Redox-active Centers Research Articles

Long-Range Charge Transport in Molecular Wires.

Long-range charge transport (LRCT) in molecular wires is crucial for the advancement of molecular electronics but remains insufficiently understood due to complex transport mechanisms and their dependencies on molecular structure. While short-range charge transport is typically dominated by off-resonant tunneling, which decays exponentially with molecular length, recent studies have highlighted certain molecular structures that facilitate LRCT with minimal attenuation over several nanometers. This Perspective reviews the latest progress in understanding LRCT, focusing on chemical designs and mechanisms that enable this phenomenon. Key strategies include π-conjugation, redox-active centers, and stabilization of radical intermediates, which support LRCT through mechanisms such as coherent resonant tunneling or incoherent hopping. We discuss how the effects of molecular structure, length, and temperature influence charge transport, and highlight emerging techniques like the Seebeck effect for distinguishing between transport mechanisms. By clarifying the principles behind LRCT and outlining future challenges, this work aims to guide the design of molecular systems capable of sustaining efficient long-distance charge transport, thereby paving the way for practical applications in molecular electronics and beyond.

Hydrogen bonding-stabilized bipolar organic cathode achieved all-round enhancement in zinc batteries

Bipolar organic small molecules with dual advantages of n/p-type redox reactions can achieve high-capacity-voltage zinc-organic batteries (ZOBs), but are plagued by inevitable dissolution in aqueous electrolytes. Here a hydrogen bonding-stabilized bipolar quinacridone (QA) molecule with synergetic coupling effects of two redox-active centers of n-type carbonyl moieties and p-type amine sites is proposed towards superior ZOBs. Compared with unipolar organics, bipolar QA molecule delivers low-energy-barrier redox kinetics and strong resistance to dissolution in aqueous electrolytes to avoid capacity decay due to the stable H-bond structure and extended π-conjugated aromatic plane. Consequently, Zn||QA battery harvests a high capacity of 212 mAh g−1 at 0.2 A g−1, an outstanding energy density (161 Wh kg−1), along with superior electrochemical stability (20,000 cycles). The high-kinetics hybrid anion-cation two-electron co-coordination mechanism in QA cathode is the root of excellent electrochemical metrics. This study opens novel insights to design multielectron bipolar cathodes for high-performance ZOBs.

Extending the π-Conjugation of a Donor-Acceptor Covalent Organic Framework for High-Rate and High-Capacity Lithium-Ion Batteries.

Realizing high-rate and high-capacity features of Lihium-organic batteries is essential for their practical use but remains a big challenge, which is due to the instrinsic poor conductivity, limited redox kinetics and low utility of organic electrode mateials. This work presents a well-designed donor-acceptor Covalent Organic Framework (COFs) with extended conjugation, mesoscale porosity, and dual redox-active centers to promote fast charge transfer and multi-electron processes. As anticipated, the prepared cathode with benzo [1,2-b:3,4-b':5,6-b''] trithiophene (BTT) as p-type and pyrene-4,5,9,10-tetraone (PTO) as n-type material (BTT-PTO-COF) delivers impressive specific capacity (218 mAh g-1 at 0.2 A g-1 in ether-based electrolyte and 275 mAh g-1 at 0.2 A g-1 in carbonate-based electrolyte) and outstanding rate capability (79 mAh g-1 at 50 A g-1 in ether-based electrolyte and 124 mAh g-1 at 10 A g-1 in carbonate-based electrolyte). In addition, the potential of BTT-PTO-COF electrode for prototype batteries has been demonstrated by full cells of dual-ion (FDIBs), which attain comparable electrochemical performances to the half cells. Moreover, mechanism studies combining ex situ characterization and theoratical calculations reveal the efficient dual-ion storage process and facile charge transfer of BTT-PTO-COF. This work not only expands the diversity of redox-active COFs but also provide concept of structure design for high-rate and high-capacity organic electrodes.

CuO and spinel CuCo2O4 supported on mesoporous nanocubes with dual redox-active centers for electrocatalytic H2O2 reduction and sensing

The copper hexacyanoferrate (CuHCF) nanocube and its cobalt-substituted analogue (CoCuHCF) possess numerous micropores; however, they demonstrate limited electrocatalytic performance for H2O2 reduction. Heat treatment of CuHCF in air (CuHCF-ht) results in the formation of CuO and Fe3O4 with noticeable aggregation. In contrast, the heat treatment of CoCuHCF in air (CoCuHCF-ht) preserves its nanocube structure while also forming a spinel CuCo2O4 phase. The partial substitution of cobalt ions for copper ions in CuHCF enhances both thermal and structural stability, leading to the transformation of micropores into mesopores during heat treatment. This transformation improves electrolyte accessibility and structural integrity. The CoCuHCF-ht electrode features dual redox-active centers (Cu+/Cu2+ couple) derived from the CuO and CuCo2O4 phases, serving as catalytic redox mediators for H2O2 reduction. This dual redox activity results in superior performance for H2O2 reduction compared to CuHCF-ht, which only exhibits a single pair of Cu+/Cu2+ couple from CuO as an active mediator. This enhanced performance is evidenced by a lower Tafel slope, a higher catalytic rate constant, and improved mass transfer of analytes. The CoCuHCF-ht catalyst, characterized by dual redox-active centers in mesoporous nanocubes, demonstrates a high sensitivity of 234.03 μA mM−1 cm−2 and a low limit of detection (0.26 μM), along with high selectivity, stability, repeatability, and reproducibility.

Dual Redox-active Covalent Organic Framework-based Memristors for Highly-efficient Neuromorphic Computing.

Organic memristors based on covalent organic frameworks (COFs) exhibit significant potential for future neuromorphic computing applications. The preparation of high-quality COF nanosheets through appropriate structural design and building block selection is critical for the enhancement of memristor performance. In this study, a novel room-temperature single-phase method was used to synthesize Ta-Cu3 COF, which contains two redox-active units: trinuclear copper and triphenylamine. The resultant COF nanosheets were dispersed through acid-assisted exfoliation and subsequently spin-coated to fabricate a high-quality COF film on an indium tin oxide (ITO) substrate. The synergistic effect of the dual redox-active centers in the COF film, combined with its distinct crystallinity, significantly reduces the redox energy barrier, enabling the efficient modulation of 128 non-volatile conductive states in the Al/Ta-Cu3 COF/ITO memristor. Utilizing a convolutional neural network (CNN) based on these 128 conductance states, image recognition for ten representative campus landmarks was successfully executed, achieving a high recognition accuracy of 95.13 % after 25 training epochs. Compared to devices based on binary conductance states, the memristor with 128 conductance states exhibits a 45.56 % improvement in recognition accuracy and significantly enhances the efficiency of neuromorphic computing.

P/n-Type Polyimide Covalent Organic Frameworks for High-Performance Cathodes in Sodium-Ion Batteries.

Covalent organic frameworks (COFs) are viewed as promising organic electrode materials for metal-ion batteries due to their structural diversity and tailoring capabilities. In this work, firstly using the monomers N,N,N',N'-tetrakis(4-aminophenyl)-1,4-phenylenediamine (TPDA) and terephthaldehyde (TA), p-type phenylenediamine-based imine-linked TPDA-TA-COF is synthesized. To construct a bipolar redox-active, porous and highly crystalline polyimide-linked COF, i.e., TPDA-NDI-COF, n-type 1,4,5,8-naphthalene tetracarboxylic dianhydride (NDA) molecules are incorporated into p-type TPDA-TA-COF structure via postsynthetic linker exchange method. This tailored COF demonstrated a wide potential window (1.03.6 V vs Na+/Na) with dual redox-active centers, positioning it as a favorable cathode material for sodium-ion batteries (SIBs). Owing to the inheritance of multiple redox functionalities, TPDA-NDI-COF can deliver a specific capacity of 67 mAh g-1 at 0.05 A g-1, which is double the capacity of TPDA-TA-COF (28 mAh g-1). The incorporation of carbon nanotube (CNT) into the TPDA-NDI-COF matrix resulted in an enhancement of specific capacity to 120 mAh g-1 at 0.02 A g-1. TPDA-NDI-50%CNT demonstrated robust cyclic stability and retained a capacity of 92 mAh g-1 even after 10 000 cycles at 1.0 A g-1. Furthermore, the COF cathode exhibited an average discharge voltage of 2.1 V, surpassing the performance of most reported COF as a host material.

Co‐Based Metal–Organic Frameworks With Dual Redox Active Centers for Lithium‐Ion Batteries With High Capacity and Excellent Rate Capability

AbstractSmall molecule organic materials are widely used as anode materials for lithium‐ion batteries (LIBs) due to their high reversible capacity, designable structure, and environmental friendliness. However, they suffer from poor intrinsic electronic conductivity, severe dissolution, and low initial coulombic efficiency. Recently, metal–organic frameworks (MOFs) have demonstrated in metal‐ion batteries, outperforming some small molecule organic materials in terms of both cycling stability and rate capability. Herein, the first rational design and synthesis of a new cobalt‐based MOF (CoBPDCA), are reported employing cobalt(II) nitrate as the metal source and small organic molecules (2,2‐bipyridyl‐4,4‐dicarboxylic acid, BPDCA) as the ligands, which shows exceptional chemical stability and impressive electron conductivity. Most importantly, CoBPDCA electrodes deliver a high reversible capacity of 1112.9 mAh g−1 at 0.05 C (1 C = 1000 mA g−1) and outstanding high‐rate durability (166.3 mAh g−1 after 2500 cycles at 10 C). Employing a series of spectroscopic and morphological characterizations and density functional theory (DFT) calculations, it is revealed that these impressively electrochemical performances are contributed to the dual active redox centers of Co cations and BPDCA ligands (CN and CO groups), and the superb electron conductivity. This work might provide a new strategy and a deeper understanding of the other MOFs' development for LIBs.

High-capacity and high-stability capacitive desalination with dual redox-active MnCo2S4/g-C3N4 heterojunction electrode

Capacitive deionization (CDI) utilizing dual redox-active electrodes has shown significant promise in desalinating low-concentration brackish waters, which is crucial for addressing global freshwater shortages. However, improvements in salt adsorption capacity and long-term operational stability are still required. In this study, we introduce, for the first time, a binary metal sulfide MnCo2S4/g-C3N4 (MCS-CN) heterojunction as a CDI electrode. The innovative integration of dual redox-active centers and heterojunction structures in the MCS-CN composite electrodes underscores their potential for achieving both high capacity and stability in capacitive desalination applications. The microscopic morphology, crystal structure, pore structure, and surface valence properties of the MCS-CN heterojunction composite materials were extensively characterized using various analytical techniques. In-situ X-ray diffraction, refined ex-situ X-ray photoelectron spectroscopy, three-electrode electrochemical analysis, and theoretical calculations were employed to elucidate the electrosorption and desorption mechanisms of sodium ions with the MCS-CN electrodes and to clarify the enhanced desalination performance of the binary metal sulfide-based heterojunction structure. As a result, the optimal MCS-CN-40 electrode exhibited the highest salt removal capacity of 71.64 mg g−1 and maintained stable desalination performance over 100 cycles in a 500 mg L−1 NaCl solution. This innovative approach provides a noval perspective for developing heterojunction electrodes with high redox activity and efficient desalination performance.

Strong Covalent Metal-Ligand Interaction Enables a Fast Kinetic and Structurally Stable Na-Ion Layered Cathode.

Anionic redox chemistry has attracted increasing attention for the improvement in the reversible capacity and energy density of cathode materials in Li/Na-ion batteries. However, adverse electrochemical behaviors, such as voltage hysteresis and sluggish kinetics resulting from weak metal-ligand interactions, commonly occur with anionic redox reactions. Currently, the mechanistic investigation driving these issues still remains foggy. Here, we chemically designed Na0.8Fe0.4Ti0.6S2 and Na0.8Fe0.4Ti0.6O2 as model cathodes to explore the covalency effects on metal-ligand interactions during anionic redox process. Na0.8Fe0.4Ti0.6S2 with strengthened covalent interaction of metal-ligand bonds exhibits smaller voltage hysteresis and faster kinetics than Na0.8Fe0.4Ti0.6O2 during (de)sodiation process. Theoretical calculations suggest that Fe is the dominant redox-active center in Na0.8Fe0.4Ti0.6S2, whereas the redox-active center moves from Fe to O with the removal of Na+ in Na0.8Fe0.4Ti0.6O2. We attribute the above different redox behaviors between Na0.8Fe0.4Ti0.6S2 and Na0.8Fe0.4Ti0.6O2 to the charge transfer kinetics from ligand to metal. Moreover, the structural stability of Na0.8Fe0.4Ti0.6S2 is enhanced by increasing the cation migration barriers through strong metal-ligand bonds during desodiation. These insights into the originality of metal-ligand interactions provide guidance for the design of high-capacity and structurally stable cathode materials for Li/Na-ion batteries.

Exploring the role of redox-active species on NiS-NiS2 incorporated sulfur-doped graphitic carbon nitride nanohybrid as a bifunctional electrocatalyst for overall water splitting

The advancement of effective and robust catalysts that can replace the precious-metal-based benchmarks for electrocatalytic overall water splitting is a highly fashionable approach to exploring sustainable energy utilization and addressing environmental issues. Herein, we demonstrate a facile single-step fabrication of NiS-NiS2 nanoparticles in situ grown on the layered porous sulfur-doped graphitic carbon nitride nanosheets (NiS-NiS2/SGCN) act as bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in an alkaline medium.Further, an increasing the Ni precursor (fixed sulfur source) increases the formation of NiS-NiS2 on the surface of SGCN (labeled as NS-1.0, NS-2.0, and NS-3.0). Structural and morphological studies such as PXRD, XPS, FTIR, HR-TEM, etc., revealed a uniform distribution of NiS/NiS2 on the surface of S-g-C3N4 nanosheets. The OER and HER activity of NiS-NiS2/SGCN nanohybrid were significantly enhanced by increasing the affinity of accessible surface-active edge sites, interfacial contact allows effective modification of electronic structure, high structural porosity, redox-active centers of Ni2+/Ni3+, and rich in sulfur vacancy. In addition, the enhancement of the exposure of the edge sites is improved by their attachment to a sulfur-doped g-C3N4 framework. As a result, in the half-cell experiments, the NiS-NiS2/SGCN (NS-3.0) catalyst exhibited enhanced performance in both OER and HER, requiring an overpotential of 298 mV to reach 50 mA/cm2 for OER and −144 mV to reach 10 mA/cm2 for HER. Furthermore, the constructed electrolyzer using Ni-S 3.0 as the cathode and anode required a cell voltage of 1.66 V to yield 50 mA/cm2 in overall water splitting. This study may inspire the in-situ synthesis of different metal sulfides on SGCN to provide a unique interfacial approach for improving the performance of bifunctional non-precious electrocatalysts.

Pyrene-4,5,9,10-tetraone-based covalent organic framework/carbon nanotube composite as sodium-ion cathodes with high-rate capability

Covalent organic frameworks (COFs) have attracted significant interest in the field of rechargeable batteries on account of their unique properties, including robust frameworks, well-defined porosity, abundant redox-active sites, and flexible structure designability. However, the limited active site utilization and low electrical conductivity always bring about poor electrochemical performance, thereby hindering practical applications. Herein, we reported a pyrene-4,5,9,10-tetraone-based covalent organic framework composite (Tp-PTO-COF@CNTs) grown on multi-walled carbon nanotubes via in-situ polycondensation. The Tp-PTO-COF@CNTs with numerous active sites (C=O groups) and strong π-π interaction between CNTs and COFs could accommodate more Na-ions and boost structural stability. Moreover, the existence of 1D CNTs could improve electronic conductivity, which facilitates fast transport of electrons and enhances reaction kinetics. In view of the two synergistic effects, Tp- PTO-COF@CNTs cathode displays an outstanding sodium-ion storage property with high initial capacity of 223.2 mA h/g at 0.1 A/g, remarkable rate capability at as high as 20 A/g, and ultra-long cycling stability (163.0 mA h/g exceeding 5000 cycles at 5 A/g). Furthermore, the ex-situ measurements are proposed to better confirm the role of carbonyl groups as redox-active centers. Such ultra-stable structural advantage might inspire the development of COF cathode materials for sodium-ion batteries.

Development of Organic Positive Electrodes for Rechargeable Aqueous Zinc-Ion Batteries for Stationary Energy Storage Systems

The rising concern towards the amount of anthropogenic carbon emissions and their effects on the environment has motivated the transition away from fossil fuels. The implementation of renewable energy sources has promoted the decarbonization of the power sector, but the variability of wind and solar energy necessitates the parallel deployment of high-performance energy storage infrastructure to ensure a reliable on-demand supply of green electricity to the grid. To this end, aqueous rechargeable zinc-ion batteries (ZIBs) are a promising new option for stationary energy storage owing to their high safety and low cost, when compared to established storage technologies like lithium-ion batteries. However, ZIBs are still limited by several technological challenges that hinder their uptake for commercial energy storage installations. In particular, the positive electrode materials, mostly comprised of metal oxides, are restraining the energy density and stability of the batteries and are yet under investigation. Low cost, long cycle life, and natural abundance are required properties of materials used in batteries for stationary energy storage. The search for materials with these properties has motivated the investigation of alternatives to metal oxides. One group of materials attracting considerable research attention is organic compounds which offer advantages like their natural abundance and high theoretical capacities. In this work, we developed an organic positive electrode for ZIBs using carbonyl groups as redox-active centers to provide energy storage capacity at round-trip energy efficiencies that are higher than the conventionally utilized manganese oxide electrodes. Furthermore, we investigated the effects of the conductive carbon additive on the deliverable capacity of the battery and elucidated the charge storage mechanism of this positive electrode material through detailed characterization. The results indicated that both Zn2+ and H+ are involved in the energy storage process of this organic electrode material. In addition to studying the energy storage process, degradation mechanisms were identified through nuclear magnetic resonance (NMR) and mass spectroscopy (MS). Both dissolution of the reduced form of the active material from the electrode and the formation of unwanted decomposition products are important contributors to the capacity fade of the battery. The technological and scientific insights provided in this presentation will thereby contribute towards the development of high-performance and naturally abundant electrode materials for the next generation of ZIBs to enable clean energy grid storage.

Boosting capacitive performance of electrode material by facile incorporation of phthalic acid ligand-based bimetallic MOF for supercapacitors

The bimetallic approach can offer a synergistic effect and energy-efficient metal-organic frameworks (MOFs) based electrode materials for electrochemical applications. The interconnected frameworks convert themselves into a two-dimensional conducting stacked structure with the favorable entity of metal ions bonded to metal-ligand covalence. This network prioritizes charge transport within the plane, minimizing the importance of out-plane transport. To achieve this, we propose a straightforward and efficient method for integrating Ni2+ and Co2+ metal ions with 1, 2-benzene dicarboxylic acid (phthalic acid) ligands. By employing this strategy, we can fabricate a bimetallic metal-organic framework electrode using a simple aromatic ligand, thus avoiding the complex synthesis typically associated with MOFs. This specially designed MOF electrode material enhances the electrochemical activities of the selected ligands. The presence of redox-active centers and conductive structure provides excellent behavior of the material with a high performance of 1755 Fg−1 (754.82 Cg−1) at 2 Ag−1. Also, the fabricated asymmetric device delivers (58.25 Whkg−1 energy density and 1299.90 Wkg−1 power density) with only a 9 % reduction in capacitance over 22 K cycles. The strategic design of a bimetallic Ni-Co-based metal-organic framework with bidentate ligand offers the potential to increase the electrochemical activities relative to single metallic MOFs.

Novel noble-metal-free NiCo2O4/CdIn2S4 S-scheme heterojunction photocatalyst with redox center for highly efficient photocatalytic H2 evolution

The exploration of efficient and noble-metal-free photocatalysts for photocatalytic H2 production is still a hot topic. Herein, a novel NiCo2O4/CdIn2S4 S-scheme heterojunction composites were successfully synthesized. Morphology characterization manifests that CdIn2S4 nanoparticles are tightly bound to the rod NiCo2O4 to form S-scheme heterojunction. Besides, X-ray photoelectron spectroscopy (XPS) results verify the existence of co-existing cobalt and nickel ions with multivalent state, which could provide H2 evolution active site and form redox centers. The S-scheme heterojunction and redox centers prompt the transfer of photogenerated e− from CdIn2S4 to NiCo2O4 for improving carrier separation efficiency, which is confirmed by a series of electrochemical experiments and photoluminescence (PL). The specific surface area and transmission electron microscope (TEM) results show that NiCo2O4 possesses a high specific surface area and porous structure, which could provide more active sites and facilitate H2O adsorption for further reduction reaction. The optimal NiCo2O4/CdIn2S4 composites possess maximum H2 evolution rate of 624 μmol·g−1·h−1, which is about 3.2 times than CdIn2S4-1 %Pt (198 μmol·g−1·h−1). After three cycles, the activity of NiCo2O4/CdIn2S4 shows no significant weaken. Moreover, the possible H2 evolution mechanism of NiCo2O4/CdIn2S4 S-scheme heterojunction composites is discussed. This research offers an idea for bimetallic oxide (NiCo2O4 etc.) modified sulfide to improve photocatalytic hydrogen production.

Masking the Bioactivity of Hydroxamic Acids by Coordination to Cobalt: Towards Bioreductive Anticancer Agents.

The clinical use of many potent anticancer agents is limited by their non-selective toxicity to healthy tissue. One of these examples is vorinostat (SAHA), a pan histone deacetylase inhibitor, which shows high cytotoxicity with limited discrimination for cancerous over healthy cells. In an attempt to improve tumor selectivity, we exploited the properties of cobalt(III) as a redox-active metal center through stabilization with cyclen and cyclam tetraazamacrocycles, masking the anticancer activity of SAHA and other hydroxamic acid derivatives to allow for the complex to reach the hypoxic microenvironment of the tumor. Biological assays demonstrated the desired low in vitro anticancer activity of the complexes, suggesting effective masking of the activity of SAHA. Once in the tumor, the bioactive moiety may be released through the reduction of the CoIII center. Investigations revealed long-term stability of the complexes, with cyclic voltammetry and chemical reduction experiments supporting the design hypothesis of SAHA release through the reduction of the CoIII prodrug. The results highlight the potential for further developing this complex class as novel anticancer agents by masking the high cytotoxicity of a given drug, however, the cellular uptake needs to be improved.

Are Redox-Active Centers Bridged by Saturated Flexible Linkers Systematically Electrochemically Independent?

The extent to which electrophores covalently bridged by a saturated linker are electrochemically independent was investigated considering the charge/spin duality of the electron and functionality of the electrophore as a spin carrier upon reduction. By combining computational modeling with electrochemical experiments, we investigated the mechanism by which tethered electrophores react together within 4,4'-oligo[n]methylene-bipyridinium assemblies (with n=2 to 5). We show that native dicationic electrophores (redox state Z=+2) are folded prior to electron injection into the system, allowing the emergence of supra-molecular orbitals (supra-MOs) likely to support the process of the reductive σ bond formation giving cyclomers. Indeed, for Z=+2, London Dispersion (LD) forces contribute to flatten the potential energy surface such that all-trans and folded conformers are approximately isoenergetic. Then, upon one-electron injection, for radical cations (Z=+1), LD forces significantly stabilize the folded conformers, except for the ethylene derivative deprived of supra-MOs. For radical cations equipped with supra-MOs, the unpaired electron is delocalized over both heterocycles through space. Cyclomer completion (Z=0) upon the second electron transfer occurs according to the inversion of redox potentials. This mechanism explains why intramolecular reactivity is favored and why pyridinium electrophores are not independent.

Unravelling the effects of active site density and energetics on the water oxidation activity of iridium oxides

Understanding what controls the reaction rate on iridium-based catalysts is central to designing better electrocatalysts for the water oxidation reaction in proton exchange membrane electrolysers. Here we quantify the densities of redox-active centres and probe their binding strengths on amorphous IrOx and rutile IrO2 using operando time-resolved optical spectroscopy. We establish a quantitative experimental correlation between the intrinsic reaction rate and the active-state energetics. We find that adsorbed oxygen species, *O, formed at water oxidation potentials, exhibit repulsive adsorbate–adsorbate interactions. Increasing their coverage weakens their binding, thereby promoting O–O bond formation, which is the rate-determining step. These analyses suggest that although amorphous IrOx exhibits a higher geometric current density, the intrinsic reaction rates per active state on IrOx and IrO2 are comparable at given potentials. Finally, we present a modified volcano plot that elucidates how the intrinsic water oxidation kinetics can be increased by optimizing both the binding energy and the interaction strength between the catalytically active states.

High performance phenothiazine-based battery materials achieved by synergistic steric blocking and extended conjugation

Phenothiazine derivatives, as sustainable cathode materials for batteries, show significant promise for large-scale electrical energy storage applications. However, the irreversibility of the sulfur redox active center severely limits their energy density and discharge voltage in rechargeable organic batteries. This study concentrates on identifying the potential side reactions that contribute to the irreversibility of phenothiazine-based materials. We propose a molecular design strategy that synergistically blocks the active site and extends the conjugation, markedly improving the electrochemical reversibility of these materials. Our findings reveal that phenothiazine derivatives can undergo two successive, one-electron redox reactions without interference from chemical bond rearrangement and reactive radicals. The assembled cells, utilizing lithium (Li) and sodium (Na) as anodes, display two distinct discharge plateaus at 3.3/3.9 V vs. Li+/Li and 3.1/3.7 V vs. Na+/Na, respectively, with nearly identical contributions from the nitrogen (N) and sulfur (S) redox centers. The phenothiazine derivatives designed via this strategy rank among the few phenothiazine-based organic electrode materials capable of achieving highly reversible, two-electron transfer redox reactions in organic batteries.

Polyoxometalates for carbon dioxide activation: Current progress and perspective

AbstractThe elevated level of carbon dioxide (CO2) in the troposphere is playing a crucial role in changing the earth's climatic system. Excess CO2 concentration is trapping a substantial proportion of heat imparting adverse range of environmental effects like extreme weather events, agricultural impacts and disruption of the ecosystem. Strategies such as CO2 capturing and storage are being pursued using polyoxometalates (POMs) to mitigate the elevated level of CO2 and their efficient conversion into useful chemicals. This review outlines the importance of CO2 reduction by electrocatalytic and photocatalytic reduction of CO2. POMs have several advantages for CO2 activation because of their promising catalytic activity, high selectivity and versatility for incorporating different metal ions and ligands. The efficient redox active metal center facilitates the conversion of CO2 by providing sufficient activation energy. Their robust structure also enables them resist harsh conditions, including high temperature and acidic or basic conditions. Owing to these properties, POMs can be a catalyst, co‐catalyst, photosensitizer or multielectron donor for CO2 reduction. Finally, we stated the challenges and future insight for CO2 activation using POMs.

Structural modulation of covalent organic frameworks based on redox active center of π-conjugated amine unit for enhanced electrochromic properties

The covalent organic frameworks (COFs) possess high designability, which provides a platform for synthesizing novel optoelectric materials, including electrochromic materials. In this work, N,N,N′,N′-tetra-(4-aminophenyl)-1,4-phenylenediamine (TPDA) with π-electron conjugate system and redox active centers was used as the main building block. The 4,4′-biphenyl diformaldehyde (BDA) and the 2,2′-thiophene-5,5′-diformaldehyde (TDA) were used as the aldehyde building units. Two imine-based COF electrochromic active materials (COFTPDA-BDA and COFTPDA-TDA) were prepared by Schiff base reaction. The test results reveal that COFTPDA-TDA has superior electrochromic properties than COFTPDA-BDA. Their contrasts are 0.46 and 0.40, and the coloring/fading times are 3.1/10.6 s and 10.9/14.3 s, respectively. The COFTPDA-TDA material possessing a bifunctional group structure was synthesized by replacing the biphenyl group with the bithiophene group with redox activity, which can enhance the optical properties and inter-layer π-π conjugation. Besides the electrochromic properties, the morphology and crystal lattice parameters are different. Therefore, it is expected to develop COFs-based electrochromic materials with rich colors, excellent performance, and wide application by introducing construction units with different redox active centers.