Optimization of Sulfurization Process of Cobalt Sulfide and Nitrogen Doped Carbon Material for Boosting the Oxygen Reduction Reaction Catalytic Activity in Alkaline Medium.

Bing-Ye Song,Sen Yao

doi:10.3389/fchem.2020.00314

Abstract

In order to reduce fuel cell material cost and promote its application, it is urgent to develop non-noble metal materials to replace platinum as the cathode catalysts in fuel cells. The cobalt sulfide and nitrogen co-doped carbon (S-Co-N/C) materials with metal-organic frameworks as precursors have shown good oxygen reduction reaction (ORR) catalytic activity. Benefiting from the protection of catalytic active sites by sulfur atoms, the stability and alcohol-tolerance of the S-Co-N/C catalyst can be significantly improved. In order to fully understand the effect of the sulfurization process on the properties of the material, zeolite imidazole frameworks (ZIF)-8, and ZIF-67 are used as precursors to prepare a novel material of S-Co-N/C by using a sulfurization-pyrolysis method. Another S-Co-N/C material by using a pyrolysis- sulfurization method is prepared for comparison. The effects of the sulfurization process in the preparation on the morphology, chemical structure, specific surface area, and ORR catalytic activity of the final material are investigated. The experimental results show that the surface of the S-Co-N/C material tends to be rough due to the sulfurization reaction of the metal elements. The porosity of the material is reduced to some extent due to the remaining Zn elements in the final product. Interestingly, some carbon nanotubes are found to be generated on the surface of the S-Co-N/C material because of the synergistic effect of Zn and Co on the carbon material during the pyrolysis process, which is beneficial to accelerate the adsorption of oxygen on the S-Co-N/C surface and the electron transportation during the oxygen reduction reaction. In addition, the generated CoS during the sulfurization process can further protect the Co elements from agglomeration, which can effectively increase the ORR catalytic active sites in the final material. The S-Co-N/C material prepared by the sulfurization-pyrolysis method performs a superior ORR catalytic activity to the one synthesized by the pyrolysis-sulfurization method.

Highlights

In the past 5 years, the technology of fuel cells has tended to be mature, which has accelerated its industrial development in the transport sector
On the other hand, during the synthetic process of S-Co-N/C-I, the prior sulfurization can promote the reaction of sulfur elements with Co and Zn to form CoS and ZnS, which can increase the roughness of the material after pyrolysis
The Zn elements remaining in the material after the sulfurization would cooperate with Co elements to generate carbon nanotubes on the surface of the carbon material

Summary

Introduction

In the past 5 years, the technology of fuel cells has tended to be mature, which has accelerated its industrial development in the transport sector. The high cost of fuel cell stacks is still one of the main factors limiting the commercialization of fuel cell applications (Alaswad et al, 2016; Sun et al, 2019). Due to the increasing scarcity of platinum and the maturity of fuel cell process technology, the cost of platinumbased materials relative to fuel cell system cost would gradual increase (Sun et al, 2011). In order to reduce the cost and achieve sustainable development of fuel cells, it is urgent to develop nonprecious metal materials for replacing platinum completely as the catalysts in the electrodes of fuel cells (Meng et al, 2019; Xu et al, 2019; Yang et al, 2019). In the cathodes, the sluggish oxygen reduction reaction (ORR) kinetics make cathode electrodes rely on platinum-based materials with excellent catalytic activity

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Results

Conclusion