Enhanced proton conductivity in a layered coordination polymer

Abstract

[Gd(H₄nmp)(H₂O)₂]Cl·2H₂O (1) converts into [Gd₂(H₃nmp)₂]·xH₂O (2) (x = 1 to 4) with a notable increase in proton conductivity. 1 is a charged layered material counter balanced by chloride ions, with proton conductivity values of 1.23 × 10⁻⁵ S cm⁻¹ at 98% relative humidity (RH) and 40 °C. At 98% RH and 94 °C the observed conductivity is 0.51 S cm⁻¹, being to date one of the highest values ever reported for a proton-conducting coordination polymer. This increase is observed during a structural transformation into 2 that occurs at high temperature and RH. While this remarkable conductivity is observed only after transformation and by maintaining high humidity conditions, as-synthesized 2 also shows a conductivity value of 3.79 × 10⁻² S cm⁻¹ at 94 °C and 98% RH, still ranked as one of the highest reported values. Moreover, it is shown that the key factor for high proton conduction is the unusual dynamic structural transformation with water insertion and release of chloride ions.