Graphite-like Charge Storage Mechanism in a 2D π–d Conjugated Metal–Organic Framework Revealed by Stepwise Magnetic Monitoring

Abstract

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.

Bipolar Molecular Torque Wrench Modulates the Stacking of Two-Dimensional Metal–Organic Framework Nanosheets

“…The stacking of two-dimensional (2-D) semiconductive nanosheets, such as graphene, MoS 2 , and h-BN, provides an unprecedented chance to produce functional Moiré materials with the exceptional electronic properties, optronic properties, and catalytic activities. − New 2-D porous materials, − for example, 2-D metal–organic frameworks (2-D MOFs) − and 2-D covalent-organic frameworks (2-D COFs) − also arouse remarkable attention due to their highly exposed active sites and designable pore environment that affords high-performance adsorption, separation, sensing, and catalysis. Their pore characteristics conventionally depend on the intrinsic pore sizes and shapes from intralayer constitutions .…”

Copper and conjugated carbonyls of metal–organic polymers as dual redox centers for Na storage

“…The Cu 2p spectrum is still dominated by Cu + further discharged to 1.0 V, which is similar to the redox reaction of Cu 2+ /Cu + in other conjugated coordination compounds. 31,37 Aer being fully charged to 3.0 V, the Cu-TABQ can be returned to the initial state. Besides, the C 1s spectra are presented in Fig.…”

“…). 21,33–37 Therefore, it is desirable to combine TABQ and Cu 2+ into a MOP skeleton to achieve high energy-density organic cathodes for SIBs and explore the Na + /e − transfer mechanism and structural evolution during the charge/discharge process.…”

Intertwining of Localized (d) and Delocalized (π) Spins in Magnetically Frustrated Two-Dimensional Metal–Organic Frameworks

“…The EPR spectra of Cu 3 (HHTP) 2 exhibited a broad isotropic signal which is a characteristic of the Cu­(II) ions arranged in the square planar coordination environment, along with the presence of a weak signal corresponding to the HHTP radicals (Figure c). Furthermore, the radical concentration in Cu 3 (HHTP) 2 was estimated from the EPR spectra, and it was observed that only ∼0.03% of spins originated from the ligand radicals (can be assigned as defect states) (Figure S15). Nonetheless, in such systems, the estimation of radical concentration cannot be precise, as the radicals in Cu 3 (HHTP) 2 are not static due to the redox activity of the HHTP in M-HHTP systems (multielectron redox process within −0.009 to −0.865 V) as well as the small redox potential for Cu­(II)/Cu­(I) couple [E 0 (Cu 2+ /Cu + ) = 0.153 V] which leads to continuous charge delocalization between the Cu d orbitals and the π orbitals from HHTP.…”

“…In this direction, Awaga and co-workers recently investigated a π−d conjugated 2D MOF Cu-THQ (THQ = tetrahydroxy-1,4-benzoquinone), where they observed voltage-induced charge delocalization between the metal d-orbitals and ligand π-orbitals. 36 The spin interaction was studied in-depth using cyclic voltammetry and EPR spectroscopy. Taking inspiration from their work, here, we have introduced an innovative design strategy of selectively introducing diamagnetic S = 0 Zn(II) ions in place of paramagnetic S = 1/2 Cu(II) ions, to experimentally observe the intertwining of the localized (d-electrons) and delocalized (π-electrons) S = 1/2 spins toward the emergence of the possible spin frustrated state in M 3 (HHTP) 2 -based materials (Figures S2 and S3).…”