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Low and Intermediate Temperature Electrocatalysis
Ever increasing energy demands and environmental concerns necessitate development of technologies which are clean, efficient and environmental friendly such as hydrogen based proton exchange membrane (PEM) fuel cells. One of the technological challenges identified here is the slow kinetics of the oxygen reduction reaction (ORR) at the cathode. Platinum based catalysts currently used to catalyze ORR suffer from their high costs, limited availability and susceptibility to presence of poisons in the fuel. Our research effort has focused on development of alternative electrocatalyst materials for PEM fuel cells.
Carbon materials doped with hetero-atoms, with or without an active metal center offer promise to substitute Pt. Early phase of our work was focused on development of nitrogen containing carbon nanostructures (CNx) synthesized by pyrolyzing C and N containing precursors on a variety of metal-doped supports. By controlling the carbon growth process, we were able to synthesize different nano-geometries with different levels of basal and edge planes.
We also used several poisons such as CO, cyanide and H2S as probes to investigate the nature of ORR active sites in these materials. It was found that these catalysts do not show any decrease in their activity even after exposure to these poisons whereas commercial platinum sample supported on Vulcan carbon showed marked decrease in its activity under identical conditions. These results support the assertion that the metal is not an essential part of the ORR active site in these materials. In addition to PEM fuel cells, these materials were also found to be suitable for Direct Methanol Fuel Cells (DMFCs) as they are not active for methanol oxidation and do not lose ORR activity even in presence of methanol. Furthermore, CNx catalysts were found to be resistant to carbon corrosion unlike commercial Vulcan carbon support suggesting their possible application as supports for platinum.
We have also compared the CNx materials with so called iron-nitrogen coordinated catalysts supported on carbon (FeNC) with an aim to resolve existing debate on the nature of ORR active sites in NNMC(expand) catalysts. Extensive and systematic experimentation from our group has revealed that FeNC and CNx are infact fundamentally different materials and very different ORR active sites with iron playing a key role to catalyze ORR in the former whereas remaining encased within carbon in case of CNx catalysts.
Our most recent research has focused on understanding the formation of ORR active sites in FeNC electrocatalysts as they evolve from their crude (inactive) form to the most active form and the effect of acid-washing on their ORR performance.