Ion-Exchanged UPG-1 as Potential Electrolyte for Fuel Cells

Proton-exchange membrane fuel cells are an attractive green technology for energy production. However, one of their major drawbacks is instability of the electrolytes under working conditions (i.e., temperature and humidity). Some metal-organic frameworks (MOFs) have recently emerged as promising alternative electrolyte materials because of their higher stability (compared with the organic polymers currently used as electrolytes), proton conductivity, and outstanding porosity and versatility. Here, we present ionic exchange in a microporous zirconium phosphonate, UPG-1, as an efficient strategy to enhance its conductivity and cyclability. Thus, labile protons of the hybrid structure were successfully replaced by different alkali cations (Li⁺, Na⁺, and K⁺), leading to 2 orders of magnitude higher proton conductivity than the pristine UPG-1 (up to 2.3 × 10^-2 S·cm^-1, which is comparable with those of the commercial electrolytes). Further, the proton conductivity was strongly influenced by the MOF hydrophilicity and the polarization strength of the cation, as suggested by molecular simulation. Finally, a mixed-matrix membrane containing the best-performing material (the potassium-exchanged one) was successfully prepared, showing moderate proton conductivity (up to 8.51 × 10^-3 S·cm^-1).