electron spin resonance (esr) spectra of cu-thq mof

Magnetometric Characterization of Redox-Active MOFs Solid-State Electrochemistry

Since the term metal-organic framework (MOF) was introduced in the late 1990s, and especially in the 2010s, the most representative scientific concepts concerning MOFs have arisen from their structural porosity, including guest encapsulation, separation, catalytic properties, and molecule sensing1,2,3,4. Meanwhile, scientists were quick to realize that it is essential for MOFs to possess stimuli-responsive electronic properties in order to integrate them into modern smart devices. This idea triggered the spawning and flourishing of the conductive two-dimensional (2D) MOF family in the past 10 years, thereby opening the gate for MOFs to play key roles in electronics5 and, more attractively, in electrochemical energy storage devices6. These 2D MOFs have been incorporated as active materials in alkali metal batteries, aqueous batteries, pseudocapacitors, and supercapacitors7,8,9, and have exhibited tremendous capacity as well as excellent stability. However, to design better-performing 2D MOFs, it is crucial to understand their charge storage mechanisms in detail. Therefore, this article aims to provide a comprehensive understanding of the electrochemical mechanisms of MOFs, which can aid in the rational design of better-performing MOFs for energy storage applications.
In 2014, we first reported the solid-state electrochemical mechanisms of MOFs with redox-active sites on both metal cations and ligands10,11.&#160;These mechanisms were interpreted with the help of various in situ and ex situ spectroscopic techniques, such as X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS), X-ray diffraction (XRD), and solid-state nuclear magnetic resonance (NMR). Since then, this research paradigm has become a trend in studies of the solid-state electrochemistry of molecular-based materials12. These methods work fine for identifying the redox events of conventional MOFs with carboxylate bridging ligands, as the molecular orbitals and energy levels of metal cluster building blocks and organic ligands are almost independent of each other in such MOFs12,13.
However, when encountering the strongly correlated 2D MOFs with significant &#960;-d conjugation, the limitations of these spectroscopic methods were exposed. One of these limitations is that the band levels of most aforementioned 2D MOFs cannot be considered as a simple combination of metal clusters and ligands, but are rather a hybridization of them, while most of the spectroscopic methods only provide averaged, qualitative information about the oxidation states14. The other limitation is that the interpretation of these data is always based on the assumption of localized atomic orbitals. Therefore, the intermediate states with metal-ligand hybridization and delocalized electronic states are usually overlooked and described incorrectly with only these spectroscopic methods15. It is necessary to develop new probes for the electronic states of these electrochemical intermediates of not only 2D MOFs, but also other materials with similar conjugated or strongly correlated electronic structures, such as covalent organic frameworks16, molecular conductors, and conjugated polymers17.
The most common and powerful tools for assessing the electronic structures of materials are electron spin resonance (ESR) and superconducting quantum interference device (SQUID) magnetic susceptibility measurements18,19. As both rely on unpaired electrons in the system, these tools can provide tentative information about the spin densities, spin distributions, and spin-spin interactions. ESR offers sensitive detection of unpaired electrons, while magnetic susceptibility measurement gives more quantitative signals for upper properties20. Unfortunately, both techniques unavoidably face great challenges when used to analyze the electrochemical intermediates. This is because target samples are not pure, but rather a mixture of target material, conductive additive, binder, and byproduct from the electrolyte, so the obtained data21,22 are the sum of contributions from both the material and the impurities. Meanwhile, most intermediates are sensitive to the environment, including air, water, certain electrolytes, or any other unpredictable perturbations; extra care is necessary while handling and measuring intermediates. Trial and error is normally necessary while dealing with a new combination of electrode material and electrolyte.
Here, we present a new paradigm, called electrochemical magnetometry, for analyzing the electronic states or spin states of 2D MOFs and similar materials using a series of techniques, utilizing electrochemistry and temperature-variable ex situ ESR spectroscopy as well as ex situ magnetic susceptibility measurements20. To demonstrate the effectiveness of this approach, we use Cu3THQ2 (THQ = 1,2,4,5-tetrahydroxybenzoquinone; referred to as Cu-THQ), a representative 2D MOF, as an example. We explain the selection of conductive additives and electrolytes, the fabrication of electrodes and electrochemical cells, as well as details on sample handling and measurement, including possible issues during measurement. By comparing with classic characterizations such as XRD and XAFS, electrochemical magnetometry can provide a comprehensive understanding of the electrochemical mechanism of most MOFs. This approach is capable of capturing unique intermediate states and avoiding incorrect assignment of redox events. The elucidation of energy storage mechanisms using electrochemical magnetometry can also contribute to a better understanding of the structure-function relationships in MOFs, leading to more intelligent synthetic strategies for MOFs and other conjugated materials.

Author Spotlight: Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks  

Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks

Our research interest lies in understanding the electronic and spin states of 2D pi-d conjugated metal organic frameworks and their correlation with their electrochemical behavior of this MOFs in solid-state energy storage devices. In the last five years, many new 2D-conjugating MOF materials have been reported and studied for their use as active materials in electrochemical cells. However, the mechanism of their charge storage processes is still unclear.Spectroscopic methods, including X-ray diffraction, X-ray photoelectron spectroscopy, and the X-ray absorption fine structure are the mostly common used techniques in this field. Those techniques are crucial for categorizing the specific element's crystal structure and oxygen states. It is not possible to separate a 2D MOF from the device while analyzing its electrochemical intermediate state.Measurement must be taken on a mixture that includes conductive additives and binders. However, to accurately determine the MOF's electrochemical behavior, it is necessary to calibrate the contribution of these additives. Most spectroscopic measures assume well localized electrons in MOFs, but our protocol provides a physical view and reveals strongly-correlated phenomena in these materials.We will continue to better understand the electronic, magnetic, and quantum properties of 2D-conjugated MOFs and bridge the gap between coordination chemistry and solid-state physics through physical insights.

Watch this Scientific Journal Video about Electron Spin Resonance ESR Spectra of Cu-THQ MOF at JoVE.com

Electron Spin Resonance (ESR) Spectra of Cu-THQ MOF  

Connect the sample tube to a rubber tube and seal it with a valve. Then, flame the seal head of the sample tube under a vacuum. Set up the ESR spectrometer.Then, insert the prepared sample tube into the microwave cavity and center the sample. Then, to register the ESR of the sample, choose the optimal parameters such as microwave power, magnetic field sweep time, center field sweep width, modulation frequency and width, and channel amplitude and time constant. Then, sweep the magnetic field and record the ESR spectrum.Adjust the manganese marker insertion amount to 800 and capture the ESR spectrum with the manganese marker as previously demonstrated. Calibrate the magnetic field using six hyperfine lines for manganese divalent ions. The copper THQ MOF exhibited a broad ESR line.When copper THQ was reduced to 1.5 volts, the copper ion signal disappeared and the radical signal remained, indicating that the ions were reduced to copper. Quantitative ESR measurement confirmed that the copper THQ MOF had 96%copper ions in the paramagnetic state. ESR line width of copper ion narrowed with decreasing temperature, while chemical shift remained constant, indicating significant anti-ferromagnetic interaction.A significant temperature-independent paramagnetic term was observed in ESR spin susceptibility compared to squid magnetic susceptibility.

electron-spin-resonance-esr-spectra-of-cu-thq-mof

Research

JoVE Journal

Chemistry

333 Views.  Nagoya University. Connect the sample tube to a rubber tube and seal it with a valve. Then, flame the seal head of the sample tube under a vacuum. Set up the ESR spectrometer.Then, insert the prepared sample tube into the microwave cavity and center the sample. Then, to register the ESR of the sample, choose the optimal parameters such as microwave power, magnetic field sweep time, center field sweep width, modulation frequency and width, and channel amplitude and time constant. Then, sweep the magnetic ...

Video: Electron Spin Resonance (ESR) Spectra of Cu-THQ MOF  

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.

Ex situ magnetic surveys can directly provide bulk and local information on a magnetic electrode to reveal its charge storage mechanism step by step. Herein, electron spin resonance (ESR) and magnetic susceptibility are demonstrated to monitor the evaluation of paramagnetic species and their concentration in a redox-active metal-organic framework (MOF).

Electrochemical energy storage has been a widely discussed application of redox-active metal-organic frameworks (MOFs) in the past 5 years. Although MOFs ...

Fabrication of Copper Tetrahydroxyquinone (Cu-THQ) Electrode and Electrochemical Cycling

To begin, prepare the copper THQ/CB/PVDF electrode by dissolving 10 milligrams of PVDF in 1.4 milliliters of NMP. Disperse 50 milligrams of pre-synthesized copper THQ MOF followed by 40 milligrams of carbon black in the solution and leave it to stir vigorously overnight. Coat the homogenous slurry onto an aluminum disc of 15 millimeter diameter and approximately 9.7 milligrams mass.Next, assemble the lithium copper THQ coin cells from the bottom to the top, starting with a negative shell, a 0.5-millimeter spacer, lithium, a separator, the prepared copper THQ electrode, a spacer, a spring, and a positive shell. Add a drop of 0.04 milliliters of electrolyte before and after placing the separator. To prepare the electrochemical intermediates, use a homemade device to compress the coin cell by tightening the screw.Then, connect the device to the measuring cables in the glove box. Next, connect the instrument outside the glove box to the ports corresponding to the coin cell. Finally, perform cyclic voltammetry and galvanostatic charge or discharge measurements to obtain the intermediates at various potentials.After electrochemical cycling, carefully disassemble the coin cell to avoid short circuits. Rinse the cycled copper THQ electrode with five milliliters of battery-grade dimethyl carbonate and dry it for 30 minutes at room temperature. Using a clean spatula, transfer the sample from the aluminum disc to aluminum foil.Transfer the sample powder into a sample tube and seal it tightly with a cap and transparent film or with a vacuum until further use. Electrochemical performances of the lithium copper THQ batteries demonstrated that the carbon and binder did not affect the electron transfer, and the battery delivered with a specific capacity of 390 milliampere-hours per gram in the first discharge process. Differential capacity analysis of copper THQ/CB/PVDF electrode demonstrated three electronic states, namely copper state, pi-d conjugated state, and delocalized pi-electron state account for the three redox peaks, and cyclic voltammetry curves varied from 4.0 to 1.5 volts.