High‐dispersion Co‐Fe‐NC electrocatalyst based on leaf‐shaped zeolite imidazole framework for oxygen–reduction reaction in acidic medium

Abstract

Platinum-free electrocatalysts for oxygen-reduction reactions in acidic medium are challenging as the cathode of PEMFC. Here, we synthesized coreshell-type leaf-shaped CoFe-NC (L-CoFe-NC) catalysts using 2D ZIFs as cores and 3D ZIFs as shells. The amounts of Co and Fe were carefully controlled to obtain highly dispersed CoFe-doped carbon. This strategy made it easy to tune the catalyst while maintaining the advantages of the 2D structures. The prepared L-CoFe-NC showed a half-wave potential (E 1/2 ) of 0.77 V and an onset potential (E onset ) of 0.88 V in 0.5 M H 2 SO 4 . In addition, the results confirmed high durability even after 10 000 tests while retaining excellent performance. This outstanding stability was achieved owing to the high dispersion of Co and Fe in the 2D carbon frame. Furthermore, PEMFC was evaluated using an MEA containing L-CoFe-NC catalysts as a cathode.

Atomic Edge Fe-N4 Active Sites on N-Doped Porous Carbon Nanosheets Derived from Zeolitic-Imidazolate Frameworks for High-Efficiency Oxygen Reduction

“…The sluggish kinetics of oxygen reduction reaction (ORR) immensely limits the industrial application of fuel cells due to the high overpotential of electrode reactions. − Compared with commercial Pt-group catalysts, metal-nitrogen codoped carbon materials (M-N/C), with advantages of extensive resources, low cost, and outstanding durability, are considered as the most promising next-generation ORR catalysts. , Numerous studies have been carried out on Fe-N/C catalysts, and the major active sites are Fe-N 4 sites, which could adsorb O 2 on the Fe center and break the OO subsequently, thus accelerating the ORR reaction process . Especially, when the size of the metal is reduced to the atomic level to form the atomic Fe-N 4 sites, the atom utilization of the metal could be maximized. , Zeolitic imidazolate frameworks (ZIFs) have been widely used as a preferred precursor for preparing Fe-N/C due to their high N contents and abundant ordered porous structure, − and atomically dispersed Fe-N 4 sites in Fe-N/C derived from ZIFs could be constructed by precisely controlling the adding concentration of Fe salts, precoordinating Fe, removing Fe species clusters with acid etching, and so on. To obtain high-efficiency Fe-N/C catalysts, considerable efforts have been dedicated to promoting the active-site densities, such as using the cage-confined effect of ZIFs and the steric hindrance effect of macromolecular iron sources to avoid the formation of Fe clusters and increase the atomic metal loading, , and strengthening the interaction of Fe and N by introducing extra N source to enhance the Fe-N 4 sites density. − Fundamentally, blindly increasing the active-site densities can only increase the activity to a certain bottleneck due to the unchanged intrinsic activity of Fe-N 4 sites, that is, its inherent simplicity and symmetric structure endow Fe-N 4 sites with a strong adsorption energy of *OOH and *OH, making the desorption of OH* in Fe-N 4 sites to OH – difficult. − Therefore, the focus of synthesizing Fe-N/C has gradually shifted from increasing the densities of Fe-N 4 sites to modulating the configurations of Fe-N x sites in recent years.…”