Additive-assisted oriented growth of cobalt oxide: controlled morphology and enhanced supercapacitor performance

This research investigates the supercapacitor properties of cobalt oxide (Co₃O₄) thin films enhanced by five different additives: urea, ammonium chloride (NH₄Cl), ammonium hydroxide (NH₄OH), ammonium fluoride (NH₄F), and hexamethylenetetramine (HMT). The thin films are synthesized using a double hydrothermal approach on stainless steel substrates. Morphological and XRD analyses reveal well-separated Co₃O₄ nanowires stacked together, with diameters ranging from 10 to 34 nm and an average crystallite size between 19 and 23 nm. The additives serve as complexing agents, influencing the pH of the solution and facilitating the formation of cobalt-containing complexes, thereby promoting the uniform growth of Co₃O₄. Notably, the C-HMT nanowires exhibit superior supercapacitive performance, achieving a specific capacitance of 468.68 F g^-1 at a scan rate of 5 mV s^-1 and an impressive retention rate of 98.31% after 10 000 cycles at a scan rate of 100 mV s^-1. Additionally, a symmetric device composed of two C-HMT electrodes is developed, demonstrating practical application by effectively illuminating five parallel-connected LEDs for approximately 20 seconds. In conclusion, this study presents a pioneering application of C-HMT as a symmetric supercapacitor, showcasing significant advancements in performance for future flexible energy storage devices.