Tunable Bifunctional Activity of MnxCo3-xO4 Nanocrystals Decorated at Carbon Nanotubes for Oxygen Electrocatalysis

Noble-metal free electrocatalysts are attractive for cathodic oxygen catalysis in alkaline membrane fuel cells, metal-air batteries and electrolysers. However, much of the structure-activity relationship is poorly understood. Here, comprehensive development of manganese cobalt oxide/nitrogen-doped multi-walled carbon nanotube hybrids (MnxCo3-xO4@NCNTs) is reported for highly reversible oxygen reduction and evolution reactions (ORR and OER). 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, MnxCo3-xO4@NCNTs. A nanocrystalline MnCo2O4@NCNTs (MCO@NCNTs) hybrid shows superior ORR activity, with a favourable onset potential and a high current density response, equivalent to the commercial Pt@C standard. Moreover, the hybrid structure exhibits tuneable 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