Even though Fe-N/C electrocatalysts with abundant Fe-N x active sites have been developed as one of the most promising alternatives to precious metal materials for oxygen reduction reaction (ORR), further improvement of their performance requires precise control over Fe-N x sites at the molecular level and deep understanding of the catalytic mechanism associated with each particular structure. Herein, we report a host−guest chemistry strategy to construct Fe-mIm nanocluster (NC) (guest)@zeolite imidazole framework-8 (ZIF-8) (host) precursors that can be transformed into Fe-N/C electrocatalysts with controllable structures. The ZIF-8 host network exhibits a significant host−guest relationship dependent confinement effect for the Fe-mIm NCs during the pyrolysis process, resulting in different types of Fe-N x sites with two-to five-coordinated configurations on the porous carbon matrix confirmed by X-ray absorption near edge structure (XANES) and Fourier transform (FT) extended X-ray absorption fine structure (EXAFS) spectra. Electrochemical tests reveal that the five-coordinated Fe-N x sites can significantly promote the reaction rate in acid media, due to the small ORR energy barrier and the low adsorption energy of intermediate OH on these sites suggested by density functional theory (DFT) calculations. Such a synthesis strategy provides an effective route to realize the controllable construction of highly active sites for ORR at the molecular level.
Metal organic framework (MOF)‐derived nitrogen‐enriched nanocarbons have been proposed as promising metal‐free electrocatalysts for oxygen reduction reaction. However, the characteristic microporous feature of MOF‐derived carbon determined by the MOF structure significantly hinders the mass transfer and exposure of active sites, resulting in unsatisfactory electrocatalytic performance. Here an in situ confinement pyrolysis strategy that can simply but efficiently transform monodisperse ZIF‐8 polyhedrons to nitrogen‐enriched meso‐microporous carbon (NEMC) frameworks is reported. Using this strategy, 3D NEMC frameworks, 1D NEMC fibers, and 2D NEMC on graphene (NEMC/G) can be successfully obtained. As a metal‐free elctrocatalyst, optimized NEMC/G can reach a comparable electrocatalytic activity with superior stability and methanol resistance to commercial 30 wt% Pt/C catalyst in 0.1 m KOH solution. Such enhanced performance can be ascribed to the stable and highly open network consisting of NEMC and G with fully exposed active sites, thereby leading to durable catalytic activity.
The commercialization of lithium-sulfur (Li-S) batteries has so far been limited by their rapid capacity fading, which is induced by dissolution of intermediate polysulfides and the pulverization of the sulfur cathode due to volume expansion. Herein, we reported an efficient strategy to confine active sulfur in chemically tailored graphene nanosheets, which were prepared via modified chemical activation of hydrothermal reduced graphene oxide hydrogels. Due to its high specific surface area, large pore volume, controllable size and distribution of nanopores, the two-dimensional (2D) highly porous activated graphene nanosheets (AGNs) were proved to be a promising scaffold to uniformly confine elemental sulfur (S) in their nanopores with high loading. The resultant AGNs/S nanocomposites exhibited a reversible capacity up to 1379 mA h g À1 at 0.2 C as well as remarkable cycling stability, which may contribute to the desirable structural features. The dense nanopores of AGNs, as "microreactors" for the electrochemical reactions of sulfur, minimized polysulfide dissolution and shuttling in the electrolyte, and also reserved fast transport of lithium ions to the sequestered sulfur by ensuring good electrolyte penetration. Furthermore, the AGNs with good electronic conductivity allowed good transport of electrons from/to the poorly conducting sulfur for electrochemical reactions at high rates.
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