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

Coexistence of Redox-Active Metal and Ligand Sites in Copper-Based Two-Dimensional Conjugated Metal-Organic Frameworks as Active Materials for Battery-Supercapacitor Hybrid Systems

Two-dimensional (2D) conjugated metal-organic frameworks (c-MOFs) are promising materials for supercapacitor (SC) electrodes due to their high electrochemically accessible surface area coupled with superior electrical conductivity compared to traditional MOFs. In this work, porous and non-porous HHB-Cu (HHB=hexahydroxybenzene), derived through surfactant-assisted synthesis are studied as representative 2D c-MOF models with different characteristics, showing diverse reversible redox reactions with Na+ and Li+ in aqueous (10 M NaNO3) and organic (1.0 M LiPF6 in ethylene carbonate and dimethyl carbonate) electrolytes, respectively. These redox activities were here deployed to design negative electrodes for hybrid SCs (HSCs), combining the battery-like property of HHB-Cu at the negative electrode and the high capacitance and robust cyclic stability of activated carbon (AC) at the positive electrodes. In the organic electrolyte, porous HHB-Cu-based HSC achieves a maximum cell specific capacity (Cs) of 22.1 mAh g-1 at 0.1 A g-1, specific energy (Es) of 15.55 Wh kg-1 at specific power (Ps) of 70.49 W kg-1, and 77 % cyclic stability after 3000 gravimetric charge-discharge (GCD) cycles at 1 A g-1 (specific metrics calculated on the mass of both electrode materials). In the aqueous electrolyte, porous HHB-Cu-based HSC displays a Cs of 13.9 mAh g-1 at 0.1 A g-1, Es of 6.13 Wh kg-1 at 44.05 W kg-1, and 72.3 % Cs retention after 3000 GCD cycles. The non-porous sample, interesting for its superior electrical conductivity despite its limited surface area compared to its porous counterpart, shows lower Es performance but better rate capability compared to the porous one. This study indicates the potential of assembling a battery-SC hybrid system by rationally exploiting the battery-like behavior of 2D c-MOFs and the electrochemical double-layer capacitance of AC.

Interface Storage Mechanism in Aqueous Ammonium-Ion Supercapacitors with Keggin-Type Polyoxometalates-Modified Ag-BTC

Ammonium-ion supercapacitors (AISCs) offer considerable potential for future development owing to their low cost, high safety, environmental sustainability, and efficient electrochemical energy storage capabilities. The rapid and efficient charge-transfer process at the AISC can endow them with high capacitive and cycling stabilities. However, the prolonged intercalation/deintercalation of NH4 + in layered and framework materials often results in the cleavage of the active sites and the deconstruction of the framework, which makes it difficult to achieve long-term stable energy storage while maintaining high capacitance in the electrode materials. Herein, highly redox-active polyoxometalates (POMs) modified [Ag3(µ-Hbtc)(µ-H2btc)]n (Ag-BTC) is used as electrode materials. POMs effectively promote the pseudocapacitance storage of NH4 + through a similar interface storage mechanism. At a current density of 1 A g-1, {PMo12}@Ag-BTC exhibited a specific capacitance of 619.4 mAh g-1 and retained 100% of its capacitance after 20,000 charge-discharge cycles. An asymmetrical battery with {PMo12}@Ag-BTC and {PW12}@Ag-BTC as positive and negative electrode materials, respectively, achieved an energy density of 125.3 Wh kg-1. The interface-capacitance process enables the full utilization of metal-Ox (x = b, c, t) sites within the POMs, significantly enhancing charge storage. This study emphasizes the considerable potential of POM-based electrode materials for NH4 + intercalation/deintercalation energy storage.

Challenges and opportunities in 2D materials for high-performance aqueous ammonium ion batteries

Aqueous ammonium ion batteries (AAIBs) have attracted considerable attention due to their high safety and rapid diffusion kinetics. Unlike spherical metal ions, NH4 + forms hydrogen bonds with host materials, leading to a unique storage mechanism. A variety of electrode materials have been proposed for AAIBs, but their performance often falls short in terms of future energy storage needs. Hence, there is a critical need to design and develop advanced electrode materials for AAIBs. 2D materials, with their tunable interlayer spacing, remarkable interfacial chemistry and abundant surface functional groups, are an ideal choice for electrode materials for NH4 + storage. This review highlights the latest research on 2D electrode materials for AAIBs, providing insights into their working principles, NH4 + storage mechanisms and control strategies for designing high-performance AAIBs. Furthermore, a summary and future perspectives on 2D electrode materials in the development of AAIBs are provided, aiming to promote the advancement of high-performance AAIBs.

Intermolecular Forces-Oriented Growth of MOF-on-MOF Heterostructures

Well-controlled conjugation of distinct metal-organic frameworks (MOFs) with dissimilar components and structures into a multi-component MOF system is important for the design of myriad materials with structural complexity, integrated properties, and desired applications. Herein, we developed an interesting integration strategy that relies on intermolecular force, and which can overcome the difference in lattice parameters. The strategy uses the pre-formed MOF matrixes as seeds to direct the self-assembly of secondary MOF particles for the construction of hybrid MOFs. As a proof of concept, the representative core-satellite-type Ni-MOF@ZIF-8 and core-shell-type MOF-74@ZIF-8 heterostructures were achieved. Theory calculations elucidate the uncommon formation of well-defined hybrid MOFs by self-assembly of secondary ZIF-8 particles to anchor on Ni-MOF or MOF-74 seed surface via hydrogen bonds and van der Waals interactions. We anticipate that this work opens an avenue to design the multi-component MOF systems from different MOFs with lattice mismatching.

Enhancing the Electrochemical Energy Storage of Metal-Organic Frameworks: Linker Engineering and Size Optimization

The electric conductivity and charge transport efficiency of metal-organic frameworks (MOFs) dictate the effective utilization of built-in redox centers and electrochemical redox kinetics and therefore electrochemical performance. Reticular chemistry and the tunable microcosmic shape of MOFs allow for improving their electric conductivity and charge transfer efficiency. Herein, we synthesized two Ni-MOFs (Ni-tdc-bpy and Ni-tdc-bpe) by the solvothermal reaction of Ni2+ ions with 2,5-thiophenedicarboxylic acid (H2tdc) in the presence of conjugated 4,4'-bipyridyl (bpy) and 1,2-di(4-pyridyl)ethylene (bpe) coligands, respectively. We also synthesized two thinning Ni-MOFs (Ni-tdc-bpy(0.5) and Ni-tdc-bpe(0.5)) by adjusting the amounts of bpy and bpe, respectively. Experimental investigations revealed that linker engineering by tuning the delocalization of the N-donor dipyridyl coligands and size optimization by controlling the amount of the coligand rendered the Ni-MOF with significantly improved electrical conductivity and charge transport efficiency. Among them, Ni-tdc-bpe(0.5) possessing the bpe coligand with more strong delocalization and an optimized size exhibited an enhanced specific capacitance of 650 F g-1 at 0.5 A g-1. Moreover, the hybrid supercapacitor constructed from Ni-tdc-bpe(0.5) and activated carbon delivered an excellent energy density of 33.6 Wh kg-1 at a power density of 232.6 W kg-1.

Two-Dimensional (2D) Conductive Metal-Organic Framework Thin Films: The Preparation and Applications in Electrochemistry

Two-dimensional conductive MOF thin films have attracted attention due to their rich pore structure and unique electrical properties, and their applications in many fields, including batteries, sensing, supercapacitors, electrocatalysis, etc. This paper discusses several preparation methods for 2D conductive MOF thin films. And the applications of 2D conductive MOF thin films are summarized. In addition, the current challenges in the preparation of 2D conductive MOF thin films and the great potential in practical applications are discussed.

Enhancing Energy Density in Aqueous Ammonium-Ion Supercapacitors via CuCo2S4@MoS2 Core@Shell Heterostructure Design

Aqueous ammonium-ion supercapacitors (AASCs) are recognized for their rapid charge-discharge capability, long cycle life, and excellent power density. However, they still confront the challenges of low energy density. To address the above issue, this work proposes a novel strategy involving the establishment of CuCo2S4@MoS2 core@shell heterostructures to enhance the capacity of electrode material. The double electric layer energy storage mechanism of the MoS2 shell facilitates the storage and provision of a substantial ammonium source for NH4 + insertion into CuCo2S4, thereby enhancing the electrochemical performance of AASCs. The density functional theory (DFT) calculations demonstrate that the CuCo2S4@MoS2 core@shell heterostructures exhibit better affinity for NH4 + and improved conductivity. Furthermore, the internal electric field at the heterojunction accelerates NH4 + transfer, thereby enhancing the pseudocapacitive behavior of CuCo2S4. Owing to the abundant active sites and pronounced pseudo-capacitance, the CuCo2S4@MoS2 electrode achieves a specific capacity of 2045 C g-1 at 1 A g-1. With activated carbon (AC) as the negative electrode, the fabricated CuCo2S4@MoS2//AC AASC device attains a specific capacity of 591 C g-1 and an energy density of 83.23 Wh kg-1. This work presents a promising new strategy for the next generation of AASCs.

Efficient asymmetric diffusion channel in MnCo2O4 spinel for ammonium-ion batteries

Transition metal oxides ion diffusion channels have been developed for ammonium-ion batteries (AIBs). However, the influence of microstructural features of diffusion channels on the storage and diffusion behavior of NH4+ is not fully unveiled. In this study, by using MnCo2O4 spinel as a model electrode, the asymmetric ion diffusion channels of MnCo2O4 have been regulated through bond length optimization strategy and investigate the effect of channel size on the diffusion process of NH4+. In addition, the reducing channel size significantly decreases NH4+ adsorption energy, thereby accelerating hydrogen bond formation/fracture kinetics and NH4+ reversible diffusion within 3D asymmetric channels. The optimized MnCo2O4 with oxygen vacancies/carbon nanotubes composite exhibits impressive specific capacity (219.2 mAh g-1 at 0.1 A g-1) and long-cycle stability. The full cell with 3,4,9,10-perylenetetracarboxylic diimide anode demonstrates a remarkable energy density of 52.3 Wh kg-1 and maintains 91.9% capacity after 500 cycles. This finding provides a unique approach for the development of cathode materials in AIBs.

Iodine intercalation-assisted alkali activation constructs coal-based porous carbon for high-performance supercapacitors

Supercapacitors have the advantages of fast charging and discharging speeds, high power density, long cycle life, and wide operating temperature range. They are widely used in portable electronic equipment, rail transit, industry, military, aerospace, and other fields. The design and preparation of low-cost, high-performance electrode materials still pose a bottleneck that hinders the development of supercapacitors. In this paper, coal was used as the raw material, and the coal-based porous carbon electrode material was constructed using the iodine intercalation-assisted activation method and used for supercapacitors. The CK-700 electrode exhibits excellent charge storage performance in a 6 M potassium hydroxide (KOH) electrolyte, with a maximum specific capacitance of 350 F/g at a current density of 0.5 A/g. In addition, it has an excellent rate performance (310 F/g at 1 A/g) and cycle stability (capacitance retention up to 91.7 % after 30000 cycles). This work provides a method for realizing high-quality, high-yield and low-cost preparation of coal-based porous carbon, and an idea for improving the performance of supercapacitors.

Iodine Adsorption in Nanoporous Carbon to Fabricate Assimilated Battery Electrodes for Durable Hybrid Supercapacitors

A hybrid supercapacitor is designed by coupling a battery electrode with a capacitive electrode in a single device/cell to enhance energy density. In iodine-based hybrid supercapacitors, the nanoporous carbon serves as the electrode material; however, the cathode or positive electrode is charged with iodine via electrodeposition from a redox aqueous electrolyte, while a negative electrode stores charges at the electric double-layer. In this work, iodine is loaded via physical adsorption into the porosity of a carbon electrode, keeping the aqueous electrolyte free from iodide redox moieties. By this way, the risk of polyiodide (I3- and I5-) generation at the positive electrode leading to a shuttling-related performance loss of the hybrid supercapacitor is prevented. Chemical interactions of iodine with the carbon surface and within the pores have been investigated with Raman spectroscopy, thermogravimetry and electron microscopy. Electrochemical methods have been used to test individual electrodes and hybrid supercapacitors in aqueous NaNO3 and aqueous LiTFSI at 5 mol/L concentration for performance parameters such as energy efficiency, capacitance, self-discharge and cyclability. The hybrid supercapacitor in aqueous LiTFSI exhibits stable capacitance and energy efficiency during long-term aging tests at 1.5 V. Carbon nanoarchitecturing with iodine as shown in the present work offers an economical approach to enhance the performance of hybrid supercapacitors.

Morphology Control of Mixed Metallic Organic Framework for High-Performance Hybrid Supercapacitors

The study presents the binder-free synthesis of mixed metallic organic frameworks (MMOFs) supported on a ternary metal oxide (TMO) core as an innovative three-dimensional (3D) approach to enhance electron transport and mass transfer during the electrochemical charge-discharge process, resulting in high-performance hybrid supercapacitors. The research demonstrates that the choice of organic linkers can be used to tailor the morphology of these MMOFs, thus optimizing their electrochemical efficiency. Specifically, a NiCo-MOF@NiCoO2@Ni electrode, based on terephthalic linkers, exhibits highly ordered porosity and a vast internal surface area, achieving a maximum specific capacity of 2320 mC cm-2, while maintaining excellent rate capability and cycle stability. With these performances, the hybrid supercapacitor (HSC) achieves a maximum specific capacitance of 424.6 mF cm-2 (specific capacity 653.8 mC cm-2) and 30.7 F cm-3 with energy density values of 10.1 mWh cm-3 at 167.4 mW cm-3 (139.8 µWh cm-2 at 2310 µW cm-2), which are higher than those of previously reported MMOFs based electrodes. This research introduces a novel approach for metal organic framework based HSC electrodes, diverging from the traditional emphasis on metal ions, in order to achieve the desired electrochemical performance.

Enhanced Energy Storage Performance through Controlled Composition and Synthesis of 3D Mixed Metal-Oxide Microspheres

Binary transition metal oxide complexes (BTMOCs) in three-dimensional (3D) layered structures show great promise as electrodes for supercapacitors (SCs) due to their diverse oxidation states, which contribute to high specific capacitance. However, the synthesis of BTMOCs with 3D structures remains challenging yet crucial for their application. In this study, we present a novel approach utilizing a single-step hydrothermal technique to fabricate flower-shaped microspheres composed of a NiCo-based complex. Each microsphere consists of nanosheets with a mesoporous structure, enhancing the specific surface area to 23.66 m2 g-1 and facilitating efficient redox reactions. When employed as the working electrode for supercapacitors, the composite exhibits remarkable specific capacitance, achieving 888.8 F g-1 at 1 A g-1. Furthermore, it demonstrates notable electrochemical stability, retaining 52.08% capacitance after 10,000 cycles, and offers a high-power density of 225 W·kg-1, along with an energy density of 25 Wh·kg-1, showcasing its potential for energy storage applications. Additionally, an aqueous asymmetric supercapacitor (ASC) was assembled using NiCo microspheres-based complex and activated carbon (AC). Remarkably, the NiCo microspheres complex/AC configuration delivers a high specific capacitance of 250 F g-1 at 1 A g-1, with a high energy density of 88 Wh kg-1, for a power density of 800 W kg-1. The ASC also exhibits excellent long-term cyclability with 69% retention over 10,000 charge-discharge cycles. Furthermore, a series of two ASC devices demonstrated the capability to power commercial blue LEDs for a duration of at least 40 s. The simplicity of the synthesis process and the exceptional performance exhibited by the developed electrode materials hold considerable promise for applications in energy storage.

Non-Metal Ion Storage in Zinc-Organic Batteries

Zinc-organic batteries (ZOBs) are receiving widespread attention as up-and-coming energy-storage systems due to their sustainability, operational safety and low cost. Charge carrier is one of the critical factors affecting the redox kinetics and electrochemical performances of ZOBs. Compared with conventional large-sized and sluggish Zn2+ storage, non-metallic charge carriers with small hydrated size and light weight show accelerated interfacial dehydration and fast reaction kinetics, enabling superior electrochemical metrics for ZOBs. Thus, it is valuable and ongoing works to build better ZOBs with non-metallic ion storage. In this review, versatile non-metallic cationic (H+, NH4 +) and anionic (Cl-, OH-, CF3SO3 -, SO4 2-) charge carriers of ZOBs are first categorized with a brief comparison of their respective physicochemical properties and chemical interactions with redox-active organic materials. Furthermore, this work highlights the implementation effectiveness of non-metallic ions in ZOBs, giving insights into the impact of ion types on the metrics (capacity, rate capability, operation voltage, and cycle life) of organic cathodes. Finally, the challenges and perspectives of non-metal-ion-based ZOBs are outlined to guild the future development of next-generation energy communities.

Delocalization Engineering of Heme-Mimetic Artificial Enzymes for Augmented Reactive Oxygen Catalysis

Developing artificial enzymes based on organic molecules or polymers for reactive oxygen species (ROS)-related catalysis has broad applicability. Herein, inspired by porphyrin-based heme mimics, we report the synthesis of polyphthalocyanine-based conjugated polymers (Fe-PPc-AE) as a new porphyrin-evolving structure to serve as efficient and versatile artificial enzymes for augmented reactive oxygen catalysis. Owing to the structural advantages, such as enhanced π-conjugation networks and π-electron delocalization, promoted electron transfer, and unique Fe-N coordination centers, Fe-PPc-AE showed more efficient ROS-production activity in terms of Vmax and turnover numbers as compared with porphyrin-based conjugated polymers (Fe-PPor-AE), which also surpassed reported state-of-the-art artificial enzymes in their activity. More interestingly, by changing the reaction medium and substrates, Fe-PPc-AE also revealed significantly improved activity and environmental adaptivity in many other ROS-related biocatalytic processes, validating the potential of Fe-PPc-AE to replace conventional (poly)porphyrin-based heme mimics for ROS-related catalysis, biosensors, or biotherapeutics. It is suggested that this study will offer essential guidance for designing artificial enzymes based on organic molecules or polymers.

De Novo Design and Facile Synthesis of Highly Crystalline 2D Conductive Metal-Organic Frameworks: A "Rotor-Stator" Strategy

Two-dimensional conductive metal-organic frameworks (2D c-MOFs), which feature high electrical conductivity and large charge carrier mobility, hold great promise in electronics and optoelectronics. Nevertheless, the limited solubility of commonly used planar ligands inevitably brings challenges in synthesis and purification and causes laborious coordination conditions for screening. Moreover, most reported 2D c-MOFs are polycrystalline powders with relatively low crystallinity and irregular morphology, hindering the unveiling of the detailed structure-function relationship. Herein, we developed a "rotor-stator" molecular design strategy to construct 2D c-MOFs using a delicately designed nonplanar biscarbazole ligand (8OH-DCB). Benefiting from the special "rotor-stator" structure of the ligand, crystals of Cu-DCB-MOF were successfully prepared, allowing for the precise determination of their crystal structure. Interestingly, the crystals of Cu-DCB-MOF can be obtained in various organic solvents, indicating excellent solvent compatibility. The versatility of the "rotor-stator" molecular design strategy was further demonstrated by another two new ligands with a "rotor-stator" structure, and afford corresponding 2D c-MOF crystals (Cu-DCBT-MOF and Cu-DCBBT-MOF). The current work presents a facile approach toward the rational design and direct construction of highly crystalline 2D c-MOFs using nonplanar ligands.

N-Doped Carbon Nanonecklaces with Encapsulated BiOCl Nanoparticles as High-Rate Anodes for Lithium-Ion Batteries

The unique two-dimensional layered structure of BiOCl makes it highly promising for energy storage applications. In this study, we successfully synthesized BiOCl nanoparticles encapsulated in N-doped carbon nanonecklaces (BiOCl NPs/N-CNNs) using well-established electrospinning and solvothermal substitution. As an anode material for lithium-ion batteries, BiOCl NPs/N-CNNs exhibited enhanced rate performance, delivering a capacity of 220.2 mA h g-1 at 8 A g-1. Furthermore, it demonstrated remarkable long cycle stability, retaining a capacity of 200.5 mA h g-1 after 9000 cycles with a discharge rate of 8.0 A g-1. The superior electrochemical performance can be attributed to the stacked layered structure of BiOCl, facilitated by van der Waals force, as well as the ingenious nanonecklace structures. These structures not only provide fast ion diffusion pathways but also enhance electrolyte penetration and offer more active sites for Li+ insertion and extraction. Additionally, the nanonecklace structure prevents the aggregation of nanopolyhedra, promoting the complete reaction of BiOCl with Li+. Moreover, the unique nanopolyhedron structure alleviates the stress caused by the volume expansion of Bi nanoparticles during cycling and reduces the internal resistance of the electrode.