Atomically Dispersed Iron Catalysts for Oxygen Reduction and Carbon Dioxide Reduction Reactions
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The transition from non-renewable energy to renewable energy has driven the development of polymer electrolyte fuel cells (PEFCs) using hydrogen fuels to power vehicles free from emissions for sustainable future. However, the high cost of PEFCs is one of the bottlenecks for marketing fuel cell vehicles in large-scale due to the heavy use of expensive platinum group metal catalysts (PGM) for boosting oxygen reduction reaction (ORR), a sluggish electrochemical reaction. Iron-nitrogen-carbon (Fe-N-C) catalysts, a replacement of PGM catalysts, have shown promising activity for ORR in PEFCs as PGM-free catalysts because they contain highly ORR-active Fe-N4 sites formed in the high temperature treatment. However, the ORR activity of current Fe-N-C catalysts is still insufficient to replace PGM catalysts mainly due to their low density of Fe-N4 active sites. This is because Fe in precursors typically tends to form Fe metallic phases over Fe-N4 active sites during high temperature treatment.This dissertation focus on developing Fe-N-C catalysts in high density of Fe-N4 active sites and favorable morphology to achieve high activity and stability for ORR in fuel cells. Chapter 1 summarizes metal-organic frameworks (MOFs) as ideal precursors for preparing Fe-N-C catalysts with decent ORR activity due to their well-defined crystal structure, high surface area and flexible chemistry. In Chapter 2, a facile doping chemistry has been employed to prepare well-defined Fe chemically doped MOF precursor in a controlled manner. Atomically dispersed Fe in Fe-N-C catalysts can be exclusively obtained to build high density of Fe-N4 sites without any Fe metallic phases at optimum Fe content. Such atomically dispersed Fe catalysts with high density of Fe-N4 sites achieve high ORR activity approaching commercial Pt/C catalysts. The Fe content in the synthesis of precursors has be found to be critical to obtain high density of Fe-N4 catalysts with high ORR activity. The promising durability of this atomically dispersed Fe catalyst has been also observed in fuel cells at the practical operation voltages. In addition to increasing the density of active sites, the particle size of these atomically dispersed Fe catalysts can be readily controlled by tuning the crystal size of Fe doped MOFs in Chapter 3. The particle size in catalysts plays an important role on affecting the utilization of active sites for ORR as well as the porous structure of catalysts for mass transport. The atomically dispersed Fe catalyst with particle size in 50 nm has been identified with best ORR activity at rotating ring-disk electrode measurement at the same Fe content since it could expose the most numbers of Fe-N4 accessible for ORR. A reduction to 20 nm particle size in catalysts results in the decreasing in ORR activity because of the severe particle agglomeration under this dimension. The formation of Fe-N4 active sties in atomically dispersed Fe catalysts has been also investigated on various temperature at high temperature treatment. The Fe-N4 active sites doped in the carbon structure can be directly transformed from tetrahedral Fe-N4 structure in Fe doped MOF precursors, suggesting that this doping chemistry is effective approach to controlling the formation of Fe-N4 at high temperature. In Chapter 4, the morphology of atomically dispersed Fe catalysts with various shapes and dimensions has been also prepared by tuning the ligand chemistry in the synthesis of Fe doped MOFs with dual ligands, showing promising activity and stability enhancement. In Chapter 5, these atomically dispersed Fe catalysts have been explored as electro-catalysts for carbon dioxide reduction reaction (CO2RR), possessing higher CO selectivity over H2 than atomically Co dispersed catalysts and nitrogen doped carbon. Overall, this dissertation provides an effective strategies to prepare these atomically dispersed Fe catalysts with high density of active sites and controlled morphology, which will play an essential role for the electrocatalysis of ORR, CO2RR and other emerging electrochemical reactions for sustainable energy future.