Mixed-valence NaSb₃O₇ support toward improved electrocatalytic performance in the oxygen-reduction reaction

Nanocrystals of sodium antimony oxide, NaSb₃O₇ (pyrochlore structure, a = 1.030 nm), act as an efficient catalyst support for the electrocatalytic oxygen-reduction reaction (ORR) in acidic media. The NaSb₃O₇ nanocrystals (edge length ∼ 150 nm) were synthesized by hydrothermal decomposition of SbCl₅ in aqueous solution of NaOH. The NaSb₃O₇ nanocrystals were then decorated with Pt nanoparticles by chemical reduction of H₂PtCl₆ in water to yield an ORR catalyst, Pt/NaSb₃O₇. The Pt/NaSb₃O₇ exhibited higher ORR performance than the state-of-the-art Pt/TiO₂- or Pt/C catalysts in terms of the +40 mV higher half-wave reduction potential and the retained electrochemical surface area than the Pt/TiO₂ after 10000-times repeated ORR in an acidic electrolyte. Unlike NaSb₃O₇, Pt-decorated Sb₂O₅ (Pt/Sb₂O₅) was much less active than the Pt/TiO₂ or Pt/C. The enhanced ORR activity of the Pt/NaSb₃O₇ may be attributed to the promoted electron hopping between the Sb³⁺ and Sb⁵⁺ ions in mixed-valence Na¹⁺(Sb³⁺Sb₂⁵⁺)O₇, which is inhibited in single-valence Sb₂⁵⁺O₅.