Bottom-up pore-generation strategy modulated active nitrogen species for oxygen reduction reaction

Nitrogen-doped graphitic carbon materials (NC) have been regarded as one of the most promising alternatives for commercial Pt/C catalyst. However, it is still a challenge to modulate the porous structures and specific reactive nitrogen species. Herein, we designed the hierarchically holey N-doped carbon materials (holey-NC) through an in situ HF etching strategy during the pyrolysis. A hierarchically porous structure can be indicated by N2 adsorption-desorption. X-Ray absorption structure and X-ray photoelectron spectroscopy confirm the dominating graphitic N species. The correlation between the graphitic N ratio and the pore size indicates the effectiveness of the synthetic strategy in optimizing the amount of graphitic N of carbon materials by opening up pore structure within multiple length scale. The optimal catalyst (holey-NC1000) shows outstanding ORR performance with an onset potential of 1.0 V (vs. RHE) and half-wave potential of 0.86 V. It is verified that the optimized graphitic N content and hierarchically porous structure can accelerate the catalytic activation and the mass transfer of oxygen species, and hence promote their electrocatalytic activity. This work develops an effective strategy to generate hierarchical porous structure and modulate the highly active nitrogen species, which can serve as a general bottom-up pore-generation scheme for the design of carbon-based ORR catalyst.