Exploration of single-crystal proton conduction in ordered networks

Recently, to obtain high-performance proton conductors for electrochemical applications such as proton exchange membranes, impedance sensing, proton sieving, etc., there is an urgent need to get insight into the proton-conducting mechanisms. The emergence of crystalline solid materials, especially single-crystal products, offers new possibilities to solve this challenge. Compared with the impedance spectra obtained from the powder pellet sample, the intrinsic conductivity achieved from the anisotropic impedance measurement of a single crystal sample can acquire more accurate information about the conducting mechanism, avoid the influence of grain boundary, crystal orientation perturbation, and extrinsic proton transfer across interparticle spaces for the powder sample, which will provide a theoretical and experimental basis for more accurate design and assembly of proton-conductive crystalline materials. Thus, it is essential to provide an overview and summary of the single-crystal proton conduction research. In this review, the single crystal materials are divided into metal–organic frameworks (MOFs)/coordination polymers (CPs), hydrogen-bonded organic frameworks (HOFs), and proton acids according to their components to demonstrate their ordered structures, proton conductivity, and proton-conducting mechanism. Moreover, future research priorities and trends regarding single-crystal proton conductivity are also highlighted.