Iron Encapsulated within Pod-like Carbon Nanotubes for Oxygen Reduction Reaction

摘要

Efforts to explore non-precious-metal cathode catalysts to replace the precious Pt in polymer electrolyte membrane fuel cells (PEMFC) have been going on for several decades to bring this efficient technology into real applications. Progress has been made through the use of macrocyclic molecules particularly those containing Fe-N4 and Co-N4 moieties, or high-temperature synthetic materials containing separately or collectively transition-metalitrogen/carbon elements. However, the non-precious-metal based catalysts frequently suffer from poor stability in acids owing to leaching.'3 Recently, Wu et al. reported an encouraging catalyst synthesized from high-surface-area carbon, polyaniline, iron and cobalt at a high temperature, which exhibited a high activity and stability as an oxygen reduction reaction (ORR) catalyst in PEMFC. The metal-containing particles were to a large extent encapsulated in onion-like carbon nanoshells or attached to carbon nanosheets. However, the nature of the active ORR catalytic sites was not clear. Our previous study demonstrated that the coordinatively unsaturated ferrous sites, when confined on the interface of Pt, are highly active and stable in activating oxygen. Herein we intend to design active and stable non-precious metal ORR catalysts with features of coordinatively unsaturated metal sites or low valence state metals physically isolated from the acid environment to avoid leaching, which however should not impede the catalytic properties of metals, and on this basis, to understand the behavior of active sites.