Atomically dispersed Co–N5 sites in graphene nanofoams for piezocatalytic N2 fixation driven by mild hydromechanical energy

Abstract

Novel Cobalt/Nitrogen co-doped graphene piezoelectric nanofoams (CoNG), featuring a confined Co-N₅ configuration with four coordinating N atoms being from the same NG layer and an additional coordinating N atom coming from an adjacent nitrogen doped graphene (NG) layer, have been synthesized for nitrogen fixation under low-frequency hydromechanical energy. The Co–N₅ configuration could increase local dipole moments in the a, b and c directions, thereby significantly improving the piezoelectric properties. On the other hand, the Co–N bonds bridging between neighboring NG layers effectively serve as interlayer electronic transport channels, enhancing carrier spatial separation in CoNG. It is due to the introduction of the versatile Co–N₅ conformation that CoNG is indeed capable of piezocatalytic nitrogen fixation under low-frequency stirring force, yielding NH₄⁺ and H₂ at rates of 0.64 and 14.10 µmol·g⁻¹·h⁻¹, respectively. Which are markedly higher than those achieved with NG (0.32 μmol·g⁻¹·h⁻¹ and 9.51 μmol·g⁻¹·h⁻¹). Isotopic ¹⁵N₂ labeling confirms the nitrogen source of ammonia. Furthermore, DFT calculations reveal that nitrogen molecules adsorb predominantly in an end-on manner at the Co sites and form NH₃ in a distal hydrogenation pathway. Moreover, CoNG demonstrates 16.5-fold and 3.4-fold enhancements in water splitting and rhodamine B degradation efficiency, respectively, in comparison with NG. This work establishes a sustainable pathway for ambient-condition ammonia synthesis and expands the application scope of piezocatalysis in green chemistry.