Wenhan Niu

Mesoporous N-Doped Carbons Prepared with Thermally Removable Nanoparticle Templates: An Efficient Electrocatalyst for Oxygen Reduction Reaction

Thermally removable nanoparticle templates were used for the fabrication of self-supported N-doped mesoporous carbons with a trace amount of Fe (Fe-N/C). Experimentally Fe-N/C was prepared by pyrolysis of poly(2-fluoroaniline) (P2FANI) containing a number of FeO(OH) nanorods that were prepared by a one-pot hydrothermal synthesis and homogeneously distributed within the polymer matrix. The FeO(OH) nanocrystals acted as rigid templates to prevent the collapse of P2FANI during the carbonization process, where a mesoporous skeleton was formed with a medium surface area of about 400 m(2)/g. Subsequent thermal treatments at elevated temperatures led to the decomposition and evaporation of the FeO(OH) nanocrystals and the formation of mesoporous carbons with the surface area markedly enhanced to 934.8 m(2)/g. Electrochemical measurements revealed that the resulting mesoporous carbons exhibited apparent electrocatalytic activity for oxygen reduction reactions (ORR), and the one prepared at 800 °C (Fe-N/C-800) was the best among the series, with a more positive onset potential (+0.98 V vs RHE), higher diffusion-limited current, higher selectivity (number of electron transfer n > 3.95 at +0.75 V vs RHE), much higher stability, and stronger tolerance against methanol crossover than commercial Pt/C catalysts in a 0.1 M KOH solution. The remarkable ORR performance was attributed to the high surface area and sufficient exposure of electrocatalytically active sites that arose primarily from N-doped carbons with minor contributions from Fe-containing species.

Graphene‐Supported Mesoporous Carbons Prepared with Thermally Removable Templates as Efficient Catalysts for Oxygen Electroreduction

Graphene-supported mesoporous carbons with rich nitrogen self-doped active sites (N-MC/rGO) are prepared by direct pyrolysis of a graphene-oxide-supported polymer composite embedded with massive, evenly distributed amorphous FeOOH that serve as efficient thermally removable templates. The resulting N-MC/rGO catalysts exhibit high surface areas and apparent electrocatalytic activity for oxygen reduction reaction in alkaline media. Among the series, the sample prepared at 800 °C displays the best performance with a more positive onset potential, higher limiting currents, much higher stability, and stronger poison resistance than commercial Pt/C. This is ascribed to the synergetic functions of the highly conductive graphene support and the mesoporous N-doped carbons that effectively impede the restacking of the graphene sheets and enhance the exposure of the rich nitrogen self-doped active sites.

Volatilizable template-assisted scalable preparation of honeycomb-like porous carbons for efficient oxygen electroreduction

The electrocatalytic activity of nitrogen-doped carbons towards the oxygen reduction reaction is largely determined by the concentration of active nitrogen dopants and the electrochemically accessible surface area. Herein we report a novel, facile route for the preparation of N-doped carbons based on direct pyrolysis of polypyrrole nanosheet precursors synthesized by confining the polymerization on the surface of NaCl crystals using FeCl3 as both the initiator and dopant. In the heating-up process of pyrolysis, a large amount of homogeneously distributed FeCl3 dopant and its derivatives gradually evolved into volatile nanoparticles which helped to generate abundant hierarchical macro- and meso-pores, resulting in honeycomb-like porous carbons with a high content of nitrogen dopants ranging from 7 to 18 at%, a large surface area, and an ORR activity superior to that of commercial Pt/C in alkaline electrolytes. Significantly, by using the best sample that was prepared at 800 °C (HPC-800) as the air electrode, a Zn–air battery was found to display a specific capacity of 647 mA h g−1 at 10 mA cm−2 and a negligible loss of voltage even after continuous operation for 110 h, a performance markedly better than that with Pt/C as the air cathode. The results not only highlight the significance of precursor engineering in the synthesis of highly efficient nitrogen-doped carbon catalysts for oxygen electroreduction, but also suggest the high potential of the interfacially confined polymerization method in the scalable preparation of cost-effective, highly porous carbons for electrochemical energy storage and conversion devices.

Ordered mesoporous carbons-supported gold nanoparticles as highly efficient electrocatalysts for oxygen reduction reaction

Unlike bulk gold, gold nanoparticles (AuNPs) have been found to exhibit apparent electrocatalytic activity for oxygen reduction reaction (ORR). In this work, glutathione-capped AuNPs of ca. 4 nm in diameter were prepared by a wet chemical method and embedded into a mesoporous carbon matrix synthesized by using an SBA-15 amorphous silica template. Pyrolysis at elevated temperatures led to removal of the organic capping ligands and the formation of ordered mesoporous carbons loaded with a selected amount of AuNPs which exhibited no apparent agglomeration. The obtained nanocomposites exhibited apparent ORR activity in alkaline media, and the sample with 20 wt% AuNPs stood out as the best among the series, within the context of onset potential, kinetic current density and stability, in comparison with AuNPs alone, ordered mesoporous carbons, and commercial Pt/C catalysts. The remarkable performance was ascribed to the intimate interactions between the AuNPs and mesoporous carbons that facilitated fast electron transfer and rapid mass transport. This is the first time that ordered mesoporous carbons were employed to support AuNPs as ORR catalysts. The strategy may be exploited to prepare a wide range of electrocatalysts based on mesoporous carbons-supported metal nanoparticles with extraordinary reactivity and enhanced stability for ORR.