1 Introduction

Due to the severe limitations imposed by the scarcity of fossil fuels and the pressing problems posed by pollution, scientists worldwide are motivated to investigate new forms of clean energy [1,2,3]. Hydrogen gas has been considered a modern way of delivering green energy, since it is a harmless fuel with high energy density. In addition, it may improve the efficiency of energy converters [4,5,6,7]. Electrocatalytic water splitting is an effective and widely used process to obtain hydrogen as water is 70% of the earth crust and is again regenerated by hydrogen burning. Thus, hydrogen’s high energy density and low environmental impact have made it a popular candidate as a perfect energy carrier [8, 9]. Numerous energy technologies have explored electrochemical water splitting, which can split water into O2 and H2, as a viable way to generate hydrogen in a viable manner [10]. However, the total effectiveness of water splitting is decisively governed by OER, the half-cell reaction of the water electrolysis [11]. The universal mechanism for the production of O2 and H2 is discussed as

$${2{\mathrm{H}}_{2}\mathrm{O}}_{(\mathrm{l})}\to {{\mathrm{O}}_{2}}_{(\mathrm{g})} + {2{\mathrm{H}}_{2}}_{(\mathrm{g})},$$
(1)
$${2{\mathrm{H}}_{2}\mathrm{O}}_{(\mathrm{l})} + 2{\mathrm{e}}^{-} \to {{\mathrm{H}}_{2}}_{(\mathrm{g})} + {2{\mathrm{OH}}^{-}}_{(\mathrm{aq})},$$
(2)
$${4{\mathrm{OH}}^{-}}_{(\mathrm{aq})} \to {{\mathrm{O}}_{2}}_{(\mathrm{g})} + {2{\mathrm{H}}_{2}\mathrm{O}}_{(\mathrm{l})}+ 4{e}^{-}.$$
(3)

The general process of water splitting begins with the oxidation of water into molecular oxygen at the anode. On the other hand, an applied membrane and an external circuit are used to convey the e− and H+ produced during water oxidation to the cathode area. Therefore, widespread marketing is hindered by the reaction’s slow kinetics and high overpotential [12]. Most cutting-edge OER catalysts, including IrO2 and RuO2, are noble metal oxides due to their ability to vastly enhance reaction rates and decrease the overpotential [13,14,15]. Widespread use of these materials is only limited by scarcity, increasing the cost of catalyst preparation [16]. Clean and regenerated energy is a growing area that requires the development of improved activity of electrocatalysts for OER.

For this purpose, transition metal oxides [17], carbon-based compounds [18], perovskites [19], and all others have all been generated as catalysts in recent years in the increment of reaction rates and lessen its overpotential. Thus, the metal oxide consists of divalent and trivalent metals [20]. High-redox processes, adaptability, and compositional tunability are reasons to be used in electrochemical energy catalysis reactions that make these promising materials. It was found that metal oxide consisting of transition metals exhibited superior catalytic activity. For instance, Mugheri et al. have reported Co3O4 decorated on MWCNT’s, which displayed overpotential (η) of 262 mV@10 mAcm−2 and Tafel value around 81 mVdec−1 [21]. Jun Wu. et al. reported ultrasonication synthesis of CoNi/LDH/CoO, demonstrating a Tafel value around 123 mVdec−1 and η of 300 mV@10 mAcm−2 [22]. Tauseef Munawar et al. have reported sol–gel synthesis of Pr2CeSmO3, which exhibited a Tafel value of 75 mVdec−1 and 189 mV η to obtain 10 mAcm−2 [23]. Tianyun Zhang et al. have depicted the creation of Co3O4 exhibiting an overpotential around 268 mV@10 mAcm−2 and a Tafel value of 74 mVdec−1 [24]. Xiaomin Xu et al. have reported Pr0.5(Ba0.5Sr0.5)0.5Co0.8Fe0.2O3–δ (Pr0.5BSCF) for HER, very little amount of Praseodymium lowered the η required by 100 mV and composite material exhibited Tafel value of 45 mVdec−1 and η of around 242 mV [25]. However, poor conductivity became a limiting factor in improving OER performance. To eliminate this, opting for materials with excellent electrical conductivity, such as praseodymium (Pr) [26], cobalt (Co) [27], iron (Fe) [28] etc. oxides based on these metals exhibit higher stability and possess tunable electrical properties. Herein, Co3O4/Pr2O3, thanks to the combined efforts of cobalt and praseodymium, possesses a particularly potent redox chemistry, which helps it perform admirably as an OER electrocatalyst.

From the above-motivated point of view, this study proposes an efficient and cost-effective sol–gel technique for preparing novel Co3O4/Pr2O3 nanocomposite exhibiting good electrical properties due to synergy between metal oxides. Higher surface and porosity with smaller crystal sizes raise the quantity of operational sites for the reaction. This is confirmed by the higher electrochemically active surface area (ECSA) of the nanocomposite of 1825 cm2. In this case, the Pr2O3 has a peculiar two-dimensional structure that confers flexibility and lengthens and complicates the electrochemical process, while the Co3O4 NPs prevent the ultrathin Pr2O3 from stacking back up. On the other hand, the Co3O4 are the principal activity donors and may use the majority of the catalytic activity. Therefore, combining two components and producing a well-designed Co3O4/Pr2O3 nanocomposite reduced the OER overpotential to 257 mV@10 mAcm−2, with a Tafel value around 78 mVdec−1, including all other parameters been discussed below.

2 Experimental section

2.1 Materials

For the manufacture of Co3O4, Pr2O3, and Co3O4/Pr2O3 nanostructures, praseodymium nitrate hexahydrates (Pr(NO3)3.6H2O/Sigma-Aldrich, 99.9%), ethanol (C2H5OH/Merck ≥ 99%), potassium hydroxide (KOH/ACS reagent, 98.0%), cobalt chloride (CoCl2/Merck, 97%) nitric acid (HNO3, Analar, 68%), and citric acid monohydrate (C6H8O7·H2O, 99.0%/Sigma Aldrich) were utilized as precursors, and all of these compounds were utilized as obtained without undergoing any additional refinement procedures.

2.2 Synthesis of Pr2O3 and Co3O4

Simple sol–gel method was employed to construct the nanostructures of Pr2O3 using praseodymium nitrate as a precursor. In the typical process, 0.1 M solution of praseodymium nitrate salt was solvated in 100.0 mL of distilled water. Following that, a 0.05 M solution of citric acid (used as a complexing agent) was introduced to the above solution. The components were then permitted to stir for 1 h on the magnetic hotplate at 80 °C. The pH was then attained upto7–8 by adding 2.0 M ammonia solution gradually while stirring continually. After maintaining the pH, the fusion was swirled at 80 °C until the solvent evaporated, which finally converted the sol into a gel-like appearance. This resulting gel been burned at 350 °C, and finally, the obtained ash was annealed for 5h at 700 °C and then crushed into a fine powder. The same procedure was adopted to synthesize the Co3O4 using 0.1 M cobalt chloride as a source of cobalt.

2.3 Synthesis of Co3O4/Pr2O3 nanocomposite

For the generation of Co3O4/Pr2O3 nanocomposite, the same sol–gel process was employed, and the praseodymium nitrate (0.1 M) solution was prepared in the deionized water. After 5 h of stirring, the praseodymium nitrate solution was reacted with the previously made Co3O4 (0.5 g). Following that, citric acid was introduced as a chelating agent while keeping the pH around 7–8 to deprotonate the ions, so that they could readily chelate with the citric acid. The rest of the procedure was the same as described for the preparation of the individuals.

2.4 Characterization

To do the structural study, a cutting-edge laboratory diffractometer (Bruker-D8) was used, which was outfitted with a Cu-Kα radiation origin (λ = 1.5408), functioned at 40 kV/35 mA, and scanned at a rate of 1º/min across a 2θ-range of 10º–80º. Nanostructure was also investigated via transmission electron microscope (TEM) using a Tecnai F20. A field emission scanning electron microscope (FE-SEM Emcrafts cube-2020) was employed to evaluate its surface morphology and elemental composition, and an energy dispersive X-ray spectrometer was attributed to assess the elemental composition (EDX). Its chemical make-up and valence states of all elements were validated using X-ray photoelectron spectroscopy (XPS) on a Thermo-Scientific K-Alpha apparatus with Mg–K as the X-ray source and operating at 300 W.

2.5 Electrode synthesis and electrochemical measurements

The working electrode was made by drop cost method using nickel foam (NF) as substrate, and to remove any leftover oxidized particles, the NF was first washed with DI water and sulfuric acid. After collecting the sample, 5.0 mg of the samples were mixed with 5% of 1 μL of the Nafion as a binder and then dissolved in 10.0 μL of DI water to make the electrode ink, and then the fabricated ink was placed on the NF electrode using a micropipette. After that, the deposited ink was dried in a drying oven. The Pyrex glass reactor was cleaned using distilled water, aqua regia, and acetone before putting it in an oven at 60 °C. For electrochemical measurements, the Metrohm Autolab apparatus (model: PGSTAT-204, computer-controlled) has utilized. On an electrochemical workstation, all electrochemical experiments featuring cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and chronoamperometry (CA) were carried out. The three-electrode configuration included a nickel foam (NF) working electrode that had been modified with Co3O4, Pr2O3, and a Co3O4/Pr2O3 nanohybrid, a silver/silver chloride reference electrode (Ag/AgCl) along with a platinum wire used as counter electrode. These potentials were calculated into a reversible hydrogen electrode potential (RHE) by comparing them to the standard Ag/AgCl containing 3.0 M saturated KCl. Utilizing cyclic voltammetry with a sweep rate around 10 mVs−1 and a potential range of − 1 to 1.0 V versus Ag/AgCl, the OER activity of Co3O4/Pr2O3 was assessed. A frequency ranges of 10 mHz to 100 kHz and an oscillating voltage of 5.0 mV were used in electrochemical impedance spectroscopy (EIS) measurements. These electrochemical studies were conducted at ambient temperature using a KOH electrolyte solution with a concentration of 1.0 M. Each sample’s cyclic voltammograms were recorded concerning the Ag/AgCl reference before being transformed into the reversible hydrogen electrode (RHE) employing the following equation:

$${E}_{\mathrm{RHE}}= {E}_{\mathrm{Ag}/\mathrm{AgCl}}+ 0.059\times \mathrm{pH}+ {E}^{0}.$$
(4)

The potential obtained through CV and LSV was corrected for IR compensation using the following equation:

$${E}_{\mathrm{actual}} = {E}_{\mathrm{experimental}}-\mathrm{IR}.$$
(5)

The linear portion of the polarization curve was determined via Tafel analysis using the following equation:

$$\eta =a+ \frac{2.303\mathrm{RT}}{\alpha nF}\left(\mathrm{log}j\right).$$
(6)

In this, n is the sum of electrons, j is the current density, F is the Faraday constant, 2.303 RT/αnF describes the slope.

For electrochemical active surface area (ECSA), the double layer capacitance (Cdl) values for all electrocatalysts were recorded by taking CV in non-faradic regions at various sweep rates (υ). A specified value of current density (j) is taken at each scan rate and then plotted according to Eq. 7 to obtain Cdl directly from the slope of the equation. The electrochemical active surface area (ECSA) was employed to figure out all catalytic regions of the materials using double-layer capacitance using Eq. 7. The ratio of the specific capacitance (Csp) to the Cdl gives the ECSA as given in Eq. 8. The value of Cs is generally ranges from 0.022 to 0.14 mFcm−2, but in the present case we used 0.04 mFcm−2:

$$j= v{C}_{dl},$$
(7)
$$\mathrm{ECSA}= \frac{{C}_{dl}}{{C}_{s}}.$$
(8)

3 Results and discussion

3.1 Structural analysis

The Co3O4, Pr2O3, and Co3O4/Pr2O3 nanocomposite were fabricated via the sol–gel method with a 1:1 ratio and annealed at 700 ºC. Its XRD pattern was utilized to analyze the crystal phase and particle dimension of as-synthesized materials, as given in Fig. 1a. The Pr2O3 displays a cubic structure well-matched with JCPDS #00-006-0410, confirming the purity as well. Furthermore, the diffraction peaks of Pr2O3 positioned at 2-theta around 30°, 40 o, 47°, 55°, 56.5°, 60°, 68°, and 77o correspond to the planes of (002), (102), (110), (103), (200), (112), (004), (104), and (211), respectively. Furthermore, the formation of Co3O4 was fully matched with the JCPDS #00-009-0418 having a space group of Fd-3m with cubic structure [29]. Alternatively, the peaks positioned at 2θ = 18°, 31°, 36°, 44°, 55°, 65°, and 73.7° corresponds to the planes of (111), (220), 311), (400), (422), (440), and (620), respectively. Moreover, the Co3O4/Pr2O3 nanocomposite contains both phases, such as Co3O4 (*), and Pr2O3 (#). Scherrer’s equation determined the standard crystallite dimensions for all the fabricated samples in the 19.2–34.2 nm range. The investigation of porosity, dislocation density and microstrain analysis might be done using XRD information shown in Table S1. The increased dislocation density porosity leads to an increase in the micro strain capacity of the prepared composite indicated in Table S2. The following plotted graphs are used to analyze the consequence of particle dimension and micro-strain on the peak broadening of the synthesized samples. Williams Hall graphs are plotted in Fig. 1b. The micro strain of any material is calculated using the formulas listed below. The increased dislocation density, porosity and micro strains in the composite material are beneficial to catalyze the oxygen evolution reaction:

Fig. 1
figure 1

a X-ray diffractograms of Pr2O3, Co3O4, and Pr2O3/Co3O4, b Williamson Hall plots of Pr2O3, Co3O4 and Pr2O3/Co3O4

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$$\varepsilon ={\beta \mathrm{Cot\theta }}_{s}/4,$$
(9)
$${B}_{hkl}(\mathrm{Cos\theta })=k\uplambda /\mathrm{D}+(4\mathrm{\varepsilon Sin\theta }).$$
(10)

To confirm the internal and external morphology of all synthesized products, the SEM and TEM approaches were employed, respectively. The SEM image in Fig. 2a, b demonstrates that the morphological profile of Co3O4 reveals the shape of nanospheres-like clusters. Figure 2c, d depicts a typical SEM micrograph of Pr2O3, which is composed of a large number of trigonal nanoparticles. These trigonal particles have approximately 0.3 mm length and 3.2 mm wide edges. Faces and straight edges of the trigonal particles are evident in the magnified SEM image. On the other hand, the morphology of the nanocomposite exhibits the trigonal shaped and nanospheres, confirming the presence of both phases. Thus, the morphology of Pr2O3/Co3O4 nanocomposite depicts that it consists of numerous small inter-combined crystalline nanoparticles joined to form large nanocluster-like appearance, and these structures are composed of several flexible nanoparticles linked together having numerous holes, fissures, and interstices on their exteriors, as illustrated in Fig. 2e, f. Transmission electron microscopy (TEM) has employed to precisely depict their intricate microstructure of Co3O4, Pr2O3 and the nanocomposite. Figure 2g–i depicts a typical TEM image of Co3O4, Pr2O3 and the nanocomposite, respectively. Furthermore, the crystalline and well-defined surface morphology was indicated via TEM images, which agreed with the SEM images. Energy dispersive X-ray (EDX) spectroscopy meticulously examines the elemental composition to confirm the presence of desired elements. The EDX graphs analysis displayed in Fig. S1 suggests the existence of Co, Pr, and O in the crystal structure of pristine relative oxides and composite Co3O4/Pr2O3. The elemental mapping images specify the unvarying dispersal of Co, Pr and O atoms on exterior of the fabricated electrocatalysts (Fig. S1).

Fig. 2
figure 2

Scanning electron micrograph for the a, b Co3O4, c, d Pr2O3, e, f Co3O4/Pr2O3 and g–i transmission electron micrograph analysis of Co3O4, Pr2O3, and Co3O4/Pr2O3

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XPS analysis was done to learn more about the chemical makeup of the surface and oxidation states of all produced samples. Figure 3a shows the XPS survey spectrum of the nanocomposite and shows that all the related elements exist in the composite, authenticating the purity as well. The higher resolution Co 2p spectra of Co3O4/Pr2O3 nanocomposite and individuals in its purest form are compared. The Co 2p spectra are deconvoluted into two fitting peaks around 779.0 and 780.1 eV with varying oxidation states of Co3+ and Co2+, Fig. 3b. Likewise, in Fig. 3c, two fitted peaks with the oxidation states of Pr3+ at 933.4 and 976.7 eV and an inherent shake-up peak might be ascribed to the successful fabrication of Pr2O3 phase in both pristine and composite samples [30, 31]. The O 1s spectra of the designed materials reveal the metal–oxygen link at about 529 eV, and the peak around 532.2 eV accedes to the oxygen species from surface-adsorbed water molecules, Fig. 3d. The XPS results depict that the Co Pr-oxide structural system is composed of Co3+, Co2+, and Pr3+ ions and that the ratios of these ions were determined in the Co–Pr oxides because redox pairs of Co3+/Co2+ may function as active sites for the redox processes. The redox couple’s ratio for Co3+/Co2+ and Pr3+/Pr2+ ions in the Co–Pr oxide system, respectively, is 0.27/6.63 in the details. When a composite of Co3O4/Pr2O3 is created, the different active sites can be added, which ultimately improves the oxygen evolution reaction.

Fig. 3
figure 3

Comparison of XPS analysis Co3O4/Pr2O3 vs. pristine oxides Co3O4 and Pr2O3. a Survey spectrum, b Co2p, c Pr 3d, d O1s

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The electrocatalytic behavior of all the fabricated electrocatalysts was estimated for the oxygen evolution mechanism, and the results are presented in Fig. 4. Following the OER activity, the pre-oxidation peak is evident in Fig. 4a. The CV curve refers to the characteristic’s redox activity of Co2+ to Co3+ and back to Co2+ ion, also reported in cobalt-based electrocatalysts. As shown, the Co3O4/Pr2O3 have shown exceptional OER activity, i.e., it requires 200 mV overpotential to initiate oxygen evolution reaction, which is lower than pristine Co3O4 (302 mV) and Pr2O3 (297 mV). The superior activity is also indicated when Co3O4/Pr2O3 (257 mV) achieve higher current densities at low overpotentials, such as 256 mV, 278 mV, 281 mV at 10, 200, 400 and 800 mAcm−2, which is lower than Co3O4 and Pr2O3, according to the LSV polarization curve, Fig. 4b. The unique morphology and presence of various active sites of Pr ions are considered to be credited for such astonishing OER performance in 1.0 M KOH solution. The overpotential required at 10 mAcm−2 for all the fabricated samples are depicted in Fig. 4c. Among all the Co3O4/Pr2O3 oxides, they offer an outstanding OER performance in comparison with the overwhelming majority of metal oxide catalysts that have previously been reported (Table S3). The comparative study indicates that the resultant nanocomposite has higher and comparable results with the earlier reported results.

Fig. 4
figure 4

Comparison of electrochemical activity for Co3O4/Pr2O3, Co3O4 and Pr2O3. a Cyclic voltammograms, b linear sweep voltammograms, c bar graph showing overpotential required for current density 10 mAcm−2 and d Tafel plot analysis

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The kinetic analysis of Co3O4/Pr2O3 is shown in Fig. 4d. It was previously reported that a smaller value of the Tafel slope is beneficiary for the oxygen evolution mechanism. It might also have a shallow Tafel slope due to a bigger active surface area and mesopore volume. The electrocatalyst Co3O4/Pr2O3 has a smaller Tafel slope value of 78 mVdec−1, which is lower than the pristine Co3O4 and Pr2O3. It is formerly well-documented that the Tafel slope indicates the rate-determining steps and transport of electrons across the electrode–electrolyte contact [32]. As the composite has a lesser Tafel value as compared to pristine oxides, it further provides the evidence for the enhanced OER for Co3O4/Pr2O3, as given in Table 1. The enhanced kinetics rate can be credited to the myriad active sites available for OER in its composite. The addition of praseodymium oxide (Pr2O3) to cobalt oxide (Co3O4) systems has resulted in an unexpected and significant increase in catalytic activity. It is believed that the increased OER activity is the result of a synergistic effect between the two oxides, with praseodymium Oxide acting as a promoter to increase electron transfer and charge carrier mobility within the Co3O4 lattice. In addition, it is anticipated that the addition of praseodymium ions to the surface will generate favorable active sites for the OER, thereby enhancing the overall reaction kinetics.

Table 1 Comparison of electrochemical activity of Co3O4/Pr2O3, Co3O4, and Pr2O3
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The presence of a large number of active sites has further confirmed by estimating the electrochemical active surface area, and their outcomes are depicted in Fig. 5. The ECSA is calculated by estimating double layer capacitance value within the applied potential window of 1.1–1.2 V vs. RHE. For it, CV curves have taken at different sweep rates within the given potential range and are shown in Fig. 5a–c for Pr2O3, Co3O4 and Pr2O3/Co3O4, respectively. Then, current density (j) was recorded from the CV curves for each scan rate (υ) and by plotting j vs. υ according to Eq. 7, Cdl was directly noted from the slope. The ECSA was estimated using Eq. 8, showing that it is highest for Co3O4/Pr2O3 as compared to pristine oxides. The larger ESCA value for the composite reflects that Co3O4/Pr2O3 provides a large number of active sites for catalyzing OER [33], as given in Table S4.

Fig. 5
figure 5

a–c Comparison of CV curves obtained at different sweep rates and d–f linear plot of change in current densities vs. scan rates giving the value of double layer capacitance Pr2O3, Co3O4 and Co3O4/Pr2O3 nanocomposite

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The proposed feasible mechanism for the OER reaction to occur over an electrocatalyst in an alkaline medium, and OER is a process that takes place during water electrolysis in which oxidation of water yields oxygen and protons. The Co3O4/Pr2O3 catalyst is a commonly used catalyst for OER in an alkaline medium, the proposed mechanisms include:

Cobalt oxidation mechanism:

$${\text{Co}}_{{3}} {\text{O}}_{{4}} \to {\text{ 3CoO }} + \, \raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} {\text{ O}}_{{2}} ,$$
(11)
$${\text{CoO }} + {\text{ H}}_{{2}} {\text{O }} \to {\text{ Co}}\left( {{\text{OH}}} \right)_{{2}} + e^{ - } ,$$
(12)
$${\text{Co}}\left( {{\text{OH}}} \right)_{{2}} \to {\text{ CoO }} + {\text{ H}}_{{2}} {\text{O }} + e^{ - } .$$
(13)

Overall reaction:

$${\text{Co}}_{{3}} {\text{O}}_{{4}} + {\text{ 4H}}_{{2}} {\text{O }} + { 4}e^{ - } \to {\text{ 3Co}}\left( {{\text{OH}}} \right)_{{2}} + {\text{ O}}_{{2}} .$$
(14)

Praseodymium enhancement mechanism:

$${\text{Pr}}_{{2}} {\text{O}}_{{3}} + {\text{ 3H}}_{{2}} {\text{O }} + { 3}e^{ - } \to {\text{ 2Pr}}\left( {{\text{OH}}} \right)_{{3}} .$$
(15)
$${\text{Co}}_{{3}} {\text{O}}_{{4}} + {\text{ 3H}}_{{2}} {\text{O }} + { 3}e^{ - } \to {\text{ 3Co}}\left( {{\text{OH}}} \right)_{{2}} .$$
(16)

Overall reaction:

$${\text{Co}}_{{3}} {\text{O}}_{{4}} + {\text{ Pr}}_{{2}} {\text{O}}_{{3}} + {\text{ 6H}}_{{2}} {\text{O }} + { 6}e^{ - } \to {\text{ 3Co}}\left( {{\text{OH}}} \right)_{{2}} + {\text{ 2Pr}}\left( {{\text{OH}}} \right)_{{3}} + {\text{ O}}_{{2}} .$$
(17)

In the presence of an alkaline electrolyte, the cobalt oxide goes through a series of oxidation and reduction processes, while the praseodymium oxide boosts the catalytic activity of the cobalt oxide. These reactions take place, while the cobalt oxide is in contact with the alkaline electrolyte. Because of this, highly active catalytic sites are formed on the interface of the electrocatalyst, which makes the OER reaction easier to carry out [34]. In present on the modification of Co3O4 with Pr2O3 and its effect on increasing certain reactions, the authors provide a comprehensive explanation for this phenomenon by evaluating the potentials at which these reactions occur. The observed boost does not appear to be due solely to Pr2O3 catalyzing the processes, but rather to a complicated interplay between Co and Pr active sites. Doping with Pr2O3 introduces more active sites into the structure of Co3O4, which may facilitate the desired reactions. This enhancement could be the result of a variety of mechanisms, such as an increase in the availability of active sites due to the incorporation of Pr2O3, changes in the electronic structure of Co3O4 due to Pr2O3 doping, or synergistic effects between Co and Pr species. Thus, while Pr2O3 may play a significant role in accelerating certain reactions, this does not necessarily imply a complete transfer of activity from Co to Pr active sites, but rather a cooperative interaction between the two that ultimately enhances overall catalytic efficiency. This granular comprehension of the reaction processes is indispensable for elucidating the fundamental principles underlying the observed enhancements in the Co3O4–Pr2O3 system. This study not only enhances our knowledge of OER catalysts, but also holds great promise for the development of more efficient and sustainable energy conversion systems.

The promising kinetics of Co3O4/Pr2O3 is further evaluated by EIS (Fig. 6). The Nyquist plot is obtained at overpotential of 200 mV, which is the onset potential for OER in the case of Co3O4/Pr2O3. The fitted Randle circuit on the Nyquist plot gives the values of solution and charge transfer resistance across its contact area, Fig. 6b–d, inset. Significantly, the smaller charge transfer resistance, as given in the circuit diagram of Co3O4/Pr2O3 for the oxygen evolution process, indicates the sufficient conductivity of the prepared electrode, which contributes to fast transport kinetics across the electrode–electrolyte interface as contrasted to pristine Co3O4 and Pr2O3. Convenient electron transfer from the electrolyte to the anode is represented by a low Rct.

Fig. 6
figure 6

Comparison of Impedance spectroscopy showing Nyquist plot with fitted Randle circuit values at applied potential a Nyquist plot, b–d Fitted Randle circuit for Co3O4, Pr2O3, and Co3O4/Pr2O3

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For commercial-scale applications, the reliability and durability of prepared catalyst is essential to investigate. Therefore, the prepared Co–Pr oxide nanocomposite and pristine oxide were put to the test to estimate its capability of producing oxygen gas for a prolonged time. For it, the repeated cyclic voltammetry technique was used for 5000 CV cycles at a scan rate of 100 mVs−1. Afterwards, the 1st, 2000th and 5000th CV cycle was compared in Fig. 7a. The superimposable overlap of CV curves suggests the remarkable stability of the prepared electrocatalyst. It indicates that the prepared oxide can generate redox species over and over again at the same potentials that can catalyze the oxygen evolution reaction for a prolonged time. In addition to, chronoamperometry was used to investigate the stability of the electrocatalyst to produce oxygen gas bubbles for a longer period of time, as shown in Fig. 7b. The prepared Co–Pr oxide system can generate oxygen gas exceptionally for approximately 100 h without significant loss in activity. The comparison of all CV curve before and after their stability test (chronoamperometry) is given in Fig. 7c, which confirms the remarkable stability of the catalyst for an extended period. Furthermore, the XRD analysis as post stability test suggests that the prepared oxides have sustained its crystal phase structure after such rigorous OER activity. The higher OER activity can be linked to the porous, inter-combined morphology having higher values of ECSA, the higher redox species of Co3+/Co2+ ions in the Co–Pr oxide nanocomposite, which exposed numerous active sites for OER and consequently suitable for scaling up the process of energy conversion and for storage devices.

Fig. 7
figure 7

a Comparison of CV curves at 1st, 2000th and 5000th, b chronoamperometry test for 100 h, c comparison of CV curve before and after prolonged activity of OER, d XRD analysis after stability test of the nanocomposite

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4 Conclusion

Herein, we developed an excellent Co3O4/Pr2O3 nanohybrid electrocatalyst for the OER and analyzed it by employing a variety of analytical methodologies (XRD, SEM, EDX) to confirm the structural, morphological and compositional properties. Owing to a synergistic effect among Co3O4 and Pr2O3 nanocomposite and due to this combined effect, the conductive nature performs well for OER. The Co3O4/Pr2O3 material has a minimal overpotential around 257 mV and a reduced Tafel value of 78 mVdec−1 to yield the current density around 10 mAcm−2 in a 1M KOH solution. Finally, a Co3O4/Pr2O3 composite with a good morphology was successfully produced, and the interaction between Co3O4 nanoparticles and Pr2O3 was favorable. Owing to its enhanced electrocatalytic performance, excellent durability, and favorable reaction kinetics under both alkaline and neutral conditions, Co3O4/Pr2O3 is an effective water oxidation catalyst. According to the boosted catalytic activity, the composite possesses a high electroactive surface area, a rapid electron transfer rate, and enhanced electrical and chemical interaction. Due to its unique structure and good electrochemical performance, the Co3O4/Pr2O3 catalyst might serve as a precious metal-free oxygen evolution catalyst. Hence, our research also indicates that Co3O4/Pr2O3 hybrid materials have been utilized in several electrocatalytic processes.