We report on the synthesis and electrochemical properties of oriented NiO-TiO(2) nanotube (NT) arrays as electrodes for supercapacitors. The morphology of the films prepared by electrochemically anodizing Ni-Ti alloy foils was characterized by scanning and transmission electron microscopies, X-ray diffraction, and photoelectron spectroscopies. The morphology, crystal structure, and composition of the NT films were found to depend on the preparation conditions (anodization voltage and postgrowth annealing temperature). Annealing the as-grown NT arrays to a temperature of 600 °C transformed them from an amorphous phase to a mixture of crystalline rock salt NiO and rutile TiO(2). Changes in the morphology and crystal structure strongly influenced the electrochemical properties of the NT electrodes. Electrodes composed of NT films annealed at 600 °C displayed pseudocapacitor (redox-capacitor) behavior, including rapid charge/discharge kinetics and stable long-term cycling performance. At similar film thicknesses and surface areas, the NT-based electrodes showed a higher rate capability than the randomly packed nanoparticle-based electrodes. Even at the highest scan rate (500 mV/s), the capacitance of the NT electrodes was not much smaller (within 12%) than the capacitance measured at the slowest scan rate (5 mV/s). The faster charge/discharge kinetics of NT electrodes at high scan rates is attributed to the more ordered NT film architecture, which is expected to facilitate electron and ion transport during the charge-discharge reactions.

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We report on the synthesis and electrochemical properties of oriented anatase TiO2 nanotube (NT) arrays as electrodes for Li-ion batteries. The TiO2 NT electrodes displayed both pseudocapacitive Li+ storage associated with the NT surface and the Li+ storage within the bulk material. The relative contribution of the pseudocapacitive and bulk storages depends strongly on the scan rate. While the charges are stored primarily in the bulk at low scan rates (≪1 mV/s), the surface storage dominates the total storage capacity at higher scan rates (>1 mV/s). The storage capacity of the NT electrodes as a function of charge/discharge rates showed no dependence on the NT film thickness, suggesting that the Li+ insertion/extraction processes occur homogeneously across the entire length of NT arrays. These results indicated that the electron conduction along the NT walls and the ion conduction within the electrolyte do not cause significant hindering of the charge/discharge kinetics for NT electrode architectures. As a result of the surface pseudocapacitive storage, the reversible Li+ storage capacities for TiO2 NT electrodes were higher than the theoretical storage capacity for bulk anatase TiO2 materials.