Tunable Bifunctional Activity of Mn(x)Co(3−x)O(4) Nanocrystals Decorated on Carbon Nanotubes for Oxygen Electrocatalysis

Noble‐metal‐free electrocatalysts are attractive for cathodic oxygen catalysis in alkaline membrane fuel cells, metal–air batteries, and electrolyzers. However, much of the structure–activity relationship is poorly understood. Herein, the comprehensive development of manganese cobalt oxide/nitrogen‐doped multiwalled carbon nanotube hybrids (Mn(x)Co(3−x)O(4)@NCNTs) is reported for highly reversible oxygen reduction and evolution reactions (ORR and OER, respectively). The hybrid structures are rationally designed by fine control of surface chemistry and synthesis conditions, including tuning of functional groups at surfaces, congruent growth of nanocrystals with controllable phases and particle sizes, and ensuring strong coupling across catalyst–support interfaces. Electrochemical tests reveal distinctly different oxygen catalytic activities among the hybrids, Mn(x)Co(3−x)O(4)@NCNTs. Nanocrystalline MnCo(2)O(4)@NCNTs (MCO@NCNTs) hybrids show superior ORR activity, with a favorable potential to reach 3 mA cm(−2) and a high current density response, equivalent to that of the commercial Pt/C standard. Moreover, the hybrid structure exhibits tunable and durable catalytic activities for both ORR and OER, with a lowest overall potential of 0.93 V. It is clear that the long‐term electrochemical activities can be ensured by rational design of hybrid structures from the nanoscale.