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[en] Highlights: • Mn silicate hollow spheres are enclosed in reduced graphene oxide (rGO). • Manganese silicates provide advantageous de-/lithiation potentials with regard to energy density. • Mn silicate hollow spheres remain unaltered even after 350 full dis-/charge cycles. • The incorporation of rGO enhances the achievable capacity and rate capability. - Abstract: Herein is presented a new composite material consisting of nanostructured Mn silicate hollow spheres enclosed in a matrix of reduced graphene oxide (rGO), synthesized via a facile and low-cost hydrothermal method. The hollow structure provides free space to accommodate the volume expansion occurring upon lithiation, while the rGO facilitates the electron transport, thus enhancing the lithiation kinetics. Remarkably, the composite provides a continuously increasing reversible capacity up to ca. 1300 mAh g−1 after 350 cycles. This increase in capacity is ascribed in part to the steadily rising fraction of Mn2+/Mn3+ being oxidized to Mn4+ as well as the reversible formation of the solid electrolyte interphase. The particle morphology, in fact, remains unaltered, as evidenced by ex situ scanning electron microscopy – even after 350 cycles. Additionally, the implementation of manganese as transition metal for the reversible conversion reaction appears advantageous with regard to the overall electrochemical performance and the relatively lower lithiation potential.
[en] Graphical abstract: Three-dimentional Fe, N-doped carbon foams prepared by two steps exhibited comparable catalytic activity for oxygen reduction reaction to commercial Pt/C due to the unique structure and the synergistic effect of Fe and N atoms. - Highlights: • Three-dimensional Fe, N-doped carbon foam (3D-CF) were prepared. • 3D-CF exhibits comparable catalytic activity to Pt/C for oxygen reduction reaction. • The enhanced activity of 3D-CF results of its unique structure. - Abstract: Three-dimensional (3D) Fe, N-doped carbon foams (3D-CF) as efficient cathode catalysts for the oxygen reduction reaction (ORR) in alkaline solution are reported. The 3D-CF exhibit interconnected hierarchical pore structure. In addition, Fe, N-doped carbon without porous strucuture (Fe-N-C) and 3D N-doped carbon without Fe (3D-CF’) are prepared to verify the electrocatalytic activity of 3D-CF. The electrocatalytic performance of as-prepared 3D-CF for ORR shows that the onset potential on 3D-CF electrode positively shifts about 41 mV than those of 3D-CF’ and Fe-N-C respectively. In addition, the onset potential on 3D-CF electrode for ORR is about 27 mV more negative than that on commercial Pt/C electrode. 3D-CF also show better methanol tolerance and durability than commercial Pt/C catalyst. These results show that to synthesize 3D hierarchical pores with high specific surface area is an efficient way to improve the ORR performance
[en] Highlights: • ZnO/ZnFe2O4/N-doped C micro-polyhedrons with hierarchical hollow structure (ZZFO-C). • ZZFO-C material exhibits remarkable energy storage performance as LIB anode. • ZZFO-C shows synergy between the two active components and the N-doped carbon matrix. • The reaction mechanism of ZZFO-C is revealed via in situ X-ray diffraction studies. In this work, a facile and potentially scalable self-template synthesis of bi-component ZnO/ZnFe2O4/N-doped C micro-polyhedrons with hierarchical hollow structure (ZZFO-C) is presented. These are obtained through calcination of a single bi-metallic metal-organic framework (MOF) precursor (ZIF-ZnFe, molar ratio of 3:1). The resulting material shows a high surface area and is constituted by the organized assembly of numerous nanoparticles sub-unit (with size in the range of 20 nm). By tuning the annealing conditions, porous ZnO/ZnFe2O4 (ZZFO) micro-polyhedrons are obtained. The ZZFO-C composite materials are studied as anodes for LIBs exhibiting remarkable energy storage performance, such as, large reversible capacity (ca. 1000 mA h g−1 after 100 cycles at 200 mA g−1), excellent rate capability and cycling stability. After high-rate capacity testing (1000 cycles at 2.0 A g−1), ZZFO-C shows an excellent reversible capacity of 620 mA h g−1. The excellent performance of ZZFO-C arises from its unique hierarchical hollow structure and the synergy between the two active components and the N-doped carbon matrix. The electrochemical reaction mechanism and structure phase changes upon the initial lithiation are identified via in situ X-ray diffraction studies.