Morphology evolution of urchin-like NiCo ₂O ₄ nanostructures and their applications as psuedocapacitors and photoelectrochemical cells

Urchin-like NiCo ₂O ₄ nanostructures were synthesized on a large scale via a simple hydrothermal method free of any template and catalyst. As-synthesized NiCo ₂O ₄ urchins have uniform diameters of 5 μm with numerous small nanorods radially grown from the center. Typical nanorods have diameters of 100-200 nm and lengths of about 2 μm. Studies found that urea plays an important role to determine the morphology of the products and a "rods-to-straw-bundles-to-urchins" mechanism was proposed. With a porous structure and a large surface area of 99.3 m ² g ^-1, the prepared NiCo ₂O ₄ urchins exhibited superior specific capacitance of 1650 and 1348 F g ^-1 at current densities of 1 and 15 A g ^-1, respectively. The capacitance loss after 2000 cycles is only 9.2% at the current density of 8 A g ^-1, indicating their excellent cycling stability. Photoelectrochemical cells were also fabricated on the urchin-like NiCo ₂O ₄ nanostructures with the features of fast photocurrent response and excellent stability. A high photocurrent response of about 70 μA cm ^-2 was observed.