Similar electronic state effect enables excellent activity for nitrate-to-ammonia electroreduction on both high- and low-density double-atom catalysts

Double-atom catalysts (DACs) for harmful nitrate (NO₃⁻) electroreduction to valuable ammonia (eNO₃RR) is attractive for both environmental remediation and energy transformation. However, the limited metal loading in most DACs largely hinders their applications in practical catalytic applications. Therefore, exploring ultrahigh-density (UHD) DACs with abundant active metal centers and excellent eNO₃RR activity is highly desired under the site-distance effect. Herein, starting from the experimental M₂N₆ motif deposited on graphene, we firstly screened the low-density (LD) Mn₂N₆ and Fe₂N₆ DACs with high eNO₃RR activity and then established an appropriate activity descriptor for the LD-DAC system. By utilizing this descriptor, the corresponding Mn₂N₆ and Fe₂N₆ UHD-DACs with dynamic, thermal, thermodynamic, and electrochemical stabilities, are identified to locate at the peak of activity volcano, exhibiting rather-low limiting potentials of −0.25 and −0.38 V, respectively. Further analysis in term of spin state and orbital interaction, confirms that the electronic state effect similar to that of LD-DACs enable the excellent eNO₃RR activity to be maintained in the UHD-DACs. These findings highlight the promising application of Mn₂N₆ and Fe₂N₆ UHD-DACs in nitrate electroreduction for NH₃ production and provide impetus for further experimental exploration of ultrahigh-density DACs based on their intrinsic electronic states.