The study of the structure and conductive properties of an iron gallate MOF: [FeIII(C7H4O5)]n·2nH2O

Abstract

Gall ink, a metal–organic framework, has been described in chronicles since the middle ages. The literature contains extensive data on its biocompatibility and biomedical applications, as well as adsorption properties. This paper focuses on the electrochemical properties of gall ink, specifically proton conductivity, which was investigated for the first time. We studied iron(III) gallate, synthesizing it using the hydrothermal method as a crystal hydrate in the form of a single-phase sample. We characterized its structure, composition, charge state, thermal stability, and morphology. For the obtained structure, we used the bond valence site energy method and kinetic Monte Carlo method to calculate the proton migration energy (E_m) and proton conductivity in the temperature range between 250 K and 375 K, since the substance showed high thermal stability before heating at this temperature. We confirmed the 1D H⁺-ion diffusion with E_m = 0.4 eV. The conductivity was found to be in the order of 10⁻³ S cm⁻¹ in the specified temperature range. We then performed electrochemical measurements using impedance spectroscopy to measure the total conductivity of iron(III) gallate in various solvents, including water, methanol, ethanol, n-propranol, and i-propanol, and also in a humid atmosphere. We found that the highest conductivity was in water and methanol, with a room temperature conductivity of 1.5 × 10⁻⁴ S cm⁻¹ and 2.4 × 10⁻⁴ S cm⁻¹, respectively. The conductivity of gallate in the gas phase at 50% relative humidity was ca. 10⁻⁸ S cm⁻¹ (at 30 °C), and the experimental activation energy was 0.47 eV, in agreement with the simulation results. Therefore, iron gallate could be a promising material for proton-conducting membranes and could be used in electrochemical devices due to its safety, stability and high conductive properties.