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
[M
2
(dobdc); M = Mg
2+
, Zn
2+
, Ni
2+
; dobdc = 2,5-dioxido-1,4-benzenedicarboxylate] materials
loaded with various stoichiometric amounts of NH
4
SCN. 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.