Co3?xMnxO4 is a bimetal oxide with excellent electrochemical activity in alkaline solution, has been regarded as a promising alternative in the field of ion-air batteries and proton exchange membrane fuel cell (PEMFC). Herein, we report a simple solvothermal-calcination method to fabricate Co3?xMnxO4 with tunable external Co3+/Co2+ and Mn3+/Mn2+ ratio. The tunable ratio of element valence in the bimetal results in a higher exposure of active center for oxygen redox reaction (ORR), and thus lead to a better ORR activity, which was confirmed by X-ray photoelectron spectroscopy characterizations and electrochemical measurements. Specially, Co1.8Mn1.2O4 with a Co3+/Co2+ ratio of 2.08 showed an overpotential of 0.37 V at benchmark ORR current density of 3 mA/cm2 in 0.1 M KOH, which is lower than that of pure oxide (Mn3O4 0.53 V and Co3O4 0.56 V). In addition, the as prepared Co1.8Mn1.2O4 exhibited a positive half-wave potential (0.83 V vs RHE) due to their more active sites, promotes charge transfer, adsorption and desorption of oxygen species. This work provides a strategy for the design and fabrication of earth-abundant, low-cost electrocatalysts for PEMFC in practical applications.
Graphic Abstract
Co3?xMnxO4 was fabricated by tuning external Co3+/Co2+ and Mn3+/Mn2+ ratio, and the activity initially shows a positive correlation with the ration of Co3+/Co2+ in Co3?xMnxO4.
Various lamellar orientations of high-density polyethylene (HDPE), due to competition between bulk nucleation and interfacial nucleation, have been realized in its melt drawn blends with isotactic polypropylene (iPP) upon cooling after subjected to 160 °C for 30 min. Directed crystallization, with heterogeneous nucleation in the bulk (within domains), is defined as lamellar growth along boundary of anisotropic domains and is favored in larger domains at higher temperature (slow cooling), since overgrowth of lamellae can feel the interface rather than impingement with neighbor ones as a result of scare nuclei at higher temperature. Moreover, lamellar growth caused by directed crystallization is dependent of dimension of confinement. Due to 2D confinement of cylindrical domains, lamellae can only grow along the axis of cylinder and thus b-axis orientation is formed. While in the layered domains with 1D confinement, however, lamellae grow with the normal of (110) plane along the melt drawn direction. On the other hand, epitaxial growth of HDPE chains onto iPP lamellae is related to the surface-induced crystallization and dominated by the interfacial nucleation. Only interfacial nucleation is preferred can epitaxial growth occur. Therefore, retarded crystallization, realized by either strong confinement in finer domains or rapid cooling or both, is favorable for it. 相似文献