Room-Temperature Growth of Co(OH)₂ Nanosheets on Nanobelt-like Cu(OH)₂ Arrays for a Binder-Free High-Performance All-Solid-State Supercapacitor

Herein, a solution-based route is presented to fabricate Cu(OH)2/Co(OH)2 heterostructures directly on copper foil (CF) as a potential electrode material for supercapacitor application. First a CF is etched to form uniform Cu(OH)2 nanobelt-like structures on its surface. Using the same CF as the substrate, Co(OH)2 nanosheets are electrochemically grown on Cu(OH)2 nanobelts to form a Cu(OH)2/Co(OH)2 heterostructure at room temperature. The electrodeposition duration determines the density of Co(OH)2 nanosheets on the substrate. With an electrodeposition duration of 900 s, a maximum areal capacitance is achieved and thus the same duration is used for forming the heterostructure for device fabrication. An areal capacitance of 413 mF cm–2 is obtained for CF/Cu(OH)2/Co(OH)2, which is higher than those for CF/Co(OH)2 (344 mF cm–2) and CF/Cu(OH)2 (44 mF cm–2) at 5 mA cm–2. This demonstrates the direct use of a surface-modified binder-free electrode in supercapacitor applications. A more detailed analysis following Trasatti and Dunn’s method reveals the primary charge storage through the diffusion-controlled process. Additionally, the performance of an asymmetric supercapacitor device is demonstrated with CF/Cu(OH)2/Co(OH)2 as a positive electrode, activated carbon on CF as a negative electrode, and polyvinyl alcohol–KOH gel as the solid-state electrolyte. An areal capacitance of 113 mF cm–2 at 2.5 mA cm–2 is achieved with an energy density of 3.5 × 10–2 mW h cm–2 at a power density of 1.87 mW cm–2 with a 1.5 V voltage window. A capacitance retention of 81% is measured after 12,000 galvanostatic charge–discharge cycles.