Metal-organic frameworks (MOFs), a class of crystalline nano-porous materials consisting of metal-containing clusters and organic linkers, are known to retain their polyhedral shape after carbonization accompanied by loss of crystallinity. Researchers from Department of Materials Science and Metallurgy, University of Cambridge have used a porous MOF as a host for loading metal-containing salts as guests and converted to a 3D multilevel hierarchical carbon-based architecture, after a simple carbonization and washing process.

The guest salt used was (NH₄)₂MoS₄ into a readily available MOF host, Cu-based HKUST-1, and achieved a carbon-based material with three levels of hierarchy: (i) micron-sized cage containing (ii) nanofibers with (iii) mesopores distributed on them. They named such type of materials as “nano-diatoms” due to the morphological analogy to the diatomaceous species in nature. They also found that Mo and S can be homogeneously distributed into the carbon matrix after carbonization. Meanwhile, using the nano-diatoms the researchers demonstrated among the highest performing electrodes for Li-ion batteries that can retain high capacity at a high charging rate.  

By understanding the formation process of the nano-diatoms, they produced a range of nano-diatoms with different hierarchical structures and elements incorporated in the carbon matrix.  As there are thousands of available MOFs and metal-containing compounds, the options for designing nano-diatoms are very large. This is a promising strategy to chemically functionalizing the carbon by incorporating other active elements, the researchers are actively in collaboration with the Solid State Research group in Max Planck Institute in order to collaboratively develop advanced electrocatalysts for battery electrodes.

Figure: Fabrication summary and cyclic stability tests at 2A /g.