Confinement of Organic Molecules in Microporous Electrodes for Enhanced Energy Storage

Microporous electrodes (pores <2 nm in width) can confine molecules into uniquely packed, charged volumes that exhibit characteristics different from molecules from a bulk solution interacting with an electrode surface. By using surface and electrochemical characterizations, we show the confinement of organic molecules in micropores can shift their redox potentials beyond the classical Nernstian regime, with a shift as large as 252 mV. We identify an excess contribution to the electrochemical potentials of ions that leads to the thermodynamic limit for these shifts and use continuum-scale simulations from a modified Donnan model to confirm this limit and derive deviations from it. Density functional theory simulations confirm that micropore confinement can change the mechanism of charge transfer. We find trends in behavior in micropore environments for organic and metalorganic molecules in aqueous solutions based on their electrophilicity, charge, core molecules, and molecular functionalizations (i.e., side chains). Finally, using micropore confinement on the high and low potential sides of an enclosed secondary battery to increase the open circuit voltage, we demonstrate an increase in average discharge cell voltage of 39% and a corresponding increase in discharge energy density of 36% by replacing macroporous electrodes with microporous electrodes.