All-Printed High-Performance Flexible Supercapacitors Using Hierarchical Porous Nickel–Cobalt Hydroxide Inks

Supercapacitors have received great attention in the energy storage of flexible wearable electronic products due to their fast charging and discharging. However, low energy density and poor rate performance have always been the main factors limiting their application. Herein, by adjusting the ratio of Ni and Co, optimized porous nanoflower-like Ni/Co-layered double hydroxides (LDH) deliver a large specific capacitance value of 1575 F g–1 at 1 A g–1 and superior rate performance with 86.3% capacity retention from 1 to 50 A g–1. The superior electrochemical performance is mainly attributed to the transfer of electrons from Co to Ni, resulting in an increase in more conductive Co3+ and more active Ni2+. Simultaneously, rich porosity and wider interlayer spacing of Ni/Co LDH facilitate easy electrolyte access and fast ion diffusion within active materials. By combining with screen printing, a flexible Ni3Co1 LDH@graphene//activated carbon (Ni3Co1 LDH@G//AC) asymmetric supercapacitor (ASC) is fabricated, exhibiting outstanding specific areal capacitance of 599 mF cm–2 at 1 mA cm–2, excellent areal energy density of 0.27 mW h cm–2, and power density of 49.9 mW cm–2. Moreover, the capacity of ASC remains 95.8% even after bending to different angles and for 400 times, which shows excellent flexibility. After two ASCs are connected in series and packaged, they can power a watch for more than 60 min after only charging for 50 s, and even if the device is worn on the hand and completely submerged in water, they can still power the watch normally. This work provides inspiration for large-scale production of high-performance supercapacitors and the integration of supercapacitors into wearable electronic products.