Abstract
Oxygen reduction reaction (ORR) remains a pivotal factor in assessing the overall efficiency of energy conversion and storage technologies. A promising family of ORR electrocatalysts is mixed transition-metal oxides (MTMOs), which have recently gained a growing research interest. In this study, we developed MTMOs with different compositions (designated as A (x) B(3-x) O(4); A = Cu, B = Co or Mn) anchored on two different carbon supports (activated carbon Vulcan XC-72 (AC) and graphene (G)) for catalyzing ORR in neutral media. Four different MTMO electrocatalysts (i.e., MnO(2)-CuO/AC, CoO-CuO/AC, CoO-CuO/G, and MnO(2)-CuO/G) were synthesized by a simple and scalable co-precipitation method. We documented the morphology and electrocatalytic properties of MTMO electrocatalysts using transmission and scanning electron microscopy, X-ray diffraction (XRD), X-ray photoelectron spectrometer (XPS), energy dispersive X-ray (EDX), and electrochemical techniques. Generally, MTMOs exhibited remarkably high ORR electrocatalytic activity with MTMOs anchored on an activated carbon support outperforming their respective MTMOs anchored on a graphene support, highlighting the importance of the catalyst support in determining the overall ORR activity of electrocatalysts. MnO(2)-CuO/AC has the highest diffusion limiting current density (j) value of 4.2 mA cm(-2) at -600 mV (vs. SHE), which is ∼1.1-1.7-fold higher than other tested electrocatalysts (i.e., 3.9, 3.5, and 2.7 mA cm(-2) for CoO-CuO/AC, CoO-CuO/G, and MnO(2)-CuO/G, respectively), and slightly lower than Pt/C (5.1 mA cm(-2)) at the same potential value. Moreover, all electrocatalysts exhibited good linearity and parallelism of the Koutechy-Levich (K-L) plots, suggesting that ORR followed first-order reaction kinetics with the number of electrons involved being close to four. Benefiting from their remarkable ORR electrochemical activities and low cost, our results reveal that non-precious MTMOs are efficient enough to replace expensive Pt for broad applications in energy conversion and electrocatalysis in neutral media, such as microbial fuel cells.