Nevertheless, perfluorinated polymers usually involve complex synthesis process. Besides, they are amorphous polymers without clear structures, which make it challenging to uncover their proton transport mechanism for directionally improving the proton conducting performances. [4] Recently, crystalline porous materials such as metal-organic frameworks (MOFs) and metal-oxo clusters have been extensively investigated in the pursuit of new proton conductors. [5] In particular, porous MOFs have attracted great attention because of their diverse structures and high porosity. Besides, their proton conducting performances can be effectively improved through structural functionalization, such as decorating functionalized groups (e.g., SO 3 H, COOH, and PO 3 H 2 ) on the linkers or adding proton carriers (e.g., imidazole and sulfuric acid) into the porous channels. [6] Nevertheless, many porous MOFs are sensitive to water and easily to lose guest/ water molecules, especially under high temperature, which make a great challenge for their application under these conditions. [7] Compared with MOFs, crystalline metal-oxo clusters, especially those constructed by high valence metal ions and O-contained ligands (e.g., SO 4 2− and PO 4 2−) usually show more stable in aqueous media. [8] In addition, their hydrophilic O-rich surface can function as proton hopping sites and binding sites Metal-oxo clusters have emerged as advanced proton conductors with welldefined and tunable structures. Nevertheless, the exploitation of metal-oxo clusters with high and stable proton conductivity over a relatively wide temperature range still remains a great challenge. Herein, three sulfate groups decorated zirconium-oxo clusters (Zr 6 , Zr 18 , and Zr 70 ) as proton conductors are reported, which exhibit ultrahigh bulk proton conductivities of 1.71 × 10 −1 , 2.01 × 10 −2 , and 3.73 × 10 −2 S cm −1 under 70 °C and 98% relative humidity (RH), respectively. Remarkably, Zr 6 and Zr 70 with multiple sulfate groups as proton hopping sites show ultralow activation energies of 0.22 and 0.18 eV, respectively, and stable bulk conductivities of >10 −2 S cm −1 between 30 and 70 °C at 98% RH. Moreover, a time-dependent proton conductivity test reveals that the best performing Zr 6 can maintain high proton conductivity up to 15 h with negligible loss at 70 °C and 98% RH, representing one of the best crystalline cluster-based proton conducting materials.
