Porous organic polymers for high-performance supercapacitors

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.

Boosting supercapacitive performance of pristine covalent organic frameworks via phenolic hydroxyl groups: A two-in-one strategy

Two-dimensional covalent organic frameworks (COFs) are ideal electrode materials for electrochemical energy storage devices due to their unique structures and properties, and the accessibility and utilization efficiency of the redox-active sites within COFs are critical determinants of their pseudocapacitive performance. Via introducing meticulously designed phenolic hydroxyl (Ar-OH) groups with hydrogen-bond forming ability onto the imine COF skeletons, DHBD-Sb-COF exhibited improved hydrophilicity and crystallinity than the parent BD-Sb-COF, the redox-active sites (SbPh3 moieties) in COF electrodes could thus be highly accessed by aqueous electrolyte with a high active-site utilization of 93%. DHBD-Sb-COF//AC provided an excellent supercapacitive performance with an energy density of 78 Wh Kg-1 at the power density of 2553 W Kg-1 and super cycling stability, exceeding most of the previously reported pristine COF electrode-based supercapacitors. The "two-in-one" strategy of introducing hydroxyl groups onto imine COF skeletons to enhance both hydrophilicity and crystallinity provides a new avenue to improve the electrochemical performance of COF-based electrodes for high-performance supercapacitors.

Enhanced Conductivity in Conjugated Microporous Polymers via Integrating of Carbon Nanotubes for Ultrasensitive NO2 Chemiresistive Sensor

Conjugated microporous polymers (CMPs) present high promise for chemiresistive gas sensing owing to their inherent porosities, high surface areas, and tunable semiconducting properties. However, the poor conductivity hinders their widespread application in chemiresistive sensing. In this work, three typical CMPs (PSATA, PSATB, and PSATT) are synthesized and their chemiresistive gas sensing performance is investigated for the first time. To further improve performance, PSATT are modified on the surface of amino-functionalized multi-walled carbon nanotubes (NH2-MWCNTs) to improve the conductivity. As a result, the obtained material, PSATT-7NC exhibited a high sensitivity of 9766% toward 4 ppm NO2, which is 2.5 times higher than that of pristine PSATT. It also demonstrated remarkable selectivity and excellent long-term stability. Furthermore, the lowest limit of detection (0.79 ppb) among all polymers-based sensors is achieved at a low operating temperature of 100 °C. This work provides a valuable strategy into the development of a new material platform for advancing high-performance gas sensing applications.

Hydroxyl-functionalized cationic porous organic polymers for efficient enrichment and detection of phenolic endocrine disrupting chemicals in water and snapper

Endocrine-disrupting chemicals (EDCs) can disrupt the normal functioning of the endocrine system in organisms, leading to various health issues. Therefore, monitoring EDCs in the environment and food is of significant importance. In this study, a hydroxyl-functionalized ionic porous organic polymer (OH-IPOP) has been synthesized for the first time using 2-benzimidazolemethanol as a monomer. The OH-IPOP exhibited excellent adsorption performance towards phenolic EDCs. An efficient method for determination of phenolic EDCs (p-tert-butylphenol, bisphenol B, bisphenol A and bisphenol F) in environmental water and snapper samples was successfully established by with OH-IPOP as solid-phase extraction sorbent and determination with high-performance liquid chromatography-ultraviolet detection. The method showed good linearity (r2 > 0.998), low detection limits (0.008-0.020 ng mL-1 for lake water, 1.00-3.00 ng/g for snapper), high recovery rates (82.3-106 %), and good precision (relative standard deviation < 6.6 %), making it a highly efficient adsorbent for the enrichment of EDCs in complex sample matrices.

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.

Engineering π-Electron Bridge Enables Low-Potential 2D Redox Polymer Anodes for High-Voltage Aqueous All-Organic Batteries

Aqueous all-organic batteries (AAOBs) have emerged as a hot topic but their development was plagued by limited choices of anode materials, generally facing an intractable trade-off between low potential and high stability. Here, we propose a novel π-electron bridge engineering strategy to explore a class of 2D dioxin-bridged redox covalent organic polymer (RCOP) as trade-off-breaking anodes for high-voltage AAOBs. By establishing a tunable RCOP platform, we perform theoretical study to scrutinize how bridge units between active sites affect the electrode potential and redox activity for the first time. We discover that compared to common pyrazine bridge, the weakened conjugation and strong electron donor character of the proposed dioxin bridge can induce elevated LUMO level and enriched π-electron populations in active sites, heralding a low electrode potential and enhanced redox activity. Besides, the nonaromaticity-induced molecular flexibility of dioxin bridge mitigates intermolecular stacking for sufficient active sites exposure. To experimentally corroborate this, a new dioxin-bridged 2D RCOP (D-HATN) and its pyrazine-bridged analogue (P-HATN) are synthesized for proof-of-concept demonstration. D-HATN displays excellent compatibility with Na+/Zn2+/NH4 +/H3O+ and obviously lower redox potentials in various dilute electrolytes compared to P-HATN and most reported organic anodes, while featuring rapid Grotthuss-type proton conduction and unprecedented durability in acid - 91.8 % capacity retention after 20000 cycles. Thus, the D-HATN-involved all-organic proton battery delivers an average output voltage of 0.75 V, which can be further elevated to 1.63 V with alkaline-acidic hybrid electrolyte design, affording markedly-increased specific energy.

N/O Co-doped Porous Carbon with Controllable Porosity Synthesized via an All-in-One Step Method for a High-Rate-Performance Supercapacitor

A green and economical methodology to fabricate carbon-based materials with suitable pore size distributions is needed to achieve rapid electrolyte diffusion and improve the performance of supercapacitors. Here, a method combining in situ templates with self-activation and self-doping is proposed. By variation of the molar ratio of magnesium folate and potassium folate, the pore size distribution was effectively adjusted. The optimal carbon materials (Kx) have a high specific surface area (1021-1676 m2 g-1) and hierarchical pore structure, which significantly promotes its excellent capacitive properties. Notably, K2 shows an excellent mass specific capacitance of 233 F g-1 at 0.1 A g-1. It still retained 113 F g-1 at 55 A g-1. The assembled symmetric supercapacitor exhibited an outstanding cyclic stability. It maintains 100% capacitance after 100 000 cycles at 10 A g-1. The symmetric supercapacitor demonstrated a maximum power density of 99.8 kW kg-1. This study focuses on the preparation of layered pore structures to provide insights into the sustainable design of carbon materials.

Emerging Two-Dimensional Materials for Proton-Based Energy Storage

The rapid diffusion kinetics and smallest ion radius make protons the ideal cations toward the ultimate energy storage technology combining the ultrafast charging capabilities of supercapacitors and the high energy densities of batteries. Despite the concept existing for centuries, the lack of satisfactory electrode materials hinders its practical development. Recently, the rapid advancement of the emerging two-dimensional (2D) materials, characterized by their ultrathin morphology, interlayer van der Waals gaps, and distinctive electrochemical properties, injects promises into future proton-based energy storage systems. In this perspective, we comprehensively summarize the current advances in proton-based energy storage based on 2D materials. We begin by providing an overview of proton-based energy storage systems, including proton batteries, pseudocapacitors and electrical double layer capacitors. We then elucidate the fundamental knowledge about proton transport characteristics, including in electrolytes, at electrolyte/electrode interfaces, and within electrode materials, particularly in 2D material systems. We comprehensively summarize specific cases of 2D materials as proton electrodes, detailing their design concepts, proton transport mechanism and electrochemical performance. Finally, we provide insights into the prospects of proton-based energy storage systems, emphasizing the importance of rational design of 2D electrode materials and matching electrolyte systems.

Porous Organic Polymers as Active Electrode Materials for Energy Storage Applications

Eco-friendly and efficient energy production and storage technologies are highly demanded to address the environmental and energy crises. Porous organic polymers (POPs) are a class of lightweight porous network materials covalently linked by organic building blocks, possessing high surface areas, tunable pores, and designable components and structures. Due to their unique structural and compositional advantages, POPs have recently emerged as promising electrode materials for energy storage devices, particularly in the realm of supercapacitors and ion batteries. In this work, a comprehensive overview of recent progress and applications of POPs as electrode materials in energy storage devices, including the structural features and synthesis strategies of various POPs, as well as their applications in supercapacitors, lithium batteries, sodium batteries, and potassium batteries are provided. Finally, insights are provided into the future research directions of POPs in electrochemical energy storage technologies. It is anticipated that this work can provide readers with a comprehensive background on the design of POPs-based electrode materials and ignite more research in the development of next-generation energy storage devices.

Straightforward Production Methods for Diverse Porous PEDOT:PSS Structures and Their Characterization

Porous conductive polymer structures, in particular Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) structures, are gaining in importance due to their versatile fields of application as sensors, hydrogels, or supercapacitors, to name just a few. Moreover, (porous) conducting polymers have become of interest for wearable and smart textile applications due to their biocompatibility, which enables applications with direct skin contact. Therefore, there is a huge need to investigate distinct, straightforward, and textile-compatible production methods for the fabrication of porous PEDOT:PSS structures. Here, we present novel and uncomplicated approaches to producing diverse porous PEDOT:PSS structures and characterize them thoroughly in terms of porosity, electrical resistance, and their overall appearance. Production methods comprise the incorporation of micro cellulose, the usage of a blowing agent, creating a sponge-like structure, and spraying onto a porous base substrate. This results in the fabrication of various porous structures, ranging from thin and slightly porous to thick and highly porous. Depending on the application, these structures can be modified and integrated into electronic components or wearables to serve as porous electrodes, sensors, or other functional devices.

Layered S-Bridged Covalent Triazine Frameworks via a Bifunctional Template-Catalytic Strategy Enabling High-Performance Zinc-Ion Hybrid Supercapacitors

Exploring covalent triazine frameworks (CTFs) with high capacitative activity is highly desirable and challenging. Herein, the S-rich CTFs cathode is pioneeringly introduced in Zn-ion hybrid supercapacitors (ZSC), achieving outstanding capacity and energy density, and satisfactory anti-freezing flexibility. Specifically, the S-bridged CTFs are synthesized by a bifunctional template-catalytic strategy, where ZnCl2 serves as both the catalyst/solvent and in situ template to construct triazine frameworks with interconnected pores and layered gaps. The resultant CTFs (CTFS-750) are employed as a reasonable pattern-like system to more deeply scrutinize the synergistic effect of S-bridged triazine and layered porous architecture for polymer-based cathodes in Zn-ion storage. The experimental results indicate that the adsorption barriers of Zn-ions on CTFS-750 are effectively weakened, and accessible Zn2+-absorption sites provided by the C─S─C and C═N bonds have been confirmed via DFT calculations. Consequently, the CTFS-750 cathode-assembled ZSC displays an ultra-high capacity of 211.6 mAh g-1 at 1.0 A g-1, an outstanding energy density of 202.7 Wh kg-1, and attractive cycling performance. Moreover, the resulting flexible ZSC device shows superior capacity, good adaptability, and satisfactory anti-freezing behavior. This approach sheds new light on constructing advanced polymer-based cathodes at the atom level and paves the way for fabricating high-performance ZSC and beyond.

Electrochemical Formation of Ionic Porous Organic Polymers Based on Viologen for Electrochromic Applications

The systematic study of two ionic porous organic polymers (iPOPs) based on viologens and their first applications in the electrochromic field are reported. The viologen-based iPOPs are synthesized by electrochemical polymerization with cyano groups, providing a simple and controllable method for iPOPs that solves the film preparation problems common to viologens. After the characterization of these iPOPs, a detailed study of their electrochromic properties is conducted. The iPOP films based on viologens structure exhibit excellent electrochromic properties. In addition, the resulting iPOP films show high sensitivity to electrolyte ions of different sizes in the redox process. Electrochemical and electrochromic data of the iPOPs explain this phenomenon in detail. These results demonstrate that iPOPs of this type are ideal candidates as electrochromic materials due to their inherent porous structures and ion-rich properties.

Hierarchical porous carbon materials for lithium storage: preparation, modification, and applications

Secondary battery as an efficient energy conversion device has been highly attractive for alleviating the energy crisis and environmental pollution. Hierarchical porous carbon (HPC) materials with multiple sizes pore channels are considered as promising materials for energy conversion and storage applications, due to their high specific surface area and excellent electrical conductivity. Although many reviews have reported on carbon materials for different fields, systematic summaries about HPC materials for lithium storage are still rare. In this review, we first summarize the main preparation methods of HPC materials, including hard template method, soft template method, and template-free method. The modification methods including porosity and morphology tuning, heteroatom doping, and multiphase composites are introduced systematically. Then, the recent advances in HPC materials on lithium storage are summarized. Finally, we outline the challenges and future perspectives for the application of HPC materials in lithium storage.

Green synthesis of N-rich carbon dot-derived crosslinked covalent organic nanomaterial for multipurpose chromatographic applications

Carbon dots (CDs) derived crosslinked covalent organic nanomaterials (CONs) possessing high specific surface area and abundant surface functional groups are considered to be potential candidates for multimodal chromatographic separations. Typically, the synthesis of CDs and CONs requires harsh reaction conditions and toxic organic solvents, hence, the pursuit of facile and mild preparation strategies is the goal of researchers. In this work, 3-aminopropyltriethoxysilane and D-glucose were used as nitrogen and carbon sources, respectively, to prepare amino-CDs (AmCDs) by rapid low-temperature polymerization rather than the common high-temperature and high-pressure reaction. Then, surface functionalization of the aminated silica gel was carried out in a deep eutectic solvent by using hydrophilic AmCDs and 1,3,5-triformylbenzene (TFB) as the functional monomers. Consequently, a novel N-rich CDs derived CON surface-functionalized silica gel (AmCDs-CON@SiO2) was obtained under mild reaction conditions. The combination of AmCDs and TFB created an ideal CON based chromatographic stationary phase. The incorporation of TFB not only contributed to the successful construction of a crosslinked CON, but also enhanced the interaction forces. The developed AmCDs-CON@SiO2 has a great potential for versatile applications in liquid chromatography. This study proposes a simple stationary phase preparation strategy by the surface modification of silica gel with CDs-based CON. Moreover, this study verified the application potential of CDs derived CON in chromatographic separation. This not only promotes the development of CDs in the field of liquid chromatographic stationary phase, but also provides some reference value for the wide application of cross-linked CON.

Optimizing Performance in Supercapacitors through Surface Decoration of Bismuth Nanosheets

Bismuth (Bi) exhibits a high theoretical capacity, excellent electrical conductivity properties, and remarkable interlayer spacing, making it an ideal electrode material for supercapacitors. However, during the charge and discharge processes, Bi is prone to volume expansion and pulverization, resulting in a decline in the capacitance. Deposition of a nonmetal on its surface is considered an effective way to modulate its morphology and electronic structure. Herein, we employed the chemical vapor deposition technique to fabricate Se-decorated Bi nanosheets on a nickel foam (NF) substrate. Various characterizations indicated that the deposition of Se on Bi nanosheets regulated their surface morphology and chemical state, while sustaining their pristine phase structure. Electrochemical tests demonstrated that Se-decorated Bi nanosheets exhibited a 51.1% improvement in capacity compared with pristine Bi nanosheets (1313 F/g compared to 869 F/g at a current density of 5 A/g). The energy density of the active material in an assembled asymmetric supercapacitor could reach 151.2 Wh/kg at a power density of 800 W/kg. These findings suggest that Se decoration is a promising strategy to enhance the capacity of the Bi nanosheets.

Morphology Tuning via Linker Modulation: Metal-Free Covalent Organic Nanostructures with Exceptional Chemical Stability for Electrocatalytic Water Splitting

The development of synthetic routes for the formation of robust porous organic polymers (POPs) with well-defined nanoscale morphology is fundamentally significant for their practical applications. The thermodynamic characteristics that arise from reversible covalent bonding impart intrinsic chemical instability in the polymers, thereby impeding their overall potential. Herein, a unique strategy is reported to overcome the stability issue by designing robust imidazole-linked POPs via tandem reversible/irreversible bond formation. Incorporating inherent rigidity into the secondary building units leads to robust microporous polymeric nanostructures with hollow-spherical morphologies. An in-depth analysis by extensive solid-state NMR (1D and 2D) study on 1H, 13C, and 14N nuclei elucidates the bonding and reveals the high purity of the newly designed imidazole-based POPs. The nitrogen-rich polymeric nanostructures are further used as metal-free electrocatalysts for water splitting. In particular, the rigid POPs show excellent catalytic activity toward the oxygen evolution reaction (OER) with long-term durability. Among them, the most efficient OER electrocatalyst (TAT-TFBE) requires 314 mV of overpotential to drive 10 mA cm-2 current density, demonstrating its superiority over state-of-the-art catalysts (RuO2 and IrO2).

A Tröger's base-linked aluminium phthalocyanine polymer for discriminative electrochemical sensing of the antibiotic isoniazid

Isoniazid is a first-line drug used to treat tuberculosis. However, its excessive use can lead to serious adverse effects. Thus, strict monitoring of the isoniazid levels in medications and human systems is required. In this study, a new polymer (AlPc-TB POP) containing a metal phthalocyanine and Tröger's base was synthesized and explored as an electrocatalyst for the oxidation of isoniazid. The results indicated that the polymer is an excellent electron-transfer medium for isoniazid oxidation. The AlPc-TB POP-based sensor quantified isoniazid in the linear range of 0.1-130 μM, with a detection limit of 0.0185 μM. The response of the developed sensor to isoniazid was reproducible and stable. Furthermore, this method can accurately determine isoniazid levels by ignoring the influence of common interfering species in tablets and biological samples. This study contributes to the development of nitrogen-rich porous organic polymers and offers a novel strategy for addressing challenges in disease therapeutic efficacy and public safety monitoring.

In situ polymerization of a melamine-based microsphere into 3D nickel foam for supercapacitors

An in situ synthesis approach is used to directly grow a microsphere of melamine-glutaraldehyde (MAGA) polymer over three-dimensional (3D) nickel foam (NF). The materials are used to produce nitrogen-doped carbon (NC) with and without NF. These precursors undergo carbonization at various temperatures, namely 400 °C, 500 °C, and 700 °C. The electrochemical properties of the materials would be significantly improved by directly growing MAGA polymer on the surface of NF. The electrochemical performance of NC/NF-400 was excellent, with a capacitance of 297 F g-1 achieved at a current density of 1 A g-1. The in situ growing approach does not necessitate the use of additional chemical agents, such as binders or conductive compounds when preparing the electrode. In addition, the material exhibits only 10% reduction in capacitance after undergoing 5000 cycles, indicating excellent cycling performance. The outstanding electrochemical performance achieved by using the in situ method of MAGA microsphere polymer on NF may be attributed to the rapid transit of ions to the electrode surfaces, facilitating effortless redox reactions.

Relationships Between Defectivity and Porosity in High Surface Area Porous Aromatic Frameworks

Porous aromatic framework (PAF) microporosity is known to be strongly dependent on synthetic approach but little is known about why certain reactions yield significantly and consistently more porous PAFs. This article explores the connections between synthetic pathway, PAF defectivity, and microporosity. Using a network disassembly strategy, we show that defectivity is highly dependent on synthetic approach and that more defective PAFs are associated with lower surface areas and pore volumes. This empirical association is corroborated through systematic introduction of defects to a modelPAF, which results in significant reduction of apparent surface area and pore volumes. Taken together, these data suggest that only highly efficient coupling reactions should be targeted for the synthesis of ultra-high surface area porous aromatic frameworks.

Atomic Layer Deposition-A Versatile Toolbox for Designing/Engineering Electrodes for Advanced Supercapacitors

Atomic layer deposition (ALD) has become the most widely used thin-film deposition technique in various fields due to its unique advantages, such as self-terminating growth, precise thickness control, and excellent deposition quality. In the energy storage domain, ALD has shown great potential for supercapacitors (SCs) by enabling the construction and surface engineering of novel electrode materials. This review aims to present a comprehensive outlook on the development, achievements, and design of advanced electrodes involving the application of ALD for realizing high-performance SCs to date, as organized in several sections of this paper. Specifically, this review focuses on understanding the influence of ALD parameters on the electrochemical performance and discusses the ALD of nanostructured electrochemically active electrode materials on various templates for SCs. It examines the influence of ALD parameters on electrochemical performance and highlights ALD's role in passivating electrodes and creating 3D nanoarchitectures. The relationship between synthesis procedures and SC properties is analyzed to guide future research in preparing materials for various applications. Finally, it is concluded by suggesting the directions and scope of future research and development to further leverage the unique advantages of ALD for fabricating new materials and harness the unexplored opportunities in the fabrication of advanced-generation SCs.

Micron-sized H2MoO3/PANI for superfast proton batteries in frozen electrolyte through Grotthuss mechanism

Aqueous proton battery is considered as a promising candidate for the electrochemical energy storage system with the merits of safety, environmental benignity, fast kinetics and low cost. The realization of these advantages relies on the development of suitable and easy-access electrode materials. Herein, micron-sized H2MoO3/Polyaniline (PANI) is developed as a high-rate and stable anode material in proton battery. Contrary to the pseudocapacitive nature of most anode materials, the H2MoO3/PANI presents diffusion-controlled charge storage mechanism with both high capacity and high rate-capability. The H2MoO3/PANI electrode shows a rather high capacity of 268.2 mAh g-1 at 1.0 A g-1, and a surprisingly high rate-capability with ∼50% capacity retention even at an extremely high current density of 200.0 A g-1. Detailed analyses demonstrate the Grotthuss mechanism of ultrafast proton conduction in H2MoO3/PANI. The constructed proton full cell based on H2MoO3/PANI delivers a high energy density of 42.1 Wh kg-1 at 800.0 W kg-1. Impressively, the proton full cell shows fast proton transportation even in the frozen electrolyte, and ∼70% of the room temperature capacity is retained at -20 °C. These excellent proton storage behaviors provide insights into the practical applications of micron-sized electrode materials in proton batteries at low temperatures.

Conductive phthalocyanine-based porous organic polymer as sensing platform for rapid determination of vanillin

Vanillin (Van) is widely utilized in processed foods and medicines for its appealing scent and multiple therapeutic benefits. However, its overconsumption poses a risk to public health, making its quantification essential for ensuring food and medicine safety and quality. This study introduces a stable and conductive phthalocyanine-based porous organic polymer (NiPc-CC POP), synthesized through a straightforward electrophilic substitution of nickel tetra-amine phthalocyanine (NiTAPc) with cyanuric chloride (CC). Appropriate characterization techniques were employed to determine the morphologies and structures of the synthesized materials. Furthermore, the NiPc-CC POP was applied to devise a sensitive Van detection method. Leveraging the high electrocatalytic activity of NiPc-CC POP toward Van oxidation, a linear response of 0.15-32 μmol L-1 was achieved, along with an exceptional detection limit of 0.10 μmol L-1. The sensor demonstrated high selectivity and stability. Samples of human serum and tablets spiked with Van were analyzed, yielding satisfactory recoveries. Consequently, this work contributes to the advancement of sensitive detection platforms for Van at minimal concentrations.

Thermally Induced Persistent Covalent-Organic Frameworks Radicals

Persistent covalent-organic framework (COF) radicals hold important applications in magnetics and spintronics; however, their facile synthesis remains a daunting challenge. Here, three p-phenylenediacetonitrile-based COFs (named CityU-4, CityU-5, and CityU-6) were synthesized. Upon heat treatment (250 °C for CityU-4 and CityU-5 or 220 °C for CityU-6), these frameworks were brought into their persistent radical forms (no obvious changes after at least one year), together with several observable factors, including color changes, red-shifted absorption, the appearance of electron spin resonance (ESR) signals, and detectable magnetic susceptibility. The theoretical simulation suggests that after heat treatment, lower total energy and nonzero spin density are two main factors to guarantee persistent COFs radicals and polarized spin distributions. This work provides an efficient method for the preparation of persistent COF radicals with promising potentials.

Mobile energy storage technologies for boosting carbon neutrality

Carbon neutrality calls for renewable energies, and the efficient use of renewable energies requires energy storage mediums that enable the storage of excess energy and reuse after spatiotemporal reallocation. Compared with traditional energy storage technologies, mobile energy storage technologies have the merits of low cost and high energy conversion efficiency, can be flexibly located, and cover a large range from miniature to large systems and from high energy density to high power density, although most of them still face challenges or technical bottlenecks. In this review, we provide an overview of the opportunities and challenges of these emerging energy storage technologies (including rechargeable batteries, fuel cells, and electrochemical and dielectric capacitors). Innovative materials, strategies, and technologies are highlighted. Finally, the future directions are envisioned. We hope this review will advance the development of mobile energy storage technologies and boost carbon neutrality.

Flexible All-Solid-State Asymmetric Supercapacitors Based on PPy-Decorated SrFeO3-δ Perovskites on Carbon Cloth

The defective structure and high oxygen vacancy concentration of SrFeO3-δ perovskite enable fast ion-electron transport, but its low conductivity still hinders the high electrochemical performance. Herein, to enhance the conductivity of SrFeO3-δ-based electrodes, polypyrrole-modified SrFeO3-δ perovskite on carbon cloth (PPy@SFO@CC) has been successfully fabricated by electrodeposition of polypyrrole (PPy) on the surface of SFO@CC. The optimal PPy700@SFO@CC electrode exhibits a specific capacitance of 421 F g-1 at 1 A g-1. It was found that the outside PPy layer not only accelerates the electron transport and ion diffusion but also creates more oxygen vacancies in SrFeO3-δ, enhancing the charge storage performance significantly. Moreover, the NiCo2O4@CC//PPy700@SFO@CC device maintains a specific capacitance of 63.6% after 3000 cycles, which is ascribed to the weak adhesion forces between the active materials and carbon cloth. Finally, the all-solid-state flexible supercapacitor NiCo2O4@CC//PPy700@SFO@CC is constructed with PVA-KOH as the solid electrolyte, delivering an energy density of 16.9 W h kg-1 at a power density of 984 W kg-1. The flexible supercapacitor retains 69% of its specific capacitance after 1000 bending and folding times, demonstrating a certain degree of foldability. The present study opens new avenues for perovskite oxide-based flexible all-solid-state supercapacitors.

Unusual 3,4-Oxidative Coupling Polymerization on 1,2,5-Trisubstituted Pyrroles for Novel Porous Organic Polymers

Porous organic polymers (POPs) have demonstrated promising task-specific applications due to their structure designability and thus functionality. Herein, an unusual 3,4-polymerization on 1,2,5-trisubstituted pyrroles has been developed to give linear polypyrrole-3,4 in high efficiency, with Mn of 20000 and PDI of 1.7. This novel polymerization technique was applied to prepare a series of polypyrrole-based POPs (PY-POP-1-4), which exhibited high BET surface areas (up to 762 m2 g-1) with a meso-micro-supermicro hierarchically porous structure. Furthermore, PY-POPs were doped in the mixed matrix membranes based on the polysulfone matrix to enhance the gas permeability and gas pair selectivity, with H2/N2 selectivity up to 84.6 and CO2/CH4 and CO2/N2 selectivity up to 46.8 and 39.6.

Constructing Interconnected Microporous Structures in Carbon by Homogeneous Activation as a Sustainable Electrode Material for High-Performance Supercapacitors

Microporous carbon attracts attention as an electrode material for supercapacitors. However, a large number of deep and distorted mesoporous and macroporous structures are usually created by non-uniform etching, resulting in underutilized internal space. Homogeneous activation has been considered by researchers as a necessary condition for the formation of interconnected microporous structures in carbon materials. Herein, a simple strategy of hydrothermal introduction of defects followed by homogeneous activation for the preparation of microporous carbon was developed for the synthesis of electrode materials for high-performance supercapacitors. The optimized sample with defect-enriched microporous structure and large specific surface area has a specific capacity of 315 F g-1 (1 A g-1) in KOH solution, and the assembled symmetric supercapacitor achieves a high energy density of 7.3 Wh kg-1 at a power density of 250 W kg-1. This work is interesting because it not only demonstrates that rational design of electrode materials is important to boost the performance of supercapacitors, but also provides inspiration for the design of efficient supercapacitors in the future.

Dendritic Solid Polymer Electrolytes: A New Paradigm for High-Performance Lithium-Based Batteries

Li-ions battery is widely used and recognized, but its energy density based on organic electrolytes has approached the theoretical upper limit, while the use of organic electrolytes also brings some safety hazards (leakage and flammability). Polymer electrolytes (PEs) are expected to fundamentally solve the safety problem and improve energy density. Therefore, Li-ions battery based on solid PE has become a research hotspot in recent years. However, low ionic conductivity and poor mechanical properties, as well as a narrow electrochemical window limit its further development. Dendritic PEs with unique topology structure has low crystallinity, high segmental mobility, and reduced chain entanglement, providing a new avenue for designing high-performance PEs. In this review, the basic concept and synthetic chemistry of dendritic polymers are first introduced. Then, this story will turn to how to balance the mechanical properties, ionic conductivity, and electrochemical stability of dendritic PEs from synthetic chemistry. In addition, accomplishments on dendritic PEs based on different synthesis strategies and recent advances in battery applications are summarized and discussed. Subsequently, the ionic transport mechanism and interfacial interaction are deeply analyzed. In the end, the challenges and prospects are outlined to promote further development in this booming field.

From energy storage to pathogen eradication: unveiling the antibacterial and antiviral capacities of flexible solid-state carbon cloth supercapacitors

With the emergence of deadly viral and bacterial infections, preventing the spread of microorganisms on surfaces has gained ever-increasing importance. This study investigates the potential of solid-state supercapacitors as antibacterial and antiviral devices. We developed a low-cost and flexible carbon cloth supercapacitor (CCSC) with highly efficient antibacterial and antiviral surface properties. The CCSC comprised two parallel layers of carbon cloth (CC) electrodes assembled in a symmetric, electrical double-layer supercapacitor structure that can be charged at low potentials between 1 to 2 V. The optimized CCSC exhibited a capacitance of 4.15 ± 0.3 mF cm-2 at a scan rate of 100 mV s-1, high-rate capability (83% retention of capacitance at 100 mV s-1 compared to its value at 5 mV s-1), and excellent electrochemical stability (97% retention of the initial capacitance after 1000 cycles). Moreover, the CCSC demonstrated outstanding flexibility and retained its full capacitance even when bent at high angles, making it suitable for wearable or flexible devices. Using its stored electrical charge, the charged CCSC disinfects bacteria effectively and neutralizes viruses upon surface contact with the positive and negative electrodes. The charged CCSC device yielded a 6-log CFU reduction of Escherichia coli bacterial inocula and a 5-log PFU reduction of HSV-1 herpes virus. Antibacterial and antiviral carbon cloth supercapacitors represent a promising platform technology for various applications, including electronic textiles and electronic skins, health monitoring or motion sensors, wound dressings, personal protective equipment (e.g., masks) and air filtration systems.

A Polyzwitterion-Mediated Polymer Electrolyte with High Oxidative Stability for Lithium-Metal Batteries

To achieve high-performance solid-state lithium-metal batteries (SSLMBs), solid electrolytes with high ionic conductivity, high oxidative stability, and high mechanical strength are necessary. However, balancing these characteristics remains dramatically challenging and is still not well addressed. Herein, a simple yet effective design strategy is presented for the development of high-performance polymer electrolytes (PEs) by exploring the synergistic effect between dynamic H-bonded networks and conductive zwitterionic nanochannels. Multiple weak intermolecular interactions along with ample nanochannels lead to high oxidative stability (over 5 V), improved mechanical properties (strain of 1320%), and fast ion transport (ionic conductivity of 10-4 S cm-1 ) of PEs. The amphoteric ionic functional units also effectively regulate the lithium ion distribution and confine the anion transport to achieve uniform lithium ion deposition. As a result, the assembled SSLMBs exhibit excellent capacity retention and long-term cycle stability (average Coulombic efficiency: 99.5%, >1000 cycles with LiFePO4 cathode; initial capacity: 202 mAh g-1 , average Coulombic efficiency: 96%, >230 cycles with LiNi0.8 Co0.1 Mn0.1 O2 cathode). It is exciting to note that the corresponding flexible cells can be cycled stably and can withstand severe deformation. The resulting polyzwitterion-mediated PE therefore offers great promise for the next-generation safe and high-energy-density flexible energy storage devices.

Structurally-stable Mg-Co-Ni LDH grown on reduced graphene by ball-milling and ion-exchange for highly-stable asymmetric supercapacitor

As an electrode for energy storage, the inherently poor conductivity of metal hydroxides (MHs) can be improved by in situ growth of MHs on conductive carbon based substrates so that their performances on energy storage could be enhanced to a high level. However, the incompatibility of hydrophilic component (metal hydroxides) and hydrophobic counterpart (carbon based materials) makes it difficult to be accomplished. Herein, we presented a scalable and easy-operated strategy by ball-milling combined with ion-exchange technique to grow Mg-Co-Ni LDH (layered double hydroxides) on reduced graphene, in which ball-milling was utilized to disperse the staring material of magnesium acetate on graphene oxide (GO) to obtain the composite of Mg(Ac)₂/GO. The composite can be in situ transformed to MgO/reduced grapheme (rG) by following heat treatment. While, the ion-exchange reaction could enables the in situ growth of Mg-Co-Ni LDHs on the reduced graphene. The derived products (denoted as Mg-Co-Ni LDH/rG-x) owns nanosheet morphology, surface area of 59-115 m²/g, homogenous elements distribution. As electrode for supercapacitor, the maximum capacitance of 1204F/g@1.0 A/g was achieved and the corresponding asymmetric supercapacitor device shows a large energy density of 44.3 Wh/kg@800 W/kg. Particularly, a superlong cycling stability with 90.5% capacitance retention of the first cycle was attained after continuous charge/discharge for 20 000 cycles at current density of 5.0 A/g, promising great potential for practical energy storage application. The present strategy is simple and scalable that can be widely applied to the synthesis of various hydroxides/oxides or multi-component hydroxides/oxides on carbon substrates forming a composite structure, thus offers a great potential for broad application areas including catalysis, adsorption, energy storage, etc.

Nitrogen-Rich Porous Organic Polymers from an Irreversible Amine-Epoxy Reaction for Pd Nanocatalyst Carrier

Nitrogen-rich porous organic polymers were fabricated through a nonreversible ring-opening reaction from polyamines and polyepoxides (PAEs). The epoxide groups reacted with both primary and secondary amines provided by the polyamines at different epoxide/amine ratios with polyethylene glycol as the solvent to form the porous materials. Fourier-transform infrared spectroscopy confirmed the occurrence of ring opening between the polyamines and polyepoxides. The porous structure of the materials was confirmed through N₂ adsorption-desorption data and scanning electron microscopy images. The polymers were found to possess both crystalline and noncrystalline structures, as evidenced by X-ray diffraction and high-resolution transmission electron microscopy (HR-TEM) results. The HR-TEM images revealed a thin, sheet-like layered structure with ordered orientations, and the lattice fringe spacing measured from these images was consistent with the interlayer of the PAEs. Additionally, the selected area electron diffraction pattern indicated that the PAEs contained a hexagonal crystal structure. The Pd catalyst was fabricated in situ onto the PAEs support by the NaBH₄ reduction of the Au precursor, and the size of the nano-Pd was about 6.9 nm. The high nitrogen content of the polymer backbone combined with Pd noble nanometals resulted in excellent catalytic performance in the reduction of 4-nitrophenol to 4-aminophenol.

High-energy-density flexible graphene-based supercapacitors enabled by atypical hydroquinone dimethyl ether

Supercapacitor is an electrochemical energy-storage technology that can meet the green and sustainable energy needs of the future. However, a low energy density was a bottleneck that limited its practical application. To overcome this, we developed a heterojunction system composed of two-dimensional (2D) graphene and hydroquinone dimethyl ether- an atypical redox-active aromatic ether. This heterojunction displayed a large specific capacitance (Cs) of 523 F g^-1 at 1.0 A g^-1, as well as good rate capability and cycling stability. When assembled in symmetric and asymmetric two-electrode configuration, respectively, supercapacitors can work in voltage windows of 0 ∼ 1.0 V and 0 ∼ 1.6 V, accordingly, and exhibited attractive capacitive characteristics. The best device can deliver an energy density of 32.4 Wh Kg^-1 and a power density of 8000 W Kg^-1, and suffered a small capacitance degradation. Additionally, the device showed low self-discharge and leakage current behaviors during long time. This strategy may inspire exploration of aromatic ether electrochemistry and pave a way to develop electrical double-layer capacitance (EDLC)/pseudocapacitance heterojunctions to boost the critical energy density.

Insight into Electrochemical Performance of Nitrogen-Doped Carbon/NiCo-Alloy Active Nanocomposites

Nanocomposites containing Ni or Co or NiCo alloy and nitrogen-doped carbon with diverse ratios have been prepared and utilized as active elements in supercapacitors. The atomic contents of nitrogen, nickel, and cobalt have been adjusted by the supplement amount of Ni and Co salts. In virtue of the excellent surface groups and rich redox active sites, the NC/NiCo active materials exhibit superior electrochemical charge-storage performances. Among these as-prepared active electrode materials, the NC/NiCo1/1 electrode performs better than other bimetallic/carbon electrodes and pristine metal/carbon electrodes. Several characterization methods, kinetic analyses, and nitrogen-supplement strategies determine the specific reason for this phenomenon. As a result, the better performance can be ascribed to a combination of factors including the high surface area and nitrogen content, proper Co/Ni ratio, and relatively low average pore size. The NC/NiCo electrode delivers a maximum capacity of 300.5 C g^-1 and superior capacity retention of 92.30% after 3000 unceasing charge-discharge cycles. After assembling it into the battery-supercapacitor hybrid device, a high energy density of 26.6 Wh kg^-1 (at 412 W kg^-1 ) is achieved, comparable to the recent reports. Furthermore, this device can also power four light-emitting-diode (LED) demos, suggesting the potential practicability of these N-doped carbon compositing with bimetallic materials.

Rational Design of Bifunctional Microporous Organic Polymers Containing Anthracene and Triphenylamine Units for Energy Storage and Biological Applications

In this study, we synthesized two conjugated microporous polymers (CMPs), An-Ph-TPA and An-Ph-Py CMPs, using the Suzuki cross-coupling reaction. These CMPs are organic polymers with p-conjugated skeletons and persistent micro-porosity and contain anthracene (An) moieties linked to triphenylamine (TPA) and pyrene (Py) units. We characterized the chemical structures, porosities, thermal stabilities, and morphologies of the newly synthesized An-CMPs using spectroscopic, microscopic, and N₂ adsorption/desorption isotherm techniques. Our results from thermogravimetric analysis (TGA) showed that the An-Ph-TPA CMP displayed better thermal stability with T_d10 = 467 °C and char yield of 57 wt% compared to the An-Ph-Py CMP with T_d10 = 355 °C and char yield of 54 wt%. Furthermore, we evaluated the electrochemical performance of the An-linked CMPs and found that the An-Ph-TPA CMP had a higher capacitance of 116 F g^-1 and better capacitance stability of 97% over 5000 cycles at 10 A g^-1. In addition, we assessed the biocompatibility and cytotoxicity of An-linked CMPs using the MTT assay and a live/dead cell viability assay and observed that they were non-toxic and biocompatible with high cell viability values after 24 or 48 h of incubation. These findings suggest that the An-based CMPs synthesized in this study have potential applications in electrochemical testing and the biological field.

Macrocycle-Based Covalent Organic Frameworks

Macrocycles with well-defined cavities and the ability to undergo supramolecular interactions are classical materials that have played an essential role in materials science. However, one of the most substantial barriers limiting the utilization of macrocycles is their aggregation, which blocks the active regions. Among many attempted strategies to prevent such aggregation, installing macrocycles into covalent organic frameworks (COFs), which are porous and stable reticular networks, has emerged as an ideal solution. The resulting macrocycle-based COFs (M-COFs) preserve the macrocycles' unique activities, enabling applications in various fields such as single-atom catalysis, adsorption/separation, optoelectronics, phototherapy, and structural design of forming single-layered or mechanically interlocked COFs. The resulting properties are unmatchable by any combination of macrocycles with other substrates, opening a new chapter in advanced materials. This review focuses on the latest progress in the concepts, synthesis, properties, and applications of M-COFs, and presents an in-depth outlook on the challenges and opportunities in this emerging field.

Design strategies of covalent organic framework-based electrodes for supercapacitor application

Supercapacitors (SCs) have been recognized as a promising electrochemical energy storage (EES) device, thanks to their high-power density, long lifespan, fast charge-discharge capability, and eco-friendliness. The breakthrough of electrode materials that determine the electrochemical performance of SCs is urgently desired. Covalent organic frameworks (COFs), an emerging and burgeoning class of crystalline porous polymeric materials, have been found to have huge potential for application in EES devices by virtue of their unique properties including atomically adjustable structures, robust and tunable skeletons, well-defined and open channels, high surface areas, etc. In this feature article, we aim at summarizing the design strategies of COF-based electrode materials for SCs based on the representative advances. The current challenges and future perspectives of COFs for SC application are highlighted as well.

In Situ Raman Study of Voltage Tolerance Up to 2.2 V of Ionic Liquid Analogue Supercapacitor Electrolytes Immune to Water Adsorption Conferred by Amphoteric Imidazole Additives

Ionic liquid analogues (ILAs) are promising electrolytes for supercapacitors due to their low cost and considerable voltage (>2.0 V). However, the voltage is <1.1 V for water-adsorbed ILAs. Herein for the first time, an amphoteric imidazole (IMZ) additive is reported to address this concern by reconfiguring the solvent shell of ILAs. Addition of only 2 wt % IMZ increases the voltage from 1.1 to 2.2 V, with an increase in capacitance from 178 to 211 F g^-1 and an increase in energy density from 6.8 to 32.6 Wh kg^-1. In situ Raman reveals that the strong H-bonds formed by IMZ with completive ligands 1,3-propanediol and water induce a reversal of the polarity of the solvent shells, suppressing absorbed water electrochemical activity and thus increasing the voltage. This study solves the problem of low voltage for water-adsorbed ILAs and reduces the equipment cost of ILA-based supercapacitor assembly (e.g., assembly in air without a glovebox).

Trinitroaromatic Salts as High-Energy-Density Organic Cathode Materials for Li-Ion Batteries

Even though organic molecules with designed structures can be assembled into high-capacity electrode materials, only limited functional groups such as -C═O and -C═N- could be designed as high-voltage cathode materials with enough high capacity. Here, we propose a common chemical raw material, trinitroaromatic salt, to have promising potential to develop organic cathode materials with high discharge voltage and capacity through a strong delocalization effect between -NO₂ and aromatic ring. Our first-principles calculations show that electrochemical reactions of trinitroaromatic potassium salt C₆H₂(NO₂)₃OK are a 6-electron charge-transfer process, providing a high discharge capacity of 606 mAh g^-1 and two voltage plateaus of 2.40 and 1.97 V. Electronic structure analysis indicates that the discharge process from C₆H₂(NO₂)₃OK to C₆H₂(NO₂Li₂)₃OK stabilizes oxidized [C₆]ⁿ⁺ to achieve a stable conjugated structure through electron delocalization from -NO₂ to [C₆]ⁿ⁺. The ordered layer structure C₆H₂(NO₂)₃OK can provide large spatial pore channels for Li-ion transport, achieving a high ion diffusion coefficient of 3.41 × 10^-6 cm² s^-1.

Ion-in-Conjugation: A Promising Concept for Multifunctional Organic Semiconductors

Most organic semiconductors (OSCs) consist of conjugated skeletons with flexible peripheral chains. Their weak intermolecular interactions from dispersion and induction forces result in environmental susceptibilities and are unsuitable for many multifunctional applications where direct exposure to external environments is unavoidable, such as gas absorption, chemical sensing, and catalysis. To exploit the advantages of inorganic semiconductors in OSCs, ion-in-conjugation (IIC) materials are proposed. An IIC material refers to any conjugated material (molecules, polymers, and crystals) in Kekule's structural formula containing stoichiometric ionic states in its conjugated backbone in the electronic ground state. In this review, the definitions, structures, synthesis, properties, and applications of IIC materials are described briefly. Four types of IIC material, including zwitterionic conjugated molecules/polymers, conjugated ionic dyes, π-d conjugated molecules and polymers, and coordinatively doped polymers, are reported. Their applications in gas sensing, humidity sensing, resistive memory devices, and thermal/photo-/electro-catalysis are demonstrated. The challenges and opportunities for future research are also discussed. It is expected that this work will inspire the design of new organic electronic information materials.

Jujube Shell Based-Porous Carbon Composites Double-Doped by MnO2 and Ti3C2Tx: The Effect of Double Pseudocapacitive Doping on Electrochemical Properties

In this study, manganese-containing porous carbon was synthesized from jujube shells by two-step carbonization and activation and was then covered with Ti₃C₂Tx to obtain double-doped biomass composites. In order to improve the interfacial properties (surface tension and wettability) between Ti₃C₂Tx and porous carbon, the effects of two media (deionized water and acetone solution) on the electrochemical properties of the composites were compared. The acetone solution changed the surface rheology of Ti₃C₂Tx and porous carbon, and the decreased surface tension and the increased wettability contributed to the ordered growth of 2D-Ti₃C₂Tx on the surface of the porous carbon. Raman analysis shows the relatively higher graphitization degree of JSPC&Ti₃C₂Tx (acetone). Compared with JSPC&Ti₃C₂Tx, JSPC&Ti₃C₂Tx (acetone) can maintain better rectangle-like properties even at a higher scanning rate. Under the effect of the acetone solution, the pseudocapacitive ratio of JSPC&Ti₃C₂Tx (acetone) increased from 10.1% to 30.7%. At the current density of 0.5 A/g, the specific capacitance of JSPC&Ti₃C₂Tx (acetone) achieved 96.83 F/g, and the specific capacitance of 58.17 F/g was maintained even at the high current density (10 A/g), which shows excellent magnification. Under the condition of the current density of 10 A/g, JSPC&Ti₃C₂Tx (acetone) can obtain a power density of 52,000 W/kg while maintaining an energy density of 8.74 Wh/kg. After 2000 cycles, the symmetrical button battery assembled with this material can still have a capacitance retention rate of more than 90%. This method realized the deep utilization of green and low-cost raw materials by using biomass as the precursor of composite materials and promoted the further development of carbon-based supercapacitor electrode materials.

Covalent Organic Frameworks with Ionic Liquid-Moieties (ILCOFs): Structures, Synthesis, and CO2 Conversion

CO₂, an acidic gas, is usually emitted from the combustion of fossil fuels and leads to the formation of acid rain and greenhouse effects. CO₂ can be used to produce kinds of value-added chemicals from a viewpoint based on carbon capture, utilization, and storage (CCUS). With the combination of unique structures and properties of ionic liquids (ILs) and covalent organic frameworks (COFs), covalent organic frameworks with ionic liquid-moieties (ILCOFs) have been developed as a kind of novel and efficient sorbent, catalyst, and electrolyte since 2016. In this critical review, we first focus on the structures and synthesis of different kinds of ILCOFs materials, including ILCOFs with IL moieties located on the main linkers, on the nodes, and on the side chains. We then discuss the ILCOFs for CO₂ capture and conversion, including the reduction and cycloaddition of CO₂. Finally, future directions and prospects for ILCOFs are outlined. This review is beneficial for academic researchers in obtaining an overall understanding of ILCOFs and their application of CO₂ conversion. This work will open a door to develop novel ILCOFs materials for the capture, separation, and utilization of other typical acid, basic, or neutral gases such as SO₂, H₂S, NOx, NH₃, and so on.

Nanoconfined Space: Revisiting the Charge Storage Mechanism of Electric Double Layer Capacitors

The electric double layer capacitor (EDLC) has been recognized as one of the most appealing electrochemical energy storage devices. Nanoporous materials with relatively high specific surface areas are generally used as the electrode materials for electric double layer capacitors (EDLCs). The past decades have witnessed anomalous phenomena of EDLCs under nanoconfined space, which to a large degree doubt the conventional recognition. However, there are currently still no deep insights and consensus on the mechanism of these striking discoveries. In this Perspective, we start with a brief introduction to contextualize the significance of EDLCs, especially with electrode materials of nanoconfined space. Next, we briefly review the landmark studies in light of the charge storage mechanism of EDLCs, mainly focusing on the study of nanoporous materials for EDLCs. Subsequently, we reexamine the basic concepts under nanoconfined space and some representative in situ characterization techniques applied to understand the charge storage mechanism of EDLCs. Finally, we provide general conclusions and insights into the future research directions in the field of EDLCs.