Supercritical deposition coupled with ammonia treatment: a new route to co-promoted n-doped carbon aerogels with high oxygen reduction reaction activity

Abstract

A new highly active family of Co-promoted N-doped carbon aerogel catalysts is prepared by a supercritical CO2-assisted technique combined with NH3 treatment. The active sites are created during the complex transformation of an organic aerogel to a carbon aerogel in the presence of cobalt by NH3 as the source of nitrogen and pyrolysis medium. The mass activity for oxygen reduction reaction (ORR) is highest for the catalyst pyrolyzed at 800 degrees C. The catalyst displays similar activity for ORR as a commercial Pt/C catalyst. Moreover, the catalyst exhibits nearly double the mass activity of N-doped carbon aerogel pyrolyzed at the same temperature. The nature of the active sites for ORR is also investigated by selectively adding or removing various species (i.e., Co-N-x and Co-O) via post acid-base treatments. After removing the Co species from the catalyst, the same activity as the parent catalyst is achieved suggesting that the Co species in the catalyst are spectators in ORR catalysis and they serve only to promote the formation of catalytically active graphitic N sites during pyrolysis in NH3. A strong correlation is found between the relative distribution of graphitic N species and the mass activity.