Twofold Effects of Zirconium Doping into TiN on Durability and Oxygen Reduction Reactivity in an Acidic Environment

Carbon-support-free TiN-based catalysts have recently been developed to avoid both carbon oxidation and the use of scarce platinum group metals in acidic polymer electrolyte fuel cell cathodes. However, providing sufficient durability at high potentials above 1.0 V remains a challenge. Herein, zirconium doping is revealed as a new route that enhances catalyst activity and selectivity toward the four-electron oxygen reduction reaction (ORR) in an acidic environment. The TiN surface is oxidized to form rutile TiO₂ layers, and some zirconium atoms are dissolved into both the bulk TiN and the surface rutile layers. The zirconium atoms distort the rutile lattice to increase the number of oxygen vacancies which are critical for the ORR, whereas some others segregate to form monoclinic and tetragonal ZrO₂ phases to inhibit the TiN crystallite growth. The optimized Ti_0.8Zr_0.2O_xN_y catalysts exhibit excellent durability during 5000 start-up and shut-down cycles between 1.0 and 1.5 V versus the reversible hydrogen electrode in 0.1 mol dm^–3 H₂SO₄ solution. The decrease in the halfwave potential during the 5000 cycles is only 0.04 V, which is half that of the previous best phosphorus-doped TiN catalyst.