Specific Cu2O surfaces for electrocatalytic oxygen reduction reaction

Abstract

Examination of the facet effects of metal oxide crystals on the oxygen reduction reaction (ORR) has been inadequately investigated due to the limited availability of polyhedra that expose only specific surfaces. Here, cuprous oxide cubes, octahedra, and rhombic dodecahedra, exposing the respective {100}, {111}, and {110} surfaces, were incorporated into a matrix of carbon nanotubes (CNTs) to enhance electrical conductivity. The composites were evaluated for their electrocatalytic ORR activity. The rhombic dodecahedra/CNTs composite exhibited the highest ORR activity, followed by the octahedra/CNTs and then the cubes/CNTs. Commercial Cu₂O powder/CNTs showed notably lower ORR activity, demonstrating the importance of catalyst surface control for ORR performance. Koutecký–Levich plots showed that these Cu₂O polyhedra were highly selective towards the four-electron pathway in the ORR, whereas the commercial Cu₂O powder/CNTs catalyst proceeded via the two-electron pathway. Durability tests revealed a reversed trend, with the cubes/CNTs being the most stable electrocatalyst. Density functional theory (DFT) calculations indicated the weakest O₂ adsorption on the Cu₂O {110} surface. The free energy diagrams and 2D volcano plot further established the {110} surface as the most active towards ORR, while strong OH intermediate binding on the {100} and {111} surfaces led to lower theoretical limiting potentials and higher overpotentials. DFT results provided mechanistic insights to explain the experimental facet effects.