Design of Highly Active and Durable Nanostructured Carbon-Based Oxygen Reduction Electrocatalysts

Abstract

The development of highly active, durable, and low-cost oxygen reduction reaction (ORR) catalysts is central to making polymer electrolyte fuel cell and Li-air battery technologies commercially viable. In this regards, tremendous recent efforts have been devoted to replacing expensive, scarce Pt-based electrocatalysts with non-precious metal or metal-free materials for the ORR. In this presentation, we show our recent efforts toward new electrocatalysts for the ORR based on nanoporous/nanostructured carbons including mesoporous carbon, carbon nanotubes (CNTs), and graphene [1-6]. We present transition metal and nitrogen co-doped ordered mesoporous porphyrinic carbons (M-OMPC) with high surface areas and tunable pore structures, which were prepared by nanocasting mesoporous silica templates [1]. A family of M-OMPC catalysts generally showed very high ORR activity in an acidic medium. Particularly, the most active FeCo-OMPC represented one of the best performances among the non-precious metal catalysts (NPMCs). The characterizsation by EXAFS and high-resolution TEM revealed the molecular face-to-face porphyrin-like active site structure in the FeCo-OMPC catalyst. We further demonstrate that the nanocasting with Ni and Fe macrocyclic compounds generate graphitic shell embedded mesoporous carbon (GNS/MC) structure, which exhibits very high activity and durability for both oxygen evolution and reduction reactions in an alkaline medium [2]. We also show highly active, durable CNT-based core-sheath structure catalysts [4,5]. CNTs wrapped with ionic liquid-driven, heteroatom-doped carbon (CNT/HDC) structure showed an excellent ORR activity in an alkaline solution, which is one of the best performances among the metal-free, heteroatom-doped nanocarbon catalysts [4]. The CNT/HDC also showed high current and power densities when employed as cathode catalysts in alkaline fuel cell. In addition, CNTs with metallomacrocycle-driven sheath layer exhibited very high ORR activity in both alkaline and acidic medium, and demonstrated excellent performance in both anion and proton exchange membrane fuel cells [5]. [1] J. Y. Cheon et al., Scientific Reports 3, 2715 (2013). [2] J. Y. Cheon et al., Adv. Energy Mater. Revised (2015). [3] J. Y. Cheon et al., J. Am. Chem. Soc. 136, 8875 (2014). [4] Y. J. Sa et al., Angew. Chem. Int. Ed. 53, 4102 (2014). [5] J. Y. Cheon et al., Manuscript in preparation (2015). [6] J. Han et al., Angew. Chem. Int. Ed. 54, 12622 (2015).