Extra Highways for Proton Diffusion in TiO2@MIL-101-Cr/Nafion Composite Membranes with High Single-Cell Performance

Buffer layers of TiO2 nanoparticles are introduced on the surface of the MIL-101-Cr, a kind of metal-organic frameworks (MOFs), to improve performances of MOFs/Nafion proton exchanging membranes. A series of Nafion composite membranes are fabricated by doping TiO2-wrapped MIL-101-Cr (TiO2@MIL-101-Cr). The water uptakes of composite membranes reasonably increase while their swelling ratios unexpectedly reduce in comparison to pristine Nafion. Nevertheless, the swelling ratios of composite membranes gradually increase with the increment of TiO2@MIL-101-Cr due to their enlarged free volumes and more amorphous phases. The introduction of additive facilitates water molecules diffusion as well as overall hydration of the composite membranes, resulting in a significant increment in the proton conductivity. As a result of the formation of extra proton conducting highways both around/in TiO2@MIL-101-Crs and the Nafion matrix, a composite membrane in single-cell possesses a maximum power density of 0.946 W/cm2. Furthermore, higher thermal and oxidative stabilities are observed for the composite membranes. This work demonstrates that the addition of encapsulated structural TiO2@MIL-101-Crs in Nafion is promising for fabricating proton exchanging membranes with superior electrochemical performance and high thermal, oxidative stabilities.