Comparative Study of Morphological Variation in Bi-functional ZnCo<inf>2</inf>O<inf>4</inf> Nanostructures for Supercapacitor and OER Applications

ZnCo2O4 has been studied as an electrode material for supercapacitor applications because of its excellent redox activity. It is known that morphology plays an important role in deciding the electrochemical behavior of material through factors such as specific surface area and active surface states. Surfactant plays a significant role in determining the morphology and hence surface area of the material. Here, we present an extensive comparative study on the effect of two surfactants, urea and hexamethylenetetramine (HMTA), on the morphology of ZnCo2O4 and its impact on both supercapacitor and oxygen evolution reaction (OER) performance. Synthesis of ZnCo2O4 using urea yields urchin-like morphology, which exhibits a specific capacity of 94 mAh g−1 at 2 A g−1 , which is much higher than the 34 mAh g−1 achieved through hexagonal sheet-type morphology obtained from HMTA as a surfactant. We fabricated a coin cell with the urea-derived material, which shows an energy density of 19 W h kg−1, a power density of 4125 W kg−1 , and a Coulombic efficiency of 97%. Further, a comparative study of OER for both types of morphology yields a lower overpotential of 336 mV for urea-derived urchin morphology compared to 362 mV for the HMTA-based sample. Other electrochemical parameters, such as Tafel slope, double-layer capacitance, and turn-over frequency, are also evaluated for both types of morphology. The better overall performance of urea-based urchin-like morphology is attributed to higher specific surface area and oxygen vacancies, as confirmed through Brunauer–Emmett–Teller (BET) and x-ray photoelectron spectroscopy (XPS).