Conductive Metal-Organic Frameworks for Supercapacitors

Elaborate Designed Three-Dimensional Hierarchical Conductive MOF/LDH/CF Nanoarchitectures for Superior Capacitive Deionization

Rational exploration of cost-effective, durable, and high-performance electrode materials is imperative for advancing the progress of capacitive deionization (CDI). The integration of multicomponent layered double hydroxides (LDHs) with conjugated conductive metal-organic frameworks (c-MOFs) to fabricate bifunctional heterostructure electrode materials is considered a complex but promising strategy. Herein, the fabrication of elaborately designed three-dimensional hierarchical conductive MOF/LDH/CF nanoarchitectures (M-CAT/LDH/CF) as CDI anodes via a controllable grafted-growth strategy is reported. In this assembly, carbon fiber (CF) provides exceptional electrical conductivity facilitating rapid ion transfer and acts as a sturdy foundation for even distribution of NiCoCu-LDH nanosheets. Moreover, the well-ordered NiCoCu-LDH further acts as interior templates to create an interface by embedding c-MOFs and aligning two crystal lattice systems, facilitating the graft growth of c-MOFs/LDH heterostructures along the LDH nanosheet arrays on CF, leading to accelerated ion diffusion kinetics. Density functional theory (DFT) confirms the unique structure of M-CAT/LDH/CF promotes interfacial charge transfer from NiCoCu-LDH to M-CAT. This enhancement accelerates ion transfer, decreases ion migration energy, and leads to better ion diffusion kinetics and a smoother Cl- shuttle. Accordingly, the asymmetrical M-CAT/LDH/CF cell exhibited superior specific capacitance (315 F g-1), excellent salt adsorption capacity (147.8 mg g-1), rapid rate (21.1 mg g-1 min-1), and impressive cyclic stability (91.4 % retention rate). This work offers valuable insights for designing heterostructure electrode materials based on three-dimensional interconnected networks, contributing to further advancements in CDI technology.

From 0D to 2D: microwave-assisted synthesis of electrically conductive metal-organic frameworks with controlled morphologies

Morphology control of electrically conductive metal-organic frameworks (EC-MOFs) can be a powerful means to tune their surface area and carrier transport pathways, particularly beneficial for energy conversion and storage. However, controlling EC-MOFs' morphology is underexplored due to the uncontrollable crystal nucleation and rapid growth kinetics. This work introduces a microwave-assisted strategy to readily synthesize Cu-HHTP (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) with controlled morphologies. We controlled solvent compositions to facilitate particles' directional growth to 1D and 2D crystals. Meanwhile, we found that ultrasonication can manipulate crystal seeding, yielding 0D spherical Cu-HHTP crystals. Electronic conductivity measurements suggest that the isotropic nature of the 0D crystals allows a conductivity of 7.34 × 10-1 S cm-1, much higher than 1D and 2D counterparts. Additionally, the controlled 0D morphology enhanced the material's capacitance and effective surface area and significantly improved its photocurrent response. These findings underscore the pivotal impact of controlled morphology in optimizing EC-MOFs' physicochemical properties.

Two-Dimensional Conjugated Metal-Organic Frameworks for Photochemical Transformations

Photochemical transformation represents an attractive pathway for the conversion of earth-abundant resources, such as H2O, CO2, O2, and N2, into valuable chemicals by utilizing sunlight as an energy source. Recently, two-dimensional conjugated metal-organic frameworks (2D c-MOFs) have emerged as the focal points in the field of photo-to-chemical conversion due to their advantages in light harvesting, electrical conductivity, mass transport, tunable electronic and porous structures, as well as abundant active sites. In this review, we highlight various physical and chemical features of 2D c-MOFs that can contribute to enhanced photo-induced exciton generation, charge transport, proton migration and redox catalysis. Then, the existing strategies to integrate suitable light absorbers and/or co-catalysts onto 2D c-MOFs for photochemical transformations (with a particular focus on H2 evolution, CO2 reduction and O2 reduction) have been discussed. Finally, the challenges and opportunities of using 2D c-MOFs in other photochemical applications (e.g., N2 fixation, organic synthesis, and environmental remediation) are assessed.

High-Entropy Metal-Organic Frameworks and Their Derivatives: Advances in Design, Synthesis, and Applications for Catalysis and Energy Storage

As a nascent class of high-entropy materials (HEMs), high-entropy metal-organic frameworks (HE-MOFs) have garnered significant attention in the fields of catalysis and renewable energy technology owing to their intriguing features, including abundant active sites, stable framework structure, and adjustable chemical properties. This review offers a comprehensive summary of the latest developments in HE-MOFs, focusing on functional design, synthesis strategies, and practical applications. This work begins by presenting the design principles for the synthesis strategies of HE-MOFs, along with a detailed description of commonly employed methods based on existing reports. Subsequently, an elaborate discussion of recent advancements achieved by HE-MOFs in diverse catalytic systems and energy storage technologies is provided. Benefiting from the application of the high-entropy strategy, HE-MOFs, and their derivatives demonstrate exceptional catalytic activity and impressive electrochemical energy storage performance. Finally, this review identifies the prevailing challenges in current HE-MOFs research and proposes corresponding solutions to provide valuable guidance for the future design of advanced HE-MOFs with desired properties.

Construction of 1D Molecular Conductive Wires Through a Polarized Gene Weaving Strategy for Efficient Electromagnetic Wave Absorption

The growing threat of electromagnetic pollution has become a pressing safety concern. Metal-organic framework (MOF) derivatives are considered ideal candidates for mitigating electromagnetic radiation. However, due to the limitations imposed by complex post-processing and disruption of pristine crystal structures, the mechanisms of electromagnetic wave absorption remain unclear, let alone achieving atomic-level regulation in MOF derivatives. Moreover, research on MOF-based electromagnetic wave absorbers (EMWA) has predominantly focused on 2D and 3D structures, leaving 1D MOFs largely unexplored. To address these challenges, a bottom-up polarization gene weaving strategy is proposed to integrate polarizable conjugated groups, thieno(3,2-b)thiophene (TBTT), into two types of conductive MOFs by fine-tuning self-assembly conditions. As expected, both MOFs exhibited strong natural polarization effects. Among them, the 1D linear coordination mode of CuTBTT-1D demonstrated enhanced charge carrier mobility and geometric effects compared to the 2D structure, CuTBTT-2D. The synthesized 1D molecular polarization wire, with a thickness of 2.2 mm, achieved ultra-high reflection loss (-77 dB) and super-wide absorption bandwidth (6.52 GHz). Its performance surpasses that of all known MOF-based EMWAs. This study provides a valuable strategy for the rational design of next-generation 1D MOF EMWA with atomic precision.

Thiol-functionalized conductive Co-MOF and its derivatives S-doped Co(OH)2 nanoflowers for high-performance supercapacitors

The low conductivity of many traditional metal-organic-framework (MOF)-based electrode limits their developments in the field of electrochemical energy storage and still of great challenge. The controllable preparation of various kinds of nanomaterials using thiol-functionalized MOF shows great prospects. In this work, a thiol-functionalized metal-organic framework sheet structure (Co-MOF/NF) on nickel foam was successfully prepared by in situ interfacial growth synthesis, which was transformed into its derivatives S-doped β-Co(OH)2 nanoflowers Co-x/NF (x = 1, 2 and 6) in different concentrations of KOH solutions through ion etching/exchange reaction. The pristine thiol-functionalized Co-MOF/NF and its derivatives Co-x/NF (x = 1, 2 and 6) nanoflower-like arrays could be used as positive electrode materials for effective supercapacitors. Among them, the transformation of the nanoflower-like Co-1/NF electrode exhibits excellent electrochemical properties with high areal capacitance (1925 ± 23 mF/cm2 at 1 mA/cm2), good rate performance, excellent conductivity and decent cycling stability. The Co-1//AC ASC device provides a high energy density of 0.176 mWh/cm2 (92.6 Wh/kg) at a power density of 0.745 mW/cm2 (392.1 W/kg). And this Co-1//AC ASC device exhibits a good cycling stability and practical application in energy storage field. This study provides a new strategy for the pristine thiol-functionalized MOF and its conversion nanostructures for energy storage applications.

Synthesis of 2D NiCo-MOF/GO/CNTs flexible films for high-performance supercapacitors

Flexible two-dimensional nickel-cobalt metal-organic frameworks/graphene oxide/carbon nanotubes (2D NiCo-MOF/GO/CNTs) hybrid films have been designed and prepared as high-performance supercapacitor electrode materials via vacuum filtration. The 2D NiCo-MOF nanosheets serve as the main source of capacitance for the hybrid films, while CNTs function as both the conductive network, enhancing the electrical conductivity of the MOFs, and the binder, linking the 2D NiCo-MOF nanosheets and GO. When the mass ratio of 2D NiCo-MOF, GO, and CNTs is 2 : 1 : 0.5, the hybrid film exhibits a high specific capacitance of 40.3 F g-1 at 0.4 A g-1. Furthermore, the film electrode demonstrates outstanding cycling stability, with a capacitance retention of 82.8% after 5000 cycles at 1 A g-1. Notably, the CV curves of the asymmetric supercapacitor (ASC) show almost no change after multiple bending, indicating excellent flexibility. Additionally, two devices connected in series can light an LED, demonstrating significant potential for practical applications.

Sacrificial MOF-derived MnNi hydroxide for high energy storage supercapacitor electrodes via DFT-based quantum capacitance study

Electrochemical energy storage plays a critical role in the transition to clean energy. With the growing demand for efficient and sustainable energy solutions, supercapacitors have gained significant attention due to their high specific capacitance, rapid charge/discharge capabilities, long lifespan, safe operation across various temperatures, and minimal maintenance needs. This study introduces a novel approach for the synthesis of high-performance supercapacitor electrodes by using MnNi-MOF-74 as a precursor. Bimetallic Mn(OH)₂/Ni(OH)₂ hydroxides (MnNi-x, where x = 2, 6, 12) with tailored morphologies were successfully fabricated by treating MnNi-MOF-74 anchored on nickel foam with different concentrations of KOH. Among the various synthesized samples, MnNi-6 exhibited the best performance, with a remarkable specific capacitance of 4031.51 mF cm⁻2 at 2 mA cm⁻2, attributed to its high surface area of 186 m2/g, optimized particle size, and abundant micropores. Furthermore, MnNi-6 demonstrated exceptional thermal stability, positioning it as a promising candidate for high-temperature supercapacitors. It also exhibited excellent cycling stability, retaining 86.34 % of its capacity after 10,000 cycles at 10 mA cm⁻2, highlighting its remarkable durability. Density functional theory (DFT) calculations were conducted to explore the quantum capacitance of the bimetallic hydroxide. The DFT results revealed electron density near the Fermi level, which directly contributes to the high quantum capacitance of Mn(OH)₂/Ni(OH)₂ with a Mn:Ni molar ratio of 3:1. This work underscores the potential of MOF-derived materials as a promising route for the development of high-performance supercapacitor electrodes, paving the way for future advances in electrochemical energy storage technologies.

The synergistic effect of NiCo2S4 and carbon nanosheets for supercapacitor: Enhanced adsorption/desorption of OH- on Ni and Co active sites

To improve the low energy density, low conductivity, and poor cycling stability of NiCo2S4 in supercapacitors, a two-step hydrothermal method was used to prepare a composite material of NiCo2S4 and carbon nanosheets (NiCo2S4/CNs). The electrochemical tests revealed a high specific capacitance of 1576 F g-1 at 1 A/g for the composite, and the NiCo2S4/CNs//AC asymmetric supercapacitor showed a energy density of 49.7 Wh kg-1 at 818 W kg-1. This study confirmed the phase transformation of NiCo2S4 during charge/discharge in alkaline solution through ex-situ X-ray diffraction (ex-situ XRD) for the first time, and proposed a potential reaction pathway. Moreover, Density Functional Theory (DFT) confirmed that the NiCo2S4/CNs heterostructure enhances OH- adsorption/desorption on Ni and Co active sites and improves electronic conductivity. In conclusion, this study advances the application of transition metal sulfide in high-performance energy storage.

A multi-scale circuit model bridges molecular modeling and experimental measurements of conductive metal-organic framework supercapacitors

A multi-scale model is crucial for combining experiments and simulations to reveal the energy storage mechanism. As novel electrode materials, conductive metal-organic frameworks (c-MOFs) provide an ideal platform for understanding the energy storage process in supercapacitors. However, the prevailing circuit models lack consideration of the distinctive transmission path of c-MOFs, which hinders accurate descriptions of c-MOF supercapacitors. By proposing a concept for representing the c-MOF electrode as a crystal-matrix electrode according to the crystallinity, we developed a universal multi-scale circuit model considering crystal shape and porosity to describe the impedance and capacitance of c-MOF electrodes. For supercapacitors with c-MOF electrodes and ionic liquid electrolytes, results predicted from the new multi-scale circuit model, based on microscale parameters obtained from molecular dynamics simulations, demonstrate quantitative agreement with experimental data for electrodes with different crystallinities.

Nano-Metal-Organic Frameworks and Nano-Covalent-Organic Frameworks: Controllable Synthesis and Applications

Nanoscale framework materials have attracted extensive attention due to their diverse morphology and good properties, and synthesis methods of different size structures have been reported. Therefore, the relationship between different sizes and performance has become a research hotspot. This paper reviews the controllable synthesis strategies of nano-metal-organic frameworks (nano-MOFs) and nano-covalent-organic frameworks (nano-COFs). Firstly, the synthetic evolution of nano-frame materials is summarized. Due to their special surface area, regular pores and adjustable structural functions, nano-frame materials have attracted much attention. Then the preparation methods of nanostructures with different dimensions are introduced. These synthetic strategies provide the basis for the design of novel energy storage and catalytic materials. In addition, the latest advances in the field of energy storage and catalysis are reviewed, with emphasis on the application of nano-MOFs/COFs in zinc-, lithium-, and sodium-based batteries, as well as supercapacitors.

Ligand-Insertion Strategy for Constructing 2D Conjugated Metal-Organic Framework with Large Pore Size for Electrochemical Analytics

Two-dimensional conjugated metal-organic frameworks (2D c-MOFs) have shown great promise in various electrochemical applications due to their intrinsic electrical conductivity. A large pore aperture is a favorable feature of this type of material because it facilitates the mass transport of chemical species and electrolytes. In this work, we propose a ligand insertion strategy in which a linear ligand is inserted into the linkage between multitopic ligands, extending the metal ion into a linear unit of -M-ligand-M-, for the construction of 2D c-MOFs with large pore apertures, utilizing only small ligands. As a proof-of-concept trial of this strategy, a 2D c-MOF with mesopores of 3.2 nm was synthesized using commercially available ligands hexahydrotriphenylene and 2,5-dihydroxybenzoquinone. The facilitation of the diffusion of redox species by the large pore size of this MOF was demonstrated through a series of probes. With this feature, it showed superior performance in the electrochemical analysis of a variety of biological species.

Solution-Processable MOF-on-MOF System Constructed via Template-Assisted Growth for Ultratrace H2S Detection

Conductive metal-organic frameworks (c-MOFs) hold promise for highly sensitive sensing systems due to their conductivity and porosity. However, the fabrication of c-MOF thin films with controllable morphology, thickness, and preferential orientation remains a formidable yet ubiquitous challenge. Herein, we propose an innovative template-assisted strategy for constructing MOF-on-MOF (Ni3(HITP)2/NUS-8 (HITP: 2,3,6,7,10,11-hexamino-tri (p-phenylene))) systems with good electrical conductivity, porosity, and solution processability. Leveraging the 2D nature and solution processability of NUS-8, we achieve the controllable self-assembly of Ni3(HITP)2 on NUS-8 nanosheets, producing solution-processable Ni3(HITP)2/NUS-8 nanosheets with a film conductivity of 1.55×10-3 S ⋅ cm-1 at room temperature. Notably, the excellent solution processability facilitates the fabrication of large-area thin films and printing of intricate patterns with good uniformity, and the Ni3(HITP)2/NUS-8-based system can monitor finger bending. Gas sensors based on Ni3(HITP)2/NUS-8 exhibit high sensitivity (LOD~6 ppb) and selectivity towards ultratrace H2S at room temperature, attributed to the coupling between Ni3(HITP)2 and NUS-8 and the redox reaction with H2S. This approach not only unlocks the potential of stacking different MOF layers in a sequence to generate functionalities that cannot be achieved by a single MOF, but also provides novel avenues for the scalable integration of MOFs in miniaturized devices with salient sensing performance.

Construction of Solution Processable NUS-8/PANI Nanosheets via Template-Directed Polymerization for Ultratrace Gas Sensing

The advancement of wireless gas sensing signifies a substantial leap forward in gas detection and intelligent monitoring technologies. This necessitates stringent design criteria for gas sensitive materials with good solution processability, conductivity, and porosity, whose design and synthesis remain challenging yet highly sought-after. Herein, the fabrication of NUS-8/polyaniline (PANI) nanosheets is presented with excellent solution processability, high porosity, triboelectric property, and superior electrical conductivity via a template-directed polymerization strategy. Solution processable NUS-8 nanosheets, synthesized directly by a "one-pot" approach, serve as templates to enhance the "on-site" polymerization of aniline, resulting in the formation of PANI layer on NUS-8 nanosheets with a thickness of 7 nm. The resultant NUS-8/PANI nanosheets exhibit outstanding solution processability, and a film conductivity of 8.6 S m-1. The solution processability enables the facile fabrication of homogeneous and compact NUS-8/PANI films and thus their integration onto electronic devices targeted for multifunctional sensing. The NUS-8/PANI coated sensors demonstrate sensitive and selective detection at room temperature toward ultratrace ammonia with a detection limit of 120 ppb. A wireless sensing system based on the NUS-8/PANI-coated sensor is capable to monitor the spoilage process of meat. This study paves novel avenues for designing and synthesizing gas-sensitive materials for practical applications.

A highly conducting tetrathiafulvalene-tetracarboxylate based dysprosium(iii) 2D metal-organic framework with single molecule magnet behaviour

The synthesis and whole characterization by a multitechnique approach of an unprecedented dysprosium(iii) 2D metal organic framework (MOF), involving the redox-active tetrathiafulvalene (TTF)-based linker TTF-tetracarboxylate (TTF-TC), are herein reported. The single-crystal X-ray structure, formulated as [Dy6(TTF-TC)5(H2O)22]·21H2O (1), reveals a complex 2D topology, with hexanuclear Dy6 clusters as secondary building units (SBUs) interconnected by five linkers, stacked almost parallel in each layer and eclipsed along the [111] direction, leading to the formation of 1D channels filled by water molecules. The mixed valence of the TTF units is confirmed by both bond distance analysis, Raman microscopy and diffuse reflectance spectroscopy, and further supported by band structure calculations, which also predict activated conductivity for this material. Thanks to efficient TTF stacking and partial oxidation, 1 shows semiconducting behavior, with, however, a record conductivity value of 1 mS cm-1 at room temperature, when compared to the previously reported TTF-based MOFs. Furthermore, temperature and magnetic field dependent ac (alternative current) magnetic susceptibility measurements demonstrate field induced slow relaxation of magnetization, accounting for two independent relaxation processes, with an energy barrier (U eff/K) of around 12 K, typical for dysprosium carboxylate complexes. The herein reported 2D Dy-MOF provides a valuable master plan for coexistence of conducting π-TTF stacks and highly anisotropic DyIII SMM properties.

A composite of pineapple leaf-derived porous carbon integrated with ZnCo-MOF for high-performance supercapacitors

Electrochemical energy storage heavily depends on the activity and stability of electrode materials. However, the direct use of metal-organic frameworks (MOFs) as supercapacitor electrode materials poses challenges due to their low electrical conductivity. In this study, pineapple leaf-derived biochar (PLB) was employed as a carrier for bimetallic ZnCo-MOF, resulting in the composite ZnCo-MOF@PLB-800, synthesized through in situ growth and pyrolysis at 800 °C. The highly porous structure of PLB alleviated the aggregation of ZnCo-MOF particles, thereby enhancing the electron transfer rate and improving the conductivity of the electrode material. Electrochemical testing revealed that ZnCo-MOF@PLB-800 achieved a specific capacitance of 698.5 F g-1 at a current density of 1 A g-1. The assembled asymmetric supercapacitor (ASC) demonstrated excellent specific capacitance and electrochemical stability, delivering a high energy density of 35.85 W h kg-1 at a power density of 350 W kg-1, with robust cycle stability, retaining 90.4% capacitance after 8000 cycles. This work offers an effective integration of bimetallic MOFs with waste biomass-derived porous carbon for electrode materials, supporting both energy storage applications and environmental sustainability.

A Two-Step Synthesis of Porous Nitrogen-Doped Graphene for Electrochemical Capacitors

Porous nitrogen-doped graphene (PNG) materials with high conductivity, high surface area, and chemical stability have displayed superior performance in electrochemical capacitors. However, previously reported methods for fabricating PNG render the processes expensive, hard to control, limited in production, and unsafe as well, thus largely restricting their practical applications. Herein, we present a facile two-step calcination method to prepare PNG using petroleum asphalt as the carbon source to provide the original three-dimensional porous structure directly and using environmentally friendly and high nitrogen content urea as the nitrogen source without adding any etching agent. The porous structure in PNG can largely increase its specific surface area, and the introduction of nitrogen atoms can effectively increase the degree of defects and improve the wettability of PNG. As a result, PNG displays a high specific capacitance of 157 F g-1 at a current density of 1 A g-1 and cycling stability while maintaining 98.68% initial capacitance after 10,000 cycles.

Investigating the potential of pyrazine dioxide based-compounds as organic electrodes for batteries

Understanding structure-property relationship in redox-active molecular species is of central importance in various fields, including many medicinal and chemical applications. The quest for performant organic electrodes in the context of energy storage calls for pioneering studies to develop new and possibly optimal materials. Beyond modifying the molecular design of the existing compounds through functionalization, expansion of the search enabling the advent of efficient new backbones can potentially lead to breakthroughs in this research area. The number of already identified families able to constitute negative organic electrodes is much lower than that of their positive counterparts, which calls for finding ways to bridge this gap. To expand the dataset of known predicted redox potentials and in view of reaching an educated guess about the abilities of some eventual new redox active electrodes, we examined the properties of pyrazine N,N'-dioxide (PZDO) and its fully methylated functionalized derivative (TeMePzDO). The aspects and mechanisms driving the various features characteristic of these compounds were unraveled through molecular and periodic DFT calculations combined with accurate electronic structure analysis. The predicted molecular redox/crystalline intercalation potentials lead to the classification of PZDO and TeMePzDO systems within the class of negative electrodes, with features that are significantly appealing compared to those of some existing systems with backbones suited for such kind of application.

Electrodeposition of Ni/Cu Bimetallic Conductive Metal-Organic Frameworks Electrocatalysts with Boosted Oxygen Reduction Activity for Zinc-Air Batteries

Zinc-air batteries employing non-Pt cathodes hold significant promise for advancing cathodic oxygen reduction reaction (ORR). However, poor intrinsic electrical conductivity and aggregation tendency hinder the application of metal-organic frameworks (MOFs) as active ORR cathodes. Conductive MOFs possess various atomically dispersed metal centers and well-aligned inherent topologies, eliminating the additional carbonization processes for achieving high conductivity. Here, a novel room-temperature electrochemical cathodic electrodeposition method is introduced for fabricating uniform and continuous layered 2D bimetallic conductive MOF films cathodes without polymeric binders, employing the organic ligand 2,3,6,7,10,11-hexaiminotriphenylene (HITP) and varying the Ni/Cu ratio. The influence of metal centers on modulating the ORR performance is investigated by density functional theory (DFT), demonstrating the performance of bimetallic conductive MOFs can be effectively tuned by the unpaired 3d electrons and the Jahn-Teller effect in the doped Cu. The resulting bimetallic Ni2.1Cu0.9(HITP)2 exhibits superior ORR performance, boasting a high onset potential of 0.93 V. Moreover, the assembled aqueous zinc-air battery demonstrates high specific capacity of 706.2 mA h g-1, and exceptional long-term charge/discharge stability exceeding 1250 cycles.

Boosting the Electrical Transfer by Molybdenum Doping for Robust and Flexible NiSe-Based Supercapacitor

NiSe is a promising electrode material for enhancing the energy density of supercapacitors, but it faces challenges such as sensitivity to electrolyte anions, limited specific capacity, and unstable cycling. This study employs a strategy of metal atom doping to address these issues. Through a hydrothermal reaction, Mo-doped NiSe demonstrates significant improvement in electrochemical performance, exhibiting high capacity (799.90 C g-1), splendid rate performance, and excellent cyclic stability (90% capacity retention). The introduction of Mo induces charge redistribution in NiSe, leading to a reduction in the band gap. Theoretical calculation reveals that Mo doping can effectively enhance the electrical conductivity and the adsorption energy of NiSe. A flexible printed hybrid Mo-doped NiSe-based supercapacitor is fabricated, demonstrating superior electrochemical performance (367.04 mF cm-2) and the ability to power timers, LEDs, and toy fans. This research not only deepens the understanding of the electrochemical properties of metal-doped NiSe but also highlights its application potential in high-performance supercapacitors.

Molecular-Level Pore Tuning in 2D Conductive Metal-Organic Frameworks for Advanced Supercapacitor Performance

Two-dimensional (2D) electrically conductive metal-organic frameworks (MOFs) have emerged as viable candidates for active electrode materials in supercapacitors due to their high electrical conductivity, high specific surface area, and intrinsic redox-active sites. Despite their promising electrochemical performance, their pseudocapacitive behavior via fast and reversible charge transfer reactions remains yet to be fully exploited. Here, we investigate the electrochemical energy storage mechanism of Cu3(HHTATP)2 (HHTATP = 2,3,6,7,10,11-hexahydroxy-1,5,9-triaminotriphenylene), a 2D conductive MOF featuring characteristic redox-active pendant aromatic amines. Cu3(HHTATP)2 exhibited pseudocapacitive charge storage with an average gravimetric capacitance of 340 ± 15 F g-1 at a discharge rate of 0.2 A g-1 and maintained a capacitance retention over 90% after 7000 galvanostatic cycles at 5 A g-1. The polar pendant amines not only enhanced capacitance via additional amine/imine redox activity but also reduced interfacial charge transfer resistance through modified electrode-electrolyte interactions. These results highlight the potential of molecular-level pore environment tuning as a strategic approach in materials design for energy storage applications.

Metal Effect on the Proton Conduction of Three Isostructural Metal-Organic Frameworks and Pseudo-Capacitance Behavior of the Cobalt Analogue

Three isostructural transition metal-organic frameworks, [M(bta)0.5(bpt)(H2O)2]·2H2O (M = Co (1), Ni (2), Zn (3), H4bta = 1,2,4,5-benzenetetracarboxylic acid, bpt = 4-amino-3,5-bis(4-pyridyl)-1,2,4-triazole), were successfully constructed using different metal cations. These frameworks exhibit a three-dimensional network structure with multiple coordinated and lattice water molecules within the framework, contributing to high stability and a rich hydrogen-bond network. Proton conduction studies revealed that, at 333 K and 98% relative humidity, the proton conductivities (σ) of MOFs 1-3 reached 1.42 × 10-2, 1.02 × 10-2, and 6.82 × 10-3 S cm-1, respectively. Compared to the proton conductivity of the initial ligands, the σ values of the complexes increased by 2 orders of magnitude, with the activation energies decreasing from 0.36 to 0.18 eV for 1, 0.09 eV for 2, and 0.12 eV for 3. An in-depth analysis of the correlation between different metal centers and proton conduction performance indicated that the varying coordination abilities of the metal cations and the water absorption capacities of the frameworks might account for the differences in conductivity. Additionally, the potential of 1 as a supercapacitor electrode material was assessed. 1 exhibited a specific capacitance of 61.13 F g-1 at a current density of 0.5 A g-1, with a capacitance retention of 82.4% after 5000 cycles, making it a promising candidate for energy storage applications.

Upgrading Structural Conjugation in Three-Dimensional Ni-Based Metal-Organic Frameworks for Promoting Electrical Conductivity and Specific Capacitance

Metal-organic frameworks (MOFs) have emerged as promising candidates for electrochemical energy storage and conversion due to their high specific surface areas, abundant active sites, and excellent chemical and structural tunability. However, the direct utilization of MOFs as electrochemical materials is a challenge because of the poor electroconductivity induced by the insulating nature of most organic linkers. Herein, a conjugated three-dimensional Ni-MOF {Ni(HBTC)(BPE)}n (Ni-BPE) with a 2-fold interpenetrating structure was developed via the coordination polymerization of Ni2+, a H3BTC ligand (1,3,5-benzenetricarboxylic acid), and a vinyl-functionalized bipyridine linker (1,2-di(4-pyridyl)ethylene, BPE). Ni-BPE displayed an enhanced conjugation system compared to analogous and insulated Ni-BPY that is constructed by the Ni-BTC layer and ordinary bipyridine linker (4,4'-bipyridine, BPY). Notably, upgrading structural conjugation promoted a dramatical ∼204 times increase in the electroconductivity of Ni-BPE compared to Ni-BPY. More importantly, Ni-BPE displayed a higher specific capacitance of 633.2 F g-1 (316.6 C g-1) at 1 A g-1, which exhibited a significant ∼1.5-fold enhancement than Ni-BPY. Furthermore, the asymmetric supercapacitor can reach a good energy density of 25.2 Wh kg-1 with a reasonable cycle stability of 71.0% over 5000 cycles. This work provides an effective method for optimizing the structure of insulating MOFs to enhance the electroconductivity and specific capacitance.

2D Conductive Metal-Organic Frameworks Based on Tetraoxa[8]circulenes as Promising Cathode for Aqueous Zinc Ion Batteries

Aqueous zinc-ion batteries (ZIBs) are the new generation electrochemical energy storage systems. Recently, two-dimensional conductive metal-organic frameworks (2D c-MOFs) are attractive to serve as cathode materials of ZIBs due to their compositional diversity, abundant active sites, and excellent conductivity. Despite the growing interest in 2D c-MOFs, their application prospects are still to be explored. Herein, a tetraoxa[8]circulene (TOC) derivative with unique electronic structure and interesting redox-active property are synthesized to construct c-MOFs. A series of novel 2D c-MOFs (Cu-TOC, Zn-TOC and Mn-TOC) with different conductivities and packing modes are obtained by combining the linker tetraoxa[8]circulenes-2,3,5,6,8,9,11,12-octaol (8OH-TOC) and corresponding metal ions. Three c-MOFs all exhibit typical semiconducting properties, and Cu-TOC exhibits the highest electrical conductivity of 0.2 S cm-1 among them. Furthermore, their electrochemical performance as cathode materials for ZIBs have been investigated. They all performed high reversible capacity, decent cycle stability and excellent rate capability. This work reveals the key insights into the electrochemical application potential of 2D c-MOFs and advances their development as cathode materials in ZIBs.

A Highly Robust and Conducting Ultramicroporous 3D Fe(II)-Based Metal-Organic Framework for Efficient Energy Storage

Exploitation of metal-organic framework (MOF) materials as active electrodes for energy storage or conversion is reasonably challenging owing to their poor robustness against various acidic/basic conditions and conventionally low electric conductivity. Keeping this in perspective, herein, a 3D ultramicroporous triazolate Fe-MOF (abbreviated as Fe-MET) is judiciously employed using cheap and commercially available starting materials. Fe-MET possesses ultra-stability against various chemical environments (pH-1 to pH-14 with varied organic solvents) and is highly electrically conductive (σ = 0.19 S m-1) in one fell swoop. By taking advantage of the properties mentioned above, Fe-MET electrodes give prominence to electrochemical capacitor (EC) performance by delivering an astounding gravimetric (304 F g-1) and areal (181 mF cm-2) capacitance at 0.5 A g-1 current density with exceptionally high cycling stability. Implementation of Fe-MET as an exclusive (by not using any conductive additives) EC electrode in solid-state energy storage devices outperforms most of the reported MOF-based EC materials and even surpasses certain porous carbon and graphene materials, showcasing superior capabilities and great promise compared to various other alternatives as energy storage materials.

Superior Charge Transport in Ni-Diamine Conductive MOFs

Two-dimensional conductive metal-organic frameworks (2D cMOFs) are an emerging class of crystalline van der Waals layered materials with tunable porosity and high electrical conductivity. They have been used in a variety of applications, such as energy storage and conversion, chemiresistive sensing, and quantum information. Although designing new conductive 2D cMOFs and studying their composition/structure-property relationships have attracted significant attention, there are still very few examples of 2D cMOFs that exhibit room-temperature electrical conductivity above 1 S cm-1, the value exhibited by activated carbon, a well-known porous and conductive material that serves in myriad applications. When such high conductivities are achieved, Ni-diamine linkages are often involved, yet Ni-diamine MOFs remain difficult to access. Here, we report two new 2D cMOFs made through ortho-diamine connections: M3(HITT)2 (M = Ni, Cu; HITT = 2,3,7,8,12,13-hexaiminotetraazanaphthotetraphene). The electrical conductivity of Ni3(HITT)2 reaches 4.5 S cm-1 at 298 K, whereas the conductivity of Cu3(HITT)2 spans from 0.05 (2Cu+Cu2+) to 10-6 (3Cu2+) upon air oxidation, much lower than that of Ni3(HITT)2. Spectroscopic analysis reveals that Ni3(HITT)2 exhibits significantly stronger in-plane π-d conjugation and higher density of charge carriers compared to Cu3(HITT)2, accounting for the higher electrical conductivity of Ni3(HITT)2. Cu2+/Cu+ mixed valency modulates the energy level and carrier density of Cu3(HITT)2, allowing for a variation of electrical conductivity over 4 orders of magnitude. This work provides a deeper understanding of the influence of metal nodes on electrical conductivity and confirms ortho-diamine linkers as privileged among ligands for 2D cMOFs.

Organic Solvent Boosts Charge Storage and Charging Dynamics of Conductive MOF Supercapacitors

Conductive metal-organic frameworks (c-MOFs) and ionic liquids (ILs) have emerged as auspicious combinations for high-performance supercapacitors. However, the nanoconfinement from c-MOFs and high viscosity of ILs slow down the charging process. This hindrance can, however, be resolved by adding solvent. Here, constant-potential molecular simulations are performed to scrutinize the solvent impact on charge storage and charging dynamics of MOF-IL-based supercapacitors. Conditions for >100% enhancement in capacity and ≈6 times increase in charging speed are found. These improvements are confirmed by synthesizing near-ideal c-MOFs and developing multiscale models linking molecular simulations to electrochemical measurements. Fundamentally, the findings elucidate that the solvent acts as an "ionophobic agent" to induce a substantial enhancement in charge storage, and as an "ion traffic police" to eliminate convoluted counterion and co-ion motion paths and create two distinct ion transport highways to accelerate charging dynamics. This work paves the way for the optimal design of MOF supercapacitors.

Two-Dimensional Conjugated Metal-Organic Frameworks with a Ring-in-Ring Topology and High Electrical Conductance

Electrically conducting two-dimensional (2D) metal-organic frameworks (MOFs) have garnered significant interest due to their remarkable structural tunability and outstanding electrical properties. However, the design and synthesis of high-performance materials face challenges due to the limited availability of specific ligands and pore structures. In this study, we have employed a novel highly branched D3h symmetrical planar conjugated ligand, dodechydroxylhexabenzotrinaphthylene (DHHBTN) to fabricate a series of 2D conductive MOFs, named M-DHHBTN (M=Co, Ni, and Cu). This new family of MOFs offers two distinct types of pores, elevating the structural complexity of 2D conductive MOFs to a more advanced level. The intricate tessellation patterns of the M-DHHBTN are elucidated through comprehensive analyses involving powder X-ray diffraction, theoretical simulations, and high-resolution transmission electron microscope. Optical-pump terahertz-probe spectroscopic measurements unveiled carrier mobility in DHHBTN-based 2D MOFs spanning from 0.69 to 3.10 cm2 V-1 s-1. Among M-DHHBTN famility, Cu-DHHBTN displayed high electrical conductivity reaching 0.21 S cm-1 at 298 K with thermal activation behavior. This work leverages the "branched conjugation" of the ligand to encode heteroporosity into highly conductive 2D MOFs, underscoring the significant potential of heterogeneous double-pore structures for future applications.

Successful In Situ Growth of Conductive MOFs on 2D Cobalt-Based Compounds and Their Electrochemical Performance

Conductive metal-organic frameworks (cMOFs), as a kind of porous material, are considered to be highly promising materials in the field of electrochemistry due to their excellent conductivity. However, due to the low specific capacitance of pure cMOFs, their application in supercapacitors is limited. By virtue of the high theoretical capacity and excellent chemical stability of Co-based compounds, in this work, cMOFs' M-HHTP (M = Ni, Co, NiCo, HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) are grown in situ on Co(OH)2, CoP, and Co3O4 nanosheets, resulting in a series of electroactive compounds as electrode materials used in supercapacitors. Among all of the compounds, Ni-HHTP@Co(OH)2 shows the most excellent energy storage performance and outstanding cyclic stability in the application of aqueous asymmetric supercapacitors.

Toward Enhancing Performance of Electromagnetic Wave Absorption for Conductive Metal-Organic Frameworks: Nanostructure Engineering or Crystal Morphology Controlling

Conductive metal-organic frameworks (cMOFs), which have high porosity and intrinsic electron conductivity, are regarded as ideal candidates for electromagnetic wave (EMW) absorption materials. Controlling the nanostructure of absorbers may be one of the effective strategies to improve the electromagnetic wave (EMW) absorption performance. Herein, a series of conductive Cu-HHTP MOFs (HHTP = 2,3,6,7,10,11-hexahydroxytriphenyl hydrates) with different nanostructures or crystal morphologies were successfully synthesized by using different structural inducers to regulate the changes in the morphology, thereby improving the EMW absorption performance. Specifically, when ammonia was used as an inducer, the obtained A-Cu-HHTP with a nanosheet structure exhibited excellent EMW absorption performance. The minimum reflection loss (RLmin) can reach -51.08 dB at 7.25 GHz with a thickness of 4.4 mm, and the maximum effective absorption bandwidth (EAB) can cover 5.73 GHz at 2.5 mm. The influence of the nanostructures of the cMOFs on the dielectric and EMW absorption performance was clarified. The nanosheet structure of A-Cu-HHTP increases its specific surface area, which expands multiple scattering and reflection paths of incident EMW; Meanwhile, the unique structure facilitates the formation of more heterogeneous interfaces, optimizing impedance matching. The significant improvement in EMW performance is mainly attributed to multiple reflections and scattering as well as impedance matching. This work not only provides a simple and effective strategy for improving electromagnetic wave absorption performance but also offers guidelines for preparing morphology functional cMOF materials.

Topologically Tunable Conjugated Metal-Organic Frameworks for Modulating Conductivity and Chemiresistive Properties for NH3 Sensing

Electrically conductive metal-organic frameworks (cMOFs) have garnered significant attention in materials science due to their potential applications in modern electrical devices. However, achieving effective modulation of their conductivity has proven to be a major challenge. In this study, we have successfully prepared cMOFs with high conductivity by incorporating electron-donating fused thiophen rings in the frameworks and extending their π-conjugated systems through ring-closing reactions. The conductivity of cMOFs can be precisely modulated ranging from 10-3 to 102 S m-1 by regulating their dimensions and topologies. Furthermore, leveraging the inherent tunable electrical properties based on topology, we successfully demonstrated the potential of these materials as chemiresistive gas sensors with an outstanding response toward 100 ppm NH3 at room temperature. This work not only provides valuable insights into the design of functional cMOFs with different topologies but also enriches the cMOF family with exceptional conductivity properties.

In situ synthesis of self-supporting conductive CuCo-based bimetal organic framework for sensitive nonenzymatic glucose sensing in serum and beverage

Most MOFs are associated with the inherent defect of low conductivity, limiting their further application in electrochemical sensing. Herein, a self-supporting conductive CuCo-based bimetal organic framework with HHTP as the organic ligand was in situ synthesized on carbon cloth via a one-step hydrothermal method, namely CuCo-MOF/CC. Benefiting from the advantages of electrical conductivity and bimetallic synergies, CuCo-MOF/CC exhibited remarkable electrocatalytic performance toward glucose. Consequently, the prepared sensor demonstrated an outstanding sensitivity of 9317 μA mM-1 cm-2, a wide range of 0.25-2374.5 μM, a low determination limit (0.27 μM), and a rapid response time (1.6 s). The reproducibility, stability, and selectivity were also proved to be satisfactory. Furthermore, the remarkable feasibility of proposed sensor was confirmed in serum and beverages. With the convenience of the one-step hydrothermal method and portability of self-supporting electrode, CuCo-MOF/CC has emerged as a promising candidate for commercial glucose sensors.

Designed Synthesis and Electrochemical Performance Regulation of the Hierarchical Hollow Structure Cu2S/Cu7S4/NC Anode for Hybrid Supercapacitors

Copper-based compounds have attracted increasing attention as electrode materials for rechargeable devices, but their poor conductivity and insufficient stability inhibit their further development. Herein, an effective method has been proposed to improve the electrochemical properties of the copper-based electrodes by coating carbon materials and generating unique micro/nanostructures. The prepared Cu2S/Cu7S4/NC with hierarchical hollow structure possesses excellent electrochemical performance, attributing to the composition and structure optimization. The superior charge storage performance has been assessed by theoretical and experimental research. Specifically, the Cu2S/Cu7S4/NC exhibits remarkably higher electrical conductivity and lower adsorption-free energy for O* and OH* than those of Cu2O. Moreover, the Cu2S/Cu7S4/NC delivers a high specific capacitance of 1261.3 F·g-1 at the current density of 1 A·g-1 and also has great rate performance at higher current densities, which are much better than those of the Cu2O nanocubes. In addition, the assembled hybrid supercapacitor using Cu2S/Cu7S4/NC as the anode exhibits great energy density, power density, and cycling stability. This study has proposed a novel and feasible method for the synthesis of high-performance copper-based electrodes and their electrochemical performance regulation, which is of great significance for the advancement of high-quality electrode materials and rechargeable devices.

In situ selective selenization of ZIF-derived CoSe2 nanoparticles on NiMn-layered double hydroxide@CuBr2 heterostructures for high performance supercapacitors

As an emerging energy storage device, the practical application of supercapacitors (SCs) is currently constrained by their low energy density. Enhancing the capacitance of supercapacitors by leveraging the synergistic effect of multiple components in composite electrodes with well-designed structures can effectively increase their energy density. Here, a wire-sheet-particle hierarchical heterostructured CoSe2@NiMn-layered double hydroxide (NiMn-LDH) @Cu1.8Se/Copper foam (CF) electrode is synthesized via phase pseudomorphic transformation process achieved by selective selenization for Cu and Co elements. Benefiting from the stable support structure of CuBr2, the large specific surface area of NiMn-LDH, and the excellent conductivity of CoSe2, the prepared binder-free electrode shows excellent electrochemical properties. The CoSe2@NiMn-LDH@Cu1.8Se hybrid electrode exhibits a superior specific areal capacitance of 7064 mF cm-2 at 2 mA cm-2 and a stable cyclic performance with 80.11 % capacitance retention after 10,000 cycles. Furthermore, the assembled CoSe2@NiMn-LDH@Cu1.8Se/CF//AC (activated carbon) asymmetric supercapacitor (ASC) achieves an energy density of 36.6 Wh kg-1 when the power density is 760.6 W Kg-1 and retains 87.35 % of the initial capacitance after 5000 cycles. Overall, this pioneering research provided new insight for preparing supercapacitor electrode materials by selective selenization and ration design of the structures.

Fabrication of Multi-Layered Paper-Based Supercapacitor Anode by Growing Cu(OH)2 Nanorods on Oxygen Functional Groups-Rich Sponge-Like Carbon Fibers

This work addresses the challenges in developing carbon fiber paper-based supercapacitors (SCs) with high energy density by focusing on the limited capacity of carbon fiber. To overcome this limitation, a sponge-like porous carbon fiber paper enriched with oxygen functional groups (OFGs) is prepared, and Cu(OH)2 nanorods are grown on its surface to construct the SC anode. This design results in a multi-layered carbon fiber paper-based electrode with a specific structure and enhanced capacitance. The Cu(OH)2 @PCFP anode exhibits an areal capacitance of 547.83 mF cm-2 at a current density of 1 mA cm-2 and demonstrates excellent capacitance retention of 99.8% after 10 000 cycles. Theoretical calculations further confirm that the Cu(OH)2 /OFGs-graphite heterostructure exhibits higher conductivity, facilitating faster charge transfer. A solid-state SC is successfully assembled using Ketjen Black@PCFP as the cathode and KOH/PVA as the gel electrolyte. The resulting device exhibits an energy density of 0.21 Wh cm-2 at 1.50 mW cm-2 , surpassing the performance of reported Cu(OH)2 SCs. This approach, combining materials design with an understanding of underlying mechanisms, not only expands the range of electrode materials but also provides valuable insights for the development of high-capacity energy storage devices.

Challenges and strategies faced in the electrochemical biosensing analysis of neurochemicals in vivo: A review

Our brain is an intricate neuromodulatory network, and various neurochemicals, including neurotransmitters, neuromodulators, gases, ions, and energy metabolites, play important roles in regulating normal brain function. Abnormal release or imbalance of these substances will lead to various diseases such as Parkinson's and Alzheimer's diseases, therefore, in situ and real-time analysis of neurochemical interactions in pathophysiological conditions is beneficial to facilitate our understanding of brain function. Implantable electrochemical biosensors are capable of monitoring neurochemical signals in real time in extracellular fluid of specific brain regions because they can provide excellent temporal and spatial resolution. However, in vivo electrochemical biosensing analysis mainly faces the following challenges: First, foreign body reactions induced by microelectrode implantation, non-specific adsorption of proteins and redox products, and aggregation of glial cells, which will cause irreversible degradation of performance such as stability and sensitivity of the microsensor and eventually lead to signal loss; Second, various neurochemicals coexist in the complex brain environment, and electroactive substances with similar formal potentials interfere with each other. Therefore, it is a great challenge to design recognition molecules and tailor functional surfaces to develop in vivo electrochemical biosensors with high selectivity. Here, we take the above challenges as a starting point and detail the basic design principles for improving in vivo stability, selectivity and sensitivity of microsensors through some specific functionalized surface strategies as case studies. At the same time, we summarize surface modification strategies for in vivo electrochemical biosensing analysis of some important neurochemicals for researchers' reference. In addition, we also focus on the electrochemical detection of low basal concentrations of neurochemicals in vivo via amperometric waveform techniques, as well as the stability and biocompatibility of reference electrodes during long-term sensing, and provide an outlook on the future direction of in vivo electrochemical neurosensing.

Molecular Understanding of Charging Dynamics in Supercapacitors with Porous Electrodes and Ionic Liquids

Porous electrodes and ionic liquids could significantly enhance the energy storage of supercapacitors. However, they may reduce the charging dynamics and power density due to the nanoconfinement of porous electrodes and the high viscosity of ionic liquids. A comprehensive understanding of the charging mechanism in porous supercapacitors with ionic liquids provides a crucial theoretical foundation for their design optimization. Here, we review the progress of molecular simulations of the charging dynamics in supercapacitors consisting of porous electrodes and ionic liquids. We highlight and delve into the breakthroughs in the ion transport and charging mechanism for electrodes with subnanometer pores and realistic porous structures. We also discuss future directions for the charging dynamics of supercapacitors.

Stable zinc anode interface and environmentally adaptable hydrogel electrolytes for stable operation of zinc-ion hybrid supercapacitors

Hydrogel-based zinc ion hybrid supercapacitors (ZIHS) have stood out from many energy storage device candidates due to their battery-level energy density, inherent flexibility, and safety. Nevertheless, the inevitable dendrite growth of Zn anodes and sharp capacity degradation at low-temperature seriously hinder their practical application. Herein, a dense ZnF2 solid electrolyte interface protective layer was constructed in situ on the Zn electrode surface by a simple chemical deposition method, effectively isolating the water molecules and alleviating the water-induced dendrite growth and parasitic reaction. To achieve the flexible ZIHS with environmental adaptability, a self-adhesion and anti-freezing zwitterionic hydrogel electrolyte was fabricated to afford superior ionic conductivity (97.1 mS cm-1), excellent anti-drying ability, and robust interfacial adhesion. Benefitting from the integrated merits of the as-designed electrolyte and electrode, the ZIHS delivered excellent mechanical adaptability, favorable energy density (103.9 Wh kg-1 at 270.1 W kg-1), broad operating temperature range (-40 to 40 °C), along with long-term cycling stability (12,000 cycles) with 90.3 % capacity retention at -25 °C. Notably, the unencapsulated ZIHS achieved exceptional electrochemical stability in an open environment. This finding provides valuable insights for constructing durable, flexible, and environmentally adaptable zinc-based energy storage systems.

High-Performance Electrochemical Actuator under an Ultralow Driving Voltage with a Mixed Electronic-Ionic Conductive Metal-Organic Framework

Although versatile deformation, high flexibility, and environmental friendliness of electrochemical actuators (EAs) have made them promising in bioinspired soft robots and biomedical devices, the relatively high driving voltages unfortunately impose great restrictions on their applications in low-energy and human-friendly electronics. Here, we find that the uses of a mixed electronic-ionic conductive metal-organic framework (c-MOF), i.e., Ni3(hexaiminotriphenylene)2 (Ni3(HITP)2), largely lower the driving voltage of EAs. The as-fabricated EA can work under a driving voltage as low as 0.1 V, representing the lowest value among those for the c-MOF-based EAs reported so far. The Ni3(HITP)2-based EA shows an excellent actuation performance such as a high bending strain difference of 0.48% (±0.5 V, 0.1 Hz) and long-term durability of >99% after 15,000 cycles due to the improved conductivity up to 1000 S·cm-1 and double-layer capacitance as high as 176.3 F·g-1 stemming from the mixed electronic-ionic conduction of Ni3(HITP)2.

Metal Ion-controlled Growth of Different Metal-Organic Framework Micro/nanostructures for Enhanced Supercapacitor Performance

We report a metal ion-modulated effective strategy to achieve different metal-organic framework (MOF) micro/nanostructures using different metal precursors like CoCl2  ⋅ 6H2 O, CoCl2  ⋅ 6H2 O and NiCl2  ⋅ 6H2 O, and NiCl2  ⋅ 6H2 O with pyridine-3,5-dicarboxylate (3,5-pdc). The structural characterizations confirm that different morphological structures, hollow microsphere, hierarchical nanoflower, and solid nanosphere are for Co-(3,5-pdc), Co0.19 Ni0.81 -(3,5-pdc), and Ni-(3,5-pdc), respectively. These different MOF micro/nanostructures correlate with the coordination ability of Co and Ni with 3,5-pdc. Benefitting from the synergistic effect of the alloying metal nodes of Co and Ni producing rapid and rich redox reactions and the hierarchical nanoflower with higher surface area enabling excellent ion kinetics, the Co0.19 Ni0.81 -(3,5-pdc) exhibits higher specific capacitance of 515 F g-1 /273 C g-1 at 0.5 A g-1 than that of Ni-(3,5-pdc) (290 F g-1 /153.7 C g-1 ) and Co-(3,5-pdc) (132 F g-1 /67 C g-1 ), good rate capability and cycling stability. Moreover, the asymmetric supercapacitor device (Co0.19 Ni0.81 -(3,5-pdc)//AC) assembled from Co0.19 Ni0.81 -(3,5-pdc) and activated carbon (AC) achieves a maximum energy density of 42.6 Wh kg-1 at a power density of 277.3 W kg-1 .

Engineering the crystal facets of α-MnO2 nanorods for electrochemical energy storage: experiments and theory

Crystal facet engineering is an effective strategy for precisely regulating the orientations and electrochemical properties of metal oxides. However, the contribution of each crystal facet to pseudocapacitance is still puzzling, which is a bottleneck that restricts the specific capacitance of metal oxides. Herein, α-MnO2 nanorods with different exposed facets were synthesized through a hydrothermal route and applied to pseudocapacitors. XRD and TEM results verified that the exposure ratio of active crystal facets was significantly increased with the assistance of the structure-directing agents. XPS analysis showed that there was more adsorbed oxygen and Mn3+ on the active crystal facets, which can provide strong kinetics for the electrochemical reaction. Consequently, the α-MnO2 nanorods with {110} and {310} facets exhibited much higher pseudocapacitances of 120.0 F g-1 and 133.0 F g-1 than their α-MnO2-200 counterparts (67.5 F g-1). The theoretical calculations proved that the {310} and {110} facets have stronger adsorption capacity and lower diffusion barriers for sodium ions, which is responsible for the enhanced pseudocapacitance of MnO2. This study provides a strategy to enhance the electrochemical performance of metal oxide, based on facet engineering.

Hierarchical Nickel Cobalt Phosphide @ Carbon Nanofibers Composite Microspheres: Ultrahigh Energy Densities of Electrodes for Supercapacitors

Supercapacitors (SCs) are widely used in energy storage devices due to their superior power density and long cycle lifetime. However, the limited energy densities of SCs hinder their industrial application to a great extent. In this study, we present a new combination of metallic phosphide-carbon composites, synthesized by directly carbonizing (Ni1-xCox)5TiO7 nanowires via thermal chemical vapor deposition (TCVD) technology. The new method uses one-dimensional (1D) (Ni1-xCox)TiO7 nanowires as precursors and supporters for the in situ growth of intertwined porous CNF microspheres. These 1D nanowires undergo microstructure transformation, resulting in the formation of CoNiP nanoparticles, which act as excellent interconnected catalytic nanoparticles for the growth of porous 3D CNF microspheres. Benefiting from the synergistic effect of a unique 1D/3D structure, the agglomeration of nanoparticles can effectively be prevented. The resulting CNF microspheres exhibit an interconnected conductive matrix and provide a large specific surface area with abundant ion/charge transport channels. Consequently, at a scanning rate of 10 mV s-1, its specific capacitance in 1.0 M Na2SO4 + 0.05 M Fe(CN)63-/4- aqueous solution is as high as 311.7 mF cm-2. Furthermore, the CoNiP@CNFs composite film-based symmetrical SCs show an ultrahigh energy density of 20.08 Wh kg-1 at a power density of 7.20 kW kg-1, along with outstanding cycling stability, with 87.2% capacity retention after 10,000 cycles in soluble redox electrolytes. This work provides a new strategy for designing and applying high-performance binary transition metal phosphide/carbon composites for next-generation energy storage devices.

A mini review on metal-organic framework-based electrode materials for capacitive deionization

Capacitive deionization (CDI) is an electrochemical method of extracting ions from solution at potentials below electrolysis. It has various applications ranging from water remediation and desalination to heavy metal removal and selective resource recovery. A CDI device applies an electrical charge across two porous electrodes to attract and remove ions without producing waste products. It is generally considered environmentally friendly and promising for sustainability, yet ion removal efficiency still falls short of more established filtration methods. Commercially available activated carbon is typically used for CDI, and its ion adsorption capacity is low at approximately 20-30 mg g-1. Recently, much interest has been in the highly porous and well-structured family of materials known as metal-organic frameworks (MOFs). Most MOFs are poor conductors of electricity and cannot be directly used to make electrodes. A common workaround is to pyrolyze the MOF to convert its organic components to carbon while maintaining its underlying microstructure. However, most MOF-derived materials only retain partial microstructure after pyrolysis and cannot inherit the robust porosity of the parent MOFs. This review provides a systematic breakdown of structure-performance relationships between a MOF-derived material and its CDI performance based on recent works. This review also serves as a starting point for researchers interested in developing MOF-derived materials for CDI applications.

Three-Dimensional Polypyrrole-Decorated CuCo2S4 Nanowires Anchored on Nickel Foam: A Promising Electrode for High-Performance Supercapacitors

The exploitation of high-performance supercapacitors is crucial to promote energy storage technologies. Benefiting from the three-dimensional conductive micronanostructures and high specific capacity of the PPy@CuCo2S4@NF (polypyrrole/copper cobalt sulfide/nickel foam) composite electrode, this electrode exhibits a high specific capacity of 1403.21 C g-1 at 1 A g-1 and a capacitance retention of 85.79% after 10,000 cycles at 10 A g-1. The assembled PPy@CuCo2S4@NF//AC aqueous hybrid supercapacitor (AHSC) reveals a wide operating potential window of 1.5 V and achieves a high specific capacity of 322.52 C g-1 at 1 A g-1 and a capacitance retention of 86.84% after 15,000 cycles at 10 A g-1. The AHSC also exhibits a high power density of 733.69 W kg-1 at an energy density of 67.19 W h kg-1, surpassing those of previously reported spinel-based supercapacitors. Ex situ X-ray diffraction and X-ray photoelectron spectroscopy results show that the CuCo2S4 spinel structure changes to CuS2 and CoS2 cube structures, and the oxidation states of Cu and Co increase during charging and discharging processes. Density functional theory calculations suggest a superior conductivity for CuCo2S4 compared to that for CuCo2O4, demonstrating that CuCo2S4 has superior electrochemical performance. These findings attest to the considerable potential of the spinel materials for advanced energy storage applications.

NiGA MOF-based dispersive micro solid phase extraction coupled to temperature-assisted evaporation using low boiling point solvents for the extraction and preconcentration of butylated hydroxytoluene and some phthalate and adipate esters

The first-ever attempt to apply nickel gallic acid metal-organic framework (NiGA MOF) in analytical method development was done in this research by the extraction of some plasticizers from aqueous media. The greenness of the method is owing to the use of gallic acid and nickel as safe reagents and water as the safest solvent. Low boiling point solvents were applied as desorption solvents that underwent temperature-assisted evaporation in the preconcentration step. Performing the evaporation using a low-temperature water bath for a short period of time streamlines the preconcentration section. Into the solution of interest enriched with sodium sulfate, a mg amount of NiGA MOF was added alongside vortexing to extract the analytes. Following centrifugation and discarding the supernatant, a μL level of diethyl ether was added onto the analyte-loaded NiGA MOF particles and vortexed. The analyte-enriched diethyl ether phase was transferred into a conical bottom glass test tube and located in a water bath set at the temperature of 35 °C under a laboratory hood. After the evaporation, a μL level of 1,2-dibromoethane was added to the test tube and vortexed to dissolve the analytes from the inner perimeter of the tube. One microliter of the organic phase was injected into a gas chromatograph equipped with flame ionization detection. Appreciable extraction recoveries (61-98%), high enrichment factors (305-490), low limits of detection (0.80-1.74 μg L-1) and quantification (2.64-5.74 μg L-1), and wide linear ranges (5.74-1000 μg L-1) were obtained at the optimum conditions.

Mesoporous Iron Family Element (Fe, Co, Ni) Molybdenum Disulfide/Carbon Nanohybrids for High-Performance Supercapacitors

As the demand for fuel continues to increase, the development of energy devices with excellent performance is crucial. Supercapacitors (SCs) are attracting attention for their advantages of high specific energy and a long cycle life. At present, the development of high-performance electrode materials is the main point for research and development of SCs. Transition metal sulfides have the advantages of a large interlayer space and high theoretical capacity, making them promising electrode materials. Herein, we reported a series of ultrathin mesoporous iron family element (Fe, Co, Ni) molybdenum disulfide (MxMo1-xS2/C, M = Fe, Co, and Ni) by a template method. The original monolayer mesoporous structure of MoS2/C was maintained, and accumulation and agglomeration of MoS2/C were avoided. Based on our investigations, the best performance was that of CoxMo1-xS2/C nanohybrids. Furthermore, the concentrations of Co and Mo ions were modulated to obtain the best performance, in which Mo and Co ions were released at 1:1, 1:2, and 1:3 ratios and they were named CoxMo1-xS2/C-1, CoxMo1-xS2/C-2, and CoxMo1-xS2/C-3, respectively. Overall, these materials represent a significant improvement and show promise as high-performance SC electrode materials due to their enhanced capacitance and stability. At a current density of 0.5 A g-1, CoxMo1-xS2/C-2 has the optimal specific capacitance of 184 F g-1. CoxMo1-xS2/C-2 as an SC electrode exhibited better reversible capacity and cycling stability than MoS2/C, which is an improvement over MoS2/C regarding reversible capacity and cycling stability.

Thiacalix[4]arene-based metal-organic framework/reduced graphene oxide composite for electrochemical detection of chlorogenic acid

A novel metal-organic framework [Co2LCl4]·2DMF (Co-L) based on thiacalix[4]arene derivative was synthesized using the solvothermal method. Then Co-L was respectively mixed with reduced graphene oxide (RGO), multi-walled carbon nanotubes (MWCNT) and mesoporous carbon (MC) to prepare corresponding composite materials. PXRD, SEM and N2 adsorption-desorption illustrated that composite materials have been successfully prepared. After optimizing experimental conditions for detecting chlorogenic acid (CGA), the Co-L@RGO(1:1) composite material showed the optimal electrocatalytic activity for CGA, which may be because RGO possessed large specific surface area and hydroxyl and carboxyl groups that could form hydrogen-bonding with the oxide of CGA. Benefiting from the synergetic effect of Co-L and RGO, the glassy carbon electrode modified with Co-L@RGO(1:1) (Co-L@RGO(1:1)/GCE) exhibited a low limit of detection (LOD) of 7.24 nM for CGA within the concentration of 0.1-2 μM and 2-20 μM. Co-L@RGO(1:1)/GCE also showed excellent selectivity, stability, and reproducibility for the CGA detection. Co-L@RGO(1:1)/GCE could detect the CGA in honeysuckle with satisfactory results. This work provided a great example for the thiacalix[4]arene-based MOF in the application of electrochemical sensors.

2D Conjugated Metal-Organic Frameworks Embedded with Iodine for High-Performance Ammonium-Ion Hybrid Supercapacitors

Ammonium ions (NH4 + ) are emerging non-metallic charge carriers for advanced electrochemical energy storage devices, due to their low cost, elemental abundance, and environmental benignity. However, finding suitable electrode materials to achieve rapid diffusion kinetics for NH4 + storage remains a great challenge. Herein, a 2D conjugated metal-organic framework (2D c-MOF) for immobilizing iodine, as a high-performance cathode material for NH4 + hybrid supercapacitors, is reported. Cu-HHB (HHB = hexahydroxybenzene) MOF embedded with iodine (Cu-HHB/I2 ) features excellent electrical conductivity, highly porous structure, and rich accessible active sites of copper-bis(dihydroxy) (Cu─O4 ) and iodide species, resulting in a remarkable areal capacitance of 111.7 mF cm-2 at 0.4 mA cm-2 . Experimental results and theoretical calculations indicate that the Cu─O4 species in Cu-HHB play a critical role in binding polyiodide and suppressing its dissolution, as well as contributing to a large pseudocapacitance with adsorbed iodide. In combination with a porous MXene anode, the full NH4 + hybrid supercapacitors deliver an excellent energy density of 31.5 mWh cm-2 and long-term cycling stability with 89.5% capacitance retention after 10 000 cycles, superior to those of the state-of-the-art NH4 + hybrid supercapacitors. This study sheds light on the material design for NH4 + storage, enabling the development of novel high-performance energy storage devices.

Applications of electrically conductive membranes in water treatment via membrane distillation: Joule heating, membrane fouling/scaling/wetting mitigation and monitoring

Membrane distillation (MD) is a thermally driven separation process that is driven by phase change. The core of this technology is the hydrophobic microporous membrane that prevents mass transfer of the liquid while allowing the vapor phase to pass through the membrane's pores. Currently, MD is challenged by its high energy consumption and membrane degradation due to fouling, scaling and wetting. The use of electrically conductive membranes (ECMs) is a promising alternative method to overcome these challenges by inducing localized Joule heating, as well as mitigating and monitoring membrane fouling/scaling/wetting. The objective of this review is to consolidate recent advances in ECMs from the standpoint of conductive materials, membrane fabrication methodologies, and applications in MD processes. First, the mechanisms of ECMs-based MD processes are reviewed. Then the current trends in conductive materials and membrane fabrication methods are discussed. Thereafter, a comprehensive review of ECMs in MD applications is presented in terms of the different processes using Joule heating and various works related to membrane fouling, scaling, and wetting control and monitoring. Key insights in terms of energy consumption, economic viability and scalability are furnished to provide readers with a holistic perspective of the ECMs potential to achieve better performances and higher efficiencies in MD. Finally, we illustrate our perspectives on the innovative methods to address current challenges and provide insights for advancing new ECMs designs. Overall, this review sums up the current status of ECMs, looking at the wide range of conductive materials and array of fabrication methods used thus far, and putting into perspective strategies to deliver a more competitive ECMs-based MD process in water treatment.

Mixed solvent-assisted synthesis of high mass loading amorphous NiCo-MOF as a promising electrode material for supercapacitors

The pursuit of high mass loading metal-organic framework (MOF) materials via a simple method is crucial to achieve high-performance supercapacitors. Herein, an amorphous NiCo-MOF material with a high mass loading of up to 10.3 mg cm-2 was successfully prepared using a mixed solvent system of ethanol and water. In addition, by adjusting the volume ratio of ethanol to water, amorphous NiCo-MOFs with three different morphologies including nanospheres, nanopores, and ultra-thick plates were obtained. It was found that the different solvent systems not only affected the growth rate of MOFs, but also controlled their nucleation rate by changing the coordination environment of the metal ions, and thus achieved morphology and mass loading regulation, thereby influencing their energy storage behavior. Notably, the optimum NiCo-MOF exhibited the superior specific capacitance of up to 9.7 F cm-2 (941.8 F g-1) at a current density of 5 mA cm-2 and high-rate capability of 71.1% even at 20 mA cm-2. Moreover, the corresponding assembled solid-state supercapacitor exhibited an excellent energy density of 0.65 mW h cm-2 at a power density of 2 mW cm-2 and capacity retention of 84.7% after 8000 cycles at 30 mA cm-2. Overall, this work proposes a feasible and effective strategy to achieve high mass loading NiCo-MOFs, impacting their ultimate electrochemical performance, which can possibly be further extended to other MOFs with superior capacitance.