Trojan Horse Thiocyanate: Induction and Control of High Proton Conductivity in CPO-27/MOF-74 Metal–Organic Frameworks by Metal Selection and Solvent-Free Mechanochemical Dosing

Proton-conducting
metal–organic frameworks (MOFs) have been
gaining attention for their role as solid-state electrolytes in various
devices for energy conversion and storage. Here, we present a convenient
strategy for inducing and tuning of superprotonic conductivity in
MOFs with open metal sites via postsynthetic incorporation of charge
carriers enabled by solvent-free mechanochemistry and anion coordination.
This scalable approach is demonstrated using a series of CPO-27/MOF-74 [M2(dobdc); M = Mg2+, Zn2+, Ni2+; dobdc = 2,5-dioxido-1,4-benzenedicarboxylate] materials
loaded with various stoichiometric amounts of NH4SCN. The
modified materials are not achievable by conventional immersion in
solutions. Periodic density functional theory (DFT) calculations,
supported by infrared (IR) spectroscopy and powder X-ray diffraction,
provide structures of the modified MOFs including positions of inserted
ions inside the [001] channels. Despite the same type and concentration
of proton carriers, the MOFs can be arranged in the increasing order
of conductivity (Ni < Zn < Mg), which strongly correlates with
amounts of water vapor adsorbed. We conclude that the proton conductivity
of CPO-27 materials can be controlled over a few orders
of magnitude by metal selection and mechanochemical dosing of ammonium
thiocyanate. The dosing of a solid is shown for the first time as
a useful, simple, and ecological method for the control of material
conductivity.