Metal–organic frameworks with a large breathing effect to host hydroxyl compounds for high anhydrous proton conductivity over a wide temperature range from subzero to 125 °C

Abstract

It is important but still challenging to develop high-performance proton conducting materials for proton exchange membrane fuel cells (PEMFCs), as such materials should meet the following requirements: stable proton-transport pathway over a wide temperature range, high conductivity at work temperature, and small activation energy E_a to maintain high conductivity at start temperature. Here, we firstly demonstrated that flexible metal–organic frameworks (FMOFs) are good hosts to seek out better proton carriers for such high-performance proton conducting materials. A FMOF [Zn₃(tz)₂(bdc)₂]_n (FJU-31, Htz = 1H-1,2,3-triazole, H₂bdc = terephthalic acid) with high thermal stability up to 400 °C, which can be readily synthesized from the Zn₅(tz)₆(NO₃)₄ precursor and H₂bdc, has been employed to host various organic hydroxyls as new proton carriers. Three resulting FMOFs Zn₃(tz)₂(bdc)₂@G (FJU-31@G, G = hydroquinone (Hq), cyclohexanol (Ch) or butanol (Bu)) show large breathing effect amplitudes up to 65% and guest-related single-crystal to single-crystal structural transformations by temperature stimulus. Most importantly, FJU-31@Hq hosting hydroquinone with a high melting point and small pK_a exhibits a high anhydrous proton conductivity of 2.65 × 10⁻⁴ S cm⁻¹, low activation energy E_a of 0.18 eV, and the widest temperature range from −40 to 125 °C for stable proton conduction among the crystalline porous materials.