High Proton Conductivity in Two Stable Bi(III)-Organic Frameworks Based on 2,6-Pyridinedicarboxylic Acid

The exploration of proton-conducting metal-organic frameworks (MOFs) has attracted sustained interest fueled by their potential in energy conversion and storage. To expand the range of proton-conductive MOFs, this study focuses on bismuth-based MOFs, utilizing Bi(III)'s unique properties: low toxicity, strong affinity for O/N-containing ligands, and flexible coordination. Two stable Bi(III) MOFs, [Bi₂(Hpdc)₂(pdc)₂]·2H₂O (1) and [pip][Bi(Hpdc)(pdc)]·H₂O (2) (H₂pdc = 2,6-pyridinedicarboxylic acid; pip = piperazine), were hydrothermally synthesized, and their proton conduction was systematically studied. Both exhibit excellent structural, thermal, and water stability with frameworks featuring abundant H-bonded networks enabling proton transport. Alternating-current impedance spectroscopy showed conductivities highly dependent on temperature (30-100 °C) and relative humidity (68%-98% RH). Under optimal conditions (100 °C/98% RH), 1 and 2 achieved maximum conductivities of 3.38 × 10^-4 and 0.65 × 10^-3 S·cm^-1, respectively. Activation energy (E_a) analysis revealed a mechanism transition: the Vehicle mechanism at 68% RH (E_a: 0.79 eV for 1, 0.97 eV for 2) and the Grotthuss mechanism at 98% RH (E_a: 0.22 eV for 1, 0.31 eV for 2). Powder X-ray diffraction confirmed structural integrity post-tests. This work advances research on crystalline proton-conducting MOFs and provides insights for next-generation system design.