A mini-review: emerging all-solid-state energy storage electrode materials for flexible devices

Development of the all-solid-state flexible supercapacitor membranes via RAFT-mediated grafting and electrospun nanofiber modification of track-etched membranes

Developing novel membranes marks a significant advancement in flexible energy storage systems. In this work, a hybrid track-etched membrane (TeM) was synthesized through RAFT-mediated polymerization, where poly(acrylic acid) (PAA) was grafted onto both the nanopore walls and surface of PET-based TeMs (PET-g-PAA), creating a stable and functionalized matrix for further enhancements. The membrane was then modified by incorporating electrospun composite nanofibers made from poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) as the polymer matrix, ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate, EM-IMBF4) as the supporting electrolyte, and graphene oxide (GO) as the ionic conductivity enhancer. The nanofibers (PVDF-HFP_GO) were deposited on either one or both surfaces of the grafted membrane. These modifications substantially improved the membrane's active surface area, porosity, and electrochemical performance, positioning it as a strong candidate for flexible energy storage applications. Comprehensive characterizations verified the successful modification and enhanced properties, including FTIR, SEM-EDX, XPS, TGA, porosity analysis, and contact angle measurements. Electrochemical performance was evaluated through cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). Among the tested membranes, the one modified with 0.5% GO-containing nanofibers demonstrated the highest capacitance and coulombic efficiency. Although the membrane showed strong charge/discharge efficiency and high initial performance, performance degradation was observed after extended cycling, particularly at higher current densities. The ionic conductivity of the hybrid membranes (with a GO concentration of 0.5%) reaches 14.83 × 10-3 mS cm-1 for single-sided nanofiber-covered membranes and 39.08 × 10-3 mS cm-1 for double-sided nanofiber-covered membranes, while for similar samples without addition of GO this values were found to be of 1.42 × 10-3 mS cm-1, which is significantly higher than conventional polymer-based electrolyte membranes (∼10-4 to 10-2 mS cm-1), and comparable to advanced ionic gel-based systems (∼10-2 to 10-1 mS cm-1). The synergistic effects of PAA grafting and PVDF-HFP_GO fibers delivered competitive charge/discharge efficiency when compared to similar systems, though further optimization of current density and cycling stability is required. This study highlights the potential of combining the RAFT-mediated grafting technique with electrospun composite nanofibers in modifying TeMs to develop durable and flexible supercapacitor membranes with promising electrochemical performance.

Inkjet-printed flexible V2CTx film electrodes with excellent photoelectric properties and high capacities for energy storage device

Energy storage devices are progressively advancing in the light-weight, flexible, and wearable direction. Ti3C2Tx flexible film electrodes fabricated via a non-contact, cost-effective, high-efficiency, and large-scale inkjet printing technology were capable of satisfying these demands in our previous report. However, other MXenes that can be employed in flexible energy storage devices remain undiscovered. Herein, flexible V2CTx film electrodes (with the low formula weight vs Ti3C2Tx film electrodes) with both high capacities and excellent photoelectric properties were first fabricated. The area capacitances of V2CTx film electrodes reached 531.3-5787.0 μF⋅cm-2 at 5 mV⋅s-1, corresponding to the figure of merits (FoMs) of 0.07-0.15. Noteworthy, V2CTx film electrode exhibited excellent cyclic stability with the capacitance retention of 83 % after 7,000 consecutive charge-discharge cycles. Furthermore, flexible all solid-state symmetric V2CTx supercapacitor was assembled with the area capacitance of 23.4 μF⋅cm-2 at 5 mV⋅s-1. Inkjet printing technology reaches the combination of excellent photoelectric properties and high capacities of flexible V2CTx film electrodes, which provides a new strategy for manufacturing MXene film electrodes, broadening the application prospect of flexible energy storage devices.

Functional metal-organic frameworks derived electrode materials for electrochemical energy storage: a review

Pristine metal-organic frameworks (MOFs) are built through self-assembly of electron rich organic linkers and electron deficient metal nodes via coordinate bond. Due to the unique properties of MOFs like highly tunable frameworks, huge specific surface areas, flexible chemical composition, flexible structures and a large volume of pores, they are being used to design the electrode materials for electrochemical energy storage devices. As per the literature, MOFs (including manganese, nickel, copper, and cobalt-based zeolitic imidazolate frameworks (ZIFs), University of Oslo (UiO) MOFs, Hong Kong University of Science and Technology (HKUST) MOFs and isoreticular MOFs (IRMOFs)) have attracted much attention in the field of supercapacitors (SCs)/batteries. According to their dimensionality such as 1D, 2D and 3D, pristine MOFs are mainly used as SC materials. Highly porous materials and their composites are capable for intercalation of metal ions (Na+/Li+). Moreover, the supramolecular features (π⋯π, C-H⋯π, hydrogen bond interactions) of redox stable MOFs provide better insight for electrochemical stability. So, this review provides an in-depth analysis of pure MOFs and MOF derived composites (MOF composites and MOF derived porous carbon) as electrode materials and also discusses their metal ion charge storage mechanism. Finally, we provide our perspectives on the current issues and future opportunities for supercapacitor materials.

Review on recent advances in cellulose nanofibril based hybrid aerogels: Synthesis, properties and their applications

With the depletion of fossil fuels and growing environmental concerns, the modernized era of technology is in desperate need of sustainable and eco-friendly materials. The industrial sector surely has enough resources to produce cost-effective, renewable, reusable, and sustainable raw materials. The family of very porous solid materials known as aerogels has a variety of exceptional qualities, such as high porosity, high specific surface area, ultralow density, and superior thermal, acoustic, and dielectric properties. As a result, aerogels have the potential to be used for many different purposes, such as absorbents, supercapacitors, energy storage, and catalytic supports. Recently, cellulose nanofibril (CNF) aerogels have attracted remarkable attention for their large-scale utilization because of their high absorption capacity, low density, biodegradability, large surface area, high porosity, and biocompatibility. Recent advancements have confirmed that CNF-based hybrid aerogels can be proposed as the most privileged and promising novel material in various applications. This comprehensive review highlights the recent reports of the CNF-based hybrid aerogels, including their properties and frequent preparation approaches, in addition to their new applications in the areas of fire retardant, water and oil separation, supercapacitors, environmental, and CO2 capture. It is also assumed that this article will promote additional investigation and establish innovative capabilities to enhance novel CNF-based hybrid aerogels with new and exciting applications.

Recent advances in zinc-air batteries: self-standing inorganic nanoporous metal films as air cathodes

Zinc-air batteries (ZABs) have promising prospects as next-generation electrochemical energy systems due to their high safety, high power density, environmental friendliness, and low cost. However, the air cathodes used in ZABs still face many challenges, such as the low catalytic activity and poor stability of carbon-based materials at high current density/voltage. To achieve high activity and stability of rechargeable ZABs, chemically and electrochemically stable air cathodes with bifunctional oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) activity, fast reaction rate with low platinum group metal (PGM) loading or PGM-free materials are required, which are difficult to achieve with common electrocatalysts. Meanwhile, inorganic nanoporous metal films (INMFs) have many advantages as self-standing air cathodes, such as high activity and stability for both the ORR/OER under highly alkaline conditions. The high surface area, three-dimensional channels, and porous structure with controllable crystal growth facet/direction make INMFs an ideal candidate as air cathodes for ZABs. In this review, we first revisit some critical descriptors to assess the performance of ZABs, and recommend the standard test and reported manner. We then summarize the recent progress of low-Pt, low-Pd, and PGM-free-based materials as air cathodes with low/non-PGM loading for rechargeable ZABs. The structure-composition-performance relationship between INMFs and ZABs is discussed in-depth. Finally, we provide our perspectives on the further development of INMFs towards rechargeable ZABs, as well as current issues that need to be addressed. This work will not only attract researchers' attention and guide them to assess and report the performance of ZABs more accurately, but also stimulate more innovative strategies to drive the practical application of INMFS for ZABs and other energy-related technologies.

Secured Nanosynthesis-Deposition Aerosol Process for Composite Thin Films Incorporating Highly Dispersed Nanoparticles

Application of nanocomposites in daily life requires not only small nanoparticles (NPs) well dispersed in a matrix, but also a manufacturing process that is mindful of the operator and the environment. Avoiding any exposure to NPs is one such way, and direct liquid reaction-injection (DLRI) aims to fulfill this need. DLRI is based on the controlled in situ synthesis of NPs from the decomposition of suitable organometallic precursors in conditions that are compatible with a pulsed injection mode of an aerosol into a downstream process. Coupled with low-pressure plasma, DLRI produces nanocomposite with homogeneously well-dispersed small nanoparticles that in the particular case of ZnO-DLC nanocomposite exhibit unique properties. DLRI favorably compares with the direct liquid injection of ex situ formed NPs. The exothermic hydrolysis reaction of the organometallic precursor at the droplet-gas interface leads to the injection of small and highly dispersed NPs and, consequently, the deposition of fine and controlled distribution in the nanocomposite. The scope of DLRI nanosynthesis has been extended to several metal oxides such as zinc, tin, tungsten, and copper to generalize the concept. Hence, DLRI is an attractive method to synthesize, inject, and deposit nanoparticles and meets the prevention and atom economy requirements of green chemistry.

Constructing N-doped and 3D Hierarchical Porous graphene nanofoam by plasma activation for supercapacitor and Zn ion capacitor

Traditional electrode materials still face vital challenges of few active sites, low porosity, complex synthesis process, and low specific capacitance. Herein, N-doped and 3D hierarchical porous graphene nanofoam (N-GNF) is created on carbon fibers (CFs) by employing a facile, fast, and environmentally friendly strategy of N₂ plasma activation. After an appropriated N₂ plasma activation, the graphene nanosheets (GNSs) synthesized by Ar/CH₄ plasma deposition transform into N-GNF successfully. N doping donates rich active sites and increases the hydrophilia, while hierarchical nanoarchitecture exposes an enlarged effective contact area at the interface between electrode and electrolyte and affords sufficient space for accommodating more electrolytes. The as-assembled flexible N-GNF@CFs//Zn NSs@CFs Zn ion capacitor delivered a high energy density of 105.2 Wh kg^-1 at 378.6 W kg^-1 and initial capacity retention of 87.9% at the current of 2 A g^-1 after a long cycle of 10,000.

Flexible Solid Supercapacitors of Novel Nanostructured Electrodes Outperform Most Supercapacitors

Sustainable and scalable fabrication of electrode materials with high energy and power densities is paramount for the development of future electrochemical energy storage devices. The electrode material of a supercapacitor should have high electrical conductivity, good thermal and chemical stability, and a high surface area per unit volume (or per unit mass). Researchers have made great efforts to use two-dimensional (2D) nanomaterials, but the separated 2D plates are re-stacked during processing for electrode fabrication, impeding the transport of ions and reducing the number of active sites. We developed a novel process for manufacturing thin and flexible electrodes using a 2D material (MXene,Ti₃AlC₂) and a conducting polymer (poly(3,4-ethylenedioxythiophene), PEDOT). Because the PEDOT layer is electrochemically synthesized, it does not contain the activator poly(styrene sulfonate). The electrospray deposition technique solves the restacking problem and facilitates the infilling of the gel electrolyte by forming a highly porous open structure across the entire electrode. In the PEDOT/MXene multilayered electrode, the double-layer capacitance increased substantially because of a dramatic increase in the number of accessible sites through the MXene layer. Although applied to solid supercapacitors, these new supercapacitors outperform most aqueous electrolyte supercapacitors as well as other solid supercapacitors.

Lamellar structured Ni3P2O8: first-ever use to design 1.8 V operated flexible all-solid-state symmetric supercapacitor

Increasing demand for microelectronic devices necessitates the development of highly flexible energy storage technologies with a wide operating voltage. Thus, flexible electrodes and their devices with the requisite mechanical and electrochemical characteristics have prime importance. In this regard, the present article demonstrates the feasibility of designing a flexible all-solid-state supercapacitor using a chemically grown Ni₃P₂O₈ lamellar microstructured electrode embedded with carboxy methyl cellulose-Na₂SO₄ (CMC-Na₂SO₄) gel electrolyte. The formed symmetric device impressively exhibited a maximum working voltage window of 1.8 V with a high specific energy of 44.7 W h kg^-1 and specific power of 3.3 kW kg^-1 along with prolonged cycle life. Also, the device's high deformation tolerance (95%) when bent at 170° with a flashing light-emitting diode (LED) working demonstration showcases its viability for advanced flexible energy storage applications.

Flexible and Freestanding MoS2 Nanosheet/Carbon Nanotube/Cellulose Nanofibril Hybrid Aerogel Film for High-Performance All-Solid-State Supercapacitors

Flexible supercapacitors assembled with two-dimensional materials have become a research hotspot in recent years. Here, we have prepared two-dimensional nanomaterial MoS₂ and SWCNT, CNF aerogel composite electrode, and its flexible all-solid-state supercapacitor. SWCNT can inhibit the accumulation of MoS₂ nanosheets and enhance the conductivity of the composite electrode. CNF can improve the dispersion uniformity of MoS₂ and SWCNT, and endow the composite electrode with a high specific surface area (328.86 m² g^-1) and excellent flexibility. MoS₂-SWCNT/CNF supercapacitor has a good rectangular CV curve and symmetrical triangular GCD curve. The CV curve of the MoSCF3 supercapacitor with the highest MoS₂-SWCNT content remains rectangular even at the scanning rate of 2000 mV s^-1. Its voltage window can reach 1.5 V. MoS₂-SWCNT/CNF supercapacitor has a specific capacity of 605.32 mF cm^-2 (scanning rate of 2 mV s^-1) and 30.34 F g^-1 (0.01 A g^-1), an area specific energy of 35.61 mWh cm^-2 (area specific power of 0.03 mW cm^-2), and extremely high cycle stability (91.01% specific capacity retention rate after 10 000 cycles) and good flexibility. The fine nanocomposite structure gives MoS₂-SWCNT/CNF supercapacitor impressive electrochemical performance and excellent flexibility, which can be used in the field of portable electronic devices and flexible devices.

Foldable batteries: from materials to devices

Wearable electronics is a growing field that has important applications in advanced human-integrated systems with high performance and mechanical deformability, especially foldable characteristics. Although foldable electronics such as rollable TVs (LG signature OLED R) or foldable smartphones (Samsung Galaxy Z fold/flip series) have been successfully established in the market, these devices are still powered by rigid and stiff batteries. Therefore, to realize fully wearable devices, it is necessary to develop state-of-the-art foldable batteries with high performance and safety in dynamic deformation states. In this review, we cover the recent progress in developing materials and system designs for foldable batteries. The Materials section is divided into three sections aimed at helping researchers choose suitable materials for their systems. Several foldable battery systems are discussed and the combination of innovative materials and system design that yields successful devices is considered. Furthermore, the basic analysis process of electrochemical and mechanical properties is provided as a guide for researchers interested in the evaluation of foldable battery systems. The current challenges facing the practical application of foldable batteries are briefly discussed. This review will help researchers to understand various aspects (from material preparation to battery configuration) of foldable batteries and provide a brief guideline for evaluating the performance of these batteries.

Enhanced ionic conductivity of a Na3Zr2Si2PO12 solid electrolyte with Na2SiO3 obtained by liquid phase sintering for solid-state Na+ batteries

NASICON-type Na₃Zr₂Si₂PO₁₂ (NZSP) is supposed to be one of the best potential solid electrolytes with the characteristics of high ionic conductivity and safety for use in solid-state sodium batteries. Many methods have been used to enhance the ionic conductivity of NZSP, among which liquid phase sintering is a simple and rapid method. However, the transport mechanism of sodium ions in a NZSP electrolyte obtained by liquid phase sintering is not clear, and its application in solid-state batteries has not been confirmed. In this study, we synthesized NZSP with Na₂SiO₃ additives by liquid phase sintering to reduce the sintering temperature and improve the ionic conductivity. NZSP with 5 wt% Na₂SiO₃ (NZSP-NSO-5) achieves the highest ionic conductivity of 1.28 mS cm^-1 and the lowest activation energy of 0.21 eV. Furthermore, the DFT study proves the Na⁺ diffusion mechanism and the decline in activation energy after addition. Lastly, the Na/Na₃V₂(PO₄)₃ battery with a Na₂SiO₃-added NZSP solid electrolyte exhibits a remarkably extended cycling capacity of 96.6% capacity retention after being cycled at 0.1 C 100 times. The liquid phase sintering with addition of low melting point salt compounds to electrolyte powder represents a rapid and straightforward technique for improving other ceramic electrolytes.

3D Printing for Solid-State Energy Storage

Ever-growing demand to develop satisfactory electrochemical devices has driven cutting-edge research in designing and manufacturing reliable solid-state electrochemical energy storage devices (EESDs). 3D printing, a precise and programmable layer-by-layer manufacturing technology, has drawn substantial attention to build advanced solid-state EESDs and unveil intrinsic charge storage mechanisms. It provides brand-new opportunities as well as some challenges in the field of solid-state energy storage. This review focuses on the topic of 3D printing for solid-state energy storage, which bridges the gap between advanced manufacturing and future EESDs. It starts from a brief introduction followed by an emphasis on 3D printing principles, where basic features of 3D printing and key issues for solid-state energy storage are both reviewed. Recent advances in 3D printed solid-state EESDs including solid-state batteries and solid-state supercapacitors are then summarized. Conclusions and perspectives are also provided regarding the further development of 3D printed solid-state EESDs. It can be expected that advanced 3D printing will significantly promote future evolution of solid-state EESDs.

Flexible core/shelled PPy@PANI nanotube porous films for hybrid supercapacitors

Flexibility of the films and the limited ion transport in the vertical direction of film highly restrict the development of flexible supercapacitors. Herein, we have developed hybrid porous films consisting of N-doped holey graphene nanosheets (NHGR) with abundant in-plane nanopores and the vertically aligned polyaniline nanowires arrays on polypyrrole nanotubes (PPy@PANI) via a two-step oxidative polymerization strategy and vacuum filtration. The rational design can efficiently shorten the diffusion path of electrons/ions, alleviate volume variation of electrodes during cycling, enhance electric conductivity of the hybrids, and while offer abundant active interfacial sites for electrochemical reaction. Benefiting from the distinctive structural and compositional merits, the obtained PPy@PANI/NHGR film electrode manifests an excellent electrochemical properties in terms of specific capacity (1348 mF cm^-2at a current density of 1 mA cm^-2), rate capability (81.2% capacitance retention from 1 to 30 mA cm^-2), and cycling stability (capacitance retention of 73.7% at 20 mA cm^-2after 7000 cycles). Matched with NHGR negative electrode, the assembled flexible all-solid-state asymmetric supercapacitor displays a remarkable areal capacitance of 359 mF cm^-2at 5 mA cm^-2, maximum areal energy density of 112.2μWh cm^-2at 3.747 mW cm^-2, and good flexibility at various bending angles while preserving stable cycling performance. The result shows the PPy@PANI/NHGR film with high flexibility and 3D ions transport channels is highly attractive for flexible energy storage devices.

Recent Development of Flexible and Stretchable Supercapacitors Using Transition Metal Compounds as Electrode Materials

Flexible and stretchable supercapacitors (FS-SCs) are promising energy storage devices for wearable electronics due to their versatile flexibility/stretchability, long cycle life, high power density, and safety. Transition metal compounds (TMCs) can deliver a high capacitance and energy density when applied as pseudocapacitive or battery-like electrode materials owing to their large theoretical capacitance and faradaic charge-storage mechanism. The recent development of TMCs (metal oxides/hydroxides, phosphides, sulfides, nitrides, and selenides) as electrode materials for FS-SCs are discussed here. First, fundamental energy-storage mechanisms of distinct TMCs, various flexible and stretchable substrates, and electrolytes for FS-SCs are presented. Then, the electrochemical performance and features of TMC-based electrodes for FS-SCs are categorically analyzed. The gravimetric, areal, and volumetric energy density of SC using TMC electrodes are summarized in Ragone plots. More importantly, several recent design strategies for achieving high-performance TMC-based electrodes are highlighted, including material composition, current collector design, nanostructure design, doping/intercalation, defect engineering, phase control, valence tuning, and surface coating. Integrated systems that combine wearable electronics with FS-SCs are introduced. Finally, a summary and outlook on TMCs as electrodes for FS-SCs are provided.

Design Principles and Applications of Next-Generation High-Energy-Density Batteries Based on Liquid Metals

Increasing need for the renewable energy supply accelerated the thriving studies of Li-ion batteries, whereas if the high-energy-density Li as well as alkali metals should be adopted as battery electrodes is still under fierce debate for safety concerns. Recently, a group of low-melting temperature metals and alloys that are in liquid phase at or near room-temperature are being reported for battery applications, by which the battery energy could be improved without significant dendrite issue. Besides the dendrite-free feature, liquid metals can also promise various high-energy-density battery designs on the basis of unique materials properties. In this review, the design principles for liquid metals-based batteries from mechanical, electrochemical, and thermodynamical aspects are provided. With the understanding of the theoretical basis, currently reported relevant designs are summarized and analyzed focusing on the working mechanism, effectiveness evaluation, and novel application. An overview of the state-of-the-art liquid metal battery developments and future prospects is also provided in the end as a reference for further research explorations.

Modern Nanocomposites and Hybrids as Electrode Materials Used in Energy Carriers

Over the past decades, the application of new hybrid materials in energy storage systems has seen significant development. The efforts have been made to improve electrochemical performance, cyclic stability, and cell life. To achieve this, attempts have been made to modify existing electrode materials. This was achieved by using nano-scale materials. A reduction of size enabled an obtainment of changes of conductivity, efficient energy storage and/or conversion (better kinetics), emergence of superparamagnetism, and the enhancement of optical properties, resulting in better electrochemical performance. The design of hybrid heterostructures enabled taking full advantage of each component, synergistic effect, and interaction between components, resulting in better cycle stability and conductivity. Nowadays, nanocomposite has ended up one of the foremost prevalent materials with potential applications in batteries, flexible cells, fuel cells, photovoltaic cells, and photocatalysis. The main goal of this review is to highlight a new progress of different hybrid materials, nanocomposites (also polymeric) used in lithium-ion (LIBs) and sodium-ion (NIBs) cells, solar cells, supercapacitors, and fuel cells and their electrochemical performance.

S, O dual-doped porous carbon derived from activation of waste papers as electrodes for high performance lithium ion capacitors

To circumvent the imbalances of electrochemical kinetics and charge-storage capacity between Li⁺ ion battery anodes and capacitive cathodes in lithium-ion capacitors (LICs), dual carbon based LICs are constructed and investigated extensively. Herein, S, O dual-doped 3D net-like porous carbon (S-NPC) is prepared using waste paper as the carbon source through a facile solvothermal treatment and chemical activation. Benefiting from the combination effect of the rich S,O-doping (about 2.1 at% for S, and 9.0 at% for O), high surface area (2262 m² g^-1) and interconnected porous network structure, the S-NPC-40 material exhibits excellent electrochemical performance as both cathode material and anode material for LICs. S, O doping not only increases the pseudocapacity but also improves the electronic conductivity, which is beneficial to reduce the mismatch between the two electrodes. The S-NPC-40//S-NPC-40 LIC delivers high energy densities of 176.1 and 77.8 W h kg^-1 at power densities of 400 and 20 kW kg^-1, respectively, as well as superior cycling stability with 82% capacitance retention after 20 000 cycles at 2 A g^-1. This research provides an efficient method to convert waste paper to porous carbon electrode materials for high performance LIC devices.

Hybrid dual-function thermal energy harvesting and storage technologies: towards self-chargeable flexible/wearable devices

The worldwide energy scarcity arising from the massive consumption of nonrenewable energy sources raised a global awareness of the need for cleaner and affordable energy solutions to mitigate climate change and ensure the world sustainable development. The rise of the Internet of Things and the fast growth of the groundbreaking market of wearable electronics boosted a major quest for self-powered technologies merging energy harvesting and energy storage functionalities to meet the demands of a myriad of market segments, such as healthcare, transportation, defense and sports. Thermoelectric devices are a green energy harvesting solution for wearable electronics since they harness the low-grade waste heat from ubiquitous thermal energy sources and convert it into electrical energy. However, these systems generate electrical energy in an intermittent manner, depend on the local heat release availability and require an additional unit to store energy. Flexible and wearable supercapacitors are a safe and eco-friendly energy storage solution to power wearables, offering advantages of security, longer cycle life, higher power density and faster charging over batteries. However, an additional unit that generates energy or that is able to charge the storage device is required. More recently, a new class of all-in-one thermally-chargeable supercapacitors blossomed to meet the requirements of the next generation of autonomous wearable electronics and ensure an endurable energy supply. This self-chargeable hybrid technology combines the functionalities of thermal energy harvesting and supercapacitive energy storage in a single multitasking device. In this Perspective, the advances in the burgeoning field of all-in-one thermally-chargeable supercapacitors for flexible/wearable applications will be critically examined, ranging from their structure and working principle to the rational design of the composing materials and of tailor-made architectures. It will start by introducing the foundations of single flexible/wearable thermoelectric generators and supercapacitors and will evolve into the pioneering venture of fully-integrated thermal energy harvesting/storage systems. It will end by highlighting the current bottlenecks and future pathways for advancing the development of this sophisticated smart technology.

Flexible/Stretchable Supercapacitors with Novel Functionality for Wearable Electronics

With the miniaturization of personal wearable electronics, considerable effort has been expended to develop high-performance flexible/stretchable energy storage devices for powering integrated active devices. Supercapacitors can fulfill this role owing to their simple structures, high power density, and cyclic stability. Moreover, a high electrochemical performance can be achieved with flexible/stretchable supercapacitors, whose applications can be expanded through the introduction of additional novel functionalities. Here, recent advances in and future prospects for flexible/stretchable supercapacitors with innate functionalities are covered, including biodegradability, self-healing, shape memory, energy harvesting, and electrochromic and temperature tolerance, which can contribute to reducing e-waste, ensuring device integrity and performance, enabling device self-charging following exposure to surrounding stimuli, displaying the charge status, and maintaining the performance under a wide range of temperatures. Finally, the challenges and perspectives of high-performance all-in-one wearable systems with integrated functional supercapacitors for future practical application are discussed.

Rational Design and in-situ Synthesis of Ultra-Thin β-Ni(OH)2 Nanoplates for High Performance All-Solid-State Flexible Supercapacitors

The all-solid-state flexible supercapacitor (AFSC), one of the most flourishing energy storage devices for portable and wearable electronics, attracts substantial attentions due to their high flexibility, compact size, improved safety, and environmental friendliness. Nevertheless, the current AFSCs usually show low energy density, which extremely hinders their practical applications. Herein, ultra-thin β-Ni(OH)₂ nanoplates with thickness of 2.4 ± 0.2 nm are in-situ grown uniformly on Ni foam by one step hydrothermal treatment. Thanks to the ultra-thin nanostructure, β-Ni(OH)₂ nanoplates shows a specific capacitance of 1,452 F g⁻¹ at the scan rate of 3 mV s⁻¹. In addition, the assembled asymmetric AFSC [Ni(OH)₂//Activated carbon] shows a specific capacitance of 198 F g⁻¹. It is worth noting that the energy density of the AFSC can reach 62 Wh kg⁻¹ while keeping a high power density of 1.5 kW kg⁻¹. Furthermore, the fabricated AFSCs exhibit satisfied fatigue behavior and excellent flexibility, and about 82 and 86% of the capacities were retained after 5,000 cycles and folding over 1,500 times, respectively. Two AFSC in series connection can drive the electronic watch and to run stably for 10 min under the bending conditions, showing a great potential for powering portable and wearable electronic devices.

Sb nanoparticle decorated rGO as a new anode material in aqueous chloride ion batteries

An aqueous chloride ion battery (CIB) is an emerging technology for electrochemical energy storage as well as battery desalination systems. However, the instability and decomposition of electrode materials in an aqueous medium is a major issue in CIBs. Herein, in one step, we synthesized fine antimony nanoparticles with a size of ∼20 nm on reduced graphene oxide (Sb@rGO) sheets using a hydrothermal route with facile and cost-effective processes. It is proposed as a new anode material and coupled with the AgCl cathode in an aqueous CIB. The specific capacity is maintained constantly at 51.6 mA h g-1 at a current density of 400 mA g-1 even after 200 cycles. In addition, characterization methods such as electrochemical analysis, X-ray diffraction, etc. were used to confirm the reaction mechanism. The chloride ion capture material developed in this research work will be significant for CIBs as an energy storage technology or battery desalination system.