Optimizing the Proton Conductivity of Fe-Diphosphonates by Increasing the Relative Number of Protons and Carrier Densities

Proton conductive materials have attracted extensive interest in recent years due to their fascinating applications in sensors, batteries, and proton exchange membrane fuel cells. Herein, two Fe-diphosphonate chains (H₄-BAPEN)_0.5·[Fe^III(H-HEDP)­(HEDP)_0.5(H₂O)] (1) and (H₄-TETA)₂·[Fe^III₂Fe^II(H-HEDP)₂(HEDP)₂(OH)₂]·2H₂O (2) (HEDP = 1-hydroxyethylidenediphosphonate, BAPEN = 1,2-bis­(3-aminopropylamino)­ethane, and TETA = triethylenetetramine) with different templating agents were prepared by hydrothermal reactions. The valence states of the Fe centers were demonstrated by ⁵⁷Fe Mössbauer spectra at 100 K, with a high-spin Fe^III state for 1 and mixed high-spin Fe^III/Fe^II states for 2. Their magnetic properties were determined, which featured strong antiferromagnetic couplings in the chain. Importantly, the proton conductivity of both compounds at 100% relative humidity was explored at different temperatures, with 2.79 × 10^–4 S cm^–1 at 80 °C for 1 and 7.55 × 10^–4 S cm^–1 at 45 °C for 2, respectively. This work provides an opportunity for improving proton conductive properties by increasing the relative number of protons and the carrier density using protonated flexible aliphatic amines.