High Proton Conduction in a Hydrogen-Bonded Co(II) Framework through Pendant Carboxylic Acid-Decorated 1D Channels

Abstract

A two-dimensional Co(II) metal–organic framework, FCG-7 (FCG = Framework Chemistry Group of CSIR-CSMCRI), containing pendant carboxylic acid (−COOH) groups was rationally constructed to investigate the role of intrinsic proton-donating sites in solid-state proton transport. Crystallographic study divulged a hydrogen-bonded layered framework in which the uncoordinated −COOH groups project toward one-dimensional channels, providing accessible sites for interaction with guest water molecules. The activated framework (FCG-7a) exhibits good hydrolytic and thermal stability, a lower water contact angle of 48.6°, and a proton conductivity of 1.25 × 10–3 S cm–1 at 80 °C and 95% relative humidity (RH). The low activation energy (0.22 eV) is in agreement with a predominantly Grotthuss-type proton-hopping mechanism. Proton transport is strongly humidity-dependent, with conductivity increasing approximately 1600-fold from 60% to 95% RH at 80 °C, highlighting the participation of confined water molecules in establishing extended hydrogen-bonding pathways. To evaluate the contribution of pendant −COOH groups, a structurally analogous framework, FCG-10, lacking pendant carboxylic acid functionality, was prepared through a molecular-scissoring strategy. FCG-10a exhibits a lower conductivity of 3.5 × 10–4 S cm–1 under identical conditions. These results highlight a structure-guided strategy wherein accessible acidic sites and water-mediated hydrogen-bonding networks promote proton transport in crystalline frameworks