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

Abstract

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 × 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 °C), and reach 10–2 S/cm at 60 °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.