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As key parameters of electrocatalysts, the density and utilization of active sites determine the electrocatalytic performance toward oxygen reduction reaction. Unfortunately, prevalent oxygen electrocatalysts fail to maximize the utilization of active sites due to inappropriate nanostructural design. Herein, a nano-emulsion induced polymerization self-assembly strategy is employed to prepare hierarchical meso-/microporous N/S co-doped carbon nanocage with atomically dispersed FeN4 (denoted as Meso/Micro-FeNSC). In situ scanning electrochemical microscopy technology reveals the density of available active sites for Meso/Micro-FeNSC reach to 3.57 × 1014 sites cm-2 , representing more than threefold improvement compared to micropore-dominant Micro-FeNSC counterpart (1.07 × 1014 sites cm-2 ). Additionally, the turnover frequency of Meso/Micro-FeNSC is also improved to 0.69 from 0.50 e- site-1 s-1 for Micro-FeNSC. These properties motivate Meso/Micro-FeNSC as efficient oxygen electroreduction electrocatalyst, in terms of outstanding half-wave potential (0.91 V), remarkable kinetic mass specific activity (68.65 A g-1 ), and excellent robustness. The assembled Zn-air batteries with Meso/Micro-FeNSC deliver high peak power density (264.34 mW cm-2 ), large specific capacity (814.09 mA h g-1 ), and long cycle life (>200 h). This work sheds lights on quantifying active site density and the significance of maximum utilization of active sites for rational design of advanced catalysts.
Tang et al. (Wed,) studied this question.