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Novel, porous NiCo2O4 nanotubes (NCO-NTs) are prepared by a single-spinneret electrospinning technique followed by calcination in air. The obtained NCO-NTs display a one-dimensional architecture with a porous structure and hollow interiors. The effect of precursor concentration on the morphologies of the products is investigated. Due to their unique structure, the prepared NCO-NT electrode exhibits a high specific capacitance (1647 F g(-1) at 1 A g(-1)), excellent rate capability (77.3 % capacity retention at 25 A g(-1)), and outstanding cycling stability (6.4 % loss after 3000 cycles), which indicates it has great potential for high-performance electrochemical capacitors. The desirable enhanced capacitive performance of NCO-NTs can be attributed to the relatively large specific surface area of these porous and hollow one-dimensional nanostructures.

Electrochemical conversion reactions of transition metal compounds create opportunities for large energy storage capabilities exceeding modern Li-ion batteries. However, for practical electrodes to be envisaged, a detailed understanding of their mechanisms is needed, especially vis-à-vis the voltage hysteresis observed between reduction and oxidation. Here, we present such insight at scales from local atomic arrangements to whole electrodes. NiO was chosen as a simple model system. The most important finding is that the voltage hysteresis has its origin in the differing chemical pathways during reduction and oxidation. This asymmetry is enabled by the presence of small metallic clusters and, thus, is likely to apply to other transition metal oxide systems. The presence of nanoparticles also influences the electrochemical activity of the electrolyte and its degradation products and can create differences in transport properties within an electrode, resulting in localized reactions around converted domains that lead to compositional inhomogeneities at the microscale.

The structural and electrochemical properties of the mixed transition metallic oxides Zn1-xMnxFe2O4 nanofibers, which crystallize in a cubic spinel AFe2O4 structure, are investigated systematically with a gradual substitution of Zn by Mn. The crystal structural information studied by X-ray diffraction (XRD) depicts the formation of single phase spinel structure, while electron-dispersive X-ray spectroscopy (EDS) reveals the stoichiometric ratio between Zn and Mn. ZnFe2O4 exhibits a good capacity of ~532 mAh g(-1) at 50th cycle through the interbeneficial conversion reaction and alloy-dealloy mechanism, with a first discharge working voltage of ~0.83 V. Subsequently, the characteristic redox potential of each spinel is gradually reduced with the replacement of Mn. Furthermore, Zn0.3Mn0.7Fe2O4 demonstrates the highest capacity of ~612 mA h g(-1) at 50th cycle among the solid solution series. Ex situ characterization by high-resolution transmission electron microscope (TEM) and electron energy loss spectroscopy (EELS) is conducted to study the participation of Mn in the battery performance. This report represents an example of how the electrochemical performance could be flexibly adjusted by tuning the ratio of transition metals within the spinel.