Ultrahigh Proton Conductivities of Postmodified Hf(IV) Metal-Organic Frameworks and Related Chitosan-Based Composite Membranes

Recently, researchers have focused on preparing and studyingprotonexchange membranes. Metal-organic frameworks (MOFs) are candidatesfor composite membrane fillers due to their high crystallinity andstructural characteristics, and Hf-based MOFs have attracted our attentionwith their high porosity and high stability. Therefore, in this study,Hf-based MOFs were doped into a cost-effective chitosan matrix asfillers to fabricate composite films having excellent proton conductivity(& sigma;). First, the nanoscale MOFs Hf-UiO-66-(OH)(2) (1) and Hf-UiO-66-NH2 (2) were chemicallymodified by a ligand design strategy to obtain SA-1 and CBD-2 bearing free -COOH units. The proton conductivitiesof SA-1 and CBD-2 under optimal test conditionsreached 1.23 x 10(-2) and 0.71 x 10(-2) S cm(-1). After that, we preparedcomposite membranes CS/SA-1 and CS/CBD-2 by the casting method; tests revealed that the introduction of MOFsimproved the stabilities and & sigma; values of the membranes, andtheir best & sigma; could reach above 10(-2) S cm(-1) under 100 & DEG;C/98% RH. Further structural characterizationand activation energy calculation revealed the conductive mechanismof the composite films. This investigation not only proposes a novelchemical modification method for optimizing the & sigma; of MOFs butalso promotes the development of MOF-doped composite membranes andprovides a basis for future applications of MOFs in fuel cells.