Design and Analysis of POM-Guanidine Compounds: Achieving Ultra-High Single-Crystal Proton Conduction

The "visible" proton-conducting pathway offers a distinct advantage for researching the mechanism of proton conducting materials at the molecular level and developing new materials. To achieve this, three crystalline materials are constructed via acid-base chemistry based on phosphomolybdic acid and diversfied guanidine, namely, (CN3H6)6(PMo12O40)2 center dot H2O (GH-PMo12), (CN4H7)3(PMo12O40)center dot H2O (AGH-PMo12), and (CN5H8)3(PMo12O40)center dot 3.5H2O (DAGH-PMo12). Proton conductivity of GH-PMo12 in the [001] direction reaches up to 0.19 S cm-1 at 85 degrees C and 98% RH, as elucidated by impedance studies of single crystals. The clear proton transport path is proposed through the analysis of single crystal structure. Moreover, impedance studies of powder crystals reveal that the proton conductivity of GH-PMo12 is higher than that of the other two compounds. The underlying reasons for this result are clarified through the analysis of the pKa, proton density, and spatial structure. Additionally, GH-PMo12 is fabricated into composite membrane with a peak proton conductivity of 5.31 x 10-2 S cm-1, which exhibits promising potential for real-world applications. Three unique polyoxometalate-based crystalline materials via acid-base chemistry are constructed by hydrogen bonds, in which compound (CN3H6)6(PMo12O40)2 center dot H2O (GH-PMo12) exhibits highly anisotropic single-crystal proton conduction with ultra-high conductivity along [001] direction, resulting from a continuous zigzag hydrogen bonds chain.image