Name: Author Spotlight: Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks
Uploaded: 2023-06-09T14:10:00
Description: Watch this Scientific Journal Video about Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks at JoVE.com

This study was supported by a Japan Society for the Promotion of Science (JSPS) KAKENHI Grant (JP20H05621). Z. Zhang also thanks the Tatematsu Foundation and Toyota Riken scholarship for financial support.

1. Electrode fabrication
<ol>
	<li>Synthesizing Cu-THQ MOF 
	NOTE: Cu-THQ MOF polycrystalline powder was synthesized via a hydrothermal method following previously published procedures14,20,23.
	<ol>
		<li>Put 60 mg of tetrahydroxyquinone into a 20 mL ampule, then add 10 mL of degassed water. In a separate glass vial, dissolve 110 mg of copper (II) nitrate trihydrate in another 10 mL of degassed ...

Magnetometric Characterization of Redox-Active MOFs Solid-State Electrochemistry

<ol>
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To produce cathodes, it is necessary to mix the active material with conductive carbon to achieve a low polarization during the electrochemical process. The carbon additive is the first critical point for ex situ magnetometry; if the carbon has radical defects, the emergence of the electrochemically induced organic radical cannot be observed in the ESR spectrum. This makes it difficult to precisely determine the spin concentration or organic radical concentration, since these two types of radicals have similar g...

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.

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	<tbody>
		<tr>
			<td>1-Methyl-2-pyrrolidone</td>
			<td>FUJIFILM Wako Chemicals</td>
			<td>139-17611</td>
			<td>Super Dehydrated</td>
		</tr>
		<tr>
			<td>1mol/L LiBF4 EC:DEC (1:1 v/v%)</td>
			<td>Kishida</td>
			<td>LBG-96533</td>
			<td>electrolyte</td>
		</tr>
		<tr>
			<td>4-Hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl</td>
			<td>FUJIFILM Wako Chemicals</td>
			<td>089-04191</td>
			<td>TEMPOL, for Spin Labeling&#160;</td>
		</tr>
		<tr>
			<td>Ampule tube</td>
			<td>Maruemu Corporation</td>
			<td>5-124-05</td>
			<td>20mL</td>
		</tr>
		<tr>
			<td>Carbon black, Super P Conductive</td>
			<td>Alfa Aesar</td>
			<td>H30253</td>
			<td></td>
		</tr>
		<tr>
			<td>Conductive Carbon Black</td>
			<td>Mitsubishi Chemical</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Copper (II) Nitrate Trihydrate</td>
			<td>FUJIFILM Wako Chemicals</td>
			<td>033-12502</td>
			<td>deleterious substances</td>
		</tr>
		<tr>
			<td>Dimethyl Carbonate</td>
			<td>FUJIFILM Wako Chemicals</td>
			<td>046-31935</td>
			<td>battery grade</td>
		</tr>
		<tr>
			<td>Ethylenediamine</td>
			<td>FUJIFILM Wako Chemicals</td>
			<td>053-00936</td>
			<td>deleterious substances</td>
		</tr>
		<tr>
			<td>Graphene Nanoplatelets</td>
			<td>Tokyo Chemical Industry</td>
			<td>G0442</td>
			<td>6-8nm(thick), 15&#181;m(wide)</td>
		</tr>
		<tr>
			<td>Poly(vinylidene fluoride)</td>
			<td>Sigma Aldrich</td>
			<td>182702</td>
			<td></td>
		</tr>
		<tr>
			<td>Potassium Bromide</td>
			<td>FUJIFILM Wako Chemicals</td>
			<td>165-17111</td>
			<td>for Infrared Spectrophotometry</td>
		</tr>
		<tr>
			<td>Sodium Alginate&#160;</td>
			<td>FUJIFILM Wako Chemicals</td>
			<td>199-09961</td>
			<td>500-600 cP</td>
		</tr>
		<tr>
			<td>SQUID Magnetometer</td>
			<td>Quantum Design</td>
			<td>MPMS-XL 5</td>
			<td></td>
		</tr>
		<tr>
			<td>Tetrahydroxy-1,4-benzoquinone Hydrate</td>
			<td>Tokyo Chemical Industry</td>
			<td>T1090</td>
			<td></td>
		</tr>
		<tr>
			<td>X-Band ESR</td>
			<td>JEOL</td>
			<td>JES-F A200</td>
			<td></td>
		</tr>
	</tbody>
</table>

magnetometric characterization of intermediates in the solid-state electrochemistry of redox-active metal-organic frameworks

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.

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.

Our previous work included a detailed discussion of ex situ ESR spectroscopy and ex situ magnetic susceptibility measurements for electrochemically cycled CuTHQ20.&#160;Here, we present the most representative and detailed results that can be obtained following the protocol described in this paper.
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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 ...