Three-Dimensional Hierarchically Porous Graphene Fiber-Shaped Supercapacitors with High Specific Capacitance and Rate Capability

Effect of graphene sheets on the physico-chemical properties of nanocrystallite ceria

Currently, replacing expensive and short-lived materials for supercapacitors based on RuO2 with more cost-effective and high-performance materials that remain operational after a large number of cycles is a challenge. Cerium-based materials are the most attractive alternative because of cerium's ability to quickly change oxidation state. This work proposes the synthesis of nanostructured graphene-ceria composite and studies its morphological features arising under the impact of oxygen-free graphene. The mechanism of formation of nano-ceria crystallites when the sol-gel transition occurs on the surface of graphene sheets is also considered. It has been proven that the introduction of 0.5-0.6 wt% graphene sheets into nano-ceria ensures the preservation of its single-phase state, while simultaneously increasing its dispersion. Using the dilatometry method, it has been determined that uniformly distributed sheets of oxygen-free graphene lead to a decrease in temperature of the beginning of composite sintering by 175°C compared with pure nano-ceria and increase the shrinkage value by two times, which, in turn, should promote better sintering. This study fills a gap in the synthesis and characterization of oxygen-free graphene composites, promising raw materials for small devices and large power plants.

An Energy-Adjustable, Deformable, and Packable Wireless Charging Fiber Supercapacitor

Wireless charging energy storage devices eliminate bulky wires of wearable electronics. However, rigid shape and specific charging energy restrict their applications in space-limited portable electronics. Herein, an all-carbon fiber supercapacitor is presented that features shape-adjustable, packable, and energy-controllable wireless charging functions. With the unique on-dimensional circuit structure, the maximum energy transfer efficiency from the electrical energy received by the wireless charging unit to the output energy of the fiber supercapacitor can reach up to ≈60.8%, and meanwhile this integrated fiber device exhibits an outstanding area capacity of 803 mF cm-2 and energy density of 1004 µWh cm-2, superior to most of the fiber supercapacitors. Moreover, this unique device can endure significant deformation in shape of circles ranging from 2 to 20 cm diameter, and can be packed into narrow spaces, such as smart bracelet and disk-shaped pet global positioning system (GPS). By altering the device shape, the wireless charging current, voltage, and power can be adjusted in the range of 0.5-20 mA, 1.4-15.5 V, and 0.003-313 mW, accommodating the energy requirements for nearly all existing micro-electronics. This work offers unprecedented opportunities for packable, space-confined and energy harvesting controllable wearable electronics.

Effects of Crumpling Stage and Porosity of Graphene Electrode on the Performance of Electrochemical Supercapacitor

The performance characteristics of supercapacitors composed of crumpled graphene electrodes and aqueous NaCl electrolytes are investigated through Molecular Dynamics (MD) simulations using a newly developed crumpled graphene-based supercapacitor model. Results suggest that the three-dimensional configuration of crumpled graphene boosts electrolyte-electrode interaction. This improved interaction, which includes a larger ion-accessible zone, increases the specific capacitance of the supercapacitor by roughly 400% (16.4 μF/cm2) compared to planar graphene electrodes. Examining the effect of different stages of crumpling and the inclusion of pores on the electrode surface shows that the stages of crumpling substantially influence the supercapacitor performance. A smaller crumpling radius, meaning fully crumpled stage, improves the performance as increased crumpling leads to better packing efficiency, which aids in more ion separation. Furthermore, adding pores on the surface of crumpled graphene improves the ion accessibility by creating additional adsorption sites. An exceptional capacitance of 19.8 μF/cm2 is obtained for a porosity of 20%. However, the results suggest that the in-plane-porosity of the electrode needs to be optimized as there is a decline in specific capacitance after that point (20% porosity), indicating a suboptimal relationship between the charge distribution, specific surface area (SSA) and the porosity of the electrode.

Multiscale Structural Design of 2D Nanomaterials-based Flexible Electrodes for Wearable Energy Storage Applications

2D nanomaterials play a critical role in realizing high-performance flexible electrodes for wearable energy storge devices, owing to their merits of large surface area, high conductivity and high strength. The electrode is a complex system and the performance is determined by multiple and interrelated factors including the intrinsic properties of materials and the structures at different scales from macroscale to atomic scale. Multiscale design strategies have been developed to engineer the structures to exploit full potential and mitigate drawbacks of 2D materials. Analyzing the design strategies and understanding the working mechanisms are essential to facilitate the integration and harvest the synergistic effects. This review summarizes the multiscale design strategies from macroscale down to micro/nano-scale structures and atomic-scale structures for developing 2D nanomaterials-based flexible electrodes. It starts with brief introduction of 2D nanomaterials, followed by analysis of structural design strategies at different scales focusing on the elucidation of structure-property relationship, and ends with the presentation of challenges and future prospects. This review highlights the importance of integrating multiscale design strategies. Finding from this review may deepen the understanding of electrode performance and provide valuable guidelines for designing 2D nanomaterials-based flexible electrodes.

Recent Progress in Multifunctional Graphene-Based Nanocomposites for Photocatalysis and Electrocatalysis Application

The global energy shortage and environmental degradation are two major issues of concern in today's society. The production of renewable energy and the treatment of pollutants are currently the mainstream research directions in the field of photocatalysis. In addition, over the last decade or so, graphene (GR) has been widely used in photocatalysis due to its unique physical and chemical properties, such as its large light-absorption range, high adsorption capacity, large specific surface area, and excellent electronic conductivity. Here, we first introduce the unique properties of graphene, such as its high specific surface area, chemical stability, etc. Then, the basic principles of photocatalytic hydrolysis, pollutant degradation, and the photocatalytic reduction of CO₂ are summarized. We then give an overview of the optimization strategies for graphene-based photocatalysis and the latest advances in its application. Finally, we present challenges and perspectives for graphene-based applications in this field in light of recent developments.

Investigation of Electrical and Wearing Properties of Wool Fabric Coated with PEDOT:PSS

The way to improve the properties (resistance to washing, delamination, and rubbing off) of the PEDOT:PSS coating applied on wool fabric without reduction of its electrical conductivity by introducing a commercially available combination of low formaldehyde content melamine resins into the printing paste is presented in this paper. Primarily, to improve the hydrophilicity and dyeability of wool fabric, the samples were modified using low-pressure nitrogen (N₂) gas plasma. Two commercially available PEDOT:PSS dispersions were used to treat wool fabric by the exhaust dyeing and screen printing methods, respectively. Spectrophotometric measurements of the color difference (ΔE*_ab) and visual evaluation of woolen fabric dyed and printed with PEDOT:PSS in different shades of the blue color showed that the sample modified with N₂ plasma obtained a more intense color compared to the unmodified one. SEM was used to examine the surface morphology and a cross-sectional view of wool fabric that had undergone various modifications. SEM image shows that the dye penetrates deeper into the wool fabric after plasma modification using dyeing and coating methods with a PEDOT:PSS polymer. In addition, with a Tubicoat fixing agent, HT coating looks more homogeneous and uniform. The chemical structure spectra of wool fabrics coated with PEDOT:PSS were investigated using FTIR-ATR characterization. The influence of melamine formaldehyde resins on the electrical properties, resistance to washing, and mechanical effects of PEDOT:PSS treated wool fabric was also evaluated. The resistivity measurement of the samples containing melamine-formaldehyde resins as an additive did not show a significant decrease in electrical conductivity, while the electrical conductivity was maintained after the washing and rubbing test as well. The best results of electrical conductivity for investigated wool fabrics before and after washing and mechanical action were determined for samples subjected to the combined processing-surface modification by low-pressure N₂ plasma, dyeing by exhaust with PEDOT:PSS, and coating by the screen-printing method of PEDOT:PSS and a 3 wt.% melamine formaldehyde resins mixture.

A hierarchical integrated 3D carbon electrode derived from gingko leaves via hydrothermal carbonization of H3PO4 for high-performance supercapacitors

Electrochemical ultracapacitors derived from green and sustainable materials could demonstrate superior energy output and an ultra-long cycle life owing to large accessible surface area and obviously shortened ion diffusion pathways. Herein, we have established an efficient strategy to fabricate porous carbon (GLAC) from sustainable gingko leaf precursors by a facile hydrothermal activation of H3PO4 and low-cost pyrolysis. In this way, GLAC with a hierarchically porous structure exhibits extraordinary adaptability toward a high energy/power supercapacitor (∼709 F g-1 at 1 A g-1) in an aqueous electrolyte (1 M KOH). Notably, the GLAC-2-based supercapacitor displays an ultra-high stability of ∼98.24% even after 10 000 cycles (10 A g-1) and an impressive energy density as large as ∼71 W h kg-1 at a power density of 1.2 kW kg-1. The results provide new insights that the facile synthetic procedure coupled with the excellent performance contributes to great potential for future application in the electrochemical energy storage field.

Nitrogen-Doped Porous Core-Sheath Graphene Fiber-Shaped Supercapacitors

In this study, a strategy to fabricate nitrogen-doped porous core-sheath graphene fibers with the incorporation of polypyrrole-induced nitrogen doping and graphene oxide for porous architecture in sheath is reported. Polypyrrole/graphene oxide were introduced onto wet-spun graphene oxide fibers by dip-coating. Nitrogen-doped core-sheath graphene-based fibers (NSG@GFs) were obtained with subsequently thermally carbonized polypyrrole/small-sized graphene oxide and graphene oxide fiber slurry (PPY/SGO@GOF). Both nitrogen doping and small-sized graphene sheets can improve the utilization of graphene layers in graphene-based fiber electrode by preventing stacking of the graphene sheets. Enhanced electrochemical performance is achieved due to the introduced pseudo-capacitance and enhanced electrical double-layered capacitance. The specific capacitance (38.3 mF cm⁻²) of NSG@GF is 2.6 times of that of pure graphene fiber. The energy density of NSG@GF reaches 3.40 μWh cm⁻² after nitrogen doping, which is 2.59 times of that of as-prepared one. Moreover, Nitrogen-doped graphene fiber-based supercapacitor (NSG@GF FSSC) exhibits good conductivity (155 S cm⁻¹) and cycle stability (98.2% capacitance retention after 5000 cycles at 0.1 mA cm⁻²).

Surface Modification of Commercial Cotton Yarn as Electrode for Construction of Flexible Fiber-Shaped Supercapacitor

In this study, we report on the rational design and facile preparation of a cotton-reduced graphene oxide-silver nanoparticle (cotton-RGO-AgNP) hybrid fiber as an electrode for the building of a flexible fiber-shaped supercapacitor (FSSC). It was adequately characterized and found to possess a well-defined core−shell structure with cotton yarn as a core and a porous RGO-AgNP coating as a shell. Thanks to the unique morphological features and low electrical resistance (only 2.3 Ω·cm−1), it displayed attractive supercapacitive properties. When evaluated in a three-electrode setup, this FSSC electrode delivered the highest linear and volumetric specific capacitance of up to ca. 12.09 mF·cm−1 and ca. 9.67 F·cm−3 with a satisfactory rate capability as well as a decent cycling stability. On the other hand, an individual parallel symmetric FSSC cell constructed by this composite fiber fulfilled the largest linear and volumetric specific capacitance of ca. 1.67 mF·cm−1 and ca. 0.67 F·cm−3 and offered the maximum energy density, as high as ca. 93.1 μWh·cm−3, which outperformed a great number of graphene- and textile yarn-based FSSCs. Impressively, bending deformation brought about quite a limited effect on its electrochemical behaviors and almost no capacitance degradation took place during the consecutive charge/discharge test for over 10,000 cycles. Consequently, these remarkable performances suggest that the currently developed cotton-RGO-AgNP fiber has considerable application potential in flexible, portable and wearable electronics.

Graphene Fiber-Based Wearable Supercapacitors: Recent Advances in Design, Construction, and Application

Fiber-based supercapacitors (FSCs) that display small volume, robust weavability, high power density, and long-term stability, have urgently become the indispensable power supplies in smart wearable industries. Graphene fiber is regarded as an ideal FSCs electrode due to its remarkable natures of anisotropic framework, adjustable layer spacing, porous structures, large specific-surface-area, processable electroactivity, and high electronical and mechanical properties. This review, mainly focuses on the graphene fiber-based supercapacitors (GFSCs), with respect to fiber preparation, micro-nanostructure modulation, supercapacitor construction, performance optimization, and wearable applications. Various fiber fabrication strategies, including wet-spinning, dry-spinning, film conversion, confined hydrothermal self-assembly, and microfluidic-spinning are presented for fiber's structure manipulation and large-scale production. Advanced nanostructures and electroactivity with various building principles, such as oriented alignment, porous network, hierarchical, and heterogeneous skeleton, engineered active-sites, and mechanical regulation are discussed for boosting charge transfer, and ionic kinetic diffusion and storage. Especially, the optimizing approaches for regular unit alignment, enhanced interlayer interactions, modulated structural nano-architecture are presented to deliver high capacitance and energy density. Moreover, the flexibility and stretchability of graphene fiber, together with wearable applications of power supply are highlighted. Finally, a short summary, current challenges and future perspectives for designing high energy density GFSCs are proposed.

Solving Gravimetric-Volumetric Capacitive Paradox of 2D Materials through Dual-Functional Chemical Bonding-Induced Self-Constructing Graphene-MXene Monoliths

High electrical conductivity and all-open microstructure characteristics intrinsically endow both graphene and MXenes with superior electrochemical energy storage capability. However, the above two-dimensional (2D) thicker electrodes (>20 μm) severely dilute their unique rapid electronic-ionic transferring characteristic, posing a paradox of high gravimetric and high volumetric capacitive properties due to massively excessive macropores or an unduly restacked issue. Herein, we elaborately construct novel monolithic NH₂-graphene and Ti₃C₂T_x MXene (NG@MX) composites through dual-functional induced self-assembly with the help of both covalent and hydrogen bonding interactions. Notably, much thicker monolithic NG@MX electrodes (>90 μm) fabricated by a conventional roll-coating method without any further compaction treatment can simultaneously deliver two times gravimetric (gra.) and volumetric (vol.) performance than those of pure graphene (in vol.) or MXene (in gra.) materials. Moreover, monolithic NG@MX-based supercapacitors can remarkably present two times energy density as that of graphene and four times as MXene, respectively. Such greatly enhanced electrochemical properties are closely related to the appropriate equilibrium of the volumetric density and the open structure, which can effectively guarantee the rapid transfer of both electrons and ions in the thick monolithic NG@MX electrodes. Undoubtedly, dual-functional chemical bonding-induced self-constructing NG@MX monoliths efficiently solve the long-existing gra. and vol. capacitive paradox of the thicker 2D materials used in supercapacitors, which will guide the design of high-performance capacitive materials and promote their practical application in electrochemical energy storage.

Zeolitic imidazolate framework (ZIF)-derived porous carbon materials for supercapacitors: an overview

The present analysis focuses on the synthetic methods used for the application of supercapacitors with various mysterious architectures derived from zeolitic imidazolate frameworks (ZIFs). ZIFs represent an emerging and unique class of metal–organic frameworks with structures similar to conventional aluminosilicate zeolites, consisting of imidazolate linkers and metal ions. Their intrinsic porous properties, robust functionalities, and excellent thermal and chemical stabilities have resulted in a wide range of potential applications for various ZIF materials. In this rapidly expanding area, energetic research activities have emerged in the past few years, ranging from synthesis approaches to attractive applications of ZIFs. In this analysis, the development of high-performance supercapacitor electrodes and recent strategies to produce them, including the synthesis of various heterostructures and nanostructures, are analyzed and summarized. This analysis goes via the ingenuity of modern science when it comes to these nanoarchitecture electrodes. Despite these significant achievements, it is still difficult to accurately monitor the morphologies of materials derived from metal–organic frameworks (MOFs) because the induction force during structural transformations at elevated temperatures is in high demand. It is also desirable to achieve the direct synthesis of highly functionalized nanosized materials derived from zeolitic imidazolate frameworks (ZIFs) and the growth of nanoporous structures based on ZIFs encoded in specific substrates for the construction of active materials with a high surface area suitable for electrochemical applications. The latest improvements in this field of supercapacitors with materials formed from ZIFs as electrodes using ZIFs as templates or precursors are discussed in this review. Also, the possibility of usable materials derived from ZIFs for both existing and emerging energy storage technologies is discussed.

The present analysis focuses on the synthetic methods used for the application of supercapacitors with various mysterious architectures derived from zeolitic imidazolate frameworks (ZIFs).

Boosting storage properties of reduced graphene oxide fiber modified with MOFs-derived porous carbon through a wet-spinning fiber strategy

Supercapacitors that are light weight and flexible, while occupying a low volume and demonstrating good mechanical properties are in demand for portable energy storage devices. Graphene composite fibers are supposed to be ideal electrodes for flexible fiber-shaped supercapacitors. Integration of MOFs-derived porous carbon into graphene fibers provides desirable electrochemical and mechanical properties. Herein, a general strategy is shown for the preparation of MOFs-derived porous carbon/reduced graphene oxide fibers. Close-packed and aligned graphene sheets along with porous MOFs-derived porous carbon can achieve outstanding mechanical properties through synergistic effects. Consequently, a large specific capacitance of 56.05 F cm^-3, a good tensile property of 86.5 MPa and a high retention of 96.6% after 10 000 cycles can be achieved with the composite fibers. Moreover, a further deposition of polyaniline (PANI) and manganese dioxide (MnO₂) by in situ growth on the fabricated composite fibers provide an improvement in specific capacitance with value of 74.21 F cm^-3 and 65.08 F cm^-3, respectively. The above results demonstrate the promising application of composite fibers as a flexible and stable electrode and substrate for energy storage devices.

Self-supporting hybrid silica membranes with controlled porous architectures

Transferrable, self-supporting membranes with controlled and ordered pore architectures have been developed for potential applications in the fields of filtration, sensing, separation and catalysis.