Advancements in Flexible and Stretchable Electronics for Resistive Hydrogen Sensing: A Comprehensive Review.

Flexible and stretchable electronics have emerged as a groundbreaking technology with wide-ranging applications, including wearable devices, medical implants, and environmental monitoring systems. Among their numerous applications, hydrogen sensing represents a critical area of research, particularly due to hydrogen's role as a clean energy carrier and its explosive nature at high concentrations. This review paper provides a comprehensive overview of the recent advancements in flexible and stretchable electronics tailored for resistive hydrogen sensing applications. It begins by introducing the fundamental principles underlying the operation of flexible and stretchable resistive sensors, highlighting the innovative materials and fabrication techniques that enable their exceptional mechanical resilience and adaptability. Following this, the paper delves into the specific strategies employed in the integration of these resistive sensors into hydrogen detection systems, discussing the merits and limitations of various sensor designs, from nanoscale transducers to fully integrated wearable devices. Special attention is paid to the sensitivity, selectivity, and operational stability of these resistive sensors, as well as their performance under real-world conditions. Furthermore, the review explores the challenges and opportunities in this rapidly evolving field, including the scalability of manufacturing processes, the integration of resistive sensor networks, and the development of standards for safety and performance. Finally, the review concludes with a forward-looking perspective on the potential impacts of flexible and stretchable resistive electronics in hydrogen energy systems and safety applications, underscoring the need for interdisciplinary collaboration to realize the full potential of this innovative technology.

Structured nanoporous surfaces from hybrid block copolymer micelle films with metal ions.

We present a novel method for producing structured nanoporous thin films using block copolymer (BCP) micelles loaded with metallic ions. The BCP micellar thin films containing gold (Au) ions were prepared by spin-coating poly(styrene-block-4-vinylpyridine) (PS-b-P4VP) micelle solutions in which Au precursors (AuCl4(-)) were selectively loaded onto the P4VP core. When the micellar films were exposed to cetyltrimethylammonium bromide (CTAB) solutions, the Au precursors were selectively extracted from the P4VP domains due to their strong electrostatic interaction with CTAB, leading to the formation of pores in the micelles. Consequently, regularly patterned nanoporous surfaces were formed. By controlling the molecular weight (Mn) of PS-b-P4VP and the amount of Au precursors (λ) that were loaded in the P4VP domains, the pore size and depth could be tuned precisely. In particular, when a sufficient amount of Au precursors was loaded (λ  ≥ 0.3), the porous surface nanostructure was well developed. In addition, the pore size and depth of the nanostructure increased as the λ value increased. For instance, when the λ value increased from 0.3 to 1.0, the pore size increased from 22.8 nm to 28.8 nm, and the pore depth increased from 2.1 nm to 3.2 nm. Interestingly, the transition from the nonporous structures to the porous structures in the micellar film could be reversibly controlled by adding and removing the Au precursors in the film. Moreover, our method for the preparation of nanoporous films can be extended to micellar film by incorporating other metal ions such as silver (Ag) and iron (Fe).

Size-controlled nanoparticle-guided assembly of block copolymers for convex lens-shaped particles.

The tuning of interfacial properties at selective and desired locations on the particles is of great importance to create the novel structured particles by breaking the symmetry of their surface property. Herein, a dramatic transition of both the external shape and internal morphology of the particles of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) was induced by precise positioning of size-controlled Au nanoparticle surfactants (Au NPs). The size-dependent assembly of the Au NPs was localized preferentially at the interface between the P4VP domain at the particle surface and the surrounding water, which generated a balanced interfacial interaction between two different PS/P4VP domains of the BCP particles and water, producing unique convex lens-shaped BCP particles. In addition, the neutralized interfacial interaction, in combination with the directionality of the solvent-induced ordering of the BCP domains from the interface of the particle/water, generated defect-free, vertically ordered porous channels within the particles. The mechanism for the formation of these novel nanostructures was investigated systemically by varying the size and the volume fraction of the Au NPs. Furthermore, these convex lens-shaped particles with highly ordered channels can be used as a microlens, in which the light can be concentrated toward the focal point with enhanced near-field signals. And, these particles can possess additional optical properties such as unique distribution of light scattering as a result of the well-ordered Au cylinders that filled into the channels, which hold great promise for use in optical, biological-sensing, and imaging applications.

Advanced Triboelectric Applications of Biomass-Derived Materials: A Comprehensive Review.

The utilization of triboelectric materials has gained considerable attention in recent years, offering a sustainable approach to energy harvesting and sensing technologies. Biomass-derived materials, owing to their abundance, renewability, and biocompatibility, offer promising avenues for enhancing the performance and versatility of triboelectric devices. This paper explores the synthesis and characterization of biomass-derived materials, their integration into triboelectric nanogenerators (TENGs), and their applications in energy harvesting, self-powered sensors, and environmental monitoring. This review presents an overview of the emerging field of advanced triboelectric applications that utilize the unique properties of biomass-derived materials. Additionally, it addresses the challenges and opportunities in employing biomass-derived materials for triboelectric applications, emphasizing the potential for sustainable and eco-friendly energy solutions.

In Situ Analysis of Binder Degradation during Catalyst-Accelerated Stress Test of Polymer Electrolyte Membrane Fuel Cells.

High-oxygen-permeability ionomers (HOPIs) are being actively developed to enhance the performance and durability of high-power polymer electrolyte membrane fuel cells (PEMFCs). While methods for evaluating binder performance are well-established, techniques for assessing binder durability and measuring its degradation in situ during the AST process remain limited. This study examines the distribution of relaxation times (DRT) and Warburg-like response (WLR) methods as in situ analysis techniques during the catalyst-accelerated stress test (AST) process. We conducted catalyst-ASTs (0.6-0.95 V cycling) for 20,000 cycles, monitoring changes using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and linear sweep voltammetry (LSV). Contrary to expectations, during the catalyst-AST, the ion transport resistance of the binder decreased, indicating no binder degradation. Scanning electron microscopy/energy dispersive spectrometer (SEM/EDS) analysis revealed that the degradation rate of the catalyst and the support was relatively higher than that of the binder, leading to a reduction in catalyst layer thickness and improved binder network formation. By applying the DRT method during the catalyst-AST process, we were able to measure the increase in oxygen reduction reaction (ORR) resistance and the decrease in proton transport resistance in situ. This allowed for the real-time detection of the reduction in catalyst layer thickness and improvements in ionomer networks due to catalyst and support degradation. These findings provide new insights into the complex interplay between catalyst degradation and binder performance, contributing to the development of more durable PEMFC components.

Recent Developments in Materials for Physical Hydrogen Storage: A Review.

The depletion of reliable energy sources and the environmental and climatic repercussions of polluting energy sources have become global challenges. Hence, many countries have adopted various renewable energy sources including hydrogen. Hydrogen is a future energy carrier in the global energy system and has the potential to produce zero carbon emissions. For the non-fossil energy sources, hydrogen and electricity are considered the dominant energy carriers for providing end-user services, because they can satisfy most of the consumer requirements. Hence, the development of both hydrogen production and storage is necessary to meet the standards of a "hydrogen economy". The physical and chemical absorption of hydrogen in solid storage materials is a promising hydrogen storage method because of the high storage and transportation performance. In this paper, physical hydrogen storage materials such as hollow spheres, carbon-based materials, zeolites, and metal-organic frameworks are reviewed. We summarize and discuss the properties, hydrogen storage densities at different temperatures and pressures, and the fabrication and modification methods of these materials. The challenges associated with these physical hydrogen storage materials are also discussed.

Multilayered Functional Triboelectric Polymers for Self-Powered Wearable Applications: A Review.

Multifunctional wearable devices detect electric signals responsive to various biological stimuli and monitor present body motions or conditions, necessitating flexible materials with high sensitivity and sustainable operation. Although various dielectric polymers have been utilized in self-powered wearable applications in response to multiple external stimuli, their intrinsic limitations hinder further device performance enhancement. Because triboelectric devices comprising dielectric polymers are based on triboelectrification and electrostatic induction, multilayer-stacking structures of dielectric polymers enable significant improvements in device performance owing to enhanced interfacial polarization through dissimilar permittivity and conductivity between each layer, resulting in self-powered high-performance wearable devices. Moreover, novel triboelectric polymers with unique chemical structures or nano-additives can control interfacial polarization, allowing wearable devices to respond to multiple external stimuli. This review summarizes the recent insights into multilayered functional triboelectric polymers, including their fundamental dielectric principles and diverse applications.

Ferroelectricity-Coupled 2D-MXene-Based Hierarchically Designed High-Performance Stretchable Triboelectric Nanogenerator.

Triboelectric nanogenerators based on the state-of-the-art functional materials and device engineering provide an exciting platform for future multifunctional electronics, but it remains challenging to realize due to the lack of in-depth understanding on the functional properties of nanomaterials that are compatible with microstructural engineering. In this study, a high-performance stretchable (∼60% strain) triboelectric nanogenerator is demonstrated via an interlocked microstructural device configuration sandwiched between silver-nanowire-(Ag-NW) electrodes and hierarchically engineered spongy thermoplastic polyurethane (TPU) polymer composite with ferroelectric barium-titanate-coupled (BTO-coupled) 2D MXene (Ti3C2Tx) nanosheets. The use of MXene results in an increase in the dielectric constant whereas the dielectric loss is lowered via coupling with the ferroelectricity of BTO, which increases the overall output performance of the nanogenerator. The spongy nature of the composite film increases the capacitance variation under deformation, which results in improved energy-conversion efficiency (∼79%) and pressure sensitivity (4.6 VkPa-1 and 2.5 mAkPa-1) of the device. With the quantum-mechanically calculated electronic structure, the device converts biomechanical energy to electrical energy and generates an open-circuit output voltage of 260 V, short-circuit output current of 160 mA/m2, and excellent power output of 6.65 W/m2, which is sufficient to operate several consumer electronics. Owing to its superior pressure sensitivity and efficiency, the device enables a broad range of applications including real-time clinical human vital-sign monitoring, acoustic sensing, and multidimensional gesture-sensing functionality of a robotic hand. Considering the ease of fabrication, excellent functionality of the hierarchical polymer nanocomposite, and outstanding energy-harvesting performance of nanogenerators, this work is expected to stimulate the development of next-generation self-powered technology.

Dynamic and Reprocessable Fluorinated Poly(hindered urea) Network Materials Containing Ionic Liquids to Enhance Triboelectric Performance.

Triboelectric nanogenerators (TENGs), a newly developed energy harvesting device that converts surrounding environmental mechanical stimuli into electricity, have been significantly explored as an ideal long-term power source for electrical devices. Despite recent advances, the development of advanced TENG devices with sufficient outputs to sustainably power electronic devices and rapid self-healability under mild conditions to improve their lifetime and function is highly demanded. Here, we report a robust self-healable and reprocessable TENG fabricated with a covalent adaptive network based on mechanically strong fluorinated poly(hindered urea) (F-PHU) integrated with ionic liquid as an efficient dielectric material to improve its triboelectric efficiency and self-healing capability simultaneously. The synthesis and integration of a well-defined reactive copolymer having both pendant fluorinated and t-butylamino bulky groups are the key to fabricate robust F-PHU networks containing fluorinated dangling chains that can interact with ionic liquids to induce ionic polarization, which raises the dielectric constant and thus increases triboelectric performance. They also are cross-linked with dynamic bulky urea linkages for rapid self-healability and high reprocessability through their reversible exchange reactions at moderate temperatures. The developed ionic F-PHU materials exhibit a high TENG output performance (power density of 173.0 mW/m2) as well as high TENG output recovery upon repairing their surface damages. This work demonstrates that such a synergistic design of triboelectric ionic F-PHU materials could have great potential for applications requiring high-performance and long-lasting energy harvesting.

Interdigitated Three-Dimensional Heterogeneous Nanocomposites for High-Performance Mechanochromic Smart Membranes.

Mechanochromic smart membranes capable of optical modulation have great potential in smart windows, artificial skins, and camouflage. However, the realization of high-contrast optical modulation based on light scattering activated at a low strain remains challenging. Here, we present a strategy for designing mechanochromic scattering membranes by introducing a Young's modulus mismatch between the two interdigitated polydimethylsiloxane phases with weak interfaces in a periodic three-dimensional (3D) structure. The refractive index-matched interfaces of the nanocomposite provide a high optical transparency of 93%. Experimental and computational studies reveal that the 3D heterogeneity facilitates the generation of numerous nanoscale debonds or "nanogaps" at the modulus-mismatching interfaces, enabling incident light scattering under tension. The heterogeneous scatterer delivers both a high transmittance contrast of >50% achieved at 15% strain and a maximum contrast of 82%. When used as a smart window, the membrane demonstrates effective diffusion of transmitting sunlight, leading to moderate indoor illumination by eliminating extremely bright or dark spots. At the other extreme, such a 3D heterogeneous design with strongly bonded interfaces can enhance the coloration sensitivity of mechanophore-dyed nanocomposites. This work presents insights into the design principles of advanced mechanochromic smart membranes.

High-Performance Triboelectric Devices via Dielectric Polarization: A Review.

Energy harvesting devices based on the triboelectric effect have attracted great attention because of their higher output performance compared to other nanogenerators, which have been utilized in various wearable applications. Based on the working mechanism, the triboelectric performance is mainly proportional to the surface charge density of the triboelectric materials. Various approaches, such as modification of the surface functional group and dielectric composition of the triboelectric materials, have been employed to enhance the surface charge density, leading to improvements in triboelectric performances. Notably, tuning the dielectric properties of triboelectric materials can significantly increase the surface charge density because the surface charge is proportional to the relative permittivity of the triboelectric material. The relative dielectric constant is modified by dielectric polarization, such as electronic, vibrational (or atomic), orientation (or dipolar), ionic, and interfacial polarization. Therefore, such polarization represents a critical factor toward improving the dielectric constant and consequent triboelectric performance. In this review, we summarize the recent insights on the improvement of triboelectric performance via enhanced dielectric polarization.

A Fully Biodegradable Ferroelectric Skin Sensor from Edible Porcine Skin Gelatine.

High-performance biodegradable electronic devices are being investigated to address the global electronic waste problem. In this work, a fully biodegradable ferroelectric nanogenerator-driven skin sensor with ultrasensitive bimodal sensing capability based on edible porcine skin gelatine is demonstrated. The microstructure and molecular engineering of gelatine induces polarization confinement that gives rise the ferroelectric properties, resulting in a piezoelectric coefficient (d33) of ≈24 pC N-1 and pyroelectric coefficient of ≈13 µC m-2K-1, which are 6 and 11.8 times higher, respectively, than those of the conventional planar gelatine. The ferroelectric gelatine skin sensor has exceptionally high pressure sensitivity (≈41 mV Pa-1) and the lowest detection limit of pressure (≈0.005 Pa) and temperature (≈0.04 K) ever reported for ferroelectric sensors. In proof-of-concept tests, this device is able to sense the spatially resolved pressure, temperature, and surface texture of an unknown object, demonstrating potential for robotic skins and wearable electronics with zero waste footprint.

Highly Stretchable Sound-in-Display Electronics Based on Strain-Insensitive Metallic Nanonetworks.

The growing importance of human-machine interfaces and the rapid expansion of the internet of things (IoT) have inspired the integration of displays with sound generation systems to afford stretchable sound-in-display devices and thus establish human-to-machine connections via auditory system visualization. Herein, the synchronized generation of sound and color is demonstrated for a stretchable sound-in-display device with electrodes of strain-insensitive silver nanowires (AgNWs) and emissive layers of field-induced inorganic electroluminescent (EL) phosphors. In this device, EL phosphors embedded in a dielectric elastomer actuator (DEA) emit light under alternating-current bias, while audible sound waves are simultaneously generated via DEA actuation along with input sound signals. The electroluminescence and sound-generation performances of the fabricated device are highly robust and reliable, being insensitive to stretch-release cycling because of the presence of the AgNW stretchable electrodes. The presented principle of integrating light emission and acoustic systems in a single stretchable device can be further expanded to realize sound-in-display electronics for IoT and human-machine interface applications.

Binary Spiky/Spherical Nanoparticle Films with Hierarchical Micro/Nanostructures for High-Performance Flexible Pressure Sensors.

Flexible pressure sensors have been widely explored for their versatile applications in electronic skins, wearable healthcare monitoring devices, and robotics. However, fabrication of sensors with characteristics such as high sensitivity, linearity, and simple fabrication process remains a challenge. Therefore, we propose herein a highly flexible and sensitive pressure sensor based on a conductive binary spiky/spherical nanoparticle film that can be fabricated by a simple spray-coating method. The sea-urchin-shaped spiky nanoparticles are based on the core-shell structures of spherical silica nanoparticles decorated with conductive polyaniline spiky shells. The simple spray coating of binary spiky/spherical nanoparticles enables the formation of uniform conductive nanoparticle-based films with hierarchical nano/microstructures. The two differently shaped particles-based films (namely sea-urchin-shaped and spherical) when interlocked face-to-face to form a bilayer structure can be used as a highly sensitive piezoresistive pressure sensor. Our optimized pressure sensor exhibits high sensitivity (17.5 kPa-1) and linear responsivity over a wide pressure range (0.008-120 kPa), owing to the effects of stress concentration and gradual deformation of the hierarchical microporous structures with sharp nanoscale tips. Moreover, the sensor exhibits high durability over 6000 repeated cycles and practical applicability in wearable devices that can be used for healthcare monitoring and subtle airflow detection (1 L/min).

Highly Transparent, Flexible, and Self-Healable Thermoacoustic Loudspeakers.

Thermoacoustic (TA) loudspeakers have garnered significant attention in recent times as a novel film speaker that utilizes temperature oscillation to vibrate the surrounding air. Conventional film-type TA loudspeakers are known to experience problems when external environments damage their conductive networks, causing them to malfunction. Therefore, introducing self-healing polymers in TA loudspeakers could be an effective way to restore the surface damage of conductive networks. In this study, we present transparent, flexible, and self-healable TA loudspeakers based on silver nanowire (AgNW)-poly(urethane-hindered urea) (PUHU) conductive electrodes. Our self-healable AgNW/PUHU electrodes exhibit significant self-healing for repairing the surface damages that are caused due to the dynamic reconstruction of reversible bulky urea bonds in PUHU. The fabricated self-healable TA loudspeakers generate a sound pressure level of 61 dB at 10 kHz frequency (alternating current (AC) 7 V/direct current (DC) 1 V). In particular, the TA speakers are able to recover the original sound after healing the surface damages of electrodes at 95 °C and 80% relative humidity within 5 min. We believe that the technique proposed in this study provides a robust and powerful platform for the fabrication of transparent and flexible TA loudspeakers with excellent self-healing, which can be applied in flexible and wearable acoustic electronics.

Self-Healable Reprocessable Triboelectric Nanogenerators Fabricated with Vitrimeric Poly(hindered Urea) Networks.

In recent years, the advent of highly deformable and healable electronics is exciting and promising for next-generation electronic devices. In particular, self-healable triboelectric nanogenerators (SH-TENGs) serve as promising candidates based on the combination of the triboelectric effect, electrostatic induction, and self-healing action. However, the majority of SH-TENGs have been devised with weak polymeric networks that are healed with reversible supramolecular interactions or disulfides, thus resulting in poor mechanical properties and low resistance to creeping. To address this issue, we demonstrate the integration of mechanically strong and self-healable poly(hindered urea) (PHU) network in the fabrication of effective TENGs. The designed PHU network is flexible but shows greater mechanical property of tensile strength as high as 1.7 MPa at break. The network is capable of self-healing quickly and repeatedly as well as being reprocessable under mild conditions, enabling the recovery of triboelectric performances after the complete healing of the damaged surfaces. Furthermore, the interfacial-polarization-induced enhancement of dielectric constant endows our SH-TENG with the highest triboelectric output performance (169.9 V/cm2) among the reported healable TENGs. This work presents an avenue to the development of mechanical energy-harvesting devices and self-powered sensors with excellent stretchability, high recoverability, and good mechanical strength.

Ferroelectric Multilayer Nanocomposites with Polarization and Stress Concentration Structures for Enhanced Triboelectric Performances.

Although ferroelectric composites have been reported to enhance the performance of triboelectric (TE) devices, their performances are still limited owing to randomly dispersed particles. Herein, we introduce high-performance TE sensors (TESs) based on ferroelectric multilayer nanocomposites with alternating poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE) and BaTiO3 (BTO) nanoparticle (NP) layers. The multilayers comprising alternating soft/hard layers can induce stress concentration and increase the effective stress-induced polarization and interfacial polarization between organic and inorganic materials, leading to a dielectric constant (17.06) that is higher than those of pure PVDF-TrFE films (13.9) and single PVDF-TrFE/BTO nanocomposites (15.9) at 10 kHz. As a result, the multilayered TESs with alternating BTO NP layers exhibit TE currents increased by 2.3 and 1.5 times compared to pure PVDF-TrFE without BTO NPs and PVDF-TrFE/BTO nanocomposites without multilayer structures, respectively. The multilayered TESs exhibit a high pressure sensitivity of 0.94 V/kPa (48.7 nA/kPa) and output power density of 29.4 µWcm-2, enabling their application in the fabrication of highly sensitive healthcare monitoring devices and high-performance acoustic sensors. The suggested architecture of ferroelectric multilayer nanocomposites provides a robust platform for TE devices and self-powered wearable electronics.

High-Resolution Filtration Patterning of Silver Nanowire Electrodes for Flexible and Transparent Optoelectronic Devices.

Silver nanowire (AgNW) electrodes attract significant attention in flexible and transparent optoelectronic devices; however, high-resolution patterning of AgNW electrodes remains a considerable challenge. In this study, we have introduced a simple technique for high-resolution solution patterning of AgNW networks, based on simple filtration of AgNW solution on a patterned polyimide shadow mask. This solution process allows the smallest pattern size of AgNW electrodes, down to a width of 3.5 µm. In addition, we have demonstrated the potential of these patterned AgNW electrodes for applications in flexible optoelectronic devices, such as photodetectors. Specifically, for flexible and semitransparent UV photodetectors, AgNW electrodes are embedded in sputtered ZnO films to enhance the photocurrent by light scattering and trapping, which resulted in a significantly enhanced photocurrent (up to 800%) compared to devices based on AgNW electrodes mounted on top of ZnO films. In addition, our photodetector could be operated well under extremely bent conditions (bending radius of approximately 770 µm) and provide excellent durability even after 500 bending cycles.

Gate-Tunable and Programmable n-InGaAs/Black Phosphorus Heterojunction Diodes.

Semiconductor heterostructures have enabled numerous applications in diodes, photodetectors, junction field-effect transistors, and memory devices. Two-dimensional (2D) materials and III-V compound semiconductors are two representative materials providing excellent heterojunction platforms for the fabrication of heterostructure devices. The marriage between these semiconductors with completely different crystal structures may enable a new heterojunction with unprecedented physical properties. In this study, we demonstrate a multifunctional heterostructure device based on 2D black phosphorus and n-InGaAs nanomembrane semiconductors that exhibit gate-tunable, photoresponsive, and programmable diode characteristics. The device exhibits clear rectification with a large gate-tunable forward current, which displays rectification and switching with a maximum rectification ratio of 4600 and an on/off ratio exceeding 105, respectively. The device also offers nonvolatile memory properties, including large hysteresis and stable retention of storage charges. By combining the memory and gate-tunable rectifying properties, the rectification ratio of the device can be controlled and memorized from 0.06 to 400. Moreover, the device can generate three different electrical signals by combining a photoresponsivity of 0.704 A/W with the gate-tunable property, offering potential applications, for example, multiple logic operator. This work presents a heterostructure design based on 2D and III-V compound semiconductors, showing unique physical properties for the development of multifunctional heterostructure devices.