Incorporating Metal-Organic Frameworks into Graphene-Based Electrodes for High-Performance Capacitive Energy Storage
The high porosity and openness of metal-organic frameworks (MOFs) have been extensively studied in gas adsorption, catalysis, and the use as templates for nanoporous materials synthesis; but its potential in electrochemical energy storage is not well understood. MOFs consist of redox active metal clusters and tunable pore size, which enable the ability to contribute to pseudocapacitance in electrochemical supercapacitors. Although MOFs are generally considered as poor conductors, doping MOFs with highly conductive graphene sheets can potentially enhance the capacitance in pure graphene capacitive devices. In this study, a four-metal MOF-74 (M4M-MOF-74) was selected to investigate the capacitance enhancement due to its unique coordinately unsaturated metal sites. The MOF-74 bears one dimensional channels that can adsorb H+ and Li+ very well, but not Na+ in aqueous electrolytes, allowing ions of appropriate size to access the framework and to fully interact with the metal sites. Moreover, the MOF/graphene hybrid electrodes demonstrate great conductivity, high areal capacitance, and good stability in an H2SO4 aqueous electrolyte. At a scan rate of 10 mV/s, the hybrid electrode exhibits a high areal capacitance of 54.1 mF/cm2, which is about four times higher than a pure graphene electrode. This work could potentially open up a new application for MOFs in electrochemical capacitors.