Preparation of Co1-xS-MnS@CNTs/CNFs for Electrocatalytic Oxygen Reduction Reaction
- 2021年第27卷 
氧还原反应是燃料电池中重要的阴极反应,但由于动力学迟缓等问题导致其效率低。碳基材料具有导电性高、稳定性好、比表面积大等优点,常被应用于电催化氧还原反应。然而其在电催化氧还原反应中效率较低,对碳基材料进行Co、Mn掺杂有望提高其氧还原效率。本文采用静电纺丝技术制备出含有Co,Mn双金属的碳纳米纤维,经热解和硫化后碳纳米纤维上形成许多包裹Co1-xS和MnS纳米颗粒的碳纳米管（记为Co1-xS-MnS@CNTs/CNFs）,垂直生长在碳纳米纤维表面。通过X射线衍射、场发射扫描电子显微镜、高分辨透射电子显微镜、X射线光电子能谱对Co1-xS-MnS@CNTs/CNFs的形貌、结构和组成进行表征,发现仅在Co1-xS和MnS同时存在的情况下碳纳米纤维表面才能生长碳纳米管。小颗粒的MnS为碳纳米管的生成提供成核位点,大颗粒的Co1-xS促进碳纳米管生长,最终形成Co1-xS-MnS@CNTs/CNFs。碳纳米管的形成不仅在金属颗粒表面形成一道屏障,防止其聚集、溶解,而且提高了碳纳米纤维的导电性,使其电催化性能及稳定性得到很大提升。电催化测试证明,Co1-xS-MnS@CNTs/CNFs相较于不含金属的碳纳米纤维（CNFs）及含单金属的硫化锰碳纳米纤维（MnS/CNFs）或硫化钴碳纳米纤维（Co1-xS/CNFs）具有更优异的电催化氧还原（ORR）性能,且在氧还原反应过程中表现出高效的四电子转移。其甲醇耐受性及长期稳定性显著优于商业Pt/C电催化剂。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 Co1-xS and MnS nanoparticles (denoted as Co1-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 Co1-xS and MnS nanoparticles co-existed. During the growth of Co1-xS-MnS@CNTs/CNFs, MnS particles served as nucleation sites for the formation of carbon nanotubes and Co1-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 Co1-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 Co1-xS and MnS nanoparticles play as barriers, preventing Co1-xS and MnS nanoparticles from aggregation and dissolution. Electrocatalytic test displays that Co1-xS-MnS@CNTs/CNFs exhibited higher electrocatalytic activity toward ORR compared with CNFs, MnS/CNFs and Co1-xS/CNFs, revealing an efficient overall four-electron transfer process. The onset potential of Co1-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 Co1-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 Co1-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.