An Organofunctionalized Polyoxovanadium Cluster as a Molecular Model of Interfacial Pseudocapacitance

To design new materials for efficient and energy-dense electrochemical energy storage, it is critical to understand the interactions between metal oxides and alkali ions. Here, we discuss the solution-phase interactions of lithium, sodium, potassium, and alkylammonium cations with the Lindqvist-type polyoxovanadate alkoxide (POV alkoxide) cluster, [V6O7(OCH3)12]. In all cases, the presence of alkali cations positively shifts the half-wave potentials of the reduction events of the POV alkoxide cluster relative to alkylammonium. In contrast, the two cluster oxidation events are not affected by the presence of alkali ions, indicating that the observed changes in reduction potentials are the result of unique interactions with charge-compensating cations. Further analysis of the shift in reduction potential shows that the energetics of cation binding to the reduced cluster depend both on the charge state of the complex and the charge density of the compensating ion. Single-crystal X-ray diffraction studies indicate that two {Li}+ ions undergo site-selective coordination to opposite faces of the octahedron upon complete reduction, manifesting in sluggish reoxidation of this tightly associated, ion-paired species. Thus, this single molecular complex demonstrates redox behavior that spans the range from nonspecific to highly specific cation binding, which is directly analogous to the transition from double-layer capacitance to pseudocapacitance in bulk energy storage electrodes.