Ultrahigh-Density Double-Atom Catalyst with Spin Moment as an Activity Descriptor for the Oxygen-Reduction Reaction

One of the great challenges facing atomically dispersed catalysts, including single-atom catalysts (SACs) and double-atom catalysts (DACs), is their ultralow metal loading (typically less than 5 wt %), which limits their practical catalytic applications, such as in the oxygen-reduction reaction (ORR), which is crucial for hydrogen fuel cells and metal-air batteries. Although some important progress has been achieved in ultrahigh-density (UHD) SACs, reports on UHD DACs with stable uniform dispersion are still lacking. Herein, based on the experimentally synthesized ${M}_{2}{\mathrm{N}}_{6}$ motif ($M\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}\mathrm{Sc}$-$\mathrm{Zn}$), we theoretically demonstrate the existence of UHD DACs, with a metal loading of >40 wt %, which are confirmed by systematic analyses of dynamic, thermal, mechanical, thermodynamic, and electrochemical stabilities. Furthermore, the ORR activities of the UHD DACs are comparable to or even better than those of their experimentally synthesized low-density counterparts, and the ${\mathrm{Fe}}_{2}{\mathrm{N}}_{6}$ and ${\mathrm{Co}}_{2}{\mathrm{N}}_{6}$ UHD DACs are located at the peak of the activity volcano with ultralow overpotentials of 0.31 and 0.33 V, respectively. Finally, the spin magnetic moment of the active center is found to be a catalytic descriptor for the ORR on DACs. Our work can stimulate the experimental exploration of ultrahigh-density DACs and provides an insight into the relationship between the ORR activity of DACs and their spin states.