Electron-Conductive Metal-Organic Framework, Fe(dhbq)(dhbq = 2,5-Dihydroxy-1,4-benzoquinone): Coexistence of Microporosity and Solid-State Redox Activity

Effective Stabilization of Organic Cathodes Through Formation of a Protective Solid Electrolyte Interface Layer via Reduction

Organic electrode materials offer a promising alternative for lithium-ion batteries due to their lower costs, reduced environmental impact, renewability, and high theoretical capacity. Among them, 2,5-dihydroxy-1,4-benzoquinone (DHBQ) is a promising cathode material, but its high solubility in electrolytes leads to rapid capacity degradation of the battery. This study investigates the dilithium salt of DHBQ, Li2DHBQ, as a cathode material for LIBs. Despite its minimal solubility in the electrolyte, Li2DHBQ cathodes suffer rapid capacity decay due to severe morphological damage within the voltage range of 1.5-3.0 V. To achieve morphological stabilization, we promoted the formation of a protective solid electrolyte interphase (SEI) layer on Li2DHBQ particles by lowering the discharge cutoff voltage. Cycling the battery with a 0.5 V discharge cutoff voltage achieved the optimal thickness and organic-rich composition of the SEI layer, leading to significantly improved morphological stability of Li2DHBQ. Consequently, the battery maintained 170 mAh g-1 with a low decay rate of 0.16 % within a voltage range of 0.5-3.0 V after 200 cycles at 500 mA g-1. Furthermore, initial cycling with a discharge cutoff voltage of 0.5 V for 20 cycles to form an SEI layer, followed by cycling at a normal discharge cutoff voltage of 1.5 V, retained an even higher capacity of 187 mAh g-1 after 200 cycles at 500 mA g-1. These are significant improvements compared to the battery cycled only in the normal range of 1.5-3.0 V, which retained a capacity of 87 mAh g-1. This study demonstrates the effectiveness of forming a cathode SEI layer at low discharge voltages as a new approach to stabilizing organic cathode materials.

Recent Advances in Pristine Iron Triad Metal-Organic Framework Cathodes for Alkali Metal-Ion Batteries

Pristine iron triad metal-organic frameworks (MOFs), i.e., Fe-MOFs, Co-MOFs, Ni-MOFs, and heterometallic iron triad MOFs, are utilized as versatile and promising cathodes for alkali metal-ion batteries, owing to their distinctive structure characteristics, including modifiable and designable composition, multi-electron redox-active sites, exceptional porosity, and stable construction facilitating rapid ion diffusion. Notably, pristine iron triad MOFs cathodes have recently achieved significant milestones in electrochemical energy storage due to their exceptional electrochemical properties. Here, the recent advances in pristine iron triad MOFs cathodes for alkali metal-ion batteries are summarized. The redox reaction mechanisms and essential strategies to boost the electrochemical behaviors in associated electrochemical energy storage devices are also explored. Furthermore, insights into the future prospects related to pristine iron triad MOFs cathodes for lithium-ion, sodium-ion, and potassium-ion batteries are also delivered.

Expanding the horizons of porphyrin metal-organic frameworks via catecholate coordination: exploring structural diversity, material stability and redox properties

Porphyrin based Metal-Organic Frameworks (MOFs) have generated high interest because of their unique combination of light absorption, electron transfer and guest adsorption/desorption properties. In this study, we expand the range of available MOF materials by focusing on the seldom studied porphyrin ligand H10TcatPP, functionalized with tetracatecholate coordinating groups. A systematic evaluation of its reactivity with M(iii) cations (Al, Fe, and In) led to the synthesis and isolation of three novel MOF phases. Through a comprehensive characterization approach involving single crystal and powder synchrotron X-ray diffraction (XRD) in combination with the local information gained from spectroscopic techniques, we elucidated the structural features of the solids, which are all based on different inorganic secondary building units (SBUs). All the synthesized MOFs demonstrate an accessible porosity, with one of them presenting mesopores and the highest reported surface area to date for a porphyrin catecholate MOF (>2000 m2 g-1). Eventually, the redox activity of these solids was investigated in a half-cell vs. Li with the aim of evaluating their potential as electrode positive materials for electrochemical energy storage. One of the solids displayed reversibility during cycling at a rather high potential (∼3.4 V vs. Li+/Li), confirming the interest of redox active phenolate ligands for applications involving electron transfer. Our findings expand the library of porphyrin-based MOFs and highlight the potential of phenolate ligands for advancing the field of MOFs for energy storage materials.

Redox-active ligands enhance oxygen evolution reaction activity: Regulating the spin state of ferric ions and accelerating electron transfer

Metal-organic frameworks (MOFs) are considered as one of the most promising catalysts for oxygen evolution reaction (OER). However, only a few have introduced redox-active ligands into MOFs and explored their role in the OER process. In this work, we synthesized FeNi DHBQ/NF using the redox-active ligand 2,5-dihydroxy-1,4-benzoquinone (DHBQ), which exhibited excellent redox activity and required only 207 and 242 mV overpotentials to achieve current densities of 10 and 100 mA cm-2. Our research confirms that (i) the doping of Fe leads to the formation of Ni → O → Fe electron transfer channels in the MOFs and stronger electron transfer, attributed to the stronger d-π conjugation between the metal center and the ligand and reduced the d-orbital crystal field splitting energy of Fe3+; (ii) the rate determination step (RDS) in the OER process of the catalyst is the formation of O*, while Fe and redox-active ligands effectively regulate the adsorption energy of oxygen-containing intermediates, reducing the energy barrier of the RDS; (iii) the redox-active ligands can act as "electron reservoirs" in the electrochemical process, making Ni more readily oxidized to Ni3+ or even Ni4+ at low potentials, which is beneficial to the subsequent OER process.

Insulator-to-metal-like transition in thin films of a biological metal-organic framework

Temperature-induced insulator-to-metal transitions (IMTs) where the electrical resistivity can be altered by over tens of orders of magnitude are most often accompanied by structural phase transition in the system. Here, we demonstrate an insulator-to-metal-like transition (IMLT) at 333 K in thin films of a biological metal-organic framework (bio-MOF) which was generated upon an extended coordination of the cystine (dimer of amino acid cysteine) ligand with cupric ion (spin-1/2 system) - without appreciable change in the structure. Bio-MOFs are crystalline porous solids and a subclass of conventional MOFs where physiological functionalities of bio-molecular ligands along with the structural diversity can primarily be utilized for various biomedical applications. MOFs are usually electrical insulators (so as our expectation with bio-MOFs) and can be bestowed with reasonable electrical conductivity by the design. This discovery of electronically driven IMLT opens new opportunities for bio-MOFs, to emerge as strongly correlated reticular materials with thin film device functionalities.

Quinoid-Based Three-Dimensional Metal-Organic Framework Fe2(dhbq)3: Porosity, Electrical Conductivity, and Solid-State Redox Properties

We report the synthesis, characterization, and electronic properties of the quinoid-based three-dimensional metal-organic framework [Fe₂(dhbq)₃]. The MOF was synthesized without using cations as a template, unlike other reported X₂dhbq₃-based coordination polymers, and the crystal structure was determined by using single-crystal X-ray diffraction. The crystal structure was entirely different from the other reported [Fe₂(X₂dhbq₃)]^2-; three independent 3D polymers were interpenetrated to give the overall structure. The absence of cations led to a microporous structure, investigated by N₂ adsorption isotherms. Temperature dependence of electrical conductivity data revealed that it exhibited a relatively high electrical conductivity of 1.2 × 10^-2 S cm^-1 (E_a = 212 meV) due to extended d-π conjugation in a three-dimensional network. Thermoelectromotive force measurement revealed that it is an n-type semiconductor with electrons as the majority of charge carriers. Structural characterization and spectroscopic analyses, including SXRD, Mössbauer, UV-vis-NIR, IR, and XANES measurements, evidenced the occurrence of no mixed valency based on the metal and the ligand. [Fe₂(dhbq)₃] upon incorporating as a cathode material for lithium-ion batteries engendered an initial discharge capacity of 322 mAh/g.

Advances of Electroactive Metal-Organic Frameworks

The preparation of electroactive metal-organic frameworks (MOFs) for applications of supercapacitors and batteries has received much attention and remarkable progress during the past few years. MOF-based materials including pristine MOFs, hybrid MOFs or MOF composites, and MOF derivatives are well designed by a combination of organic linkers (e.g., carboxylic acids, conjugated aromatic phenols/thiols, conjugated aromatic amines, and N-heterocyclic donors) and metal salts to construct predictable structures with appropriate properties. This review will focus on construction strategies of pristine MOFs and hybrid MOFs as anodes, cathodes, separators, and electrolytes in supercapacitors and batteries. Descriptions and discussions follow categories of electrochemical double-layer capacitors (EDLCs), pseudocapacitors (PSCs), and hybrid supercapacitors (HSCs) for supercapacitors. In contrast, Li-ion batteries (LIBs), Lithium-sulfur batteries (LSBs), Lithium-oxygen batteries (LOBs), Sodium-ion batteries (SIBs), Sodium-sulfur batteries (SSBs), Zinc-ion batteries (ZIBs), Zinc-air batteries (ZABs), Aluminum-sulfur batteries (ASBs), and others (e.g., LiSe, NiZn, H⁺ , alkaline, organic, and redox flow batteries) are categorized for batteries.

Nine-Electron Transfer of Binder Synergistic π-d Conjugated Coordination Polymers as High-Performance Lithium Storage Materials

To solve the problems such as the dissolution and the poor conductivity of organic small molecule electrode materials, we construct π-d conjugated coordination polymer Ni-DHBQ with multiple redox-active centers as lithium storage materials. It exhibits an ultra-high capacity of 9-electron transfers, while the π-d conjugation and the laminar structure inside the crystal ensure fast electron transport and lithium ion diffusion, resulting in excellent rate performance (505.6 mAh g^-1 at 1 A g^-1 after 300 cycles). The interaction of Ni-DHBQ with the binder CMC synergistically inhibits its dissolution and anchors the Ni atoms, thus exhibiting excellent cycling stability (650.7 mAh g^-1 at 0.1 A g^-1 after 100 cycles). This work provides insight into the mechanism of lithium storage in π-d conjugated coordination polymers and the synergistic effect of CMC, which will contribute to the molecular design and commercial application of organic electrode materials.

Geometry-dependent valence tautomerism, magnetism, and electrical conductivity in 1D iron–tetraoxolene chains

Here we show how a simple change in the geometry of 1D iron–tetraoxolene chains dramatically alters the observed physical properties, including the presence of valence tautomerism, strong magnetic coupling, and electrical conductivity.

Deciphering the Thermal and Electrochemical Behaviors of Dual Redox-Active Iron Croconate Violet Coordination Complexes

Interest in coordination compounds based on non-innocent ligands (NILs) for electrochemical energy storage has risen in the last few years. We have focused our attention on an overlooked redox active linker, croconate violet, which has not yet been addressed in this field although closely related to standard NILs such as catecholate and tetracyanoquinodimethane. Two anionic complexes consisting of Fe(II) and croconate violet (-2) with balancing potassium cations were isolated and structurally characterized. By a combination of in situ and ex situ techniques (powder and single-crystal X-ray diffraction, infrared, and ⁵⁷Fe Mössbauer spectroscopies), we have shown that their dehydration occurs through complex patterns, whose reversibility depends on the initial crystal structure but that the structural rearrangements around the iron cations occur without any oxidation. While electrochemical studies performed in solution clearly show that both the organic and inorganic parts can be reversibly addressed, in the solid state, poor charge storage capacities were initially measured, mainly due to the solubilization of the solids in the electrolyte. By optimizing the formulation of the electrode and the composition of the electrolyte, a capacity of >100 mA h g^-1 after 10 cycles could be achieved. This suggests that this family of redox active linkers deserves to be investigated for solid-state electrochemical energy storage, although it requires the solving of the issues related to the solubilization of the derived coordination compounds.