Tuning the interfacial chemistry between 2D conductive metal-organic frameworks and graphene derivatives for unlocking energy storage performance

Abstract

Two-dimensional conductive metal-organic frameworks (2D-CMOFs) are emerging materials for electrochemical energy storage. However, their performance is often hindered by insulating interfaces between the CMOF particles. To address this limitation, we introduced and experimentally demonstrated the development of interfacial bridges through the coordination of the metal nodes of a nickel 2D-CMOF (2DNi) and the carboxyl groups from a densely and selectively functionalized graphene (graphene acid, GA). To achieve this, the 2DNi MOF was grown in the presence of GA at low temperatures and ambient pressure in water. The obtained 2DNiGA electrode material shows more than 100% capacitance improvement compared to electrodes synthesized under identical conditions using either unfunctionalized pristine graphene or GA-added post-synthesis via physical mixing. In the latter case no formation of coordination bonds with the nickel nodes of the MOF is observed, confirming our initial hypothesis on the importance of the seamless interfacial bridging. Moreover, a hybrid, flexible supercapacitor using 2DNiGA as a positive and GA as a negative electrode delivers a superior gravimetric energy density of 71 Wh kg−1 at 0.8 kW kg−1. Notably, it also delivers a similarly high volumetric energy density of 73.8 Wh L-1 at 0.836 kW L-1. The data indicate that the asymmetric supercapacitor not only favorably compares with top-tier supercapacitor electrodes, but also offers a qualitative advantage through its combined superior gravimetric and volumetric energy content. The supercapacitor is also robust, retaining more than 94% of its initial performance after 10,000 charge/discharge cycles, or when bent at 180°. The introduced concept of such tailored interfacial bridging offers broad potential for improving the properties of MOF materials in electrochemical energy storage.