Mixed metal oxides in synergy at nanoscale: Electrospray induced porosity of in situ grown film electrode for use in electrochemical capacitor

A heterogeneous film electrode with mixed metal oxides of nickel and cobalt constituting the active layer is developed by first electrospraying metal acetate precursors on nickel foam, followed by calcination. The formation of unique cubic phases of nickel oxide (NiO) and cobalt oxide (Co3O4) are confirmed through X-ray diffraction. The growth of two oxides in nanoscale is confirmed from high-resolution transmission electron microscopy, whereas the images from scanning electron microscope show synergistic growth leading to three dimensional porosity. The synergy is further demonstrated from the areal capacity of 267 mC cm−2 at 1 mA cm−2 for mixed oxide electrode, which is much higher than the respective values for pure NiO film (124 mC cm−2), and Co3O4 film (174 mC cm−2) electrodes. The faradaic behaviour of Co3O4 film electrode in cyclic voltammetry and chronopotentiometry scans is significantly modified to a more capacitive charge transfer due to the presence of NiO, resulting in better rate capability. The electrochemical performance of film electrodes is augmented utilizing hybrid mode, where in situ grown carbon film based on electrospray coating of resorcinol formaldehyde on nickel foam serves as counter electrode. The hybrid cell delivers specific energy and specific power to the extent of 22.7 Wh kg−1, and 2.8 kW kg−1 respectively, with the capacitance retention of 89% after 2000 cycles at 10 mA cm−2.