Coordination derived stable Ni–Co MOFs for foldable all-solid-state supercapacitors with high specific energy

In this work, polycrystalline flower-like Ni–Co metal–organic frameworks (Ni–Co MOFs) have been fabricated by a simple hydrothermal method with further electrochemical activation. The mixed metal centers and polycrystalline nature are beneficial for the transport of electrons and ions. The Ni–Co MOF exhibits a high specific capacity of 833 C g−1 at 0.5 A g−1 and maintains 714 C g−1 at 20 A g−1. Meanwhile, a very good cycling stability is achieved after 5000 cycles. All-solid-state supercapacitors have been assembled using the Ni–Co MOF, polybenzimidazole (PBI) and activated carbon (AC). A high specific capacitance of 172.7 F g−1 at 0.5 A g−1 in a large potential window of 1.8 V has been achieved, which corresponds to a high specific energy of 77.7 W h kg−1 and 0.45 kW kg−1, respectively. Meanwhile, the devices also exhibit very good flexibility under bending and folding conditions with no obvious capacitance change due to the good mechanical properties of PBI. The influence of temperature on the performance of the all-solid-state devices has been evaluated. To avoid side reactions at high temperature, the device works better at relatively low temperature. This work suggests that the Ni–Co MOF and PBI solid polymer electrolyte based foldable all-solid-state supercapacitors are very promising for application in wearable and portable electronic devices.