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.