Two birds with one stone: A ferrocene-based zirconium(IV)-organic framework nanosheet denoting intrinsic high proton conductivity and acting as a fine dopant to chitosan-based composite membranes

Recently, it was demonstrated that employing metal-organic frameworks (MOFs) with prominent proton conductivity (σ) as fillers with organic substrates such as chitosan (CS) or Nafion is an effective approach for preparing composite membranes (CMs) with outstanding functionalities. Inspired by this, one extremely stable Zr(IV)-MOF (namely Zr-FDC) with a nanosheet structure produced by 1,1′-ferrocene dicarboxylic acid (H₂FDA) was successfully manufactured in this research and subsequently used as a filler to synthesize a series of CS-based CMs via casting method. The alternating current (AC) impedance determinations manifested that both Zr-FDC and the related CS-based CMs (CS/MOF-x; x = 2, 4, 6, 8 being the mass percentage of Zr-MOF in the CM) showed ultrahigh σ values, indicating the structural advantages of the Zr-MOF. Further research verified that when the MOF doping quantity is 4 %, the CM's (CS/MOF-4) σ is the greatest, being 2.22 × 10⁻² S/cm, which is boosted by nearly tenfold at 100 °C and 98 % relative humidity (RH) compared to the original MOF (3.2 × 10⁻³ S/cm). Furthermore, the CM exhibits superior thermal stability and tensile resistance. Finally, considering the structural features of the MOF and CS, activation energy data, and other determinations, we thoroughly theorized the proton conduction process within the MOF framework and CMs, referencing the subsequent proton exchange membrane design.