Development of Nanosized Mn 3 O 4 -Co 3 O 4 on Multiwalled Carbon Nanotubes for Cathode Catalyst in Urea Fuel Cell

: Double-oxide Mn 3 O 4 -Co 3 O 4 nanoparticles were synthesized and anchored on multiwalled carbon nanotubes (MWCNTs) via a single-step solvothermal method. The largest speciﬁc area (99.82 m 2 g − 1 ) of the catalyst was conﬁrmed via a nitrogen adsorption isotherm. Furthermore, the uniform coating of the Mn 3 O 4 -Co 3 O 4 nanoparticles on the surface of the MWCNTs was observed via scanning electron microscopy and transmission electron microscopy; the uniform coating provided an e ﬀ ective transport pathway during the electrocatalytic activities. The rotating disk electrode and rotating ring disk electrode measurements indicated that the electron transfer number was 3.96 and the evolution of H 2 O 2 was 2%. In addition, the Mn 3 O 4 -Co 3 O 4 / MWCNT catalyst did not undergo urea poisoning and remained stable in an alkaline solution. Conversely, commercial Pt / C could not withstand urea poisoning for long. The performance cell achieved a power density of 0.4226 mW cm − 2 at 50 ◦ C. Therefore, Mn 3 O 4 -Co 3 O 4 / MWCNT is an e ﬃ cient and inexpensive noble-metal-free cathodic catalyst for direct urea fuel cells.


Introduction
In recent years, the demand for renewable energy has been gradually increasing. As a type of renewable energy carrier, urea is considered as a hydrogen carrier form because it is non-flammable, non-toxic, biodegradable, and abundant. Urea can be utilized as a power source with direct urea fuel cells (DUFCs). DUFCs represent novel technology for the efficient generation of electricity because of their numerous advantages, such as low cost, high energy-storage densities, and high levels of safety [1]. In addition, DUFCs generate electricity without releasing toxic products, thus limiting the level of environmental pollution. Hence, it constitutes an attractive power generation technology [2]. Furthermore, urea is widely available in industrial wastewater and human/animal urine. However, untreated urea could pose danger to both human health and the environment because it can hydrolyze into toxic ammonia and nitrite [3,4]. Therefore, DUFCs have two-fold benefits-they can generate electricity and produce clean water. In a DUFC, a urea electrooxidation reaction and oxygen reduction reaction (ORR) occur at the interface of the electrolyte and the electrocatalytic layers, and are expressed as follows [5]: Anode: CO(NH 2 ) 2 + 6OH − → N 2 + 5H 2 O+ CO 2 + 6e − E 0 = −0.746 V Cathode: 2 H 2 O + O 2 + 4e− → 4OH − E 0 = 0.4 V Overall: 2CO(NH 2 ) 2 + 3O 2 → 2N 2 + 4H 2 O + 2CO 2 E 0 = 1.146 V

Material Characterization
The morphology and particle sizes of the composite were observed via field emission scanning electron microscopy (SEM) (FE-SEM, Hitachi S-4200 system, Tokyo, Japan) and transmission electron microscopy (TEM) (HR-TEM, CRYO-ARM200F, JEOL, Massachusetts, USA). A Brunauer-Emmett-Teller (BET) measurement was performed to investigate the specific surface areas of the obtained samples (Autosorb iQ Station 2). The X-ray diffraction (XRD) spectra were recorded using the Panoalytical Empyrean XRD apparatus with Cu 1.8 KW (Cu k-α 1.54 Å, max 60 kV, and 55 mA) and PICXEL 3D with a prefix interface Xenon proportional detector (0D point detector).

Electrochemical Measurements
Rotating ring disk electrode (RRDE) measurements were performed with an RRDE-3A instrument in the SP-240 Bio-Logic SAS mode to evaluate the activities of Co 3 O 4 /MWCNT, Mn 3 O 4 /MWCNT, and Mn 3 O 4 -Co 3 O 4 /MWCNT in ORR electrocatalysis. For this measurement, a three-electrode cell was used, which consisted of an RRDE, Ag-AgCl/KCl (0.973 V vs. RHE (reversible hydrogen electrode)), and a Pt wire employed as the working, reference, and counter electrodes, respectively. Typically, 10 mg of a prepared catalyst powder was ultrasonically suspended in 1 mL isopropanol and 100 µL of Nafion for 30 min. Subsequently, 10 µL of the resulting slurry was coated on the surface of the RRDE. All measurements, including cyclic voltammetry (CV), chronoamperometry, and linear sweep voltammetry (LSV) were carried out in 1 M KOH at room temperature. The measuring system was purged with O 2 for at least 30 min before measurements. We also analyzed the scan rate at 20 mV s −1 between 0.4 and 1.2 V vs. RHE for CV and 0.5 to 1.2 V vs. RHE at rotating speeds ranging from 400 to 2000 rpm for LSV. The commercial Pt/C (Pt 20%, from Premetek) was subjected to the same method as a reference standard.

Fabrication and Examination of Membrane Electrode Assembly
A single fuel cell was fabricated to evaluate the practical application of the Mn 3 O 4 -Co 3 O 4 /MWCNT composite using the as-prepared nanostructure as a cathode catalyst and the commercial Ni/C (Ni 20%, from Premetek) as an anode catalyst with Fumasep FAA-3-50 (Fumatech) anion exchange membranes. For this, 50 mg of the as-prepared catalyst (in powder form) and the commercial Ni/C were dispersed in a mixture of 5% Nafion (200 µL) and isopropanol (800 µL), and sonicated for 30 min to form a uniform ink. The catalyst inks (2 mg cm −2 ) were then loaded onto a 5.0 cm 2 carbon paper. Both electrodes were attached to the membrane electrolyte by hot-pressing for 2 min at 100 • C. The membrane electrode assemblies were examined in a fuel cell station (Scitech Korea Inc., Seoul, Korea). The experiments were performed in a 0.33 M urea/1 M KOH solution fed as an anolyte and wet O 2 as the catholyte at room temperature. The successful synthesis of manganese-cobalt oxide nanoparticles on the MWCNT composite was confirmed via TEM and SEM, as shown in Figure 2. SEM and TEM images of the obtained products showed that the Mn3O4 and Co3O4 nanoparticles were homogeneously coated in a high quantity on the surface of the MWCNT template without any aggregation. According to the TEM images shown in Figure 2, the average size of both oxides was small, ranging from 8 to 10 nm, which could be due to the low hydrolysis rate of the precursor [21].  The successful synthesis of manganese-cobalt oxide nanoparticles on the MWCNT composite was confirmed via TEM and SEM, as shown in Figure 2. SEM and TEM images of the obtained products showed that the Mn 3 O 4 and Co 3 O 4 nanoparticles were homogeneously coated in a high quantity on the surface of the MWCNT template without any aggregation. According to the TEM images shown in Figure 2, the average size of both oxides was small, ranging from 8 to 10 nm, which could be due to the low hydrolysis rate of the precursor [21]. The successful synthesis of manganese-cobalt oxide nanoparticles on the MWCNT composite was confirmed via TEM and SEM, as shown in Figure 2. SEM and TEM images of the obtained products showed that the Mn3O4 and Co3O4 nanoparticles were homogeneously coated in a high quantity on the surface of the MWCNT template without any aggregation. According to the TEM images shown in Figure 2, the average size of both oxides was small, ranging from 8 to 10 nm, which could be due to the low hydrolysis rate of the precursor [21].    The structures and crystallographic phase of the synthesized catalyst were studied through XRD, as shown in Figure 3. For the Mn3O4/MWCNT and Co3O4/MWCNT catalysts, the diffraction peaks were indexed according to JCPDS 24-0734 and JCPDS 43-1003, respectively. The XRD patterns of the prepared Mn3O4-Co3O4/MWCNT nanoparticles exhibited well-defined peaks at 18.96° (111), 31.09° (220), 36.86° (311), 44.78° (400), 59.11° (511), and 65.28° (440), which were indexed to the Co3O4 spinel structure (JCPDS 43-1003). No residual peaks were observed. The individual Co3O4 phase had a crystalline structure. In addition, the metal-oxygen-carbon (M-O-C) composite presented a peak at 55° (303), which corresponded to the formed Mn3O4, in reference to JCPDS 27-0734. This observation suggests the formation of Mn3O4 in addition to the Co3O4 phase in the composite, indicating the presence of a multiphase structure. To study the chemical bonding of the synthesized catalysts, we performed X-ray photoelectron spectroscopy (XPS). From the survey spectrum, we confirmed the existence of cobalt, manganese, oxygen, and carbon. The XPS profiles of Co2p 3/2 and Co2p 1/2 are presented in Figure 4b. The fitting peaks were observed at 780.2 and 795.3 eV for the binding energy at a spin energy gap of 15.1 eV, which further verified the presence of Co3O4 in the composite [22]. The presence of Mn3O4 was verified by two peaks at 641.8 and 653.4 eV with a separating energy of 11.6 eV, indicating the Mn2p 3/2 and Mn2p 1/2 spin-orbit states, respectively [23,24]. The C1s deconvolution spectrum contained three distinguished peaks at 284.5, 286.04, and 282.1 eV, which corresponded to carbon atoms bonded to carbon (C-C), oxygen (C-O), and oxygen in carboxyl groups (O-C=O), respectively [25]. The O1s spectrum was divided into four distinct peaks, which indicated the formation of oxygen bonds at the interface between the oxides and carbon.

Results and Discussion
In addition, the O1s XPS profiles of the individual components Mn3O4/MWCNT and Co3O4/MWCNT were recorded for reference and are shown in Figure 4e,f. The two peaks at 531.84 and 533.04 eV corresponded to O-C=O and C-O bonding, respectively, which corroborated previously reported results [26,27]. The third peak observed at 529.7 eV was consistent with an M-O bond (M = Co, Mn) [28,29], which is slightly positively shifted. Furthermore, the intensity of this peak decreased compared to that of the individual oxides alone on MWCNTs, demonstrating the coexistence of Mn-O and Co-O in the composite catalysts. The formation of the M-O-C bonding at the interface of the oxide and MWCNTs was also confirmed by comparing the binding energy and intensity in the O1s of the synthesized catalysts. The characteristic peak at 530.14 eV in the spectrum of the Mn3O4-Co3O4/MWCNT composite was similar to that of Mn3O4/MWCNT and Co3O4/MWCNT. Nonetheless, this peak showed an increased intensity and a slight change in the binding energy, indicating the formation of M-O-C bonding, including Mn-O-C and Co-O-C [30]. In addition, the bonding energy of the well-defined peaks shifted positively from 1 to 3 eV in contrast with that in the M-O bond, which further proved the formation of M-O-C in Mn3O4-Co3O4/MWCNT [20]. To study the chemical bonding of the synthesized catalysts, we performed X-ray photoelectron spectroscopy (XPS). From the survey spectrum, we confirmed the existence of cobalt, manganese, oxygen, and carbon. The XPS profiles of Co2p 3/2 and Co2p 1/2 are presented in Figure 4b. The fitting peaks were observed at 780.2 and 795.3 eV for the binding energy at a spin energy gap of 15.1 eV, which further verified the presence of Co 3 O 4 in the composite [22]. The presence of Mn 3 O 4 was verified by two peaks at 641.8 and 653.4 eV with a separating energy of 11.6 eV, indicating the Mn2p 3/2 and Mn2p 1/2 spin-orbit states, respectively [23,24]. The C1s deconvolution spectrum contained three distinguished peaks at 284.5, 286.04, and 282.1 eV, which corresponded to carbon atoms bonded to carbon (C-C), oxygen (C-O), and oxygen in carboxyl groups (O-C=O), respectively [25]. The O1s spectrum was divided into four distinct peaks, which indicated the formation of oxygen bonds at the interface between the oxides and carbon.
In addition, the O1s XPS profiles of the individual components Mn 3 O 4 /MWCNT and Co 3 O 4 /MWCNT were recorded for reference and are shown in Figure 4e,f. The two peaks at 531.84 and 533.04 eV corresponded to O-C=O and C-O bonding, respectively, which corroborated previously reported results [26,27]. The third peak observed at 529.7 eV was consistent with an M-O bond (M = Co, Mn) [28,29], which is slightly positively shifted. Furthermore, the intensity of this peak decreased compared to that of the individual oxides alone on MWCNTs, demonstrating the   The XPS results demonstrated that the Co3O4 and Mn3O4 nanoparticles were decorated over the surface of the MWCNTs, and this was highly related to the enhancement of the electrochemical performance. The high concentration of oxygen vacancies could facilitate oxygen adsorption and covalent metal oxide-nanocarbon bonding and enable improved electron transfer across the interface. The specific areas of Mn3O4-Co3O4/MWCNT were 99.82 m 2 /g as obtained from the BET analysis shown as Figure 5. The large specific surface areas of the synthesized composite could facilitate the adsorption and transportation of O2 and H2O during the ORR. To assess the catalytic activity of Mn3O4-Co3O4/MWCNT, we evaluated the individual components using commercial Pt/C 20% as a comparative sample via CV on an RRDE in O2-saturated 1.0 M KOH. The observed ORR peaks for the obtained sample are depicted in Figure 6. The CV curves of the Mn3O4-Co3O4/MWCNT composite suggest much higher activity than those of Co3O4 and  The XPS results demonstrated that the Co3O4 and Mn3O4 nanoparticles were decorated over the surface of the MWCNTs, and this was highly related to the enhancement of the electrochemical performance. The high concentration of oxygen vacancies could facilitate oxygen adsorption and covalent metal oxide-nanocarbon bonding and enable improved electron transfer across the interface. The specific areas of Mn3O4-Co3O4/MWCNT were 99.82 m 2 /g as obtained from the BET analysis shown as Figure 5. The large specific surface areas of the synthesized composite could facilitate the adsorption and transportation of O2 and H2O during the ORR. To assess the catalytic activity of Mn3O4-Co3O4/MWCNT, we evaluated the individual components using commercial Pt/C 20% as a comparative sample via CV on an RRDE in O2-saturated 1.0 M KOH. The observed ORR peaks for the obtained sample are depicted in Figure 6. The CV curves of the Mn3O4-Co3O4/MWCNT composite suggest much higher activity than those of Co3O4 and  Mn3O4. Additionally, the curves were compared with those of Pt/C. The reduction peaks of all synthesized samples had the same position peak (0.7 V vs. RHE), but the diffusion limiting current density followed this sequence: Mn3O4-Co3O4/MWCNT > Co3O4/MWCNT > Mn3O4/MWCNT. The reduction peak reached approximately 3.55 mA cm −2 for the Mn3O4-Co3O4/MWCNT composite and was approximately 1.3 times higher than that of Co3O4 (2.54 mA cm −2 ) and Mn3O4 (2.49 mA cm −2 ), indicating that the combination of metal oxides can significantly improve the ORR activity. In addition, the current density of the Mn3O4-Co3O4/MWCNT composite was close to that of Pt/C; however, the more positive the peak potential was (0.7 V), the more positively shifted the onset potential (0.85 V) was, as shown in Figure 6b. Thus, the Mn3O4-Co3O4/MWCNT composite was an active electrocatalyst for the ORR. For comparison, the polarization curves for the ORR were also recorded in a 1 M KOH solution at 1600 rpm, as shown in Figure 6c. The electrocatalytic activity of the composite catalyst was higher than the activities of its individual components and approximately equal to that of the commercial Pt/C owing to their higher current density and positive half-wave potential. The results illustrated that the onset potential of the composite (0.87 V) was higher than that of Mn3O4 and Co3O4 (0.84 V) and lower than that of the Pt/C catalyst (0.97 V). The current density on the composite was 4.97 mA cm −2 at 0.6 V, which was higher than that of Co3O4 (4.70 mA cm −2 ) and Mn3O4 (4.53 mA cm -2 ) and similar to that of Pt/C. To gain insight into the kinetics of the ORR, the LSV of Mn3O4-Co3O4/MWCNT was recorded in O2-saturated 1.0 M KOH at different rotating rates. As shown in Figure 7a, the polarization curves suggested that the measured current intensity increased with the high-speed rotation rates because of the enhanced diffusion. Based on the diffusion in the kinetically limited For comparison, the polarization curves for the ORR were also recorded in a 1 M KOH solution at 1600 rpm, as shown in Figure 6c. The electrocatalytic activity of the composite catalyst was higher than the activities of its individual components and approximately equal to that of the commercial Pt/C owing to their higher current density and positive half-wave potential. The results illustrated that the onset potential of the composite (0.87 V) was higher than that of Mn 3 O 4 and Co 3 O 4 (0.84 V) and lower than that of the Pt/C catalyst (0.97 V). The current density on the composite was 4.97 mA cm −2 at 0.6 V, which was higher than that of Co 3 O 4 (4.70 mA cm −2 ) and Mn 3 O 4 (4.53 mA cm -2 ) and similar to that of Pt/C. To gain insight into the kinetics of the ORR, the LSV of Mn 3 O 4 -Co 3 O 4 /MWCNT was recorded in O 2 -saturated 1.0 M KOH at different rotating rates. As shown in Figure 7a, the polarization curves suggested that the measured current intensity increased with the high-speed rotation rates because of the enhanced diffusion. Based on the diffusion in the kinetically limited regions, we used the Koutecky-Levich (K-L) plot to determine the electron transfer number. The K-L equation is [31]: where J is the measured current density; J k is the kinetic current density; B is the Levich constant or proportionality coefficient, which could be determined from the slope of the K-L plot; ω is the electrode rotating rate, F = 96485 C mol −1 ; ν is the kinematic viscosity of the electrolyte (ν = 0.01 cm 2 s −1 ); where J is the measured current density; Jk is the kinetic current density; B is the Levich constant or proportionality coefficient, which could be determined from the slope of the K-L plot; ω is the electrode rotating rate, F = 96485 C mol −1 ; ν is the kinematic viscosity of the electrolyte (ν = 0.01 cm 2 s −1 ); Co = 7.8 × 10 −7 mol cm −1 is the concentration of O2 in the electrolyte; and Do = 1.8 × 10 −5 m 2 s −1 is the diffusion coefficient of O2 in 1.0 M KOH. The electron transfer number was calculated using Equations (1) and (2)   To gain further insight into the activities of the catalysts in the ORR, we investigated the reaction kinetics at a rotation rate of 1600 rpm. During the experiment, the disk potential was scanned at 0.55-0.9 V vs. RHE, while the ring potential was maintained at 0.627 V. As shown in Figure 7c, both ring and disk currents increased rapidly because the oxygen supply was sufficient on the surface of the working electrode to enhance the acceleration rate of the ORR. Co3O4/MWCNT and Mn3O4/MWCNT had very similar disk currents, whereas the composite catalyst exhibited a higher ring current. The onset potential and current density peaked at 0.855 V and 0.465 mA, which were higher than those of the other catalysts. The half-wave potential and diffusion-limited current increased in the following order: Co3O4/MWCNT ≤ Mn3O4/MWCNT < Mn3O4-Co3O4/MWCNT, illustrating the increase in the ORR activity.
In addition, the electron transfer number and peroxide percentages were obtained as a function of both disk and ring currents using the following equation from the RRDE technique [31].
where ID and IR are the currents at the disk and ring electrodes, respectively, and N = 0.317 (ID/IR) is the RRDE collection efficiency. For the as-prepared catalysts Mn3O4-Co3O4/MWCNT, Co3O4/MWCNT, and Mn3O4/MWCNT, the χ H2O2 were 2%, 4.46%, and 2.5%, and the n values were To gain further insight into the activities of the catalysts in the ORR, we investigated the reaction kinetics at a rotation rate of 1600 rpm. During the experiment, the disk potential was scanned at 0.55-0.9 V vs. RHE, while the ring potential was maintained at 0.627 V. As shown in Figure 7c, both ring and disk currents increased rapidly because the oxygen supply was sufficient on the surface of the working electrode to enhance the acceleration rate of the ORR. Co 3 O 4 /MWCNT and Mn 3 O 4 /MWCNT had very similar disk currents, whereas the composite catalyst exhibited a higher ring current. The onset potential and current density peaked at 0.855 V and 0.465 mA, which were higher than those of the other catalysts. The half-wave potential and diffusion-limited current increased in the following order: Co 3 O 4 /MWCNT ≤ Mn 3 O 4 /MWCNT < Mn 3 O 4 -Co 3 O 4 /MWCNT, illustrating the increase in the ORR activity.
In addition, the electron transfer number and peroxide percentages were obtained as a function of both disk and ring currents using the following equation from the RRDE technique [31].
where I D and I R are the currents at the disk and ring electrodes, respectively, and N = 0.  [34]. In addition, the Co 3+ species of Co 3 O 4 provide a surface electronic state, which carries electrons from bulk oxides to produce an excited cationic state, facilitating the ORR [37]. Moreover, the uniform metal oxides decorated on MWCNTs confine agglomerations, which contribute to the active surface areas. Additionally, this process can enhance the electronic conductivity due to the efficient generation of ions in the alkaline media [38,39].
Energies 2020, 13, x FOR PEER REVIEW 9 of 13 3.96, 3.91, and 3.94, respectively. These values were similar to those obtained from the K-L plot, but a small difference remained due to the calculation process. However, the results presented a preference for a direct four-electron reduction pathway. Mn3O4-Co3O4/MWCNT showed the highest n value (3.96) and the lowest χ H2O2 (2%), which proved its superior electrochemical activity compared to Co3O4/MWCNT and Mn3O4/MWCNT. The improved performance (higher electron transfer and lower ring current) of the Mn3O4-Co3O4/MWCNT catalyst can be attributed to the following reasons ( Figure 8). First, the uniformity of the Co3O4 and Mn3O4 metal oxides decorated on the surface of the MWCNTs enabled C-O-M covalent bonding at the interface [32][33][34][35]. The fine decoration of the Mn3O4-Co3O4 nanoparticles contributed to this activation [36]: In this process, a trace amount of the H2O2 intermediate undergoes chemical disproportionation to generate O2 species, which can be used as reactants for a further ORR [37]. Similarly, OHis produced by catalyzing the decomposition of H2O2 with Co3O4 nanoparticles [34]. In addition, the Co 3+ species of Co3O4 provide a surface electronic state, which carries electrons from bulk oxides to produce an excited cationic state, facilitating the ORR [37]. Moreover, the uniform metal oxides decorated on MWCNTs confine agglomerations, which contribute to the active surface areas. Additionally, this process can enhance the electronic conductivity due to the efficient generation of ions in the alkaline media [38,39]. Fuel tolerance and stability are vital characteristics of a high-performing ORR catalyst. Urea crossover tests were performed in a 1.0 M KOH solution containing 5 mL of 0.33 M urea, as shown in Figure 9a,b. The current density immediately decreased by 43% at 0.7 V for Pt/C after injecting the urea solution, whereas that of the Mn3O4-Co3O4/MWCNT catalyst showed a minor decrease and retained 92% of its initial value. This behavior demonstrated that Pt/C could not withstand the urea crossover. With regard to the long-time durability of the catalysts, the voltammograms showed high stability of Mn3O4-Co3O4/MWCNT with no distinct current change; the initial limiting current density was 0.672 V (vs. RHE). Figure 9c,d presents the chronoamperometric responses of the commercial Pt/C and the as-prepared Mn3O4-Co3O4/MWCNT catalyst at a rotation rate of 1000 rpm in a 1M KOH solution for 5000 s. The commercial Pt/C dropped approximately 10% of its initial current due to the  solution for 5000 s. The commercial Pt/C dropped approximately 10% of its initial current due to the formation of Pt hydroxide on its surface. It is clear that Mn 3 O 4 -Co 3 O 4 /MWCNT underwent an inconsiderable decrease and remained stable at 97% of its initial current after 5000 s. The obtained results demonstrate that Mn 3 O 4 -Co 3 O 4 /MWCNT has a superior long-term stability in comparison with commercial Pt/C. [39]. All chronoamperometric responses demonstrate that the Mn-Co oxide composite possessed high durability and favorable kinetics. Therefore, the composite can be used effectively as a cathode catalyst in an alkaline fuel cell.
Energies 2020, 13, x FOR PEER REVIEW 10 of 13 formation of Pt hydroxide on its surface. It is clear that Mn3O4-Co3O4/MWCNT underwent an inconsiderable decrease and remained stable at 97% of its initial current after 5000 s. The obtained results demonstrate that Mn3O4-Co3O4/MWCNT has a superior long-term stability in comparison with commercial Pt/C. [39]. All chronoamperometric responses demonstrate that the Mn-Co oxide composite possessed high durability and favorable kinetics. Therefore, the composite can be used effectively as a cathode catalyst in an alkaline fuel cell. Furthermore, we evaluated the cell performance of the prepared sample used as a catholyte in DUFC. Figure 10 shows the polarization and power density curves of Mn3O4-Co3O4/MWCNT at room temperature. A current density of 2.13 mA cm −2 and a maximum power density of 0.4226 mW cm −2 at 50 °C were obtained.  Furthermore, we evaluated the cell performance of the prepared sample used as a catholyte in DUFC. Figure 10 shows the polarization and power density curves of Mn 3 O 4 -Co 3 O 4 /MWCNT at room temperature. A current density of 2.13 mA cm −2 and a maximum power density of 0.4226 mW cm −2 at 50 • C were obtained.
Energies 2020, 13, x FOR PEER REVIEW 10 of 13 formation of Pt hydroxide on its surface. It is clear that Mn3O4-Co3O4/MWCNT underwent an inconsiderable decrease and remained stable at 97% of its initial current after 5000 s. The obtained results demonstrate that Mn3O4-Co3O4/MWCNT has a superior long-term stability in comparison with commercial Pt/C. [39]. All chronoamperometric responses demonstrate that the Mn-Co oxide composite possessed high durability and favorable kinetics. Therefore, the composite can be used effectively as a cathode catalyst in an alkaline fuel cell. Furthermore, we evaluated the cell performance of the prepared sample used as a catholyte in DUFC. Figure 10 shows the polarization and power density curves of Mn3O4-Co3O4/MWCNT at room temperature. A current density of 2.13 mA cm −2 and a maximum power density of 0.4226 mW cm −2 at 50 °C were obtained.