Promotion of Low-Humidity
Proton Conduction by Controlling
Hydrophilicity in Layered Metal–Organic Frameworks
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Abstract
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,
{NR<sub>3</sub>(CH<sub>2</sub>COOH)}[MCr(ox)<sub>3</sub>]·<i>n</i>H<sub>2</sub>O (abbreviated as <b>R-MCr</b>, where
R = Me (methyl), Et (ethyl), or Bu (<i>n</i>-butyl), and
M = Mn or Fe): <b>Me-FeCr</b>, <b>Et-MnCr</b>, <b>Bu-MnCr</b>, and <b>Bu-FeCr</b>. The cationic components have a carboxyl
group that functions as the proton carrier. The hydrophilicity of
the cationic ions was tuned by the NR<sub>3</sub> residue to decrease
with increasing bulkiness of the residue: {NMe<sub>3</sub>(CH<sub>2</sub>COOH)}<sup>+</sup> > {NEt<sub>3</sub>(CH<sub>2</sub>COOH)}<sup>+</sup> > {NBu<sub>3</sub>(CH<sub>2</sub>COOH)}<sup>+</sup>. The
proton conduction of the MOFs increased with increasing hydrophilicity
of the cationic ions. The most hydrophilic sample, <b>Me-FeCr</b>, adsorbed a large number of water molecules and showed a high proton
conductivity of ∼10<sup>–4</sup> S cm<sup>–1</sup>, 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
