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The metal-free carbon-based catalyst with low cost and high oxygen reduction reaction (ORR) activity is urgently desired to satisfy the demands of microbial fuel cells (MFCs). However, it is still a great challenge to develop a facile and feasible strategy to construct efficient active sites of heteroatom doping for carbon-based electrocatalyst. Herein, we report a strategy based on an ammonium fluoride (NH 4 F) gas-steamed metal-organic frameworks (MOFs) to heighten structural defects and density of N, F active sites of metal-free catalyst. Oxygen temperature-programmed deposition and density functional theory results confirm that the NH 4 F gas-steamed process greatly enhances the adsorption affinity of O 2 and oxygen intermediates on the catalysts. The resulted N and F co-doped porous carbon cage (FNC-15) demonstrates outstanding ORR catalytic activity and long-term stability in alkaline and neutral electrolytes. This work proposes a facile and efficient in situ gas-steamed strategy to develop metal-free cathode catalysts with superior performance. [Display omitted] • NH 4 F gas-steamed strategy introduce defects and N, F active sites within catalysts. • Defect sites enhance the oxygen molecules adsorption affinity of catalysts. • Remarkably enhanced ORR catalytic activity and stability. • The metal-free FNC-15 exhibits superior performance in microbial fuel cell. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fe, Cu co-doped promotes electron transfer of ORR. • Fe, Cu co-doped adjust d-band of elements. • Fe, Cu act synergistically through redox coupling. • FeCu@CN shows the remarkable stability in application of MFC. Air cathode microbial fuel cell (AC-MFC) cannot be used on a large scale because of its low oxygen reduction reaction (ORR) efficiency. Despite the fact that bimetallic catalysts can greatly enhance the oxygen reduction rate by regulating the electronic structure of the active site, the flaws of insufficient exposure of the active site and easy metal agglomeration limit its catalytic activity. Herein, we report on the preparation of a stable heteroatomic substrate using a copper material organic framework as a precursor, covered by Fe-based active sites. As a result of dipole-dipole interactions, the reduced product Fe2+ forms a weak Fe-O surface that is conducive to the adsorption of active substances. The presence of Fe0 enhances the electrical conductivity of the catalytic, thus promoting ORR efficiency. Through redox coupling, the D-band center of Fe at FeCu@CN is optimized and brought close to the Fermi level to facilitate electron transfer. Notably, FeCu@CN demonstrates a superior power density of 2796.23 ± 278.58 mW m−3, far exceeding that of Pt/C (1363.93 ± 102.56 mW m−3), in the application of microbial fuel cells (MFCs). Meanwhile, the MFC-loaded FeCu@CN maintains excellent stability and outstanding output voltage after 1000 h, which provides feasibility for large-scale application. [ABSTRACT FROM AUTHOR]

Cathode electrocatalyst is quite critical to realize the application of microbial fuel cells (MFCs). Perovskite oxides have been considered as potential MFCs cathode catalysts to replace Pt/C. Herein, Cu-doped perovskite oxide with a stable porous structure and excellent conductivity was successfully prepared through a sol-gel method. Due to the incorporation of Cu, CaFe 0.9 Cu 0.1 O 3 has more micropores and a larger surface area, which are more conducive to contact with oxygen. Doping Cu resulted in more Fe3+ in B-site and thus enhanced its binding capability to oxygen molecules. The data from electrochemical test demonstrated that the as-prepared catalyst has good conductivity, high stability, and excellent ORR properties. Compared with Pt/C catalyst, CaFe 0.9 Cu 0.1 O 3 exhibits a lower overpotential, which had an onset potential of 0.195 V and a half-wave potential of −0.224 V, respectively. CaFe 0.9 Cu 0.1 O 3 displays an outstanding four-electron pathway for ORR mechanism and demonstrates superiors corrosion resistance and stability. The MFC with CaFe 0.9 Cu 0.1 O 3 has a greater maximum power density (1090 mW m−3) rather than that of Pt/C cathode (970 mW m−3). This work demonstrated CaFe 0.9 Cu 0.1 O 3 is an economic and efficient cathodic catalyst for MFCs. • Cu-doped double B-site perovskite forms microporous structure. • Cu promotes the combination of Fe–O and O. • CaFe 0.9 Cu 0.1 O 3 shows outstanding and highly stable ORR catalytic performance. • CaFe 0.9 Cu 0.1 O 3 can be used as a highly efficient cathode catalyst in MFCs. [ABSTRACT FROM AUTHOR]

Rational design and assembly of metal-organic frameworks (MOFs) is an effective strategy to develop high-performance electrocatalysts for oxygen reduction reaction in the microbial fuel cells (MFCs). In this work, a novel strategy to fabricate hierarchically porous bimetallic-carbon nanofibers (Mn–Fe@g-C 3 N 4) via pyrolyzing Mn-doped g-C 3 N 4 assisted Fe-based MOFs (MIL-101) is proposed. The Mn–Fe@g-C 3 N 4 exhibits superior oxygen reduction reaction (ORR) onset potential (0.393 V vs. Ag/AgCl) and half-wave potential (−0.042 V vs. Ag/AgCl) under neutral condition, exceeding the state-of-the-art Pt/C catalyst. Mn–Fe@g-C 3 N 4 displays a direct four-electron-transfer pathway, excellent stability and methanol tolerance in alkaline media. Mn–Fe@g-C 3 N 4 cathode exhibits the ohmic resistance (8.07 Ω) and charge transfer resistance (5.44 Ω), which is slightly lower than Pt/C catalyst. Mn–Fe@g-C 3 N 4 catalyst is an outstanding air-cathode in MFC with a power density of 413 ± 7 mW m−2, outperforms the MFC with 20 wt% Pt/C catalyst (333 ± 9 mW m−2). The outstanding catalytic activity for Mn–Fe@g-C 3 N 4 is mainly due to 3D interconnected porous architectures, highly conductive framework and the synergistic effects between nitrogen and metal ion center. The present work not only provides an efficient conductive-skeleton-assisted synthetic strategy to construct high-performance electrocatalyst, but also improves the electrochemical performance of MOF-derived materials for practical MFCs application. Image 1 • Efficient catalyst based on MIL-101 and conductive skeleton graphitic-C 3 N 4. • Mn–Fe@g-C 3 N 4 shows porous core−shell structure and active nitrogen species. • Mn–Fe@g-C 3 N 4 exhibits superior performance in alkaline and neutral media. • Microbial fuel cell with Mn–Fe@g-C 3 N 4 shows power density of 413 ± 7 mW m−2. [ABSTRACT FROM AUTHOR]