Transition metal ion-chelating ordered mesoporous carbons as noble metal-free fuel cell catalysts
Polymer electrolyte membrane fuel cells offer promising possibilities for clean and efficient chemical to electrical energy conversion, though their high price makes them financially unappealing. This is to a large part due to the high cost of the noble metal catalyst used in fuel cells, which makes development of new, cheap and active catalyst materials necessary. Iron- and cobalt-containing metalorganic materials have shown great promise in this respect and major performance improvements of such materials were reported lately.
This work focuses on a new synthesis approach for non-noble metal-containing fuel cell catalysts. The catalysts were synthesised in a one pot approach, by a modified synthesis route for highly ordered mesoporous carbons (OMCs). This is the first study where the concepts of OMCs and transition metal metalorganic catalysts are combined. Nitrogen-containing carbon precursors and iron/cobalt salts were included in the OMC synthesis, thus the formed active sites were directly incorporated in the highly porous, electron conducting carbon support material. Advantages of such a synthesis approach are the high specific surface area relative to conventional Vulcan support materials and the high tuneability of the synthesis route.
Changes of the meso- and atomic structure were studied with respect to their influence on catalyst activity. Electrochemical characterisation was done in a single cell fuel cell setup, including a dedicated study concerning membrane electrode assembly preparation for functionalized OMC catalysts. Nitrogen physisorption, SAXS, HR-TEM, SEM, XANES and EXAFS were used to examine the meso- and atomic structure. EDX and EELS were used for elemental analysis.
The results presented here illustrate that it is possible to synthesize carbon-supported metalorganic oxygen reduction reaction (ORR) catalysts in a one pot approach where Fe and Co ions are organically bound instead of present as metallic or oxide particles. It was also shown that it is possible to vary to the material structure on the mesoscale without disturbing the atomic-scale active site structure. This makes efficient tuning of the surface area, pore size and pore wall thickness of the OMC possible.