The sequential structural transformation of a heptanuclear zinc cluster towards hierarchical porous carbon for supercapacitor applications

Abstract

The peripheral N/O chelating of Schiff base ligands, inner bridges, counterions, and metal centers gave rise to a brucite disk cluster [Zn₇L₆(OCH₃)₆](NO₃)₂ (Zn₇, (HL = 2-methoxy-6-((methylimino)-methyl)phenolate)) which crystallized into hexagonal prismatic plates. The combination of crystallographic studies, in situ TG-MS, and other characterization techniques showed that with a fixed metal and ligand composition in the precursors, weak correlative interactions (e.g., electrostatic interactions) and shape matching between the cluster core and counterions determine the cluster packing modes in the crystals and affect their phase and morphological changes during pyrolysis. The tracking of the pyrolysis process showed that the peripheral ligands, inner bridge, and counterion decompose first, followed by the Zn₇O₆ core merging with cubic ZnO, which was then reduced by carbon and eventually evaporated, leaving behind a porous carbon structure. In this process, the solid material composition change was in the sequence {Zn₇}-{Zn–O core@C}-{ZnO@C}-{Zn@C}-{C}, which was accompanied by a porosity change from micropores to hierarchical pores, and then to micropores again. The core structure and packing modes of Zn₇ evolved into micropores and mesopores, respectively. Micro-mesoporous carbon Zn₇-1000 featured a capacitance of 1797 F g⁻¹ at 1 A g⁻¹, where the BET specific surface area was 3119.18 m² g⁻¹, which, to the best of our knowledge, is the highest value reported for a porous carbon electrode. This work represents an important benchmark for the analysis of dynamic chemical processes involving coordination clusters at high temperatures, and it could lead to important applications in high-performance devices.