Metal-Organic-Framework-Mediated Fast Self-Assembly 3D Interconnected Lignin-Based Cryogels in Deep Eutectic Solvent for Supercapacitor Applications

A cost-effective and sustainable method is successfully developed to produce lignin-based cryogels with a mechanically robust 3D interconnected structure. A choline chloride-lactic acid (ChCl-LA)-based deep eutectic solvent (DES) is used as a cosolvent to promote the synthesis of lignin-resorcinol-formaldehyde (LRF) gels that can self-assemble a robust string-bead-like framework. The molar ratio of LA to ChCl in DES has a significant influence on the gelation time and properties of the ensuing gels. Moreover, it is discovered that doping the metal-organic framework (MOF) during the sol-gel process can greatly accelerate the gelation of lignin. It takes a mere 4 h to complete the LRF gelation process at a DES ratio of 1:5 combined with 5% MOF. This study yields LRF carbon cryogels doped with copper that exhibit 3D interconnected bead-like carbon spheres with a prominent micropore of 1.2 nm. A specific capacitance as high as 185 F g(-1) can be obtained for the LRF carbon electrode at a current density of 0.5 A g(-1), and it has an excellent long-term cycling stability. This study provides a novel method of synthesizing high-lignin-content carbon cryogels with promising potential for application in the field of energy storage devices.