In situ observation Zn volatilization and microstructural resolution in ZIF-8-derived porous carbon for supercapacitors

Zn-based metal–organic frameworks (MOFs) are promising self-templates to fabricate metal-free porous carbon with large surface area and high porosity due to the abundant pore structure of their MOF parents and low boiling point of zinc metal during pyrolysis. However, the Zn volatilization process and microstructural resolution are still not clearly defined. Herein, we studied the pore structure formation mechanism of zeolitic imidazolate framework-8 (ZIF-8) derived carbon. The detailed temperature-dependent Zn volatilization process, Zn–N coordination configuration, and microstructural resolution processes were dynamically studied by in situ heating transmission electron microscope (TEM) and synchrotron radiation techniques. We revealed that the volatilization of Zn and N elements during pyrolysis process leads to porous carbon with large specific surface area and high microporosity. However, trace amounts of residual Zn still exist above the boiling point of Zn (907 °C), even at 1100 °C, which refreshes the viewpoint from previous literature. The residual zinc species were characterized by Cs-corrected high-angle annular dark-field scanning TEM (HAADF-STEM) image and synchrotron radiation. The results showed that the residual Zn element is uniformly anchored in the carbon skeleton as single Zn atom with Zn–N₁ configuration. Further experiments and density functional theory (DFT) calculations revealed that Zn–N₁ configuration has higher electrochemical activity than structurally symmetrical Zn–N₄. Besides, a symmetric supercapacitor was assembled using the porous carbon, which shows relatively high energy density and power density with excellent cycling stability. Electrochemical studies indicate that the specific capacitance is mainly determined by the specific surface area. This work is of great significance to deeply understand the microstructural resolution and properties of Zn-MOFs derived porous carbon, guiding their practical applications.