Carbon-isolation induced indirect metal-metal interaction regulates the effect of hierarchical cobalt core on anchored palladium for bifunctional oxygen electrocatalysis

Herein, palladium (Pd)-anchored hierarchical structure metallic Co coated by nitrogen-doped carbon (Pd-NC/Co HS) is constructed as a bifunctional catalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). For Pd-NC/Co HS, the carbon shell isolates the metallic Co core from the electrolyte for avoiding corrosion, while the hierarchical metallic Co enhances the overall structural stability, mass transmission, and intrinsic electron transfer for beneficial electrocatalysis. Notably, the metallic Co core underneath carbon suitably tunes the microstructure of interface Pd nanocrystals, forming electron-deficient Pd2+ which is highly active for OER. Moreover, this carbon-isolation induced indirect metal-metal interaction (IMMI) between metallic Co core and Pd boosts the ORR of the hybrid Pd-NC/Co HS. Theoretical calculations further show that, compared to Pd/C, Pd-NC/Co HS possesses an obvious decreased energy barrier for ORR potential limiting step of *OH desorption and for OER potential limiting step of the conversion from *OH to *O, further confirming the appropriate regulation of Co on Pd via carbon shell. The zinc-air battery (168.25 mW/cm2) and magnesium-air battery (89.98 mW/cm2) driven by Pd-NC/Co HS deliver a higher peak power density than the one driven by precious metal catalysts, along with a longer operation time, showing promising application potentials.