Controllable Synthesis and Ultrahigh Anisotropic Single-Crystal Proton Conduction of a Hydrogen-Bonded Organic Framework

Hydrogen-bonded organic frameworks (HOFs) with inherent well-defined hydrogen-bond networks are promising proton conduction materials. Herein, four three-dimensional HOFs were controllably assembled from 1,2,4,5-benzenetetracarboxylic acid and guanidinium of different chain lengths with certain ratios, in which GC-1 has undergone a water-induced single-crystal-to-single-crystal (SCSC) transformation to a more stable GC-2 with successive π–π stacking interactions. Notably, the ideal single-crystal sample of GC-2 exhibits an ultrahigh proton conductivity of 1.78 × 10–2 S cm–1 along the [100] direction at ambient temperature and 98% RH. This sample is highly anisotropic with 3–5 orders of magnitude higher than those along the [010] and [001] directions, which is closely related to favorable proton-transfer paths of 1D highly hydrophilic channels formed by the consecutive hydrogen-bonded network between protonated guanidinium cations and carboxylic acid anions along the a-axis direction. Compared with the pelletized samples of GC-2, GC-3, and GC-4, their composite membranes with Nafion show significant enhancement of proton conduction with remarkable values of 1.33 × 10–1, 1.92 × 10–1, and 1.98 × 10–1 S cm–1 at 338 K and 98% RH, indicating great application potential.