The exploration of composition−structure−function relationship in proton-conducting solids remains a challenge in materials chemistry. Polyoxometalate-based compounds have been long considered as candidates for proton conductors; however, their low structural stability and a large decrease in conductivity under reduced relative humidity (RH) have limited their applications. To overcome such limitations, the hybridization of polyoxometalates with proton-conducting polymers has emerged as a promising method. Besides, 4f lanthanide ions possess a high coordination number, which can be utilized to attract water molecules and to build robust frameworks. Herein, a Preyssler-type polyoxometalate functionalized with a 9-coordinate Eu 3+ (Eu-[P 5 W 30 O 110 K] 11− ) is newly synthesized and combined with poly(allylamine) with amine moieties as protonation sites. The resulting robust crystalline composite exhibits an ultrahigh proton conductivity >10 −2 S cm −1 at 368 K and 90% RH, which is still >10 −3 S cm −1 at 50% RH, due to the strengthened and extended hydrogen-bonding network.

Single-crystal X-ray structure analysis revealed the precise structure of a Ce-encapsulated Preyssler-type phosphotungstate which was first synthesized by Pope et al. Antonio et al. reported that encapsulated Ce has a 3 + oxidation state and is redox-inactive, which is unlike other Ce-containing phosphotungstates. Therefore, precise structural analysis about Ce position is needed. The encapsulated Ce was located in one of the side cavities, and was coordinated to one H 2 O molecule. The position of Ce was similar to that of Y 3 + and Eu 3 + in similar compounds. This result, together with the bond valence sum calculation, provides additional evidence that the oxidation state of the redox-inactive Ce in the Preyssler-type compound is 3 + .