Systematic Study of Effective Hydrothermal Synthesis to Fabricate Nb-Incorporated TiO₂ for Oxygen Reduction Reaction

Fuel cells are expected to serve as next-generation energy conversion devices owing to their high energy density, high power, and long life performance. The oxygen reduction reaction (ORR) is important for determining the performance of fuel cells; therefore, using catalysts to promote the ORR is essential for realizing the practical applications of fuel cells. Herein, we propose Nb-incorporated TiO₂ as a suitable alternative to conventional Pt-based catalysts, because Nb doping has been reported to improve the conductivity and electron transfer number of TiO₂. In addition, Nb-incorporated TiO₂ can induce the electrocatalytic activity for the ORR. In this paper, we report the synthesis method for Nb-incorporated TiO₂ through a hydrothermal process with and without additional load pressures. The electrocatalytic activity of the synthesized samples for the ORR was also demonstrated. In this process, the samples obtained under various load pressures exceeding the saturated vapor pressure featured a high content of Nb and crystalline TiNb₂ O₇, resulting in an ellipsoidal morphology. X-ray diffraction results also revealed that, on increasing the Nb doping amounts, the diffraction peak of the anatase TiO₂ shifted to a lower angle and the full width at half maximum decreased. This implies that the Ti atom is exchanged with the Nb atom during this process, resulting in a decrease in TiO₂ crystallinity. At a doping level of 10%, Nb-incorporated TiO₂ exhibited the best electrocatalytic activity in terms of the oxygen reduction current (i_ORR) and onset potential for the ORR (E_ORR); this suggests that 10% Nb-doped samples have the potential for enhancing electrocatalytic activity.