MnCo2O4/NiCo2O4/rGO as a Catalyst Based on Binary Transition Metal Oxide for the Methanol Oxidation Reaction

The demands for alternative energy have led researchers to find effective electrocatalysts in fuel cells and increase the efficiency of existing materials. This study presents new nanocatalysts based on two binary transition metal oxides (BTMOs) and their hybrid with reduced graphene oxide for methanol oxidation. Characterization of the introduced three-component composite, including cobalt manganese oxide (MnCo2O4), nickel cobalt oxide (NiCo2O4), and reduced graphene oxide (rGO) in the form of MnCo2O4/NiCo2O4/rGO (MNR), was investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive X-ray (EDX) analyses. The alcohol oxidation capability of MnCo2O4/NiCo2O4 (MN) and MNR was evaluated in the methanol oxidation reaction (MOR) process. The crucial role of rGO in improving the electrocatalytic properties of catalysts stems from its large active surface area and high electrical conductivity. The alcohol oxidation tests of MN and MNR showed an adequate ability to oxidize methanol. The better performance of MNR was due to the synergistic effect of MnCo2O4/NiCo2O4 and rGO. MN and MNR nanocatalysts, with a maximum current density of 14.58 and 24.76 mA/cm2 and overvoltage of 0.6 and 0.58 V, as well as cyclic stability of 98.3% and 99.7% (at optimal methanol concentration/scan rate of 20 mV/S), respectively, can be promising and inexpensive options in the field of efficient nanocatalysts for use in methanol fuel cell anodes.


Introduction
Due to the high dependence of the modern world on electricity, the energy supply is a major concern for many countries. The severe reduction in fossil fuel sources and the harmful effects of excessive use of these fuels on the environment has led the world to use clean and renewable fuel sources. Energy storage and energy production from these renewable sources are two important topics that have attracted the attention of many researchers in various fields [1,2].
Supercapacitors and electrochemical batteries as energy storage equipment [3,4] and solar cells, wind turbines, and fuel cells as energy production-conversion types of equipment [5] have attracted the attention of researchers [6]. Among fuel cells, alcoholic fuel cells have specific properties such as lower operating temperature [7], ease of use, and portability [8]. Moreover, in alcoholic fuel cells, the problems caused by the storage and conversion of hydrogen in fuel cells have been solved. Therefore, due to these characteristics, alcoholic fuel cells have been considered by researchers more than other fuel cells [9].
In a direct methanol fuel cell, methanol oxidation and oxygen reduction occur at the anode and cathode [10], and a Nafion is placed between the anode and cathode as a 464 mg of Co (NO 3 ) 2 ·6H 2 O, 150 mg of urea, and 28 mg of NH 4 F in a solution containing 60 mL of DI water and 10 mL of ethanol were stirred and dissolved for 30 min. Finally, the cleaned nickel foam (1 × 1 cm 2 ) was placed in an autoclave containing the prepared solution and maintained for 12 h at 140 • C. The autoclave cooled, and the electrode was washed several times with DI water and ethanol and dried for 4 h at 45 • C.

Synthesis of MnCo 2 O 4 /NiCo 2 O 4
To synthesize MnCo 2 O4/NiCo 2 O 4 , 3 mmol Co (NO 3 ) 2 ·6H 2 O with 1.5 mmol Ni (NO 3 ) 2 ·6H 2 O and 18 mmol urea were dissolved in a solution with 30 mL of distilled water and 20 mL of pure ethanol and stirred for 45 min. Then, the nickel foam coated with MnCo was placed in an autoclave containing the solution at 145 • C for 11 h. Finally, after washing and drying at 40 • C, the electrode was annealed at 400 • C.

Synthesis of MnCo 2 O 4 /NiCo 2 O 4 /rGO
To synthesize MnCo 2 O 4 /NiCo 2 O 4 /rGO, 3 mg of rGO was prepared by the Hummers method and was added gradually to the solution that was previously prepared for MnCo 2 O 4 /NiCo 2 O 4 .

Characterizations
To investigate the crystal structure of the synthesized nanomaterials, XRD analysis was performed using a 3000 EQUINOX INEL device with a CuKα lamp at room temperature. The corresponding peaks were observed at angles 2θ from 10 • to 80 • . Figure 1  Firstly, nickel foam was washed through the three-time sonication process with 0.5 M hydrochloric acid and deionized water (DI) for 10 min. Then, 234 mg of Mn (NO3) 2 with 464 mg of Co (NO3)2⋅6H2O, 150 mg of urea, and 28 mg of NH4F in a solution containing 60 mL of DI water and 10 mL of ethanol were stirred and dissolved for 30 min. Finally, the cleaned nickel foam (1 × 1 cm 2 ) was placed in an autoclave containing the prepared solution and maintained for 12 h at 140 °C. The autoclave cooled, and the electrode was washed several times with DI water and ethanol and dried for 4 h at 45 °C.

Synthesis of MnCo2O4/NiCo2O4
To synthesize MnCo2O4/NiCo2O4, 3 mmol Co (NO3)2⋅6H2O with 1.5 mmol Ni (NO3)2⋅6H2O and 18 mmol urea were dissolved in a solution with 30 mL of distilled water and 20 mL of pure ethanol and stirred for 45 min. Then, the nickel foam coated with MnCo was placed in an autoclave containing the solution at 145 °C for 11 h. Finally, after washing and drying at 40 °C, the electrode was annealed at 400 °C.

Synthesis of MnCo2O4/NiCo2O4/rGO
To synthesize MnCo2O4/NiCo2O4/rGO, 3 mg of rGO was prepared by the Hummers method and was added gradually to the solution that was previously prepared for MnCo2O4/NiCo2O4.

Characterizations
To investigate the crystal structure of the synthesized nanomaterials, XRD analysis was performed using a 3000 EQUINOX INEL device with a CuKα lamp at room temperature. The corresponding peaks were observed at angles 2θ from 10° to 80°. Figure 1 shows the XRD pattern for MnCo2O4/NiCo2O4. As can be seen, the index peaks at 18.   cess. Considering the MNR, Figure 2c,d also clearly shows the MnCo2O4/NiCo2O4 hybrid placed on the surface of rGO nanosheets. The uniform dispersion of the MN catalyst on the surface of the rGO is also clearly visible, and rGO has been marked in Figure 2d.
The EDX mapping analysis is an appropriate way to investigate the presence of manganese, nickel, cobalt, and oxygen. In this system, the number of X-rays emitted is counted against their energy. The amount of energy of each X-ray determines the characteristic of the element from which the beam is emitted. Therefore, by obtaining the energy spectrum against the counted X-rays, the quantitative and qualitative estimates of the elements in the sample are examined. Figure 3 also shows the EDX diagram, which confirms the presence of the elements Mn, Co, O, and Ni on the nickel foam substrate.  The EDX mapping analysis is an appropriate way to investigate the presence of manganese, nickel, cobalt, and oxygen. In this system, the number of X-rays emitted is counted against their energy. The amount of energy of each X-ray determines the characteristic of the element from which the beam is emitted. Therefore, by obtaining the energy spectrum against the counted X-rays, the quantitative and qualitative estimates of the elements in the sample are examined. Figure 3 also shows the EDX diagram, which confirms the presence of the elements Mn, Co, O, and Ni on the nickel foam substrate. Nanomaterials 2022, 12, 4072 5 of 12

Electrode Preparation
The electrochemical properties of the catalysts were studied on a Potentiostat/Galvanostat (AUTOLAB3202N, Wuhan, China) device with a typical three-electrode set-up, involving MnCo2O4/NiCo2O4 and MnCo2O4/NiCo2O4/rGO coated on NF as working electrodes, Ag/AgCl as the reference electrode, and 0.5 mm diameter platinum wire as the auxiliary electrode.

Electrochemical Investigation of Catalysts for Methanol Oxidation
According to many studies, metal oxide-based nanocatalysts are more efficient in the oxidation process of alcohols in alkaline media. Therefore, we investigated the capability of MN and MNR nanocatalysts in the methanol oxidation process. In the initial part, the cyclic voltammetric analysis of the nanocatalysts was performed in a 2 M KOH solution at a scan rate of 20 mV/s.
As can be seen in Figure 4a, both nanocatalysts have capacitive behavior, in that these materials are used as supercapacitor electrodes for electrochemical energy storage devices. Based on the CV analysis of catalysts in KOH, the MNR catalyst has a higher current density than the MN catalyst. The electrochemical impedance spectroscopy (EIS) analysis was performed to further study the electrical properties of catalysts. According to Figure  4b, the EIS analysis was accomplished at open circuit potential (OCP) over the frequency range of 0.01 Hz-100 kHz in a 2 M KOH aqueous solution. The charge transfer resistance (RCT) for MN and MNR in an alkaline medium obtained 9 and 6 ohms, respectively.

Electrode Preparation
The electrochemical properties of the catalysts were studied on a Potentiostat/ Galvanostat (AUTOLAB3202N, Wuhan, China) device with a typical three-electrode set-up, involving MnCo 2 O 4 /NiCo 2 O 4 and MnCo 2 O 4 /NiCo 2 O 4 /rGO coated on NF as working electrodes, Ag/AgCl as the reference electrode, and 0.5 mm diameter platinum wire as the auxiliary electrode.

Electrochemical Investigation of Catalysts for Methanol Oxidation
According to many studies, metal oxide-based nanocatalysts are more efficient in the oxidation process of alcohols in alkaline media. Therefore, we investigated the capability of MN and MNR nanocatalysts in the methanol oxidation process. In the initial part, the cyclic voltammetric analysis of the nanocatalysts was performed in a 2 M KOH solution at a scan rate of 20 mV/s.
As can be seen in Figure 4a, both nanocatalysts have capacitive behavior, in that these materials are used as supercapacitor electrodes for electrochemical energy storage devices. Based on the CV analysis of catalysts in KOH, the MNR catalyst has a higher current density than the MN catalyst. The electrochemical impedance spectroscopy (EIS) analysis was performed to further study the electrical properties of catalysts. According to Figure 4b The ability of catalysts for MOR was evaluated by performing CV analysis in 2 M KOH solution in the presence of 0.5 M methanol in the potential range of 0 to 0.8 mV.  Figure 4c shows that both nanocatalysts have a methanol oxidation peak in their CV analysis, which confirms the efficiency of the catalysts in the oxidation of methanol. It can also be seen that the oxidation peaks observed in MNR are much higher than in MN. Two important points can be concluded from Figure 4a-c: both nanocatalysts have the ability for MOR and can be used in the anode of methanol fuel cells, and the performance of MNR is better than MN in the methanol oxidation reaction. The MNR nanocatalyst has lower charge transfer resistance than MN and has a higher oxidation current density. This advantage can be related to the presence of rGO in the structure of the catalyst, which improved the performance of the catalyst by increasing the electrical conductivity [48] as well as by increasing the electrochemically active surface area [49][50][51]. In the following, we examine the influence of various parameters such as methanol concentration, scan rates, and temperature in MOR by MNR and MN, and we optimize these parameters for two nanocatalysts.

The Effect of Methanol Concentration on MOR Process by MN and MNR
To study the effect of methanol concentration on the MOR process of MN and MNR nanocatalysts, the CV analysis was performed in the constant concentration of 2 M KOH with different concentrations of methanol (0.3, 0.5, 0.7, 1, 1.5, 2, 2.5, and 3 M) at a scan rate of 20 mV/s. According to Figure 5a, for the MN catalyst, the oxidation peak has an upward behavior up to 1 M methanol, and then at 1.5 and 2 M methanol concentrations, the peak oxidation current density decreases. In the same behavior, the MNR catalyst showed an upward trend in methanol oxidation activity up to 2 M methanol, and then from 2.5 M methanol, the oxidation current density trend decreased (Figure 5b). It appears that at concentrations higher than the critical concentration of methanol, the surface of the catalyst is saturated and the methanol oxidation process is disrupted.  The ability of catalysts for MOR was evaluated by performing CV analysis in 2 M KOH solution in the presence of 0.5 M methanol in the potential range of 0 to 0.8 mV. Figure 4c shows that both nanocatalysts have a methanol oxidation peak in their CV analysis, which confirms the efficiency of the catalysts in the oxidation of methanol. It can also be seen that the oxidation peaks observed in MNR are much higher than in MN. Two important points can be concluded from Figure 4a-c: both nanocatalysts have the ability for MOR and can be used in the anode of methanol fuel cells, and the performance of MNR is better than MN in the methanol oxidation reaction. The MNR nanocatalyst has lower charge transfer resistance than MN and has a higher oxidation current density. This advantage can be related to the presence of rGO in the structure of the catalyst, which improved the performance of the catalyst by increasing the electrical conductivity [48] as well as by increasing the electrochemically active surface area [49][50][51]. In the following, we examine the influence of various parameters such as methanol concentration, scan rates, and temperature in MOR by MNR and MN, and we optimize these parameters for two nanocatalysts.

The Effect of Methanol Concentration on MOR Process by MN and MNR
To study the effect of methanol concentration on the MOR process of MN and MNR nanocatalysts, the CV analysis was performed in the constant concentration of 2 M KOH with different concentrations of methanol (0.3, 0.5, 0.7, 1, 1.5, 2, 2.5, and 3 M) at a scan rate of 20 mV/s. According to Figure 5a, for the MN catalyst, the oxidation peak has an upward behavior up to 1 M methanol, and then at 1.5 and 2 M methanol concentrations, the peak oxidation current density decreases. In the same behavior, the MNR catalyst showed an upward trend in methanol oxidation activity up to 2 M methanol, and then from 2.5 M methanol, the oxidation current density trend decreased (Figure 5b). It appears that at concentrations higher than the critical concentration of methanol, the surface of the catalyst is saturated and the methanol oxidation process is disrupted. The ability of catalysts for MOR was evaluated by performing CV analysis in 2 M KOH solution in the presence of 0.5 M methanol in the potential range of 0 to 0.8 mV.  Figure 4c shows that both nanocatalysts have a methanol oxidation peak in their CV analysis, which confirms the efficiency of the catalysts in the oxidation of methanol. It can also be seen that the oxidation peaks observed in MNR are much higher than in MN. Two important points can be concluded from Figure 4a-c: both nanocatalysts have the ability for MOR and can be used in the anode of methanol fuel cells, and the performance of MNR is better than MN in the methanol oxidation reaction. The MNR nanocatalyst has lower charge transfer resistance than MN and has a higher oxidation current density. This advantage can be related to the presence of rGO in the structure of the catalyst, which improved the performance of the catalyst by increasing the electrical conductivity [48] as well as by increasing the electrochemically active surface area [49][50][51]. In the following, we examine the influence of various parameters such as methanol concentration, scan rates, and temperature in MOR by MNR and MN, and we optimize these parameters for two nanocatalysts.

The Effect of Methanol Concentration on MOR Process by MN and MNR
To study the effect of methanol concentration on the MOR process of MN and MNR nanocatalysts, the CV analysis was performed in the constant concentration of 2 M KOH with different concentrations of methanol (0.3, 0.5, 0.7, 1, 1.5, 2, 2.5, and 3 M) at a scan rate of 20 mV/s. According to Figure 5a, for the MN catalyst, the oxidation peak has an upward behavior up to 1 M methanol, and then at 1.5 and 2 M methanol concentrations, the peak oxidation current density decreases. In the same behavior, the MNR catalyst showed an upward trend in methanol oxidation activity up to 2 M methanol, and then from 2.5 M methanol, the oxidation current density trend decreased (Figure 5b). It appears that at concentrations higher than the critical concentration of methanol, the surface of the catalyst is saturated and the methanol oxidation process is disrupted.

Investigation of the Effect of Scan Rates on MOR Process by MN and MNR
By choosing one and two molar concentrations of methanol as the optimal concentrations for the two nanocatalysts, MN and MNR, in the process of MOR at different scan rates (20,40,60,80,100, and 120 mV/s), we investigate the behavior of nanocatalysts by increasing the scan rate. Figure 6a,b shows the CV analysis of MN and MNR at an optimal concentration of methanol in the presence of a 2 M alkaline solution of KOH at different scan rates. As seen in the behavior of both nanocatalysts, capacitive and faradic currents increase with the increasing scan rate. The square root of the scan rate is plotted in terms of the maximum current density for two catalysts in Figure 3c. The linear relationship between these two parameters, with R 2 = 0.998 and 0.997 for the two catalysts, indicates the diffusion control mechanism in the methanol oxidation process.

Investigation of the Effect of Scan Rates on MOR Process by MN and MNR
By choosing one and two molar concentrations of methanol as the optimal concentrations for the two nanocatalysts, MN and MNR, in the process of MOR at different scan rates (20,40,60,80,100, and 120 mV/s), we investigate the behavior of nanocatalysts by increasing the scan rate. Figure 6a,b shows the CV analysis of MN and MNR at an optimal concentration of methanol in the presence of a 2 M alkaline solution of KOH at different scan rates. As seen in the behavior of both nanocatalysts, capacitive and faradic currents increase with the increasing scan rate. The square root of the scan rate is plotted in terms of the maximum current density for two catalysts in Figure 3c. The linear relationship between these two parameters, with R 2 = 0.998 and 0.997 for the two catalysts, indicates the diffusion control mechanism in the methanol oxidation process. The proposed mechanism of methanol oxidation by nanocatalysts can be proposed in the form of a six-electron mechanism, as follows: Catalyst + CH 3 OH → Catalyst − CH 3 OH ads Catalyst − CH 3 OH ads + 4OH¯→ Catalyst − (CO) ads + 4H 2 O + 4e¯ Catalyst + OH¯→ Catalyst − OH ads + e¯ Catalyst − CO ads + Catalyst − OH ads + OH¯ → Catalyst + CO 2 + H 2 O + e¯

Investigation of the Effect of Temperature on the MOR Process by MN and MNR
As mentioned in the introduction, one of the most important features of methanol fuel cells is the performance of these cells at relatively lower temperatures than other fuel cells. So, to investigate the behavior of MN and MNR catalysts at various temperatures, a series of CV analyses were performed at optimal concentrations and scan rates of 20 mV/s for two nanocatalysts at different temperatures (ambient temperature, 30, 40, and 50 °C). It can be seen from Figure 7a,b that the oxidation current increases with increasing peak temperature. This increase in the current density can be related to the increase in temperature to facilitate OH − adsorption by the catalyst [38]. Moreover, the rising temperature can affect the activation energy of reactants and provide sufficient energy to break chemical bonds of the methanol in the methanol oxidation reaction. Therefore, it can be said that with increasing temperature, the oxidation process of methanol is facilitated by two nanocatalysts. The linear relationship between the temperature rise and maximum oxidation peak current density is shown in the inset of Figure 7a,b. The proposed mechanism of methanol oxidation by nanocatalysts can be proposed in the form of a six-electron mechanism, as follows: Investigation of the Effect of Temperature on the MOR Process by MN and MNR As mentioned in the introduction, one of the most important features of methanol fuel cells is the performance of these cells at relatively lower temperatures than other fuel cells. So, to investigate the behavior of MN and MNR catalysts at various temperatures, a series of CV analyses were performed at optimal concentrations and scan rates of 20 mV/s for two nanocatalysts at different temperatures (ambient temperature, 30, 40, and 50 • C). It can be seen from Figure 7a,b that the oxidation current increases with increasing peak temperature. This increase in the current density can be related to the increase in temperature to facilitate OH − adsorption by the catalyst [38]. Moreover, the rising temperature can affect the activation energy of reactants and provide sufficient energy to break chemical bonds of the methanol in the methanol oxidation reaction. Therefore, it can be said that with increasing temperature, the oxidation process of methanol is facilitated by two nanocatalysts. The linear relationship between the temperature rise and maximum oxidation peak current density is shown in the inset of Figure 7a

MN and MNR Stability Evaluation in MOR Process at Optimal Scan Rate and Concentration
To evaluate the stability of nanocatalysts in the methanol oxidation process, 500 consecutive CV cycles of MN and MNR were performed in an alkaline solution at an optimal concentration and scan rate of 60 mV/s. As can be seen, the MNR is almost 99.7% stable after this number of cycles and there is no decrease in its oxidation current density peak (Figure 8a), and this stability rate for MN is about 98.3% (Figure 8b). In order to check the stability of the synthesized nanocatalysts in the MOR process, chronoamperometry analysis was performed at a potential of 0.6 V for a duration of 4000 s. As seen in Figure 8c, MNR and MN have 89.8% and 75.9% stability in current density, respectively. Although both nanocatalysts have adequate stability in the MOR process, the superiority of MNR over MN can also be related to its higher electrical conductivity and higher electrochemical active surface area, which is due to the presence of rGO in the structure of the nanocatalyst. The performance of the MNR catalyst in the process of methanol oxidation is compared with other reported studies in Table 1. Examining Table 1 shows that the proposed catalyst is competitive with other research in terms of overvoltage and current density.

MN and MNR Stability Evaluation in MOR Process at Optimal Scan Rate and Concentration
To evaluate the stability of nanocatalysts in the methanol oxidation process, 500 consecutive CV cycles of MN and MNR were performed in an alkaline solution at an optimal concentration and scan rate of 60 mV/s. As can be seen, the MNR is almost 99.7% stable after this number of cycles and there is no decrease in its oxidation current density peak (Figure 8a), and this stability rate for MN is about 98.3% (Figure 8b). In order to check the stability of the synthesized nanocatalysts in the MOR process, chronoamperometry analysis was performed at a potential of 0.6 V for a duration of 4000 s. As seen in Figure 8c, MNR and MN have 89.8% and 75.9% stability in current density, respectively. Although both nanocatalysts have adequate stability in the MOR process, the superiority of MNR over MN can also be related to its higher electrical conductivity and higher electrochemical active surface area, which is due to the presence of rGO in the structure of the nanocatalyst.

MN and MNR Stability Evaluation in MOR Process at Optimal Scan Rate and Concentration
To evaluate the stability of nanocatalysts in the methanol oxidation process, 500 consecutive CV cycles of MN and MNR were performed in an alkaline solution at an optimal concentration and scan rate of 60 mV/s. As can be seen, the MNR is almost 99.7% stable after this number of cycles and there is no decrease in its oxidation current density peak (Figure 8a), and this stability rate for MN is about 98.3% (Figure 8b). In order to check the stability of the synthesized nanocatalysts in the MOR process, chronoamperometry analysis was performed at a potential of 0.6 V for a duration of 4000 s. As seen in Figure 8c, MNR and MN have 89.8% and 75.9% stability in current density, respectively. Although both nanocatalysts have adequate stability in the MOR process, the superiority of MNR over MN can also be related to its higher electrical conductivity and higher electrochemical active surface area, which is due to the presence of rGO in the structure of the nanocatalyst. The performance of the MNR catalyst in the process of methanol oxidation is compared with other reported studies in Table 1. Examining Table 1 shows that the proposed catalyst is competitive with other research in terms of overvoltage and current density. The performance of the MNR catalyst in the process of methanol oxidation is compared with other reported studies in Table 1. Examining Table 1 shows that the proposed catalyst is competitive with other research in terms of overvoltage and current density.

Conclusions
In this study, we introduced nanocatalysts based on transition metal oxides. In this regard, we synthesized the MN and MNR nanocatalysts deposited on the surface of NF by a simple hydrothermal method. After the physical characterization of the nanocatalysts, we studied the capability of nanocatalysts in the MOR process. Although both nanocatalysts performed relatively well in methanol oxidation, the MNR catalyst is more efficient in the MOR process due to the presence of rGO as a two-dimensional material with suitable conductivity and an active surface area. The two proposed nanocatalysts, MN and MNR, with a current density of 14.58 and 24.76 mA/cm 2 and 98.3% and 99.7% cyclic stability, respectively, after 500 consecutive CV cycles in the MOR process, can be inexpensive and attractive candidates for use in the anode of methanol fuel cells.