Highly Durable, Self-Standing Solid-State Supercapacitor Based on an Ionic Liquid-Rich Ionogel and Porous Carbon Nanofiber Electrodes

A biospecies-derived genomic DNA hybrid gel electrolyte for electrochemical energy storage

Intrinsic impediments, namely weak mechanical strength, low ionic conductivity, low electrochemical performance, and stability have largely inhibited beyond practical applications of hydrogels in electronic devices and remains as a significant challenge in the scientific world. Here, we report a biospecies-derived genomic DNA hybrid gel electrolyte with many synergistic effects, including robust mechanical properties (mechanical strength and elongation of 6.98 MPa and 997.42%, respectively) and ion migration channels, which consequently demonstrated high ionic conductivity (73.27 mS/cm) and superior electrochemical stability (1.64 V). Notably, when applied to a supercapacitor the hybrid gel-based devices exhibit a specific capacitance of 425 F/g. Furthermore, it maintained rapid charging/discharging with a capacitance retention rate of 93.8% after ∼200,000 cycles while exhibiting a maximum energy density of 35.07 Wh/kg and a maximum power density of 193.9 kW/kg. This represents the best value among the current supercapacitors and can be immediately applied to minicars, solar cells, and LED lightning. The widespread use of DNA gel electrolytes will revolutionize human efforts to industrialize high-performance green energy.

Fabrication of Gelatin-Derived Gel Electrolyte Using Deep Eutectic Solvents through In Situ Derivatization and Crosslinking Strategy for Supercapacitors and Flexible Sensors

The facile fabrication of gel polymer electrolytes is crucial to the development of flexible electronics, and the use of natural polymers as sources has obtained great attention due to their abundant, low-cost, biodegradable, easy modification, and biocompatible features. In this article, a facile fabrication protocol to engineer gelatin into gel electrolytes was developed by taking the advantages of both deep eutectic solvent (DES) (including its good solubility to gelatin and satisfactory electrochemical properties) and rich active functional groups of gelatin, through in situ derivatization and crosslinking strategy. A double-crosslinked DES gel electrolyte was prepared with the dissolution of gelatin in choline chloride and alcohol-based DES and a further crosslinking with Fe3+ ions. The obtained DES gel presented outstanding mechanical properties, excellent ionic conductivity (up to 101-102 mS/cm), a wide operating temperature range (-40 to 80 °C), satisfactory self-healing property, and good degradability. Moreover, the obtained DES gel electrolyte was successfully applied to supercapacitors and flexible sensors, showing excellent electrochemical performance and strain-response properties. In a word, our study provides a facile protocol to engineer gelatin into gel electrolytes by using deep eutectic solvent, showing significant insights into the design and preparation of sustainable gel polymer electrolytes and having great application potential in next-generation high-performance flexible electronics.

Opportunities of Flexible and Portable Electrochemical Devices for Energy Storage: Expanding the Spotlight onto Semi-solid/Solid Electrolytes

The ever-increasing demand for flexible and portable electronics has stimulated research and development in building advanced electrochemical energy devices which are lightweight, ultrathin, small in size, bendable, foldable, knittable, wearable, and/or stretchable. In such flexible and portable devices, semi-solid/solid electrolytes besides anodes and cathodes are the necessary components determining the energy/power performances. By serving as the ion transport channels, such semi-solid/solid electrolytes may be beneficial to resolving the issues of leakage, electrode corrosion, and metal electrode dendrite growth. In this paper, the fundamentals of semi-solid/solid electrolytes (e.g., chemical composition, ionic conductivity, electrochemical window, mechanical strength, thermal stability, and other attractive features), the electrode-electrolyte interfacial properties, and their relationships with the performance of various energy devices (e.g., supercapacitors, secondary ion batteries, metal-sulfur batteries, and metal-air batteries) are comprehensively reviewed in terms of materials synthesis and/or characterization, functional mechanisms, and device assembling for performance validation. The most recent advancements in improving the performance of electrochemical energy devices are summarized with focuses on analyzing the existing technical challenges (e.g., solid electrolyte interphase formation, metal electrode dendrite growth, polysulfide shuttle issue, electrolyte instability in half-open battery structure) and the strategies for overcoming these challenges through modification of semi-solid/solid electrolyte materials. Several possible directions for future research and development are proposed for going beyond existing technological bottlenecks and achieving desirable flexible and portable electrochemical energy devices to fulfill their practical applications. It is expected that this review may provide the readers with a comprehensive cross-technology understanding of the semi-solid/solid electrolytes for facilitating their current and future researches on the flexible and portable electrochemical energy devices.

Flexible Sustained Ionogels with Ionic Hyperbranched Polymers for Enhanced Ion-Conduction and Energy Storage

Flexible and mechanically robust gel-like electrolytes offer enhanced energy storage capabilities, versatility, and safety in batteries and supercapacitors. However, the trade-off between ion conduction and mechanical robustness remains a challenge for these materials. Here, we suggest that the introduction of ionic hyperbranched polymers in structured sustained ionogels will lead to both enhanced ion conduction and mechanical performance because of the hyperbranched polymers' ionically conductive groups and the complementary interfacial interactions with ionic liquids. More specifically, we investigate the effect of hyperbranched polymers with carboxylate terminal groups and imidazolium counterions with various ionic group densities on the properties of ionogels composed of coassembled cellulose nanofibers (CNFs) and cellulose nanocrystals (CNCs) as sustainable open pore frame for ionic liquid immersion. The addition of hyperbranched polymers leads to the formation of highly interconnected openly porous, lightweight, and shape-persistent materials by harnessing hydrogen bonding between the polymers and the CNFs/CNCs "frame". Notably, these materials possess a 2-fold improvement in ionic conductivity combined with many-fold increase in Young's modulus, tensile strength, and toughness, making them comparable to common reinforced nanocomposite materials. Furthermore, the corresponding thin-film gel supercapacitors possess enhanced electrochemical cycling stability upon repeated bending with an 85% capacitance retention after 10 000 cycles, promising new insight in the development of simultaneously conductive and flexible gel electrolytes with sustained performance.

A Transient Supercapacitor with a Water-Dissolvable Ionic Gel for Sustainable Electronics

We develop an environmentally benign supercapacitor, which decomposes by contact with water, incorporating an ionic liquid, carbon powder, a cellulose separator, and a molybdenum electrode. The ionic liquid is dispersed into a water-dissolvable polymer, poly(vinyl alcohol), to produce a solid electrolyte, so-called ionic gel. A carbon composite mixed with the ionic liquid maintains a gel form. The ionic gel and the carbon composite enable an all-solid-state supercapacitor, which can be charged at a voltage of 1.5 V. The supercapacitor shows areal and volumetric capacitances of 65 mF/cm2 and 2.2 F/cm3, respectively. A cycle test reveals that capacitance retention and Coulombic efficiency are 77 and 90%, respectively. As for the dissolution test, the ionic gel and carbon composite dissolves in phosphate buffer solution in 18 days, and the Mo electrode is able to fully dissolve in 500-588 days. Potential applications of the environmentally benign supercapacitor include smart agriculture by monitoring of soil and disaster prevention by a wireless sensor network without the need for retrieval of devices after use.

Thiol-ene ionogels based on polymerizable imidazolium ionic liquid

In this work, we report the synthesis of polymerizable ionic liquids (PILs) and the synthesis of ionogels by thiol-ene photopolymerization. A series of gemini imidazolium-based bis(trifluoromethylsulfonyl)imide polymerizable ionic liquids with...

Flexible electrodes with high areal capacity based on electrospun fiber mats

The ever-growing portable, flexible, and wearable devices impose new requirements from power sources. In contrast to gravitational metrics, areal metrics are more reliable performance indicators of energy storage systems for portable and wearable devices. For energy storage devices with high areal metrics, a high mass loading of the active species is generally required, which imposes formidable challenges on the current electrode fabrication technology. In this regard, integrated electrodes made by electrospinning technology have attracted increasing attention due to their high controllability, excellent mechanical strength, and flexibility. In addition, electrospun electrodes avoid the use of current collectors, conductive additives, and polymer binders, which can essentially increase the content of the active species in the electrodes as well as reduce the unnecessary physically contacted interfaces. In this review, the electrospinning technology for fabricating flexible and high areal capacity electrodes is first highlighted by comparing with the typical methods for this purpose. Then, the principles of electrospinning technology and the recent progress of electrospun electrodes with high areal capacity and flexibility are elaborately discussed. Finally, we address the future perspectives for the construction of high areal capacity electrodes using electrospinning technology to meet the increasing demands of flexible energy storage systems.

Cellulose ionogels, a perspective of the last decade: A review

Cellulose ionogels have been extensively studied due to the variability of their properties and applications. The capability of trapping an ionic liquid in a biodegradable solid matrix without losing its properties makes this type of material a promising substitute for fossil fuel-derived materials. The possibility to formulate ionogels chemically or physically, to choose between different ionic liquids, cellulose types, and the possibility to add a wide range of additives, make these ionogels an adaptable material that can be modified for each target application in many fields such as medicine, energy storage, electrochemistry, etc. The aim of this review is to show its versatility and to provide a summary picture of the advances in the field of cellulose ionogels formulation (chemical or physical methods), as well as their potential applications, so this review will serve as a stimulus for research on these materials in the future.

Transition metal dichalcogenide (TMDs) electrodes for supercapacitors: a comprehensive review

As globally, the main focus of the researchers is to develop novel electrode materials that exhibit high energy and power density for efficient performance energy storage devices. This review covers the up-to-date progress achieved in transition metal dichalcogenides (TMDs) (e.g. MoS₂, WS₂, MoSe_2,and WSe₂) as electrode material for supercapacitors (SCs). The TMDs have remarkable properties like large surface area, high electrical conductivity with variable oxidation states. These properties enable the TMDs as the most promising candidates to store electrical energy via hybrid charge storage mechanisms. Consequently, this review article provides a detailed study of TMDs structure, properties, and evolution. The characteristics technique and electrochemical performances of all the efficient TMDs are highlighted meticulously. In brief, the present review article shines a light on the structural and electrochemical properties of TMD electrodes. Furthermore, the latest fabricated TMDs based symmetric/asymmetric SCs have also been reported.

The Influence of Water Vapor on the Electrochemical Shift of an Ionic Liquid Measured by Ambient Pressure X-ray Photoelectron Spectroscopy

Ionic liquids (ILs) are considered to be one of the steppingstones to fabricate next generation electrochemical devices given their unique physical and chemical properties. The addition of water to ILs significantly impact electrochemical related properties including viscosity, density, conductivity, and electrochemical window. Herein we utilize ambient pressure X-ray photoelectron spectroscopy (APXPS) to examine the impact of water on values of the electrochemical shift (S), which is determined by measuring changes in binding energy shifts as a function of an external bias. APXPS spectra of C 1s, O 1s and N 1s regions are examined for the IL 1-butyl-3-methylimidazolium acetate, [C₄ mim][OAc], at the IL/gas interface as a function of both water vapor pressure and external bias. Results reveal that in the absence of water vapor there is an IL ohmic drop between the working electrode and quasi reference electrode, giving rise to chemical specific S values of less than one. Upon introducing water vapor, S values approach one as a function of increasing water vapor pressure, indicating a decrease in the IL ohmic drop as the IL/water mixture becomes more conductive and the potential drop is driven by the electric double layer at the electrode/IL interface.

In Situ Growth of Oriented Polyaniline Nanorod Arrays on the Graphite Flake for High-Performance Supercapacitors

Polyaniline with oriented nanorod arrays could provide high surface area and relaxed nanostructure to optimize ion diffusion paths, thus enhancing the performance of the device. In this paper, we designed an all-solid symmetrical supercapacitor with good performance based on polyaniline nanorod arrays in situ-grown on a graphite flake free-standing substrate. The specific capacitance, cycle stability, and energy density of the prepared supercapacitor device were 135 F/g, 75.4% retention after 1500 cycles, and the energy density is 18.75 W h/kg at a power density of 500 W/kg. The good performance of the supercapacitor device was obviously related to the oriented nanorod arrays of polyaniline/graphite flakes. In order to find the application of the prepared supercapacitor device, the tandem device consisting of three single supercapacitor devices connected in series had been used to drive small electronic equipment. The red light-emitting diode and chronograph could be easily driven by the 3-series supercapacitor devices. These results indicated that the prepared supercapacitor device based on the polyaniline/graphite flake electrode had potential applications in energy storage devices.

Transparent Stretchable Dual-Network Ionogel with Temperature Tolerance for High-Performance Flexible Strain Sensors

A stretchable transparent double network ionogel composed of physically cross-linked poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-co-HFP)) and chemically cross-linked poly(methyl methacrylate-co-butylmethacrylate) (P(MMA-co-BMA)) elastomer networks within [EMIM][TFSI] ionic liquid was fabricated through a facile one-pot thermal polymerization. The dual-network (DN) ionogel presents good mechanical performance (failure tensile stress 2.31 MPa, strain 307%) with a high loading of ionic liquid (70 wt %) for achieving required ionic conductivity (>0.1 S/m at room temperature). The transparent chemical cross-linked P(MMA-co-BMA) elastomer network endows high transparency (>93%) and high stretchability to the DN ionogel. The DN ionogel maintains good toughness, elasticity, and transparency in a wide temperature range (-40 to 80 °C) for the application in a harsh environment. In addition, the sensitivity of the DN ionogel to the change of environment temperature and deformation was detected and described. The practical potential of the DN ionogel in flexible electronic devices is further revealed by fabricating DN ionogel strain sensors to detect the movement of different human limbs including the bending of the finger, wrist, and elbow as well as the slight throat jitter during the swallowing and vocalization, showing fast response, high sensitivity, and good repeatability.

Key issues facing electrospun carbon nanofibers in energy applications: on-going approaches and challenges

Electrospun carbon nanofibers (CNFs), with one-dimensional (1D) morphology, tunable size, mechanical flexibility, and functionalities by themselves and those that can be added onto them, have witnessed the intensive development and extensive applications in energy storage and conversion, such as supercapacitors, batteries, and fuel cells. However, conventional solid CNFs often suffer from a rather poor electrical conductivity and low specific surface area, compared with the graphene and carbon nanotube counterparts. A well-engineered porous structure in CNFs increases their surface areas and reactivity, but there is a delicate balance between the level and type of pores and mechanical robustness. In addition, CNFs by themselves often show unsatisfactory electrochemical performance in energy storage and conversion, where, to endow them with high and durable activity, one effective approach is to dope CNFs with certain heteroatoms. Up to now, various activation strategies have been proposed and some of them have demonstrated great success in addressing these key issues. In this review, we focus on the recent advances in the issue-oriented schemes for activating the electrospun CNFs in terms of enhancing the conductivity, modulating pore configuration, doping with heteroatoms, and reinforcing mechanical strength, in close reference to their applications in supercapacitors. The basic scientific principles involved in these activation processes and their effectiveness in boosting the electrochemical performance of CNFs are examined. Finally, some of the on-going challenges and future perspectives in engineering CNFs for better performance are highlighted.

Porous g-C3N4 and MXene Dual-Confined FeOOH Quantum Dots for Superior Energy Storage in an Ionic Liquid

Owing to their unique nanosize effect and surface effect, pseudocapacitive quantum dots (QDs) hold considerable potential for high-efficiency supercapacitors (SCs). However, their pseudocapacitive behavior is exploited in aqueous electrolytes with narrow potential windows, thereby leading to a low energy density of the SCs. Here, a film electrode based on dual-confined FeOOH QDs (FQDs) with superior pseudocapacitive behavior in a high-voltage ionic liquid (IL) electrolyte is put forward. In such a film electrode, FQDs are steadily dual-confined in a 2D heterogeneous nanospace supported by graphite carbon nitride (g-C3N4) and Ti-MXene (Ti3C2). Probing of potential-driven ion accumulation elucidates that strong adsorption occurs between the IL cation and the electrode surface with abundant active sites, providing sufficient redox reaction of FQDs in the film electrode. Furthermore, porous g-C3N4 and conductive Ti3C2 act as ion-accessible channels and charge-transfer pathways, respectively, endowing the FQDs-based film electrode with favorable electrochemical kinetics in the IL electrolyte. A high-voltage flexible SC (FSC) based on an ionogel electrolyte is fabricated, exhibiting a high energy density (77.12 mWh cm-3), a high power density, a remarkable rate capability, and long-term durability. Such an FSC can also be charged by harvesting sustainable energy and can effectively power various wearable and portable electronics.

Homologous Hierarchical Porous Hollow Carbon Spheres Anode and Bowls Cathode Enabling High-Energy Sodium-Ion Hybrid Capacitors

It is a highly expected avenue to construct dual-carbon sodium-ion hybrid capacitors (SIHCs) using hierarchical porous carbon with interconnected pores, high accessible surface area, and disordered carbon frameworks for ameliorating the sluggish kinetics of SIHCs. In this work, a novel dual-carbon SIHCs system with homologous enhanced kinetics hierarchical porous hollow carbon spheres (HPCS) and hierarchical porous hollow carbon bowls (HPCB) as the anode and cathode is constructed for the first time. In a Na half-cell configuration, the HPCS anode synthesized through a facile one-pot in-situ template route demonstrates a superior reversible capacity as well as outstanding rate capability and cycleability, and the HPCB cathode fabricated by chemical activation of HPCS exhibits excellent capacitive behaviors. Thanks to superior properties and structures of the anode and cathode, the constructed novel dual-carbon SIHCs present an exceptionally high energy/power density (128.5 Wh kg^-1 and 11.9 kW kg^-1), along with a long cycling lifespan with retained morphology. This study on the kinetics of enhanced dual-carbon SIHCs opens a new avenue for optimizing the microstructure of hierarchical porous carbon and constructing new type of high-performance SIHCs systems.

Graphene/transition metal dichalcogenides hybrid supercapacitor electrode: status, challenges, and perspectives

Supercapacitors, based on fast ion transportation, are among the most promising energy storage solutions that can deliver fast charging-discharging within seconds and exhibit excellent cycling stability. The development of a good electrode material is one of the key factors in enhancing supercapacitor performance. Graphene (G), an allotrope of carbon that consists of a single layer of carbon atoms arranged in a hexagonal lattice, elicits research attention among scientists in the field of energy storage due to its remarkable properties, such as outstanding electrical conductivity, good chemical stability, and excellent mechanical behavior. Furthermore, numerous studies focus on 2D materials that are analogous to graphene as electrode supercapacitors, including transition metal dichalcogenides (TMDs). Recently, scientists and researchers are exploring TMDs because of the distinct features that make 2D TMDs highly attractive for capacitive energy storage. This study provides an overview of the structure, properties, synthesis methods, and electrochemical performance of G/TMD supercapacitors. Furthermore, the combination of G and TMDs to develop a hybrid structure may increase their energy density by introducing an asymmetric supercapacitor system. We will also discuss the future prospect of this system in the energy field.

A Water Dissolvable Electrolyte with an Ionic Liquid for Eco-Friendly Electronics

A water-dissolvable electrolyte is developed by combining an ionic liquid (IL) with poly(vinyl alcohol) (PVA), which decays over time by contact with water. An IL generally consists of two species of ions (anion and cation), and forms an electrical double layer (EDL) of a large electrostatic capacitance due to the ions accumulated in the vicinity of a conductive electrode when voltage is applied. In a similar manner, the ionic gel developed in this work forms an EDL due to the ions suspended in the conjugated polymer network while maintaining the gel form. Test measurements show a large capacitance of 13 µF cm^-2 within the potential window of the IL. The ionic gel shows an electrical conductance of 20 µS cm^-1 due to the ionic conduction, which depends on the weight ratio of the IL with respect to the polymer. The developed ionic gel dissolves into water in 16 h. Potential application includes the electrolyte in disposable electronics such as distributed sensors and energy harvesters that are supposed to be harmless to environment.

Microporous carbons derived from organosilica-containing carbon dots with outstanding supercapacitance

Microporous carbons with outstanding supercapacitance were synthesized from hydrothermally-synthesized organosilica-modified carbon dots.

Organosilica-based ionogel derived nitrogen-doped microporous carbons for high performance supercapacitor electrodes

A new preparation process is developed to yield stable, yellowish and transparent organosilica ionogels, which after pyrolysis yields N-doped microporous carbon with remarkable capacity.