Polystyrene Template Used to Synthesize Hollow COFs for Supercapacitors

Covalent organic frameworks (COFs) are crystalline porous organic materials connected by covalent bonds. They can be useful, e.g., in gas adsorption/separation, sensing, drug delivery, catalysis, or energy storage. For example, COFs could be promising as electrode materials in supercapacitors and metal-ion batteries. Their porosity, high surface area, and redox chemistry could be useful in this context. The topology of the COFs is important for their electrochemical properties, but can be difficult to predict. Templated synthesis methods can provide control over the topology of porous materials such as COFs.

Paolo Samorì, Université de Strasbourg, CNRS, France, Artur Ciesielski, Université de Strasbourg and Adam Mickiewicz University, Poznań, Poland, and colleagues have synthesized anthraquinone-based hollow COFs for applications in supercapacitors using polystyrene nanospheres as hard templates. The team prepared the COF (called COF DAAQ-Tp) via a condensation reaction between the two building blocks 1,3,5-triformylphloroglucinol (Tp) and 2,6-diaminoanthraquinone (DAAQ), which form imine links, using p-toluenesulfonic acid as the catalyst. To obtain a hollow version of the COF, the synthesis was performed in the presence of polystyrene (PS) nanospheres with sizes of about 300 nm. After the condensation of the monomers, the PS nanospheres were removed via extraction with tetrahydrofuran (THF) to obtain the hollow COF.

The researchers found that the COF prepared using the PS nanosphere templates has larger pores on average and a larger specific surface area than a control COF prepared from the same monomers without the templates. This higher porosity could provide better access to the COFs redox-active quinone units. The team tested the COFs electrochemical properties and their performance as cathode materials in zinc-ion hybrid supercapacitors. They found that the hollow, templated COF showed improved electrochemical performance, e.g., a higher specific capacitance, energy density, and long-term stability. According to the researchers, the approach could be extended to other COFs.