Interfacial nanoflower-nanosheet heterojunction engineering via in-situ ion exchange strategy for ultra-stable asymmetric supercapacitors.

Designing highly dispersed and tightly bound heterojunction to achieve rapid ion transport and electron transfer is a promising strategy for improving electrochemical performance of energy storage system. However, weak interfacial interactions derived from two phase-heterostructure usually enhance charge transfer resistance, which makes poor system efficiency. In this work, an ion exchange strategy was employed to derive iron cobalt layered double hydroxide (FeCo-LDH) from zeolitic imidazolate framework (ZIF-67) on nickel cobalt oxide (NiCo2O4) substrate. The resulting nanoflower-nanosheet FeCo-LDH/NiCo2O4 heterojunction was grown on Ni foam and denoted as FC0.5-LDH/NC-NF. Benefiting from interfacial charge redistribution and enhanced electron transfer, the FeCo-LDH/NiCo2O4 heterointerface lowers the deprotonation energy barrier and thereby improves the charge-storage kinetics. Significantly, the heterostructured electrode materials display large specific surface area, abundant mesoporous structures, outstanding rate capability and superior specific capacitance of 1935 F g-1 at 0.5 A g-1. Moreover, an asymmetric supercapacitor was assembled using FC0.5-LDH/NC-NF as the positive electrode and activated carbon as the negative electrode (FC0.5-LDH/NC-NF//AC). The device exhibits a wide operating window of 1.5 V, an energy density of 76.6 Wh kg-1 at a power density of 375 W kg-1, and excellent cycling stability, retaining 83.6% of its capacitance with a Coulombic efficiency of 99.6% after 20,000 cycles (620 h). This interfacial engineering strategy not only offers a promising approach for the construction of heterojunction to improve electrochemical performance, but also provides broad prospects for the practical application of SCs in high-efficiency storage systems.