Elsevier

Journal of Power Sources

Volume 360, 31 August 2017, Pages 106-113
Journal of Power Sources

N, S co-doped carbon spheres with highly dispersed CoO as non-precious metal catalyst for oxygen reduction reaction

https://doi.org/10.1016/j.jpowsour.2017.05.120Get rights and content

Highlights

  • N, S co-doped CSs with highly dispersed CoO were prepared via one-step pyrolysis.

  • CoO@NS-CSs exhibits excellent ORR activity through a four-electron pathway.

  • CoO@NS-CSs has much better long-term stability and methanol-tolerance than Pt/C.

  • High surface area of CoO@NS-CSs facilitates exposure of more active sites.

  • Pyridinic N, graphitic N and highly dispersed CoO play essential roles for ORR.

Abstract

It is still a great challenge in preparing non-precious metal catalysts with high activity and long-term stability to substitute for precious metal catalysts for oxygen reduction reaction (ORR) in fuel cells. Herein, we report a novel and facile catalyst-N, S co-doped carbon spheres with highly dispersed CoO (CoO@NS-CSs), where biomass glucose spheres act as carbon precursor and H2S, NH3 derived from the decomposition of thiourea not only provide N, S sources but also can etch carbon spheres to produce nanoporous structure. CoO@NS-CSs catalyst exhibits excellent ORR activity with a high onset potential of 0.946 V vs. RHE (reversible hydrogen electrode) and a half-wave potential of 0.821 V vs. RHE through a four-electron pathway in alkaline solution, which is comparable to commercial Pt/C catalyst (onset potential: 0.926 V vs. RHE, half-wave potential: 0.827 V vs. RHE). Furthermore, both the long-term stability and methanol-tolerance of CoO@NS-CSs catalyst are superior to those of commercial Pt/C catalyst. The excellent ORR performance of CoO@NS-CSs catalyst can be attributed to its micro-mesopore structure, high specific surface area (667 m2 g−1), and highly dispersed CoO. This work manifests that the obtained CoO@NS-CSs catalyst is promising to be applied to fuel cells.

Introduction

Fuel cells, as clean and efficient energy conversion devices, have attracted more and more attention. Nevertheless, at the cathode of fuel cells, the oxygen reduction reaction (ORR) is a sluggish step, which restricts the efficiency of the overall fuel cells [1], [2], [3]. Therefore, it is great significant to develop an effective catalyst to boost ORR. Up to now, there are many great achievements to improve ORR. Among the catalysts for ORR, precious metals (Pt) and their alloys-based catalysts are considered the most effective one [4], [5], [6]. However, Pt-based catalysts cannot be extensively applied to the commercial products due to their disadvantages including high-cost, scarcity, poor stability and methanol tolerance [6], [7]. Therefore, there is a strong popular interest in developing high effective and long-term stability non-precious metal catalysts to solve this bottleneck, and thus promote ORR.

Carbon materials are widely applied to the material fields due to their good electronic conductivity. Heteroatoms doping carbon materials can effectively change the electron state of carbon materials and increase the structure defects which can contribute to the formation of active sites, and then improve the ORR performance. In the past few decades, heteroatoms (Nitrogen [8], [9], [10], [11], [12], Phosphorus [7], [13], [14], Sulfur [15], [16], [17], [18], Boron [14], [19], Iodine [20], [21]) doped and co-doped carbon materials, such as carbon nanotube, nanocarbon foam, graphene, have been reported for ORR. Due to the larger electronegativity of N atom than C atom (N: 3.04, C: 2.55), N-doped carbon array can result in the charge delocalization and then promote the adsorption of oxygen on catalysts [11], [22]. In the aspect of electronegativity, there is no much difference between C and S (2.58). Therefore, S-doping can give rise to asymmetric electron spin redistribution rather than charge delocalization induced by N-doping [23]. Thus, it is expected that N, S co-doped carbon materials could further promote ORR performance through the synergetic effect between N and S atoms.

Recently, heteroatoms doping carbon materials containing transition metal ions have become the research hotspots. Transition metal compounds possess good ORR catalytic activity. Therefore, non-precious metal catalysts (e.g. Fe [24], [25], [26], [27], [28], Co [29], [30], [31], [32], [33], [34], Mn [35], [36] etc.) are showing promising potential to replace the precious metal catalysts. However, the poor electronic conductivity limits the application of non-precious metal compounds in electrocatalysis [30], [37]. Hence, an accepted method to overcome this problem is loading transition metal compounds on the conductive carbon materials. The common method for preparing metal based catalysts is via pyrolysis of precursors including metal and heteroatom doping carbon materials. Glucose, a low-cost and green biomass material, can polymerize into glucose spheres without any template through hydrothermal treatment. Glucose spheres can be excellent carbon precursor due to the high surface area, good thermal stability and good conductivity [38]. Moreover, the surface of glucose spheres is rich in negative hydroxyl groups, which facilitates to combine with metal cations.

Herein, we report a new and facile strategy for fabricating N, S co-doped carbon spheres with highly dispersed CoO (CoO@NS-CSs) via one-step pyrolysis, where biomass glucose spheres (GSs) acts as carbon precursor and H2S, NH3 derived from the decomposition of thiourea serve as N, S sources. Meanwhile, NH3 and H2S can etch GSs to optimize the nanoporous structure. The prepared CoO@NS-CSs exhibits high electrocatalytic activity for ORR in the alkaline solution through a four-electron pathway, which could be comparable to commercial Pt/C (20 wt%). Moreover, compared to Pt/C, CoO@NS-CSs exhibits better durability and methanol tolerance.

Section snippets

Materials

d-glucose, cobalt nitrate hexahydrate (Co(NO3)2·6H2O), thiourea, formaldehyde, ethanol, and sodium hydroxide were purchased from Sinopharm chemical reagent Co. Ltd (Shanghai, China). Nafion solution (5.0 wt%) was purchased from DuPont. Pt/C (20 wt% platinum supported on Vulcan XC-72 carbon) was purchased from Sigma. All the reagents were analytical grade and used without further purification.

Synthesis of glucose spheres (GSs)

Glucose spheres were prepared using a modified method [39], [40]. Typically, 2.48 g glucose and 2 g

Results and discussion

Scheme 1 illustrates the synthetic process of CoO@NS-CSs. Glucose molecules were polymerized into GSs by hydrothermal treatment with formaldehyde as surfactant and dispersant. In weak base condition, aldol condensation reaction between the formyl group of formaldehyde and the hydroxyl of GSs would occur. Therefore, formaldehyde contributes to modify the surface morphology, control the size of GSs and prevent from GSs agglomeration. Due to rich in hydroxyl on the surface of GSs, GSs could

Conclusions

In summary, we have successfully developed a facile method to prepare N, S co-doped carbon spheres with highly dispersed CoO (CoO@NS-CSs) as non-precious metal catalyst for ORR, where biomass glucose spheres (GSs) act as carbon precursor and H2S, NH3 derived from the decomposition of thiourea serve as N, S sources and pore-forming gases. CoO@NS-CSs catalyst exhibits excellent ORR activity in alkaline solution, which is comparable to commercial Pt/C catalyst. Furthermore, both the long-term

Acknowledgments

The authors thank the support of National Natural Science Foundation of China (Nos. 51371086). The authors also thank the support of analytical and testing center of Huazhong University of Science and Technology.

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