Engineering Ultrathin Co(OH)₂ Nanosheets for High-Performance All-Solid-State Flexible Transparent Supercapacitors

Flexible and transparent energy storage devices are crucial for smart electronics. Herein, we developed a facile strategy to fabricate a flexible, transparent supercapacitor (FTSC) with high performance. The electrode’s areal capacitance is directly proportional to the thickness of the electrode, but its optical transparency decreases inevitably with the number of layers/materials’ final depth. In this study, we prepared electrodes by depositing Co(OH) 2 on the ITO/PET substrate and optimized the number of Co(OH) 2 depositions to attain the best-performing electrode. The optimized Co(OH) 2 /ITO/PET electrode exhibited excellent optoelectronic and electrochemical performances with 95.4% optical transmittance in the visible region and the areal capacitance of 2021 μF cm –2 at 50 μA cm –2 . The fabricated all-solid-state FTSC device showed good optical transparency ( T 550nm = 86.7%) and a remarkable capacitance of 149.2 μF cm –2 at 10 μA cm –2 current density. The device demonstrated the highest energy density of 0.040 μW h cm –2 and power density of 27.4 μW cm –2 with outstanding mechanical flexibility under various bending angles and long-term cyclic stability over 40,000 GCD cycles, revealing flexibility and stability.