Interconnected Hierarchically Porous Fe, N‑Codoped Carbon Nanofibers as Efficient Oxygen Reduction Catalysts for Zn–Air Batteries

Developing porous carbon-based non-precious-metal catalysts for an oxygen reduction reaction (ORR) is a suitable approach to significantly reduce the costs of fuel cells or metal–air batteries. Herein, interconnected hierarchically porous carbon nanofibers simultaneously doped with nitrogen and iron (HP-Fe-N/CNFs) were fabricated by facile pyrolysis of polypyrrole-coated electrospun polystyrene/FeCl₃ fibers. The obtained carbon nanofibers have a high specific surface area (569.6 m²/g) and large pore volume (1.00 cm³/ g) as well as effective doping of N and Fe. Benefiting from the improved mass transfer and utilization of active sites attributed to interconnected hierarchical porous structures, HP-Fe-N/CNFs display excellent ORR catalytic activity in alkaline media, with a comparable onset potential and half-wave potential but superior selectivity, stability, and tolerance against methanol to commercial 30 wt % Pt/C. Particularly, when applied in an assembled Zn–air battery, HP-Fe-N/CNFs outperform 30 wt % Pt/C in power density and long-term stability, explicitly showing their promising practical application

Metal–Organic-Framework-Derived Fe-N/C Electrocatalyst with Five-Coordinated Fe‑N_<i>x</i> Sites for Advanced Oxygen Reduction in Acid Media

Even though Fe-N/C electrocatalysts with abundant Fe-N_<i>x</i> 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_<i>x</i> 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_<i>x</i> 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_<i>x</i> 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

Manipulating K‑Storage Mechanism of Soft Carbon via Molecular Design-Driven Structure Transformation

The emerging potassium-ion batteries (PIBs) have been placing stratospheric expectations for realizing grid-scale electrochemical storage of renewable energy. However, the unsatisfactory K-storage of PIB anode materials, especially promising carbonaceous materials, significantly limited the development of PIBs. Here, a molecular design strategy was proposed to realize controllable structure transformation of soft carbon (SC) materials for enhanced K-storage performance. The optimized SC-PCN material delivered a high reversible K-storage capacity of 838 mAh/g at 50 mA/g, outstanding rate capability (213 mAh/g at 1000 mA/g), and excellent long-term cycling performance (301 mAh/g maintained after 300 cycles at 500 mA/g), superior to most previously reported carbon-based PIB anodes materials. Reaction kinetic analysis revealed that the proposed molecular design strategy can achieve the transformation from a surface capacitive-dominated mechanism to a capacitive-diffusion hybrid mechanism for SC-PCN, benefiting from its unique microstructures with highly defective surface generated via the synergistic effect from template removal, N doping, and surface reconstruction. The optimal hybrid K-storage mechanism should be responsible for the excellent K-storage properties of the prepared SC-PCN

Structure Manipulation of C₁N₁‑Derived N‑Doped Defective Carbon Nanosheets to Significantly Boost K‑Storage Performance

Nanocarbon materials demonstrated huge advantages for K-storage applications due to their wide range of structural tunabilities. However, their K-storage performance was still limited by the underutilization of disordered and ordered carbon structures simultaneously. Here, we developed a C1N1-based reconstruction strategy to fabricate N-doped defective carbon nanosheet (NdC) materials for K-storage. The disordered carbon defects and ordered carbon interlayers were well balanced via choosing suitable precursors for self-condensation generation of the C1N1 skeleton as well as subsequently regulating the high-temperature reconstruction process, resulting in a significantly enhanced intercalation-adsorption K-storage mechanism. As a result, the optimized G-NdC materials delivered a high reversible discharging capacity of 620 mA h/g at 50 mA/g and 241 mA h/g even at 1000 mA/g as well as 210 mA h/g after 300 cycles at 500 mA/g. These excellent K-storage properties should be ascribed to the unique order–disorder balanced microstructures with fast surface capacitive-controlled reaction kinetics. This study emphasized the important roles of carbon defects in the K-storage process and provides a deep insight into the understanding of nanocarbon-based K-storage mechanisms

<i>In Situ</i> Self-Sacrificed Template Synthesis of Fe-N/G Catalysts for Enhanced Oxygen Reduction

To facilely prepare high-performance Fe-N/G oxygen reduction catalysts via a simple and controllable route from available and low-cost materials is still a challenge. Herein, we develop an <i>in situ</i> self-sacrificed template strategy to synthesize Fe-N/G catalysts from melamine, glucose, and FeSO₄·7H₂O. Fe/Fe₃C@graphitic carbon nanocapsules are uniformly formed on the NG surface to create a highly opened and stable mesoporous framework structure. Furthermore, effectively doped N sites and high active Fe-N_<i>x</i> sites are synchronously constructed on such structures, leading to an enhanced synergistic effect for ORR and promising the Fe-N/G catalyst a similar catalytic activity and four-electron selectivity, but superior stability to commercial 30 wt % Pt/C catalysts in 0.1 M KOH solution under the same loading