Fabrication of Nitrogen-Doped Porous Carbon from Covalent Organic Frameworks and the Electrocatalytic Activities for Oxygen Reduction Reaction

Abstract

Introduction Nitrogen-doped carbon materials have been attracting great attention as one of the promising metal-free oxygen reduction reaction (ORR) catalysts due to the abundant resources, chemical stability, and high electronic conductivity, which could replace the high cost platinum-based materials limiting expansion of application fields of the fuel cells. Generally, nitrogen-doped carbons have been prepared by chemical vapor deposition with precursors containing nitrogen (pyridine or acetonitrile, etc.)[i], or by thermal/plasma treatment using graphitic carbons.[ii] The active site of nitrogen-doped carbons is yet controversial, though it has been suggested that incorporation of nitrogen atoms in the sp2 carbon network structures enhances ORR activity through active site formation by produced pyridinic N or graphitic N.1 Together with the nitrogen doped structures, the amount of the doped nitrogen atoms, surface area and degrees of graphitization can affect the ORR performance[iii] and control techniques of these factors are crucial to achieve the higher catalytic activity. Herein we fabricated the nitrogen-doped carbon from covalent organic frameworks (COFs) which have molecularly-ordered structures constructed by covalent linkage of organic building blocks. Due to the above unique structures and properties, COFs are promising for potential applications in gas storage, optoelectronic devices and catalysis.[iv] In this study, we focus on the structural features of COFs, which are versatile molecular design based on various combination of the building block molecules and the covalent bonding and highly porous structures. Thus we have been examining carbonization of COFs having hetero atoms such as N and B to create hetero atom doped carbon materials from the molecularly-ordered starting materials, COFs. Herein we synthesized two types of the nitrogen atom-containing COFs as the precursors. After carbonization, we found that the nitrogen-doped porous carbons were fabricated with high surface areas. The obtained materials would be useful to examine the effect of structural differences of the resulting nitrogen-doped structures on ORR activity to develop highly active carbon catalysts. Experiment Synthesis of COF1[v]Benzene-1,3,5-tricarboxylate (1, 24 mg) and phenylendiamine (2, 24 mg) were dissolved in dioxane (2.5 ml), respectively. After mixing the solutions, 3.0 M aqueous acetic acid (0.25 ml) was added. Two-day storage at 25 °C was found to generate a precipitate, which was collected by filtration, then washed with acetone. The obtained solid was dried overnight in vacuo at 80 °C to provide a yellow powder (COF1). Synthesis of ACOF-1 1 (24 mg) and hydrazine (3, 19 ml) were dissolved in dioxane (1.0 ml). Then 6.0 M tetrafluoroacetic acid (0.13 ml) was added. The solution was heated at 120 °C in a pressure resistant vessel yielding a precipitate, which was collected by filtration, then washed with acetone. The obtained solid was dried overnight in vacuo at 80 °C to provide a pale yellow powder (ACOF-1). Carbonization of COFs The obtained COF1 and ACOF-1 were heated at 800 °C for 3 h under N2 flow. Result and Discussion The XRD patterns of the obtained COFs show obvious diffraction peaks, which indicate crystalline structure formation. For porous structure investigation, N2 gas adsorption measurements were conducted. Based on the BET analysis, the surface area of COF1 and ACOF-1 were estimated to be 1528 m2/g and 1484 m2/g respectively. These results reveal that the prepared COFs have highly ordered structures with large surface areas. After carbonization of the both COFs, in Raman spectra, two peaks were observed at around 1300 cm-1 and 1590 cm-1, which are attributed to D band and G band of graphitic carbon structures. From deconvoluted XPS spectra of the N1s, three peaks were observed at 398, 400, and 402 eV ascribable to pyridinic, pyrrolic, and graphitic N. The results exhibit that the carbonized COFs have nitrogen-doped graphitic structures. The BET surface area of the carbonized COF1 and the carbonized ACOF-1 were calculated to be 538 m2/g and 1257 m2/g, respectively. Interestingly, the carbonized ACOF-1 showed a similar surface area to one before carbonization. This result indicates that COFs would be capable of producing porous carbon structures. The electrocatalytic properties of the carbonized ACOF-1 for ORR were investigated by cyclic voltammetry (CV) in 0.1M KOH saturated with N2 or O2 gas. Though no peak was observed in a N2 atmosphere condition, a significant reduction peak appeared at -0.198 V under an O2 atmosphere, which indicates ORR catalytic activity from the material. [i] M. Kawaguchi et al. Chem. Mater. 1996, 8, 1197. [ii] X. Wang et al. Science, 2009, 324, 768. [iii] C. H. Choi et al. ACS nano, 2 012, 6, 7084. [iv] A. P. Cote et al. Science, 2005, 310, 1166. [v] T. Shiraki et al. Chem. Lett. 2015, 44, 1488. Figure 1