Proton Conduction Study on Water Confined in Channel or Layer Networks of LaIIIMIII(ox)3·10H2O (M = Cr, Co, Ru, La)

Proton conduction of the LaIIIMIII compounds, LaM­(ox)3·10H2O (abbreviated to LaM; M = Cr, Co, Ru, La; ox2– = oxalate) is studied in view of their networks. LaCr and LaCo have a ladder structure, and the ladders are woven to form a channel network. LaRu and LaLa have a honeycomb sheet structure, and the sheets are combined to form a layer network. The occurrence of these structures is explained by the rigidness versus flexibility of [M­(ox)3]3– in the framework with large LaIII. The channel networks of LaCr and LaCo show a remarkably high proton conductivity, in the range from 1 × 10–6 to 1 × 10–5 S cm–1 over 40–95% relative humidity (RH) at 298 K, whereas the layer networks of LaCr and LaCo show a lower proton conductivity, ∼3 × 10–8 S cm–1 (40–95% RH, 298 K). Activation energy measurements demonstrate that the channels filled with water molecules serve as efficient pathways for proton transport. LaCo was gradually converted to LaIIICoII(ox)2.5·4H2O, which had no channel structure and exhibited a low proton conductivity of less than 1 × 10–10 S cm–1. The conduction–network correlation of LaCo­(ox)2.5·4H2O is reported.本研究针对系列La³⁺M³⁺型化合物LaM(ox)₃·10H₂O（缩写为LaM；其中M=Cr、Co、Ru、La，ox²⁻为草酸根（oxalate））的质子传导（proton conduction）性能，结合其骨架结构展开系统探究。LaCr与LaCo化合物呈现梯形结构，多条梯形链相互编织形成通道型骨架网络；LaRu与LaLa化合物则具有蜂窝状片层结构，片层间相互堆叠构筑为层状骨架网络。这类结构的形成可归因于骨架中与大尺寸三价镧离子结合的[M(ox)₃]³⁻配体的刚性与柔性差异。在298 K、相对湿度（relative humidity, RH）40%~95%的测试条件下，LaCr与LaCo的通道型骨架网络展现出极高的质子传导率，范围为1×10⁻⁶至1×10⁻⁵ S·cm⁻¹；而其层状骨架网络的质子传导率则较低，约为3×10⁻⁸ S·cm⁻¹（测试条件：40%~95% RH，298 K）。活化能测试结果表明，填充水分子的通道可作为质子传输的高效通路。LaCo化合物会逐步转化为LaCo(ox)₂.₅·4H₂O，该产物无通道型结构，质子传导率低于1×10⁻¹⁰ S·cm⁻¹。本研究同时报道了LaCo(ox)₂.₅·4H₂O的传导性能与骨架结构的关联规律。