To reduce greenhouse gases (GHG) emission, we need to move away from fossil-based energy sources. Electrochemical energy conversion systems (EECSs), like fuel cells, are broadly considered to be alternative systems to the currently dominant internal combustion energy-generation systems (ICESs). Among several types of fuel cells, polymer electrolyte fuel cells (PEFCs) are the most suitable one for vehicle applications. Nanoparticle platinum (Pt) catalysts are now used as state-of-the-art catalyst for the PEFCs. However high price, scarcity, and monopolized global distribution of Pt place significant limitations on these energy conversion systems. Particularly oxygen reduction reaction (ORR) at the cathode is inherently slower by six orders of magnitude than hydrogen oxidation reaction at the anode, thus requiring higher Pt loading. Platinum group metal free (PGM-free) ORR catalyst development thus has been a continuous research theme for several decades by many research groups.

Metal-nitrogen-carbon (M-N-C) type catalysts have demonstrated the highest activity and durability among several types of PGM-free catalysts. Recently there has been a significant improvement in ORR activity, however further improvement in activity is still needed to compete with Pt catalyst. Furthermore good durability of these catalysts has not been demonstrated. Understanding of active site that has not been clarified yet is a core for solving these issues.

Recently we directly observed FeN₄ moiety in (CM+PANI)-Fe-C catalysts [1]. If FeN₄ is an active site for ORR, increasing the number of this moiety, i.e., making atomically dispersed (AD) Fe, will be a pathway for improving the ORR activity. Thus to achieve high density of FeN₄, we synthesized fiber-type zeolitic imidazolate framework (ZIF-F) as a precursor for (AD)-Fe-N-C catalysts (Fig. 1 (a)), in which FeN₄ structure already exist. Heat-treatment converts this ZIF-F into fibrous N-doped carbons (Fig. 1(b). Fe atoms are dispersed atomically without aggregation in the fibrous N-doped carbons (Fig. 1(c)). Importantly, electron energy loss spectroscopy (EELS) demonstrates that N is cordinated to the Fe atoms (Fig. 1(d)). Thus we could construct FeN_x moieties within the (AD)-Fe-N-C catalysts.

In this presentation, we will present the activity and durability of this (AD)-Fe-N-C catalyst in rotating disk electrode (RDE) and fuel cells in conjunction with diverse analysis tools. This will give some insights for the nature of activity/durability for M-N-C type ORR catalysts.

1. Hoon T, Chung, David A. Cullen, Drew Higgins, Brian T. Sneed, Edward F. Holby, Karren L. More, Piotr Zelenay, “Direct atomic-level insignt into the active sites of a high-performance PGM-free ORR catalyst”, Science, 357, 479 (2017).

Acknowledgments

This research is supported by DOE Fuel Cell Technologies Office, through the Electrocatalysis Consortium (ElectroCat).