and storage devices, such as fuel cells and metal-air batteries. [1][2][3][4][5][6][7][8] Especially, the polymer electrolyte membrane fuel cells, used in automotive applications, have already achieved the level of commercialization. [5] Although the Pt group metal (PGM) catalysts can efficiently address the kinetics-related challenges, the high cost and the limited resource of PGM greatly hinder the widespread commercialization of these energy conversion and storage devices. Therefore, the development of low-cost, highly active, and stable PGM-free catalysts, to replace the PGM ones, toward ORR, is highly desired. Over the past decades, extensive research was devoted to the development of alternative and low-cost catalysts, including i) non-precious Fe and Co-based metal catalysts, [3,4,[9][10][11] ii) metal oxides, nitrides, and oxynitrides, [12][13][14] and iii) metal-free catalysts such as carbon-based materials. [15] Oxides from metals belonging to groups 4 and 5 (Ti, V, Zr, Nb, Hf, Ta) have attracted interest in these fields, mainly due to their low cost compared to PGM catalysts, environmental compatibility, and excellent stability in acid medium. [12,[16][17][18][19] It has been demonstrated that these oxides can have some activity toward ORR, if their stoichiometry is conveniently tailored to generate a high concentration The oxygen reduction reaction (ORR) is one of the most important reactions in renewable energy conversion and storage devices. The full deployment of these devices depends on the development of highly active, stable, and low-cost catalysts. Herein, a new hybrid material consisting of Na 2 Ta 8 O 21−x / Ta 2 O 5 /Ta 3 N 5 nanocrystals grown on N-doped reduced graphene oxide is reported. This catalyst shows a significantly enhanced ORR activity by ≈4 orders of magnitude in acidic media and by ≈2 orders of magnitude in alkaline media compared to individual Na 2 Ta 8 O 21−x on graphene. Moreover, it has excellent stability in both acid and alkaline media. It also has much better methanol tolerance than the commercial Pt/C, which is relevant to methanol fuel cells. The high ORR activity arises not only from the synergistic effect among the three Ta phases, but also from the concomitant nitrogen doping of the reduced graphene oxide nanosheets. A correlation between ORR activity and nitrogen content is demonstrated. Deep insights into the mechanism of the synergistic effect among these three Ta-based phases, which boosts the ORR's kinetics, are acquired by combining specific experiments and density functional theory calculations.
