Superior proton conductivity of amino acid-modified UiO-66-(COOH)2 embedded in chitosan: Mechanistic insights into the acid-base interactions

We focus on optimizing acid-base interactions and hydrogen bonding networks to achieve enhanced proton conductivity under low relative humidity (RH) and high temperature. By functionalizing UiO-66-(COOH)₂ with glutamic acid (Glu) and lysine (Lys), we generate Glu-UiO-66-(COOH)₂ and Lys-UiO-66-(COOH)₂. These modified materials are subsequently incorporated into chitosan (CS) to produce the composites Glu-UiO-66-(COOH)₂@CS and Lys-UiO-66-(COOH)₂@CS. The successful incorporation of amino acids and cross-linking between -COOH and -NH₂ groups in the composites, confirmed by FTIR and PXRD analyses, significantly enhances the structural integrity and durability by strengthening the network, and reducing polymer chain mobility. Proton conductivity assessments reveal that Lys-UiO-66-(COOH)₂@CS-7 exhibits a remarkable conductivity of 0.022 S/cm at 100 % RH and 363 K, outperforming Glu-UiO-66-(COOH)₂@CS-7. The lower activation energy (E_a) of 0.28 eV for Lys-UiO-66-(COOH)₂@CS-7, compared to 0.336 eV for Glu-UiO-66-(COOH)₂@CS-7, highlights the significant improvement in intramolecular acid-base interactions and hydrogen bonding. Furthermore, Lys-UiO-66-(COOH)₂@CS-7 maintains notable proton conductivity at 43 % RH, with an E_a of 0.216 eV, demonstrating its efficacy in low-humidity conditions. These findings underscore the profound impact of amino acid modifications and cross-linking on proton conductivity by reinforcing acid-base interactions, hydrogen bonding networks, and proton transfer efficiency.