Rectification effect: A universal strategy for single-atom electrocatalysts to enhance oxygen reduction reaction

Abstract The M-Nx single-atom catalysts (SACs) are critical for efficient energy conversion technologies. However, most SACs with M-Nx moiety (M: Fe, Co, or/and Mn) suffer the strong binding ability with OH* intermediates in oxygen reduction reaction (ORR), which becomes a bottleneck in accelerating the kinetics. Herein, a universal “space-charged localization effect” strategy is proposed by constructing a p-n junction, where an n-type ZnS semiconductor longitudinally bridges with p-type M-Nx moiety to weaken the interaction of M-Nx with OH*. As expected, the a-ZnS/Fe-NSC electrocatalyst exhibits remarkable intrinsic activity in alkaline media with a half-wave potential of 0.90 V vs. RHE, and long-term durability (a shift of only 10 mV in E1/2 after 8,000 cycles). This phenomenon can be ascribed to the optimization of electronic structure, the S-MN4 site can effectively activate the M center with the intermediate spin state which possesses one eg electron (t2g4 eg1) readily penetrating the antibonding π-orbital of oxygen. Moreover, it offers a superior power density and higher discharge voltage in Al-air batteries. This universal strategy provides a rational perspective for the design of SACs and electronic structure engineering to construct robust active sites for high-performance oxygen reduction.