Proton-conduction under mild humid conditions in [NH4][M(HCOO)3] (M=Mn2+, Co2+) frameworks

Room temperature proton conduction was investigated in ammonium-based formate frameworks [NH4][M(HCOO)3] (M = Mn2+, Co2+) well-known for their ferroelectric and magnetic properties at low temperature. The synthesis was achieved through mechanosynthesis and proton conduction measurements were performed in ambient as well as in humidity and temperature controlled conditions. The proton conduction is activated by reaching the relative humidity (RH) limit of 50 % for both compounds. At 294 K a maximal conductivity of 2.2 ± 0.3 × 10−4 S/cm is observed for [NH4][Mn(HCOO)3] under RH = 70 %. The lack of proton conduction behavior in the isostructural compound Cs [Mn(HCOO)3] corroborates the role played by the NH4+ units as proton vehicle. Ammonium-based formate frameworks [NH4][M(HCOO)3] (M = Mn2+, Co2+) are characterized by room temperature proton conduction property. The proton conduction is activated by reaching the relative humidity (RH) threshold of 50 % for both compounds. At 303 K the maximal conductivity of 2.9 ± 0.3 × 10-4 S/cm is observed for [NH4][Mn(HCOO)3] under RH = 70 %. The lack of proton conduction behavior in the isostructural compound Cs [Mn(HCOO)3] corroborates the role played by the NH4+ units as proton carriers. [Display omitted] •Proton conductivity in ammonium metal formate-based frameworks.•Measurements of proton conductivity at different relative humidity conditions.•Suppression of proton conductivity in absence of acidic NH4+ hosted in MOF cavities.•The proton conduction is ruled by vehicle mechanism involving both water adsorbed and NH4+.