High-Performance Electrocatalysts for Oxygen Reduction Based on Nitrogen-Doped Porous Carbon from Hydrothermal Treatment of Glucose and Dicyandiamide

Carbon materials have been used in a wide range of applications, for instance, as electrodes, electrocatalysts, or electrocatalyst supports in energy-storage and -conversion devices, because of low costs, high electrical conductivity, chemical inertness, and thermal stability. Among the various carbon materials, nitrogen-doped carbon is of particular interest because it exhibits excellent electrocatalytic activity in the oxygen reduction reaction (ORR), which is a key reaction at fuel-cell cathodes. For instance, Gong et al. reported that N-doped carbon nanotube arrays acted as efficient metal-free electrocatalysts for oxygen reduction in alkaline media with a performance even better than that of commercial platinum catalysts. Remarkable activity has also been observed with other N-doped carbon catalysts that are based on carbon nanotubes, graphene, and porous carbon. Of these, N-doped porous carbons have been attracting considerable attention for their high activity and long-term stability. For instance, Asefa and co-workers synthesized N-doped porous carbons by polymerizing polyamine in situ within the pores of SBA-15 mesoporous silica. The doping of N atoms enhanced the ORR activity with an increased current density and decreased overpotential compared to commercial Pt/C catalysts. Yang et al. prepared N-doped porous carbon through the carbonization of nucleobases in an all-organic ionic liquid (1ethyl-3-methylimidazolium dicyanamide), which showed a high surface area and an ORR performance that was almost identical to that of commercial 20 wt% Pt/C catalysts. At least two critical factors have been identified to account for the high ORR performance of these heteroatom-doped carbon catalysts, 1) elemental composition and interactions between the components, which determine the intrinsic nature of the active sites, and 2) specific surface area and porous structure, which determine accessible active sites and transport properties of ORR-relevant species. However, to the best of our knowledge, the preparation of high-performance N-doped porous carbon materials for the ORR, as mentioned above, usually requires hard templates to guide and control the formation of mesopores, which inevitably involve several complicated and tedious processes, such as synthesis of monodispersed templating nanoparticles, delicate control over the dispersion of template nanoparticles in carbon matrix, subsequent purification, leaching, and so on. Consequently, many previously reported methods are not beneficial for scale-up production and cost reduction. Therefore, explorations of novel and facile methods for the preparation of N-doped porous carbon catalysts without the aid of commonly used hard templates, such as carbon nanotubes, graphene, silica nanoparticles, and so forth, is still highly in demand. This is the primary motivation of the present work. In this study, we developed a new and facile hydrothermal method to prepare uniform N-doped carbon spheres (THNC) by using glucose and dicyandiamide as precursors. These were then activated by ZnCl2 at an elevated temperature, ranging A synthetic method was developed for the preparation of Ndoped porous carbon through hydrothermal treatment at controlled temperatures by using glucose and dicyandiamide as precursors and ZnCl2 as an activation reagent. Nitrogen doping was quantitatively determined by using XPS measurements and identified in the forms of pyridinic, pyrrolic, graphitic, and pyridinic N+¢O¢nitrogen atoms. Further structural analysis by using SEM, TEM, XRD, Raman, FTIR, and BET measurements showed that the N-doped porous carbons exhibited a microcrystalline graphite structure and a high specific surface area up to 1000 m2g¢1. Electrochemical studies showed that the samples all exhibited a remarkable ORR catalytic activity in alkaline media, which was comparable to that of state-of-the-art Pt/C catalysts, and the one prepared at 800 8C was found to be the best among the series with an onset potential of +0.96 V, almost complete reduction of oxygen to OH¢ , and superior methanol tolerance and cycling stability.