Polymer electrolyte fuel cells (PEFCs) have been regarded as a promising renewable energy technology to meet future societal demands, attributed to their environmentally friendly by-products, high power densities and energy conversion efficiency. Oxygen reduction reaction (ORR) in cathode is one of the key reactions in PEFCs, however, efficient ORR currently requires platinum group metal (PGM) based electrocatalysts. Transition-metal/nitrogen/carbon, carbide, and metal oxide catalysts have thus been intensively explored over the past decade for the replacement of the PGM. Recently crystalline iron nitrides are regarded as a significant ORR catalyst due to their high onset potential and good durability. Iron nitrides can be prepared by ammonolysis of iron oxide or iron complexes and these show various crystal structures and phases, such as ζ-Fe2N, ε-Fe2N, ε-Fe3N, and γ'-Fe4N. Among them, ε-Fe2N has been deduced as the most active nitride species from both experimental results and DFT calculations. However, ε-Fe2N with anisotropic distortion prefers to transform to the ζ-Fe2N with orthorhombic structure due to the thermodynamic stability, thus, it has been challenging to retain the catalyst in theε-Fe2N phase. Here, a facile approach to produce ε-Fe2N incorporated carbon catalysts from iron-based metal-organic framework (MOF) crystal is developed and the relationship between the iron nitride structure and their ORR activity investigated. The formation of ε-Fe2N sites could be systematically controlled by tuning of the ammonolysis temperature. These ε-Fe2N materials exhibited high ORR activity in acidic media and the best performing material exhibits an onset potential of 0.86 V and electron transfer number of 3.9. They also provided a high kinetic-limiting current density and a higher cycling stability compared with the benchmarked Pt/C.

Fabrication of ε-Fe2N catalytic sites in porous carbons derived from an iron−triazolate crystal

256th National Meeting (2018)

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