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, {NR ₃(CH ₂COOH)}[MCr(ox) ₃]•nH ₂O (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 NR ₃ residue to decrease with increasing bulkiness of the residue: {NMe ₃(CH ₂COOH)} ⁺ > {NEt ₃(CH ₂COOH)} ⁺ > {NBu ₃(CH ₂COOH)} ⁺. 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.