Preparation and characterization of Nb-doped TiO2 nanoparticles used as a conductive support for bifunctional CuCo2O4 electrocatalyst

Nb-doped TiO2 powders with various Nb contents (0%, 5% and 10%) were prepared using a sol–gel process and annealed at 450, 650 and 1050 °C with the aim to be used as a conductive support for bifunctional electrocatalysts in rechargeable metal-air batteries, substituting the more conventional carbon particles which slowly corrode at high potentials during cell recharge. X-ray diffraction analyses revealed that niobium hinders the anatase–rutile transformation and prevents the grain growth, which is important to achieve a conductive support with high specific surface area. X-ray photoelectron spectroscopy measurements have shown that the powders possess surface Ti4+and Nb4+species, and that the Nb0.05Ti0.95O2 (1050 °C) sample also contains Ti3+and Nb5+ ions. Composite film electrodes containing CuCo2O4 particles (60 w/w%), Nb0.05Ti0.95O2 or Vulcan XC72R or Ebonex powder (25%), and poly(vinylidene fluoride-co-hexafluoropropylene) (15%) were formed on the glassy carbon disk surface of a rotating ring-disk electrode and studied for both the O2 evolution and O2 reduction reactions in 1 M KOH solution and O2-saturated 1 M KOH solution, respectively. Rutile Nb0.05Ti0.95O2 (1050 °C) powder was chosen owing to its larger overpotential for H2 evolution (detrimental to the performance of bifunctional electrodes) and its low and stable anodic activity, whereas anatase Nb0.05Ti0.95O2 (450 °C) powder was mainly selected due to its higher specific surface area. The results have shown that the Nb0.05Ti0.95O2 powders are more suitable than Vulcan XC72R for use as a conductive support material, based on the higher electrode intrinsic current densities recorded for O2 evolution and reduction reactions.