Structure-function relationship of Metal-organic Frameworks

The basic elements of MOFs that can be used as proton-conducting materials are proton sources, hydrophilic pore channels, and hydrogen-bonding networks. The proton source can be provided by acid groups (-COOH, -HSO3, -PO3H2, etc.), H+ and NH4+ embedded in pores. However, when MOFs act as proton conductors, the proton-conducting receptors are dependent on exogenous water or Bronsted base molecules, thus most proton-conducting MOFs exhibit a heavy dependence on water molecules. However, the humidification process of MOFs as a PEM membrane layer is not like that of Nafion membranes, the wetting of MOFs could not be accomplished only through the reaction gas carrying water vapour in a short time due to the capillary effect. This exogenous wetting is limited by the physicochemical properties of water, and the vaporization of water at 100 °C can drastically reduce the water content of the membrane layer and make the overall device work abnormally. Therefore, this project aims to solve the current problem of MOFs applied as PEMs, where the water content drops sharply when the operating temperature is higher than 100 ℃, resulting in extremely poor device performance and stability. By introducing water-metal coordination bonds near the carboxylic acid-metal coordination bond, a carboxylic acid-water proton donor-acceptor pair is formed to enhance the proton conductivity of MOFs. Meanwhile, the newly introduced coordinated water, which is more stable and more resistant to high temperatures compared with the conventional water molecules in the cavities of MOFs, will undoubtedly compensate for the lack of high-performance PEMs at wide operating temperatures.