Oxygen-Coordinated Cr Single-Atom Catalyst for Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells

Carbon-supported metal single-atom catalysts (M-SACs) are promising oxygen reduction reaction (ORR) catalysts. Their ORR activity and selectivity are significantly affected by the heteroatoms that coordinate the central metal atoms. Previous reports found that oxygen-coordinated M-SACs promoted a 2e- ORR rather than the 4e- ORR that is more desirable for fuel cells. Herein, we report for the first time that oxygen-coordinated M-SACs are capable of promoting the 4e- ORR in acid media. We prepared a Cr(acac)-NC catalyst with the central Cr atom coordinated by two O atoms. The Cr(acac)-NC catalyst not only exhibits excellent ORR activity and stability in acid media, but also delivers high proton exchange membrane fuel cell (PEMFC) performance comparable to N-coordinated M-SACs. Density functional theory (DFT) calculations reveal that Cr-O2 moieties located on the zigzag edge of the carbon support are the ORR-active sites.