Promotion of Low-Humidity Proton Conduction by Controlling Hydrophilicity in Layered Metal–Organic Frameworks

We controlled the hydrophilicity of metal–organic
frameworks
(MOFs) to achieve high proton conductivity and high adsorption of
water under low humidity conditions, by employing novel class of MOFs,
{NR3(CH2COOH)}­[MCr­(ox)3]·nH2O (abbreviated as R-MCr, where
R = Me (methyl), Et (ethyl), or Bu (n-butyl), and
M = Mn or Fe): Me-FeCr, Et-MnCr, Bu-MnCr, and Bu-FeCr. The cationic components have a carboxyl
group that functions as the proton carrier. The hydrophilicity of
the cationic ions was tuned by the NR3 residue to decrease
with increasing bulkiness of the residue: {NMe3(CH2COOH)}+ > {NEt3(CH2COOH)}+ > {NBu3(CH2COOH)}+. The
proton conduction of the MOFs increased with increasing hydrophilicity
of the cationic ions. The most hydrophilic sample, Me-FeCr, adsorbed a large number of water molecules and showed a high proton
conductivity of ∼10–4 S cm–1, even at a low humidity of 65% relative humidity (RH), at ambient
temperature. Notably, this is the highest conductivity among the previously
reported proton-conducting MOFs that operate under low RH conditions.