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.