Single copper sites dispersed on defective TiO_2-x as a synergistic oxygen reduction reaction catalyst

Catalysts containing isolated single atoms have attracted much interest due to their good catalytic behavior, bridging the gap between homogeneous and heterogeneous catalysts. Here, we report an efficient oxygen reduction reaction (ORR) catalyst that consists of atomically dispersed single copper sites confined by defective mixed-phased TiO_2-x. This synergistic catalyst was produced by introducing Cu²⁺ to a metal organic framework (MOF) using the Mannich reaction, occurring between the carbonyl group in Cu(acac)₂ and the amino group on the skeleton of the MOF. The embedding of single copper atoms was confirmed by atomic-resolution high-angle annular dark-field scanning transmission electron microscopy and x-ray absorption fine structure spectroscopy. Electronic structure modulation of the single copper sites coupling with oxygen vacancies was further established by electron paramagnetic resonance spectroscopy and first-principles calculations. Significantly enhanced ORR activity and stability were achieved on this special Cu single site. The promising application of this novel electrocatalyst was demonstrated in a prototype Zn-air battery. This strategy of the stabilization of single-atom active sites by optimization of the atomic and electronic structure on a mixed matrix support sheds light on the development of highly efficient electrocatalysts.