Controlled synthesis of single cobalt atom catalysts via a facile one-pot pyrolysis for efficient oxygen reduction and hydrogen evolution reactions.

Metal-nitrogen doped carbon catalysts (M-N/C) with abundantly accessible M-N sites, particularly single metal atom M-N/C (SAM-N/C), have been developed as a substitute for expensive Pt-based catalysts. These catalysts are used to increase the efficiency of otherwise sluggish oxygen reduction reactions (ORR) and hydrogen evolution reactions (HER). However, although the agglomerated metal nanoparticles are usually easy to form, they are very difficult to remove due to the protective surface-coating carbon layers, a factor that significantly hampers SAM-N/C fabrication. Herein, we report a one-step pyrolysis approach to successfully fabricate single cobalt atom Co-N/C (SACo-N/C) by using a Co-SCN coordination compound as the metal precursor. Thanks to the decomposition of Co-SCN compound at lower temperature than that of carbon layer deposition, Co-rich particles grow up to larger ones before carbon layers formation. Even though encapsulated by the carbon layers, it is difficult for the large Co-rich particle to be completely sealed. And thus, it makes the Co atoms possible to escape from incomplete carbon layer, to coordinate with nitrogen atoms, and to form SACo-N/C catalysts. This SACo-N/C exhibits excellent performances for both ORR (half-wave potential of 0.878 V) and HER (overpotential at 10 mA/cm of 178 mV), and is thus a potential replacement for Pt-based catalysts. When SACo-N/C is integrated into a Zn-O battery, battery with high open-circuit voltage (1.536 V) has high peak power density (266 mW/cm) and large gravimetric energy density (755 mA h/g) at current densities of 100 mA/cm. Thus, we believe that this strategy may offer a new direction for the effective generation of SAM-N/C catalysts.