Sulfonic Groups Lined along Channels of Metal-Organic Frameworks (MOFs) for Super-Proton Conductor

Designing high-performance proton-conducting metal-organic frameworks simultaneously having highly hydrothermal stability and a high-density proton carrier remains a great challenge. Fe-MIL-88B is a classic metal-organic framework (MOF) with a large-size one-dimensional (1D) channel lined with a high-density uncoordinated metal atom for postfunctionalization; however this MOF cannot act as a proton conductor due to the weak hydrothermal stability. Here, we prepared an ultrastable isostructure Cr-MIL-88B, which is subsequently functionalized by anchoring 3-pyridinesulfonic acid and 2-(4-pyridyl) ethanesulfonic acid on the naked Cr atoms exposed on the surface of the host-framework, producing two new MOFs, i.e. Cr-MIL-88B-pyridine sulfonic acid (abbreviated as Cr-MIL-88B-PSA) and Cr-MIL-88B-pyridine ethanesulfonic acid (abbreviated as Cr-MIL-88B-PESA). Thus, Cr atoms on the host framework were modified by functional sulfonic groups, which stick out toward the center of the channel forming ordered high-density sulfonic groups as proton donors along the open channel and achieving the highest proton conductivity of 4.50 × 10^-2 S cm^-1 for Cr-MIL-88B-PESA and 1.58 × 10^-1 S cm^-1 for Cr-MIL-88B-PSA, surpassing that of the Nafion membrane.