A hydrothermal process was employed to create a variety of Co3O4 nanostructures. We demonstrate that nominally minor differences in the synthesis temperature (50, 70°, or 90 °C) yield profound variations in the oxide microstructure, with lath-like, necklace-like and net-like morphologies of different scales resulting. This in turn resulted in significant variations in the supercapacitive performance that ranged from mediocre to superb. Specifically, the mesoporous net-like Co3O4 nanostructures that were synthesized at 50 °C exhibited very favorable electrochemical properties: The net-like Co3O4 had a specific capacitance of 1090 F/g at a mass loading of 1.4 mg/cm2. At this high mass loading, such performance has not been previously reported. SEM and TEM analysis of these samples revealed an interconnected array of sub-10 nm crystallites interspersed with a high volume fraction of similar scale pores. The poorer performing microstructures were both coarser and much less porous.

A simple methodology is developed to directly synthesize three-dimensional (3D) electrochemically supercapacitive arrays, consisting of multiwalled carbon nanotubes conformally covered by nanocrystalline vanadium nitride, firmly anchored to glassy carbon or Inconel electrodes. These nanostructures demonstrate a respectable specific capacitance of 289 F g–1, which is achieved in 1 M KOH electrolyte at a scan rate of 20 mV s–1. The well-connected highly electrically conductive structures exhibit a superb rate capability; at a very high scan rate of 1000 mV s–1 there is less than a 20% drop in the capacitance relative to 20 mV s–1. Such rate capability has never been reported for VN and is highly unusual for any other oxide or nitride. These 3D arrays also display nearly ideal triangular voltage profiles during constant current charge–discharge cycling. Analysis of the post-electrochemically cycled samples indicates negligible changes occurring in the VN nanocrystallite morphology, but a modification in the structure of the oxidized surface. We envision that the direct synthesis approach employed in this study may serve as a “drop-in” platform for large-scale commercial fabrication of a variety of carbon nanotube-supported functional materials that require excellent electrical conductivity to the underlying support.