Proton conduction via lattice water molecules in oxalato-bridged lanthanide porous coordination polymers

Abstract

The proton conducting properties of two different structural types of porous coordination polymers [La₂(ox)₃(H₂O)₆]·4H₂O (1) and [Er₂(ox)₃(H₂O)₆]·12H₂O (2), where ox²⁻ = oxalate, were investigated. 1 has a two-dimensional layered structure, whereas 2 has a three-dimensional structure. Both 1 and 2 have hydrophilic one-dimensional channels filled by lattice water molecules with hydrogen-bonding networks. The coordinated H₂O molecules are Lewis acidic due to the lanthanoid ions donating protons to lattice-filling H₂O molecules, thereby forming efficient proton conduction pathways. Alternating-current impedance analyses of 1 and 2 indicated significant proton conduction (σ = 3.35 × 10⁻⁷ S cm⁻¹ at 368 K for 1, 1.79 × 10⁻⁶ S cm⁻¹ at 363 K for 2 under RH = 100%, with E_a = 0.35 eV for 1 and 0.47 eV for 2), which was attributed to the Grotthuss mechanism via the lattice H₂O molecules.