Charge Up in Wired Covalent Organic Frameworks

Crystalline porous covalent organic frameworks (COFs) with suitable redox-active sites were found to be useful as electrodes for capacitive energy storage. However, the bottleneck issues of COFs, i.e., their poor electrical conductivity and instability, greatly limit their practical implementation. Now, a strategy has been developed to overcome these obstacles and enable the use of COFs for high performance energy storage. Conducting polymers, such as polyacetylene, polyaniline, polypyrrole, and polyethylenedioxythiophene (PEDOT), have been widely used as pseudocapacitive electrode materials. However, these polymers usually show low cycle stability as a result of repeated swelling and shrinking during charge−discharge cycles. By confining these polymers within porous frameworks such as metal−organic frameworks, cycling stability can be improved while retaining their electrical conductivity and mechanical stability. By virtue of well-defined one-dimensional open channels, two-dimensional (2D) COFs are suitable candidates for encapsulating conducting polymers within their pores. The function of such spatial confinement in COFs is 2-fold. One is that the stability of the conjugated polymers can be enhanced. Another one is that the conductivity of the COFs can be improved. These two effects play a vital role in capacitive energy storage. Recently, Dichtel and co-workers have explored this strategy by using a PEDOT-modified redox-active COF (DAAQ-TFP COF) to achieve extremely high-rate charging and high volumetric energy storage capacity, as reported in ACS Central Science. The DAAQ-TFP COF is chemically stable and possesses anthraquinone redox-active sites on each edge of the hexagonal polygons. The ordered DAAQ-TFP COF film was grown on gold electrode with controlled thickness (∼1 μm), and then 3,4-ethylenedioxythiophene (EDOT) was electropolymerized within the pores to prepare PEDOT-modified COF films (Figure 1). Interestingly, the PEDOT-modified DAAQ-TFP COF composite films exhibited a dramatically enhanced current response in cyclic voltammetry compared with the pristine DAAQ-TFP COF film. The improved current response was attributed to the wiring effect that PEDOT chains facilitate for the electron transport between electrodes and the redox-active sites of the DAAQ-TFP COF, while vertical ordered pore channels help to improve ion transport by shortening the diffusion distance. The composite COF film (1 μm thick) exhibited