  One of the great challenges facing atomically dispersed catalysts, including
single-atom catalyst (SAC) and double-atom catalyst (DAC) is their ultra-low
metal loading (typically less than 5 wt%), basically limiting the practical
catalytic application, such as oxygen reduction reaction (ORR) crucial to
hydrogen fuel cell and metal-air battery. Although some important progresses
have been achieved on ultra-high-density (UHD) SACs, the reports on UHD-DACs
with stable uniform dispersion is still lacking. Herein, based on the
experimentally synthesized M2N6 motif (M = Sc-Zn), we theoretically
demonstrated the existence of the UHD-DACs with the metal loading > 40 wt%,
which were confirmed by systematic analysis of dynamic, thermal, mechanical,
thermodynamic, and electrochemical stabilities. Furthermore, ORR activities of
the UHD-DACs are comparable with or even better than those of the
experimentally synthesized low-density (LD) counterparts, and the Fe2N6 and
Co2N6 UHD-DACs locate at the peak of the activity volcano with ultra-low
overpotentials of 0.31 and 0.33 V, respectively. Finally, spin magnetic moment
of active center is found to be a catalytic descriptor for ORR on the DACs. Our
work will stimulate the experimental exploration of the ultra-high-density DACs
and provides the novel insight into the relationship between ORR activity of
the DACs and their spin states.
