Stretchable Supercapacitors Based on Carbon Nanotubes-Deposited Rubber Polymer Nanofibers Electrodes with High Tolerance against Strain

Mini Review of Reliable Fabrication of Electrode under Stretching for Supercapacitor Application

Currently, there is an increasing demand for portable and wearable electronics. This has necessitated the development of stretchable energy storage devices, while simultaneously maintaining performance. Hence, the electrodes and electrolyte materials used in stretchable supercapacitors should be robust under severe mechanical deformation. Polymers are widely used in the fabrication of stretchable supercapacitors. It is not only crucial to choose good polymer candidates with inherent advantages, but it is also important to design suitable polymer materials for both electrodes and electrolytes. This mini-review explains the concept of stretchable supercapacitors, the theoretical background of polymer-based electrodes for supercapacitors, and the fabrication strategies of stretchable electrodes for supercapacitors. Finally, we present the drawbacks and areas that still need to be developed.

Supercapacitor based on polymeric binary composite of polythiophene and single-walled carbon nanotubes

The aim of this work is to fabricate supercapacitor electrode based on poly (3-hexyl-thiophene-2, 5-diyl) (P3HT) and single-walled carbon nanotubes (SWCNTs) nanocomposites with different ratios onto a graphite sheet as a substrate with a wide voltage window in nonaqueous electrolyte. Structural, morphological and electrochemical properties of the prepared nanocomposites of P3HT/SWCNTs were studied and discussed. The electrochemical properties included cyclic voltammetry (CV), galvanostatic charging-discharging (GCD), and electrochemical impedance spectroscopy (EIS) were investigated. The obtained results indicated that P3HT/SWCNTs nanocomposite possesses higher specific capacitance than that present in its individual component. The high electrochemical performance of the nanocomposite was due to formation of microporous structure which facilitates ions diffusion and electrolyte penetration in these pores. The morphological micrographs of the purified SWCNTs had buckypaper structure while the photomicrographs of P3HT/SWCNTs showed that SWCNTs appear behind and front of the P3HT nanospheres. The specific capacitance of 50% SWCNTs at 0.5 Ag⁻¹ was found to be 245.8 Fg⁻¹ compared with that of pure P3HT of 160.5 Fg⁻¹.

Ionic Gelatin-Based Flexible Thermoelectric Generator with Scalability for Human Body Heat Harvesting

The prosperity of intelligent wearables brings an increasingly critical problem of power supply. Regular rechargeable lithium or disposable button batteries have some problems, such as limited capacity, frequent replacement, environmental pollution, etc. Wearable energy harvester (WEH) can fundamentally solve these problems. Among WEHs, thermoelectric generator (TEG) is a promising option due to its independence of light condition or the motion of the wearer, and thermoelectric conversion (TEC) has the characteristics of quietness and continuity. Therefore, TEG has become a suitable choice for harvesting low-grade heat energy such as human body heat. Ionic thermoelectric gel (iTEG) has the advantages of a large Seebeck coefficient, freely defined shape and size, low processing cost, wide material sources, easy encapsulation, etc. In this paper, the gelatin-based iTEG is regulated and optimized by silica nanoparticles (SiO2 NPs). The optimal compound quantity of SiO2 NPs is determined, and the optimization mechanism is discussed through a series of characterization tests. Based on the iTEG, a kind of scalable flexible TEGs is proposed, and its preparation method is described in detail. A small wristband TEG (STEG) was made, and its Seebeck coefficient is 74.5 mV/K. Its bendability and stretchability were verified, and the impedance matching experiment was carried out. By charging a capacitor, the STEG successfully lights up an LED at a temperature difference (ΔT) of ~15.5 K. Subsequently, a large extended oversleeve TEG (LTEG) was prepared, and a set of heat sinks was added at the cooling end of the LTEG. Being worn on a volunteer’s forearm, the LTEG output a voltage of more than 3 V at ~20 °C. Through storing the converted energy in a capacitor, the LTEG directly drove a calculator without a DC–DC booster. The proposed iTEG and TEGs in this paper have the prospect of mass production, extending to people’s clothes, harvesting human body heat and directly powering wearable electronics.

Prospects and Challenges of Flexible Stretchable Electrodes for Electronics

The application of flexible electronics in the field of communication has made the transition from rigid physical form to flexible physical form. Flexible electrode technology is the key to the wide application of flexible electronics. However, flexible electrodes will break when large deformation occurs, failing flexible electronics. It restricts the further development of flexible electronic technology. Flexible stretchable electrodes are a hot research topic to solve the problem that flexible electrodes cannot withstand large deformation. Flexible stretchable electrode materials have excellent electrical conductivity, while retaining excellent mechanical properties in case of large deformation. This paper summarizes the research results of flexible stretchable electrodes from three aspects: material, process, and structure, as well as the prospects for future development.

Stretchable Supercapacitors: From Materials and Structures to Devices

Stretchable supercapacitors have received widespread attention due to their potential applications in wearable electronics and health monitoring. Stretchable supercapacitors not only possess advantages such as high power density, long cycle life, safety, and low cost of conventional supercapacitors but also have excellent flexibility and stretchability, which make them well integrated with other wearable systems. In this review, various strategies to fabricate stretchable supercapacitors are focused. The preparation methods for stretchable electrodes/devices in the literature are carefully classified and analyzed. Three strategies for preparing stretchable electrodes/devices are summarized in detail-the design of elastic polymer substrates, stretchable electrode structures, and composite electrodes combined with elastic polymers and stretchable structures. Meanwhile, the interface problem of electrodes/devices in the stretching process is studied in depth. The research progress of multifunctional stretchable supercapacitors is also introduced. Finally, challenges and possible solutions that still need to be addressed in the future development of stretchable supercapacitors are highlighted and prospected. This review comprehensively discusses the latest research progress in the field of stretchable supercapacitors and systematically elucidates the electrochemical and mechanical properties of these devices, hoping to improve the roadmap for scientists and engineers to develop supercapacitors with high electrochemical performance and good stretchability.

Highly stretchable superhydrophobic surface by silica nanoparticle embedded electrospun fibrous mat

HYPOTHESIS

Obtaining simultaneous stretchability and superhydrophobicity remains a great challenge in stretchable electronics, and wearable devices. Inspired by natural surfaces, such as lotus leaf, surface roughness and coating materials are the fundamental requirements to achieve superhydrophobicity.

EXPERIMENTS

We prepared an elastic fibrous mat by electrospinning of a composite solution made of thermoplastic elastomer as an organic polymer matrix, and silica nanoparticles as inorganic additives to support surface roughness. To enhance hydrophobicity, the pristine mat was immersed into a solution of fluorinated material, which can decrease the surface energy.

FINDINGS

The pristine fibrous mat showed high stretchability (with more than 1000% strain), and superhydrophobicity (with a contact angle of 156°, and a sliding angle of 7.8°). Superhydrophobicity did not disappear when the fibrous mat was stretched up to 1000%. Sliding angles were less than 10° under different strain levels only in longitudinal direction, suggesting the stretchable superhydrophobic surface is effective in rolling off the water droplet in one direction. The fibrous mat was repeatedly stretched 1000 times to 1000% strain; the material showed stable stretchability and superhydrophobicity. Based on these observations, the resulting fibrous mat appears to be in the Cassie-Baxter wetting state.

Highly Stretchable All-Rubber-Based Thread-Shaped Wearable Electronics for Human Motion Energy-Harvesting and Self-Powered Biomechanical Tracking

The development of stretchable smart electronics has attracted great attentions due to their potential applications in human motions energy collection systems and self-powered biomechanical tracking technologies. Here, we present a newly stretchable all-rubber-based thread-shaped triboelectric nanogenerator (TENG) composed of the silver-coated glass microspheres/silicone rubber as the stretchable conductive thread (SCT) and the silicone rubber-coated SCT (SSCT) as the other triboelectric thread. The stretchable all-rubber-based thread-shaped TENG (SATT) generates an open-circuit voltage of 3.82 V and short-circuit current of 65.8 nA under the 100% strain and can respond to different finger motion states. Furthermore, the self-powered smart textile (SPST) woven by the SCT and SSCT units has two kinds of working mechanisms about stretch-release and contact-separation modes. The stretching-releasing interaction between knitting units can generate an open-circuit voltage of 8.1 V and short-circuit current of 0.42 μA, and the contacting-separating mode occurs between cotton and two types material outside the SPST producing peak voltage of 150 V and peak current of 2.45 μA. To prove the promising applications, the SPST device is capable to provide electrical energy to commercial electronics and effectively scavenge full-range biomechanical energy from human joint motions. Therefore, this work provides a new approach in the applications of stretchable wearable electronics for power generation and self-powered tracking.

Tuning Porosity and Functionality of Electrospun Rubber Nanofiber Mats by Photo-Crosslinking

The present work proposes a versatile and efficient method to fabricate rubber nanofiber membranes with a controlled morphology and tailored functionality, based on the application of photoinduced thiol-ene cross-linking reactions to electrospun mats. Besides preventing the polymer cold flow and freezing the structure obtained by electrospinning, the photocuring step finely controls the morphology of the nanofiber mats, in terms of the fiber diameter up to the nanometer range and of the membrane porosity. Nanofiber membranes are also made chemically resistant, while retaining their flexibility. Finally, the proposed approach allows imparting specific functionalities to the rubber nanofibers: the type and concentration of the functional groups can be precisely tuned by changing process parameters (i.e., thiol/ene stoichiometric ratio and irradiation dose). Active chemical groups that remain available on the surface of the nanofibers can be used for further material modifications, as here proven by two target reactions. This key result is also demonstrated with electrospun membranes embedded into a microfluidic chip, opening the way to advanced functional flexible devices.

The Relationships between Process Parameters and Polymeric Nanofibers Fabricated Using a Modified Coaxial Electrospinning

The concrete relationship between the process parameters and nanoproduct properties is an important challenge for applying nanotechnology to produce functional nanomaterials. In this study, the relationships between series of process parameters and the medicated nanofibers’ diameter were investigated. With an electrospinnable solution of hydroxypropyl methylcellulose (HPMC) and ketoprofen as the core fluid, four kinds of nanofibers were prepared with ethanol as a sheath fluid and under the variable applied voltages. Based on these nanofibers, a series of relationships between the process parameters and the nanofibers’ diameters (D) were disclosed, such as with the height of the Taylor cone (H, D = 125 + 363H), with the angle of the Taylor cone (α, D = 1576 − 19α), with the length of the straight fluid jet (L, D = 285 + 209L), and with the spreading angle of the instable region (θ, D = 2342 − 43θ). In vitro dissolution tests verified that the smaller the diameters, the faster ketoprofen (KET) was released from the HPMC nanofibers. These concrete process-property relationships should provide a way to achieve new knowledge about the electrostatic energy-fluid interactions, and to meanwhile improve researchers’ capability to optimize the coaxial process conditions to achieve the desired nanoproducts.

Electrospun Nanomaterials for Energy Applications: Recent Advances

Electrospinning is a simple, versatile, cost-effective, and scalable technique for the growth of highly porous nanofibers. These nanostructures, featured by high aspect ratio, may exhibit a large variety of different sizes, morphologies, composition, and physicochemical properties. By proper post-spinning heat treatment(s), self-standing fibrous mats can also be produced. Large surface area and high porosity make electrospun nanomaterials (both fibers and three-dimensional fiber networks) particularly suitable to numerous energy-related applications. Relevant results and recent advances achieved by their use in rechargeable lithium- and sodium-ion batteries, redox flow batteries, metal-air batteries, supercapacitors, reactors for water desalination via capacitive deionization and for hydrogen production by water splitting, as well as nanogenerators for energy harvesting, and textiles for energy saving will be presented and the future prospects for the large-scale application of electrospun nanomaterials will be discussed.

Design and Preparation of Biomass-Derived Carbon Materials for Supercapacitors: A Review

The synthesis and application of biomass-derived carbon in energy storage have drawn increasing research attention due to the ease of fabrication, cost-effectiveness, and sustainability of the meso/microporous carbon produced from various biological precursors, including plants, fruits, microorganisms, and animals. Compared to the artificial nanostructured carbons, such as fullerene, carbon nanotube and graphene, the biomass-derived carbons may obtain superior capacitance, rate performance and stability in supercapacitor applications ascribing to their intrinsic nanoporous and hierarchical structures. However, challenges remain in processing techniques to obtain biomass-derived carbons with high carbon yield, high energy density, and controllable graphitic microstructures, which may require a clear understanding over the chemical and elemental compositions, and the intrinsic microstructural characteristics of the biological precursors. Herein we present comprehensive analyses over the impacts of the chemical and elemental compositions of the precursors on the carbon yield of the biomass, as well as the mechanism of chemical activation on the nanoporous structure development of the biomass-derived carbons. The structure–property relationship and functional performance of various biomass-derived carbons for supercapacitor applications are also discussed in detail and compared. Finally, useful insights are also provided for the improvements of biomass-derived carbons in supercapacitor applications.