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 O-2 on Ti3+ may facilitate the O-2 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.

Automated summary

A hierarchical Co3O4@N-doped partly-graphitized carbon wrapped by Ti3+-self-doped TiO2 nanoparticles catalyst was synthesized for oxygen reduction reaction (ORR) in alkaline/acid media, exhibiting comparable activity to Pt/C and higher stability. Synergistic effects between Co3O4 and Ti3+ contribute to the high ORR activity, while the NGC@MP-TiO2 shell protects active sites from corrosion and deactivation. The dissociated adsorption of O-2 on Ti3+ facilitates O-2 protonation to accelerate electron-transfer process, while acid-pickling of CoOx@NGC improves stability to smooth ORR process.