Solvent-Free Mechanochemical Dense Pore Filling Yields CPO-27/MOF-74 Metal-Organic Frameworks with High Anhydrous and Water-Assisted Proton Conductivity

Proton-conducting solids operating in both anhydrous and humid conditions are of paramount importance for the development of fuel cells. We demonstrate that incorporating formamidinium or methylammonium ions in CPO-27/MOF-74 metal-organic frameworks renders them highly conductive under the two conditions. Highest proton conductivities reach 8 x 10-4 S/ cm under anhydrous conditions, exceed 10-3 S/cm already at low relative humidity (30-50% RH) and low temperatures (25-60 degrees C), and reach 10-2 S/cm at 60 degrees C and 70% RH. The dense pore filling with the protic cations is enabled by solvent-free mechanochemistry and stoichiometric saturation of open metal sites (Mg, Ni) with thiocyanate ligands. DFT modeling reveals that high anhydrous proton conductivity is associated with the presence of water-free extended networks of hydrogen bonds along the [001] channel and together with electrochemical impedance measurements (EIS) shed light on the mechanism of proton transport.

Automated summary

Proton-conducting solids that can operate under both anhydrous and humid conditions are crucial for fuel cell development. By incorporating formamidinium or methylammonium ions in CPO-27/MOF-74 metal-organic frameworks, these materials demonstrate high conductivity in both conditions. The highest proton conductivities are achieved under anhydrous conditions, but also maintain conductivity at low relative humidity and low temperatures. The mechanisms of proton transport and the influence of water-free hydrogen bond networks are revealed through DFT modeling and electrochemical impedance measurements.