Abstract
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 (MnxCo3−xO4@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, MnxCo3−xO4@NCNTs. Nanocrystalline MnCo2O4@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.
Highlights
Rechargeable metal–air batteries and regenerative fuel cells are highly desirable energy storage and conversion devices, in which the charge–discharge processes are critically defined by oxygen reduction and evolution reactions (ORR and oxygen evolution reaction (OER), respectively)
The oxygen electrocatalytic performance of the manganese cobalt oxide/nitrogen-doped multiwalled carbon nanotube hybrids (MnxCo3ÀxO4@NCNTs) system is highly dependent on the synthetic methods, compositions, sizes, and phases of MnxCo3ÀxO4, as well as the degree of oxidation of CNTs
Such activities can be further tuned by the mass loading of the MCO for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)
Summary
Rechargeable metal–air batteries and regenerative fuel cells are highly desirable energy storage and conversion devices, in which the charge–discharge processes are critically defined by oxygen reduction and evolution reactions (ORR and OER, respectively). Different morphologies of TMOs, such as mesoporous nanoflakes,[37] nanofibers,[20] and porous microspheres,[38,39] have been considered to promote the overall kinetics of reactions with enhanced specific surface area for three-phase contacts. These porous structures can provide interconnected channels for mass transport of both O2 and electrolyte necessary for sustaining the reactions. The conducting inorganic porous frameworks and carbon nanostructures are highly desirable substrate materials to further promote the catalysis reactions by enhancing the overall active sites and heterogeneous surface area. It showed very high stability over long-term operations for both the ORR and OER
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