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Respiratory masks are the primary and most effective means of protecting individuals from airborne hazards such as droplets and particulate matter during public engagements. However, conventional electrostatically charged melt-blown microfiber masks typically require thick and dense membranes to achieve high filtration efficiency, which in turn cause a significant pressure drop and reduce breathability. In this study, we have developed a multielectrospinning system to address this issue by manipulating the pore structure of nanofiber networks, including the use of uniaxially aligned nanofibers created via an electric-field-guided electrospinning apparatus. In contrast to the common randomly collected microfiber membranes, partially aligned dual-nanofiber membranes, which are fabricated via electrospinning of a random 150 nm nanofiber base layer and a uniaxially aligned 450 nm nanofiber spacer layer on a roll-to-roll collector, offer an efficient way to modulate nanofiber membrane pore structures. Notably, the dual-nanofiber configuration with submicron pore structure exhibits increased fiber density and decreased volume density, resulting in an enhanced filtration efficiency of over 97% and a 50% reduction in pressure drop. This leads to the highest quality factor of 0.0781. Moreover, the submicron pore structure within the nanofiber networks introduces an additional sieving filtration mechanism, ensuring superior filtration efficiency under highly humid conditions and even after washing with a 70% ethanol solution. The nanofiber mask provides a sustainable solution for safeguarding the human respiratory system, as it effectively filters and inactivates human coronaviruses while utilizing 130 times fewer polymeric materials than melt-blown filters. This reusability of our filters and their minimum usage of polymeric materials would significantly reduce plastic waste for a sustainable global society.

Serious climate changes and energy-related environmental problems are currently critical issues in the world. In order to reduce carbon emissions and save our environment, renewable energy harvesting technologies will serve as a key solution in the near future. Among them, triboelectric nanogenerators (TENGs), which is one of the most promising mechanical energy harvesters by means of contact electrification phenomenon, are explosively developing due to abundant wasting mechanical energy sources and a number of superior advantages in a wide availability and selection of materials, relatively simple device configurations, and low-cost processing. Significant experimental and theoretical efforts have been achieved toward understanding fundamental behaviors and a wide range of demonstrations since its report in 2012. As a result, considerable technological advancement has been exhibited and it advances the timeline of achievement in the proposed roadmap. Now, the technology has reached the stage of prototype development with verification of performance beyond the lab scale environment toward its commercialization. In this review, distinguished authors in the world worked together to summarize the state of the art in theory, materials, devices, systems, circuits, and applications in TENG fields. The great research achievements of researchers in this field around the world over the past decade are expected to play a major role in coming to fruition of unexpectedly accelerated technological advances over the next decade.

The emergence and development of thick electrodes provide an efficient way for the high-energy-density supercapacitor design. Wood is a kind of biomass material with porous hierarchical structure, which has the characteristics of a straight channel, uniform pore structure, good mechanical strength, and easy processing. The wood-inspired low-tortuosity and vertically aligned channel architecture are highly suitable for the construction of thick electrochemical supcapacitor electrodes with high energy densities. This review summarizes the design concepts and processing parameters of thick electrode supercapacitors inspired by natural woods, including wood-based pore structural design regulation, electric double layer capacitances (EDLCs)/pseudocapacitance construction, and electrical conductivity optimization. In addition, the optimization strategies for preparing thick electrodes with wood-like structures (e.g., 3D printing, freeze-drying, and aligned-low tortuosity channels) are also discussed in detail. Further, this review presents current challenges and future trends in the design of thick electrodes for supercapacitors with wood-inspired pore structures. As a guideline, the brilliant blueprint optimization will promote sustainable development of wood-inspired structure design for thick electrodes and broaden the application scopes.

Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

Smart wearable electronic textiles (e-textiles) that can detect and differentiate multiple stimuli, while also collecting and storing the diverse array of data signals using highly innovative, multifunctional, and intelligent garments, are of great value for personalized healthcare applications. However, material performance and sustainability, complicated and difficult e-textile fabrication methods, and their limited end-of-life processability are major challenges to wide adoption of e-textiles. In this review, we explore the potential for sustainable materials, manufacturing techniques, and their end-of-the-life processes for developing eco-friendly e-textiles. In addition, we survey the current state-of-the-art for sustainable fibers and electronic materials (i.e., conductors, semiconductors, and dielectrics) to serve as different components in wearable e-textiles and then provide an overview of environmentally friendly digital manufacturing techniques for such textiles which involve less or no water utilization, combined with a reduction in both material waste and energy consumption. Furthermore, standardized parameters for evaluating the sustainability of e-textiles are established, such as life cycle analysis, biodegradability, and recyclability. Finally, we discuss the current development trends, as well as the future research directions for wearable e-textiles which include an integrated product design approach based on the use of eco-friendly materials, the development of sustainable manufacturing processes, and an effective end-of-the-life strategy to manufacture next generation smart and sustainable wearable e-textiles that can be either recycled to value-added products or decomposed in the landfill without any negative environmental impacts.

The recent emerging significance of the Internet of Things (IoT) demands sensor devices to be integrated with many different functional structures and devices while conserving their original functionalities. To this end, optical transparency and mechanical flexibility of sensor devices are critical requirements for optimal integration as well as high sensitivity. In this work, a transparent, flexible, and sensitive gas sensor building platform is introduced by using multilevel self-assembly of block copolymers (BCPs) and polystyrene (PS) colloids. For the demonstration of an H2gas sensor, a hierarchically porous Pd metal mesh structure is obtained by overlaying the two different patterned template structures with synergistic, distinctive characteristic length scales. The hierarchical Pd mesh shows not only high transparency over 90% but also superior sensing performance in terms of response and recovery time owing to enhanced Pd-to-hydride ratio and short H2diffusion lengths from the enlarged active surface areas. The hierarchical morphology also endows high mechanical flexibility while securing reliable sensing performance even under severe mechanical deformation cycles. Our scalable self-assembly based multiscale nanopatterning offers an intriguing generalized platform for many different multifunctional devices requiring hidden in situmonitoring of environmental signals.

The massive production of polymer-based respiratory masks during the COVID-19 pandemic has rekindled the issue of environmental pollution from nonrecyclable plastic waste. To mitigate this problem, conventional filters should be redesigned with improved filtration performance over the entire operational life while also being naturally degradable at the end. Herein, we developed a functional and biodegradable polymeric filter membrane consisting of a polybutylene adipate terephthalate (PBAT) matrix blended with cetyltrimethylammonium bromide (CTAB) and montmorillonite (MMT) clay, whose surface properties have been modified through cation exchange reactions for good miscibility with PBAT in an organic solvent. Particularly, the spontaneous evolution of a partial core–shell structure (i.e., PBAT core encased by CTAB-MMT shell) during the electrospinning process amplified the triboelectric effect as well as the antibacterial/antiviral activity that was not observed in naive PBAT. Unlike the conventional face mask filter that relies on the electrostatic adsorption mechanism, which deteriorates over time and/or due to external environmental factors, the PBAT@CTAB-MMT nanofiber membrane (NFM)-based filter continuously retains electrostatic charges on the surface due to the triboelectric effect of CTAB-MMT. As a result, the PBAT@CTAB-MMT NFM-based filter showed high filtration efficiencies (98.3%, PM0.3) even at a low differential pressure of 40 Pa or less over its lifetime. Altogether, we not only propose an effective and practical solution to improve the performance of filter membranes while minimizing their environmental footprint but also provide valuable insight into the synergetic functionalities of organic–inorganic hybrid materials for applications beyond filter membranes.

The process of exsolution for the synthesis of strongly anchored metal nanoparticles (NPs) on host oxide lattices has been proposed as a promising strategy for designing robust catalyst-support composite systems. However, because conventional exsolution processes occur in harsh reducing environments at high temperatures for long periods of time, the choice of support materials and dopant metals are limited to those with inherently high thermal and chemical stability. Herein, we report the exsolution of a series of noble metal catalysts (Pt, Rh, and Ir) from metal oxide nanofibers (WO3NFs) supports in an entirely ambient environment induced by intense pulsed light (IPL)-derived momentary photothermal treatment (>1000 °C). Since the exsolution process spans an extremely short period of time (<20 ms), unwanted structural artifacts such as decreased surface area and phase transition of the support materials are effectively suppressed. At the same time, exsolved NPs (<5 nm) with uniform size distributions could successfully be formed. To prove the practical utility of exsolved catalytic NPs functionalized on WO3NFs, the chemiresistive gas sensing characteristics of exsolved Pt-decorated WO3NFs were analyzed, exhibiting high durability (>200 cyclic exposures), enhanced response (Rair/Rgas> 800 @ 1 ppm/350 °C), and selectivity toward H2S target gas. Altogether, we successfully demonstrated that ultrafast exsolution within a few milliseconds could be induced in ambient conditions using the IPL-derived momentary photothermal treatment and contributed to expanding the practical viability of the exsolution-based synthetic approaches for the production of highly stable catalyst systems.

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Oxygen-based electrocatalysis is an integral aspect of a clean and sustainable energy conversion/storage system. The development of economic bifunctional electrocatalysts with high activity and durability during reversible reactions remains a great challenge. The tailored porous structure and separately presented active sites for oxygen reduction and oxygen evolution reactions (ORR and OER) without mutual interference are most crucial for achieving desired bifunctional catalysts. Here, we report a hybrid composed of sheath–core cobalt oxynitride (CoOx@CoNy) nanorods grown perpendicularly on N-doped carbon nanofiber (NCNF). The brush-like CoOx@CoNynanorods, composed of metallic Co4N cores and oxidized surfaces, exhibit excellent OER activity (E= 1.69 V at 10 mA cm–2) in an alkaline medium. Although pristine NCNF or CoOx@CoNyalone had poor catalytic activity in the ORR, the hybrid showed dramatically enhanced ORR performance (E= 0.78 V at −3 mA cm–2). The experimental results coupled with a density functional theory (DFT) simulation confirmed that the broad surface area of the CoOx@CoNynanorods with an oxidized skin layer boosts the catalytic OER, while the facile adsorption of ORR intermediates and a rapid interfacial charge transfer occur at the interface between the CoOx@CoNynanorods and the electrically conductive NCNF. Furthermore, it was found that the independent catalytic active sites in the CoOx@CoNy/NCNF catalyst are continuously regenerated and sustained without mutual interference during the round-trip ORR/OER, affording stable operation of Zn–air batteries.

Conductive metal–organic frameworks (cMOFs) offer high porosity and electrical conductivity simultaneously, making them ideal for application in chemiresistive sensors. Recently, incorporating foreign elements such as catalytic nanoparticles into cMOFs has become a typical strategy to enhance their sensing properties. However, this approach has led to critical challenges, such as pore blockage that impedes gas diffusion, as well as limited improvement in reversibility. Herein, single-atom catalyst (SAC)-functionalized cMOF is presented as a robust solution to the current limitations. Facile functionalization of SACs in a cMOF can be achieved through electrochemical deposition of metal precursors. As a proof of concept, a Pd SAC-functionalized cMOF is synthesized. The Pd SACs are stabilized at the interplanar sites of cMOF with Pd–N4coordination while preserving the porosity of the MOF matrix. Notably, the microenvironment created by Pd SACs prevents irreversible structural distortion of cMOFs and facilitates a reversible charge transfer with NO2. Consequently, the cMOF exhibits a fully recoverable NO2response, which was not previously attainable with the nanoparticle functionalization. Additionally, with the combination of preserved porosity for gas diffusion, it demonstrates the fastest level of response and recovery speed compared to other 2D-cMOFs of this class.

Chemiresistive gas sensors based on semiconducting metal oxides typically rely on noble metal catalysts to enhance their sensitivity and selectivity. However, noble metal catalysts have several drawbacks for practical utilization, including their high cost, their propensity for spontaneous agglomeration, and poisoning effects with certain types of gases. As such, in the interest of commercializing the chemiresistive gas sensor technology, we propose an alternative design for a noble-metal-free sensing material through the case study of Co-doped ceria (Co–CeO2) catalysts embedded in a SnO2matrix. In this investigation, we utilized electrospinning and subsequent calcination to prepare Co–CeO2catalyst nanoparticles integrated with SnO2nanofibers (NFs) with uniform particle distribution and particle size regulation down to the sub-2 nm regime. The resulting Co–CeO2@SnO2NFs exhibited superior gas sensing characteristics toward isoprene (C5H8) gas, a significant biomarker for monitoring the onset of various diseases through breath diagnostics. In particular, we identified that the Co–CeO2catalysts, owing to the transition metal doping, facilitated the spillover of chemisorbed oxygen species to the SnO2sensing body. This resulting in the sensor having a 27.4-fold higher response toward 5 ppm of C5H8(compared to pristine SnO2), exceptionally high selectivity, and a low detection limit of 100 ppb. The sensor also exhibited high stability for prolonged response–recovery cycles, attesting to the strong anchoring of Co–CeO2catalysts in the SnO2matrix. Based on our findings, the transition metal-doped metal oxide catalysts, such as Co–CeO2, demonstrate strong potential to completely replace noble metal catalysts, thereby advancing the development of the commercially viable chemiresistive gas sensors free from noble metals, capable of detecting target gases at sub-ppm levels.

The chemical conversion of CO2into value-added products is the key technology to realize a carbon-neutral society. One representative example of such conversion is the reverse water–gas shift reaction, which produces CO from CO2. However, the activity is insufficient at ambient pressure and lower temperatures (<600 °C), making it a highly energy-intensive and impractical process. Herein, we report indium oxide nanofibers modified with palladium catalysts that exhibit significantly potent redox activities toward the reduction of CO2splitting via chemical looping. In particular, we uncover that the doped palladium cations are selectively reduced and precipitated onto the host oxide surface as metallic nanoparticles. These catalytic gems formed operando make In2O3lattice oxygen more redox-active in H2and CO2environments. As a result, the composite nanofiber catalysts demonstrate the reverse water–gas shift reaction via chemical looping at record-low temperatures (≤350 °C), while also imparting high activities (CO2conversion: 45%). Altogether, our findings expand the viability of CO2splitting at lower temperatures and provide design principles for indium oxide-based catalysts for CO2conversion.

Lithium–oxygen batteries have been considered as one of the most viable energy source options for electric vehicles due to their high energy density. However, they are still faced with technical challenges, such as low round-trip efficiency and short cycle life, which mainly originate from the cathode part of the battery. In this work, we designed a three-dimensional nanofibrous air electrode consisted of hierarchically structured carbon nanotube-bridged hollow Fe2O3nanoparticles (H-Fe2O3/CNT NFs). Composite nanofibers consisted of hollow Fe2O3NPs anchored by multiple CNTs offered enhanced catalytic sites (interconnected hollow Fe2O3NPs) and fast charge-transport highway (bridged CNTs) for facile formation and decomposition of Li2O2, leading to outstanding cell performance: (1) Swagelok cell exhibited highly reversible cycling characteristics for 250 cycles with a fixed capacity of 1000 mAh g–1at a current density of 500 mA g–1. (2) A module composed of two pouch-type cells stably powered an light-emitting diode lamp operated at 5.0 V.

In accordance with the fourth industrial revolution (4IR), thin-film all-solid-state batteries (TF-ASSBs) are being revived as the most promising energy source to power small electronic devices. However, current TF-ASSBs still suffer from the perpetual necessity of high-performance battery components. While every component, a series of a TF solid electrolyte (i.e., lithium phosphorus oxynitride (LiPON)) and electrodes (cathode and Li metal anode), has been considered vital, the lack of understanding of and ability to ameliorate the cathode (or anode)–electrolyte interface (CEI) (or AEI) has impeded the development of TF-ASSBs. In this work, we suggest an ensemble design of TF-ASSBs using LiPON (500 nm), an amorphous TF-V2O5–xcathode with oxygen vacancies (Ovacancy), a thin evaporated Li anode (evp-Li) with a thickness of 1 μm, and an artificial ultrathin Al2O3layer between evp-Li and LiPON. Well-defined Ovacancysites, such as O(II)vacancyand O(III)vacancy, in amorphous TF-V2O5–xnot only allow isotropic Li+diffusion at the CEI but also enhance both the ionic and electronic conductivities. For the AEI, we employed protective Al2O3, which was specially sputtered using the facing target sputtering (FTS) method to form a homogeneous layer without damage from plasma. In regard to the contact with evp-Li, interfacial stability, electrochemical impedance, and battery performance, the nanometric Al2O3layers (1 nm) were optimized at different temperatures (40, 60, and 80 °C). The TF-ASSB cell containing Al2O3(1 nm) delivers a high specific capacity of 474.01 mAh cm–3under 60 °C at 2 C for the 400th cycle, and it achieves a long lifespan as well as ultrafast rate capability levels, even at 100 C; these results were comparable to those of TF Li-ion battery cells using a liquid electrolyte. We demonstrated the reaction mechanism at the AEI utilizing time-of-flight secondary ion mass spectrometry (TOF-SIMS) and molecular dynamics (MD) simulations for a better understanding. Our design provides a signpost for future research on the rational structure of TF-LIBs.

The development of highly efficient catalysts in the cathodes of rechargeable Li–O2batteries is a considerable challenge. Polyelemental catalysts consisting of two or more kinds of hybridized catalysts are particularly interesting because the combination of the electrochemical properties of each catalyst component can significantly facilitate oxygen evolution and oxygen reduction reactions. Despite the recent advances that have been made in this field, the number of elements in the catalysts has been largely limited to two metals. In this study, we demonstrate the electrochemical behavior of Li–O2batteries containing a wide range of catalytic element combinations. Fourteen different combinations with single, binary, ternary, and quaternary combinations of Pt, Pd, Au, and Ru were prepared on carbon nanofibers (CNFs) via a joule heating route. Importantly, the Li–O2battery performance could be significantly improved when using a polyelemental catalyst with four elements. The cathode containing quaternary nanoparticles (Pt–Pd–Au–Ru) exhibited a reduced overpotential (0.45 V) and a high discharge capacity based on total cathode weight at 9130 mAh g–1, which was ∼3 times higher than that of the pristine CNF electrode. This superior electrochemical performance is be attributed to an increased catalytic activity associated with an enhanced O2adsorbability by the quaternary nanoparticles.

Supported metal catalysts represent one of the major milestones in heterogeneous catalysis. Such catalytic systems are feasible for use in a broad range of applications, including renewable energy devices, sensors, automotive emission control systems, and chemical reformers. The lifetimes of these catalytic platforms depend strongly on the stability of the supported nanoparticles. With this regard, nanoparticles synthesized viaex-solution process emphasize exceptional robustness as they are socketed in the host oxide. Ex-solution refers to a phenomenon which yields selective growth of fine and uniformly distributed metal nanocatalysts on oxide supports upon partial reduction. This type of advanced structural engineering is a game-changer in the field of heterogeneous catalysis with numerous studies showing the benefits of ex-solution process. In this review, we highlight the latest research efforts regarding the origin of the ex-solution phenomenon and the mechanism underpinning particle formation. We also propose research directions to expand the utility and functionality of the current ex-solution techniques.

The fusion of different electroactive components of lithium-ion batteries (LIBs) sometimes brings exceptional electrochemical properties. We herein report the reduced graphene-oxide (rGO)-coated Zn2SnO4z@NiO nanofibers (ZSO@NiO@G NFs) formed by the synergistic fusion of three different electroactive components including ZnO, SnO2, and NiO that exhibit exceptional electrochemical properties as negative electrodes for LIBs. The simple synthetic route comprised of electrospinning and calcination processes enables to form porous one-dimensional (1D) structured ZSO, which is the atomic combination between ZnO and SnO2, exhibiting effective strain relaxation during battery operation. Furthermore, the catalytic effect of Ni converted from the surface-functional NiO nanolayer on ZSO significantly contributes to improved reversible capacity. Finally, rGO sheets formed on the surface of ZSO@NiO NFs enable to construct electrically conductive path as well as a stable SEI layer, resulting in excellent electrochemical performances. Especially, exceptional cycle lifespan of more than 1600 cycles with a high capacity (1060 mAh g–1) at a high current density (1000 mA g–1), which is the best result among mixed transition metal oxide (stannates, molybdates, cobaltates, ferrites, and manganates) negative electrodes for LIBs, is demonstrated.

The colorimetric gas sensor offers an opportunity for the simple and rapid detection of toxic gaseous substances based on visually discernible changes in the color of the sensing material. In particular, the accurate detection of trace amounts of certain biomarkers in a patient’s breath provides substantial clues regarding specific diseases, for example, hydrogen sulfide (H2S) for halitosis and ammonia (NH3) for kidney disorder. However, conventional colorimetric sensors often lack the sensitivity, selectivity, detection limit, and mass-productivity, impeding their commercialization. Herein, we report an inexpensive route for the meter-scale synthesis of a colorimetric sensor based on a composite nanofiber yarn that is chemically functionalized with an ionic liquid as an effective H2S adsorbent and lead acetate as a colorimetric dye. As an eye-readable and weavable sensing platform, the single-strand yarn exhibits enhanced sensitivity supported by its high surface area and well-developed porosity to detect the breath biomarker (1 ppm of H2S). Alternatively, the yarn loaded with lead iodide dyes could reversibly detect NH3gas molecules in the ppm-level, demonstrating the facile extensibility. Finally, we demonstrated that the freestanding yarns could be sewn into patterned textiles for the fabrication of a wearable toxic gas alarm system with a visual output.

It is evident that the exhaustive use of fossil fuels for decades has significantly contributed to global warming and environmental pollution. To mitigate the harm on the environment, lithium–oxygen batteries (LOBs) with a high theoretical energy density (3458 Wh kg–1Li2O2) compared to that of Li-ion batteries (LIBs) have been considered as an attractive alternative to fossil fuels. For this purpose, porous carbon materials have been utilized as promising air cathodes owing to their low cost, lightness, easy fabrication process, and high performance. However, the challenge thus far lies in the uncontrollable formation of Li2CO3at the interface between carbon and Li2O2, which is detrimental to the stable electrochemical performance of carbon-based cathodes in LOBs. In this work, we successfully protected the surface of the free-standing carbon nanofibers (CNFs) by coating it with a layer of iridium metal through direct sputtering (CNFs@Ir), which significantly improved the lifespan of LOBs. Moreover, the Ir would play a secondary role as an electrochemical catalyst. This all-in-one cathode was evaluated for the formation and decomposition of Li2O2during (dis)charging processes. Compared with bare CNFs, the CNFs@Ir cathode showed two times longer lifespan with 0.2 VLilower overpotentials for the oxygen evolution reaction. We quantitatively calculated the contents of CO32–in Li2CO3formed on the different surfaces of the bare CNFs (63% reduced) and the protected CNFs@Ir (78% reduced) cathodes after charging. The protective effects and the reaction mechanism were elucidated by ex situ analyses, including scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy.

Lithium–oxygen (Li–O2) batteries have been intensively investigated in recent decades for their utilization in electric vehicles. The intrinsic challenges arising from O2(electro)chemistry have been mitigated by developing various types of catalysts, porous electrode materials, and stable electrolyte solutions. At the next stage, we face the need to reform batteries by substituting pure O2gas with air from Earth’s atmosphere. Thus, the key emerging challenges of Li–air batteries, which are related to the selective filtration of O2gas from air and the suppression of undesired reactions with other constituents in air, such as N2, water vapor (H2O), and carbon dioxide (CO2), should be properly addressed. In this review, we discuss all key aspects for developing Li–air batteries that are optimized for operating in ambient air and highlight the crucial considerations and perspectives for future air-breathing batteries.

Hydrogen (H2) is one of the next-generation energy sources because it is abundant in nature and has a high combustion efficiency that produces environmentally benign products (H2O). However, H2/air mixtures are explosive at H2concentrations above 4%, thus any leakage of H2must be rapidly and reliably detected at much lower concentrations to ensure safety. Among the various types of H2sensors, chemiresistive sensors are one of the most promising sensing systems due to their simplicity and low cost. This review highlights the advances in H2chemiresistors, including metal-, semiconducting metal oxide-, carbon-based materials, and other materials. The underlying sensing mechanisms for different types of materials are discussed, and the correlation of sensing performances with nanostructures, surface chemistry, and electronic properties is presented. In addition, the discussion of each material emphasizes key advances and strategies to develop superior H2sensors. Furthermore, recent key advances in other types of H2sensors are briefly discussed. Finally, the review concludes with a brief outlook, perspective, and future directions.

Personal protective equipment (PPE) is critical to protect healthcare workers (HCWs) from highly infectious diseases such as COVID-19. However, hospitals have been at risk of running out of the safe and effective PPE including personal protective clothing needed to treat patients with COVID-19, due to unprecedented global demand. In addition, there are only limited manufacturing facilities of such clothing available worldwide, due to a lack of available knowledge about relevant technologies, ineffective supply chains, and stringent regulatory requirements. Therefore, there remains a clear unmet need for coordinating the actions and efforts from scientists, engineers, manufacturers, suppliers, and regulatory bodies to develop and produce safe and effective protective clothing using the technologies that are locally available around the world. In this review, we discuss currently used PPE, their quality, and the associated regulatory standards. We survey the current state-of-the-art antimicrobial functional finishes on fabrics to protect the wearer against viruses and bacteria and provide an overview of protective medical fabric manufacturing techniques, their supply chains, and the environmental impacts of current single-use synthetic fiber-based protective clothing. Finally, we discuss future research directions, which include increasing efficiency, safety, and availability of personal protective clothing worldwide without conferring environmental problems.

Catalysis with single-atom catalysts (SACs) exhibits outstanding reactivity and selectivity. However, fabrication of supports for the single atoms with structural versatility remains a challenge to be overcome, for further steps toward catalytic activity augmentation. Here, we demonstrate an effective synthetic approach for a Pt SAC stabilized on a controllable one-dimensional (1D) metal oxide nano-heterostructure support, by trapping the single atoms at heterojunctions of a carbon nitride/SnO2heterostructure. With the ultrahigh specific surface area (54.29 m2g–1) of the nanostructure, we obtained maximized catalytic active sites, as well as further catalytic enhancement achieved with the heterojunction between carbon nitride and SnO2. X-ray absorption fine structure analysis and HAADF-STEM analysis reveal a homogeneous atomic dispersion of Pt species between carbon nitride and SnO2nanograins. This Pt SAC system with the 1D nano-heterostructure support exhibits high sensitivity and selectivity toward detection of formaldehyde gas among state-of-the-art gas sensors. Further ex situTEM analysis confirms excellent thermal stability and sinter resistance of the heterojunction-immobilized Pt single atoms.

The cost-effective direct writing of polymer nanofibers (NFs) has garnered considerable research attention as a compelling one-pot strategy for obtaining key building blocks of electrochemical and optical devices. Among the promising applications, the changes in optical response from external stimuli such as mechanical deformation and changes in the thermal environment are of great significance for emerging applications in smart windows, privacy protection, aesthetics, artificial skin, and camouflage. Herein, we propose a rational design for the mass production of customized NFs through the development of focused electric-field polymer writing (FEPW) coupled with the roll-to-roll technique. As a proof of key applications, we demonstrate multistimuli-responsive (mechano- and thermochromism) membranes with an exceptional production scale (over 300 cm2). Specifically, the membranes consist of periodically aligned ultrathin (∼60 nm) alumina nanotubes inserted in the elastomers. We performed a two-phase finite element analysis of the unit cells to verify the underlying physics of light scattering at heterogeneous interfaces of the strain-induced air gaps. By adding thermochromic dye during the FEPW, the optical modulation of transmittance change (∼83% to 37% at visible wavelength) was successfully extended to high-contrast thermal-dependent coloration.

For rapid hydrogen gas (H2) sensing, we propose the facile synthesis of the hollow structure of Pt-decorated molybdenum disulfide (h-MoS2/Pt) using ultrathin (mono- or few-layer) two-dimensional nanosheets. The controlled amphiphilic nature of MoS2surface produces ultrathin MoS2NS-covered polystyrene particles viaone-step Pickering emulsification. The incorporation of Pt nanoparticles (NPs) on the MoS2, followed by pyrolysis, generates the highly porous h-MoS2/Pt. This hollow hybrid structure produces sufficiently permeable pathways for H2and maximizes the active sites of MoS2, while the Pt NPs on the hollow MoS2induce catalytic H2spillover during H2sensing. The h-MoS2/Pt-based chemiresistors show sensitive H2sensing performances with fast sensing speed (response, 8.1 s for 1% of H2and 2.7 s for 4%; and recovery, 16.0 s for both 1% and 4% H2at room temperature in the air). These results mark the highest H2sensing speed among 2D material-based H2sensors operated at room temperature in air. Our fabrication method of h-MoS2/Pt structure through Pickering emulsion provides a versatile platform applicable to various 2D material-based hollow structures and facilitates their use in other applications involving surface reactions.

Polyelemental nanoparticles (PE NPs) containing four or more elements in a single NP have intriguing intrinsic properties compared to their single-element counterparts. The fusion of diverse elements induces synergistic effects including new physical and chemical phenomena. However, conventional methods have not offered effective strategies for the uniform creation of PE NPs with high reproducibility. Recently, with advances in nanoscience, several new methods have been developed using both thermodynamic and kinetic approaches and, often, the interplay between them. In this Perspective, we highlight recent key advances in the design of PE NPs and their underlying formation mechanisms. We discuss the potential applications of PE NPs and the outlook and future directions for this field.

A relatively low compressive strength significantly limits the practical application of sponges made from electrospun fibers because of an ultralow density <10 mg/cm3. To solve this problem, fibrous polyimide sponges with high density (HDPISG) were prepared using a “self-gluing” concept. The HDPISG have a density of up to 280 mg/cm3and porosity >80%, and showed good breathability. The compressive strength increased significantly as the sponge densities increased. The HDPISG with a density of 280 mg/cm3has the highest compressive strength of 5190 and 35,900 kPa under 50 and 80% compression, respectively. The small HDPISG can even hold weights more than ten thousand times of the weight of the sponge. The HDPISG also possess excellent mechanical properties after thermal treatments and no loss of compressive strength can be seen after heating at 300 °C for 30 h. Further study indicates that the HDPISG can maintain their main shape after carbonization.

Sponges based on short electrospun fibers have received significant attention due to their ultrahigh porosity, lightweight, and multifunctional characteristics. In particular, polyimide (PI) sponges have been researched due to their exceptional mechanical properties and thermal stability. Nevertheless, a number of sponges, including PI, are usually hydrophobic and synthesized in toxic, nonwater solvents (e.g., 1,4-dioxane). Conversely, hydrophilic sponges disintegrate upon contact with water. Here, we suggest a new strategy to fabricate PI sponges in water by introducing a suitable surfactant, sodium dodecylbenzenesulfonate (SDBS) (sPI sponges). With less than 1 wt % of SDBS with respect to PI short fibers, they can be homogeneously dispersed in water and mixed well with poly(amic acid) (PAA) solution. The synthesized sponge, depending on the concentration of SDBS, showed hydrophilic properties and substantial water uptake above 5000%. The hydrophilic properties of the sponges, which are not common, and the preparation from aqueous solution introduce new research opportunities. Such hydrophilic sponges are particularly special because they do not swell in contact with water, which makes them dimensionally stable. The methods presented here can serve as a milestone for the future development of various kinds of hydrophilic sponges applied for various applications, ranging from tissue engineering to oil/water separation.

Facile fabrication of multifunctional porous inorganic aerogels remains an outstanding challenge despite the considerable demand for extensive applications. Here, we present the production of a multifunctional porous inorganic nanomaterial aerogel by controllable surface chemistry of a functionalized SWNT (fSWNT) hydrogel platform for the first time. The versatile functional inorganic nanoparticles can be incorporated uniformly on the porous 3D fSWNT hydrogel platform through a facile dip coating method at ambient conditions. The morphology of the multifunctional inorganic aerogel is manipulated by designing the fSWNT hydrogel platform for different requirements of applications. In particular, Pt-SnO2@fSWNT aerogels exhibit high porosity and uniformly distributed ultrafine Pt and SnO2on the fSWNT platform with controllable particle size (1.5–3.5 nm), which result in significantly high surface area (393 m2g–1). The ultrafine Pt-SnO2@fSWNT aerogels exhibit highly sensitive (14.77% at 5 ppm) and selective NO2sensing performance even at room temperature due to the increased active surface area and controllable porous structure of the ultrafine aerogel, which can provide fast transport and penetration of a target gas into the sensing layers. The newly designed multifunctional inorganic aerogel with ultrahigh surface area and high open porosity is a prospective materials platform of high performance gas sensors, which could be also broadly expanded to widespread applications including catalysis and energy storages.

Conventional graphene oxide (GO)-based gas membranes, having a narrow pore-size range of less than 0.3 nm, exhibit limited gas molecular permeability because of the kinetic diameters of most volatile organic and sulfur compound (VOCs/VSCs) molecules being larger than 0.3 nm. Here, we employ GO nanosheets (NSs) with a tunable pore-size distribution as a molecular sieving layer on two-dimensional (2D) metal oxide NSs-based gas sensors, i.e., PdO-sensitized WO3NSs to boost selectivity toward specific gas species. The pore size, surface area, and pore density of GO NSs were simply manipulated by controlling H2O2concentration. In addition, the pore size-tuned GO NSs were coated on cellulose filtering paper as a free-standing nanoporous membrane. Holey GO membrane showed a highly selective H2S permeability characteristic, exhibiting superior cross-selectivity to CH3COCH3(0.46 nm), C2H5OH (0.45 nm), and C7H8(0.59 nm) with larger kinetic diameters compared with H2S (0.36 nm). Such pore-size-tuned GO nanoporous layer is scalable and robust, highlighting a great promise for designing low cost and highly efficient gas-permeable membrane for outstanding selective gas sensing platform.

Perovskite-type oxides with general stoichiometry ABO3(A is a lanthanide or alkali earth metal, and B is transition metal) constitute a rich material playground for application as resistive-type gas-sensing layers. Immense interest is triggered by, among other factors, stability of abundant elements (˜?90% in the periodic table) in this stoichiometry, relatively easy tunability of structure and chemical composition, and their off-stoichiometry stability upon doping. Moreover, their capability to host cationic and abundant oxygen vacancies renders them with excellent electrical and redox properties, and synergistic functions that influence their performance. Herein, we present an overview of recent development in the use of ABO3perovskites as resistive-type gas sensors, clearly elucidating current experimental strategies, and sensing mechanisms involved in realization of enhanced sensing performance. Finally, we provide a brief overview of limitations that hamper their potential utilization in gas sensors and suggest new pathways for novel applications of ABO3materials.

Recently, gallium (Ga), one of the liquid metals (LMs), has been explored with special attention because of its liquid phase nature as a self-healing agent and Li storage characteristics. The current challenge that restricts the practical use of Ga is handling Ga easily without loss and understanding its reaction behavior in Li-ion batteries. One solution that helps to address the problem associated with liquid phases is to make solid phases such as gallium oxides and nitrides as starting materials for a stable conversion reaction. Here, we have successfully incorporated GaN nanoparticles into carbon confiners [1D carbon nanofibers (CNFs) with the outermost carbon coating layer] as an anode for the Li-ion battery. By preserving liquid Ga derived from GaN after the conversion reaction in conductive walls, long-term cycling performance (over 5000 cycles) is achievable. This work provides an insight into the LM-relevant materials/carbon composite in the area of the rechargeable battery.

Transpiration is the process by which water is carried in plants from the roots to the leaves where evaporation takes place. Here, we report a transpiration driven electrokinetic power generator (TEPG) that exploits capillary flow of water in an asymmetrically wetted cotton fabric coated with carbon black. Accumulation of protons induced by the electrical double layer formed at the solid (carbon black)/liquid (water) interface gives rise to potential difference between the wet and dry sides. The conductive carbon black coating channels electrical current driven by the pseudostreaming mechanism. A TEPG of 90 mm × 30 mm × 0.12 mm yields a maximum voltage of 0.53 V, maximum current of 3.91 μA, and maximum energy density of 1.14 mWh cm–3, depending on the loading of the carbon black. Multiple TEPGs generate enough power to light up a light-emitting diode (20 mA × 2.2 V) or charge a 1 F supercapacitor.

Highlights:

Flash-induced porous graphene (FPG) was synthesized via a broad-spectrum flash lamp that induced synergistic photo-effects between ultraviolet and visible-near-infrared wavelengths, resulting in large-area synthesis in just a few milliseconds.

A hollow pillar graphene with low sheet resistance of 18 Ω sq−1was produced, exhibiting low density (0.0354 g cm−3) and outstanding absolute electromagnetic interference shielding effectiveness of 1.12 × 105dB cm2g−1.

A lightweight, flexible, and high-throughput FPG is applied for electromagnetic interference shielding of a drone radar system and the human body. Abstract: Porous 2D materials with high conductivity and large surface area have been proposed for potential electromagnetic interference (EMI) shielding materials in future mobility and wearable applications to prevent signal noise, transmission inaccuracy, system malfunction, and health hazards. Here, we report on the synthesis of lightweight and flexible flash-induced porous graphene (FPG) with excellent EMI shielding performance. The broad spectrum of pulsed flashlight induces photo-chemical and photo-thermal reactions in polyimide films, forming 5 × 10 cm2-size porous graphene with a hollow pillar structure in a few milliseconds. The resulting material demonstrated low density (0.0354 g cm−3) and outstanding absolute EMI shielding effectiveness of 1.12 × 105dB cm2g−1. The FPG was characterized via thorough material analyses, and its mechanical durability and flexibility were confirmed by a bending cycle test. Finally, the FPG was utilized in drone and wearable applications, showing effective EMI shielding performance for internal/external EMI in a drone radar system and reducing the specific absorption rate in the human body.

Two-dimensional (2D) inorganic nanomaterials have attracted enormous interest in diverse research areas because of their intriguing physicochemical properties. However, reliable method for the synthesis and composition manipulation of polycrystalline inorganic nanosheets (NSs) are still considered grand challenges. Here, we report a robust synthetic route for producing various kinds of inorganic porous NSs with desired multiple components and precise compositional stoichiometry by employing tunicin, i.e., cellulose extracted from earth-abundant marine invertebrate shell waste. Cellulose fibrils can be tightly immobilized on graphene oxide (GO) NSs to form stable tunicin-loaded GO NSs, which are used as a sacrificial template for homogeneous adsorption of diverse metal precursors. After a subsequent pyrolysis process, 2D metallic or metal oxide NSs are formed without any structural collapse. The rationally designed tunicin-loaded GO NS templating route paves a new path for the simple preparation of multicompositional inorganic NSs for broad applications, including chemical sensing and electrocatalysis.

Weavable sensing fibers with superior mechanical strength and sensing functionality are crucial for the realization of wearable textile sensors. However, in the fabrication of previously reported wearable sensing fibers, additional processes such as reduction, doping, and coating were essential to satisfy both requirements. The sensing fibers should be continuously synthesized in a scalable process for commercial applications with high reliability and productivity, which was challenging. In this study, we first synthesize mass-producible wearable sensing fibers with good mechanical properties and sensing functionality without additional processes by incorporating carbon nanotubes (CNTs) into distinct nanocellulose. Nanocellulose extracted from tunicate (TCNF) is homogeneously composited with single-walled CNTs, and composite fibers (TCNF/CNT) are continuously produced in aligned directions by wet spinning, facilitating liquid-crystal properties. The TCNF/CNT fibers exhibit a superior gas (NO2)-sensing performance with high selectivity and sensitivity (parts-per-billion detection). In addition, the TCNF/CNT fibers can endure complex and harsh distortions maintaining their intrinsic sensing properties and can be perfectly integrated with conventional fabrics using a direct weaving process. Our meter-scale scalable synthesis of functional composite fibers is expected to provide a mass production platform of versatile wearable sensors.

Single-walled carbon nanotubes (SWCNTs) are recognized as versatile materials for the formation of chemiresistive sensors. However, imparting high sensitivity and selectivity to SWCNTs remains a major challenge. Herein, we report a new sensory system that interfaces SWCNTs and catalytic metal nanoparticles (NPs) in a film of a porous ion exchange polymer to produce sensitive and selective sensors. The porous polymer films are based on imidazolium-functionalized triptycene polyether sulfone, and the environment created by this polymer results in the formation of ultrasmall (<5 nm) Au NPs. The polymer serves to suppress the growth of Au NPs while maintaining gas transport. The size control promotes strong interactions between Au NPs and carbon monoxide (CO), and this composition produces a robust, sensitive, and selective CO chemiresistive sensor. We further demonstrate that the modulation of a gate voltage in chemical field-effect transistor sensing devices enhances the performance by promoting the activity of Au NPs. The sensors display an increase of CO response at room temperature in air under the negative gate voltage. Our results demonstrate that the combination of a porous ion exchange matrix, NPs, and gate voltage-modulated chemiresistors can be synergistically tuned to create sensitive and selective sensors for target analytes.

We report a flexible hydrogen sensing platform based on a single-strand yarn consisting of high-density electrospun nanofibers, on which nanograined Pd or Pd@Pt is coated viayarn spinning followed by sputter deposition. In general, Pd undergoes a phase transition to PdHx(α-PdHxat [H2] < 1% and β-PdHxat [H2] > 2%), in which H atoms act as electron scattering centers, thus increasing the resistance. In our system, the sensors exhibit switchable H2sensing behaviors, that is, (i) ΔR/R0> 0 at [H2] > 1% by the active electron scattering and (ii) ΔR/R0< 0 at [H2] < 1% derived from nanograined Pd effects. Due to high mechanical stability stemming from nanogranular morphologies of Pd, which is essential for enduring a huge volume expansion upon exposure to high-concentration H2, we could obtain a wide concentration range (4–0.0001%) H2detection resolution. Moreover, an ultrathin Pt overlayer coated on Pd offers an accelerated H2detection capability based on effective gas dissociation and activation properties. Furthermore, by virtue of the core (thread)–shell (nanofiber yarn) scaffold, long cycling reliability and flexibility were achieved. This facile and low-cost yarn fabrication method offers the development of single-strand thread-type wearable chemiresistors that possess a high surface area and open porosity, facilitating gas diffusion and reaction.

The development of freestanding fiber-type chemiresistors, having high integration ability with various portable electronics including smart clothing systems, is highly demanding for the next-generation wearable sensing platforms. However, critical challenges stemming from the irreversible chemical sensing kinetics and weak reliability of the freestanding fiber-type chemiresistor hinder their practical use. In this work, for the first time, we report on the potential suitability of the freestanding and ultraporous reduced graphene oxide fiber functionalized with WO3nanorods (porous WO3NRs-RGO composite fiber) as a sensitive nitrogen dioxide (NO2) detector. By employing a tunicate cellulose nanofiber (TCNF), which is a unique animal-type cellulose, the numerous mesopores are formed on a wet-spun TCNF-GO composite fiber, unlike a bare GO fiber with dense surface structure. More interestingly, due to the superior wettability of TCNF, the aqueous tungsten precursor is uniformly adsorbed on an ultraporous TCNF-GO fiber, and subsequent heat treatment results in the thermal reduction of a TCNF-GO fiber and hierarchical growth of WO3NRs perpendicular to the porous RGO fiber (porous WO3NRs-RGO fiber). The freestanding porous WO3NRs-RGO fiber shows a notable response to 1 ppm NO2. Furthermore, we successfully demonstrate reversible NO2sensing characteristics of the porous WO3NRs-RGO fiber, which is integrated on a wrist-type wearable sensing device.

Flexible and stretchable electrochromic supercapacitor systems are widely considered as promising multifunctional energy storage devices that eliminate the need for an external power source. Nevertheless, the performance of conventional designs deteriorates significantly as a result of electrode/electrolyte exposure to atmosphere as well as mechanical deformations for the case of flexible systems. In this study, we suggest an all-transparent stretchable electrochromic supercapacitor device with ultrastable performance, which consists of Au/Ag core–shell nanowire-embedded polydimethylsiloxane (PDMS), bistacked WO3nanotube/PEDOT:PSS, and polyacrylamide (PAAm)-based hydrogel electrolyte. Au/Ag core–shell nanowire-embedded PDMS integrated with PAAm-based hydrogel electrolyte prevents Ag oxidation and dehydration while maintaining ionic and electrical conductivity at high voltage even after 16 days of exposure to ambient conditions and under application of mechanical strains in both tensile and bending conditions. WO3nanotube/PEDOT:PSS bistacked active materials maintain high electrochemical–electrochromic performance even under mechanical deformations. Maximum specific capacitance of 471.0 F g–1was obtained with a 92.9% capacity retention even after 50 000 charge–discharge cycles. In addition, high coloration efficiency of 83.9 cm2C–1was shown to be due to the dual coloration and pseudocapacitor characteristics of the WO3nanotube and PEDOT:PSS thin layer.

Pt-based catalysts are suffering from the intrinsic high cost and poor stability for practical use in fuel cells. We develop a free-standing fabric type Pt-free high-performance oxygen reduction reaction (ORR) catalyst, consisting of N, Fe-codoped carbon nanofibers and graphene (N/Fe-CG) hybrids. Electrospinning is used to synthesize the free-standing fabric catalysts. Incorporation of graphene into the nanofiber structure greatly increases the electrical conductivity and catalytic surface area. X-ray photoelectron spectroscopy (XPS) analysis provides evidence that Fe-N4active centers are possibly formed in the catalyst, as is confirmed by electrochemical tests also. Compared to commercial Pt/C, our N/Fe-CG exhibits a similar level of catalytic activity but superior stability and methanol tolerance in both alkaline and acidic media, making it a good candidate for an ORR electrocatalyst in energy storage/conversion and other relevant applications.

Wireless electronic devices require small, rechargeable batteries that can be rapidly designed and fabricated in customized form factors for shape conformable integration. Here, we demonstrate an integrated design and manufacturing method for aqueous zinc-ion batteries composed of polyaniline (PANI)-coated carbon fiber (PANI/CF) cathodes, laser micromachined zinc (Zn) anodes, and porous separators that are packaged within three-dimensional printed geometries, including rectangular, cylindrical, H-, and ring-shapes. The PANI/CF cathode possesses high surface area and conductivity giving rise to high rate (∼600 C) performance. Due to outstanding stability of Zn-PANI batteries against oxygen and moisture, they exhibit long cycling stability in an aqueous electrolyte solution. As exemplar, we demonstrated rechargeable battery packs with tunable voltage and capacity using stacked electrodes that are integrated with electronic components in customized wearable devices.

Real-time temperature monitoring of individual blood packages capable of wireless data transmission to ensure the safety of blood samples and minimize wastes has become a critical issue in recent years. In this work, we propose flexible temperature sensors using silver nanowires (NWs) and a flexible colorless polyimide (CPI) film integrated with a wireless data transmission circuit. The unique design of the temperature sensors was achieved by patterning Ag NWs using a three-dimensional printed mold and embedding the patterned Ag NWs in the CPI film (p-Ag NWs/CPI), which resulted in a flexible temperature sensor with electrical, mechanical, and temperature stability for applications in blood temperature monitoring. Indeed, a reliable resistance change of the p-Ag NWs/CPI was observed in the temperature range of −20 to 20 °C with a robust bending stability of up to 5000 cycles at 5 mm bending radius. Real-time and wireless temperature monitoring using the p-Ag NWs/CPI was demonstrated with the packages of rat blood. The result revealed that the stable and consistent temperature monitoring of individual blood packages could be achieved in a blood box, which was mainly attributed to the conformal attachment of the p-Ag NWs/CPI to different packages in a blood container.