Development of Carbon Nanofiber Based Non-Precious Metal Catalysts for Oxygen Reduction Reaction

Cost reduction is the most crucial issue in commercialization of polymer electrolyte membrane fuel cells (PEMFCs). According to DOE’s analysis, expensive platinum (Pt) catalyst & its application account for almost half of the PEMFCs stack cost. (500,000 vehicles/year, DOE Annual Merit Review, 2013). For that reason, non-precious metal catalysts (NPMCs) have been extensively developed in recent dates to replace Pt catalyst.  

Oxygen reduction reaction (ORR) is the main focus in development of NPMCs because of its sluggish reaction kinetics. Previous reports examined that carbon based catalysts, which contain transition metal-nitrogen bond, exhibit high activity, stability, and selectivity for ORR [1-5]. These catalysts are usually synthesized by high temperature pyrolysis of precursor mixture for transition metal (Co or Fe), nitrogen, carbon, and support material. However, it is difficult to prepare uniform mixture using many precursors, and accompanied by complicated synthesis route.

 Electrospinning is a facile technique to make 1-D structured nanofiber by applying high voltage between viscous polymer solution injection needle and conductive bottom plate. The electrospun nanofiber provides high surface area without support materials. And, polyacyrlronitrile (PAN), which is common precursor for CNF, can act as source for both carbon and nitrogen because PAN has nitrogen containing nitrile group. According to Kim`s doctoral thesis [6], only PAN and cobalt acetate were used for making precursor mixture, and Co-CNF catalysts after the high temperature pyrolysis exhibit high ORR activity comparable with the commercial Pt catalyst in alkaline solution as shown in Fig. 1. However, ORR activity of Co-CNF catalysts is still lower than Pt catalysts even in large amount of catalyst usage, especially in acidic solution. 

In this study, we were trying to maximize ORR active sites of CNF based catalysts by modification of CNF structure and transition metal composition. To make porous CNF, other polymer materials, such as PMMA, and PVP, were added to electrospinning solution. These polymer materials were evaporated at high temperature pyrolysis, and then remaining CNF has porous structure. The porous CNF exhibit enhanced ORR activity than CNF in alkaline solution by increase in surface area. Heat-treatment under reactive gas, such as NH₃, also enables to gain porous structure by gasification of carbon. And, Fe addition in Co-CNF catalysts leads to increase in on-set potential. The Fe,Co-CNF catalysts exhibit same on-set potential with the commercial Pt catalyst as shown in Fig. 3. By optimization of these strategies, we plans to synthesize highly active CNF based NPMCs in alkaline solution and even in acidic solution for future works. 

Fig. 1 Measured ORR activity of CNF and Co-CNF catalysts in 0.1 M KOH solution.        

Fig. 2 TEM image of porous CNF

Fig. 3 Measured ORR activity of Co-CNF and Fe,Co-CNF catalysts in 0.1 M KOH solution.             

References

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