Proton Transport in Metal–Organic Frameworks

Solid-state proton conductors (SSPCs), which are a key component for the safety and efficiency of fuel cells, have received much attention due to their broad application in electrochemical devices. In particular, the development of new materials with high conducting performance and an understanding of the conduction mechanism have become critical issues in this field. Porous metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) have recently emerged and have been extensively studied as a new type of proton conductor due to their crystallinity, designability, and high porosity. These properties are able to adsorb the guest molecules working as conducting media. During the past decade, major advances in proton-conductive MOFs have been achieved with high performance (>10^-2 S cm^-1), comparable to the conventional material, via various synthetic strategies, and the veiled conduction mechanism has been elucidated through structure analysis and spectroscopy tools such as NMR, X-ray diffraction, and neutron scattering measurement. This Review aims to summarize and provide a comprehensive understanding of proton transport in MOFs. Here, we discuss the fundamental principles and various design strategies and implementations aimed at enhancing proton conductivity with representative examples. We also deal with characterization methods used to investigate proton-conductive MOFs and computational/theoretical studies that aid in understanding the conduction mechanism. Finally, future endeavors are suggested regarding the challenges of research for practical SSPCs.