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

Abstract

N, S-doped rGO was successfully synthesized and embedded Co-doped CeO₂ 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 CeO₂ leads to a synergistic effect in catalyzing the ORR in alkaline media. Co–CeO₂/N, S–rGO displays enhanced ORR performance compared to bare CeO₂. The superior stability of the prepared catalyst implies its potential applications beyond fuel cells and metal–air batteries.

1. 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 CeO₂ [6,7,8,9], MnO₂ [10,11,12], TiO₂ [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. CeO₂ has been widely used in fuel cells (MOR, ORR) [18,19,20]. Precious metals like Pt and Pd supported on CeO₂ 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 CeO₂-supported catalysts for the ORR, there are very few studies focusing on the importance of oxygen vacancy in doped CeO₂ [27,28,29]. Furthermore, the abundance of oxygen vacancies in CeO₂ favors O₂ adsorption and oxygen conductivity due to the coexistence of Ce⁴⁺ and Ce³⁺, which contribute to the CeO₂-supported electrocatalyst in the ORR [30,31]. However, the poor electronic conductivity of CeO₂ limits its ORR performance. Depositing CeO₂ 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 graphene oxide with metal oxide hybrid materials has emerged as a promising ORR electrocatalyst with huge potential [37,38,39,40,41]. In these cases, N doping in graphene oxide provides a positive charge on neighboring carbon atoms through structural distortion, which leads to an increase of O₂ adsorption and higher defect density, resulting in more active sites to enhance the ORR performance [42,43,44]. Moreover, the atomic size of sulfur is larger than carbon and may create more defects and strain, which enhance the O₂ adsorption and accelerate the weakening of the O–O bond [45,46]. Therefore, the synergistic impact of nitrogen and sulfur co-doping on rGO enhances the electrocatalytic performance towards the ORR. According to the findings of several studies, it can be concluded that transition metal-doped CeO₂/N, S–rGO support could be an effective strategy to improve electrocatalytic activity toward the ORR.

Herein, we prepared a Co–CeO₂/N, S-doped rGO composite with the hydrothermal method. The synthesized sample’s crystal structure, morphology and elemental oxidation state were confirmed by XRD, Raman spectra, SEM, TEM and XPS analyses. Then, the electrochemical performance of the Co–CeO₂/N, S–rGO composite was performed in alkaline media.

2. Results and Discussion

2.1. XRD

The XRD patterns of Co-doped CeO₂ and Co–CeO₂/N, S–rGO composites are shown in Figure 1. The diffraction pattern of CeO₂ 2$\theta$ values at 28.47$°$, 33.0°, 47.46°, 56.31°, 59.12°, 69.41° and 76.70° correspond to the (111), (200), (220), (311), (322), (400) and (331) planes of FCC CeO₂ (JCPDS no: 34-0394) [47], respectively. The XRD patterns of Co–CeO₂ reveal that during the Co doping, the peak intensity decreases, and the peaks slightly shift to a higher angle in comparison with bare CeO₂, which indicates the doping of the Co ion into the CeO₂ lattice. The peak intensity of the Co-doped CeO₂ is much weaker than CeO₂, which can be created by the ionic radii difference between CeO₂ and Co-doped CeO₂. Additionally, the ionic radius of Co^2+/3+ is 0.75−0.9 Å and Ce⁴⁺ 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 CeO₂. 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 CeO₂ and N,S–rGO, which confirm the formation of Co-doped CeO₂ and the N, S–rGO hybrid.

2.2. 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 sp³ structural defect sites, and the G band is associated with the sp²-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–CeO₂/N, S–rGO, respectively. The 3% Co–CeO₂/N, S–rGO exhibits a higher I_D/I_G 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.

2.3. 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.

2.4. 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 3d_3/2 and Ce 3d_5/2 were labelled as U and V, respectively. The characteristic binding energy peaks of U‴ (916.2 eV), V‴ (899.7 eV), U𠌲 (907.6 eV), V″ (889.2 eV), U (901.7 eV) and V (883 eV) are ascribed to Ce⁴⁺ species. While the peaks centered at U_o (899.1 eV), V_o (881.1 eV), U′ (904.9 eV) and V′ (886.1 eV) were attributed to the Ce³⁺ [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 CeO₂ [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].

2.5. 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 Co dopant. The cathodic oxygen reduction onset potential increases in the following order as 3% Co–CeO₂/N, S–rGO > 1.5% Co–CeO₂/N, S–rGO > 4.5% Co–CeO₂/N, S–rGO > CeO₂/N, S–rGO. Moreover, the 3% Co–CeO₂/N, S–rGO exhibits a higher current density than the samples. This result demonstrates an enhanced ORR activity due to the Co doping on CeO₂ 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 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 CeO₂ improves the ORR performance, which can be attributed to the oxygen vacancy creation through Co doping and the coexistence of Ce³⁺ and Ce⁴⁺. The optimized 3% Co–CeO₂/N, S–rGO hybrid electrocatalyst shows better ORR performance (

Table 1. Comparison of ORR activity of doped CeO₂ with rGO based catalyst.

S.no	Catalyst	Onset Potential (V vs Ag/AgCl)	Halfwave Potential (V vs Ag/AgCl)	Limiting Current Density (mA/cm2)	References
1	CeO2/N–rGO	−0.15 V	-	4.0	[32]
2	5% Mn–CeO2/rGO	−0.136 V	−0.336 V	4.8	[57]
3	5% Ni–CeO2/rGO	−0.170 V	−0.300 V	3.6	[31]
4	3% Co–CeO2/rGO	−0.223 V	−0.295 V	-	[58]
5	3% Co–CeO2/N, S–rGO	−0.124 V	−0.261 V	4.2	Present work

) compared to bare CeO₂ and 3% Co–CeO₂. After the combination of Co–CeO₂ and N, S–rGO, the 3% Co–CeO₂/N, S–rGO has a high limiting current density (J_L = 4.2 mA/cm²) as well as a higher halfwave potential (E_1/2 = 0.73 V vs RHE) and onset potential (E⁰ = 0.87 V vs RHE). In comparison with the 3% Co–CeO₂/N, S–rGO (Figure 6b), other samples show electrocatalytic activity with limited current density (1.4 mA/cm² for CeO₂, 3.4 mA/cm² for CeO₂/N, S–rGO, 3.6 mA/cm² for 1.5% Co–CeO₂/N, S–rGO and 3.0 mA/cm² for 4.5% Co–CeO₂/N, S–rGO), halfwave potential (0.45 V for CeO₂, 0.67 V for CeO₂/N, S–rGO, 0.70 V for 1.5% Co–CeO₂/N, S–rGO and 0.71 V for 4.5% Co–CeO₂/N, S–rGO) and onset potential (0.67 V for CeO₂, 0.82 V for CeO₂/N, S–rGO, 0.86 V for 1.5% Co–CeO₂/N, S–rGO and 0.89 V for 4.5% Co–CeO₂/N, S–rGO). Based on the ORR results, the 3% Co–CeO₂/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 (Figure 6c and Figure 7a–c) during the ORR, RDE measurements were carried out at 10 mV s⁻¹ and different electrode rotation speeds in O₂-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 O₂ molecule during the ORR by the Koutecky–Levich equation, as follows [59]:

1/j = 1/j_k + 1/j_d = 1/j_k + 1/(Bω^1/2)

(1)

B = 0.62nFCD^2/3 ν^−1/6

(2)

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–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:

2CeO₂→2CeO_2−x + xO₂ (0 ≤ x ≤ 1)

(3)

CeO₂ possesses a remarkable ability to reversibly exchange oxygen. CeO₂ is frequently used as an oxygen buffer due to the redox reaction between Ce⁴⁺/Ce³⁺ (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 Ce⁴⁺ to Ce³⁺ occurs. The oxidation of Ce³⁺ to Ce⁴⁺ occurs while oxygen is adsorbed on the CeO₂ surface. Through N doping in graphene oxide, it creates a positive charge on neighboring carbon atoms and generates a structural distortion, leading to increased O₂ 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 O₂ 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 CeO₂ improves the ORR electrocatalytic performance. Moreover, 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 ($\mathsf{\Delta }$E_1/2 = ~4 mV), which demonstrate the long-term stability of 3% Ce–CeO₂/N, S–rGO.

3. Materials and Methods

3.1. 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.

3.2. 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.

3.3. Synthesis of Co–CeO₂/N, S–rGO Nanocomposites

Co–CeO₂/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.

3.4. 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.

3.5. 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 cm²) 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.

4. Conclusions

In summary, we have synthesized the Co–CeO₂/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 CeO₂ and the composite with N, S–rGO exhibited an apparent catalytic activity in alkaline media, and the synthesized 3% Co–CeO₂/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 N- and S-doped conductive rGO support aiding the charge transport during the ORR. We believe that the Co–CeO₂/N, S–rGO composite will produce very promising and low-cost catalyst fuel cells and metal–air batteries.