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) (pytet^4- = 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.