Co NP/NC hollow nanoparticles derived from yolk-shell structured ZIFs@polydopamine as bifunctional electrocatalysts for water oxidation and oxygen reduction reactions

The pyrolysis under inert atmosphere has been widely used for the synthesis of metal containing heteroatoms doped carbon materials, versatile catalysts for various reactions. However, it is difficult to prevent metal nanoparticles aggregation during pyrolysis process. Herein, we reported the efficient synthesis of nitrogen doped carbon hollow nanospheres with cobalt nanoparticles (Co NP, ca. 10 nm in size) distributed uniformly in the shell via pyrolysis of yolk-shell structured Zn-Co-ZIFs@polydopamine (PDA). PDA acted as both protection layer and carbon source, which successfully prevented the aggregation of cobalt nanoparticles during high-temperature pyrolysis process. The Co NP and N containing carbon (Co NP/NC) hollow nanospheres were active for both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), affording overpotential of 430 mV at 10 mA/cm 2 for OER in 1 M KOH and comparable half-wave potential to that of Pt/C (0.80 V vs RHE) for ORR in 0.1 M KOH. The superior performance of carbon hollow nanospheres for both OER and ORR was mainly attributed to its small metal nanoparticles, N-doping and hollow nanostructure. The protection and confinement effect that originated from PDA coating strategy could be extended to the synthesis of other hollow structured carbon materials, especially the ones with small metal nanoparticles.