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

Abstract 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·H₂O (GH–PMo₁₂), (CN₄H₇)₃(PMo₁₂O₄₀)·H₂O (AGH–PMo₁₂), and (CN₅H₈)₃(PMo₁₂O₄₀)·3.5H₂O (DAGH–PMo₁₂). Proton conductivity of GH–PMo₁₂ in the [001] direction reaches up to 0.19 S cm⁻¹ at 85 °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 pK_a, proton density, and spatial structure. Additionally, GH–PMo₁₂ is fabricated into composite membrane with a peak proton conductivity of 5.31 × 10⁻² S cm⁻¹, which exhibits promising potential for real‐world applications.