Improving Charge Transfer in Metal–Organic Frameworks through Open Site Functionalization and Porosity Selection for Li–S Batteries

The tunable nature of metal–organic frameworks (MOFs) enables versatility and precise control over structures and properties, making them feasible for potential applications including gas storage and separation, catalysis, molecular sensing, and energy storage. However, porous MOFs are typically insulating, greatly limiting their utility in electrochemical devices. Introducing redox activity to MOFs can promote charge conduction and provide insights into redox mechanisms in a multidimensional coordination system. Toward this end, we prepared a series of anthraquinone (AQ)-functionalized zirconium MOFs (MOF-AQ) to investigate the relationship between porosity and charge transfer reactions using the canonical MOF-808 and NU-1000 frameworks. We evaluated the ability of these frameworks as sulfur host materials to promote polysulfide redox, which are critical conversions for Li–S batteries. Li–S batteries are promising contenders as high-capacity energy storage devices, with an energy density surpassing that of Li ion batteries. We found that the incorporation of AQ on the nodal structure leads to improvement in specific capacity, particularly at high charge and discharge rates. More importantly, enhanced electrochemical behavior of NU-1000-AQ over MOF-808-AQ suggests that larger pore aperture favors overall charge transfer and diffusion. Our study demonstrates there is a delicate balance between AQ loading and available pore volume for ion flux to achieve optimized charge transfer efficiency under fast charge–discharge conditions. Our work provides insight for future designs of novel redox-active MOFs to facilitate charge transport in porous coordination networks.