دانلود رایگان مقاله اکسید کبالت محکم شده بر نیتروژن و گوگرد دوپ دوتایی فوم گرافن

Abstract

Developing highly active bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the oxygen electrocatalysis based energy conversion or storage remains challenges. Herein, an efficient catalyst of cobalt oxide (Co3O4) nanocrystals grown on nitrogen and sulfur co-doped graphene foam (NSGF) is prepared via a hydrothermal method. Scanning electron microscope (SEM) and transmission electron microscopy (TEM) measurements demonstrate Co3O4 is of regular single crystal morphology in the Co3O4/NSGF composite with 1:1 mass ratio of cobalt to graphene foam. For ORR, the hybrid favors a direct 4e− pathway, similar to that of Pt/C, but outperforms Pt/C with a long-term durability. Besides, it achieves a small overpotential of ∼0.48 V vs. SCE at the current density of 10 mA cm−2 and there is only 0.018 V vs. SCE overpotential increasing after 200 cycles for OER testing. The good electrochemical activities of our fabricated bifunctional catalyst are attributed to electrochemical contributions of the loaded Co3O4 nanocrystals, the integral structure of three-dimensional graphene foam, the heteroatoms doping effects and the synergistic effects between Co3O4 and NSGF.

3. Results and discussion

3.1. Characterizations of Co3O4/NSGF composites Fig. 1a depicts XRD patterns of the synthesized samples. Pure GF sample exhibited a broad diffraction peak (005) at 2 = 25.8◦. In the case of 1:1 Co3O4/NSGF hybrid, the (111), (220), (311), (222), (400),(422),(440) and(511)peaksmatched well withthose of cubic Co3O4 (PDF-#78-1969). And Co3O4 were also formed in 1:2 and 2:1 Co3O4/NSGF composites. The strong peaks suggested the high crystallinity of Co3O4 nanoparticles (NPs). The nitrogen adsorptiondesorption isotherm (Fig. 1b) assessing the porous nature clearly shows a type isotherm IV indicative of the existence of mesopores and macropores. The average pore size in the pore size distribution curve (Fig. 1b inset) was around 82.3 nm and tC-S-Che BET surface area was 90.2 m2 g−1, resulting from the occurrence of partial pore blockage [27]. Details of the morphology and structure of the hybrids were characterized by SEM and TEM. Both graphene foam (GF) and graphene oxide foam (GOF) exhibit intense interconnected network morphology (Fig. 2a and b). For 1:1 Co3O4/NSGF, cubic Co3O4 NPs were observed on the GF sheets or in the holes (Fig. 2c, d and f). While the Co3O4 particles did not grow well in 1:2 and 2:1 Co3O4/NSGF composites (see Supporting information Fig. S1a and b). TEM images showed that the GOF and the GF were crumbled with covered sheets (Supplementary Fig. S1c, Fig. 2e, agreeing well with SEM results. The high-resolution TEM (HRTEM) image given in Fig. 2g clearly displays that lattice fringe spacing of Co3O4 was around 0.27 nm, and the selected area electron diffraction (SAED) pattern (the insetin Fig. 2h) demonstrated Co3O4 was of single crystalline form, and the first diffraction areas corresponded to (111) surface plane of Co3O4. Details of study have demonstrated thatthe exposed Co3O4 (111) surface contributed a lot to OER [3,13].