Publisher：Journal of Materials Chemistry C  

Covalent organic frameworks (COFs) have garnered significant attention in recent years, but they generally suffer from low electrical conductivity. In our previous studies, we observed that the nano-hybrid material PEDOT@AQ-COF (PEDOT = poly(3,4-ethylenedioxythiophene) and AQ-COF = ketoenamine-linked COF with anthraquinone (AQ) moieties), formed by polymerizing the organic conductive polymer PEDOT within the hollow pores of a COF, demonstrates outstanding electrical conductivity and energy storage properties. In this study, we systematically synthesized samples with various ratios of PEDOT to AQ-COF by adjusting the concentration of the precursor molecule in AQ-COF. Elemental analysis results indicated that PEDOT exists as oligomers with a degree of polymerization between 3.4 and 5.6, and there is a saturation point for the amount of PEDOT. This saturation state suggests that the PEDOT chains are densely packed within the cavity of AQ-COF, forming molecular contacts between PEDOT and AQ-COF. We assessed the electrical conductivity, electrochemical properties, and electron paramagnetic resonance (EPR) of the PEDOT@AQ-COF series. We observed that, upon reaching the saturation point of PEDOT, a critical transition occurs to a highly conductive state. In this state, the cyclic voltammetry curves exhibit redox reactions of AQ-COF, assisted by the PEDOT guest molecules. Compared to the low-conductivity sample, the high-conductivity samples in EPR displayed a broader linewidth component with nearly temperature-independent spin susceptibility.

DOI： 10.1039/d3tc04794f

Web of Science

Scopus

Language：English   Publisher：Journal of Visualized Experiments  

Electrochemical energy storage has been a widely discussed application of redox-active metal-organic frameworks (MOFs) in the past 5 years. Although MOFs show outstanding performance in terms of gravimetric or areal capacitance and cyclic stability, unfortunately their electrochemical mechanisms are not well understood in most cases. Traditional spectroscopic techniques, such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS), have only provided vague and qualitative information about valence changes of certain elements, and the mechanisms proposed based on such information are often highly disputable. In this article, we report a series of standardized methods, including the fabrication of solid-state electrochemical cells, electrochemistry measurements, the disassembly of cells, the collection of MOF electrochemical intermediates, and physical measurements of the intermediates under the protection of inert gases. By using these methods for quantitatively clarifying the electronic and spin state evolution within a single electrochemical step of redox-active MOFs, one can provide clear insight into the nature of electrochemical energy storage mechanisms not only for MOFs, but also for all other materials with strongly correlated electronic structures.

DOI： 10.3791/65335

Web of Science

Scopus

PubMed

Language：English   Publisher：Journal of the American Chemical Society  

Quasi-two-dimensional (2D) fully π-d conjugated metal-organic frameworks (MOFs) have been widely employed as active materials of secondary batteries; however, the origin of their high charge storage capacity is still unknown. Some reports have proposed a mechanism by assuming the formation of multiple radicals on one organic ligand, although there is no firm evidence for such a mechanism, which would run counter to the resonance theory. In this work, we utilized various magnetometric techniques to monitor the formation and concentration of paramagnetic species during the electrochemical process of 2D π-d conjugated Cu-THQ MOF (THQ = tetrahydroxy-1,4-benzoquinone). The spin concentration of the fully reduced (discharged 1.5 V) electrode was estimated to be around only 0.1 spin-1/2 per CuO4 unit, which is much lower than that of the expected “diradical” form. More interestingly, a significant elevation of the temperature-independent paramagnetic term was simultaneously observed, which indicates the presence of delocalized π electrons in this discharged state. Such results were corroborated by first-principles density functional theory calculations and the electrochemically active density of states, which reveal the microscopic mechanism of the charge storage in the Cu-THQ MOF. Hence, a graphite-like charge storage mechanism, where the π-electron band accepts/donates electrons during the charge/discharge process, was suggested to explain the excessive charge storage of Cu-THQ. This graphite-like charge storage mechanism revealed by magnetic studies can be readily generalized to other π-d conjugated MOFs.

DOI： 10.1021/jacs.2c10650

Web of Science

Scopus

PubMed