关键词: 碳纳米管, 碳纳米纤维, 硫化锰, 硫化钴, 电催化, 氧还原

Abstract:

As an important cathode reaction in fuel cells and metal-air batteries, oxygen reduction reaction (ORR) is a complex reaction of slow kinetics, which severely limits performances of fuel cells and metal-air batteries. Therefore, it is of key importance to find an efficient and stable electrocatalyst to promote ORR. Carbon-based materials, which possess high conductivity, good stability and large specific surface area, are usually used in electrocatalytic ORR. However, pure carbon-based materials exhibit low efficiency. Coupling carbon-based materials with manganese (Mn) and cobalt (Co) transition metals containing 3d orbitals is an effective way to improve electrocatalytic performance. Herein, carbon nanofibers containing Co and Mn elements were crafted via an electrospinning technique, using polyacrylonitrile, cobalt acetate, manganese acetate and F127 as the carbon, cobalt, and manganese sources and pore-forming agent, respectively. After vulcanizing and carbonizing the pre-oxidized electrospun samples, carbon nanofibers coated with carbon-nanotubes-wrapping Co_1-xS and MnS nanoparticles (denoted as Co_1-xS-MnS@CNTs/CNFs) were formed. A series of measurements were carried out to characterize the morphology and structure of samples by employing X-ray diffraction technique, field emission scanning electron microscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. It is found that carbon nanotubes grew on the surface of carbon nanofibers when Co_1-xS and MnS nanoparticles co-existed. During the growth of Co_1-xS-MnS@CNTs/CNFs, MnS particles served as nucleation sites for the formation of carbon nanotubes and Co_1-xS particles promoted the growth of carbon nanotubes. In addition, the as-prepared samples were composed of Co, Mn, N, C and S elements. More specifically, nanofibers were composed of C and N elements. S element functioned as the sulfur source for both Co_1-xS and MnS nanoparticles, as well as the dopant of nanofibers. The dopings of N and S into carbon nanofibers could promote the oxygen adsorption, facilitating the oxygen breaking of the O-O bond and thus improving the ORR activity. Moreover, it is demonstrated that carbon nanotubes wrapping Co_1-xS and MnS nanoparticles play as barriers, preventing Co_1-xS and MnS nanoparticles from aggregation and dissolution. Electrocatalytic test displays that Co_1-xS-MnS@CNTs/CNFs exhibited higher electrocatalytic activity toward ORR compared with CNFs, MnS/CNFs and Co_1-xS/CNFs, revealing an efficient overall four-electron transfer process. The onset potential of Co_1-xS-MnS@CNTs/CNFs was slightly negative, while the ORR current density was higher compared with commercial Pt/C (20wt.%). It is worth noting that Co_1-xS-MnS@CNTs/CNFs possessed better stability and higher methanol tolerance than commercial Pt/C in alkaline media. It is observed that the current of Co_1-xS-MnS@CNTs/CNFs dropped slightly after the test running for 20000 s, while that of commercial Pt/C lowered significantly. This research provides a simple yet effective method to prepare non-noble metal electrocatalysts with excellent electrocatalytic performance, which presents an appealing strategy for efficient energy storage and conversion.

Key words: carbon nanotubes, carbon nanofibers, manganese sulfide, cobalt sulfide, electrocatalysis, oxygen reduction reaction