A non-polarity flexible asymmetric supercapacitor with nickel nanoparticle@ carbon nanotube three-dimensional network electrodes

Asymmetric supercapacitors are one of the promising next-generation electrochemical energy storage devices. However, the necessity to distinguish the polarity of asymmetric supercapacitors makes it more like rechargeable batteries and affects the convenience of practical applications. We herein report a novel flexible asymmetric supercapacitor (FASC) that can be charged without distinguishing the electrode polarity based on CVD-grown nickel nanoparticle@carbon nanotube (Ni@CNT) network films. The FASC is assembled by using the binder-free Ni@CNT as both the negative and positive electrodes but aqueous alkaline solution as the electrolyte. In such a design, one electrode side is involved with the Faradaic redox reactions between nickel nanoparticles and OH-, while the other side works via electric double layer charge accumulation at the interface of CNTs/electrolyte, enabling the device with an asymmetric feature and a high voltage of 1.8 V. Exactly because of the above, the FASC demonstrates almost the same charge/discharge profiles, stable cycling and comparable efficiency for powering small electronics even with frequently alternating the charging direction. With the integration of hydrogel electrolyte of PVA-KOH, our flexible quasi-solid-state FASC device delivers high volumetric energy density of 1.39 mWh cm(-3) and power density of 440 mW cm(-3), as well as excellent cycleability up to 10,000 times (100% capacitance retention). The volumetric performance is even much better than previous reported FASC devices that were designed with complicated electrode architectures. Our work presents a pioneering strategy for developing non-polarity asymmetric electrochemical energy storage systems.