pH-Dependent Structural Engineering of Sulfonate-Carboxylate Cu-MOFs for High Proton Conductivity

Metal-organic frameworks (MOFs) with free carboxylic acid (COOH) groups are promising for solid-state proton-conducting materials, owing to the Brønsted acidity, polarity, and the hydrogen-bonding ability of COOH groups. In this work, two Cu-MOFs with different dimensions were synthesized by adjusting the pH of the reaction solution using disodium-2,2'-disulfonate-4,4'-oxidibenzoic acid (Na₂H₂DSOA) and 4,4'-bipyridine (4,4'-bpy) as ligands to coordinate with Cu(II). The resulting compounds, CuDSOA-1 (([Cu(4,4'-bpy)₂(H₂O)₂][Cu(H₂DSOA)₂(4,4'-bpy)(H₂O)₂]·12H₂O)) and CuDSOA-2 ([Cu₂(DSOA)(4,4'-bpy)₂(H₂O)₂]·4H₂O), have distinct dimensionalities and structures, mainly due to the pH's effect on carboxylic acid deprotonation. Notably, CuDSOA-1 with abundant COOH groups, uncoordinated sulfonate groups, and water molecules shows a significantly enhanced proton conductivity of 2.46 × 10^-2 S cm^-1 at 95 °C and 98% RH, surpassing CuDSOA-2 (3.40 × 10^-5 S cm^-1 at 85 °C and 98% RH). The conductivity mechanism was found to be a Grotthuss mechanism, confirmed by deuterium-hydrogen isotopic effects. This study offers a method to control the coordination of sulfonic-carboxylic acid ligands with Cu(II) by pH adjustment, aiming to create MOFs with ultrahigh proton conductivity.