Porous Environmental Polarity as a Critical Descriptor for Efficient Proton Conductivity in Metal-Organic Frameworks

Recent years have witnessed growing research interest in proton-conducting metal-organic frameworks (MOFs) owing to the characteristics of highly ordered pores, oriented packing of crystals, and particularly designable structures. However, how to construct a suitable microenvironment in MOF pores to form optimal proton transport pathways remains challenging. In this study, four MOFs with similar porous diameters but different microenvironments have been screened to study how porous environments influence proton conduction for the first time. It is found that the proton conductivity exhibits a linear relationship with porous polarity evaluated by the Henry constant of water in these MOFs, and the proton-conductive mechanism is also controlled by porous polarity. Notably, MIL-53(Al)-NH₂ demonstrates significant proton conduction (1.02 × 10^-3 S cm^-1) at 80 °C and 98% relative humidity, benefiting from its high pore polarity and Grotthuss hopping mechanism. This study offers useful guidelines for designing efficient proton-conducting MOFs.