Reversible Electrodeposition of Potassium-bridged Molecular Vanadium Oxides: A New Approach Towards Multi-Electron Storage.

Molecular metal oxides, so-called polyoxometalates (POMs), have shown outstanding performance as catalysts and lately attracted interest as materials in energy conversion and storage systems due to their capability of storing and exchanging multiple electrons. Here, we report the first example of redox-driven reversible electrodeposition of molecular vanadium oxide clusters, leading to the formation of thin films. The detailed investigation of the deposition mechanism reveals that the reversibility is dependent on the reduction potential. Correlating electrochemical quartz microbalance studies with X-ray photoelectron spectroscopy (XPS) data gave insight into the redox chemistry and oxidation states of vanadium in the deposited films in dependence on the potential window. A multi-electron reduction of the polyoxovanadate cluster, which facilitates the potassium (K+ ) cation-assisted reversible formation of potassium vanadium oxide thin films was confirmed. At anodic potentials, re-oxidation of the polyoxovanadate and complete stripping of the thin film is observed for films deposited at potentials more positive than -500 mV vs. Ag/Ag+ , while electrodeposition at more negative cathodic potential reduces the electrochemical reversibility of the process and increases the stripping overpotential. As proof of principle, we demonstrate the electrochemical performance of the deposited films for potential use in potassium-ion batteries.