Nitrogen-doped graphitic porous carbons (NGPCs) have been synthesized by using a zeolite-type nanoscale metal-organic framework (NMOF) as a self-sacrificing template, which simultaneously acts as both the carbon and nitrogen sources in a facile carbonization process. The NGPCs not only retain the nanopolyhedral morphology of the parent NMOF, but also possess rich nitrogen, high surface area and hierarchical porosity with well-conducting networks. The promising potential of NGPCs as metal-free electrocatalysts for oxygen reduction reactions (ORR) in fuel cells is demonstrated. Compared with commercial Pt/C, the optimized NGPC-1000-10 (carbonized at 1000 C for 10 h) catalyst exhibits comparable electrocatalytic activity via an efficient four-electron-dominant ORR process coupled with superior methanol tolerance as well as cycling stability in alkaline media. Furthermore, the controlled experiments reveal that the optimum activity of NGPC-1000-10 can be attributed to the synergetic contributions of the abundant active sites with high graphitic-N portion, high surface area and porosity, and the high degree of graphitization. Our findings suggest that solely MOF-derived heteroatom-doped carbon materials can be a promising alternative for Pt-based catalysts in fuel cells.

Anodic aluminum oxide (AAO), a typical self-ordered nanoporous material formed by anodization of aluminum plates, has attracted intensive interest as a template for fabrication of many nanomaterials due to its interesting mechanical and dielectric properties. [1][2][3] One of the most amazing features of the AAO, the hexagonal pattern of pores, has been under discussion ever since this phenomenon was first reported more than 50 years ago.[4] However, the driving force for the self-organization of the pores still remains unclear, although extensive research has made great achievements in the past years. [5][6][7][8] Several other metals, such as Hf, Zr, Nb, Ta, and Ti, [9][10][11][12][13] can also form self-organized porous films. Among them, anodic titanium oxide (ATO) is particularly interesting with its high potential for use in various applications, e.g., being used as gas-sensing, [13] self-cleaning [14] materials, and photoanode in dye-sensitized solar cells. [15] The most apparent difference between ATO and AAO is that the former contains separated nanotubes instead of a continuous porous film in the latter (Fig. 1).To date, the most popular model for the self-adjustment of pores in AAO is based on mechanical stress associated with expansion of aluminum during the oxide formation; this is the cause of a repulsive force between neighboring pores, which leads to the self-ordering process.[7] However, the pore generation and the formation of the hemispherical pore bottom are yet to be further studied. Nielsch et al. further proposed a 10% porosity rule based on measurements of a number of AAO films. They found this commonly observed 10% porosity corresponded to a volume expansion of 1.2 times from aluminum to alumina and was independent on specific anodization conditions. [16] This empirical model now faces a challenge when the same group Lee et al., by using the so-called hard-anodization process, successfully made well ordered hexagonal pore arrays in AAO with a porosity of only 3.3%.[17]In our research on AAO and ATO, we found that the morphology and self-adjustment of pores in these films were mainly governed by electric field enhanced electrochemical reactions in the electrolyte/oxide and oxide/metal interfaces.Herein, we propose an equifield strength model based on equilibrium between the oxidation of metal by anions of O 2À and OH À (mainly produced from dissociation of water) and the dissolution of oxides, and demonstrate how to use this new model to explain the experimental observations, including the pore growth, pore morphology and porosity. The discovery of a double-layer structure of titania nanotubes enables us to elucidate the separation process of these nanotubes in ATO. It has been well known that, during anodization of aluminum, a nonporous layer forms in a near-neutral electrolyte, e.g., boric acid or ammonium tetraborate. At the electrolyte/oxide interface as indicated by A, B, C in Figure 2A, Al 2 O 3 is dissolved accompanied by dissociation of water.where the ratio of produced O 2À to OH ...

Anodic titanium dioxide films, especially anodic TiO 2 nanotube arrays, have attracted extensive interest in the past decade. A number of electrolytes, either aqueous or non-aqueous, fluoride containing or fluoride free, have been chosen to grow anodic titanium oxide films. With great improvements in the morphology control on porosity, pore size, nanotube length and pore ordering, anodic titanium oxide films have been widely applied in photochemical water splitting, hydrogen sensing, dye-sensitized solar cells, templating for low dimensional nanomaterials and biomedical research. This article presents a brief review of the progress to date in the formation mechanism, morphology control and some applications of these smart materials.

No abstract