Ultrathin 2D Ni/Co Hydroxide Heterostructures for High Energy Density Flexible Microsupercapacitor.

Abstract

Assembling 2D ultrathin nanosheets into vertical heterostructures offers significant potential for advanced energy storage due to enhanced active sites, improved ion diffusion, and increased electrical conductivity, leading to superior ion/electron transport, higher energy density, and improved rate performance. In case of transition-metal hydroxides, to overcome with challenges such as random assembly, complex synthesis, instability, and poor interfacial contact is critical. This study synthesizes large area, ultrathin, 2D Nickel/Cobalt hydroxide vertical heterostructures (nickel as the top layer) using a wet chemical process, achieving 32% higher areal charge storage compared to cobalt/nickel hydroxide vertical heterostructures, 57% higher than Ni(OH)₂ and 330% higher than individual Co(OH)₂. The synergistic interaction between nickel and cobalt hydroxides contributes to a high volumetric capacity (710 mAh cm^-3) and energy density (285 mAh cm^-3) in symmetric devices. The flexible microsupercapacitor retains 75% capacitance after 15,000 cycles and demonstrates stability under bending up to 135°, with a volumetric capacity of 393 mAh cm^-3. Density functional theory simulations complement experiments, revealing interaction energy and electronic state redistribution near the Fermi level. This integrated approach serves as a guide for enhancing electrochemical properties in 2D heterostructures, aiding in the development of next-generation, high-performance energy storage materials.