Enhanced Electrocatalytic Activity of Cobalt-Doped Ceria Embedded on Nitrogen, Sulfur-Doped Reduced Graphene Oxide as an Electrocatalyst for Oxygen Reduction Reaction

: N, S-doped rGO was successfully synthesized and embedded Co-doped CeO 2 via hydrothermal synthesis. The crystal structure, surface morphology and elemental composition of the prepared catalyst were studied by XRD, Raman spectra, SEM, TEM and XPS analyses. The synthesized electrocatalyst exhibits high onset and halfwave potential during the ORR. This result shows that a combination of N- and S-doped rGO and Co-doped CeO 2 leads to a synergistic effect in catalyz-ing the ORR in alkaline media. Co–CeO 2 /N, S–rGO displays enhanced ORR performance compared to bare CeO 2 . The superior stability of the prepared catalyst implies its potential applications beyond fuel cells and metal–air batteries.


Introduction
Developing an electrocatalyst for the oxygen reduction reaction by using nonprecious metal to replace the Pt group metal-based catalyst is an important strategic task for fuel cells or metal-air batteries because of its crucial role in both [1][2][3]. Currently, extensive research has been carried out to find effective nonprecious metal catalysts for the ORR [4,5]. Metal oxide-based catalysts are used for support in the ORR. Metal oxides such as CeO 2 [6][7][8][9], MnO 2 [10][11][12], TiO 2 [13,14] and ZnO [15] have been used as alternatives for the Pt-based electrocatalyst for the ORR. Metal oxides act as promoters during the ORR due to their facile oxygen release and storage capacity, as well as their well-known corrosion resistance [16,17].
Cerium oxide-based materials have shown promising electrocatalytic performance because of their unique structural features. CeO 2 has been widely used in fuel cells (MOR, ORR) [18][19][20]. Precious metals like Pt and Pd supported on CeO 2 exhibit good catalytic activity toward the ORR [21][22][23][24], but precious metals are quite challenging for commercialization [25,26]. Although there is much research dealing with CeO 2 -supported catalysts for the ORR, there are very few studies focusing on the importance of oxygen vacancy in doped CeO 2 [27][28][29]. Furthermore, the abundance of oxygen vacancies in CeO 2 favors O 2 adsorption and oxygen conductivity due to the coexistence of Ce 4+ and Ce 3+ , which contribute to the CeO 2 -supported electrocatalyst in the ORR [30,31]. However, the poor electronic conductivity of CeO 2 limits its ORR performance. Depositing CeO 2 on conducting carbon-based materials has proven to be the most effective method for improving its dispersibility and conductivity [32,33].
In recent years, a number of reduced graphene oxides with metal oxide hybrid catalysts have been investigated as ORR catalysts [34][35][36]. Among those, heteroatom-doped  [47], respectively. The XRD patterns of Co-CeO 2 reveal that during the Co doping, the peak intensity decreases, and the peaks slightly shift to a higher angle in comparison with bare CeO 2 , which indicates the doping of the Co ion into the CeO 2 lattice. The peak intensity of the Co-doped CeO 2 is much weaker than CeO 2 , which can be created by the ionic radii difference between CeO 2 and Co-doped CeO 2 . Additionally, the ionic radius of Co 2+/3+ is 0.75−0.9 Å and Ce 4+ is 0.97Å [48]. However, the further increase in the Co concentration to 4.5% of the peak intensity also increases, which indicates grain size increase after the 4.5% Co ion is doped into the CeO 2 . From Figure 1c, GO exhibits a typical characteristic plane of (002) appearing at 10.1 • . After the hydrothermal treatment, the synthesized N, S-rGO shows a characteristic peak of (002) at 24.5 and the strong peak (002) at 10.1 • disappears, which indicates the reduction of rGO. Figure 1b exhibits the diffraction peaks of both CeO 2 and N,S-rGO, which confirm the formation of Co-doped CeO 2 and the N, S-rGO hybrid.

Raman Analysis
The doping of the Co ion in CeO2 induces oxygen defects in CeO2 (Figure 2a), which is further characterized by the Raman analysis. The main characteristic peak at 462 cm −1 refers to the F2g vibration mode cubic CeO2, and a shoulder peak at 595 cm −1 corresponds to the oxygen vacancy. Compared to the bare CeO2, the peak intensity of 462 cm −1 decreases and the peak of 595 cm −1 is increased after the doping of Co. These results suggest that the Co doping has led to the enhancement of the oxygen vacancy in CeO2. According to the literature, the Raman peak at 830 cm −1 corresponds to the peroxide species [49]. Figure 2b shows the Raman spectra of GO, N, S-rGO and the Co-CeO2/N, S-rGO composite. All samples exhibit two characteristic peaks of the D band at 1350 cm −1 and the G band at 1579 cm −1 [50,51]. The D band is assigned to the sp 3 structural defect sites, and the G band is associated with the sp 2 -bonded graphitic carbon atom. The intensity ratio of the D and G bands (ID/IG) is typically used to measure defects in carbon materials. The ID/IG values were 0.941, 1.100 and 1.114 for GO, N, S-rGO and 3% Co-CeO2/N, S-rGO, respectively. The 3% Co-CeO2/N, S-rGO exhibits a higher ID/IG value than the rGO, suggesting that the N, S-doping and embedded Co-CeO2 create more defects in the carbon matrix and also create more active sites during the ORR. Additionally, the Raman spectra Co-CeO2/N, S-rGO exhibit the typical peak of CeO2, located at 462 cm −1 , and the D and G bands of N, S-rGO, which conform to the coexistence of CeO2 and N, S-rGO. This result demonstrates the interaction between rGO and Co-CeO2 during the synthesis.

Raman Analysis
The doping of the Co ion in CeO 2 induces oxygen defects in CeO 2 (Figure 2a), which is further characterized by the Raman analysis. The main characteristic peak at 462 cm −1 refers to the F 2 g vibration mode cubic CeO 2 , and a shoulder peak at 595 cm −1 corresponds to the oxygen vacancy. Compared to the bare CeO 2 , the peak intensity of 462 cm −1 decreases and the peak of 595 cm −1 is increased after the doping of Co. These results suggest that the Co doping has led to the enhancement of the oxygen vacancy in CeO 2 . According to the literature, the Raman peak at 830 cm −1 corresponds to the peroxide species [49]. Figure 2b shows the Raman spectra of GO, N, S-rGO and the Co-CeO 2 /N, S-rGO composite. All samples exhibit two characteristic peaks of the D band at 1350 cm −1 and the G band at 1579 cm −1 [50,51]. The D band is assigned to the sp 3 structural defect sites, and the G band is associated with the sp 2 -bonded graphitic carbon atom. The intensity ratio of the D and G bands (I D /I G ) is typically used to measure defects in carbon materials. The I D /I G values were 0.941, 1.100 and 1.114 for GO, N, S-rGO and 3% Co-CeO 2 /N, S-rGO, respectively. The 3% Co-CeO 2 /N, S-rGO exhibits a higher I D /I G value than the rGO, suggesting that the N, S-doping and embedded Co-CeO 2 create more defects in the carbon matrix and also create more active sites during the ORR. Additionally, the Raman spectra Co-CeO 2 /N, S-rGO exhibit the typical peak of CeO 2 , located at 462 cm −1 , and the D and G bands of N, S-rGO, which conform to the coexistence of CeO 2 and N, S-rGO. This result demonstrates the interaction between rGO and Co-CeO 2 during the synthesis.

FESEM and TEM Analyses
The morphologies of 3% Co-CeO2/N, S-rGO were studied by SEM and TEM analyses. Figure 3a shows the sheet-like morphology of rGO. Figure 3b shows that the Co-CeO2 nanoparticles are uniformly anchored on the N, S-rGO to form the Co-CeO2/N, S-rGO. The corresponding TEM image shows that CeO2 nanoparticles were anchored on the sheet-like structure of N, S-rGO. In the HRTEM image (Figure 3c

FESEM and TEM Analyses
The morphologies of 3% Co-CeO 2 /N, S-rGO were studied by SEM and TEM analyses. Figure 3a shows the sheet-like morphology of rGO. Figure 3b shows that the Co-CeO 2

FESEM and TEM Analyses
The morphologies of 3% Co-CeO2/N, S-rGO were studied by SEM and TEM analyses. Figure 3a shows the sheet-like morphology of rGO. Figure 3b shows that the Co-CeO2 nanoparticles are uniformly anchored on the N, S-rGO to form the Co-CeO2/N, S-rGO. The corresponding TEM image shows that CeO2 nanoparticles were anchored on the sheet-like structure of N, S-rGO. In the HRTEM image (Figure 3c), the lattice fringes of 0.31 nm and 0.26 nm correspond to the CeO2 (111) and (002) N, S-rGO, respectively.

XPS Analysis
The XPS analysis was performed to further measure the elemental composition and chemical valence of the 3% Co-CeO2/N, S-rGO. Figure 4a shows the Ce 3d core level spectra of CeO2. The spin orbit doublets of Ce 3d3/2 and Ce 3d5/2 were labelled as U and V, respectively. The characteristic binding energy peaks of U′′′ (916.2 eV), V′′′ (899.7 eV), U′′ (907.6 eV), V′′ (889.2 eV), U (901.7 eV) and V (883 eV) are ascribed to Ce 4+ species. While the peaks centered at Uo (899.1 eV), Vo (881.1eV), U′ (904.9 eV) and V′ (886.1 eV) were attributed to the Ce 3+ [52,53]. Figure 4b shows the core level spectra of Co, which exhibits the core level of 2p3/2 (800 eV) and 2p1/2 (796.5 eV) for Co-O bonding. Additionally, the higher energy level of Co 2p3/2 and Co 2p1/2 corresponds to bivalent cobalt ions. As displayed in Figure 4c, the O 1s core level spectra split into three peaks, with the binding energy peak of 529.1 eV assigned as the lattice oxygen, the peak of 530.5 eV corresponding to the chemisorbed oxygen and the binding energy peak around 532.2 eV corresponding to the oxygen vacancy in CeO2 [54,55]

XPS Analysis
The XPS analysis was performed to further measure the elemental composition and chemical valence of the 3% Co-CeO 2 /N, S-rGO. Figure [52,53]. Figure 4b shows the core level spectra of Co, which exhibits the core level of 2p 3/2 (800 eV) and 2p 1/2 (796.5 eV) for Co-O bonding. Additionally, the higher energy level of Co 2p 3/2 and Co 2p 1/2 corresponds to bivalent cobalt ions. As displayed in Figure 4c, the O 1s core level spectra split into three peaks, with the binding energy peak of 529.1 eV assigned as the lattice oxygen, the peak of 530.5 eV corresponding to the chemisorbed oxygen and the binding energy peak around 532.2 eV corresponding to the oxygen vacancy in CeO 2 [54,55]. The C 1s (Figure 4d) core level spectra exhibit three different binding energy peaks, 284.7, 286.0 and 289.1 eV, which correspond to the C = C-O, C-N/C-S and C-O, respectively. The N 1s core level spectrum (Figure 4e) displays three peaks located at 398.3 eV for pyridinic N, 400 eV for pyrrolic and 400.8 eV for graphitic N. Figure 4f is the core level spectra of S 2p, where the characteristic peaks at 163.25 eV and 164.80 eV are assigned to the S 2p 3/2 and S 2p 1/2 of thiophene S, and the peak of 167.8 eV corresponds to the SO x group [56].

Electrochemical Performance of Co-CeO2 and Co-CeO2/N, S-rGO Composites
The ORR performance of the synthesized catalyst was studied by using a cyclic voltammogram (CV), which was performed at a scan rate of 50 mV/s with O2-saturated KOH. Figure 5 shows the CV measurements of the prepared catalysts with a different percent of

Electrochemical Performance of Co-CeO 2 and Co-CeO 2 /N, S-rGO Composites
The ORR performance of the synthesized catalyst was studied by using a cyclic voltammogram (CV), which was performed at a scan rate of 50 mV/s with O 2 -saturated KOH. Figure 5 shows the CV measurements of the prepared catalysts with a different percent of Co dopant. The cathodic oxygen reduction onset potential increases in the following order as 3% Co-CeO 2 /N, S-rGO > 1.5% Co-CeO 2 /N, S-rGO > 4.5% Co-CeO 2 /N, S-rGO > CeO 2 /N, S-rGO. Moreover, the 3% Co-CeO 2 /N, S-rGO exhibits a higher current density than the samples. This result demonstrates an enhanced ORR activity due to the Co doping on CeO 2 and composite formation with N, S-rGO.  The electrocatalytic ORR performances of the synthesized catalysts were performed at a scan rate of 10 mV/s in O2-saturated 0.1 M KOH. Figure 6a shows the ORR polarization curves of the bare CeO2 and the different percentage of Co-doped CeO2. At 3% Co-CeO2, the halfwave potential shifted more positively and Jlim increased. However, when the Co doping concentration increased to 4.5%, the ORR performance decreased, showing that the optimal concentration is 3%. This result shows that Co doping to CeO2 improves the ORR performance, which can be attributed to the oxygen vacancy creation through Co doping and the coexistence of Ce 3+ and Ce 4+ . The optimized 3% Co-CeO2/N, S-rGO hybrid electrocatalyst shows better ORR performance (Table 1) Figure 6a shows the ORR polarization curves of the bare CeO 2 and the different percentage of Co-doped CeO 2 . At 3% Co-CeO 2 , the halfwave potential shifted more positively and J lim increased. However, when the Co doping concentration increased to 4.5%, the ORR performance decreased, showing that the optimal concentration is 3%. This result shows that Co doping to CeO 2 improves the ORR performance, which can be attributed to the oxygen vacancy creation through Co doping and the coexistence of Ce 3+ and Ce 4+ . The optimized 3% Co-CeO 2 /N, S-rGO hybrid electrocatalyst shows better ORR performance (Table 1) compared to bare CeO 2 and 3% Co-CeO 2 . After the combination of Co-CeO 2 and N, S-rGO, the 3% Co-CeO 2 /N, S-rGO has a high limiting current density (J L = 4.2 mA/cm 2 ) as well as a higher halfwave potential (E 1/2 = 0.73 V vs RHE) and onset potential (E 0 = 0.87 V vs RHE). In comparison with the 3% Co-CeO 2 /N, S-rGO (Figure 6b), other samples show electrocatalytic activity with limited current density (1.4 mA/cm 2 for CeO 2 , 3.4 mA/cm 2 for CeO 2 /N, S-rGO, 3.6 mA/cm 2 for 1.5% Co-CeO 2 /N, S-rGO and 3.0 mA/cm 2 for 4.5% Co-CeO 2 /N, S-rGO), halfwave potential (0.45 V for CeO 2 , 0.67 V for CeO 2 /N, S-rGO, 0.70 V for 1.5% Co-CeO 2 /N, S-rGO and 0.71 V for 4.5% Co-CeO 2 /N, S-rGO) and onset potential (0.67 V for CeO 2 , 0.82 V for CeO 2 /N, S-rGO, 0.86 V for 1.5% Co-CeO 2 /N, S-rGO and 0.89 V for 4.5% Co-CeO 2 /N, S-rGO). Based on the ORR results, the 3% Co-CeO 2 /N, S-rGO exhibits superior electrocatalytic activity among the synthesized catalysts.   In order to further understand the electron transfer kinetics of the 3% Co-CeO2/N, S-rGO and the Co-CeO2/N, S-rGO composites (Figures 6c and 7a-c) during the ORR, RDE measurements were carried out at 10 mV s −1 and different electrode rotation speeds in O2saturated 0.1 M KOH solution. When increasing the rotation speed from 400 rpm to 1600 rpm, the diffusion current densities increased, while the onset potential was unchanged. To estimate the electron transfer number per O2 molecule during the ORR by the Kou-  In order to further understand the electron transfer kinetics of the 3% Co-CeO 2 /N, S-rGO and the Co-CeO 2 /N, S-rGO composites (Figures 6c and 7a-c) during the ORR, RDE measurements were carried out at 10 mV s −1 and different electrode rotation speeds in O 2 -saturated 0.1 M KOH solution. When increasing the rotation speed from 400 rpm to 1600 rpm, the diffusion current densities increased, while the onset potential was unchanged. To estimate the electron transfer number per O 2 molecule during the ORR by the Koutecky-Levich equation, as follows [59]: where j k , j d and j l is kinetic, diffusion limiting and total current density, respectively; B and ω are the proportionality coefficient and angular velocity of the disk, respectively; C is the bulk concentration of oxygen; D is the diffusion coefficient of oxygen in 0.1 M KOH; F is the Faraday constant (96,485 C·mol −1 ); ν is the kinematic viscosity, and n is the number of electrons transferred per oxygen molecule. Figure 6d shows the K-L plots of 3% Co-CeO 2 /N, S-rGO, which exhibit a linear and parallel relationship with different potentials and rpms, suggesting that it abides by a first-order kinetic reaction during the ORR. where jk, jd and jl is kinetic, diffusion limiting and total current density, respectively; B and ω are the proportionality coefficient and angular velocity of the disk, respectively; C is the bulk concentration of oxygen; D is the diffusion coefficient of oxygen in 0.1 M KOH; F is the Faraday constant (96485 C⋅mol −1 ); ν is the kinematic viscosity, and n is the number of electrons transferred per oxygen molecule. Figure 6d shows the K-L plots of 3% Co-CeO2/N, S-rGO, which exhibit a linear and parallel relationship with different potentials and rpms, suggesting that it abides by a first-order kinetic reaction during the ORR. The enhanced ORR performance can be presumed by considering the following facts: (1) The defects such as oxygen vacancy play an essential role in the ORR; (2) The synergistic impact of nitrogen and sulfur co-doping on rGO enhances the electrocatalytic performance towards ORR: CeO2 possesses a remarkable ability to reversibly exchange oxygen. CeO2 is frequently used as an oxygen buffer due to the redox reaction between Ce 4+ /Ce 3+ (Equation (3)). It can store oxygen in the oxygen-rich environment as well as produce oxygen in oxygen insufficient conditions. When oxygen escapes from the structure of CeO2, the reduction of Ce 4+ to Ce 3+ occurs. The oxidation of Ce 3+ to Ce 4+ occurs while oxygen is adsorbed on the CeO2 The enhanced ORR performance can be presumed by considering the following facts: (1) The defects such as oxygen vacancy play an essential role in the ORR; (2) The synergistic impact of nitrogen and sulfur co-doping on rGO enhances the electrocatalytic performance towards ORR: CeO 2 possesses a remarkable ability to reversibly exchange oxygen. CeO 2 is frequently used as an oxygen buffer due to the redox reaction between Ce 4+ /Ce 3+ (Equation (3)). It can store oxygen in the oxygen-rich environment as well as produce oxygen in oxygen insufficient conditions. When oxygen escapes from the structure of CeO 2 , the reduction of Ce 4+ to Ce 3+ occurs. The oxidation of Ce 3+ to Ce 4+ occurs while oxygen is adsorbed on the CeO 2 surface. Through N doping in graphene oxide, it creates a positive charge on neighboring carbon atoms and generates a structural distortion, leading to increased O 2 adsorption and higher defect density, resulting in more active sites to improve the ORR performance. Furthermore, because sulfur has a greater atomic size than carbon, it may induce more flaws and strain, enhancing O 2 adsorption while simultaneously enhancing the weakening of the O-O bond. As a result, the synergistic effect of nitrogen and sulfur co-doping on rGO and Co-doped CeO 2 improves the ORR electrocatalytic performance. Moreover, oxygen buffering capacity is highly responsible for the enhanced ORR activity of the Co-CeO 2 /N, S-rGO composite. Durability is a crucial factor for the ORR, which can be studied by ADT (accelerated durability test) 0.1 M KOH solution and a scan rate of 10 mV s −1 (vs RHE) with an electrode rotation speed of 1600 rpm. As shown in Figure 8, after 5000 cycles the 3% Ce-CeO 2 /N, S-rGO exhibits a much less negative halfwave potential shift (∆E 1/2 =~4 mV), which demonstrate the long-term stability of 3% Ce-CeO 2 /N, S-rGO.
Catalysts 2022, 11, x 9 of 13 oxygen buffering capacity is highly responsible for the enhanced ORR activity of the Co-CeO2/N, S-rGO composite. Durability is a crucial factor for the ORR, which can be studied by ADT (accelerated durability test) 0.1 M KOH solution and a scan rate of 10 mV s −1 (vs RHE) with an electrode rotation speed of 1600 rpm. As shown in Figure 8, after 5000 cycles the 3% Ce-CeO2/N, S-rGO exhibits a much less negative halfwave potential shift ( E1/2 = ~4 mV), which demonstrate the long-term stability of 3% Ce-CeO2/N, S-rGO.

Materials
Graphite powder, cerium nitrate hexahydrate and cobalt chloride hexahydrate were purchased from Sigma-Aldrich. Thiourea was purchased from Alfa Aesar. All required solutions were prepared from Milli-Q reagent water.

Synthesis of N, S-rGO
Graphene oxide (GO) was prepared by using the modified Hummers method [60,61]. Initially, GO suspension was prepared by dispersing 60 mg of GO in 40 mL of DI water ultrasonically for 2 h. To the above suspension, 250 mg of thiourea was slowly added and kept for stirring (6 h). Then, the solution was transferred to a Teflon-lined stainless steel, and then heated at 180 °C for 12 h. The obtained precipitate was washed several times with DI water and dried at 60 °C for 24 h.

Materials
Graphite powder, cerium nitrate hexahydrate and cobalt chloride hexahydrate were purchased from Sigma-Aldrich. Thiourea was purchased from Alfa Aesar. All required solutions were prepared from Milli-Q reagent water.

Synthesis of N, S-rGO
Graphene oxide (GO) was prepared by using the modified Hummers method [60,61]. Initially, GO suspension was prepared by dispersing 60 mg of GO in 40 mL of DI water ultrasonically for 2 h. To the above suspension, 250 mg of thiourea was slowly added and kept for stirring (6 h). Then, the solution was transferred to a Teflon-lined stainless steel, and then heated at 180 • C for 12 h. The obtained precipitate was washed several times with DI water and dried at 60 • C for 24 h.

Synthesis of Co-CeO 2 /N, S-rGO Nanocomposites
Co-CeO 2 /N, S-rGO nanocomposites were synthesized through a hydrothermal method by dispersing 60 mg of GO in 30 mL of 2 M sodium hydroxide solution and kept stirring for about 1 h. To the above solution, 50 mM of cerium nitrate hexahydrate (8 mL) was added slowly and kept stirring for 30 min. To the mixture, cobalt nitrate hexahydrate was added by varying the Co (1.5% and 4.5%) concentration for doping. Then, the hydrothermal reaction was carried out at 120 • C for 12 h after transferring the solution into a Teflon-lined stainless steel autoclave. Subsequently, the precipitate was washed several times with DI water and dried at 60 • C for 24 h.

Characterization
Phase purity of the sample was examined by using powder X-ray diffraction (XRD) (PAN Analytical, Almelo, The Netherlands). Morphology of the sample was observed by using SEM (FEI Quanta FEG 200, Zürich. Switzerland). The JEM Fb-2000 instrument was used to obtain the transmission electron microscopy (TEM) images. For the XPS spectra, a Perkin Elmer PHI 550 spectrometer (Norwalk, CA, USA) was used. A Raman spectrometer (LAB RAMAN HR Evolution HORIBA, Paris, France) was used to analyze the Raman spectra of the sample.

Electrochemical Techniques
The electrochemical characterizations were studied by using a CHI 760E electrochemical workstation. The experimental setup consists of Ag/AgCl (sat.KCl) as reference electrode, a thin Pt wire as counter electrode and a glassy carbon-rotating disc electrode (GC-RDE, 0.196 cm 2 ) as a working electrode. An amount of 20% Pt/C (standard catalyst) bought from Alfa Aesar, (Chennai, india)(Pt/C), was used for the comparison of the ORR studies. Then, the catalyst ink was prepared by dispersing 2.5 mg of electrocatalyst in 1 mL of ethanol and water (1:1), 20 µL Nafion (5 wt.% solution), followed by sonication for 30 min. Then, 10 µL of the catalyst ink was dropped on the surface of the polished RDE-GC and dried under an IR lamp.

Conclusions
In summary, we have synthesized the Co-CeO 2 /N, S-rGO composite with the hydrothermal method. The structural, morphological and elemental compositions have been performed on this system by XRD, Raman spectra, FESEM, TEM and XPS. The electrochemical ORR performance of the catalyst was studied by measuring CV and LSV in oxygen-saturated 0.1 M KOH solution. The electrochemical ORR performance measurements showed that the Co doping on CeO 2 and the composite with N, S-rGO exhibited an apparent catalytic activity in alkaline media, and the synthesized 3% Co-CeO 2 /N, S-rGO exhibited the highest electrocatalytic performance. The enhancement is ascribed to the Co doping, offering oxygen vacancy and the utilization efficiency of active sites, with the Nand S-doped conductive rGO support aiding the charge transport during the ORR. We believe that the Co-CeO 2 /N, S-rGO composite will produce very promising and low-cost catalyst fuel cells and metal-air batteries.