Confinement Sulfuric Acid to Construct Acidic Microenvironment in Polyoxometalate-Based Metal–Organic Framework for Enhanced Proton Conduction

The development of stable proton-conducting materials remains a key challenge, as strong acid modification often compromises the structural stability of metal–organic frameworks (MOFs). However, this strategy is fundamentally limited by the MOF structural instability under highly acidic conditions. To overcome this challenge, a polyoxometalate (POM)-based MOF, CUST-834 (CUST = Changchun University of Science and Technology), confines sulfuric acid within its channels via the surface hydroxyl groups of POMs. This unique confinement mechanism not only establishes a stable acidic microenvironment in CUST-834, but also enables the encapsulated H2SO4 to serve as both proton source and conduction medium. The synergistic effect results in an intrinsic proton conductivity of 3.63 × 10–3 S cm–1 under conditions of 90 °C and 98% relative humidity (RH). The strong host–guest interactions between the hydroxyl groups of POMs and sulfuric acid molecules enable CUST-834 to preserve its structural integrity.