Perspectives on Working Voltage of Aqueous Supercapacitors

Theoretical insights and design of MXene for aqueous batteries and supercapacitors: status, challenges, and perspectives

Aqueous batteries and supercapacitors are promising electrochemical energy storage systems (EESSs) due to their low cost, environmental friendliness, and high safety. However, aqueous EESS development faces challenges like narrow electrochemical windows, irreversible dendrite growth, corrosion, and low energy density. Recently, two-dimensional (2D) transition metal carbide and nitride (MXene) have attracted more attention due to their excellent physicochemical properties and potential applications in aqueous EESSs. Understanding the atomic-level working mechanism of MXene in energy storage through theoretical calculations is necessary to advance aqueous EESS development. This review comprehensively summarizes the theoretical insights into MXene in aqueous batteries and supercapacitors. First, the basic properties of MXene, including structural composition, experimental and theoretical synthesis, and advantages in EESSs are introduced. Then, the energy storage mechanism of MXene in aqueous batteries and supercapacitors is summarized from a theoretical calculation perspective. Additionally, the theoretical insights into the side reactions and stability issues of MXene in aqueous EESSs are emphasized. Finally, the prospects of designing MXene for aqueous EESSs through computational methods are given.

Unveiling the charge storage mechanisms of Co-based perovskite fluoride in a mild aqueous electrolyte

This study is an in-depth exploration of the charge storage mechanisms of KCoF3 in 1 M Na2SO4 mild aqueous electrolytes via an array of ex situ/in situ physicochemical/electrochemical methods, especially the electrochemical quartz crystal microbalance (EQCM) technique, showing a combination of conversion, insertion/extraction and adsorption mechanisms. Specifically, during the first charge phase, Co(OH)2 is formed/oxidized into amorphous CoOOH and Co3O4, and then CoOOH undergoes partial proton extraction to yield CoO2, which is simultaneously accompanied by the transformation of Co3O4 into CoOOH and (hydrated) CoO2. During the first discharge process, the partial insertion of H+ into (hydrated) CoO2 leads to the formation of CoOOH and Co3O4, with the conversion of Co3O4 into CoOOH and both Co3O4 and CoOOH undergoing further transformations into (hydrated) Co(OH)2via the insertion of H+. This work offers valuable references for the development of aqueous energy storage.

Flexible Aqueous Cr-Ion Hybrid Supercapacitors with Remarkable Electrochemical Properties in all Climates

The integration of hostless battery-like metal anodes for hybrid supercapacitors is a realistic design method for energy storage devices with promising future applications. With significant Cr element deposits on Earth, exceptionally high theoretical capacity (1546 mAh g-1), and accessible redox potential (-0.74 V vs. reversible hydrogen electrode) of Cr metals, the design of Cr anodes has rightly come into our focus. This work presents a breakthrough design of a flexible Cr-ion hybrid supercapacitor (CHSC) based on a porous graphitized carbon fabric (PGCF) substrate prepared by K2FeO4 activation. In the CHSC device, PGCF acts as both a current collector and cathode material due to its high specific surface area and superior conductivity. The use of a highly concentrated LiCl-CrCl3 electrolyte with high Cr plating/stripping efficiency and excellent antifreeze properties enables the entire PGCF-based CHSC to achieve well-balanced performance in terms of energy density (up to 1.47 mWh cm-2), power characteristics (reaching 9.95 mW cm-2) and durability (95.4 % capacity retention after 30,000 cycles), while realizing it to work well under harsh conditions of -40 °C. This work introduces a new concept for low-temperature energy storage technology and confirms the potential application of Cr anodes in hybrid supercapacitors.

Recent Development on Transition Metal Oxides-Based Core-Shell Structures for Boosted Energy Density Supercapacitors

In recent years, nanomaterials exploration and synthesis have played a crucial role in advancing energy storage research, particularly in supercapacitor development. Researchers have diversified materials, including metal oxides, chalcogenides, and composites, as well as carbon materials, to enhance energy and power density. Balancing energy density with electrochemical stability remains challenging, driving intensified efforts in advancing electrode materials. This review focuses on recent progress in designing and synthesizing core-shell materials tailored for supercapacitors. The core-shell architecture offers advantages such as increased surface area, redox active sites, electrical conductivity, ion diffusion kinetics, specific capacitance, and cyclability. The review explores the impact of core and shell materials, specifically transition metal oxides (TMOs), on supercapacitor electrochemical behavior. Metal oxide choices, such as cobalt oxide as a preferred core and manganese oxide as a shell, are discussed. The review also highlights characterization techniques for assessing structural, morphological, and electrochemical properties of core-shell materials. Overall, it provides a comprehensive overview of ongoing TMOs-based core-shell material research for supercapacitors, showcasing their potential to enhance energy storage for applications ranging from gadgets to electric vehicles. The review outlines existing challenges and future opportunities in evolving TMOs-based core-shell materials for supercapacitor advancements, holding promise for high-efficiency energy storage devices.

Thiocyanogen-modulated N, S Co-doped lignin hierarchical porous carbons for high-performance aqueous supercapacitors

Constructing heteroatom-doped porous carbons with distinct charge storage properties is significant for high-energy-density supercapacitors, yet it remains a formidable challenge. Herein, we employed a thiocyanogen-modulated alkali activation strategy to synthesize N and S co-doped lignin hierarchical porous carbon (NSLHPC). In this process, thiocyanogen serves as a surface modulation mediator to substitute oxygen with nitrogen and sulfur species, while the combination of KOH activation and MgO template generates numerous nanopores within the carbon structure. The three-dimensional interconnected nanosheet architecture facilitates rapid ion transfer and enhances accessibility to active sites. Density functional theory (DFT) calculations demonstrate that introducing N and S heteroatoms through oxygen substitution reduces the adsorption energy barrier of Zn2+. Consequently, the optimized NSLHPC exhibits a remarkable specific capacitance of 438F/g at 0.5 A/g in 6 M KOH, delivering an energy density of 10.4 Wh/kg in the symmetric supercapacitor and an impressive energy density of 104.9 Wh/kg in the zinc-ion hybrid capacitor. The NSLHPC cathode also shows an excellent lifespan with capacitance retention of 99.0 % and Columbic efficiency of 100 % over 10,000 cycles. This study presents innovative strategies for engineering high-performance porous carbon electrode materials by emphasizing pore structure modulation and N, S co-doping as crucial approaches.

Preparation of Few-Layered MoS2 by One-Pot Hydrothermal Method for High Supercapacitor Performance

Molybdenum disulfide (MoS2), a typical layered material, has important applications in various fields, such as optoelectronics, catalysis, electronic devices, sensors, and supercapacitors. Extensive research has been carried out on few-layered MoS2 in the field of electrochemistry due to its large specific surface area, abundant active sites and short electron transport path. However, the preparation of few-layered MoS2 is a significant challenge. This work presents a simple one-pot hydrothermal method for synthesizing few-layered MoS2. Furthermore, it investigates the exfoliation effect of different amounts of sodium borohydride (NaBH4) as a stripping agent on the layer number of MoS2. Na+ ions, as alkali metal ions, can intercalate between layers to achieve the purpose of exfoliating MoS2. Additionally, NaBH4 exhibits reducibility, which can effectively promote the formation of the metallic phase of MoS2. Few-layered MoS2, as an electrode for supercapacitor, possesses a wide potential window of 0.9 V, and a high specific capacitance of 150 F g-1 at 1 A g-1. This work provides a facile method to prepare few-layered two-dimensional materials for high electrochemical performance.

The Mass-Balancing between Positive and Negative Electrodes for Optimizing Energy Density of Supercapacitors

Supercapacitors (SCs) are some of the most promising energy storage devices, but their low energy density is one main weakness. Over the decades, superior electrode materials and suitable electrolytes have been widely developed to enhance the energy storage ability of SCs. Particularly, constructing asymmetric supercapacitors (ASCs) can extend their electrochemical stable voltage windows (ESVWs) and thus achieve high energy density. However, only full utilization of the electrochemical stable potential windows (ESPWs) of both positive and negative electrodes can endow the ASC devices with a maximum ESVW by using a suitable mass-ratio between two electrodes (the mass-balancing). Nevertheless, insufficient attention is directed to mass-balancing, and even numerous misunderstandings and misuses have appeared. Therefore, in this Perspective, we focus on the mass-balancing: summarize theoretic basis of the mass-balancing, derive relevant relation equations, analyze and discuss the change trends of the specific capacitance and energy density of ASCs with mass-ratios, and finally recommend some guidelines for the normative implementation of the mass-balancing. Especially, the issues related to pseudocapacitive materials, hybrid devices, and different open circuit potentials (OCPs) of the positive and negative electrodes in the mass-balancing are included and emphasized. These analyses and guidelines can be conducive to understanding and performing mass-balancing for developing high-performance SCs.

Recent Advances of Flexible MXene and its Composites for Supercapacitors

MXenes have unique properties such as high electrical conductivity, excellent mechanical properties, rich surface chemistry, and convenient processability. These characteristics make them ideal for producing flexible materials with tunable microstructures. This paper reviews the laboratory research progress of flexible MXene and its composite materials for supercapacitors. And introduces the general synthesis method of MXene, as well as the preparation and properties of flexible MXene. By analyzing the current research status, the electrochemical reaction mechanism of MXene was explained from the perspectives of electrolyte and surface terminating groups. This review particularly emphasizes the composite methods of freestanding flexible MXene composite materials. The review points out that the biggest problem with flexible MXene electrodes is severe self-stacking, which reduces the number of chemically active sites, weakens ion accessibility, and ultimately lowers electrochemical performance. Therefore, it is necessary to composite MXene with other electrode materials and design a good microstructure. This review affirms the enormous potential of flexible MXene and its composite materials in the field of supercapacitors. In addition, the challenges and possible improvements faced by MXene based materials in practical applications were also discussed.

Asymmetric Supercapacitors based on 1,10-phenanthroline-5,6-dione Molecular Electrodes Paired with MXene

An efficient approach to increase the energy density of supercapacitors is to prepare electrode materials with larger specific capacitance and increase the potential difference between the positive and negative electrodes in the device. Herein, an organic molecular electrode (OME) is prepared by anchoring 1,10-phenanthroline-5,6-dione (PD), which possesses two pyridine rings and an electron-deficient conjugated system, onto reduced graphene oxide (rGO). Because of the electron-deficient conjugated structure of PD molecule, PD/rGOs exhibit a more positive redox peak potential along with the advantages of high capacitance-controlled behaviour and fast reaction kinetics. Additionally, the small energy gap between the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) leads to increased conductivity in PD/rGO. To assemble the asymmetric supercapacitor (ASC), a two-dimensional metal carbide, as known as MXene, with a chemical composition of Ti3C2Tx is selected as the negative electrode due to its exceptional performance, and PD/rGO-0.5 is employed as the positive electrode. Consequently, the working voltage is expanded up to 1.8 V. Through further electrochemical measurements, the assembled ASC (PD/rGO-0.5//Ti3C2Tx) achieves a remarkable energy density of 36.8 Wh kg-1. Remarkably, connecting two ASCs in series can power 73 LEDs, showcasing its promising potential for energy storage applications.

Controlled establishment of advanced local high-entropy NiCoMnFe-based layered double hydroxide for zinc batteries and low-temperature supercapacitors

The development of high-performance electrodes is essential for improving the charge storage performance of rechargeable devices. In this study, local high-entropy C, N co-doped NiCoMnFe-based layered double hydroxide (C/N-NiCoMnFe-LDH, C/N-NCMF) were designed using a novel method. Multi-component synergistic effects can dramatically modulate the surface electron density, crystalline structure, and band-gap of the electrode. Thus, the electrical conductivity, electron transfer, and affinity for the electrolyte can be optimized. Additionally, the C/N-NCMF yielded a high specific capacitance (1454F·g-1) at 1 A·g-1. The electrode also exhibited excellent cycling stability, with 62 % capacitance retention after 5000 cycles. Moreover, the assembled Zn||C/N-NCMF battery and the C/N-NCMF//AC hybrid supercapacitor yielded excellent energy densities of 63.1 and 35.4 Wh·kg-1 at power densities of 1000 and 825 W·kg-1, and superior cycling performance with 69 % and 88.7 % capacitance retention after 1000 and 30,000 cycles, respectively. Furthermore, the electrode maintained high electrochemical activity and stability and ensured high energy density, power density, and cycling stability of the rechargeable devices even at a low temperature (-20 °C). This study paves a new pathway for regulating the electrochemical performance of LDH-based electrodes.

Recent Advances in Low‐Temperature Liquid Electrolyte for Supercapacitors

As one of the key components of supercapacitors, electrolyte is intensively investigated to promote the fast development of the energy supply system under extremely cold conditions. However, high freezing point and sluggish ion transport kinetics for routine electrolytes hinder the application of supercapacitors at low temperatures. Resultantly, the liquid electrolyte should be oriented to reduce the freezing point, accompanied by other superior characteristics, such as large ionic conductivity, low viscosity and outstanding chemical stability. In this review, the intrinsically physical parameters and microscopic structure of low-temperature electrolytes are discussed thoroughly, then the previously reported strategies that are used to address the associated issues are summarized subsequently from the aspects of aqueous and non-aqueous electrolytes (organic electrolyte and ionic liquid electrolyte). In addition, some advanced spectroscopy techniques and theoretical simulation to better decouple the solvation structure of electrolytes and reveal the link between the key physical parameters and microscopic structure are briefly presented. Finally, the further improvement direction is put forward to provide a reference and guidance for the follow-up research.

Ionic liquid electrolyte selection for high voltage supercapacitors in high-temperature applications

Systematic analyses of electrolyte physicochemical properties are important to screen ionic liquids (ILs) and understand the electrochemical performance of supercapacitor electrolytes. This study harmonizes the evaluation of electrochemical performance and transport properties of eight shortlisted ILs from 22 commercially available hydrophobic ILs toward achieving a ≥ 5 V supercapacitor capable of high-temperature operation (up to 353.15 K). The eight ILs are N-Propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([Pyr 1, 3] [TFSI], N-Pentyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([Pyr 1, 5] [TFSI]), N-Propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide ([Pyr 1, 3] [FSI]), 1-Methyl-1-(2-methoxyethyl)pyrrolidinium Bis(trifluoromethanesulfonyl)imide ([Pyr 1, 102] [TFSI]), 1-Methyl-1-propylpiperidinium bis(trifluoromethanesulfonyl)imide ([Pip 1, 3] [TFSI]), 1-Methyl-1-propylpiperidinium bis(fluorosulfonyl)imide ([Pip 1, 3] [FSI]), N-Trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide ([N 111, 3] [TFSI]), N-Trimethyl-N-hexylammonium bis(trifluoromethanesulfonyl)imide ([N 111, 6] [TFSI]). The density, viscosity, and ionic conductivity of the eight ILs were measured between 278.15 and 373.15 K to confirm the effects of temperature and ion structure before electrochemical characterization. The [FSI]-based ILs ([Pip 1, 3] [FSI] and [Pyr 1, 3] [FSI]) showed lower densities and viscosities compared to other ILs among the eight based on [TFSI]. Consequently, the highest conductivity was obtained for [Pyr 1, 3] [FSI]. Cyclic voltammetry and impedance spectroscopy was performed on supercapacitors assembled with the eight ILs as electrolytes between 298.15-353.15 K. Conclusion from the two-electrode supercapacitors using multi-walled carbon nanotubes showed the 6 most-applicable ILs towards the targeted ≥ 5 V SC at high temperature are [Pip 1, 3] [TFSI] (5.4 V), [Pip 1, 3] [FSI] (5 V), [N 111, 3] [TFSI] (5.1 V), [N 111, 6] [TFSI] (5.2 V), [Pyr 1, 102] [TFSI] (5.2 V), and [Pyr 1, 5] [TFSI] (5.2 V).

Unlocking the Effect of Chain Length and Terminal Group on Ethylene Glycol Ether Family Toward Advanced Aqueous Electrolytes

Constructing high-performance hybrid electrolyte is important to advanced aqueous electrochemical energy storage devices. However, due to the lack of in-depth understanding of how the molecule structures of cosolvent additives influence the properties of electrolytes significantly impeded the development of hybrid electrolytes. Herein, a series of hybrid electrolytes are prepared by using ethylene glycol ether with different chain lengths and terminal groups as additives. The optimized 2 m LiTFSI-90%DDm hybrid electrolyte prepared from diethylene glycol dimethyl ether (DDm) molecule showcases excellent comprehensive performance and significantly enhances the operating voltage of supercapacitors (SCs) to 2.5 V by suppressing the activity of water. Moreover, the SC with 2 m LiTFSI-90%DDm hybrid electrolyte supplies a long-term cycling life of 50 000 cycles at 1 A g-1 with 92.3% capacitance retention as well as excellent low temperature (-40 ºC) cycling performance (10 000 times at 0.2 A g-1). Universally, Zn//polyaniline full cell with 2 m Zn(OTf)2-90%DDm electrolyte manifests outstanding cycling performance in terms of 77.9% capacity retention after 2,000 cycles and a dendrite-free Zn anode. This work inspires new thinking of developing advanced hybrid electrolytes by cosolvent molecule design toward high-performance energy storage devices.

Mechanically Robust and Highly Conductive Poly(ionic liquid)/Polyacrylamide Double-Network Hydrogel Electrolytes for Flexible Symmetric Supercapacitors with a Wide Operating Voltage Range

Flexible electronic devices, such as supercapacitors (SCs), place high demands on the mechanical properties, ionic conductivity, and electrochemical stability of electrolytes. Hydrogels, which combine flexibility and the advantages of both solid and liquid electrolytes, will meet the demand. Here, we report the synthesis of novel poly(ionic liquid)/polyacrylamide double-network (DN) (PIL/PAM DN) hydrogel electrolytes containing different metal salts via a two-step γ-radiation method. The resultant Li2SO4-1.0/PIL/PAM DN hydrogel electrolyte possesses excellent mechanical properties (tensile strength of 3.64 MPa, elongation at break of 446%) and high ionic conductivity (24.1 mS·cm-1). The corresponding flexible SC based on the Li2SO4-1.0/PIL/PAM DN hydrogel electrolyte (SC-Li2SO4) presents improved ion diffusion, ideal electrochemical double-layer capacitor behavior, good rate capability, and excellent cyclic stability. Moreover, symmetric SC-Li2SO4 achieves a wide operating voltage range of up to 1.5 V, with a maximum energy density of 26.0 W h·kg-1 and a capacitance retention of 94.1% after 10,000 galvanostatic charge-discharge cycles, owing to the deactivation of free water molecules by the synergistic effect of PIL, PAM, and SO42-. Above all, the capacitance of SC-Li2SO4 is well-maintained after overcharge, overdischarge, short circuit, extreme temperature, compression, and bending tests, indicating its high security and flexibility. This work reveals the enormous application potential of PIL-based conductive hydrogel electrolytes for flexible electronic devices.

Ammonium Folate-Reinforced Self-Assembly of Gelatin into N/B/O-Enriched Hierarchical Porous Carbons with Loosely Layered Structure for Anti-Freezing Flexible Supercapacitors

Heteroatom-doped layered porous carbons are recently regarded as promising electrode materials for high energy density supercapacitors because they can integrate high-level heteroatom-doping and layered nano-space together to provide huge pseudocapacitive reaction areas and accelerate ion diffusion/transport. Herein, an innovative strategy is reported to prepare N/B/O co-doped layered porous carbons via ammonium folate-reinforced self-assembly of gelatin and boric acid followed by carbonization. Biomass-derived ammonium folate not only acts as an N-riched precursor but also can fasten in the process of self-assembly via boric acid-assisted electrostatic adsorption and hydrogen bonding to promote the formation of stable 3D cross-linked networks, resulting in the obtained N/B/O co-doped layered porous carbon (BNLC-850) has a large specific surface area (1822 m2 g-1 ), hierarchical porous structure and super-high heteroatom contents (N, 12.65; B, 5.67; and O, 13.84 at.%). The BNLC-850 achieves an ultrahigh specific capacitance of 525.2 F g-1 in the alkaline electrolyte at 0.5 A g-1 , meanwhile, DFT calculations reveal that the high-level N/B/O-doping can effectively weaken the adsorption barriers of K-ions. Moreover, the BNLC-850 assembles anti-freezing flexible solid-state supercapacitors in MPEI-TF-IL gel polymer electrolyte deliver a high energy density of 41.2 Wh kg-1 , excellent flexibility, and long cycle-life at -20 °C.

Iron-doped nickel-cobalt bimetallic phosphide nanowire hybrids for solid-state supercapacitors with excellent electromagnetic interference shielding

The development of flexible and wearable electronics subjects to the limited energy density and accompanying electromagnetic pollution. With a high theoretical specific capacity, nickel-cobalt bimetallic phosphide (NiCoP) is considered to be potential cathode materials for supercapacitor. However, the pristine NiCoP fails to display excellent electrochemical performance due to its inferior rate performance and cycling stability. Herein, we design Fe doped NiCoP nanowire arrays on carbon cloth (Fe-NiCoP/CC) as the cathode for supercapacitors. The introduced Fe doping enable to increase in the electronic conductivity and enhance the adsorption of OH-, supported by the density functional theory (DFT) analysis. As a result, Fe-NiCoP/CC electrode displays a high areal capacity of 3.18 F cm-2 at 1 mA cm-2, superb rate capability (86.3 % capacity retention at 20 mA cm-2) and outstanding structure stability, superior to the NiCo/CC, FeNiCo/CC, and NiCoP/CC counterparts. Moreover, the assembled Fe-NiCoP/CC||VN/CNT/CC hybrid supercapacitor (HSC) device delivers a high energy density of 176.9 μWh cm-2 at the power density of 750 μW cm-2. More importantly, the designed electrodes and assembled HSC device exhibits excellent electromagnetic interference (EMI) shielding performance. This design concept presented in this paper can provide insights into the construction of multifunctional and high-performance flexible electronic devices.

A Biodegradable High-Performance Microsupercapacitor for Environmentally Friendly and Biocompatible Energy Storage

Biodegradable and biocompatible microscale energy storage devices are very crucial for environmentally friendly microelectronics and implantable medical applications. Herein, a biodegradable and biocompatible microsupercapacitor (BB-MSC) with satisfying overall performance is realized via the combination of three-dimensional (3D) printing technique and biodegradable materials. Due to the 3D-interconnected structure of electrodes and elaborated design of electrolyte, the as-prepared BB-MSC exhibits superior overall performance than most of biodegradable devices, including a wide operation voltage of 1.8 V, high areal specific capacitance of 251 mF/cm2, good cycle stability, and favorable low-temperature resistance (-20 °C), demonstrative of reliability and practicality of our devices even in frosty environments. Importantly, the smooth degradation has been realized for the BB-MSC after being buried in natural soil for ∼90 days, and its implantation does not affect the healthy status of SD rats. Therefore, this work explores avenues for the design and construction of environmentally friendly and biocompatible microscale energy storage devices.

General Design Concepts for CAPodes as Ionologic Devices

Capacitive analogues of semiconductor diodes (CAPodes) present a new avenue for energy-efficient and nature-inspired next-generation computing devices. Here, we disclose the generalized concept for bias-direction-adjustable n- and p-CAPodes based on selective ion sieving. Controllable-unidirectional ion flux is realized by blocking electrolyte ions from entering sub-nanometer pores. The resulting CAPodes exhibit charge-storage characteristics with a high rectification ratio (96.29 %). The enhancement of capacitance is attributed to the high surface area and porosity of an omnisorbing carbon as counter electrode. Furthermore, we demonstrate the use of an integrated device in a logic gate circuit architecture to implement logic operations ('OR', 'AND'). This work demonstrates CAPodes as a generalized concept to achieve p-n and n-p analogue junctions based on selective ion electrosorption, provides a comprehensive understanding and highlights applications of ion-based diodes in ionologic architectures.

High-Voltage MXene-Based Supercapacitors: Present Status and Future Perspectives

As an emerging class of 2D materials, MXene exhibits broad prospects in the field of supercapacitors (SCs). However, the working voltage of MXene-based SCs is relatively limited (typically ≤ 0.6 V) due to the oxidation of MXene electrode and the decomposition of electrolyte, ultimately leading to low energy density of the device. To solve this issue, high-voltage MXene-based electrodes and corresponding matchable electrolytes are developed urgently to extend the voltage window of MXene-based SCs. Herein, a comprehensive overview and systematic discussion regarding the effects of electrolytes (aqueous, organic, and ionic liquid electrolytes), asymmetric device configuration, and material modification on the operating voltage of MXene-based SCs, is presented. A deep dive is taken into the latest advances in electrolyte design, structure regulation, and high-voltage mechanism of MXene-based SCs. Last, the future perspectives on high-voltage MXene-based SCs and their possible development directions are outlined and discussed in depth, providing new insights for the rational design and realization of advanced next-generation MXene-based electrodes and high-voltage electrolytes.

Synergistic effect of intercalation and EDLC electrosorption of 2D/3D interconnected architectures to boost capacitive deionization for water desalination via MoSe2/mesoporous carbon hollow spheres

Transition-metal dichalcogenides can be used for capacitive deionization (CDI) via pseudocapacitive ion intercalation/de-intercalation due to their unique two-dimensional (2D) laminar structure. MoS₂ has been extensively studied in the hybrid capacitive deionization (HCDI), but the desalination performance of MoS₂-based electrodes remains only 20-35 mg g^-1 on average. Benefiting from the higher conductivity and larger layer spacing of MoSe₂ than MoS₂, it is expected that MoSe₂ would exhibit a superior HCDI desalination performance. Herein, for the first time, we explored the use of MoSe₂ in HCDI and synthesized a novel MoSe₂/MCHS composite material by utilizing mesoporous carbon hollow spheres (MCHS) as the growth substrate to inhibit the aggregation and improve the conductivity of MoSe₂. The as-obtained MoSe₂/MCHS presented unique 2D/3D interconnected architectures, allowing for synergistic effects of intercalation pseudocapacitance and electrical double layer capacitance (EDLC). An excellent salt adsorption capacity of 45.25 mg g^- 1 and a high salt removal rate of 7.75 mg g^- 1 min^-1 were achieved in 500 mg L^- 1 NaCl feed solution at an applied voltage of 1.2 V in batch-mode tests. Moreover, the MoSe₂/MCHS electrode exhibited outstanding cycling performance and low energy consumption, making it suitable for practical applications. This work demonstrates the promising application of selenides in CDI and provides new insights for ration design of high-performance composite electrode materials.

From 1D to 2D: Controllable Preparation of 2D Ni-MOFs for Supercapacitors

Controllable modulation strategies between one-dimensional (1D) and two-dimensional (2D) structures have been rarely reported for metal-organic frameworks (MOFs). Here, 1D, 1D/2D, and 2D Ni-MOFs can be facilely prepared by adjusting the ratio of Ni²⁺ and the pyromellitic acid linker. A low-dimensional structure can shorten the transmission distance, while MOFs with a high Ni²⁺ content can supply rich active sites for oxidation-reduction reactions. The 2D structure Ni-MOF with an optimized Ni²⁺/pyromellitic acid ratio presents a good performance of 1036 F g^-1 at a current density of 1 A g^-1 with a comparable rate performance of 62% at 20 A g^-1. The study may offer a facile design to control the structure of MOFs for employing in electrochemical energy storage.

Flexible aqueous supercapacitors with excellent cycling performance and high-energy density based on mesocrystalline NiCo-LDHs

Flexible aqueous supercapacitors are promising candidates as safe power sources for wearable electronic devices (WEDs). However, the absence of advanced electrode materials with high structural stability has become the most critical factor hindering the development, which is closely related to the poor interface combination between the active substances and flexible collectors. Herein, a unique rigid layered double hydroxide (LDH) nanorod array with the mesocrystalline feature is created using the NiO-Ni layer as the inducer by the electrodeposition strategy. Differing from the traditional NiCo-LDH nanosheets directly grown on a carbon cloth, an elaborately designed NiO-Ni buffer can simultaneously and effectively improve the bidirectional combination with active substances and collectors, also the mesocrystalline LDH showed enhanced intrinsic stability through the reinforcing effect of grain boundaries. Benefiting from these, the assembled supercapacitor exhibited pre-eminent cycle stability (increased from 64% of the initial capacity after 10 000 cycles to no significant attenuation after 50 000 cycles) and ultrahigh energy density. When it was used as a flexible device, a remarkable energy density of 70.4 W h kg^-1 could be harvested and processed with high flexibility in the bending state and good temperature adaptability. This study provides an excellent design strategy for the development of next-generation flexible supercapacitors with the goal of better comprehensive performances.

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.

Challenges and prospects of high-voltage aqueous electrolytes for energy storage applications

Aqueous electrolytes have attracted widespread attention as they are safe, environmentally benign and cost effective, holding great promise for future low-cost and sustainable energy storage devices. Nonetheless, the narrow electrochemical stability window caused by water electrolysis, as well as the trade-off between the stability window and other properties remain the bottleneck problem for the practical applications of aqueous electrolytes. Deep insights into the correlations between the microscopic physicochemical and electrochemical mechanisms and the macroscopic properties of aqueous electrolyte are essential for the envisaged applications, yet a systematic analysis of the recent progress in this area is still lacking. In this Perspective article, the basic mechanisms and influencing factors of water electrolysis including the hydrogen evolution and oxygen evolution reactions is critically examined. We systematically review the current state-of-the-art on high-voltage aqueous electrolytes focusing on the fundamental mechanisms of ion kinetics leading to dynamic electrolyte restructuring. Recent advances on the optimization of high-voltage aqueous electrolytes are also summarized. The existing challenges are identified and perspectives for exploring and developing future high-voltage aqueous electrolytes are provided.

Construction of Cu7KS4@NixCo1-x(OH)2 Nano-Core-Shell Structures with High Conductivity and Multi-Metal Synergistic Effect for Superior Hybrid Supercapacitors

Reasonable design of materials with complex nanostructures and diverse chemical compositions is of great significance in the field of energy storage. Cu₇KS₄ (CKS) is considered a potential electrode material for supercapacitors due to its superior electrical conductivity. Transition metal hydroxides are widely used as electrode materials for supercapacitors due to their high theoretical specific capacitance (C_s); however, single metal species with limited active sites restrict their further applications for energy storage. Herein, through a hydrothermal reaction, CKS nanorods were prepared, and then binary metal hydroxide Ni_xCo_1-x(OH)₂ nanosheets were generated directly on CKS nanorods through a one-step hydrothermal reaction to form a nano-core-shell structure (NCSS). By regulating the mole ratio of nickel nitrate to cobalt nitrate, the resulting Ni_0.75Co_0.25(OH)₂ nanosheets with the best electrochemical activity were prepared and supported on CKS nanorods to form a CKS@N_0.75C_0.25OH NCSS. The as-prepared CKS@N_0.75C_0.25OH NCSS has a larger specific surface area, which can provide more active sites, while the abundant metal species composition can generate abundant redox reactions to boost the pseudocapacitance. The prepared CKS@N_0.75C_0.25OH/NF electrode exhibits outstanding specific capacitance and cycle life. The assembled CKS@N_0.75C_0.25OH/NF//AC all-solid-state asymmetric supercapacitor achieves a high energy density of 88.7 Wh kg^-1 at a power density of 849.9 W kg^-1 with superior cycle life. Therefore, the use of polymetallic hydroxides to construct NCSS electrodes has great research significance and broad application prospects.