Synergy between Isomorphous Acid and Basic Metal–Organic Frameworks for Anhydrous Proton Conduction of Low-Cost Hybrid Membranes at High Temperatures

Metal–organic frameworks (MOFs) embedded in polymer have showed efficiency in improving proton conduction of hybrid membranes under hydrated conditions. However, anhydrous proton conduction of such hybrid membranes over 100 °C remains great challenge. Here, proton conductive hybrid membranes combined acid group (−SO3H)- and basic group (−NH2)-modified isomorphous MOFs, namely UiO-66(SO3H) (abbreviated as A, the initial of acid) and UiO-66(NH2) (abbreviated as B, the initial of basic) and a low-cost polymer (chitosan, CS) were prepared. The proton conductivity of the optimum dual MOF-cofilled hybrid membranes (CS/A + B) reached 3.78 × 10–3 S/cm at 120 °C and under anhydrous conditions, under which each component, that is MOF A, MOF B and CS, and single MOF-filled hybrid membranes (CS/A and CS/B) nearly lost proton conduction without exception, producing unprecedented results of one plus one more greater than two. The synergistic effects among UiO-66(SO3H), UiO-66(NH2), and CS on improving conductivity are also observed under hydrated conditions, the highest proton conductivity of CS/A + B reached 5.2 × 10–2 S/cm, which is 1.86, compared to that of the pure CS membrane at 100 °C and 98% relative humidity. The anhydrous proton conductivity of CS/A + B over 100 °C is one of the highest for MOF-based hybrid membranes. MOFs and hybrid membranes were extensively characterized and the proton conductive mechanism was revealed. The achievements open a new avenue for MOF-based anhydrous proton-conducting membranes and would advance the exploration of future application of these MOFs in fuel cells.