Enhanced Proton Conduction via Proton-Coupled Electron Transfer Reaction by a Keplerate-Type Polyoxometalate Capsule

Polyoxometalates (POMs), anionic metal-oxide clusters, are actively studied for their versatile structural designs and element selectivity. A series of Keplerate-type POMs with core-shell structures, known as POM capsules, that feature a Keggin-type POM core, has been reported. These POM capsules, with their neutral to negative charge and large molecular surface area, can serve as platforms for proton (H⁺) conduction. In this study, we aimed to introduce proton carriers into the POM capsule through a proton-coupled electron transfer (PCET) reaction. It is crucial to avoid excessive reduction, as it increases electrostatic interaction between the proton and the POM capsule, hindering proton conduction. The number of incorporated electrons was varied from 0 to 11, and the POM capsule containing 3 electrons exhibited proton conductivity an order of magnitude higher than that of pristine or highly reduced POM capsules. This study serves as a proof of concept, demonstrating that PCET, combined with the core-shell structure, can effectively enhance the proton conductivity without being limited by the choice of constituent elements.