Ti³⁺-self-doped TiO₂ with multiple crystal-phases anchored on acid-pickled ZIF-67-derived Co₃O₄@N-doped graphitized-carbon as a durable catalyst for oxygen reduction in alkaline and acid media

The sluggish kinetics of oxygen reduction reaction (ORR), especially in acid media, seriously restrains the commercialization of direct methanol fuel cells. Herein, we synthesize a hierarchical Co₃O₄@N-doped partly-graphitized carbon wrapped by Ti³⁺-self-doped TiO₂ nanoparticles with multiple crystal-phases (anatase and rutile TiO₂) as catalysts (Co₃O₄@NGC@MP-TiO₂) for ORR in alkaline/acid media using ZIF-67 as a precursor. Co₃O₄@NGC@MP-TiO₂-0.3 (tetrabutyl titanate of 0.3 mL) exhibits the same peak potential (E_peak of 0.83 V, vs. RHE) as Pt/C in 0.1 M KOH and the comparable E_peak (0.69 V) to Pt/C (0.7 V) in 0.5 M H₂SO₄. Synergistic effects between tetrahedral Co²⁺ (Co₃O₄) and Ti³⁺ (MP-TiO₂) chiefly contribute to the high ORR activity. ORR stabilities of Co₃O₄@NGC@MP-TiO₂-0.3 are higher than those of Pt/C in alkaline/acid media, attributing to that NGC@MP-TiO₂ shell can protect active sites (tetrahedral Co²⁺ and N species) from corrosion and deactivation. Moreover, the dissociated adsorption of O₂ on Ti³⁺ may facilitate the O₂ protonation (Ti⁴⁺–OOH_ads⁻) to accelerate electron-transfer process and ORR kinetics in acid electrolyte, while acid-pickling of CoO_x@NGC improves the stability of Co₃O₄@NGC to further smooth ORR process. This study provides a promising strategy for the design of highly-active and stable ORR catalysts in both alkaline and acid electrolytes.