Enhancement of active sites and stability by ion exchange in 3D ZIF-L for electrochemical energy storage

The practical application of metal–organic frameworks (MOFs) for energy storage is faced with great challenges, such as poor structural stability and limited active sites. Herein, we have co-designed a three-dimensional (3D) self-assembled hexagonal zeolitic imidazolate framework-L (ZIF-L) structure with a 3D conformation that greatly reduces the self-aggregation of two-dimensional (2D) layered materials. Due to the rational design of the specific morphology and atomically different coordination abilities of Ni²⁺ and Co²⁺ in the framework, the micro-nano electric field is constructed, and the structural stability and electrochemistry reaction activity of ZIF-L are obviously improved. Moreover, the consecutive hollow structure is also formed by regulating the Ni–Co ratio, which can significantly enhance the specific capacitance and cycling stability of the Ni-ZIF-L electrode through the formation of fast electrolyte ions transfer channels. Consequently, the Ni-ZIF-L-40 electrode exhibits a high specific capacity (568.9 F·g⁻¹ at 0.5 A·g⁻¹) and long cycle stability (89.5% retention after 5000 cycles at 5 A·g⁻¹). In addition, the Ni-ZIF-L-40//activated carbon (AC) asymmetric supercapacitor assembled using AC also shows an excellent cycling stability (91.1% retention after 4000 cycles at 5 A·g⁻¹). This study may open a new window for the practical application of intrinsic MOFs-based electrodes for energy storage and conversion.