Anisotropic Single-Crystal Proton Conduction in a Sodium Chain-Based Coordination Polymer

Coordination polymers (CPs) have been identified as promising
candidate
materials in the field of proton conduction owing to their customizable
and diverse structures. However, research on CPs based on alkali metal
ions has been less advanced, and the mechanism of proton transport
in these materials remains unclear. Herein, a new coordination polymer,
[Na2(pytet)(Hdat)2(H2O)3]·2H2O (abbreviated as Na-PyDat) (pytet4– = pyrene-1,3,6,8-tetrasulfonate, dat = 2-hydroxy-4,6-diamino-1,3,5-triazine),
has been synthesized based on the starting materials of Na4pytet and 2-chloro-4,6-diamino-1,3,5-triazine, with the latter undergoing
in situ transformation to dat under hydrothermal conditions. Na-PyDat is assembled from [Na–O–Na]n double strands and extends to a 3D network through
multiple inter- and intrachain hydrogen bonds. Alternate current (AC)
impedance analysis shows that Na-PyDat exhibits highly
anisotropic single-crystal proton conduction, with a conductivity
of 1.04 × 10–4 S cm–1 at
338 K and 90% relative humidity (RH) along the [100] direction, which
is 1–2 orders of magnitude higher than those along the [011]
and [01̅1] directions, as well as the powder sample under identical
conditions. The high proton conductivity along the a-axis direction is closely related to the continuous hydrogen bond
chain. To the best of our knowledge, this is one of the few studies
reporting single-crystal proton conduction in alkali metal CPs. Moreover,
by mixing Na-PyDat with Nafion, the composite membrane
presents an enhanced proton conductivity of 4.33 × 10–3 S cm–1 at 338 K and 90% RH, highlighting its potential
for future applications in fuel cells.