Pseudocapacitance: From Fundamental Understanding to High Power Energy Storage Materials

Optimizing the Ultrashort Laser Pulses for In Situ Nanostructure Generation Technique for High-Performance Supercapacitor Electrodes Using Artificial Neural Networks and Simulated Annealing Algorithms

Transition metals (TMs) are being investigated as electrodes for pseudocapacitors, where an oxide layer is necessary to allow for rapid redox reactions. In this work, we utilized an in situ, rapid, binder-free, and green method for the fast fabrication of pseudocapacitor electrodes called ultrashort laser pulses for in situ nanostructure generation (ULPING) to form oxide layers on a titanium sheet. By utilizing this fabrication technique on a titanium sheet, a specific areal capacitance of 0.3579 mF cm^-2 was achieved at a current density of 0.25 mA cm^-2. However, the laser fabrication parameters were selected experimentally and resulted in low performance of pseudocapacitors. Therefore, one of the main objectives of this study was to find the optimal laser fabrication parameters to achieve the highest specific areal capacitance. A large dataset was generated to find the relationship between the laser fabrication parameters and the electrochemical behavior performance (impedance and specific areal capacitance) of the fabricated electrodes by using an artificial neural network (ANN). We used an optimization algorithm (simulated annealing-SA) to overlook the trained ANN model as a black box and try to maximize the objective function, which in our case is a specific capacitance value, to find the most optimal laser fabrication parameters. Using SA, optimal laser fabrication parameters were found, which increased the specific areal capacitance to 0.9999 mF cm^-2 at a current density of 0.25 mA cm^-2. The results demonstrated that the conducted study has the potential to introduce effective techniques for utilizing ULPING to produce nanoscale structures on TMs. These structures have the potential to be employed as electrodes in pseudocapacitors. Additionally, the research underscores the significance of employing data-driven approaches in electrode design procedures.

Electrochemical properties and facile preparation of hollow porous V2O5 microspheres for lithium-ion batteries

Vanadium pentoxide (V₂O₅) has shown great potential to be used in lithium-ion batteries (LIBs), but it has limited applications because it has cycle instability and poor rate capability, and its lithiation mechanism is not well understood. In this work, hollow porous V₂O₅ microspheres (HPVOM) were obtained by a facile poly(vinylpyrrolidone) and ethylene glycol-assisted soft-template solvothermal method. Half cells with HPVOM exhibited good capacity, rate capability, and stability, delivering 407.9 mAh g^-1 at 1.0 A g^-1 after 700 cycles. Furthermore, a LiFePO₄/HPVOM full cell had a discharge capacity of 109.9 mAh g^-1 after 150 cycles at 0.1 A g^-1. Using an equivalent circuit model (ECM) and distribution of relaxation times (DRT), we found that the charge-transfer (including the solid-state interface resistance) and bulk resistances varied regularly with the charge/discharge state, while the electrolyte resistance was largely maintained. The bulk resistance finally vanished, indicative of dynamic activation. The method used to prepare the hollow microspheres, as well as the display of electrode kinetics via ECM and DRT, could be broadly applied in developing efficient electrodes for LIBs.

Solvent-induced structural regulation over Ni2P/CNT hybrids towards boosting the performance of supercapacitors

Although nickel-based phosphides have attracted increasing attention due to their good theoretical specific capacity, the poor rate capability weakness their advantage in electrochemical energy storage. It is, however, challenging to improve these issues by only adjusting composition. Here, we employ a synergistic strategy, both hybridizing with highly conductive materials and regulating morphology, to enhance the electrochemical performance of Ni₂P. Based on solvent-induced effects, flower/rod-like [CH₃NH₃][Ni(HCOO)₃] precursors hybridized with CNTs are prepared and then employed as templates to construct flower/rod-like Ni₂P/CNT hybrids via a gas-solid phosphorization method. Benefiting from the synergistic advantages of both structure and components, the flower-like Ni₂P/CNT hybrid, as an electrode materials for supercapacitor, exhibit outstanding specific capacitance of up to 1480 F g^-1 at 1 A g^-1, as well as improved rate capability. Additionally, the assembled asymmetric supercapacitor (Ni₂P/CNTs//AC, ASC) delivers a high capacitance retention of up to 83.5% after 5000 cycles at 10 A g^-1, and an expected energy density of 25.2 W h kg^-1 at a power density of 749.8 W kg^-1.

Engineering the First Coordination Shell of Single Zn Atoms via Molecular Design Strategy toward High-Performance Sodium-Ion Hybrid Capacitors

Atomically dispersed Zn moieties are efficient active sites for accelerating the electrode kinetics of carbons for sodium-ion hybrid capacitors (SIHCs), but the low utilization and symmetric configuration of Zn single-atom greatly hamper the Na ion storage capability. Herein, a molecular design strategy is employed to synthesize high-density Zn single atoms with asymmetric Zn-N₃ S coordination embedded in nitrogen/sulfur codoped carbon (Zn-N₃ S-NSC). The key to this strategy lies in the Zn power-catalyzed condensation of trithiocyanuric acid molecules to generate S-doped g-C₃ N₄ , which can in situ coordinate with Zn sources to form Zn-N₃ S moieties during pyrolysis. By virtue of the highly exposed Zn-N₃ S moieties, Zn-N₃ S-NSC presents ultrahigh reactivity, efficient electron transfer, and decreased ion diffusion barriers for SIHCs, rendering an impressive energy density of 215 Wh kg^-1 and a maximum power density of 15625 W kg^-1 . Moreover, the pouch cell displays a high capacity of 279 mAh g^-1 after 4000 cycles. This work provides a new avenue for the regulation of the coordination configuration of single metal atoms in carbons toward high-performance electrochemical energy technologies at the molecular level.

Safe Etching Route of Nb2SnC for the Synthesis of Two-Dimensional Nb2CTx MXene: An Electrode Material with Improved Electrochemical Performance

In this study, low-temperature synthesis of a Nb₂SnC non-MAX phase was carried out via solid-state reaction, and a novel approach was introduced to synthesize 2D Nb₂CT_x MXenes through selective etching of Sn from Nb₂SnC using mild phosphoric acid. Our work provides valuable insights into the field of 2D MXenes and their potential for energy storage applications. Various techniques, including XRD, SEM, TEM, EDS, and XPS, were used to characterize the samples and determine their crystal structures and chemical compositions. SEM images revealed a two-dimensional layered structure of Nb₂CT_x, which is consistent with the expected morphology of MXenes. The synthesized Nb₂CT_x showed a high specific capacitance of 502.97 Fg^-1 at 1 Ag^-1, demonstrating its potential for high-performance energy storage applications. The approach used in this study is low-cost and could lead to the development of new energy storage materials. Our study contributes to the field by introducing a unique method to synthesize 2D Nb₂CT_x MXenes and highlights its potential for practical applications.

Electrolyte-Wettability Issues and Challenges of Electrode Materials in Electrochemical Energy Storage, Energy Conversion, and Beyond

The electrolyte-wettability of electrode materials in liquid electrolytes plays a crucial role in electrochemical energy storage, conversion systems, and beyond relied on interface electrochemical process. However, most electrode materials do not have satisfactory electrolyte-wettability for possibly electrochemical reaction. In the last 30 years, there are a lot of literature have directed at exploiting methods to improve electrolyte-wettability of electrodes, understanding basic electrolyte-wettability mechanisms of electrode materials, exploring the effect of electrolyte-wettability on its electrochemical energy storage, conversion, and beyond performance. This review systematically and comprehensively evaluates the effect of electrolyte-wettability on electrochemical energy storage performance of the electrode materials used in supercapacitors, metal ion batteries, and metal-based batteries, electrochemical energy conversion performance of the electrode materials used in fuel cells and electrochemical water splitting systems, as well as capacitive deionization performance of the electrode materials used in capacitive deionization systems. Finally, the challenges in approaches for improving electrolyte-wettability of electrode materials, characterization techniques of electrolyte-wettability, as well as electrolyte-wettability of electrode materials applied in special environment and other electrochemical systems with electrodes and liquid electrolytes, which gives future possible directions for constructing interesting electrolyte-wettability to meet the demand of high electrochemical performance, are also discussed.

An integrated solar battery based on a charge storing 2D carbon nitride

Solar batteries capable of harvesting sunlight and storing solar energy present an attractive vista to transition our energy infrastructure into a sustainable future. Here we present an integrated, fully earth-abundant solar battery based on a bifunctional (light absorbing and charge storing) carbon nitride (K-PHI) photoanode, combined with organic hole transfer and storage materials. An internal ladder-type hole transfer cascade via a transport layer is used to selectively shuttle the photogenerated holes to the PEDOT:PSS cathode. This concept differs from previous designs such as light-assisted battery schemes or photocapacitors and allows charging with light during both electrical charge and discharge, thus substantially increasing the energy output of the cell. Compared to battery operation in the dark, light-assisted (dis)charging increases charge output by 243%, thereby increasing the electric coulombic efficiency from 68.3% in the dark to 231%, leading to energy improvements of 94.1% under illumination. This concept opens new vistas towards compact, highly integrated devices based on multifunctional, carbon-based electrodes and separators, and paves the way to a new generation of earth-abundant solar batteries.

Utilization of compressible hydrogels as electrolyte materials for supercapacitor applications

Utilization of CoO@Co₃O₄-x-Ag (x denotes 1, 3, and 5 wt% of Ag) nanocomposites as supercapacitor electrodes is the main aim of this study. A new low-temperature wet chemical approach is proposed to modify the commercial cobalt oxide material with silver nanoparticle (NP) balls of size 1-5 nm. The structure and morphology of the as-prepared nanocomposites were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and N₂ adsorption-desorption measurements. Hydrogels known to be soft but stable structures were used here as perfect carriers for conductive nanoparticles such as carbons. Furthermore, hydrogels with a large amount of water in their network can give more flexibility to the system. Fabrication of an electrochemical cell can be achieved by combining these materials with a layer-by-layer structure. The performance characteristics of the cells were examined by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charge discharge (GCD). Cobalt oxide modified with 5 wt% Ag gave the best supercapacitor results, and the cell offers a specific capacitance of ∼38 mF cm^-2 in two-electrode configurations.

Intrinsic Self-Healing Chemistry for Next-Generation Flexible Energy Storage Devices

The booming wearable/portable electronic devices industry has stimulated the progress of supporting flexible energy storage devices. Excellent performance of flexible devices not only requires the component units of each device to maintain the original performance under external forces, but also demands the overall device to be flexible in response to external fields. However, flexible energy storage devices inevitably occur mechanical damages (extrusion, impact, vibration)/electrical damages (overcharge, over-discharge, external short circuit) during long-term complex deformation conditions, causing serious performance degradation and safety risks. Inspired by the healing phenomenon of nature, endowing energy storage devices with self-healing capability has become a promising strategy to effectively improve the durability and functionality of devices. Herein, this review systematically summarizes the latest progress in intrinsic self-healing chemistry for energy storage devices. Firstly, the main intrinsic self-healing mechanism is introduced. Then, the research situation of electrodes, electrolytes, artificial interface layers and integrated devices based on intrinsic self-healing and advanced characterization technology is reviewed. Finally, the current challenges and perspective are provided. We believe this critical review will contribute to the development of intrinsic self-healing chemistry in the flexible energy storage field.

Self-Activation of Inorganic-Organic Hybrids Derived through Continuous Synthesis of Polyoxomolybdate and para-Phenylenediamine Enables Very High Lithium-Ion Storage Capacity

Inorganic-organic hybrid materials with redox-active components were prepared by an aqueous precipitation reaction of ammonium heptamolybdate (AHM) with para-phenylenediamine (PPD). A scalable and low-energy continuous wet chemical synthesis process, known as the microjet process, was used to prepare particles with large surface area in the submicrometer range with high purity and reproducibility on a large scale. Two different crystalline hybrid products were formed depending on the ratio of molybdate to organic ligand and pH. A ratio of para-phenylenediamine to ammonium heptamolybdate from 1 : 1 to 5 : 1 resulted in the compound [C₆ H₁₀ N₂ ]₂ [Mo₈ O₂₆ ] ⋅ 6 H₂ O, while higher PPD ratios from 9 : 1 to 30 : 1 yielded a composition of [C₆ H₉ N₂ ]₄ [NH₄ ]₂ [Mo₇ O₂₄ ] ⋅ 3 H₂ O. The electrochemical behavior of the two products was tested in a battery cell environment. Only the second of the two hybrid materials showed an exceptionally high capacity of 1084 mAh g^-1 at 100 mA g^-1 after 150 cycles. The maximum capacity was reached after an induction phase, which can be explained by a combination of a conversion reaction with lithium to Li₂ MoO₄ and an additional in situ polymerization of PPD. The final hybrid material is a promising material for lithium-ion battery (LIB) applications.

Design and Synthesis of Bisulfone-Linked Two-Dimensional Conjugated Microporous Polymers for CO2 Adsorption and Energy Storage

We have successfully synthesized two types of two-dimensional conjugated microporous polymers (CMPs), Py-BSU and TBN-BSU CMPs, by using the Sonogashira cross-coupling reaction of BSU-Br2 (2,8-Dibromothianthrene-5,5′,10,10′-Tetraoxide) with Py-T (1,3,6,8-Tetraethynylpyrene) and TBN-T (2,7,10,15-Tetraethynyldibenzo[g,p]chrysene), respectively. We characterized the chemical structure, morphology, physical properties, and potential applications of these materials using various analytical instruments. Both Py-BSU and TBN-BSU CMPs showed high thermal stability with thermal decomposition temperatures (Td10) up to 371 °C and char yields close to 48 wt%, as determined by thermogravimetric analysis (TGA). TBN-BSU CMPs exhibited a higher specific surface area and porosity of 391 m2 g−1 and 0.30 cm3 g−1, respectively, due to their large micropore and mesopore structure. These CMPs with extended π-conjugated frameworks and high surface areas are promising organic electroactive materials that can be used as electrode materials for supercapacitors (SCs) and gas adsorption. Our experimental results demonstrated that the TBN-BSU CMP electrode had better electrochemical characteristics with a longer discharge time course and a specific capacitance of 70 F g−1. Additionally, the electrode exhibited an excellent capacitance retention rate of 99.9% in the 2000-cycle stability test. The CO2 uptake capacity of TBN-BSU CMP and Py-BSU CMP were 1.60 and 1.45 mmol g−1, respectively, at 298 K and 1 bar. These results indicate that the BSU-based CMPs synthesized in this study have potential applications in electrical testing and CO2 capture.

Recent Progress of 2D Layered Materials in Water-in-Salt/Deep Eutectic Solvent-Based Liquid Electrolytes for Supercapacitors

Supercapacitors are candidates with the greatest potential for use in sustainable energy resources. Extensive research is being carried out to improve the performances of state-of-art supercapacitors to meet our increased energy demands because of huge technological innovations in various fields. The development of high-performing materials for supercapacitor components such as electrodes, electrolytes, current collectors, and separators is inevitable. To boost research in materials design and production toward supercapacitors, the up-to-date collection of recent advancements is necessary for the benefit of active researchers. This review summarizes the most recent developments of water-in-salt (WIS) and deep eutectic solvents (DES), which are considered significant electrolyte systems to advance the energy density of supercapacitors, with a focus on two-dimensional layered nanomaterials. It provides a comprehensive survey of 2D materials (graphene, MXenes, and transition-metal oxides/dichalcogenides/sulfides) employed in supercapacitors using WIS/DES electrolytes. The synthesis and characterization of various 2D materials along with their electrochemical performances in WIS and DES electrolyte systems are described. In addition, the challenges and opportunities for the next-generation supercapacitor devices are summarily discussed.

The Combined Effects of an External Field and Novel Functional Groups on the Structural and Electronic Properties of TMDs/Ti3C2 Heterostructures: A First-Principles Study

The stacking of Ti₃C₂ with transition metal dihalide (TMDs) materials is an effective strategy to improve the physical properties of a single material, and the tuning of the related properties of these TMDs/Ti₃C₂ heterostructures is also an important scientific problem. In this work, we systematically investigated the effects of an external field and novel functional groups (S, Se, Cl, Br) on the structural and electronic properties of TMDs/Ti₃C₂X₂ heterostructures. The results revealed that the lattice parameters and interlayer distance of TMDs/Ti₃C₂ increased with the addition of functional groups. Both tensile and compressive strain obviously increased the interlayer distance of MoS₂/Ti₃C₂X₂ (X = S, Se, Cl, Br) and MoSe₂/Ti₃C₂X₂ (X = Se, Br). In contrast, the interlayer distance of MoSe₂/Ti₃C₂X₂ (X = S, Cl) decreased with increasing compressive strain. Furthermore, the conductivity of TMDs/Ti₃C₂ increased due to the addition of functional groups (Cl, Br). Strain caused the bandgap of TMDs to narrow, and effectively adjusted the electronic properties of TMDs/Ti₃C₂X₂. At 9% compressive strain, the conductivity of MoSe₂/Ti₃C₂Cl₂ increased significantly. Meanwhile, for TMDs/Ti₃C₂X₂, the conduction band edge (CBE) and valence band edge (VBE) at the M and K points changed linearly under an electric field. This study provides valuable insight into the combined effects of an external field and novel functional groups on the related properties of TMDs/Ti₃C₂X₂.

Largely Pseudocapacitive Two-Dimensional Conjugated Metal-Organic Framework Anodes with Lowest Unoccupied Molecular Orbital Localized in Nickel-bis(dithiolene) Linkages

Although two-dimensional conjugated metal-organic frameworks (2D c-MOFs) provide an ideal platform for precise tailoring of capacitive electrode materials, high-capacitance 2D c-MOFs for non-aqueous supercapacitors remain to be further explored. Herein, we report a novel phthalocyanine-based nickel-bis(dithiolene) (NiS₄)-linked 2D c-MOF (denoted as Ni₂[CuPcS₈]) with outstanding pseudocapacitive properties in 1 M TEABF₄/acetonitrile. Each NiS₄ linkage is disclosed to reversibly accommodate two electrons, conferring the Ni₂[CuPcS₈] electrode a two-step Faradic reaction with a record-high specific capacitance among the reported 2D c-MOFs in non-aqueous electrolytes (312 F g^-1) and remarkable cycling stability (93.5% after 10,000 cycles). Multiple analyses unveil that the unique electron-storage capability of Ni₂[CuPcS₈] originates from its localized lowest unoccupied molecular orbital (LUMO) over the nickel-bis(dithiolene) linkage, which allows the efficient delocalization of the injected electrons throughout the conjugated linkage units without inducing apparent bonding stress. The Ni₂[CuPcS₈] anode is used to demonstrate an asymmetric supercapacitor device that delivers a high operating voltage of 2.3 V, a maximum energy density of 57.4 Wh kg^-1, and ultralong stability over 5000 cycles.

Bonds over Electrons: Proton Coupled Electron Transfer at Solid-Solution Interfaces

This Perspective argues that most redox reactions of materials at an interface with a protic solution involve net proton-coupled electron transfer (PCET) (or other cation-coupled ET). This view contrasts with the traditional electron-transfer-focused view of redox reactions at semiconductors, but redox processes at metal surfaces are often described as PCET. Taking a thermodynamic perspective, transfer of an electron is typically accompanied by a stoichiometric proton, much as the chemistry of lithium-ion batteries involves coupled transfers of e^- and Li⁺. The PCET viewpoint implicates the surface-H bond dissociation free energy (BDFE) as the preeminent energetic parameter and its conceptual equivalents, the electrochemical ne^-/nH⁺ potential versus the reversible hydrogen electrode (RHE) and the free energy of hydrogenation, ΔG°_H. These parameters capture the thermochemistry of PCET at interfaces better than electronic parameters such as Fermi energies, electron chemical potentials, flat-band potentials, or band-edge energies. A unified picture of PCET at metal and semiconductor surfaces is presented. Exceptions, limitations, implications, and future directions motivated by this approach are described.

Aptamer-functionalized capacitive biosensors

The growing use of aptamers as target recognition elements in label-free biosensing necessitates corresponding transducers that can be used in relevant environments. While popular in many fields, capacitive sensors have seen relatively little, but growing use in conjunction with aptamers for sensing diverse targets. Few reports have shown physiologically relevant sensitivity in laboratory conditions and a cohesive picture on how target capture modifies the measured capacitance has been lacking. In this review, we assess the current state of the field in three areas: small molecule, protein, and cell sensing. We critically analyze the proposed hypotheses on how aptamer-target capture modifies the capacitance, as many mechanistic postulations appear to conflict between published works. As the field matures, we encourage future works to investigate individual aptamer-target interactions and to interrogate the physical mechanisms leading to measured changes in capacitance. To this point, we provide recommendations on best practices for developing aptasensors with a particular focus on considerations for biosensing in clinical settings.

Charge fluctuation drives anion rotation to enhance the conductivity of Na11M2PS12 (M = Si, Ge, Sn) superionic conductors

Solid superionic conductors exhibit good battery safety and stability, promising to replace organic liquid electrolytes. However, a comprehensive understanding of the factors determining high ion mobility remains elusive. Experiments have confirmed that the Na₁₁Sn₂PS₁₂ superionic conductor has high room temperature Na⁺-ion conductivity; excellent phase stability has been demonstrated in a solid-state electrolyte. The PS₄ anion rotation exists in Na₁₁M₂PS₁₂-type superionic conductors, but this rotation is affected by the isovalent cation substitutions of the M site. In combination with ab initio molecular dynamic simulations and joint time correlation analysis of the AIMD data, we show that the transport of Na⁺ ions is directly enhanced by the charge fluctuation in their tetrahedral MS₄ anions that comprise the framework. The fundamental reason for the charge fluctuation is the material structure forming a micro-parallel capacitor with MS₄ anions, which governs the differential capacitance. Our study provides a fundamental and comprehensive understanding of the structure-controlled charge transfer of Na₁₁M₂PS₁₂-type material and can guide solid-state battery optimization and design.

A Novel Aqueous Asymmetric Supercapacitor based on Pyrene-4,5,9,10-Tetraone Functionalized Graphene as the Cathode and Annealed Ti3 C2 Tx MXene as the Anode

Asymmetric supercapacitors (ASCs), employing two dissimilar electrode materials with a large redox peak position difference as cathode and anode, have been designed to further broaden the voltage window and improve the energy density of supercapacitors. Organic molecule based electrodes can be constructed by combining redox-active organic molecules with conductive carbon-based materials such as graphene. Herein, pyrene-4,5,9,10-tetraone (PYT), a redox-active molecule with four carbonyl groups, exhibits a four-electron transfer process and can potentially deliver a high capacity. PYT is noncovalently combined with two different kinds of graphene (Graphenea [GN] and LayerOne [LO]) at different mass ratios. The PYT-functionalized GN electrode (PYT/GN 4-5) possesses a high capacity of 711 F g^-1 at 1 A g^-1 in 1 M H₂ SO₄ . To match with the PYT/GN 4-5 cathode, an annealed-Ti₃ C₂ T_x (A-Ti₃ C₂ T_x ) MXene anode with a pseudocapacitive character is prepared by pyrolysis of pure Ti₃ C₂ T_x . The assembled PYT/GN 4-5//A-Ti₃ C₂ T_x ASC delivers an outstanding energy density of 18.4 Wh kg^-1 at a power density of 700 W kg^-1 . The PYT-functionalized graphene holds great potential for high-performance energy storage devices.

Ultrafast PEDOT:PSS/H2 SO4 Electrical Double Layer Capacitors: Comparison with Polyaniline Pseudocapacitors

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is one of the most widely studied conductive polymers, owing to its excellent electrical, optical, and mechanical properties, with various applications such as organic electrochemical transistors, electrochromics, and flexible/stretchable supercapacitors. The charging mechanism of PEDOT:PSS supercapacitors has been traditionally believed to be faradaic, which involves the transfer of charge across the electrode/electrolyte interface. In the present work, however, robust experimental evidence suggests that the PEDOT:PSS supercapacitors mainly store and deliver charge nonfaradaically. The various electrochemical properties of PEDOT:PSS electrical double layer capacitors (EDLCs) are clearly distinguishable from those of polyaniline (PANI) pseudocapacitors, which store charge faradaically. Owing to the nonfaradaic mechanism, the frequency response of PEDOT:PSS supercapacitors is comparable to that of state-of-the-art ultrafast EDLCs with carbon-based electrodes, making them suitable for high-frequency applications such as 60 Hz AC line filtering. This result is of great importance for the fundamental understanding of the charging mechanism of mixed ionic-electronic conducting polymers, such as PEDOT:PSS, and is expected to contribute to the development of various electrochemical devices based on this type of material.

Recent advances in polyaniline-based micro-supercapacitors

The rapid development of the Internet of Things (IoTs) and proliferation of wearable electronics have significantly stimulated the pursuit of distributed power supply systems that are small and light. Accordingly, micro-supercapacitors (MSCs) have recently attracted tremendous research interest due to their high power density, good energy density, long cycling life, and rapid charge/discharge rate delivered in a limited volume and area. As an emerging class of electrochemical energy storage devices, MSCs using polyaniline (PANI) electrodes are envisaged to bridge the gap between carbonaceous MSCs and micro-batteries, leading to a high power density together with improved energy density. However, despite the intensive development of PANI-based MSCs in the past few decades, a comprehensive review focusing on the chemical properties and synthesis of PANI, working mechanisms, design principles, and electrochemical performances of MSCs is lacking. Thus, herein, we summarize the recent advances in PANI-based MSCs using a wide range of electrode materials. Firstly, the fundamentals of MSCs are outlined including their working principle, device design, fabrication technology, and performance metrics. Then, the working principle and synthesis methods of PANI are discussed. Afterward, MSCs based on various PANI materials including pure PANI, PANI hydrogel, and PANI composites are discussed in detail. Lastly, concluding remarks and perspectives on their future development are presented. This review can present new ideas and give rise to new opportunities for the design of high-performance miniaturized PANI-based MSCs that underpin the sustainable prosperity of the approaching IoTs era.

Achieving Ultrahigh Energy-Density Aqueous Supercapacitors via a Novel Acidic Radical Adsorption Capacity-Activation Mechanism in Ni(SeO3 )/Metal Sulfide Heterostructure

Transitional metal chalcogenide (TMC) is considered as one promising high-capacity electrode material for asymmetric supercapacitors. More evidence indicates that TMCs have the same charge storage mechanism as hydroxides, but the reason why TMC electrode materials always provide higher capacity is rare to insight. In this work, a Ni_x Co_y Mn_z S/Ni(SeO₃ ) (NCMS/NSeO) heterostructure is prepared on Ni-plated carbon cloth, validating that both NCMS and NSeO can be transformed into hydroxides in electrochemical process as accompanying with the formation of SeO₃ ^2- and SO_x ^2- in confined spaces of NCMS/NSeO/Ni sandwich structure. Based on density functional theory calculation and experimental results, a novel space-confined acidic radical adsorption capacity-activation mechanism is proposed for the first time, which can nicely explain the capacity enhancement of NCMS/NSeO electrode materials. Thanks to the unique capacity enhancement mechanism and stable NCMS/NSeO/Ni sandwich structure, the optimized electrodes exhibit a high capacity of 536 mAh g^-1 at 1 A g^-1 and the impressive rate capability of 140.5 mAh g^-1 at the amazing current density of 200 A g^-1 . The assembled asymmetric supercapacitor achieves an ultrahigh energy density of 141 Wh Kg^-1 and an impressive high-rate capability and cyclability combination with 124% capacitance retention after 10 000 cycles at a large current density of 50 A g^-1 .

Direct Probe of Electrochemical Pseudocapacitive pH Jump at a Graphene Electrode

Molecular-level insight into interfacial water at a buried electrode interface is essential in electrochemistry, but spectroscopic probing of the interface remains challenging. Here, using surface-specific heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy, we directly access the interfacial water in contact with the graphene electrode supported on calcium fluoride (CaF₂ ). We find phase transition-like variations of the HD-SFG spectra vs. applied potentials, which arises not from the charging/discharging of graphene but from the charging/discharging of the CaF₂ substrate through the pseudocapacitive process. The potential-dependent spectra are nearly identical to the pH-dependent spectra, evidencing that the pseudocapacitive behavior is associated with a substantial local pH change induced by water dissociation between the CaF₂ and graphene. Our work evidences the local molecular-level effects of pseudocapacitive charging at an electrode/aqueous electrolyte interface.

A Pseudocapacitor Diode Based on Ion-Selective Surface Redox Effect

Supercapacitor diode (CAPode) is a novel device that integrates ion diode functionality into a conventional electrical double-layer capacitor and is expected to have great applications in emerging fields such as signal propagation, microcircuit rectification, logic operations, and neuromorphology. Here, a brand new pseudocapacitor diode is reported that has both high charge storage (50.2 C g^-1 at 20 mV s^-1 ) and high rectification (the rectification ratio of 0.79 at 200 mV s^-1 ) properties, which is realized by the ion-selective surface redox reaction of spinel ZnCo₂ O₄ in aqueous alkaline electrolyte. Furthermore, an application of the integrated device is demonstrated in the logic gate of circuit system to realize the logic operations of "AND" and "OR". This work not only expands the types of CAPodes, but also provides a train of thought for constructing high-performance capacitive ionic diodes.

Hydrodynamic cavitation-assisted preparation of porous carbon from garlic peels for supercapacitors

Hydrodynamic cavitation (HC), which can effectively induce sonochemical effects, is widely considered a promising process intensification technology. In the present study, HC was successfully utilized to intensify the alkali activation of GPs for SCs, for the first time. Five BDCMs were synthesized following the method reported in the literature. For comparison, four more BDCMs with HC-treated, among which a sample was further doped with nitrogen during the HC treatment, were prepared. Then all the samples were compared from microscopical characteristics to electrochemical performance as SCs materials. The morphology study demonstrated that the HC treatment had created many defects and amorphous carbon structures on the GP-based BDCMs, with the highest SSA reaching 3272 m²/g (1:6-HCGP), which 32 folded that of the Raw carbon sample's. The HC treatment also intensified the N-doping process. XRD and XPS results manifested that the N content had been increased and consequently changed the electronic structure of the carbon atoms, leading to the increase of specific capacitance (1:6-HCGP+N-based SC, 227 F/g at 10 A/g). The cycle performance proved that the GP-based BDCMs have long-term stability, indicating that the HC-treated BDCMs were good choices for energy storage technologies. Compared with the ultrasound-assisted method, which may have a high energy density, the HC-assisted method enables high production and energy efficiency. This work is a first time attempt towards the industrial application of HC method in energy-related materials synthesis and encourages more in-depth studies in the future.

First-Principles-Based Insight into Electrochemical Reactivity in a Cobalt-Carbonate-Hydroxide Pseudocapacitor

Cobalt carbonate hydroxide (CCH) is a pseudocapacitive material with remarkably high capacitance and cycle stability. Previously, it was reported that CCH pseudocapacitive materials are orthorhombic in nature. Recent structural characterization has revealed that they are hexagonal in nature; however, their H positions still remain unclear. In this work, we carried out first-principles simulations to identify the H positions. We then considered various fundamental deprotonation reactions inside the crystal and computationally evaluated the electromotive forces (EMF) of deprotonation (V _dp). Compared with the experimental potential window of the reaction (<0.6 V (vs saturated calomel electrode (SCE)), the computed V _dp (vs SCE) value (3.05 V) was beyond the potential window, indicating that deprotonation never occurred inside the crystal. This may be attributed to the strong hydrogen bonds (H-bonds) that formed in the crystal, leading to structural stabilization. We further investigated the crystal anisotropy in an actual capacitive material by considering the growth mechanism of the CCH crystal. By associating our X-ray diffraction (XRD) peak simulations with experimental structural analysis, we found that the H-bonds formed between CCH planes (approximately parallel to the ab-plane) can result in 1-D growth (stacked along the c-axis). This anisotropic growth controls the balance between the total "non-reactive" CCH phases (inside the material) and the "reactive" hydroxide (Co(OH)₂) phases (surface layers); the former stabilizes the structure, whereas the latter contributes to the electrochemical reaction. The balanced phases in the actual material can realize high capacity and cycle stability. The results obtained highlight the possibility of regulating the ratio of the CCH phase versus the Co(OH)₂ phase by controlling the reaction surface area.

Mesoporous Cobalt Oxide (CoOx) Nanowires with Different Aspect Ratios for High Performance Hybrid Supercapacitors

Cobalt oxide (CoOx) nanowires have been broadly explored as advanced pseudocapacitive materials owing to their impressive theoretical gravimetric capacity. However, the traditional method of compositing with conductive nanoparticles to improve their poor conductivity will unpredictably lead to a decrease in actual capacity. The amelioration of the aspect ratio of the CoOx nanowires may affect the pathway of electron conduction and ion diffusion, thereby improving the electrochemical performances. Here, CoOx nanowires with various aspect ratios were synthesized by controlling hydrothermal temperature, and the CoOx electrodes achieve a high gravimetric specific capacity (1424.8 C g-1) and rate performance (38% retention at 100 A g-1 compared to 1 A g-1). Hybrid supercapacitors (HSCs) based on activated carbon anode reach an exceptional specific energy of 61.8 Wh kg-1 and excellent cyclic performance (92.72% retention, 5000 cycles at 5 A g-1). The CoOx nanowires exhibit great promise as a favorable cathode material in the field of high-performance supercapacitors (SCs).

Molecular-level uniform graphene/polyaniline composite film for flexible supercapacitors with high-areal capacitance

To increase the specific capacitance of supercapacitors, polyaniline (PANI) has been chosen as additive electrode material for the pseudocapacitive performance. Here, we synthesize a molecular-level uniform reduced graphene oxide/PANI (rGO/PANI) composite film with high flexibility and conductivity via self-assembly and specific thermal reduction, which performs great potential in flexible supercapacitors with high areal capacitance. Particularly, the electrode of rGO/PANI-42.9% exhibits a high specific areal capacitance (1826 mF cm^-2at 0.2 mA cm^-2), and it also presents a good cycling stability (it remains 76% of its initial capacitance after 10 500 cycles). Moreover, the specific gravimetric capacitance of rGO/PANI-33.3% reaches up to 256.4 F g^-1at 0.2 A g^-1, showing greatly enhanced performance compared with the pure rGO electrode (183 F g^-1). The results of various characteristic analysis demonstrate that electrochemical performance of the as-prepared rGO/PANI film is closely associated with the uniform distribution of PANI in rGO/PANI composite. Overall, our reported method is convenient and environmental-friendly, and could be beneficial for the development of high-performance capacitive energy storage materials.

Freeing Fluoride Termination of Ti3C2Tx via Electrochemical Etching for High-Performance Capacitive Deionization

Ti3C2Tx MXene is a promising Faradic capacitive deionization (CDI) electrode for high salt removal in future desalination, whereas the surface termination group of fluoride (-F) significantly impedes ion access to Ti3C2 and charge-transfer efficiency. Herein, we propose an electrochemically etched strategy to synthesize -F-free Ti3C2Tx through three-electrode cyclic voltammetry scanning within a narrowed potential window in an alkaline electrolyte. The resulting assembly of an asymmetric electrochemical-etched Ti3C2Tx//activated carbon CDI device can deliver an excellent salt removal capacity of 20.27 mg·g-1 with an adsorption rate of 1.01 mg g-1 min-1 owing to the enhanced hydrophilicity and ion transport. The tiny CDI device is demonstrated, which can generate an electric current during the electrosorption of salt ions, thus facilitating the powering of a red light-emitting diode. This study opens a new avenue for the surface chemistry of Ti3C2Tx and is expected to achieve future applications in desalination and renewable energy.

Soft Fiber Electronics Based on Semiconducting Polymer

Fibers, originating from nature and mastered by human, have woven their way throughout the entire history of human civilization. Recent developments in semiconducting polymer materials have further endowed fibers and textiles with various electronic functions, which are attractive in applications such as information interfacing, personalized medicine, and clean energy. Owing to their ability to be easily integrated into daily life, soft fiber electronics based on semiconducting polymers have gained popularity recently for wearable and implantable applications. Herein, we present a review of the previous and current progress in semiconducting polymer-based fiber electronics, particularly focusing on smart-wearable and implantable areas. First, we provide a brief overview of semiconducting polymers from the viewpoint of materials based on the basic concepts and functionality requirements of different devices. Then we analyze the existing applications and associated devices such as information interfaces, healthcare and medicine, and energy conversion and storage. The working principle and performance of semiconducting polymer-based fiber devices are summarized. Furthermore, we focus on the fabrication techniques of fiber devices. Based on the continuous fabrication of one-dimensional fiber and yarn, we introduce two- and three-dimensional fabric fabricating methods. Finally, we review challenges and relevant perspectives and potential solutions to address the related problems.

Porous Single Crystals at the Macroscale: From Growth to Application

ConspectusPorous materials have wide applications in the fields of catalysis, separation, and energy conversion and storage. Porous materials contain pores that are specifically designed to achieve expectant performance. The solid phases in porous materials are normally completely continuous to form the basic porous frame while the pores are fluid phase within the solid phase. Single crystals are macroscopic materials in three spatial dimensions with the constituent atoms, ions, molecules, or molecular assemblies arranged in an orderly repeating pattern with the ordered structures. The growth of single crystals is indeed a process to arrange these constituents in three dimensions into a repeating pattern within the materials. Today the applications of single crystals are exponentially growing in wide fields, and single crystals are therefore unacknowledged as the pillars of our modern technology. Introducing porosity into single crystals would be expected to create a new kind of porous material in which the basic porous frames are single-crystalline and free of grain boundaries. The structural symmetry is completely maintained within the basic porous frames which are a continuous solid phase, but it is completely lost inside the pores. The porous architecture is free of grain boundaries, and the fully interconnected skeletons are in single-crystalline states within the basic porous frames. Single crystals with porosities can therefore be considered to be a new kind of porous material, but they are single-crystal-like because the structural symmetry is maintained only in the skeletons and completely lost within the pores. We therefore call them porous single crystals or consider them in porous single-crystalline states to stand out with their structural features. Porous single crystals at the macroscale combine the advantages of porous materials and single crystals to incorporate both porosity and structural coherence in a porous architecture, leading to invaluable opportunities to alter the material's properties by controlling the unique structural features to enhance its performance. However, the growth of single crystals in three dimensions reduces the formation of porosities, leading to a fundamental challenge for introducing porosity into single crystals in a traditional process of crystal growth. In this Account, we report the rational design, growth methodology, and microstructural engineering of porous single crystals in a solid-solid transformation. We rationally design a high-density mother phase in a single-crystalline state and transform it into a low-density new phase in a single-crystalline state to introduce porosities into single crystals even incorporating the removal of specific compositions from the mother phase during the growth of porous single crystals. The porosity can be tailored by controlling the change in relative densities from the mother phase to the porous single crystals while the pore size can be engineered by controlling the fabrication conditions. Considering the unique structural features, we explore their functionalities and applications in photoelectrochemical energy conversion, electrochemical alkane conversion, and electrochemical energy storage. We believe that the materials, if tailored into porous single-crystalline states, would not only find a broad range of applications in other fields but also enable a new path for material innovations.

Confine, Defect, and Interface Manipulation of Fe3 Se4 /3D Graphene Targeting Fast and Stable Potassium-Ion Storage

The fast electrochemical kinetics behavior and long cycling life have been the goals in developing anode materials for potassium ion batteries (PIBs). On account of high electron conductivity and theoretical capacity, transition metal selenides have been deemed as one of the promising anode materials for PIBs. Herein, a systematic structural manipulation strategy, pertaining to the confine of Fe₃ Se₄ particles by 3D graphene and the dual phosphorus (P) doping to the Fe₃ Se₄ /3DG (DP-Fe₃ Se₄ /3DG), has been proposed to fulfill the efficient potassium-ion (K-ion) evolution kinetics and thus boost the K-ion storage performance. The theoretical calculation results demonstrate that the well-designed dual P doping interface can effectively promote K-ion adsorption behavior and provide a low energy barrier for K-ion diffusion. The insertion-conversion and adsorption mechanism for multi potassium storage behavior in DP-Fe₃ Se₄ /3DG composite has been also deciphered by combining the in situ/ex situ X-ray diffraction and operando Raman spectra evidences. As expected, the DP-Fe₃ Se₄ /3DG anode exhibits superior rate capability (120.2 mA h g^-1 at 10 A g^-1 ) and outstanding cycling performance (157.9 mA h g^-1 after 1000 cycles at 5 A g^-1 ).

In Situ Growth of Nickel-Cobalt Metal Organic Frameworks Guided by a Nickel-Molybdenum Layered Double Hydroxide with Two-Dimensional Nanosheets Forming Flower-Like Struc-Tures for High-Performance Supercapacitors

Metal organic frameworks (MOFs) are a kind of porous coordination polymer supported by organic ligands with metal ions as connection points. They have a controlled structure and porosity and a significant specific surface area, and can be used as functional linkers or sacrificial templates. However, long diffusion pathways, low conductivity, low cycling stability, and the presence of few exposed active sites limit the direct application of MOFs in energy storage applications. The targeted design of MOFs has the potential to overcome these limitations. This study proposes a facile method to grow and immobilize MOFs on layered double hydroxides through an in situ design. The proposed method imparts not only enhanced conductivity and cycling stability, but also provides additional active sites with excellent specific capacitance properties due to the interconnectivity of MOF nanoparticles and layered double hydroxide (LDH) nanosheets. Due to this favorable heterojunction hook, the NiMo-LDH@NiCo-MOF composite exhibits a large specific capacitance of 1536 F·g^-1 at 1 A·g^-1. In addition, the assembled NiMo-LDH@NiCo-MOF//AC asymmetric supercapacitor can achieve a high-energy density value of 60.2 Wh·kg^-1 at a power density of 797 W·kg^-1, indicating promising applications.

Cycling stability of Fe2O3 nanosheets as supercapacitor sheet electrodes enhanced by MgFe2O4 nanoparticles

The Fe₂O₃ material is a common active material for supercapacitor electrodes and has received much attention due to its cheap and easy availability and high initial specific capacitance. In the present study, we prepared adhesive-free Fe₂O₃ sheet electrodes for supercapacitors by growing Fe₂O₃ material on nickel foam by hydrothermal method. The sheet electrode exhibited a high initial specific capacitance of 863 F g^-1, but we found that the sheet lost its specific capacitance too quickly through cyclic stability tests. To solve this problem, Fe₂O₃/MgFe₂O₄ composites were grown on nickel foam (NF). It was found through testing that the cycling stability of the sheet electrode gradually increased as the content of MgFe₂O₄ material increased. When the molar ratio of Fe₂O₃ to MgFe₂O₄ material was 1 : 1, the initial specific capacitance of the sheet electrode was 815 F g^-1 and the capacitance remained at 81.25% of the initial specific capacitance after 1000 cycles. The better cycling stability results from the more stable structure of the composite, the synergistic effect leading to better reversibility of the reaction.

Isolation of pseudocapacitive surface processes at monolayer MXene flakes reveals delocalized charging mechanism

Pseudocapacitive charge storage in Ti₃C₂T_x MXenes in acid electrolytes is typically described as involving proton intercalation/deintercalation accompanied by redox switching of the Ti centres and protonation/deprotonation of oxygen functional groups. Here we conduct nanoscale electrochemical measurements in a unique experimental configuration, restricting the electrochemical contact area to a small subregion (0.3 µm²) of a monolayer Ti₃C₂T_x flake. In this unique configuration, proton intercalation into interlayer spaces is not possible, and surface processes are isolated from the bulk processes, characteristic of macroscale electrodes. Analysis of the pseudocapacitive response of differently sized MXene flakes indicates that entire MXene flakes are charged through electrochemical contact of only a small basal plane subregion, corresponding to as little as 3% of the flake surface area. Our observation of pseudocapacitive charging outside the electrochemical contact area is suggestive of a fast transport of protons mechanism across the MXene surface.

Nanostructured Poly(3,4-ethylenedioxythiophene) Coatings on Functionalized Glass for Energy Storage

Conducting polymers rise among some of the most promising transparent supercapacitor electrode materials due to high conductivity, environmental stability, light weight, and ease of synthesis. A major challenge for depositing conducting polymers on a glass substrate is the lack of molecular interactions between organic and inorganic moieties resulting in poor adhesion and low cycling stability of the electrode. We present a synthetic approach by covalently linking poly(3,4-ethylyenedioxythiophene) (PEDOT) and glass through Friedel-Crafts alkylation on a self-assembled diphenyldimethoxysilane monolayer. This method obviates the need for a conductive FTO or ITO coating, enabling the fabrication of current collector-free planar supercapacitor electrodes on any glass surface. The electrode produced from our vapor-phase synthesis is coated with a highly conductive nanofibrillar PEDOT film (sheet resistance 2.1 Ω/□) possessing a gravimetric capacitance of ∼200 F/g. Our PEDOT planar supercapacitor possesses outstanding stability (86% capacitance retention after 50,000 cycles). We also fabricate a proof-of-concept transparent tandem supercapacitor on PEDOT-coated glass using 3D-printed frames that supplies enough voltage and current to light up a blue light-emitting diode (LED).

Silicon/Graphite/Amorphous Carbon as Anode Materials for Lithium Secondary Batteries

Although silicon is being researched as one of the most promising anode materials for future generation lithium-ion batteries owing to its greater theoretical capacity (3579 mAh g^-1), its practical applicability is hampered by its worse rate properties and poor cycle performance. Herein, a silicon/graphite/amorphous carbon (Si/G/C) anode composite material has been successfully prepared by a facile spray-drying method followed by heating treatment, exhibiting excellent electrochemical performance compared with silicon/amorphous carbon (Si/C) in lithium-ion batteries. At 0.1 A g^-1, the Si/G/C sample exhibits a high initial discharge capacity of 1886 mAh g^-1, with a high initial coulombic efficiency of 90.18%, the composite can still deliver a high initial charge capacity of 800 mAh g^-1 at 2 A g^-1, and shows a superior cyclic and rate performance compared to the Si/C anode sample. This work provides a facile approach to synthesize Si/G/C composite for lithium-ion batteries and has proven that graphite replacing amorphous carbon can effectively improve the electrochemical performance, even using low-performance micrometer silicon and large size flake graphite.

Substrate-Enabled Room-Temperature Electrochemical Deposition of Crystalline ZnMnO3

Mixed transition metal oxides have emerged as promising electrode materials for electrochemical energy storage and conversion. To optimize the functional electrode properties, synthesis approaches allowing for a systematic tailoring of the materials' composition, crystal structure and morphology are urgently needed. Here we report on the room-temperature electrodeposition of a ternary oxide based on earth-abundant metals, specifically, the defective cubic spinel ZnMnO₃ . In this unprecedented approach, ZnO surfaces act as (i) electron source for the interfacial reduction of MnO₄ ^- in aqueous solution, (ii) as substrate for epitaxial growth of the deposit and (iii) as Zn precursor for the formation of ZnMnO₃ . Epitaxial growth of ZnMnO₃ on the lateral facets of ZnO nanowires assures effective electronic communication between the electroactive material and the conducting scaffold and gives rise to a pronounced 2-dimensional morphology of the electrodeposit forming - after partial delamination from the substrate - twisted nanosheets. The synthesis strategy shows promise for the direct growth of different mixed transition metal oxides as electroactive phase onto conductive substrates and thus for the fabrication of binder-free nanocomposite electrodes.

Surface-redox sodium-ion storage in anatase titanium oxide

Sodium-ion storage technologies are promising candidates for large-scale grid systems due to the abundance and low cost of sodium. However, compared to well-understood lithium-ion storage mechanisms, sodium-ion storage remains relatively unexplored. Herein, we systematically determine the sodium-ion storage properties of anatase titanium dioxide (TiO₂(A)). During the initial sodiation process, a thin surface layer (~3 to 5 nm) of crystalline TiO₂(A) becomes amorphous but still undergoes Ti⁴⁺/Ti³⁺ redox reactions. A model explaining the role of the amorphous layer and the dependence of the specific capacity on the size of TiO₂(A) nanoparticles is proposed. Amorphous nanoparticles of ~10 nm seem to be optimum in terms of achieving high specific capacity, on the order of 200 mAh g^-1, at high charge/discharge rates. Kinetic studies of TiO₂(A) nanoparticles indicate that sodium-ion storage is due to a surface-redox mechanism that is not dependent on nanoparticle size in contrast to the lithiation of TiO₂(A) which is a diffusion-limited intercalation process. The surface-redox properties of TiO₂(A) result in excellent rate capability, cycling stability and low overpotentials. Moreover, tailoring the surface-redox mechanism enables thick electrodes of TiO₂(A) to retain high rate properties, and represents a promising direction for high-power sodium-ion storage.

MoS2 nanosheets on plasma-nitrogen-doped carbon cloth for high-performance flexible supercapacitors

With the surging demand for flexible and portable electronic devices featuring high energy and power density, the development of next-generation lightweight, flexible energy storage devices is crucial. However, achieving the expected energy and power density of supercapacitors remains a great challenge. This work reports a facile plasma-enabled method for preparing supercapacitor electrodes made of MoS2 nanosheets grown on flexible and lightweight N-doped carbon cloth (NCC). The MoS2/NCC presents an outstanding specific capacitance of 3834.28 mF/cm2 at 1 mA/cm2 and energy density of 260.94 µWh/cm2 at a power density of 354.48 µW/cm2. An aqueous symmetric supercapacitor fitted with two MoS2/NCC electrodes achieved the maximum energy density of 138.12 µWh/cm2 and the highest power density of 7,417.33 µW/cm2, along with the excellent cycling stability of 83.3 % retention over 10,000 cycles. The high-performance energy storage ASSSs (all-solid-state supercapacitors) are demonstrated to power devices in both rigid and flexible operation modes. This work provides a new perspective for fabricating high-performance all-solid-state flexible supercapacitors for clean energy storage.

Bending Resistance Covalent Organic Framework Superlattice: "Nano-Hourglass"-Induced Charge Accumulation for Flexible In-Plane Micro-Supercapacitors

Covalent organic framework (COF) film with highly exposed active sites is considered as the promising flexible self-supported electrode for in-plane micro-supercapacitor (MSC). Superlattice configuration assembled alternately by different nanofilms based on van der Waals force can integrate the advantages of each isolated layer to exhibit unexpected performances as MSC film electrodes, which may be a novel option to ensure energy output. Herein, a mesoporous free-standing A-COF nanofilm (pore size is 3.9 nm, averaged thickness is 4.1 nm) with imine bond linkage and a microporous B-COF nanofilm (pore size is 1.5 nm, averaged thickness is 9.3 nm) with β-keto-enamine-linkages are prepared, and for the first time, we assembly the two lattice matching films into sandwich-type superlattices via layer-by-layer transfer, in which ABA-COF superlattice stacking into a "nano-hourglass" steric configuration that can accelerate the dynamic charge transportation/accumulation and promote the sufficient redox reactions to energy storage. The fabricated flexible MSC-ABA-COF exhibits the highest intrinsic CV of 927.9 F cm-3 at 10 mV s-1 than reported two-dimensional alloy, graphite-like carbon and undoped COF-based MSC devices so far, and shows a bending-resistant energy density of 63.2 mWh cm-3 even after high-angle and repeat arbitrary bending from 0 to 180°. This work provides a feasible way to meet the demand for future miniaturization and wearable electronics.

Compacting Electric Double Layer Enables Carbon Electrode with Ultrahigh Zn Ion Storage Capability

Carbon-based cathodes for aqueous zinc ion hybrid supercapacitors (ZHSCs) typically undergo low Zn ion storage capability due to their electric double layer capacitance (EDLC) energy storage mechanism that is restricted by specific surface area and thickness of electric double layer (EDL). Here, we report a universal surface charge modulation strategy to effectively enhance the capacitance of carbon materials by decreasing the thickness of EDL. Amino groups with lone pair electrons were chosen to increase the surface charge density and enhanced the interaction between carbon electrode and Zn ions, thus effectively compacting the EDL. Consequently, amino functionalized porous carbon based ZHSCs can deliver an ultrahigh capacity of 255.2 mAh g^-1 along with excellent cycling stability (95.5 % capacity retention after 50 000 cycles) in 1 M ZnCl₂ electrolyte. This study demonstrates the feasibility of EDL modified carbon as Zn²⁺ storage cathode and great prospect for constructing high performance ZHSCs.