Ultramicroporous Co(II)-Squarate framework with Co4(µ3-OH)4 cubane Structures: A pseudocapacitor electrode for advanced energy storage devices

Abstract

The increasing consumption of non-renewable resources like natural gas, coal, and crude oil has driven the urgent need for sustainable energy storage materials for batteries and supercapacitors. Using metal–organic frameworks (MOFs) as active electrodes for energy storage or conversion is very attractive; however, it is challenging due to their limited robustness in acidic/basic environments. In this study, we present the development of a chemically robust 3D Co-squarate framework (Co-SQ), a metal–organic framework (MOF) with high structural stability across a wide pH range (2 to 12), featuring a Co₄(µ₃-OH)₄ cubane secondary building unit (SBU) similar to MOF-5. This material was synthesized from inexpensive, readily available precursors without requiring calcination or chemical modification for supercapacitive applications. Co-SQ demonstrates decent gravimetric (456F g^–1) and areal (570 mF cm^–2) capacitance at a current density of 1 A g^–1 and 1.25 mA cm^–2, respectively, along with cycling stability over 10,000 cycles and decent energy and power densities. We further explored its application in advanced energy storage systems by fabricating Swagelok-type cells, coin cells, and flexible pouch-like devices (LSC@PI), which powered an LED bulb for several minutes. These results highlight Co-SQ’s superior electrochemical performance, comparable to, or exceeding, that of other MOF-based and commercial supercapacitors.