Thermoelectric Polymers and their Elastic Aerogels

Stretchable and biodegradable plant-based redox-diffusion batteries

The redox-diffusion (RD) battery concept introduces an environmentally friendly solution for stretchable batteries in autonomous wearable electronics. By utilising plant-based redox-active biomolecules and cellulose fibers for the electrode scaffold, separator membrane, and current collector, along with a biodegradable elastomer encapsulation, the battery design overcomes the reliance on unsustainable transition metal-based active materials and non-biodegradable elastomers used in existing stretchable batteries. Importantly, it addresses the drawback of limited attainable battery capacity, where increasing the active material loading often leads to thicker and stiffer electrodes with poor mechanical properties. The concept decouples the active material loading from the mechanical structure of the electrode, enabling high mass loadings, while retaining a skin-like young's modulus and stretchability. A stretchable ion-selective membrane facilitates the RD process, allowing two separate redox couples, while preventing crossovers. This results in a high-capacity battery cell that is both electrochemically and mechanically stable, engineered from sustainable plant-based materials. Notably, the battery components are biodegradable at the end of their life, addressing concerns of e-waste and resource depletion.

Cellulose-based thermoelectric composites: A review on mechanism, strategies and applications

The ever-increasing demand for energy and environmental concerns have driven scientists to look for renewable and eco-friendly alternatives. Bio-based thermoelectric (TE) composite materials provide a promising solution to alleviate the global energy crisis due to their direct conversion of heat to electricity. Cellulose, the most abundant bio-polymer on earth with fascinating structure and desirable physicochemical properties, provides an excellent alternative matrix for TE materials. Here, recent studies on cellulose-based TE composites are comprehensively summarized. The fundamentals of TE materials, including TE effects, TE devices, and evaluation on conversion efficiency of TE materials are briefly introduced at the beginning. Then, the state-of-the-art methods for constructing cellulose-based TE composites in the forms of paper/film, aerogel, liquid, and hydrogel, are highlighted. TE performances of these composites are also compared. Following that, applications of cellulose-based TE composites in the fields of energy storage (e.g., supercapacitors) and sensing (e.g., self-powered sensors) are presented. Finally, opportunities and challenges that need investigation toward further development of cellulose-based TE composites are discussed.

Construction methods and biomedical applications of PVA-based hydrogels

Polyvinyl alcohol (PVA) hydrogel is favored by researchers due to its good biocompatibility, high mechanical strength, low friction coefficient, and suitable water content. The widely distributed hydroxyl side chains on the PVA molecule allow the hydrogels to be branched with various functional groups. By improving the synthesis method and changing the hydrogel structure, PVA-based hydrogels can obtain excellent cytocompatibility, flexibility, electrical conductivity, viscoelasticity, and antimicrobial properties, representing a good candidate for articular cartilage restoration, electronic skin, wound dressing, and other fields. This review introduces various preparation methods of PVA-based hydrogels and their wide applications in the biomedical field.

eSoil: A low-power bioelectronic growth scaffold that enhances crop seedling growth

Active hydroponic substrates that stimulate on demand the plant growth have not been demonstrated so far. Here, we developed the eSoil, a low-power bioelectronic growth scaffold that can provide electrical stimulation to the plants' root system and growth environment in hydroponics settings. eSoil's active material is an organic mixed ionic electronic conductor while its main structural component is cellulose, the most abundant biopolymer. We demonstrate that barley seedlings that are widely used for fodder grow within the eSoil with the root system integrated within its porous matrix. Simply by polarizing the eSoil, seedling growth is accelerated resulting in increase of dry weight on average by 50% after 15 d of growth. The effect is evident both on root and shoot development and occurs during the growth period after the stimulation. The stimulated plants reduce and assimilate NO3- more efficiently than controls, a finding that may have implications on minimizing fertilizer use. However, more studies are required to provide a mechanistic understanding of the physical and biological processes involved. eSoil opens the pathway for the development of active hydroponic scaffolds that may increase crop yield in a sustainable manner.

Folk arts-inspired twice-coagulated configuration-editable tough aerogels enabled by transformable gel precursors

Aerogels, as famous lightweight and porous nanomaterials, have attracted considerable attention in various emerging fields in recent decades, however, both low density and weak mechanical performance make their configuration-editing capability challenging. Inspired by folk arts, herein we establish a highly efficient twice-coagulated (TC) strategy to fabricate configuration-editable tough aerogels enabled by transformable gel precursors. As a proof of concept, aramid nanofibers (ANFs) and polyvinyl alcohol (PVA) are selected as the main components of aerogel, among which PVA forms a flexible configuration-editing gel network in the first coagulation process, and ANF forms a configuration-locking gel network in the second coagulation process. TC strategy guarantees the resulting aerogels with both high toughness and feasible configuration editing capability individually or simultaneously. Altogether, the resulting tough aerogels with special configuration through soft to hard modulation provide great opportunities to break through the performance limits of the aerogels and expand application areas of aerogels.

Ti3C2Tx MXene-Based Multifunctional Tactile Sensors for Precisely Detecting and Distinguishing Temperature and Pressure Stimuli

Although skin-like sensors that can simultaneously detect various physical stimuli are of fair importance in cutting-edge human-machine interaction, robotic, and healthcare applications, they still face challenges in facile, scalable, and cost-effective production using conventional active materials. The emerging two-dimensional transition metal carbide, Ti3C2Tx MXene, integrated with favorable thermoelectric properties, metallic-like conductivity, and a hydrophilic surface, is promising for solving these problems. Herein, skin-like multifunctional sensors are designed to precisely detect and distinguish temperature and pressure stimuli without cross-talk by decorating elastic and porous substrates with MXene sheets. Because the combination of the thermoelectric and conductive MXene with the thermally insulating, elastic, and porous substrate integrates efficient Seebeck and piezoresistive effects, the resultant sensor exhibits not only an ultralow detection limit (0.05 K), high signal-to-noise ratio, and excellent cycling stability for temperature detection but also high sensitivity, fast response time, and outstanding durability for pressure detection. Based on the impressive dual-mode sensing properties and independent temperature and pressure detections, a multimode input terminal and an electronic skin are created, exhibiting great potential in robotic and human-machine interaction applications. This work provides a scalable fabrication of multifunctional tactile sensors for precisely detecting and distinguishing temperature and pressure stimuli.

Scalable-produced 3D elastic thermoelectric network for body heat harvesting

Flexible thermoelectric generators can power wearable electronics by harvesting body heat. However, existing thermoelectric materials rarely realize high flexibility and output properties simultaneously. Here we present a facile, cost-effective, and scalable two-step impregnation method for fabricating a three-dimensional thermoelectric network with excellent elasticity and superior thermoelectric performance. The reticular construction endows this material with ultra-light weight (0.28 g cm^-3), ultra-low thermal conductivity (0.04 W m^-1 K^-1), moderate softness (0.03 MPa), and high elongation (>100%). The obtained network-based flexible thermoelectric generator achieves a pretty high output power of 4 μW cm^-2, even comparable to state-of-the-art bulk-based flexible thermoelectric generators.

Boosting the Thermoelectric Performance of the Doped DPP-EDOT Conjugated Polymer by Incorporating an Ionic Additive

The thermoelectric (TE) performance of organic materials is limited by the coupling of Seebeck coefficient and electrical conductivity. Herein a new strategy is reported to boost the Seebeck coefficient of conjugated polymer without significantly reducing the electrical conductivity by incorporation of an ionic additive DPPNMe₃ Br. The doped polymer PDPP-EDOT thin film exhibits high electrical conductivity up to 1377 ± 109 S cm^-1 but low Seebeck coefficient below 30 µV K^-1 and a maximum power factor of 59 ± 10 µW m^-1 K^-2 . Interestingly, incorporation of small amount (at a molar ratio of 1:30) of DPPNMe₃ Br into PDPP-EDOT results in the significant enhancement of Seebeck coefficient along with the slight decrease of electrical conductivity after doping. Consequently, the power factor (PF) is boosted to 571 ± 38 µW m^-1 K^-2 and ZT reaches 0.28 ± 0.02 at 130 °C, which is among the highest for the reported organic TE materials. Based on the theoretical calculation, it is assumed that the enhancement of TE performance for the doped PDPP-EDOT by DPPNMe₃ Br is mainly attributed to the increase of energetic disorder for PDPP-EDOT.

Pencil-Drawn Generator Built-in Actuator for Integrated Self-Powered/Visual Dual-Mode Sensing Functions and Rewritable Display

Multifunctionality is important to the development of next-generation actuators and intelligent robots. However, current multi-functional actuating systems are achieved based on the integration of diverse functional units with complex design, especially lacking in multi-mode sensing and displaying functions. Herein, a light-driven actuator integrated with self-powered/visual dual-mode sensing functions and rewritable display function is proposed. The actuator demonstrates a bending curvature of 0.93 cm-1 under near-infrared light irradiation. Meanwhile, by embedding a pencil-drawn graphite generator and thermochromic materials, the actuator also provides two independent sensing functions. First, owing to the photo-thermoelectric effect of graphite, the actuator spontaneously outputs a self-powered voltage (Seebeck coefficient: 23 µV K-1 ), which can reflect the deformation trend of actuator. Second, color changes occur on the actuator during deformation, which provide a visual sensing due to the thermochromic property. Furthermore, the actuator can be utilized as a rewritable display, owing to the integrated color-memorizing component. Intelligent robots, switches, and smart homes are further demonstrated as applications. All of them can spontaneously provide self-powered and visual sensing signals to demonstrate the working states of actuating systems, accompanied by rewritable displays on the actuators. This study will open a new direction for self-powered devices, multi-functional actuators, and intelligent robots.

Laminated Structural Engineering Strategy toward Carbon Nanotube-Based Aerogel Films

Aerogel films with a low density are ideal candidates to meet lightweight application and have already been used in a myriad of fields; however, their structural design for performance enhancement remains elusive. Herein, we put forward a laminated structural engineering strategy to prepare a free-standing carbon nanotube (CNT)-based aerogel film with a densified laminated porous structure. By directional densification and carbonization, the three-dimensional network of one-dimensional nanostructures in the aramid nanofiber/carbon nanotube (ANF/CNT) hybrid aerogel film can be reconstructed to a laminated porous structure with preferential orientation and consecutively conductive pathways, resulting in a large specific surface area (341.9 m²/g) and high electrical conductivity (8540 S/m). Benefiting from the laminated porous structure and high electrical conductivity, the absolute specific shielding effectiveness (SSE/t) of a CNT-based aerogel film can reach 200647.9 dB cm²/g, which shows the highest value among the reported aerogel-based materials. The laminated CNT-based aerogel films with an adjustable wetting property also exhibit exceptional Joule heating performance. This work provides a structural engineering strategy for aerogel films with enhanced electric conductivity for lightweight applications, such as EMI shielding and wearable heating.

State of the Art of Chemosensors in a Biomedical Context

Healthcare is undergoing large transformations, and it is imperative to leverage new technologies to support the advent of personalized medicine and disease prevention. It is now well accepted that the levels of certain biological molecules found in blood and other bodily fluids, as well as in exhaled breath, are an indication of the onset of many human diseases and reflect the health status of the person. Blood, urine, sweat, or saliva biomarkers can therefore serve in early diagnosis of diseases such as cancer, but also in monitoring disease progression, detecting metabolic disfunctions, and predicting response to a given therapy. For most point-of-care sensors, the requirement that patients themselves can use and apply them is crucial not only regarding the diagnostic part, but also at the sample collection level. This has stimulated the development of such diagnostic approaches for the non-invasive analysis of disease-relevant analytes. Considering these timely efforts, this review article focuses on novel, sensitive, and selective sensing systems for the detection of different endogenous target biomarkers in bodily fluids as well as in exhaled breath, which are associated with human diseases.

Ultrahigh Power Factor of Ternary Composites with Abundant Se Nanowires for Thermoelectric Application

In this work, ultrahigh-performance single-walled carbon nanotube (SWCNT)/Se nanowire (NW)/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) ternary thermoelectric (TE) nanocomposite films are successfully designed by rational design of CNT/Se/PEDOT:PSS ternary nanocomposites. The addition of CNTs apparently improves the electrical conductivity of composite films, resulting in a relatively huge growth of the power factor. The PEDOT:PSS interface layers uniformly attach on both sides of the Se NWs and CNTs effectively, forming a tightly interleaving and interconnected three-dimensional network. As a consequence, a maximum power factor of 863.83 μW/(m·K2) has been achieved for the sample containing 26 wt % CNTs at 434 K. Ultimately, a flexible TE generator prototype consisting of 5-unit freestanding composite film strips is fabricated using the optimized composite films, which can generate a maximum output power of 206.8 nW at a temperature gradient of 44.7 K. Therefore, the present work has a further potential to be used for the flexible polymer/carbon TE nanocomposite films and devices.

Thermoelectrochemical Cells Based on Ferricyanide/Ferrocyanide/Guanidinium: Application and Challenges

Ferricyanide/ferrocyanide/guanidinium-based thermoelectrochemical cells have been investigated under different loading conditions in this work. Compared with ferricyanide/ferrocyanide-based devices, the device with guanidinium-added electrolytes shows higher power and energy densities. We observed that the enhanced performance is not due to the ionic Seebeck effect of guanidinium but because of the configuration entropy change resulting from the selective binding of Gdm+ to Fe(CN)64-. However, the device with guanidinium-added electrolyte does not show steady-state operation. The two possible reasons include (1) the difficult diffusion of Fe(CN)63- into the crystal layer of (Gdm+)n[Fe(CN)64-] at the hot electrode and (2) the difficult precipitation of (Gdm+)n[Fe(CN)64-] formed at the cold side upon the binding of the reduced Fe(CN)64- with Gdm+. Nevertheless, the performance recovers once the device is disconnected from the external loading. Due to the high thermopower after adding guanidinium, we successfully fabricate self-powered sensors by connecting four flexible cells in series. The sensors can transfer humidity, temperature, and air pressure data wirelessly using body heat. Therefore, ferricyanide/ferrocyanide/guanidinium is a promising electrolyte material for applications of low-grade energy harvesting.

All-polymer wearable thermoelectrochemical cells harvesting body heat

Wearable thermoelectrochemical cells have attracted increasing interest due to their ability to turn human body heat into electricity. Here, we have fabricated a flexible, cost-effective, and 3D porous all-polymer electrode on an electrical conductive polymer substrate via a simple 3D printing method. Owing to the high degree of electrolyte penetration into the 3D porous electrode materials for redox reactions, the all-polymer based porous 3D electrodes deliver an increased power output of more than twice that of the film electrodes under the same mass loading using either n-type or p-type gel electrolytes. To realize the practical application of our thermocell, we fabricated 18 pairs of n-p devices through a series connection of single devices. The strap shaped thermocell arrangement was able to charge up a commercial supercapacitor to 0.27 V using the body heat of the person upon which it was being worn and in turn power a typical commercial lab timer.

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A compatible high electrical conductivity polymer film works as underlying substrate

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3D printable polymer ink with suitable rheological properties

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A serial 18 pairs of n-p devices charged supercapacitor to power a lab timer

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3D-printed all-polymer electrode thermocell device for harvesting body heat

Printing thermoelectric inks toward next-generation energy and thermal devices

The ability of thermoelectric (TE) materials to convert thermal energy to electricity and vice versa highlights them as a promising candidate for sustainable energy applications. Despite considerable increases in the figure of merit zT of thermoelectric materials in the past two decades, there is still a prominent need to develop scalable synthesis and flexible manufacturing processes to convert high-efficiency materials into high-performance devices. Scalable printing techniques provide a versatile solution to not only fabricate both inorganic and organic TE materials with fine control over the compositions and microstructures, but also manufacture thermoelectric devices with optimized geometric and structural designs that lead to improved efficiency and system-level performances. In this review, we aim to provide a comprehensive framework of printing thermoelectric materials and devices by including recent breakthroughs and relevant discussions on TE materials chemistry, ink formulation, flexible or conformable device design, and processing strategies, with an emphasis on additive manufacturing techniques. In addition, we review recent innovations in the flexible, conformal, and stretchable device architectures and highlight state-of-the-art applications of these TE devices in energy harvesting and thermal management. Perspectives of emerging research opportunities and future directions are also discussed. While this review centers on thermoelectrics, the fundamental ink chemistry and printing processes possess the potential for applications to a broad range of energy, thermal and electronic devices.

Preparation of aerogel and its application progress in coatings: a mini overview

Aerogels are predominantly mesoporous, extremely lightweight, low density (∼ 0.003 g/cm3) and thermally insulating materials. Over the years, aerogels have gained increasing attention due to their extraordinary properties (light, heat, sound, electricity and force) and application potentials in varieties of fields. Several studies have been carried out regarding aerogel preparation and its applications in coatings on different substrates. In this review, an overview of aerogels preparation and their application progress in coatings of most common substrates is presented. Attention is paid to aerogel coatings of textiles, leather, and substrates other than leather and textiles for special functionalities that could address the application progress in coatings. This review will help to inspire scientists and engineers towards novel aerogel materials and technologies to boost the industrial fabrication of flexible advanced materials.

Recent Progress in Designing Thermoelectric Metal-Organic Frameworks

Thermoelectrics that enable direct heat-electricity conversion possess unique advantages for green and renewable energy revolution and have received rapidly growing attention in the past decade. Among various thermoelectric materials, metal-organic frameworks (MOFs) with intrinsic high porosity and tunable physical/chemical properties are emerging as a promising class of materials that have been demonstrated to exhibit many unique merits for thermoelectric applications. Their structural topologies and thermoelectric properties can be facilely regulated by precisely selecting and arranging metal centers and organic ligands. Besides, a large variety of guest molecules can be incorporated within their pores, giving rise to novel avenues of raising energy-conversion efficiency. This review focuses on the recent advances in designing conductive MOFs and MOF-based composites for thermoelectric applications. It first introduces the fundamental thermoelectric parameters and the underlying regulation mechanisms specifically effective for MOFs, then summarizes the related studies conducted in recent years, where the structural design strategies of tuning thermoelectric properties are demonstrated and discussed. In the final part, conclusions and perspectives with the envision of an outlook for this promising area are presented.

Hydropower generation by transpiration from microporous alumina

Traditional hydropower generation is one of the most sustainable energy sources; however, the local environmental impact of hydroelectric dams and reservoirs is serious, and hydroelectric power requires high-cost turbines and generators. Because these installations utilize gravitational potential energy of massive volumes of falling water, this sort of hydropower generation is unsuitable for ubiquitous, small-scale energy production. Here, we report that wetting and evaporation of pure water from a tiny block of porous alumina generates electrical current in the direction of water transpiration. The current induced in microporous alumina is associated with mass transport of water accompanying ions that accumulate near the negatively charged surface of alumina pores. Without any pre-treatment or additives, once evaporation commences, a 3 × 3 cm² piece of alumina can generate an open-circuit voltage as large as 0.27 V. The power generation scheme we propose here is simple, clean, and versatile, and it can be employed anywhere, as it utilizes only spontaneous capillary action of water and Coulombic interaction at the alumina-water interface, without requiring any input of heat or light.

On the interaction between PEDOT:PSS and cellulose: Adsorption mechanisms and controlling factors

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a conducting polymer frequently used with cellulose, to develop advanced electronic materials. To understand the fundamental interactions between cellulose and PEDOT:PSS, a quartz crystal microbalance with dissipation (QCM-D) was used to study the adsorption of PEDOT:PSS onto model films of cellulose-nanofibrils (CNFs) and regenerated cellulose. The results show that PEDOT:PSS adsorbs spontaneously onto anionically charged cellulose wherein the adsorbed amount can be tuned by altering solution parameters such as pH, ionic strength and counterion to the charges on the CNF. Temperature-dependent QCM-D studies indicate that an entropy gain is the driving force for adsorption, as the adsorbed amount of PEDOT:PSS increased with increasing temperature. Colloidal probe AFM, in accordance with QCM-D results, also showed an increased adhesion between cellulose and PEDOT:PSS at low pH. AFM images show bead-like PEDOT:PSS particles on CNF surfaces, while no such organization was observed on the regenerated cellulose surfaces. This work provides insight into the interaction of PEDOT:PSS/cellulose that will aid in the design of sustainable electronic devices.

Highly Stretchable PU Ionogels with Self-Healing Capability for a Flexible Thermoelectric Generator

With the development of thermoelectric (TE) generator, the flexible, stretchable, self-healable, and wearable TE devices have aroused great interest. Therefore, we designed a self-healable and stretchable polyurethane (PU) ionogel, composed of polyurethane main chains with double bonds in the side, cross-linkers (BDB) and nonconjugated ionic liquids (EMIM:DCA). The PU ionogels with 30 wt % ILs have a high mechanical stretchability (300%), good tensile strength (1.61 MPa), and suitable Young's modulus (0.79 MPa). The proposed materials also exhibited an excellent ionic figure of merit (ZT_i) of 0.99 ± 0.3, as well as rapid self-healability in the absence of any external stimuli. The thermoelectric capability of PU ionogels kept stable under the severe condition (50% strain) and during self-healing process, which is rarely reported in recent studies. Furthermore, a stretchable and self-healable ionic thermoelectric capacitor device is also fabricated by the PU ionogels, which can efficiently convert heat into electricity.

Nature-inspired self-powered cellulose nanofibrils hydrogels with high sensitivity and mechanical adaptability

It is still a challenge to integrate high sensitivity, mechanical adaptability, and self-powered properties for hydrogels. Herein, we report a conductive polyvinyl alcohol (PVA) hydrogel based on natural nanoclay and cellulose nanofibrils (CNFs). The CNFs and PVA chains could construct a double network structure, resulting in a high mechanical composite hydrogel. Meanwhile, the nanoclay could be well dispersed and immobilized in the network of the hydrogel, thus improving mechanical adaptability of the hydrogel for curved and dynamic surfaces. Moreover, the conductive ions (Al³⁺) imparted the hydrogel with high conductivity (6.67 S m^-1) and gauge factor (1.17). Therefore, the composite hydrogel exhibited high sensitivity to tiny pressure changes, enabling recognition of the complicated sounding and handwriting. More importantly, the composite hydrogel possessed self-powered property, which could generate an output voltage of up to 78 mV. In summary, the multi-functional composite hydrogel may have giant applications in artificial electronic skins or wearable devices.

PEDOT:PSS nano-particles in aqueous media: A comparative experimental and molecular dynamics study of particle size, morphology and z-potential

PEDOT

PSS is the most widely used conducting polymer in organic and printed electronics.

PEDOT

PSS films have been extensively studied to understand the morphology, ionic and electronic conductivity of the polymer. However, the polymer dispersion, which is used to cast or spin coat the films, is not well characterized and not well understood theoretically. Here, we study in detail the particle morphology, size, charge density and zeta potential (z-potential) by coarse-grained MD simulations and dynamic light scattering (DLS) measurements, for different pH levels and ionic strengths. The PEDOT:PSS particles were found to be 12 nm-19 nm in diameter and had a z-potential of -30 mV to -50 mV when pH was changed from 1.7 to 9, at an added NaCl concentration of 1 mM, as measured by DLS. These values changed significantly with changing pH and ionic strength of the solution. The charge density of PEDOT:PSS particles was also found to be dependent on pH and ionic strength. Besides, the distribution of different ions (PSS^-, PEDOT⁺, Na⁺, Cl^-) present in the solution is simulated to understand the particle morphology and molecular origin of z-potential in PEDOT:PSS dispersion. The trend in change of particle size, charge density and z- potential with changing pH and ionic strength are in good agreement between the simulations and experiments. Our results show that the molecular model developed in this work represents very well the PEDOT:PSS nano-particles in aqueous dispersion. With this study, we hope to provide new insight and an in-depth understanding of the morphology and z-potential evolution in PEDOT:PSS dispersion.

Chemical Coupled PEDOT:PSS/Si Electrode: Suppressed Electrolyte Consumption Enables Long-Term Stability

Huge volume changes of Si during lithiation/delithiation lead to regeneration of solid-electrolyte interphase (SEI) and consume electrolyte. In this article, γ-glycidoxypropyl trimethoxysilane (GOPS) was incorporated in Si/PEDOT:PSS electrodes to construct a flexible and conductive artificial SEI, effectively suppressing the consumption of electrolyte. The optimized electrode can maintain 1000 mAh g^-1 for nearly 800 cycles under limited electrolyte compared with 40 cycles of the electrodes without GOPS. Also, the optimized electrode exhibits excellent rate capability. The use of GOPS greatly improves the interface compatibility between Si and PEDOT:PSS. XPS Ar⁺ etching depth analysis proved that the addition of GOPS is conducive to forming a more stable SEI. A full battery assembled with NCM 523 cathode delivers a high energy density of 520 Wh kg^-1, offering good stability.

One-Minute Synthesis of a Supramolecular Hydrogel from Suspension-Gel Transition and the Derived Crystalline, Elastic, and Photoactive Aerogels

Solutions or sols are commonly employed as the starting materials for the preparation of supramolecular hydrogels; however, suspension-based synthesis has been much less reported because of inhomogeneity and quick sedimentation of large particles in a suspension. Further, it remains a technical challenge to derive supramolecular aerogels directly from the parental hydrogels owing to the ease of structural collapse during water removal. Herein, we report a suspension-gel transition for the ultrafast synthesis of a new supramolecular hydrogel simply by adding AgNO₃ into the aqueous suspension of cyanuric acid-melamine (CAM) aggregates. With the activation of preadded ammonia, Ag ions instantly reacted with the CAM particles, transforming into N-Ag(I)-N coordinating bonded supramolecular nanofibers; simultaneously, the suspension converted to a hydrogel without the use of polymer cross-linkers or external stimulus. Upon simple freeze-drying, a highly crystalline fibrous aerogel with a cellular network was obtained, which possessed a porosity up to 99.7% and a density as low as 4.8 mg/cm³, enabling remarkable oil uptake capacities (100 times of its weight). The supramolecular aerogel demonstrated intrinsic elasticity, which should arise from the cellular structure and elastic character of the nanofiber skeletons. Notably, the aerogel showed high compatibility to incorporate a range of external substances for further functionalization exemplified by polymeric carbon nitride (PCN, a metal-free semiconductor) included gels. The loaded PCN resulted in enhanced mechanical strength and endowed the aerogel unique photoactivity, i.e., in situ reducing Ag(I) into Ag nanoparticles upon light illumination and thus forming a plasmonic shell over the aerogel with potential applications in sensing and catalysis.

High Thermoelectric Performance in n-Type Perylene Bisimide Induced by the Soret Effect

Low-cost, non-toxic, abundant organic thermoelectric materials are currently under investigation for use as potential alternatives for the production of electricity from waste heat. While organic conductors reach electrical conductivities as high as their inorganic counterparts, they suffer from an overall low thermoelectric figure of merit (ZT) due to their small Seebeck coefficient. Moreover, the lack of efficient n-type organic materials still represents a major challenge when trying to fabricate efficient organic thermoelectric modules. Here, a novel strategy is proposed both to increase the Seebeck coefficient and achieve the highest thermoelectric efficiency for n-type organic thermoelectrics to date. An organic mixed ion-electron n-type conductor based on highly crystalline and reduced perylene bisimide is developed. Quasi-frozen ionic carriers yield a large ionic Seebeck coefficient of -3021 μV K^-1 , while the electronic carriers dominate the electrical conductivity which is as high as 0.18 S cm^-1 at 60% relative humidity. The overall power factor is remarkably high (165 μW m^-1 K^-2 ), with a ZT = 0.23 at room temperature. The resulting single leg thermoelectric generators display a high quasi-constant power output. This work paves the way for the design and development of efficient organic thermoelectrics by the rational control of the mobility of the electronic and ionic carriers.

Mini-Review: Mixed Ionic-Electronic Charge Carrier Localization and Transport in Hybrid Organic-Inorganic Nanomaterials

In this mini-review, a comprehensive discussion on the state of the art of hybrid organic–inorganic mixed ionic–electronic conductors (hOI-MIECs) is given, focusing on conducting polymer nanocomposites comprising inorganic nanoparticles ranging from ceramic-in-polymer to polymer-in-ceramic concentration regimes. First, a brief discussion on fundamental aspects of mixed ionic–electronic transport phenomena considering the charge carrier transport at bulk regions together with the effect of the organic–inorganic interphase of hybrid nanocomposites is presented. We also make a recount of updated instrumentation techniques to characterize structure, microstructure, chemical composition, and mixed ionic–electronic transport with special focus on those relevant for hOI-MIECs. Raman imaging and impedance spectroscopy instrumentation techniques are particularly discussed as relatively simple and versatile tools to study the charge carrier localization and transport at different regions of hOI-MIECs including both bulk and interphase regions to shed some light on the mixed ionic–electronic transport mechanism. In addition, we will also refer to different device assembly configurations and in situ/operando measurements experiments to analyze mixed ionic–electronic conduction phenomena for different specific applications. Finally, we will also review the broad range of promising applications of hOI-MIECs, mainly in the field of energy storage and conversion, but also in the emerging field of electronics and bioelectronics.

Recent Progress of Two-Dimensional Thermoelectric Materials

Thermoelectric generators have attracted a wide research interest owing to their ability to directly convert heat into electrical power. Moreover, the thermoelectric properties of traditional inorganic and organic materials have been significantly improved over the past few decades. Among these compounds, layered two-dimensional (2D) materials, such as graphene, black phosphorus, transition metal dichalcogenides, IVA-VIA compounds, and MXenes, have generated a large research attention as a group of potentially high-performance thermoelectric materials. Due to their unique electronic, mechanical, thermal, and optoelectronic properties, thermoelectric devices based on such materials can be applied in a variety of applications. Herein, a comprehensive review on the development of 2D materials for thermoelectric applications, as well as theoretical simulations and experimental preparation, is presented. In addition, nanodevice and new applications of 2D thermoelectric materials are also introduced. At last, current challenges are discussed and several prospects in this field are proposed.

An Electrochemical Gelation Method for Patterning Conductive PEDOT:PSS Hydrogels

Due to their high water content and macroscopic connectivity, hydrogels made from the conducting polymer PEDOT:PSS are a promising platform from which to fabricate a wide range of porous conductive materials that are increasingly of interest in applications as varied as bioelectronics, regenerative medicine, and energy storage. Despite the promising properties of PEDOT:PSS-based porous materials, the ability to pattern PEDOT:PSS hydrogels is still required to enable their integration with multifunctional and multichannel electronic devices. In this work, a novel electrochemical gelation ("electrogelation") method is presented for rapidly patterning PEDOT:PSS hydrogels on any conductive template, including curved and 3D surfaces. High spatial resolution is achieved through use of a sacrificial metal layer to generate the hydrogel pattern, thereby enabling high-performance conducting hydrogels and aerogels with desirable material properties to be introduced into increasingly complex device architectures.

A flexible bimodal sensor based on an electrospun nanofibrous structure for simultaneous pressure-temperature detection

We developed a flexible and multifunctional resistive sensor integrating uniform conductive coating layers with an interlaced nanofibrous structure through a large-scale and cost-efficient strategy. The elastomer nanofiber framework not only endows the multi-mode sensor with superior flexibility, but also provides abundant contact sites and contact areas, which can significantly enhance the sensitivity and operating range of the obtained sensor. More impressively, the multilevel sensing paths comprising both interlaminar and intrastratal signal transmissions fulfill the simultaneous and precise detection of pressure-temperature stimuli without interference with each other. The obtained sensor ultimately shows an ultrahigh pressure sensitivity of 1185.8 kPa-1 and superior reliability, enabling the rapid detection of a subtle stimulus as low as 2.4 Pa and superior response behavior under 5000 cyclic loading tests. Besides, high linearity and stability are achieved for the temperature sensing characteristic even under various pressure loadings. These outstanding performances are further evaluated by preparing a 4 × 5 bimodal sensor array to synchronously monitor multiple signals, consequently demonstrating precise sensing capability with negligible interference and providing an effective approach for developing multiparametric sensing platforms and wearable devices.

Facile Fabrication of Flexible, Robust, and Superhydrophobic Hybrid Aerogel

Silica aerogels, which are constructed with silica nanoparticles and numerous nanoscale pores, have many outstanding attributes, but they are usually brittle and hydrophilic. For the construction of a robust aerogel, the novel polyhedral oligomeric silsesquioxane (POSS) was introduced to prepare a series of aerogels possessing particles covered with elastic cushion to improve the mechanical property. The multialkoxy POSS, which possessed stiff Si-O-Si nanocages and flexible alkyl chains, was synthesized via thiol-ene click chemistry. After a facile and efficient approach, a partially ordered structure of SiO₂ nanoparticles and organic elastic cushion would form spontaneously within the aerogels. With the POSS as the only precursor, several outstanding attributes were achieved in a single aerogel such as high specific surface area (SSA), high compression strength, high compression modulus, and noticeable compression flexibility. Meanwhile, the aerogel was superhydrophobic of which the contact angle (CA) was higher than 153°. Moreover, the potential application of oil-water separation is also presented.

Flexible Organic Thermoelectric Materials and Devices for Wearable Green Energy Harvesting

In the past few decades, organic thermoelectric materials/devices, which can exhibit remarkable potential in green energy conversion, have drawn great attention and interest due to their easy processing, light weight, intrinsically low thermal conductivity, and mechanical flexibility. Compared to traditional batteries, thermoelectric materials have high prospects as alternative power generators for harvesting green energy. Although crystalline inorganic semiconductors have dominated the fields of thermoelectric materials up to now, their practical applications are limited by their intrinsic fragility and high toxicity. The integration of organic polymers with inorganic nanoparticles has been widely employed to tailor the thermoelectric performance of polymers, which not only can combine the advantages of both components but also display interesting transport phenomena between organic polymers and inorganic nanoparticles. In this review, parameters affecting the thermoelectric properties of materials were briefly introduced. Some recently developed n-type and p-type thermoelectric films and related devices were illustrated along with their thermoelectric performance, methods of preparation, and future applications. This review will help beginners to quickly understand and master basic knowledge of thermoelectric materials, thus inspiring them to design and develop more efficient thermoelectric devices.

A Multiparameter Pressure-Temperature-Humidity Sensor Based on Mixed Ionic-Electronic Cellulose Aerogels

Pressure (P), temperature (T), and humidity (H) are physical key parameters of great relevance for various applications such as in distributed diagnostics, robotics, electronic skins, functional clothing, and many other Internet-of-Things (IoT) solutions. Previous studies on monitoring and recording these three parameters have focused on the integration of three individual single-parameter sensors into an electronic circuit, also comprising dedicated sense amplifiers, signal processing, and communication interfaces. To limit complexity in, e.g., multifunctional IoT systems, and thus reducing the manufacturing costs of such sensing/communication outposts, it is desirable to achieve one single-sensor device that simultaneously or consecutively measures P-T-H without cross-talks in the sensing functionality. Herein, a novel organic mixed ion-electron conducting aerogel is reported, which can sense P-T-H with minimal cross-talk between the measured parameters. The exclusive read-out of the three individual parameters is performed electronically in one single device configuration and is enabled by the use of a novel strategy that combines electronic and ionic Seebeck effect along with mixed ion-electron conduction in an elastic aerogel. The findings promise for multipurpose IoT technology with reduced complexity and production costs, features that are highly anticipated in distributed diagnostics, monitoring, safety, and security applications.

Stretchable Conductive Polymers and Composites Based on PEDOT and PEDOT:PSS

The conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT), and especially its complex with poly(styrene sulfonate) (PEDOT:PSS), is perhaps the most well-known example of an organic conductor. It is highly conductive, largely transmissive to light, processible in water, and highly flexible. Much recent work on this ubiquitous material has been devoted to increasing its deformability beyond flexibility-a characteristic possessed by any material that is sufficiently thin-toward stretchability, a characteristic that requires engineering of the structure at the molecular- or nanoscale. Stretchability is the enabling characteristic of a range of applications envisioned for PEDOT in energy and healthcare, such as wearable, implantable, and large-area electronic devices. High degrees of mechanical deformability allow intimate contact with biological tissues and solution-processable printing techniques (e.g., roll-to-roll printing). PEDOT:PSS, however, is only stretchable up to around 10%. Here, the strategies that have been reported to enhance the stretchability of conductive polymers and composites based on PEDOT and PEDOT:PSS are highlighted. These strategies include blending with plasticizers or polymers, deposition on elastomers, formation of fibers and gels, and the use of intrinsically stretchable scaffolds for the polymerization of PEDOT.

Self-Assembled Three-Dimensional Bi2Te3 Nanowire-PEDOT:PSS Hybrid Nanofilm Network for Ubiquitous Thermoelectrics

Thermoelectric generation capable of delivering reliable performance in the low-temperature range (<150 °C) for large-scale deployment has been a challenge mainly due to limited properties of thermoelectric materials. However, realizing interdependence of topological insulators and thermoelectricity, a new research dimension on tailoring and using the topological-insulator boundary states for thermoelectric enhancement has emerged. Here, we demonstrate a promising hybrid nanowire of topological bismuth telluride (Bi₂Te₃) within the conductive poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) matrix using the in situ one-pot synthesis to be incorporated into a three-dimensional network of self-assembled hybrid thermoelectric nanofilms for the scalable thermoelectric application. Significantly, the nanowire-incorporated film network exhibits simultaneous increase in electrical conductivity and Seebeck coefficient as opposed to reduced thermal conductivity, improving thermoelectric performance. Based on comprehensive measurements for electronic transport of individual nanowires revealing an interfacial conduction path along the Bi₂Te₃ core inside the encapsulating layer and that the hybrid nanowire is n-type semiconducting, the enhanced thermoelectricity is ascribed to increased hole mobility due to electron transfer from Bi₂Te₃ to PEDOT:PSS and importantly charge transport via the Bi₂Te₃-PEDOT:PSS interface. Scaling up the nanostructured material to construct a thermoelectric generator having the generic pipeline-insulator geometry, the device exhibits a power factor and a figure of merit of 7.45 μW m^-1 K^-2 and 0.048, respectively, with an unprecedented output power of 130 μW and 15 day operational stability at Δ T = 60 °C. Our findings not only encourage a new approach to cost-effective thermoelectric generation, but they could also provide a route for the enhancement of other applications based on the topological nanowire.

Efficient DMSO-Vapor Annealing for Enhancing Thermoelectric Performance of PEDOT:PSS-Based Aerogel

Conducting polymer-based composite aerogel film is desired to be used as thermoelectric (TE) materials due to its good flexibility and ultralow thermal conductivity. Here, we proposed the simple freeze drying method to fabricate free-standing poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS)-based aerogel films without any crosslinker addition. The evolutions of morphology and TE performance were systemically investigated with various organic solvent addition. Furthermore, a series of the PEDOT:PSS/tellurium nanowires (Te-NWs) composite aerogel films was prepared, and the relationship between the structure and the charge-transport mechanism of the binary complex system was explored based on series and parallel models. Finally, an efficient dimethyl sulfoxide-vapor annealing was employed to further optimize the TE performance of PEDOT:PSS/Te-NWs composite aerogel films. The ZT value was estimated to be 2.0 × 10^-2 at room temperature. On the basis of the flexibility and highly enhanced TE performance, a prototype TE generator consisting of p-type PEDOT:PSS/Te-NWs aerogel films and n-type carbon nanotube fibers as legs has been fabricated with an acceptable output power of 1.28 μW at a temperature gradient of 60 K, which could be potentially applied in wearable electronics.

Ambient-dried highly flexible copolymer aerogels and their nanocomposites with polypyrrole for thermal insulation, separation, and pressure sensing

Highly flexible copolymer and copolymer/polypyrrole nanocomposite aerogels have been synthesized via ambient pressure drying for superinsulation, separation and pressure sensing.

Thermoelectric materials and applications for energy harvesting power generation

Thermoelectrics, in particular solid-state conversion of heat to electricity, is expected to be a key energy harvesting technology to power ubiquitous sensors and wearable devices in the future. A comprehensive review is given on the principles and advances in the development of thermoelectric materials suitable for energy harvesting power generation, ranging from organic and hybrid organic-inorganic to inorganic materials. Examples of design and applications are also presented.

Coaxial struts and microfractured structures of compressible thermoelectric foams for self-powered pressure sensors

Long-term operation of wearable pressure sensors to detect body movement requires self-powered human-based energy sources to minimize the need for recharging. Recently, pressure sensors with thermoelectric properties based on conducting polymers have been reported; however, these devices are limited in their ability to simultaneously achieve sufficient power generation and sensitivity of the sensor. In this article, we suggest a coaxial strut structure of poly(styrene-ethylene/butylene-styrene)(SEBS)-poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS)-melamine foam (MF) with a fractured microstructure for a highly sensitive, efficient self-powered pressure sensor. In the coaxial struts, the MF core provides a compressible and elastic framework; the intermediate PEDOT:PSS acts as a conductor and a thermoelectric material; and the SEBS shell ensures mechanical stability and resilience to stabilize the brittle PEDOT:PSS layer under high loading conditions. Additionally, by compressing the coaxial foam to 1/20, partial microfracture of PEDOT:PSS occurs only in the SEBS shell; thus, the pressure sensitivity increases significantly while maintaining high conductivity and thermoelectric performance. The coaxial foam was assembled into a wearable TEG to generate 338 nW from the forearms and demonstrate the high sensitivity of pressure sensors without an external power supply.

Biopolymer nanofibrils: structure, modeling, preparation, and applications

Biopolymer nanofibrils exhibit exceptional mechanical properties with a unique combination of strength and toughness, while also presenting biological functions that interact with the surrounding environment. These features of biopolymer nanofibrils profit from their hierarchical structures that spun angstrom to hundreds of nanometer scales. To maintain these unique structural features and to directly utilize these natural supramolecular assemblies, a variety of new methods have been developed to produce biopolymer nanofibrils. In particular, cellulose nanofibrils (CNFs), chitin nanofibrils (ChNFs), silk nanofibrils (SNFs) and collagen nanofibrils (CoNFs), as the four most abundant biopolymer nanofibrils on earth, have been the focus of research in recent years due to their renewable features, wide availability, low-cost, biocompatibility, and biodegradability. A series of top-down and bottom-up strategies have been accessed to exfoliate and regenerate these nanofibrils for versatile advanced applications. In this review, we first summarize the structures of biopolymer nanofibrils in nature and outline their related computational models with the aim of disclosing fundamental structure-property relationships in biological materials. Then, we discuss the underlying methods used for the preparation of CNFs, ChNFs, SNF and CoNFs, and discuss emerging applications for these biopolymer nanofibrils.

Conductive Polymer Protonated Nanocellulose Aerogels for Tunable and Linearly Responsive Strain Sensors

Strong and highly conductive aerogels have been assembled from cellulose nanofibrils (CNFs) protonated with conductive poly(3,4-ethylene dioxythiophene)/poly(styrene sulfonate) (PEDOT/PSS) complex at equal mass or less. Protonating CNF surface carboxylates and hydrogen-bonding CNF surface carboxyls with PSS in PEDOT/PSS generated PEDOT/PSS/CNF aerogels that were up to ten times stronger while as conductive as neat PEDOT/PSS aerogel, attributed to the transformation of PEDOT benzoid structure to the more electron transfer-preferred quinoid structure. Ethylene glycol vapor annealing further increased the conductivity of PEDOT/PSS/CNF aerogels by 2 orders of magnitude. The poly(dimethylsiloxane) (PDMS)-infused conductive PEDOT/PSS/CNF aerogel (70 wt % CNF) transform a resistance-insensitive PDMS-infused PEDOT/PSS aerogel (gauge factor of 1.1 × 10^-4) into a stretchable, sensitive, and linearly responsive strain sensor (gauge factor of 14.8 at 95% strain).