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Tremendous efforts have been dedicated into the development of high‐performance energy storage devices with nanoscale design and hybrid approaches. The boundary between the electrochemical capacitors and batteries becomes less distinctive. The same material may display capacitive or battery‐like behavior depending on the electrode design and the charge storage guest ions. Therefore, the underlying mechanisms and the electrochemical processes occurring upon charge storage may be confusing for researchers who are new to the field as well as some of the chemists and material scientists already in the field. This review provides fundamentals of the similarities and differences between electrochemical capacitors and batteries from kinetic and material point of view. Basic techniques and analysis methods to distinguish the capacitive and battery‐like behavior are discussed. Furthermore, guidelines for material selection, the state‐of‐the‐art materials, and the electrode design rules to advanced electrode are proposed. Fundamentals of the similarities and differences between electrochemical capacitors and batteries from kinetic and material point of view are provided in this review. Basic techniques and analysis methods to distinguish the capacitive and battery‐like behavior are discussed. Furthermore, guidelines for material selection, the state‐of‐the‐art materials, and the electrode design rules to advanced electrode are proposed.

To date, batteries are the most widely used energy storage devices, fulfilling the requirements of different industrial and consumer applications. However, the efficient use of renewable energy sources and the emergence of wearable electronics has created the need for new requirements such as high-speed energy delivery, faster charge–discharge speeds, longer lifetimes, and reusability. This leads to the need for supercapacitors, which can be a good complement to batteries. However, one of their drawbacks is their lower energy storage capability, which has triggered worldwide research efforts to increase their energy density. With the introduction of novel nanostructured materials, hierarchical pore structures, hybrid devices combining these materials, and unconventional electrolytes, significant developments have been reported in the literature. This paper reviews the short history of the evolution of supercapacitors and the fundamental aspects of supercapacitors, positioning them among other energy-storage systems. The main electrochemical measurement methods used to characterize their energy storage features are discussed with a focus on their specific characteristics and limitations. High importance is given to the integral components of the supercapacitor cell, particularly to the electrode materials and the different types of electrolytes that determine the performance of the supercapacitor device (e.g., storage capability, power output, cycling stability). Current directions in the development of electrode materials, including carbonaceous forms, transition metal-based compounds, conducting polymers, and novel materials are discussed. The synergy between the electrode material and the current collector is a key factor, as well as the fine-tuning of the electrode material and electrolyte.

The growing need for efficient energy storage has spurred advancements in supercapacitors (SCs), aiming to offer high power and energy density simultaneously. While SCs offer longer cycles and higher power density values, their low energy densities limit practical applications. In response, pseudocapacitive materials have emerged, leveraging reversible Faradaic reactions at or near the surface for enhanced energy storage. This approach surpasses the constraints of the electrical double layer in SCs and the mass transfer constraints of batteries. Progress in asymmetric supercapacitors and high mass loading has improved energy density values, yet maintaining high mass loading without compromising power density remains a hurdle. Advancements in pseudocapacitance through intercalation during charging/discharging processes, especially in layered structures like graphite, graphene, transition metal oxides (TMOs) transition metal dichalcogenides (TMDCs), MXenes, and metal–organic frameworks (MOFs) have proven significant. The intercalated species induce reversible or irreversible structural changes, contributing to the physicochemical characteristics of the electrode materials. Exploring the intercalation mechanism in bulk two‐dimensional (2D) materials reveals distinct differences that enhance our understanding and improve electrochemical properties for superior energy storage. Finally, an in‐depth exploration of the intercalation pseudocapacitance in 2D materials such as TMDCs and MXenes underscores their significance, setting a benchmark for future electrochemical studies in the subsequent advancement of SCs research. Intercalation pseudocapacitive electrodes store energy within the bulk of the electrode via a battery–like intercalation process, effectively bridging the gap between supercapacitors and lithium–ion batteries in terms of energy density and power density. This presents opportunities for further development of 2D materials that offer tunnels or layers for efficient intercalations. Furthermore, there has been progress in enhancing the pseudocapacitance performance of 2D redox–active materials in terms of energy density and cycle stability, to meet the increasing demand for energy storage systems. This review provides a comprehensive analysis of recent advancements in intercalation pseudocapacitors.

The interlayer modification and the intercalation pseudocapacitance have been combined in vanadium oxide electrode for aqueous zinc‐ion batteries. Intercalation pseudocapacitive hydrated vanadium oxide Mn1.4V10O24·12H2O with defective crystal structure, interlayer water, and large interlayer distance has been prepared by a spontaneous chemical synthesis method. The inserted Mn2+ forms coordination bonds with the oxygen of the host material and strengthens the interaction between the layers, preventing damage to the structure. Combined with the experimental data and DFT calculation, it is found that Mn2+ refines the structure stability, adjusts the electronic structure, and improves the conductivity of hydrated vanadium oxide. Also, Mn2+ changes the migration path of Zn2+, reduces the migration barrier, and improves the rate performance. Therefore, Mn2+‐inserted hydrated vanadium oxide electrode delivers a high specific capacity of 456 mAh g−1 at 0.2 A g–1, 173 mAh g–1 at 40 A g–1, and a capacity retention of 80% over 5000 cycles at 10 A g–1. Furthermore, based on the calculated zinc ion mobility coefficient and Zn(H2O)n2+ diffusion energy barrier, the possible migration behavior of Zn(H2O)n2+ in vanadium oxide electrode has also been speculated, which will provide a new reference for understanding the migration behavior of hydrated zinc‐ion. The strategy of interlayer modification of Mn2+ is applied to regulate the interlayer atomic coordination structure, electronic structure, and affect Zn2+ migration behavior in the vanadium oxide (V10O24·nH2O). Zinc ion intercalation pseudocapacitance, ultra‐high rate performance, and cycling stability are achieved and the Zn(H2O)n2+ migration behavior is analyzed and speculated. ​

An efficient anion‐exchange protocol was investigated for the controllable fabrication of hollow NiCo2S4 nanoboxes (NBs) from mesocrystalline nickel cobalt carbonate nanocubes as promising pseudocapacitive materials for electrochemical capacitors. The underlying processes of the formation of the hollow architecture were systematically investigated. Originating from the unique structural and compositional advantages, the resultant hollow NiCo2S4 NB electrode with a loading of 5 mg cm2 delivered a large specific capacitance of 777 F g−1 at a high rate of 4 A g−1 in a three‐electrode configuration with 6 m KOH as electrolyte. Furthermore, an asymmetric device constructed with the hollow NBs and activated carbon (AC) as positive and negative electrodes, respectively, showed extraordinary supercapacitance within an electrochemically operating voltage window from 0.0 to 1.5 V. The unique AC//NiCo2S4 NB hybrid capacitor exhibited a large specific energy density (active mass normalized) of approximately 17.1 W h kg−1 at a high power density of 2250 W kg−1, and desirable cycling durability with approximately 75 % specific capacitance retention after 5000 consecutive cycles at a current rate of 2 A g−1. These electrochemical investigations strongly indicated that the as‐fabricated hollow NiCo2S4 NBs can be elegantly utilized as powerful candidates for advanced electrode platforms. Boxing clever: Hollow NiCo2S4 nanoboxes were controllably fabricated from mesocrystalline metal carbonate nanocubes by an efficient anion‐exchange protocol, and demonstrated supercapacitance with a large energy density and desirable cycling performance at high rates (see figure).

HighlightsThe iontronic pressure sensor achieved an ultrahigh sensitivity (Smin > 200 kPa−1, Smax > 45,000 kPa−1).The iontronic pressure sensor exhibited a broad sensing range of over 1.4 MPa.Pseudocapacitive iontronic pressure sensor using MXene was proposed.Flexible pressure sensors are unprecedentedly studied on monitoring human physical activities and robotics. Simultaneously, improving the response sensitivity and sensing range of flexible pressure sensors is a great challenge, which hinders the devices’ practical application. Targeting this obstacle, we developed a Ti3C2Tx-derived iontronic pressure sensor (TIPS) by taking the advantages of the high intercalation pseudocapacitance under high pressure and rationally designed structural configuration. TIPS achieved an ultrahigh sensitivity (Smin > 200 kPa−1, Smax > 45,000 kPa−1) in a broad sensing range of over 1.4 MPa and low limit of detection of 20 Pa as well as stable long-term working durability for 10,000 cycles. The practical application of TIPS in physical activity monitoring and flexible robot manifested its versatile potential. This study provides a demonstration for exploring pseudocapacitive materials for building flexible iontronic sensors with ultrahigh sensitivity and sensing range to advance the development of high-performance wearable electronics.

The enormous demand for energy due to rapid technological developments pushes mankind to the limits in the exploration of high-performance energy devices. Among the two major energy storage devices (capacitors and batteries), electrochemical capacitors (known as ‘Supercapacitors’) play a crucial role in the storage and supply of conserved energy from various sustainable sources. The high power density and the ultra-high cyclic stability are the attractive characteristics of supercapacitors. However, the low energy density is a major downside of them, which is also responsible for the extensive research in this field to help the charge storage capabilities thrive to their limits. Discoveries of electrical double-layer formation, pseudocapacitive and intercalation-type (battery-type) behaviors drastically improved the electrochemical performances of supercapacitors. The introduction of nanostructured active materials (carbon-/metal-/redox-active-polymer/metal-organic/covalent-organic framework-based electrode materials), electrolytes (conventional aqueous and unconventional systems) with superior electrochemical stability and unprecedented device architectures further boosted their charge storage characteristics. In addition, the detailed investigations of the various processes at the electrode–electrolyte interfaces enable us to reinforce the present techniques and the approaches toward high-performance and next-generation supercapacitors. In this review, the fundamental concepts of the supercapacitor device in terms of components, assembly, evaluation, charge storage mechanism, and advanced properties are comprehensively discussed with representative examples.

The family of halide perovskite materials is extremely large and has gained huge attention because of their low manufacturing cost and extraordinary structural, optical, electrical, and optoelectronic properties. These materials also deliver a pattern for designing new materials for energy conversion and energy storage applications. Here, we synthesized potassium cadmium chloride KCdCl 3 -based halide perovskite nanocomposites with rGO and fullerene-C60 by facile solvothermal method and studied their physical and electrochemical properties. The orthorhombic phase of KCdCl 3 was confirmed from XRD spectra, and the existence of constituent elements (K, Cd, Cl, and C) was confirmed from EDX analysis. SEM images evident the successful anchoring of KCdCl 3 particles over rGO and C60. BET results revealed the high surface area, pore radius, and pore volume of the KCdCl 3 /C60 electrodes. Furthermore, the electrochemical measurements demonstrated that KCdCl 3 /C60-based electrodes have a higher specific capacitance of 1135 F/g at 5 mV/s and cyclic stability (97.6% retention over 3000th cycles) than other grown electrodes. Also, GCD measurement results revealed that KCdCl 3 /C60 electrode has a high specific capacitance of 1420 F/g, an energy density of 2052 Wh/kg, and a power density of 4.19 W/kg at 1.0 A/g than other electrodes. Finally, intensive discussion proposed that halide perovskite nanocomposite electrodes can be used efficiently as supercapacitors electrode materials for future development in this field. Graphical abstract

HighlightsLayered iron vanadate ultrathin nanosheets (FeVO UNSs) with a thickness of ~ 2.2 nm were synthesized by a sonicate-assisted method.Pseudocapacitive Na+ intercalation of FeVO UNSs anode delivers high initial coulombic efficiency (93.86%), high reversible capacity (292 mAh g−1), excellent rate capability, and remarkable cycling stability.A pseudocapacitor–battery hybrid SIC is assembled with the elimination of presodiation and delivers high energy and power densities.High-performance and low-cost sodium-ion capacitors (SICs) show tremendous potential applications in public transport and grid energy storage. However, conventional SICs are limited by the low specific capacity, poor rate capability, and low initial coulombic efficiency (ICE) of anode materials. Herein, we report layered iron vanadate (Fe5V15O39 (OH)9·9H2O) ultrathin nanosheets with a thickness of ~ 2.2 nm (FeVO UNSs) as a novel anode for rapid and reversible sodium-ion storage. According to in situ synchrotron X-ray diffractions and electrochemical analysis, the storage mechanism of FeVO UNSs anode is Na+ intercalation pseudocapacitance under a safe potential window. The FeVO UNSs anode delivers high ICE (93.86%), high reversible capacity (292 mAh g−1), excellent cycling stability, and remarkable rate capability. Furthermore, a pseudocapacitor–battery hybrid SIC (PBH-SIC) consisting of pseudocapacitor-type FeVO UNSs anode and battery-type Na3(VO)2(PO4)2F cathode is assembled with the elimination of presodiation treatments. The PBH-SIC involves faradaic reaction on both cathode and anode materials, delivering a high energy density of 126 Wh kg−1 at 91 W kg−1, a high power density of 7.6 kW kg−1 with an energy density of 43 Wh kg−1, and 9000 stable cycles. The tunable vanadate materials with high-performance Na+ intercalation pseudocapacitance provide a direction for developing next-generation high-energy capacitors.

Polyaniline‐capped mesoporous carbon nanosheets with high conductivity and porosity are synthesized by vapor deposition polymerization. The mesoporous carbon template is prepared by removing ordered cubic iron oxide nanocrystals embedded in the carbon matrix obtained by thermal decomposition of an iron‐oleate complex in a sodium chloride matrix. The evaporated aniline monomers are slowly polymerized on the carbon surface pretreated with FeCl3 as an initiator, partially filling the carbon pores to improve conductivity. The resulting products exhibit efficient hybrid energy storage mechanisms of electric double‐layer capacitance and pseudocapacitance. When the nanosheets are assembled for a symmetric supercapacitor, the device capacitance reaches 107.8 F g−1, at a current density of 0.5 A g−1, and a capacitance retention of 69.6% is achieved at a ten times higher current density of 5 A g−1. Electrochemical impedance spectroscopy reveals that the transition from resistive to capacitive behavior occurs within 0.63 s, indicating that fast ion and charge transport results in high capacitance and rate capability. The corresponding energy and power densities are 9.59 Wh kg−1 and 200.1 W kg−1 at a current density of 0.5 A g−1, demonstrating efficient energy storage in a symmetric supercapacitor. The highly ordered mesoporous carbon (mC) matrix is synthesized and polyaniline (PANI) is partially filled with the vapor deposition polymerization method. The resulting mC/PANI nanosheets‐assembled symmetric supercapacitor exhibits energy density and power density of 9.59 Wh kg−1 and 200.1 W kg−1 at a current density of 0.5 A g−1, which are higher than the majority of the reported symmetric supercapacitors.