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.

Metal-organic frameworks (MOFs) derived magnetic carbon-based nanocomposites have drawn widespread attentions due to the well distributed nanocrystals in carbon matrix. Dynamically observing the formation process is urgently needed. Herein, taking zeolitic imidazolate framework (ZIF)-67 as an example, the pyrolysis process is investigated by in-situ transmission electron microscopy (TEM) assisted with ex-situ characterizations. Co nanocrystals are evenly distributed in carbon at the initial stage of carbonization. By increasing pyrolysis temperature, the nanocrystals grow bigger and migrate to carbon surface. The carbon texture transfers from amorphous to crystalline at 600 °C, and thoroughly converts at 800 °C. In-situ heating TEM shows that more tiny Co nanocrystals move out from the carbon texture by increasing temperature from 700 to 800 °C. At 1,000 °C, some escaped tiny Co nanocrystals are volatilized and disappeared. The residual escaped Co nanocrystals catalyze the formation of carbon nanotubes (CNTs). Due to the synergistic effect between Co and carbon as well as porous structure, the nanocomposites show high-efficient microwave absorption performance, which can be tuned by pyrolysis temperature, heating rate, and mass fraction. When the mass fraction is 30 wt.%, the nanocomposites obtained at 600 or 700 °C display remarkable microwave absorption with optimal reflection loss (RL) smaller than −70 dB and effective absorption band larger than 4.9 GHz. Combining the in-situ and ex-situ techniques, some key findings were observed: (1) graphitization of carbon; (2) volatilization of Co nanocrystals; (3) formation process of CNTs by Co catalyst. These findings are helpful to understand the formation of MOFs derived carbon-based composites and expand their practical applications, especially for microwave absorption.