Hybrid inorganic-organic Nafion membranes modified with metal oxides (typically TiO 2 , ZrO 2 , WO 3 ) are a good alternative for fuel cell applications. However, one of their main limitations is associated with their relative low proton conductivity at temperatures above 80 °C. In this work, we overcome this issue using HfO 2 as a filler. HfO 2 was prepared by a sol-gel method, and it was compared with a recast Nafion membrane (named as recast). Deconvolved XPS spectra confirmed the presence of hafnia, while EDS analysis was used to determine its weight content resulting in a 1.88 wt%. FT-IR ATR experiments indicated that the HfO 2 hybrid membrane possess a higher capability to retain water than the recast. Thus, the water uptake, swelling degree, conductivity tests and fuel cell evaluations were performed. The water uptake analysis revealed that the hybrid membrane presented a higher retention percentage at 100 °C (61%) than recast (29%). This improvement enabled a higher ionic conductivity at 80 °C and 100 °C. The hybrid membrane displayed a higher conductivity at 100 °C than the recast membrane (112 versus 82 mS cm −1 ), increasing the cell performance to 0.36 W cm −2 ; being this performance almost two-fold higher to that obtained for the recast membrane. In summary, herein we demonstrated that HfO 2 can be considered as an excellent substitute to conventional fillers.

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