Metal−Organic Framework−Derived Iron and Nitrogen Co-Doped Composites as Non-Precious Catalysts for Oxygen Reduction Reaction
Authors: Wang Kai-Chin , Huang Hsin-Chih , Wang Chen-Hao 
Published: February 2020

Fuel cells can employ hydrogen and oxygen to produce water and electrical energy, which act as the alternative power sources for zero-emission electric vehicles and portable electronic devices. Several types of fuel cells have been developed during the past few years, such as the anion exchange membrane fuel cell (AEMFC). The AEMFC is attractive owing to low-cost non-precious metal catalysts (NPMCs) can be utilized in the electrodes, showing high oxygen reduction reaction (ORR) activity and enhanced durability of NPMCs in alkaline solution [1-6].

Metal–organic frameworks (MOFs) can be considered as a novel structure of porous material, which they are usually consisted of the different metal ions or second-building unit (SBU) of metal clusters combining with the organic ligands to form three-dimensional structure [7-15]. MOFs possess the different structures because of various metal ions combine with various organic ligands, and some of them have the characteristics of uniform porous and high surface area. From many studies, MOFs have acted as catalysts via different processing methods that expect to improve the activity of ORR.

This work synthesizes the porous ZIF-67 (zeolitic imidazolate framework) -800-AL and FeN-67 (iron and nitrogen-doped ZIF-67) -800-AL through the microwave hydrothermal, pyrolysis and acid leaching. FeN-67-800-AL shows high oxygen reduction reaction capability, with an almost-ideal electron transfer number of 3.99. The half-wave potential of FeN-67-800-AL only decays 6 mV after potential cycling for 30,000 cycles. From XRD pattern, ZIF-67-800-AL and FeN-67-800-AL both show strong peaks of metallic cobalt and weak peaks of carbon. N2 adsorption–desorption isotherms indicate ZIF-67-800-AL and FeN-67-800-AL both type IV curve, which suggesting the existence of both micropores and mesopores. Moreover, Fe-N67-800-AL obtains the highest BET surface areas. TEM images of FeN-67-800-AL prove the porous structure. On the other hand, TEM mapping images of FeN-67-800-AL prove the existent of Co, Fe and C. TEM results are consistent with XRD and BET analysis. X-ray photoelectron spectroscopy (XPS) results show that the catalyst has high contents of graphitic N and CoNx, which can improve the ORR performance. X-ray absorption spectroscopy (XAS) data show that the oxidation state of cobalt and/or iron as well as the interatomic distance of the heteroatoms in ZIF-67-800-AL and FeN-67-800-AL. The extended X-ray absorption fine structure spectrum (EXAFS) of ZIF-67-800-AL and FeN-67-800-AL show the bonding of Co-Co (2.36Å) and Co-N/Co-C (1.84Å). On the other hand, the bonding of Fe-Fe (2.39Å) and Fe-N/Fe-C (1.78Å) appear on the EXAFS spectrum of FeN-67-800-AL. Due to FeN-67-800-AL has additional Fe-N/Fe-C that can enhance the ORR performance. These data indicate that the presence of the porous structure, the high surface area, and the functional nitrogen can be favored for ORR.

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