Self-generated carbon nanotubes for protecting active sites on bifunctional Co/CoOx schottky junctions to promote oxygen reduction/evolution reactions via efficient valence transition

Protecting active species from aggregation and corrosion may be feasible to obtain stable catalytic activities for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, bamboo-shaped N-doped carbon nanotubes (hollow BS-NCNTs as shells) are self-generated to in situ wrap the Co/CoO_x schottky junctions (cores) to obtain the Co/CoO_x@BS-NCNTs as bifunctional ORR/OER catalysts by using the Co-chelated melamine precursor. For ORR, Co/CoO_x@BS-NCNTs (700 °C) exhibits more positive peak (0.822 V vs. RHE) and half-wave (0.842 V vs. RHE) potential than those of commercial Pt/C (10 wt%). Superior ORR activity is mainly attributed to the enriched coordination-unsaturated Co²⁺ (tetrahedral Co_Td ²⁺) in the CoO_x wrapped in the tubular structure of BS-NCNTs featuring high electrical conductivity and active N species. Moreover, the π-π bonds of CNTs are activated by N substitution, which provides a stunning electron capture and transmission capability for enhancing ORR activity. For OER, Co/CoO_x@BS-NCNTs (700 °C) obtains a smaller potential (1.590 V vs. RHE) than that of RuO₂/C at 10 mA cm⁻². The outstanding OER activity and durability of Co/CoOx@BS-NCNTs (700 °C) originates from strong interactions between C-skeleton and Co species, and efficient Co³⁺/Co⁴⁺ (Co⁴⁺OOH as active sites) transition protected by the externally-grown CNTs. Furthermore, abundant oxygen vacancies on CoO_x surface can facilitate the adsorption of OH⁻/or OER-related intermediates to improve OER activity. Therefore, this study provides a promising strategy to develop NCNTs-wrapped Co species with high catalytic activity and stability for energy conversion.