Anisotropic Proton Conductivity in LnIII–MII MOFs Mediated by Water Molecules: Experimental and Computational Insights

Isostructural metal–organic frameworks (MOFs) built from oxidiacetate, oda2–, [La2Co3(oda)6(H2O)6]·14H2O (LaCo), [Pr2Co3(oda)6(H2O)6]·14H2O (PrCo), and [La2Ni3(oda)6(H2O)6]·14H2O (LaNi) were synthesized and characterized to investigate their proton conduction properties. The presence of a hydrogen-bonding network formed by guest water molecules within the MOF channels was evidenced through crystallographic analysis and computational simulations. Powder conductivity measurements revealed a Grotthuss-type proton transport mechanism with consistent activation energies across all three compounds, but grain boundary effects limited overall performance. Critically, single-crystal conductivity measurements of PrCo revealed anisotropic conduction, exhibiting efficient proton transport along the pore channel (σ ≈ 10–1 S·cm–1 at 316 K, 100% relative humidity) that competes with leading solid-state proton electrolytes. This study highlights the potential of these MOFs as proton-conducting materials and provides a foundation for designing advanced MOF-based devices.