Kinetic Insight into the Nucleation and Growth of Nickel Hydroxide Nanostructures on Conductive Ni Foam toward Advanced Hybrid Supercapacitors

Abstract

Metal hydroxides have drawn immense attention as robust materials for supercapacitor electrodes with enhanced electrochemical performance, desired electroactive structural, and compositional properties. Among these hydroxides, nickel hydroxide nanostructures offer advantages over other nanomaterials due to their significant electrical conductivity, efficient charge transport, and increase in electroactive sites. In this work, the electrodeposition technique was adopted to fabricate nickel hydroxide Ni(OH)2 nanostructures over a nickel foam substrate, where the electrode parameters such as pH, electrolyte concentration, potential, and deposition time were systematically optimized by evaluating the areal capacity as the primary performance metric. To elucidate the fundamental mechanisms governing film formation, the nucleation and growth kinetics of the electrochemically deposited Ni(OH)2 films on Ni foam were thoroughly investigated through potentiostatic current transients recorded at a constant applied potential. The experimentally obtained transients were rigorously fitted with the Scharifker−Hill (SH) nucleation model, and the kinetic analysis revealed that the Ni(OH)2 film growth is governed by a three-dimensional (3D) instantaneous nucleation mechanism with diffusion-controlled grain growth, indicating the simultaneous activation of a finite number of nucleation sites at the onset of deposition followed by hemispherical diffusion-limited growth of the nuclei. Further, various characterization techniques were employed to analyze the morphology, structure, and composition of the electrode. Electrochemical studies for the electrodes were conducted in aqueous 6 M KOH electrolyte, and the electrodes showed considerable electrochemical properties. Among them, the well optimized electrode of Ni(OH)2 deposited over 60 min of duration (Ni(OH)2-60), identified through areal capacity optimization, demonstrated a remarkable 2.13 C cm−2 areal capacity at 2 mA cm−2 current density. Furthermore, a hybrid-supercapacitor (H-SC) device was fabricated using the Ni(OH)2-60 and the activated carbon (AC@Ni foam) as the positive and negative electrodes. The device showed a maximum specific capacitance of 97 F g−1 with a specific energy and power of 30 Wh kg−1 for 0.750 kW kg−1 and a retention of 97% after 10000 charge/discharge cycles. This enhancement can be directly attributed to the favorable nucleation kinetics achieved under the optimized deposition conditions, which promoted uniform nuclei distribution, improved electron and ion transport pathways, an increased number of electroactive sites, and the enhanced structural integrity of the Ni(OH)2 electrode, collectively making it a promising electrode material for energy storage applications.