Metal-Organic Framework-Derived Atomic Iron-Dispersed Carbon Electrocatalysts for Oxygen Reduction in Acidic Polymer Electrolyte Fuel Cells
Authors: Zhang Hanguang , Chung Hoon T , Cullen David A. , More Karren L. , Zelenay Piotr , Wu Gang 
Published: February 2020

Low cost and high performance cathode catalysts for oxygen reduction reaction (ORR) in acidic Nafion® based polymer electrolyte fuels still remain a grand challenge in the field.1-5 Here, we present an atomic iron-dispersed carbon catalyst with homogeneous microstructure. The iron atoms were found atomically embedded into partially graphitized carbon. Due to the significantly improved uniformity and increased density of active sites, the ORR activity of this new catalyst reached to a new milestone, showing a half-wave potential of 0.84 V vs RHE in 0.5 M H2SO4 by rotating disk electrode along with fuel cell performance (0. 044 A/cm2 at 0.87 ViR-free in a H2-O2 cell) and stability at a practical operation voltage of 0.7 V. The high-performance platinum group metal (PGM)-free catalyst is derived from a well-defined iron doped metal-organic framework (e.g., zeolitic imidazolate framework, ZIF-8) precursor with ordered crystal structure through a single carbonization step in N2 atmosphere. Obtaining the optimal doping content of Fe into the ZIF-8 during the precursor synthesis played a key role in achieving the atomically dispersed iron morphology associated with the improvement of activity and stability. Unlike previous studied Fe-based catalysts, the particle size and shape of this catalyst were well-controlled and highly homogeneous and can be directly transferred from the morphology of Fe-doped ZIF precursors. Notably, higher Fe doping contents yielded larger crystal sizes in the precursors. Furthermore, optimal iron doping content is crucial for activity and stability enhancement, which correlated to iron distribution, carbon structures, surface areas/porosity, and nitrogen doping. Doping high Fe content is desirable to provide more active sites. However, atomic iron tends to agglomerate and form clusters when Fe content is above 5 at%. Higher Fe content yields highly graphitized carbon, but mitigates the total pore volumes, which inhibits active formation in micropores and mass transfer through meso/macro pores. Clearly, doped Fe content affects the critical nitrogen doping including the total content and doping position. In principle, high Fe content in catalyst should coordinate more nitrogen. Oppositely, instead of coordinating with nitrogen, Fe just tends to form inactive metallic iron and iron carbides. Therefore, one future focus is to develop effective solution synthesis to chemically doped Fe into MOFs capable of forming atomic iron distribution in carbon, rather than aggregates. This work provides evidence that superior ORR activity arises from atomically and homogenously dispersed FeNx active sites located in the carbon fringes. The elucidation of structure-property correlations revealed that Fe content in the precursor is the foremost variable in controlling the synthetic chemistry of these catalysts, and affects various properties including particle size, porosity, graphitization, and nitrogen-doping. Most importantly, this work strongly supports the theoretical prediction that iron-based catalysts are able to achieve comparable catalytic activity of Pt in the highly challenging environment of acid media.
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