1 Introduction

The urgent need for energy to promote industrial and technological development and to prevent environmental degradation has led the majority of scientific research in the direction of sustainable development. Production and storage of electricity with the help of green approaches independent of fossil fuels can be an efficient solution [1]. Oxygen reduction and water-splitting reactions are two main processes in this field [2,3,4,5]. Since the only and main cathode process in fuel cells is the oxygen reduction reaction (ORR), it is called the heart of the fuel cell [6]. Although the materials required for these processes are easily available, the non-spontaneity of these reactions and the high activation energy required are serious limitations in this direction [7, 8].

The oxygen reduction reaction, especially in neutral environments, is of interest to energy researchers because it is a critical process in biofuel cells. Oxygen reduction in neutral media is kinetically slow. To facilitate this reaction and increase the efficiency of the fuel cell, it is necessary to use a suitable electrocatalyst. The development of efficient electrocatalysts is very effective in facilitating these processes [2, 8]. In recent years, researchers have been looking for inexpensive alternatives to non-precious metals as catalysts to have an activity equal to or greater than platinum, and also have a longer life and stability [9,10,11].

Although various electrocatalysts have been introduced, limitations such as the lack of stability of the modifiers, high cost, and low energy density have hindered the global development of clean energy production systems based on oxygen reduction reaction (ORR) [12]. A wide range of electrocatalysts based on transition metals and their oxides/composites have been developed to boost the ORR [13,14,15]. Unfortunately, the catalytic activity of transition metal oxides alone is low compared to noble metals and needs to be boosted [16]. On the other hand, the problem of corrosion and oxidation phenomena prevents their satisfactory electrocatalytic activity and hinders their use in making practical fuel cell electrodes [6, 17]. The construction of non-noble metal-based electrocatalysts with proper ORR activity and low production cost is the main challenge in the research field of fuel cells [18]. The use of hybrid and composite nanostructures of transition metals with other active electrocatalytic compounds can significantly improve their performance, especially their long-term stability [17, 19].

Non-precious metal oxides of Ni [20, 21], Co [22,23,24], Mn [25, 26], and Fe [27, 28] were extensively developed to overcome charge transfer restrictions and to extend the electrode stability. These compounds are more available and abundant, and more importantly, they are eco-friendly. The use of manganese dioxide in the development of electrocatalysts is not only in line with the principles of sustainable development and reduces the total price, but also brings the benefits of a wide active surface area and high specific capacity [29,30,31]. Therefore, it has been given a lot of attention. On the other hand, low electronic conductivity and cyclic stability are the main limitations of this metal oxide [30, 32]. To solve this problem, effective compounds such as cobalt oxide, which has high conductivity and suitable electrocatalytic properties, can be used [30, 33]. Till now, researchers have introduced diverse compounds to improve the electrocatalytic process of oxygen reduction. To boost the electrocatalytic performance of MnO2 for ORR, composites such as Mn3O4/CoO [34], MnO2/NiCo2O4 [35], MnO2/TiO2 [36], MnO2/Fe2O3 [37], MnO2@Co3O4 [38], and MnO2-CNTs/Co3O4 [39] have been prepared. The synergetic impact of the individual constituents of the nanocomposites led to the appearance of satisfied catalytic features [40,41,42,43].

Considering the prominent and proven characteristic of manganese oxide to catalyze the electrochemical ORR, in the present work, we focused on the development of an efficient catalyst based on manganese oxide, Scheme 1. Here, an efficient ORR catalyst based on a composite of nano MnOx and MnCo2O4 is introduced. It has been reported that spinel MnCo2O4 shows higher electronic conductivity than MnO2 and Co3O4. So, to solve the problem of low stability and low electrical conductivity of manganese oxide, spinel MnCo2O4 was used. It should be noted that carbon-based nanostructures have been commonly used to improve the stability and electrical conductivity of manganese oxide, but in current work, we have only used metal oxides. Another difference between the present work and previous reports is the use of neutral solution instead of an alkaline solution for ORR evaluations. We focused on the construction of electrodes based on Mn–Co oxides composites for ORR. The parameters involved in the electrocatalytic process of oxygen reduction were evaluated and optimized. The catalytic performance of electrodes relied on the composition of the components and their weight percentage in the texture of the electrode active layer.

Scheme 1.
scheme 1

The schematic presentation of the electrocatalyst synthesis, electrode preparation and its usage for ORR process

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2 Experimental

2.1 Chemicals and materials

Carbon black powder (Vulcan XC-72 R), polytetrafluoroethylene (PTFE-Aldrich 96%), isopropanol (Merk), Mn(NO3)2⋅2H2O, and Co(NO3)2⋅6H2O were supplied from Sigma-Aldrich Co. All chemical materials were utilized as received without further purification.

2.2 Synthesis of MnOx/MnCo2O4 nanocatalyst

The hydrothermal method was employed for the nanocatalyst synthesis. 5 mmol Mn(NO3)2⋅2H2O and 5 mmol Co(NO3)2⋅6H2O were added to an Erlenmeyer flask containing a 40 mL solution of the EtOH and ethylene glycol (with a ratio of 3:1). The content of the Erlenmeyer flask was stirred using a magnetic stirrer for 30 min. Then, the solution was purred in an autoclave that was placed in an oven and heated at 110 °C for 24 h. The content of the Erlenmeyer flask was filtered through filter paper. The obtained precipitate was washed with D.I. water and EtOH and left to dry in the laboratory environment. Finally, the resulting powder was sintered in the oven at a temperature of about 500–600 °C for 50 min. The synthesized nanocatalyst was named as MnOx/MnCo2O4.

2.3 Electrode construction

Here, the graphite rods (r = 3.5 mm) taken from the disposable batteries were utilized as an electrode substrate. The graphite rods were acid washed, and then rinsed with deionized water and dried in an oven at 60 °C. The lateral surface of the graphite rods was thoroughly covered with parafilm, which left only its cross section with 0.38 cm2 surface area exposed to the solution. The active reaction material of the electrode was prepared using carbon black type Vulcan (XC-72R), PTFE, isopropanol, and D.I. water as solvent. The prepared suspension was transferred to the oven and let the solvent was evaporated at 100 °C, until formed a sticky paste. This material was named CP. The optimum time of remaining the electrode material in the oven was determined as 5 min. To fabricate modified CPs, the synthesized nano MnOx/MnCo2O4 was added to the carbon Vulcan–PTFE mixture in various ratio percentages of 10–90% (to optimize the MnOx/MnCo2O4 value). The electrodes were prepared by covering the cross section of the graphite rods with a thin film of 10 mg of the prepared bare CP or CP-MnOx/MnCo2O4. The final electrodes were named GE (substrate), GE/CP (bare electrode), and GE/CP-MnOx/MnCo2O4 (modified electrode).

2.4 Physical and electrochemical investigations

The crystalline structure of the prepared MnOx/MnCo2O4 nano compound was evaluated with X-ray diffraction (XRD) analysis (PHILIPS-PW1730, Netherlands, the radiation of CuKα, λ = 1.5418 Å). The MnOx/MnCo2O4 morphology was characterized by scanning electron microscopy (SEM, TESCAN-MIRA III made in the Czech Republic).

A three-electrode cell assembly connected to potentiostat–galvanostat (BHP, Model 2083, Iran) was utilized for voltammetric experiments (linear sweep and cyclic voltammetry: LSV and CV, respectively) at room temperature. The ORR performance and stability tests were done in an O2 (or N2)-saturated 0.1 M phosphate buffer solution (PBS, pH 7.0). A graphite rod (bare or modified), a platinum wire, and a saturated calomel electrode (SCE) were used as the working, counter, and reference electrodes, respectively. The electrochemical impedance spectroscopy (EIS) tests were performed utilizing a single-channel potentiostat Origalys (OrigaFlex-OGF500).

3 Results and discussion

3.1 Characterization of the MnOx/MnCo2O4 nano electrocatalyst

The XRD pattern for MnOx/MnCo2O4 is shown in Fig. 1. In this figure, the presence of MnOx/MnCo2O4 peaks confirms the formation of this composite. Each of 2θ = 18.5, 21, 33, 36, 38, 44, 57, 62, 64, 65, 67, 73 in the XRD pattern of the MnOx/MnCo2O4 nanocomposite corresponds to manganese oxides with different oxidation numbers and MnCo2O4. Each of these peaks is related to plates 1, 2, and 3, respectively, which are reported according to sources. The different oxidation states of manganese oxides can be attributed to the presence of MnO, Mn2O3, and MnO2 in the composite, while MnCo2O4 corresponds to the spinel structure of the composite. The crystal structure can also affect the electronic properties of the composite. The electronic properties can influence the charge transfer kinetics and the efficiency of the electron transfer between the electrode and the reactants.

Fig. 1
figure 1

XRD pattern of MnOx/MnCo2O4

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The morphology of MnOx/MnCo2O4 nanocomposites was investigated using FE-SEM. The images with 200 Kx magnification are shown in Fig. 2. The prepared nanocomposites are accumulated next to each other in the form of spherical nanoparticles. This nanocomposite has a non-uniform particle size with an average particle size between 25 and 40 nm. The morphology of the nanocomposite is an important factor that can affect its electrocatalytic performance. The high surface area to volume ratio of the spherical nanoparticles can provide more active sites for electrocatalytic reactions. The non-uniform particle size can also create defects and exposed crystal planes, which can enhance the catalytic activity of the nanocomposite. However, the non-uniform particle size can also lead to aggregation and decrease the overall stability of the nanocomposite.

Fig. 2
figure 2

FE-SEM image of MnOx/MnCo2O4

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3.2 Electrochemical studies

The voltammetric response of the electrodes prepared in the same procedure named GE/CP (bare electrode), and GE/CP-MnOx/MnCo2O4 (modified electrode) was evaluated. The wide potential range of − 1.0 to 1.0 V, a potential sweep rate of 100 mV/s, and a phosphate buffer solution (PBS, pH 7.0) saturated with O2 (for the presence of oxygen) or N2 (for the absence of oxygen) were chosen as experiments conditions, Fig. 3. The CVs of the GE/CP were almost the same in the presence and absence of oxygen, Fig. 3a, revealing the insensitivity of the bare electrode to the oxygen molecules. The recorded CV by GE/CP did not show any peak in the N2-saturated PBS. However, when oxygen enters the cell, a very tiny reduction peak appeared at a potential of − 0.5 V. The current intensity was also very small for this reduction peak, so it can be concluded that the GE/CP does not have a good ability to electrochemically reduce oxygen molecules.

Fig. 3
figure 3

CVs of a GE/CP and b GE/CP-MnOx/MnCo2O4 in PBS, pH 7.0 in the presence and absence of oxygen

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The CV current in PBS was increased when the surface of GE/CP was modified by MnOx/MnCo2O4 nanocomposite probably due to the enhancement of electrode conductivity and electrochemically active surface area, Fig. 3b. No peak was observed for GE/CP-MnOx/MnCo2O4 in the absence of oxygen. The presence of oxygen in the solution made a sensible change in the CV response of GE/CP-MnOx/MnCo2O4 confirming the appearance of the electrocatalytic behavior for ORR. In fact, the MnOx/MnCo2O4 nanocomposite with active catalytic sites can catalyze the ORR and its high surface area provides numerous catalytic sites, thus increasing the rate of oxygen depletion through ORR and, thus, oxygen reduction current.

A series of GE/CP-MnOx/MnCo2O4 were constructed utilizing different values of the modifier to optimize MnOx/MnCo2O4 percentage in the electro-active layer of the electrode. Figure 4 displays recorded CVs of these electrodes swept from − 1.0 to + 1.0 V (vs. SCE) at a sweep rate of 100 mV/s in 0.01 M PBS saturated with O2. The presence of MnOx/MnCo2O4 nanocomposite made possible the ORR in positive potentials. An obvious shift of the open circuit voltage (OCV) of ORR to about 0.7 V reveals the fast process with very low kinetic restrictions. The best response was gained for the GE modified with 50 W% CP and 50 W% MnOx/MnCo2O4 (CP50-MnOx/MnCo2O450).

Fig. 4
figure 4

CVs of GE modified with various W% of MnOx/MnCo2O4 catalyst and CP recorded in O2-saturated PBS, pH 7.0 at a scan rate of 100 mV/s

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The optimization of the electrode is critical to achieving higher ORR electrocatalytic activity. In fact, the GE/CP50-MnOx/MnCo2O450 provided a better catalytic effect for ORR than other amounts of the nanocomposite. The value of the CP controls the electrode conductivity and porosity of the electrode material that facilitates the reactant transfer toward the electrode surface and quickly removes the products of the electrochemical reactions from the electrode surface. Here, the MnOx/MnCo2O4 percentage affects the electrocatalytic impact so the current density of the ORR and its OCV. In the presence of 50% of MnOx/MnCo2O4 nanocomposite, more electro-active species are involved in ORR. However, utilizing more than 50 W% nanocatalyst in the modification step caused the surface of the electrode to be blocked and the conductivity of the electrode surface to be reduced; thus, the efficiency of the ORR was attenuated. Therefore, the GE surface was modified with the CP50-MnOx/MnCo2O450 as the optimum composite to achieve higher ORR electrocatalytic activity.

For the kinetic study of ORR at neutral media, the linear sweep voltammogram (LSV) tests in O2-saturated PBS, pH 7.0, at a scan rate of 1 mV/s were performed to get the Tafel plot. In the case of the observed peak for ORR at − 0.04 V, the high slope of the plot of current vs. potential, in the interval of the starting point of the Faraday current to the peak current (+ 0.12 to − 0.04 V), indicates that the reaction is kinetically fast and desirable without any kinetic limitation. Based on the plot of the current logarithm vs. the overvoltage, the exchange current density for the GE modified with CP50-MnOx/MnCo2O450 under optimized situation was obtained at 0.12 mA cm−2. That indicates the ORR at GE/CP50-MnOx/MnCo2O450 is kinetically fast in neutral media. These results suggest that the electrode has a high efficiency for the ORR, which is a critical factor in fuel cells and other electrochemical applications.

Electrochemical impedance spectroscopy was used to evaluate the GE/CP modified with MnOx/MnCo2O4 nanocomposite (50%) at various voltages, Fig. 5. An equivalent circuit is proposed according to the impedance behavior in Fig. 5. In this circuit, the parameters can be described as: R1: solution resistance, C: capacitance of the dielectric layer, R2: charge transfer resistance, and W: Warburg element. The parameters of the equivalent circuit were calculated (Table 1) by fitting the model to the electrochemical impedance data. Table 1 indicates that the capacity of the double layer decreases concerning the maximum current density, which leads to a slight increase in the resistance related to species diffusion to the electrode surface. On the other hand, reducing the potential from the value of the OCV led to a decrease in the resistance to charge transfer (R2), which is a beneficial condition for a higher rate of the ORR at the peak potential. The decrease in capacitance of the double layer with increasing current density indicates that the oxygen concentration at the electrode surface is depleted as the ORR proceeds. This suggests that the ORR rate can be further improved by increasing the oxygen concentration at the electrode surface, possibly by increasing the oxygen flow rate or by optimizing the electrode design to enhance oxygen mass transport.

Fig. 5
figure 5

Nyquist diagrams of the GE/CP50-MnOx/MnCo2O450 in the PBS with pH 7 in the frequency range of 100 kHz–0.1 Hz, at different voltages

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Table 1 The best-fit impedance parameters of the GE/CP50-MnOx/MnCo2O450 at various potentials
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The surface area of the GE/CP and the GC/CP50-MnOx/MnCo2O450 was obtained using the Randles–Sevcik equation from the slope of the current diagram versus the square of the scan rate in the electrochemical probe solution. The calculated surface areas were 24.69 and 49.38 cm2, respectively. The electrode surface area was extended after introducing MnOx/MnCo2O4 nanocatalyst. The increase in surface area can lead to a higher response for ORR, as there is more surface available for the reaction to occur. This can improve the electrocatalytic activity of the electrode and enhance the ORR performance. In addition, the higher surface area can also improve the mass transport of reactants and products to and from the electrode surface, which can further enhance the ORR performance. This is because a higher surface area provides more sites for reactants to adsorb and more pathways for the transport of products away from the electrode surface. Therefore, the increase in surface area due to the introduction of the MnOx/MnCo2O4 nanocatalyst can enhance not only the electrocatalytic activity but also the mass transport properties of the electrode.

The stability of the GC/CP50-MnOx/MnCo2O450 was evaluated by the CV technique. The successive CVs in a PBS, pH 7.0, purged with oxygen gas were recorded in the potential window of 1.0 to − 1.0 V at a scan rate of 100 mV/s. After 200 CV runs no obvious variation appeared in the CV response of GC/CP50-MnOx/MnCo2O450. Some CVs depicted in Fig. 6 reveal the good maintenance of the electrocatalytic activity of the constructed electrode toward ORR. This is a promising result for practical applications, as it suggests that the electrode is durable and reliable. This can be useful in designing ORR systems that require long-term operation and reliability.

Fig. 6
figure 6

Consecutive voltammograms of the GC/CP50-MnOx/MnCo2O450 recorded in PBS, pH 7.0 at scan rate of 100 mV/s

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4 Conclusions

Electrocatalysis is an important field of study due to its potential applications in various fields, including energy conversion and storage. In this research, the electrocatalytic activity of the MnOx/MnCo2O4 electrocatalyst was evaluated in a neutral environment for ORR. The ORR is a crucial process in many electrochemical devices, including fuel cells, metal-air batteries, and air–cathode microbial fuel cells. The ORR current density can be significantly influenced by the percentage of MnOx/MnCo2O4 in the electro-active layer of the electrode. Therefore, it is important to optimize the MnOx/MnCo2O4-based modified electrode's composition to increase the oxygen reduction rate. The nano MnOx/MnCo2O4 offers numerous catalytic active sites, which provide a high surface area for the ORR to take place. The number of active sites available for the ORR is directly proportional to the reaction rate. The presence of MnO, Mn2O3, and MnO2 with varying oxidation states contribute to these active sites, providing different redox potentials that facilitate the ORR. The spinel structure of MnCo2O4 can also enhance the catalytic activity by providing a suitable crystal structure for the adsorption and activation of oxygen molecules. The high surface area of the MnOx/MnCo2O4 nanocomposite provides numerous catalytic sites, increasing the reaction rate and reducing the overpotential for ORR. The nanocomposite can enhance the electron transfer kinetics by providing a conductive pathway for the electrons to reach the active sites. The morphology and crystal structure of the MnOx/MnCo2O4 nanocomposite can also affect the electrocatalytic behavior by influencing the diffusion properties and accessibility of the active sites. In conclusion, the results of this study suggest that the MnOx/MnCo2O4 composite material is a suitable candidate for use as a fuel cell electrocatalyst for ORR in neutral media. This research provides valuable insights into the development of efficient and effective electrocatalysts for fuel cells, which are crucial for the advancement of clean energy technologies.