Hydrophilic vs Hydrophobic: Exploring Facet-Dependent Proton Transport Channels in Single Crystal of Ring-Shaped Trimeric Isopolytungstate

Research on the facet-dependent anisotropic proton conduction of single crystals is advantageous to visualize the molecular level mechanism of polyoxometalate (POM)-based crystalline proton-conducting materials. Herein, we constructed a novel ring-shaped trimeric isopolytungstate {(RuW 11 O 37 )O} 3 18– ( 1a ) with an inner hydrophobic space and an outer hydrophilic surface. After assembly through the hydrated cation linkers, polyanion 1a rings formed the single-crystal Rb 11 Na 6 H· 1a ·36H 2 O ( 1 ) with (01̅0), (101), and (1̅11) faces, which have different sizes and types (hydrophilic-Grotthuss-type and/or hydrophobic-Vehicle-type) of proton transport channels. Accordingly, proton conductivity (σ) of single-crystal 1 can reach 3.84 × 10 –2 S cm –1 along the direction perpendicular to the (01̅0) face at 358 K under 85% relative humidity (RH), which is equivalent to that of crystal pellet (2.09 × 10 –2 S cm –1 ) and accompanied by the ratio of σ (01̅0):σ (101):σ (1̅11) = 121:5:1. It reveals a visible (01̅0) facet-dependent proton-conducting pathway in the large-sized hydrophilic pore channels through a water-induced hydrogen bond network under the Grotthuss hopping mechanism.