Improving proton conductivity via crystallinity reduction and sulfonate ligand modification based on UiO-66

Abstract

As promising candidates for novel proton conductors crucial to electrchemical energy technology, metal-organic frameworks (MOFs) have attracted wide attention. The structure of MOFs was optimized via crystallinity reduction and sulfonate ligand modification method to improve the proton conductivity. Benzenesulfonic acid (-BSA), p-aminobenzene sulfonic acid (-pASA), and p-sulfobenzoic acid (-pSA) were used to substitute terephthalic acid ligand in UiO-66 reaction system. A Series of UiO-66 modified target product UiO-66-X and its corresponding metal organic gel product MOG-UiO-66-X (X=-BSA, -pASA or -pAS) were obtained and the proton conductivity was investigated at four different relative humidities, 100%, 75%, 50%, and 30%, at 303 K-353 K. The results showed that MOG-UiO-66-pASA had the maximum values of σ 9.38 × 10-2 S·cm-1 at 100% RH and 353 K with 358 times higher than UiO-66, which may be related to the reduced crystal crystallinity and the acid-base pairs, -SO3H/-NH2 in the structure from the doping modified ligand -pASA . The structural attraction and repulsion effects of acids and bases on protons accelerate the transport speed of protons in the MOG structure, thereby increasing the proton conductivity of materials. In addition, proton transfer mechanism studies reveal that the structural response of materials to variations in ambient temperature and humidity is reflected in their activation energy, prompting a transition between the Grotthuss (proton hopping) mechanism and the Vehicle mechanism. This mechanistic shift ultimately influences the resulting proton conduction performance. This investigation not only proposes a simple crystallinity-reduction strategy and ligand substitution model for MOF modifications to achieve superior proton conductivity, but also provides valuable insights into the adaptation mechanism between proton conduction and environmental conditions, offering guidance for the performance optimization of proton-conductive materials. The findings above lay a research foundation for the performance optimization of proton exchange membranes and the enhancement of their environmental durability in proton exchange membrane fuel cells.