Enhanced Proton Conductivity by Aliovalent Substitution of Cadmium for Indium in Dimethylaminium-Templated Metal Anilicates

Metal–organic
frameworks (MOFs) with excellent proton conducting
ability are crucial to fuel cells, chemical sensors, and redox flow
batteries, but achieving them remain a challenge because of the difficulty
in simultaneous fulfillment of large number of proton carriers, high
mobility of protons, and long-term durable proton conduction. To explore
a simple, efficient, and general route toward highly proton-conducting
MOFs, we propose herein an aliovalent substitution metal strategy
for isostructural aminium-templated MOFs which benefit the acquisition
of rich proton sources without modifying ligands or exchanging protic
organic molecules. This idea is verified by 100-fold enhancement of
conductivity in compounds (Me2NH2)2[Cd­(mdhbqdc)2] (Cd-BQ) and (Me2NH2) (Me2NH)­[In­(mdhbqdc)2] (In-BQ) (H2mdhbqdc = dimethyl 3,6-dihydroxy-2,5-benzoquinone-1,4-dicarboxylic
acid) that feature three-dimensional diamond-like structures with
two-dimensional intersected channels. Accompanied by the in
situ formation of an anilicate ligand, a great number of
−OH groups are grafted onto the inner wall of pores, which
interact with neutral Me2NH and/or protonated Me2NH2+ cations via N–H···O
hydrogen bonds. The high concentration of protons and dynamics of
protic amines in the porous framework readily leads to a moderate
conductivity of In-BQ (2.10 × 10–4 S cm–1, at 303 K under 95% RH) and an activation
energy of 0.73 eV (95% RH). It should be noted that the aliovalent
substitution of Cd­(II) for In­(III) results in the doubling of dimethylaminium
proton carriers in Cd-BQ, indicating more frequent hopping
and multiple proton-transfer pathways. This indication is supported
by a very high protonic conductivity of 2.30 × 10–2 S cm–1 and a reduced activation energy of 0.48
eV under the same conditions. Molecular dynamics simulations visually
elucidate the fact that compared with In-BQ, aliovalent-substituted Cd-BQ has shorter proton-migration distances, which in combination
with more proton numbers results in more frequent hopping and sliding
of protons, in agreement with the experimental results.