Highly active Fe-N-doped porous hollow carbon nanospheres as oxygen reduction electrocatalysts in both acidic and alkaline media

Iron/iron carbide coupled with S, N co-doped porous carbon as effective oxygen reduction reaction catalyst for microbial fuel cells

As a novel energy device, microbial fuel cells (MFCs) have attracted much attention for their dual functions of electricity generation and sewage treatment. However, the sluggish oxygen reduction reaction (ORR) kinetic on the cathode have hindered the practical application of MFCs. In this work, metallic organic framework derived carbon framework co-doped by Fe, S, N tri-elements was used as alternative electrocatalyst to the conventional Pt/C cathode catalyst in pH-universal electrolytes. The amount of thiosemicarbazide from 0.3 to 3 g determined the surface chemical property, and therefore the ORR activity of FeSNC catalysts. The sulfur/nitrogen doping and Fe/Fe₃C embedded in carbon shell was characterized by X-ray photoelectron spectroscopy and transmission electron microscopy. The synergy of iron salt and thiosemicarbazide contributed to the improvement of nitrogen and sulfur doping. Sulfur atoms were successfully doped into the carbon matrix and formed a certain amount of thiophene- and oxidized-sulfur. The optimal FeSNC-3 catalyst synthesized with 1.5 g of thiosemicarbazide exhibited the highest ORR activity with a positive half wave potential of 0.866 V in alkaline and 0.691 V (vs. Reversible Hydrogen Electrode) in neutral electrolyte, which both outperformed the commercial Pt/C catalyst. However, as the amount of thiosemicarbazide surpassed 1.5 g, the catalytic performance of FeSNC-4 was lowered, and this could be assigned to the decreased defects and low specific surface area. The excellent ORR performance in neutral medium urged FeSNC-3 as good cathode catalyst in single chambered MFC (SCMFC). It showed the highest maximum power density of 2126 ± 100 mW m^-2, excellent output stability of 8.14% decline in 550 h, chemical oxygen demand removal of 90.7 ± 1.6% and coulombic efficiency of 12.5 ± 1.1%, all superior to those of benchmark SCMFC-Pt/C (1637 ± 35 mW m^-2, 15.4%, 88.9 ± 0.9%, and 10.2 ± 1.1%). These outstanding results were associated to the large specific surface area and synergistic interaction of multiple active sites, like Fe/Fe₃C, Fe-N₄, pyridinic N, graphite N and thiophene-S.

Outside-in catalytic graphitization method for synthesis of dispersible and uniform graphitic porous carbon nanospheres

Common strategies to synthesize graphitic porous carbon nanospheres suffer from energy consumption, exorbitant cost and harsh condition, and lead to closed pore and polydisperse particles. The successful manipulation of adjustable graphitic skeleton, developed porosity, good monodispersity and dispersity of carbon nanospheres is essential to meet their structural varieties and practical applications. Herein, an outside-in catalytic graphitization method is reported to synthesize carbon nanospheres with abovementioned properties, which involves interfacial assembly between layered double hydroxides nanosheets and polymer nanospheres, in-situ generation of nickel nanoparticles, and outside-in catalytic graphitization. The unusual phenomenon is that the in-situ generated nickel nanoparticles are preferentially oriented to the carbon side rather than to the free open space. The interface reactions between nickel nanoparticles and amorphous carbons drive continuous etching of carbon species to form graphitic structure in the interior of spheres. The graphitic structure can be tuned by changing effective charge ratio and pyrolysis conditions and obtained carbon nanospheres possessed good dispersibility in water and ethanol. Moreover, such carbon nanospheres exhibited good performance when used as anodes in lithium-ion batteries. These findings may pave new ways for synthesizing multifarious carbon nanomaterials with adjustable graphitic skeleton, developed porosity, good monodispersity and dispersibility for various applications.