(729e) Role of Transition Metal in Non-Noble Metal Electro-Catalysts for the Oxygen Reduction Reaction

Platinum is currently the commercial cathode catalyst used in Proton Exchange Membrane (PEM) Fuel Cells. Sluggish kinetics governing the oxygen reduction reaction necessitates the use of higher platinum loadings, which leads to an increase in the cost of a fuel cell stack, impeding its large scale commercialization. As a result, significant efforts have been directed towards either lowering cathodic loadings of platinum or synthesizing non-noble metal catalysts (NNMC) as replacements for platinum.

Two groups of NNMCs have emerged as potential replacements for platinum. One of them is inspired by macrocyclic complexes such as Fe- or Co-phthalocyanine, which contain a metal-center coordinated to nitrogen atoms. These NNMCs are synthesized from separate sources of a transition metal, nitrogen and carbon, with high surface area carbon impregnated with iron and a nitrogen-containing pore filler, and are subjected to two heat treatments,  in Ar followed by NH₃ (Fe-N-C catalysts). These catalysts show high initial activity, but rapidly degrade under fuel cell conditions.  A second group of catalysts is the nitrogen-containing carbon nanostructures (CN_x), which are synthesized over a transition-metal (Fe or Co) impregnated support (MgO, SiO₂, or Vulcan carbon) by acetonitrile decomposition at high temperature and  subsequently acid-leached to remove the inactive components.  These catalysts are not as active to start with, but retain their activity through extended voltammetry and fuel cell tests.  The nature of the active site and the role of the transition metal in these two groups of catalysts continue to be debated.   Our recent work focuses on elucidating the role of the transition metal in CN_x and Fe-N-C catalysts using various in-situ and ex-situ characterization techniques, such as X-ray photon spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge spectroscopy (XANES), temperature-programmed oxidation (TPO), superconducting quantum interference device (SQUID), and transmission electron microscopy (TEM).