(621ej) Activity of Non-Precious Fuel Cell Catalysts Towards ORR

Authors: 
Poudyal, S., University of Tennessee
Laursen, S., University of Tennessee

State of the art non-noble metal ORR catalysts based on first row transition metals embedded in a N2 rich carbon support have been shown to exhibit overpotentials comparable to Pt. These materials are promising replacements for Pt in low temperature fuel cells, however they still suffer from low current-to-volume ratios due to low density of surface reaction sites, over oxidation and leaching of TM metals under reaction conditions. The lack of understanding of the nature of catalytically active site is another major hindrance in the optimization of non-noble metal ORR catalysts. A combination of the first principles quantum chemical modeling and experiments has been applied to understand the chemical composition of active sites of FeNx/C and CuNx/C catalysts, and how the chemical activity of these existing catalysts could be replicated at high reaction site density. Our results indicate that the metal-rich nature of bulk Fe3C and Fe4N make these materials highly reactive towards oxygen and OH intermediates, while the reactivity decreased remarkably if less metal rich forms such as Fe2C and Fe3N were encountered. Our results also suggest that select combinations of transition metals and non-metals could be utilized to achieve an appropriate reactivity towards oxygen. For instance, investigations on several TM sulfides and phosphides such as FeS2, TiS, Si3N4, and Ni2P already show promise as potential ORR catalysts. Finally, the enhancement of the covalent bonding of TM ceramics to the carbon support via TM-S-C, TM-P-C, or TM-Se-C bonds could ultimately improve the catalyst stability, reduce the ohmic losses, and further improve the overall current-to-volume ratio.