High‐Performance Relaxor Ferroelectric Materials for Energy Storage Applications

Relaxor ferroelectrics usually possess low remnant polarizations and slim hystereses, which can provide high saturated polarizations and superior energy conversion efficiencies, thus receiving increasing interest as energy storage materials with high discharge energy densities and fast discharge ability. In this study, a relaxor ferroelectric multilayer energy storage ceramic capacitor (MLESCC) based on 0.87BaTiO₃‐0.13Bi(Zn_2/3(Nb_0.85Ta_0.15)_1/3)O₃ (BT‐BZNT) with inexpensive Ag/Pd inner electrodes is prepared by the tape casting method. The MLESCC with two dielectric layers (layer thicknesses of 5 µm) sintered by a two‐step sintering method exhibits excellent energy storage properties with a record‐high discharge energy density of 10.12 J cm^?3, a high energy efficiency of 89.4% achieved at an electric field of 104.7 MV m^?1, a high temperature stability of the energy storage density (with minimal variation of <±5%), and energy efficiency (>90%) over a range of ?75 to 150 °C at 40 MV m^?1. These results suggest that the BT‐BZNT relaxor ferroelectric ceramic material can provide realistic solutions for high‐power energy storage capacitors.     

Fatigue‐Free Aurivillius Phase Ferroelectric Thin Films with Ultrahigh Energy Storage Performance

Dielectric capacitors have become a key enabling technology for electronics and electrical systems. Although great strides have been made in the development of ferroelectric ceramic and thin films for capacitors, much less attention has been given to preventing polarization fatigue, while improving the energy density, of ferroelectrics. Here superior capacitive properties and outstanding stability are reported over 10⁷ charge/discharge cycles and a wide temperature range of ?60 to 200 °C of ferroelectric Aurivillius phase Bi_3.25La_0.75Ti₃O₁₂‐BiFeO₃ (BLT‐BFO), which represents one of the best capacitive performances recorded for the ferroelectric materials. The modification of BLT thin films with BFO overcomes the constraints of ferroelectric Aurivillius compounds and presents an unprecedented combination of the ideal features including improved polarization, reduced ferroelectric hysteresis, and lowered leakage current for high‐energy‐density capacitors. Given the lead‐free and fatigue‐free nature of this Aurivillius phase ferroelectric, this work unveils a new approach towards high‐performance eco‐friendly ferroelectric materials for electrical energy storage applications.     

Wide Potential Window Supercapacitors Using Open‐Shell Donor–Acceptor Conjugated Polymers with Stable N‐Doped States

Supercapacitors have emerged as an important energy storage technology offering rapid power delivery, fast charging, and long cycle lifetimes. While extending the operational voltage is improving the overall energy and power densities, progress remains hindered by a lack of stable n‐type redox‐active materials. Here, a new Faradaic electrode material comprised of a narrow bandgap donor?acceptor conjugated polymer is demonstrated, which exhibits an open‐shell ground state, intrinsic electrical conductivity, and enhanced charge delocalization in the reduced state. These attributes afford very stable anodes with a coulombic efficiency of 99.6% and that retain 90% capacitance after 2000 charge–discharge cycles, exceeding other n‐dopable organic materials. Redox cycling processes are monitored in situ by optoelectronic measurements to separate chemical versus physical degradation mechanisms. Asymmetric supercapacitors fabricated using this polymer with p‐type PEDOT:PSS operate within a 3 V potential window, with a best‐in‐class energy density of 30.4 Wh kg^?1 at a 1 A g^?1 discharge rate, a power density of 14.4 kW kg^?1 at a 10 A g^?1 discharge rate, and a long cycle life critical to energy storage and management. This work demonstrates the application of a new class of stable and tunable redox‐active material for sustainable energy technologies.     

Synthesis of Pseudocapacitive Polymer Chain Anode and Subnanoscale Metal Oxide Cathode for Aqueous Hybrid Capacitors Enabling High Energy and Power Densities along with Long Cycle Life

Aqueous electrochemical energy storages are of enormous attention due to their high safety and being environmentally friendly, but they must satisfy very challenging standards in energy and power densities over long repeated charging/discharging cycles. Herein, a strategy to realize high‐performance aqueous hybrid capacitors (AHCs) using pseudocapacitive negative and positive electrodes is reported. Polymer chains, which are synthesized by in situ polymerization of polyaniline on reduced graphene sheets, show fiber‐like morphologies and the redox‐reactive surface area allowing high capacitance as anode materials even at a high current density of 20 A g^?1 and a high loading of ≈6 mg cm^?2. Additionally, subnanoscale metal oxide particles on graphene are utilized as pseudocapacitive cathode materials and they show the approximately threefold higher capacitance than nanocrystals of ≈10 nm. Assembling these polymer chain anode and subnanoscale metal oxide cathode in full‐cell AHCs is shown to give the high energy density exceeding those of aqueous batteries along with the ≈100% capacity retention over 100 000 redox cycles. Additionally, AHCs exhibit the high power density allowing ultrafast charging, so that the switching wearable display kit with two AHCs in series can be charged within several seconds by the flexible photovoltaic module and USB switching charger.     

High Energy Density Aqueous Li‐Ion Flow Capacitor

High energy density Li‐ion hybrid flow capacitors are demonstrated by employing LiMn₂O₄ and activated carbon slurry electrodes. Compared to the existing aqueous flow electrochemical capacitors, the hybrid one exhibits much higher energy densities due to the introduction of high capacity Li‐insertion materials (e.g., LiMn₂O₄ in the present work) as the flowable electrode with asymmetrical cell configuration. A record energy density, i.e., 23.4 W h kg^?1 at a power of 50.0 W kg^?1 has been achieved for aqueous flow capacitors tested at static condition reported to date. A full operational Li‐ion flow capacitor tested in an intermittent‐flow mode has also been demonstrated. The Li‐ion hybrid flow capacitor shows great promise for high‐rate grid applications.     

Functional Pillared Graphene Frameworks for Ultrahigh Volumetric Performance Supercapacitors

Supercapacitors, also known as electrochemical capacitors, can provide much faster charge–discharge, greater power density, and cyclability than batteries, but they are still limited by lower energy densities (or the amount of energy stored per unit volume). Here, a novel strategy for the synthesis of functional pillared graphene frameworks, in which graphene fragments in‐between graphene sheets, through simple thermal‐treatment of ozone (O₃)‐treated graphene oxide at very low temperature of 200 °C is reported. Due to its high packing density, high content of stable oxygen species, and continues ion transport network in‐between graphene sheets, the functional pillared‐graphene framework delivers not only high gravimetric capacitance (353 F g^?1 based on the mass of the active material) and ultrahigh volumetric capacitance (400 F cm^?3 based on total mass of electrode material) in aqueous electrolyte but also excellent cyclic stability with 104% of its initial capacitance retention after 10 000 cycles. Moreover, the assembled symmetric supercapacitor achieves as high as 27 Wh L^?1 of volumetric energy density at a power density of 272 W L^?1. This novel strategy holds great promise for future design of high volumetric capacitance supercapacitors.     

Rational Design of Nickel Hydroxide‐Based Nanocrystals on Graphene for Ultrafast Energy Storage

Compact, light, and powerful energy storage devices are urgently needed for many emerging applications; however, the development of advanced power sources relies heavily on advances in materials innovation. Here, the findings in rational design, one‐pot synthesis, and characterization of a series of Ni hydroxide‐based electrode materials in alkaline media for fast energy storage are reported. Under the guidance of density functional theory calculations and experimental investigations, a composite electrode composed of Co‐/Mn‐substituted Ni hydroxides grown on reduced graphene oxide (rGO) is designed and prepared, demonstrating capacities of 665 and 427 C g^?1 at current densities of 2 and 20 A g^?1, respectively. The superior performance is attributed mainly to the low deprotonation energy and the facile electron transport, as elaborated by theoretical calculations. When coupled with an electrode based on organic molecular‐modified rGO, the resulting hybrid device demonstrates an energy density of 74.7 W h kg^?1 at a power density of 1.68 kW kg^?1 while maintaining capacity retention of 91% after 10,000 cycles (20 A g^?1). The findings not only provide a promising electrode material for high‐performance hybrid capacitors but also open a new avenue toward knowledge‐based design of efficient electrode materials for other energy storage applications.     

Paper‐Based Energy‐Storage Devices Comprising Carbon Fiber‐Reinforced Polypyrrole‐Cladophora Nanocellulose Composite Electrodes

Composites of polypyrrole (PPy) and Cladophora nanocellulose, reinforced with 8 μm‐thick chopped carbon filaments, can be used as electrode materials to obtain paper‐based energy‐storage devices with unprecedented performance at high charge and discharge rates. Charge capacities of more than 200 C g^?1 (PPy) are obtained for paper‐based electrodes at potential scan rates as high as 500 mV s^?1, whereas cell capacitances of ～60–70 F g^?1 (PPy) are reached for symmetric supercapacitor cells with capacitances up to 3.0 F (i.e.,0.48 F cm^?2) when charged to 0.6 V using current densities as high as 31 A g^?1 based on the PPy weight (i.e., 99 mA cm^?2). Energy and power densities of 1.75 Wh kg^?1 and 2.7 kW kg^?1, respectively, are obtained when normalized with respect to twice the PPy weight of the smaller electrode. No loss in cell capacitance is seen during charging/discharging at 7.7 A g^?1 (PPy) over 1500 cycles. It is proposed that the nonelectroactive carbon filaments decrease the contact resistances and the resistance of the reduced PPy composite. The present straightforward approach represents significant progress in the development of low‐cost and environmentally friendly paper‐based energy‐storage devices for high‐power applications.     

The Effect of Crystallinity on the Rapid Pseudocapacitive Response of Nb2O5

Capacitive energy storage offers several attractive properties compared to batteries, including higher power, faster charging, and a longer cycle life. A key limitation to this electrochemical energy‐storage approach is its low energy density and, for this reason, there is considerable interest in identifying pseudocapacitor materials where faradaic reactions are used to achieve greater charge storage. This paper reports on the electrochemical properties of Nb₂O₅ and establishes that crystalline phases of the material undergo fast faradaic reactions that lead to high specific capacitance in short charging times. In particular, the specific capacitance for the orthorhombic phase at infinite sweep rate reaches ≈400 F g^?1, which exceeds that of birnessite MnO₂ in nonaqueous electrolyte and is comparable to RuO₂ at the same extrapolated rate. The specific capacitances of the orthorhombic and pseudohexagonal phases are much greater than that of the amorphous phase, suggesting that the faradaic reactions which lead to additional capacitive energy storage are associated with Li⁺ insertion along preferred crystallographic pathways. The ability for Nb₂O₅ to store charge at high rates despite its wide bandgap and low electronic conductivity is very different from what is observed with other transition metal oxides.     

Phase Transformation Induced Capacitance Activation for 3D Graphene‐CoO Nanorod Pseudocapacitor

Development of a pseudocapacitor over the integration of metal oxide on carbonaceous materials is a promising step towards energy storage devices with high energy and power densities. Here, a self‐assembled cobalt oxide (CoO) nanorod cluster on three‐dimensional graphene (CoO‐3DG) is synthesized through a facile hydrothermal method followed by heat treatment. As an additive‐free electrode, CoO‐3DG exhibits good electrochemical performance. Compared with CoO nanorod clusters grown on Ni foam (i.e., CoO‐Ni, ≈680 F g^?1 at 1 A g^?1 and ≈400 F g^?1 at 20 A g^?1), CoO‐3DG achieves much higher capacitance (i.e., ≈980 F g^?1 at 1 A g^?1 and ≈600 F g^?1 at 20 A g^?1) with excellent cycling stability of 103% retention of specific capacitance after 10 000 cycles. Furthermore, it shows an interesting activation process and instability with a redox reaction for CoO. In addition, the phase transformation from CoO nanorods to Co₃O₄ nanostructures was observed and investigated after charge and discharge process, which suggests the activation kinetics and the phase transformable nature of CoO based nanostructure. These observations demonstrate phase transformation with morphological change induced capacitance increasement in the emergent class of metal oxide materials for electrochemical energy storage device.     

3D Architecture Materials Made of NiCoAl‐LDH Nanoplates Coupled with NiCo‐Carbonate Hydroxide Nanowires Grown on Flexible Graphite Paper for Asymmetric Supercapacitors

Asymmetric supercapacitors featuring both high energy and power densities as well as a long lifespan are much sought after and may become a reality depending on the availability of cheap yet highly active electrode materials. Here, a novel flexible architecture electrode made of NiCoAl‐layered double hydroxide (NiCoAl‐LDH) nanoplates coupled with NiCo‐carbonate hydroxide (NiCo‐CH) nanowires, grown on graphite paper via an in situ, one‐step, hydrothermal method is reported. The nanowire‐like NiCo‐CH species in the nanoplate matrix function as a scaffold and support the dispersion of the NiCoAl‐LDH nanoplates, resulting in a relatively loose and open structure within the electrode matrix. Asymmetric supercapacitors fabricated using the nanohybrids as the positive electrode and a typical activated carbon (AC) as negative electrode show a high energy density of 58.9 Wh kg^?1 at a power density of 0.4 kW kg^?1, which is based on the total mass of active materials at a voltage of 1.6 V. An energy density of 14.9 Wh kg^?1 can be retained even at a high power density of 51.5 kW kg^?1. Our asymmetric supercapacitor also exhibits an excellent long cycle life, whereby a specific capacitance of 97% is retained even after 10 000 cycles.     

On‐Chip MXene Microsupercapacitors for AC‐Line Filtering Applications

Microsupercapacitors (MSCs) with high energy densities offer viable miniaturized alternatives to bulky electrolytic capacitors if the former can respond at the kilo Hertz (kHz) or higher frequencies. Moreover, MSCs fabricated on a chip can be integrated into thin‐film electronics in a compatible manner, serving the function of ripple filtering units or harvesters of energy from high‐frequency sources. In this work, wafer‐scale fabrication is demonstrated of MXene microsupercapacitors with controlled flake sizes and engineered device designs to achieve excellent frequency filtering performance. Specifically, the devices (100 nm thick electrodes and 10 µm interspace) deliver high volumetric capacitance (30 F cm^?3 at 120 Hz), high rate capability (300 V s^?1), and a very short relaxation time constant (τ₀ = 0.45 ms), surpassing conventional electrolytic capacitors (τ₀ = 0.8 ms). As a result, the devices are capable of filtering 120 Hz ripples produced by AC line power at a frequency of 60 Hz. This study opens new avenues for exploring miniaturized MXene MSCs as replacements for bulky electrolytic capacitors.     

All Pseudocapacitive MXene‐RuO2 Asymmetric Supercapacitors

2D transition metal carbides and nitrides, known as MXenes, are an emerging class of 2D materials with a wide spectrum of potential applications, in particular in electrochemical energy storage. The hydrophilicity of MXenes combined with their metallic conductivity and surface redox reactions is the key for high‐rate pseudocapacitive energy storage in MXene electrodes. However, symmetric MXene supercapacitors have a limited voltage window of around 0.6 V due to possible oxidation at high anodic potentials. In this study, the fact that titanium carbide MXene (Ti₃C₂T_x) can operate at negative potentials in acidic electrolyte is exploited, to design an all‐pseudocapacitive asymmetric device by combining it with a ruthenium oxide (RuO₂) positive electrode. This asymmetric device operates at a voltage window of 1.5 V, which is about two times wider than the operating voltage window of symmetric MXene supercapacitors, and is the widest voltage window reported to date for MXene‐based supercapacitors. The complementary working potential windows of MXene and RuO₂, along with proton‐induced pseudocapacitance, significantly enhance the device performance. As a result, the asymmetric devices can deliver an energy density of 37 µW h cm^?2 at a power density of 40 mW cm^?2, with 86% capacitance retention after 20 000 charge–discharge cycles. These results show that pseudocapacitive negative MXene electrodes can potentially replace carbon‐based materials in asymmetric electrochemical capacitors, leading to an increased energy density.     

Pushing the Energy Output and Cyclability of Sodium Hybrid Capacitors at High Power to New Limits

Hybrid capacitors, especially sodium hybrid capacitors (NHCs), have continued to gain importance and are extensively studied based on their excellent potential to serve as advanced devices for fulfilling high energy and high power requirements at a low cost. To achieve remarkable performance in hybrid capacitors, the two electrodes employed must be superior with enhanced charge storage capability and fast kinetics. In this study, a new sodium hybrid capacitor system with a sodium super ionic conductor NaTi₂(PO₄)₃ grown on graphene nanosheets as an intercalation electrode and 2D graphene nanosheets as an adsorption electrode is reported for the first time. This new system delivers a high energy density of ≈80 W h kg^?1 and a high specific power of 8 kW kg^?1. An ultralow performance fading of ≈0.13% per 1000 cycles (90%–75 000 cycles) outperforms previously reported sodium ion capacitors. The enhanced charge transfer kinetics and reduced interfacial resistance at high current rates deliver a high specific energy without compromising the high specific power along with high durability, and thereby bridge batteries and capacitors. This new research on kinetically enhanced NHCs can be a trendsetter for the development of advanced energy storage devices requiring high energy—high power.     

Battery‐like Supercapacitors from Vertically Aligned Carbon Nanofiber Coated Diamond: Design and Demonstrator

To fabricate battery‐like supercapacitors with high power and energy densities, big capacitances, as well as long‐term capacitance retention, vertically aligned carbon nanofibers (CNFs) grown on boron doped diamond (BDD) films are employed as the capacitor electrodes. They possess large surface areas, high conductivity, high stability, and importantly are free of binder. The large surface areas result from their porous structures. The containment of graphene layers and copper metal catalysts inside CNFs leads to their high conductivity. Both electrical double layer capacitors (EDLCs) in inert solutions and pseudocapacitors (PCs) using Fe(CN)₆^3?/4? redox‐active electrolytes are constructed with three‐ and two‐electrode systems. The assembled two‐electrode symmetrical supercapacitor devices exhibit capacitances of 30 and 48 mF cm^?2 at 10 mV s^?1 for EDLC and PC devices, respectively. They remain constant even after 10 000 charging/discharging cycles. The power densities are 27.3 and 25.3 kW kg^?1 for EDLC and PC devices, together with their energy densities of 22.9 and 44.1 W h kg^?1, respectively. The performance of these devices is superior to most of the reported supercapacitors and batteries. Vertically aligned CNF/BDD hybrid films are thus useful to construct high‐performance battery‐like and industry‐orientated supercapacitors for future power devices.     

Integrated Conductive Hybrid Architecture of Metal–Organic Framework Nanowire Array on Polypyrrole Membrane for All‐Solid‐State Flexible Supercapacitors

Metal–organic frameworks (MOFs) with intrinsically porous structures are promising candidates for energy storage, however, their low electrical conductivity limits their electrochemical energy storage applications. Herein, the hybrid architecture of intrinsically conductive Cu‐MOF nanowire arrays on self‐supported polypyrrole (PPy) membrane is reported for integrated flexible supercapacitor (SC) electrodes without any inactive additives, binders, or substrates involved. The conductive Cu‐MOFs nanowire arrays afford high conductivity and a sufficiently active surface area for the accessibility of electrolyte, whereas the PPy membrane provides decent mechanical flexibility, efficient charge transfer skeleton, and extra capacitance. The all‐solid‐state flexible SC using integrated hybrid electrode demonstrates an exceptional areal capacitance of 252.1 mF cm^?2, an energy density of 22.4 µWh cm^?2, and a power density of 1.1 mW cm^?2, accompanied by an excellent cycle capability and mechanical flexibility over a wide range of working temperatures. This work not only presents a robust and flexible electrode for wide temperature range operating SC but also offers valuable concepts with regards to designing MOF‐based hybrid materials for energy storage and conversion systems.     

Antimonene Engineered Highly Deformable Freestanding Electrode with Extraordinarily Improved Energy Storage Performance

Advanced 2D materials have spurred great interest as a new paradigm in pursuing improved energy storage performance. Herein, for the first time, antimonene is utilized as an effective active component for constructing highly deformable and editable freestanding film electrodes, as the basis of a supercapacitor with record‐breaking electrode performance. The insertion of antimonene is able to improve the environmental stability of the antimonene/MXene composite electrode and remarkably enhance the energy storage capability in both protic and neutral electrolytes. Notably, an ultrahigh specific volumetric capacitance of 4255 F cm^?3 is achieved by the electrode tested in a1 m H₂SO₄ electrolyte, which represents the state‐of‐the‐art value reported to date for supercapacitor electrodes based on MXenes. The flexible supercapacitors constructed by the composite electrode, also demonstrate highly competitive energy and power densities: 459.75 mWh cm^?3 and 3.12 W cm^?3 for the asymmetrical one with a much widened potential window of 2 V in neutral electrolyte; 112.52 mWh cm^?3 and 1 W cm^?3 for the symmetrical configuration with an outstanding capacitance of 1265 F cm^?3 in acidic media. This work sheds new light on the fabrication of high‐performance supercapacitor electrodes with functionalities in different electrolyte media and various device configurations.     

Nitrogen‐Doped Wrinkled Carbon Foils Derived from MOF Nanosheets for Superior Sodium Storage

Preparation of hierarchical carbon nanomaterials from metal?organicframeworks (MOFs) offers immense potential in the improvement of energy density, tunability, and stability of functional materials for energy storage and conversion. How interconnected nitrogen (N)‐doped wrinkled carbon foils derived from MOF nanosheets can serve as high‐performance sodium storage materials due to their multiscale porous structure is shown here. The novel N‐doped carbon nanomaterials are synthesized through the pyrolysis of 2D Mn‐based MOFs, which are produced through the assistance of monodentate ligands to enable the planar growth of MOFs. Subsequent acid etching creates hierarchical pores and channels to allow rapid ion transport. The resulting materials achieve high‐rate capability (165 and 150 mA h g^?1 at current densities of 8 and 10 A g^?1, respectively) and high stability (capacity retention 72.8% after 1000 cycling at 1.0 A g^?1), when they are used as anode in sodium‐ion capacitors.     

Scaling Printable Zn–Ag2O Batteries for Integrated Electronics

Printed batteries are an emerging solution for integrated energy storage using low‐cost, high accuracy fabrication techniques. While several printed batteries have been previously shown, few have designed a battery that can be incorporated into an integrated device. Specifically, a fully printed battery with a small active electrode area (<1 cm²) achieving high areal capacities (>10 mAh cm^?2) at high current densities (1–10 mA cm^?2) has not been demonstrated, which represents the minimum form‐factor and performance requirements for many low‐power device applications. This work addresses these challenges by investigating the scaling limits of a fully printed Zn–Ag₂O battery and determining the electrochemical limitations for a mm²‐scale battery. Processed entirely in air, Zn–Ag₂O batteries are well suited for integration in typical semiconductor packaging flows compared to lithium‐based chemistries. Printed cells with electrodes as small as 1 mm² maintain steady operating voltages above (>1.4 V) at high current densities (1–12 mA cm^?2) and achieve the highest reported areal capacity for a fully printed battery at 11 mAh cm^?2. The findings represent the first demonstration of a small, packaged, fully printed Zn–Ag₂O battery with high areal capacities at high current densities, a crucial step toward realizing chip‐scale energy storage for integrated electronic systems.     

Antimonene: A Novel 2D Nanomaterial for Supercapacitor Applications

In pursuing higher energy density, without compromising the power density of supercapacitor platforms, the application of an advanced 2D nanomaterial is utilized to maximize performance. Antimonene, for the first time, is characterized as a material for applications in energy storage, being applied as an electrode material as the basis of a supercapacitor. Antimonene is shown to significantly improve the energy storage capabilities of a carbon electrode in both cyclic voltammetry and galvanostatic charging. Antimonene demonstrates remarkable performance with a capacitance of 1578 F g^?1, with a high charging current density of 14 A g^?1. Hence, antimonene is shown to be a highly promising material for energy storage applications. The system also demonstrates a highly competitive energy and power densities of 20 mW h kg^?1 and 4.8 kW kg^?1_, respectively. In addition to the excellent charge storing abilities, antimonene shows good cycling capabilities.