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.