Proton conductive N-heterocyclic metal–organic frameworks

Recently, metal–organic frameworks (MOFs) have received widespread attention due to their roles as solid electrolytes in fuel cells. Herein, we will comprehensively summarize proton conductive MOFs constructed by N-heterocyclic organic ligands. The reason we are interested in these MOFs is that the main building blocks of such MOFs, N-heterocyclic organic ligands, can not only take part in coordination with the central metal ions to form stable structures, but also serve as a proton source, and can also build up complicated H-bonding systems with other components within the frameworks and adsorbed H2O units. These characteristics are very beneficial to them as promising proton conductors. Thus, we will summarize the preparation strategies, structural characteristics, proton conductivity and applications of the related MOFs in the light of the types of organic ligands (pure N-heterocyclic ligands; N-heterocyclic ligands bearing different substituted units covering carboxylate, hydroxy, phosphonate group, and sulfonic acid units, and so on). Finally, the challenges, opportunities and applications of such MOFs are elaborated and highlighted.