Celgard membrane-mediated ion diffusion for synthesizing hierarchical Co(OH)2 nanostructures for electrochemical applications

It remains a challenge to develop simple and mild strategies for synthesizing various Co(OH)2 nanostructures on substrates. Inspired by semipermeable membranes, a Celgard membrane was used as an interface for separating components of the precursor solution and for ion exchange, to construct hierarchical Co(OH)2 nanostructures. In contrast to traditional solvothermal reactions, this Celgard membrane-based method is simple and effective for preparing high-loading Co(OH)2 array (density of approximately 7 mg cm−2) on Ni foam. The permeability of the Celgard membrane was affected by the presence of an organic solvent. The morphology of Co(OH)2 could be adjusted by controlling the OH− diffusion rate across the Celgard membrane. OH− was formed via the hydrolysis of urea solution. The optimized Co(OH)2 nanosheet electrode exhibited a specific capacitance of 2406 F g−1 at 1 A g−1, and good cycling stability (89.1% capacitance retention after 5000 cycles) in 2 M aqueous KOH aqueous solution, using a typical three-electrode cell. An asymmetric supercapacitor was assembled with a Co(OH)2 nanosheet array as the positive electrode, and activated carbon as the negative electrode. The asymmetric supercapacitor exhibited a high energy density (39.2 W h kg−1), high power density (9.2 kW kg−1 at 28.9 W h kg−1), and excellent cycling stability (84.2% capacitance retention after 10,000 cycles).