Synthesis of size-controlled CoMn₂O₄ quantum dots supported on carbon nanotubes for electrocatalytic oxygen reduction/evolution

A combined hot-injection and heat-up method was developed to synthesize monodisperse and uniform CoMn₂O₄ quantum dots (CMO QDs). CMO QDs with average size of 2.0, 3.9, and 5.4 nm were selectively obtained at 80, 90, and 105 ℃, respectively. The CMO QDs supported on carbon nanotubes (CNTs) were employed as catalysts for the oxygen reduction/evolution reaction (ORR/OER) in alkaline solution to investigate their size-performance relationship. The results revealed that the amount of surface-adsorbed oxygen and the band gap energy, which affect the charge transfer in the oxygen electrocatalysis processes, strongly depend on the size of the CMO QDs. The CMO-3.9/CNT hybrid, consisting of CNT-supported CMO QDs of 3.9 nm size, possesses a moderate amount of surface- adsorbed oxygen, a lower band gap energy, and a larger charge carrier concentration, and exhibits the highest electrocatalytic activity among the hybrid materials investigated. Moreover, the CMO-3.9/CNT hybrid displays ORR and OER performances similar to those of the benchmark Pt/C and RuO₂ catalysts, respectively, due to the strong carbon-oxide interactions and the high dispersion of CoMn₂O₄ QDs on the carbon substrate; this reveals the huge potential of the CMO-3.9/CNT hybrid as a bifunctional OER/ORR electrocatalyst. The present results highlight the importance of controlling the size of metal oxide nanodots in the design of active oxygen electrocatalysts based on spinel-type, nonprecious metal oxides.