A new strategy for engineering a hierarchical porous carbon-anchored Fe single-atom electrocatalyst and the insights into its bifunctional catalysis for flexible rechargeable Zn-air batteries

Single-atom catalysts (SACs) provide the most efficient metal utilization for various catalytic reactions. Meanwhile, hierarchical pore structures play a decisive role in determining the catalytic activity of N-doped porous carbon materials, since normally micropores provide active locations and macro/mesopores serve as mass transport channels. However, it remains challenging to engineer highly dispersed SACs with hierarchical pore structures due to the complexity in synthesizing extra templates with controlled sizes and the possible exogenous impurities from templates. Herein, we develop a facile "confined recrystallization" self-template strategy to synthesize a hierarchical porous Fe SAC on N-doped carbon (Fe-NC SAC) with an iron mass loading of 1.5 wt%, by using polyvinylpyrrolidone (PVP), ferric nitrate and melamine. In this novel strategy, the introduced PVP can restrict the recrystallization of ferric nitrate owing to its large molecular weight. The formed nano-sized ferric nitrate crystals can not only serve as multifunctional self-templates to produce hierarchical pores, but also form atomically dispersed Fe-N-x sites through getting nitrogen from melamine. With the Fe-NC SAC as the catalyst, an excellent bifunctional ORR/OER performance and long-term durability superior to those of Pt/C and RuO2 are achieved in both liquid and solid Zn-air battery tests. Notably, in the flexible solid Zn-air battery, the Fe-NC SAC showed a high discharge voltage of 1.25 V and stable charge-discharge cycling at 1 mA cm(-2) under different bending conditions. Further density functional theory (DFT) calculations showed that the ORR follows the association mechanism of O-2 -> *OOH -> *O -> *OH -> OH-, while the OER goes on through the mechanism of OH- -> *OH- -> *O -> *OOH -> O-2, with a low theoretical overpotential of 1.21 V on the Fe-NC SAC. This work not only provides a novel strategy to endow SACs with a hierarchical pore structure, but also represents an example of how to effectively investigate the correlation between the structure and catalytic activity of catalysts.