A Superior Unitary Oxygen Electrode with Accelerated Mass Transfer and Highly Exposed Active Sites for Rechargeable Air-Based Batteries

Nitrogen doped carbon is a promising candidate for noble Pt and IrO2 to catalyze oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). However, its catalytic activity is principally restricted by the low density of exposed active sites. Herein, we design hierarchically structured oxygen electrodes with a hierarchical nitrogen doped carbon layer on macroporous carbon cloth (termed as CC@NCLs). The outer NCLs are well controlled with the same chemical components and defect degrees, but different geometric structures. It is noted that only catalytic moieties adjacent to mesopores and macropores can be utilized. The multi-scaled CC@NCL possesses a large oxygen diffusion coefficient (4.97 x 10(-3) cm(2) s(-1)) and thereby a high utilization of catalytic sites. Resultingly, CC@NCL exhibits excellent catalytic activity for both ORR (half-wave potential of 0.855 V) and OER (1.580 V at 10 mA cm(-2)), and shows preferable activity and stability over noble Pt and IrO2 in zinc air flow batteries. This provides an insight for developing high density and accessible active sites in the practical energy device, including, but not limited to, metal-air battery, CO2 electrolyser and N-2 electrolyser.