties and the interface. These parameters are nearly the same in KOH only and KOH plus MeOH, thus corresponding to their catalytic suitability in both the alkaline medium and methanol fuel cell applications.

4. Conclusions

Pure iron oxide and different compositions of CuO@Fe₂O₃ were successfully synthesized via facile precipitation and impregnation methods, respectively. XRD patterns confirmed the rhombohedral crystal structure of hematite, and FT-IR confirmed metal oxygen bond formation. SEM and EDX showed agglomerated shapes and the purity of the synthesized materials, respectively. EIS and CV were conducted to study the redox properties of the synthesized materials, which revealed that 10% CuO@Fe₂O₃ is the best candidate among all the catalysts. 10% CuO@Fe₂O₃ showed better catalytic behavior with the highest peak current and low onset and overpotential values. Water oxidation was observed to be diffusion-controlled using all the modified electrodes with the highest diffusion co-efficient (3.5 × 10^-8 cm²s^-1), mass transport co-efficient (3.6 × 10^-4 cms^-1) for 10% CuO@Fe₂O₃. From the electrochemical responses, the OER performance order can be visualized as: 5% CuO@Fe₂O₃ < 7% CuO@Fe₂O₃ < 10% CuO@Fe₂O₃ > 12% CuO@Fe₂O₃ > 15% CuO@Fe₂O₃. The interesting application of these simple materials as potential catalysts for electrochemical water oxidation is being reported for the first time, and the catalytic potency is found at par with the reported catalytic materials.

Author Contributions

Hira Khalid, Muhammad Asim, Akbar Hussain, Tehmeena Maryem Butt, and Sadia Kanwal: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing, Visualization. Naveed Kausar Janjua: Principal Supervisor. All authors have read and agreed to the published version of the manuscript.

Competing Interests

The authors have declared that no competing interests exist.

Data Availability Statement

The data that support the plots in this paper and other findings of this study are available from the corresponding author upon reasonable request.

AI-Assisted Technologies Statement

AI tool, ChatGPT, was used for language editing purposes only, and the author takes full responsibility for the content of the manuscript.