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

Abstract 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 (Epeak of 0.83 V, vs. RHE) as Pt/C in 0.1 M KOH and the comparable Epeak (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+–OOHads–) to accelerate electron-transfer process and ORR kinetics in acid electrolyte, while acid-pickling of CoOx@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.