Carbon-embedded mesoporous Nb-doped TiO2 nanofibers as catalyst support for the oxygen reduction reaction in PEM fuel cells

Abstract

Nanostructured titanium dioxide doped with transition metals (e.g. Nb or Cr), providing higher stability than carbon black, has been widely studied as a Pt catalyst support in Proton Exchange Membrane (PEM) fuel cells. However, the application of doped titanium dioxide supports in PEM fuel cells has been hampered by their lower electronic conductivity and often smaller surface area when compared to carbon black. Reported here is a new strategy to increase the conductivity and external surface area of mesoporous Nb-doped titanium dioxide (Nb_xTi_(1−x)O₂, x = 0.1 or 0.25) nanofibers by embedding 10–11 wt% of carbon (graphitic) via reductive calcination of the polymer coated nanofibers. A comparison between carbon-embedded and carbon-free Nb-doped titanium dioxide nanofibers reveals significantly higher electronic conductivity (0.12 vs. 5 × 10⁻⁶ S cm⁻¹) and external surface area (86 vs. 17 m² g⁻¹). Finally, carbon-embedded and carbon-free nanofibers were tested as Pt catalyst support for the oxygen reduction reaction (ORR) under acidic conditions and compared with that for a commercial carbon black support (Vulcan XC-72R). While the ORR mass activity under acidic conditions is similar for all tested supports, Pt catalysts supported by carbon-embedded nanofibers showed enhanced ORR specific activity and significantly higher durability. The lower electrochemical durability of Pt catalysts supported by carbon-free nanofibers is due to the electro-oxidation of metal oxides in lower oxidation states (e.g. NbO₂ to Nb₂O₅ and/or Ti₄O₇ to TiO₂) at the surface, which is in agreement with X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) data.