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Highly stable conducting fibers have attracted significant attention in electronic textile (e-textile) applications. Here, we fabricate highly conducting poly(vinyl alcohol) (PVA) nanocomposite fibers with high thermal and chemical stability based on silver nanobelt (AgNB)/multiwalled carbon nanotube (MWCNT) hybrid materials as conducting fillers. At 20 vol % AgNB/MWCNT, the electrical conductivity of the fiber dramatically increased (∼533 times) from 3 up to 1600 S/cm after thermal treatment at 300 °C for 5 min. Moreover, PVA/AgNB/MWCNT fiber resists the harsh conditions of good solvents for PVA as well as high temperatures over the melting point of PVA, whereas pure PVA fiber is unstable in these environments. The significantly enhanced electrical conductivity and chemical stability can be realized through the post-thermal curing process, which is attributed to the coalescence between adjacent AgNBs and additional intensive cross-linking of PVA. These remarkable characteristics make our conducting fibers suitable for applications in e-textiles such as water leakage detectors and wearable heaters. In particular, heating behavior of e-textiles by Joule heating can accelerate the desorption of physically trapped moisture from the fiber surface, resulting in the fully reversible operation of water leakage monitoring. This smart e-textile sensor based on highly stable and conductive composite fibers will pave the way for diverse e-textile applications.

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.

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/cm

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.

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

Transfer printing of electronic functions on arbitrary surfaces is essential for next-generation applications of skin-attachable electronics, wearable sensors, and implantable/medical devices. For transfer printing of electronic functions on multidimensional surfaces, such as curved regions of the skin and different objects, various strategies have been devised based on the materials and structural design of electronic components and transfer stamps, such as ultrathin membranes or in-plane structures of electronic components, soft interfacial glues or adhesives between devices and surfaces, and smart transfer adhesives with bioinspired micro/nanostructures. These techniques enable high conformity of adhesion, mechanical robustness, and high compliance of electronic devices on arbitrary surfaces under mechanical deformation. In this Perspective, we provide an overview of recent transfer printing techniques and discuss their advantages and challenges. In addition, we report a recently developed transfer printing technique based on bioinspired smart adhesives with reversible adhesion, which enables compliant electronics on various arbitrary complex surfaces without performance degradation, providing solutions for various technical challenges remaining in transfer printing. Finally, we present potential applications of transfer printing and future perspectives for this emerging field.

Synthesis of the strongly anisotropic materials, such as highly porous hollow double walled cubes are considered excellent approach to maximize the diffusion of electrolytes. Herein, hollow doubled walled (HDW) Mn2O3 nanocubes (NCs) were synthesized by facile hydrothermal method followed by calcination method. The growth of nanocubes were studied by performing hydrothermal reaction at different times ranging from 3 to 9 h. Thereafter, the feasibility of prepared HDW NCs as air cathode in hybrid Na-air battery was systematically investigated. Among all, the sample prepared by 9 h hydrothermal treatment showed superior performance than 3 and 6 h samples. The fabricated hybrid Na-air cell using HDW Mn2O3 NCs displayed 330 mV overpotential gap and 90% electrical energy efficiency at 5 mA g−1 current density, maximum of 0.2 W g−1 power density and good cyclic stability up to 75 cycles which is attributed to the highly porous nature of material that allows efficient diffusion of electrolyte ions and oxygen from air. Thus, present investigation suggests that HDW Mn2O3 NCs can be a potential air cathode and can be utilized in other metal-air battery systems.

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., A stretchable sound‐in‐display with electrodes of strain‐insensitive silver nanowires and emissive layers of field‐induced inorganic electroluminescent (EL) phosphors are proposed for generation of synchronized acoustic and visual information. The loudspeaker simultaneously emits light by the EL and generates sound by the vibration of the dielectric elastomer actuator under the same bias, allowing the synesthetic perception of both color and sound.

Hierarchical and gradient structures in biological systems with special mechanical properties have inspired innovations in materials design for construction and mechanical applications. Analogous to the control of stress transfer in gradient mechanical structures, the control of electron transfer in gradient electrical structures should enable the development of high-performance electronics. This paper demonstrates a high performance electronic skin (e-skin) via the simultaneous control of tactile stress transfer to an active sensing area and the corresponding electrical current through the gradient structures. The flexible e-skin sensor has extraordinarily high piezoresistive sensitivity at low power and linearity over a broad pressure range based on the conductivity-gradient multilayer on the stiffness-gradient interlocked microdome geometry. While stiffness-gradient interlocked microdome structures allow the efficient transfer and localization of applied stress to the sensing area, the multilayered structure with gradient conductivity enables the efficient regulation of piezoresistance in response to applied pressure by gradual activation of current pathways from outer to inner layers, resulting in a pressure sensitivity of 3.8 × 105 kPa-1 with linear response over a wide range of up to 100 kPa. In addition, the sensor indicated a rapid response time of 0.016 ms, a low minimum detectable pressure level of 0.025 Pa, a low operating voltage (100 μV), and high durability during 8000 repetitive cycles of pressure application (80 kPa). The high performance of the e-skin sensor enables acoustic wave detection, differentiation of gas characterized by different densities, subtle tactile manipulation of objects, and real-time monitoring of pulse pressure waveform.

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).

Artificial tongue for astringency perception is demonstrated with a saliva-like chemiresistive ionic hydrogel., Artificial tongues have been receiving increasing attention for the perception of five basic tastes. However, it is still challenging to fully mimic human tongue–like performance for tastes such as astringency. Mimicking the mechanism of astringency perception on the human tongue, we use a saliva-like chemiresistive ionic hydrogel anchored to a flexible substrate as a soft artificial tongue. When exposed to astringent compounds, hydrophobic aggregates form inside the microporous network and transform it into a micro/nanoporous structure with enhanced ionic conductivity. This unique human tongue–like performance enables tannic acid to be detected over a wide range (0.0005 to 1 wt %) with high sensitivity (0.292 wt %−1) and fast response time (~10 s). As a proof of concept, our sensor can detect the degree of astringency in beverages and fruits using a simple wipe-and-detection method, making a powerful platform for future applications involving humanoid robots and taste monitoring devices.

Flexible electronic devices are regarded as one of the key technologies in wearable healthcare systems, wireless communications and smart personal electronics. For the realization of these applications, wearable energy and sensor devices are the two main technologies that need to be developed into lightweight, miniaturized, and flexible forms. In this review, we introduce recent advances in the controlled design of device structures into bioinspired micro/nanostructures and 2D/3D structures for the enhancement of energy harvesting and multifunctional sensing properties of flexible electronic skins. In addition, we highlight their potential applications in flexible/wearable electronics, sensors, robotics and prosthetics, and biomedical devices.

Flexible electronic devices that are lightweight and wearable are critical for personal healthcare systems, which are not restricted by time and space. To monitor human bio-signals in a non-invasive manner, skin-conforming, highly sensitive, reliable, and sustainable healthcare monitoring devices are required. In this review, we introduce flexible and wearable sensors based on engineered functional nano/micro-materials with unique sensing capabilities for detection of physical and electrophysiological vital signs of humans. In addition, we investigate key factors for the development of user-interactive healthcare devices that are customizable, wearable, skin-conforming, and monolithic (design), and have long-term monitoring capability with sustainable power sources. Finally, we describe potential challenges of developing current wearable healthcare devices for applications in fitness, medical diagnosis, prosthetics, and robotics.

Fluorinated polymers have been widely used in triboelectric sensors, displays, and energy harvesting devices because of their superior electron affinity, which leads to the negative triboelectric materials. While previous reports have shown that the control of dielectric constants of fluorinated polymers can increase the triboelectric output performance, the exact relationship between the molecular structures of fluorinated polymers and the resulting triboelectric properties is still elusive. In this study, we demonstrate that the molecular chain structures of the fluorinated polymers depending on the number of fluorine units, the molecular weight (Mw), and conditions such as spin rate and annealing temperature directly affect the relative dielectric constants of dielectric layers and the triboelectric polarity, which are closely related to the triboelectric output performance. We observe that the polymer chain packing structures result in the increase of the relative dielectric constants, thus leading to the improvement of triboelectric output currents. Among the fluorinated polymers used in this study, a poly (2,2,2-trifluoroethyl methacrylate) polymer with three fluorine units and Mw of ~ 20 kg/mol shows the best triboelectric output performance. Our molecular engineering strategy to control the dielectric constants of fluorinated polymers can be a robust platform for the fundamental studies of triboelectric materials and their applications in diverse energy harvesting and sensing devices.

Stimuli-responsive colorimetric sensors are promising for various industrial and medical applications due to the capability of simple, fast, and inexpensive visualization of external stimuli. Here we demonstrate a thermoresponsive, smart colorimetric patch based on a thermoresponsive plasmonic microgel embedded in a stretchable hydrogel film. To achieve a fast and efficient thermoresponsive color change, raspberry-shaped plasmonic microgels were fabricated by decorating gold nanoparticles (AuNPs) on poly(N-isopropylacrylamide) (PNIPAM) microgels, which exhibit reversible and strain-insensitive color shifts (between red and grayish violet) in response to a temperature change. The smart colorimetric patch containing a plasmonic microgels exhibits a significant extinction peak shift (176 nm) in a short time (1 s), with a temperature-sensing resolution of 0.2 °C. Moreover, the transition temperature of the plasmonic microgel can be finely tuned by additives and comonomers, so that the exquisite temperature visualization can be conducted over a wide temperature range of 25‒40 °C by assembling plasmonic microgel films with different transition temperatures into an array patch. For proof-of-concept demonstrations, a freestanding smart colorimetric patch was utilized as a spatial temperature scanner and a colorimetric thermometer for a thermoresponsive actuator, which is potentially applicable in smart, wearable sensors and soft robotics. Thin flexible patches that adhere to human skin and report deviations in temperature have been developed by researchers in South Korea. Hyunhyub Ko and colleagues from the Ulsan National Institute of Science and Technology made water-filled polymers known as hydrogels into microscale beads and coated them with light-emitting gold nanoparticles. They then embedded the beads into a stretchable transparent film. Because the tiny microgels swell and shrink in response to temperature, the nanoparticles likewise move closer or further apart, an action that causes them to produce differently colored light. The film, even when stretched or bent, detected temperature changes from 25 to 45 °C by vivid color changes. A prototype “smart hand” capable of rapidly sensing heat or cold when wrapped around fingertips or wrists could find use in soft robotics. A thermoresponsive smart colorimetric patch was fabricated by embedding thermoresponsive plasmonic microgels in a stretchable hydrogel film. The stretchable and wearable colorimetric patches exhibited vivid color change, fast response time, outstanding thermal resolution, and high durability. Potential applications in wearable smart sensors and soft robotics were demonstrated through a spatial temperature scanner and a colorimetric thermometer for thermoresponsive actuators.

Highly oxygen evolution reaction (OER)-active electrocatalysts often exhibit improved OER durability in the presence of carbon corrosion or oxidation (COR) in the literature. The activity-durability coincidence of OER electrocatalysts was theoretically understood by preferential depolarization in galvanostatic situations. At constant-current conditions for a system involving multiple reactions that are independent and competitive, the overpotential is determined most dominantly by the most facile reaction so that the most facile reaction is responsible for a dominant portion of the overall current. Therefore, higher OER activity improves durability by mitigating the current responsible for COR. The activity-durability coincidence was then proved experimentally by comparing between two catalysts of the same chemical identity (MnCo2O4) in different dimensions (5 and 100 nm in size). Carbon corrosion responsible for inferior durability was suppressed in the smaller-dimension catalyst (MnCo2O4 in 5 nm) having...

Since the conventional fluorine-based electro-active polymers release toxic residues into the environment during their syntheses and decomposition processes, eco-friendly piezoelectric polymers are urgently demanded in the field of energy-related soft materials. Here, we derive a high-performance biodegradable chitin polymer from squid pen material and demonstrate its utility as a flexible piezoelectric material. The readily controlled ferroelectric chitin film confers excellent piezoelectricity under external mechanical pressure, resulting in comparable performance with that of conventional fluorine-based piezoelectric polymers. In particular, the sufficient piezoelectric behavior in chitin film allows us to not only realize a high-fidelity paper-type speaker and microphone that operates over a wide frequency range without significantly deteriorating the input and output sounds but also demonstrate transparent speaker consisting of AgNWs electrodes onto freestanding chitin film which also enables to resemble the original sound. Finally, the biodegradable chitin polymer can be successfully dissolved by chitinase enzyme within eight days without any toxic residues remained.

Electronic skins (e-skins) with high sensitivity to multidirectional mechanical stimuli are crucial for healthcare monitoring devices, robotics, and wearable sensors. In this study, we present piezoresistive e-skins with tunable force sensitivity and selectivity to multidirectional forces through the engineered microstructure geometries (i.e., dome, pyramid, and pillar). Depending on the microstructure geometry, distinct variations in contact area and localized stress distribution are observed under different mechanical forces (i.e., normal, shear, stretching, and bending), which critically affect the force sensitivity, selectivity, response/relaxation time, and mechanical stability of e-skins. Microdome structures present the best force sensitivities for normal, tensile, and bending stresses. In particular, microdome structures exhibit extremely high pressure sensitivities over broad pressure ranges (47,062 kPa−1 in the range of

Cost-effective and environmentally friendly seawater-electrolyte-based batteries exhibit high energy density and demonstrate immense potential for use in future energy storage devices; however, lack of high-performance negative and positive electrodes significantly challenges their practical applications. In this study, N-doped and N,S-doped carbon nanospheres (referred to as NCSs and NSCSs, respectively) are synthesized via the pyrolysis of melanosomes, which is a bio-inspired polymer. Electrocatalytic activity measurements reveal the bifunctionality of the prepared catalysts. NSCSs exhibit a distinctively higher performance than NCSs when used as an air electrode in seawater batteries under ambient conditions (referred to as static mode hereafter). Further, due to the introduction of air flow into the seawater electrolyte (referred to as flow mode hereafter), NSCSs exhibit an improved cell discharge potential. The high performance of the cell is attributed to the high surface area, bifunctional electrocatalytic activity, generation of new active sites, improvement of spin density in NSCSs, and continuous flow of air to the electrolyte. The cell in the flow mode exhibits an overpotential gap of 0.56 V, a round-trip efficiency of 84%, a maximum power density of 203 mW g−1, and an outstanding cycling stability up to 100 cycles. The developed synthetic method provides an effective, scalable approach for doping binary or ternary atoms into the carbon host matrix, which can motivate further experimental and theoretical studies of electrode materials in various energy storage devices. In addition, the concept and results obtained by the introduction of air flow into the electrolyte can lead to the improvement of cell performance in terms of electrical energy efficiency, which can be exploited in various metal–air batteries.

Here, we demonstrate the successful synthesis of (1T/2H) MoS2/α-MoO3 heterostructured nanoflowers at a low temperature of 200 °C by a one-step hydrothermal method. By tuning the reaction time under the influence of thiourea and hydrazine hydrate, we established a complete phase-engineered MoS2 with 1T and 2H phases on the surface of α-MoO3. Active sites associated with the phase-engineered (1T/2H) MoS2/α-MoO3 hybrid nanoflowers enable them to exhibit dual roles as a superior dye adsorbent and an electrocatalyst towards the hydrogen evolution reaction. The 2H-rich (1T/2H) MoS2/α-MoO3 hybrid heterostructured nanoflowers prepared at 16 h achieved a high surface area of 37.97 m2 g−1, and 97% of the RhB dye with an initial concentration of 47.9 mg L−1 was removed within 10 min through the adsorption process, which is the highest known removal efficiency reported in the literature. As a hydrogen evolution reaction (HER) electrocatalyst in acidic solution, the 1T-rich (1T/2H) MoS2/α-MoO3 hybrid heterostructured nanoflowers prepared at 12 h exhibited a highly efficient catalytic activity by achieving a low overpotential of 232 mV at a current density of 10 mA cm−2, which is comparable to those of previously reported HER catalysts based on MoS2. Moreover, this sample reached a low Tafel slope of 81 mV dec−1 and was stable when operated for more than 1000 cycles.

Textile-based sensors can perceive and respond to environmental stimuli in daily life, and hence are critical components of wearable devices. Herein, self-powered triboelectric wearable sensors are fabricated using polyvinylidene fluoride (PVDF) fibers stitched by using a sewing machine. The excellent mechanical properties of dry-jet wet spun PVDF fibers allow the use of a sewing machine to stitch them into diverse programmable textile patterns on various fabric substrates. Such stitches can provide remarkable triboelectric signals when in contact with the opposing surfaces of commercial fabrics, since PVDF has higher electron affinity than other polymers. In addition, PVDF stitch-based triboelectric sensors are flexible, lightweight, wearable, washable, and comfortable. Furthermore, they can detect a broad pressure range (326 Pa to 326 kPa), which is unachievable with conventional textile force sensors, enabling diverse pressure-sensor applications. To demonstrate their use in wearable devices, a smart glove and joint pads are fabricated based on PVDF stitch-based triboelectric sensors. These wearable sensors enable the detection of and distinguishing diverse hand gestures and body motions by generating intrinsic signal patterns for the specific gesture and motions. These sensors also enable real-time self-powered pulse signal monitoring. This work demonstrates a feasible fabrication approach to realize stitched textile sensors using a sewing machine, with many possible e-textile applications.

Self-healing polymers bearing reversible and bulky urea bonds were prepared by free-radical copolymerization followed by crosslinking with diisocyanates. Linear prepolymers (PtB) were first synthesized from 2-(tert-butylamino)ethyl methacrylate (tBAEMA), methyl methacrylate (MMA), and n-butyl acrylate (BA), in a molar ratio of 1 : 10 : 10, and then crosslinked with 1,6-diisocyanatohexane (HDI) or isophorone diisocyanate (IPDI) to fabricate self-healing polymers. The reversible bonding–debonding between the tBAEMA of the polymer backbone and the isocyanate units of the crosslinker in the self-healing polymer facilitates rapid, repeatable, and water-adaptive self-healing performance, which has been extensively investigated using 1H-NMR, DSC, FT-IR, AFM, optical microscopy, and rheometric analyses. Percentage recovery (%R) and self-healing efficiency (%SHE) were also studied using tensile and single-scratch tests, respectively, and compared with a control sample. It was revealed through extensive analyses that self-healing against a single scratch can be accelerated and repeated up to a certain number of times even in water, although the self-healing polymers are not water-resistant. We also demonstrated that a self-healable tube capable of containing flowing water could be fabricated from a self-healing polymer sheet by using reversible bonding–debonding characteristics of a reversible bulky urea bond.

Na-air batteries are regarded as a potential alternate to Li-air batteries due to the abundant sodium source and high theoretical energy density. However, non-aqueous Na-air battery suffers from the electrode polarization owing to the formation of insoluble discharge product, which severely limits its cyclability and performance. Herein, a high performance hybrid Na-air cell is demonstrated using a dual electrolyte (mixed aqueous and non-aqueous electrolyte) system and three dimensionally (3D) grown tin sulfide (SnS 2 ) nanopetals based air electrode. 3D SnS 2 nanopetals are synthesized by a facile solvothermal method and used as an air electrode material for hybrid Na-air battery. The vertically-grown and self-assembled ultra-thin nanosheets of 3D SnS 2 nanopetals provide exposed active sites for the efficient air and electrolyte diffusion to air electrodes, resulting in high performance hybrid Na-air cell. The fabricated hybrid Na-air cell displays low overpotential gap (0.52 V), high round trip efficiency (83%), high power density (300 mW g −1 ) and good rechargeability up to 40 cycles. The proposed 3D SnS 2 nanopetals as air electrodes can provide a robust platform for the future development of Na-air batteries and other energy storage devices.

Polyvinylidene fluoride (PVDF)/cellulose nanocrystal (CNC) nanocomposite fibers are prepared by dry-jet wet spinning at the CNC concentrations of 0, 1, and 3 wt%, followed by mechanical stretching up to the draw ratio (DR) of 8. The structure analysis results show that the strong interaction between PVDF and CNC leads to good mechanical properties and high β phase fraction (Fβ) that is beneficial for its application as a smart textile. The chain conformational variation of PVDF is analyzed as a function of processing condition, exhibiting that the tensile properties and Fβ of all the fibers are enhanced as the polymeric chains are extended with increasing DR. Among various fibers, the nanocomposite fiber containing 1 wt% CNC possesses the highest tensile strength of 298 MPa, tensile modulus of 3.3 GPa, and Fβ of 69.1% at a DR of 8. However, at a high CNC loading of 3 wt%, the tensile properties are decreased, which may be due to the defect structure evolution by CNC aggregation. In order to demonstrate the feasibility as a smart textile, triboelectric textile sensors are fabricated via cross-stitching and conventional weaving, resulting in a good pressure sensing capability over a broad range of 0.98–98 kPa.

Among non-fluorinated ferroelectric polymers, odd number Nylons have recently attracted attention because of their solution-processibility and decent ferroelectric characteristics. Here, we demonstrate a ferroelectric and piezoelectric Nylon-11 δ` phase crystal film manufactured via a humidity modulated casting and subsequent melt-quenching process. The low relative humidity (RH) below 20% during the spin casting process effectively mitigates the formation of hygroscopic holes resulting in the formation of pin-hole free Nylon-11 thin films. In addition, the subsequent melt-quenching gives rise to the evolution of the ferroelectric δ` phase crystals, which is carefully confirmed by grazing incidence X-ray diffraction and infrared spectroscopy measurements. The resulting ferroelectric thin films show decent remnant polarization of ~4.6 μC/cm2 at a low operating voltage of ±30 V, suggesting the formation of high-quality Nylon-11 δ` phases thin film. Furthermore, the centrifugal casting at low RH facilitates the fabrication of freestanding and transparent 20-μm thick ferroelectric δ` phase film after the subsequent melt-quenching process. The resulting Nylon-11 film provides excellent piezoelectric performance upon external vertical pressure, which enables a proof-of-concept demonstration as a Morse code detector.

A key requirement for wearable electronics is an adequate and sustainable power source. Accordingly, a self-powering unit that replaces rechargeable secondary batteries is a promising solution. However, to realize permanent, maintenance-free, and highly durable wearable electronics, stretchable self-powering units that can harvest and store energy should be developed. In this study, we developed a fully stretchable self-charging power unit that integrates a micro-supercapacitor and triboelectric nanogenerator using oxidized single-walled carbon nanotube/polymer electrodes. The fully stretchable micro-supercapacitor with oxidized single-walled carbon nanotube/polyvinylalcohol electrodes exhibited a double layer capacitance of 20 mF cm−2 at 0.1 mA cm−2 and improved mechanical flexibility and stretchability over 10,000 cycles of stretching tests. A stretchable, polydimethylsiloxane-based current collector employing silver nanoparticles embedded with oxidized single-walled carbon nanotubes enabled the fully stretchable, freestanding-triboelectric-layer based nanogenerators to produce a maximum instantaneous power density of 84.4 mW m−2 under periodic and round-trip sliding of a Nylon fabric while stretching up to 40% without significant performance degradation. Furthermore, a micro-supercapacitor of fully stretchable self-charging power unit could be successfully charged by the nanogenerator from 0 to 2.2 V in 1200 s and powered commercial digital clock for approximately 10 s. These results demonstrate that stretchable polymer composites with oxidized single-walled carbon nanotubes are suitable electrodes and active materials for fully stretchable and self-powered wearable electronics.

Silver nanowire (AgNW) networks are considered to be promising structures for use as flexible transparent electrodes for various optoelectronic devices. One important application of AgNW transparent electrodes is the flexible touch screens. However, the performances of flexible touch screens are still limited by the large surface roughness and low electrical to optical conductivity ratio of random network AgNW electrodes. In addition, although the perception of writing force on the touch screen enables a variety of different functions, the current technology still relies on the complicated capacitive force touch sensors. This paper demonstrates a simple and high-throughput bar-coating assembly technique for the fabrication of large-area (>20 × 20 cm2), highly cross-aligned AgNW networks for transparent electrodes with the sheet resistance of 21.0 Ω sq–1 at 95.0% of optical transmittance, which compares favorably with that of random AgNW networks (sheet resistance of 21.0 Ω sq–1 at 90.4% of optical transmi...

Binder-free and bifunctional electrocatalysts have vital roles in the development of high-performance metal–air batteries. Herein, we synthesized a vanadium oxide (VO2) nanostructure as a novel binder-free and bifunctional electrocatalyst for a rechargeable aqueous sodium–air (Na–air) battery. VO2 nanostructures were grown on reduced graphene oxide coated on carbon paper, which had a carambola morphology. We confirmed the bifunctional nature of VO2 nanostructures by analyzing their electrocatalytic activity associated with the oxygen reduction reaction and oxygen evolution reaction. The reaction pathway associated with electrocatalytic activity was also affirmed by computational modeling and simulation studies. Thereafter, an aqueous Na–air cell was built using novel binder-free VO2 nanostructures as the air electrode. The fabricated cell displayed a 0.64 V overpotential gap, 104 mW g−1 power density at 80 mA g−1 current density, 81% round trip efficiency and good cyclic stability up to 50 cycles.

van der Waals heterostructures based on stacked two-dimensional (2D) materials provide novel device structures enabling high-performance electronic and optoelectronic devices. While 2D–2D or 2D–bulk heterostructures have been largely explored for fundamental understanding and novel device applications, 2D–one-dimensional (1D) heterostructures have been rarely studied because of the difficulty in achieving high-quality heterojunctions between 2D and 1D structures. In this study, we introduce nanosheet-on-1D van der Waals heterostructure photodetectors based on a wet-transfer printing of a MoS2 nanosheet on top of a CuO nanowire (NW). MoS2/CuO nanosheet-on-1D photodetectors show an excellent photocurrent rectification ratio with an ideality factor of 1.37, which indicates the formation of an atomically sharp interface and a high-quality heterojunction in the MoS2/CuO heterostructure by wet-transfer-enhanced van der Waals bonding. Furthermore, nanosheet-on-1D heterojunction photodetectors exhibit excellent p...

Poly(vinylidene fluoride) (PVDF)/graphene oxide (GO) nanocomposite fibers were dry-jet wet spun at the GO concentrations of 0, 1, and 2 wt % with respect to the polymer. The as-spun fibers were drawn in the draw ratio (DR) range of 2-6.5, and the correlation between the PVDF chain conformation and the mechanical properties of the fibers upon drawing has been studied by two-dimensional correlation spectroscopy of Fourier-transformed infrared, wide-angle X-ray diffraction, differential scanning calorimetry, and tensile testing. The PVDF/GO nanocomposite fibers exhibited that the mobile PVDF crystals due to the conformational defects and kinks were nucleated because of the polar interaction between PVDF chains and functional groups of GO, whereas the control PVDF fiber showed the conventional conversion of crystal polymorphs (α and γ phases to β phase). As a result, the nanocomposite fiber showed dramatically improved toughness (enhanced by 1123% at a DR of 2 and 120% at a DR of 6.5) as compared to that of the control fiber. Furthermore, the tensile strength and modulus of the PVDF/GO (2 wt %) fiber were 394 MPa and 4.6 GPa, respectively, whereas those of the control PVDF fiber were 295 MPa and 3.9 GPa, respectively.

Biological tissues are multiresponsive and functional, and similar properties might be possible in synthetic systems by merging responsive polymers with hierarchical soft architectures. For example, mechanochromic polymers have applications in force-responsive colorimetric sensors and soft robotics, but their integration into sensitive, multifunctional devices remains challenging. Herein, a hierarchical nanoparticle-in-micropore (NP-MP) architecture in porous mechanochromic polymers, which enhances the mechanosensitivity and stretchability of mechanochromic electronic skins (e-skins), is reported. The hierarchical NP-MP structure results in stress-concentration-induced mechanochemical activation of mechanophores, significantly improving the mechanochromic sensitivity to both tensile strain and normal force (critical tensile strain: 50% and normal force: 1 N). Furthermore, the porous mechanochromic composites exhibit a reversible mechanochromism under a strain of 250%. This architecture enables a dual-mode mechanochromic e-skin for detecting static/dynamic forces via mechanochromism and triboelectricity. The hierarchical NP-MP architecture provides a general platform to develop mechanochromic composites with high sensitivity and stretchability.

Output performance of triboelectric devices critically depends on their surface charge density that is greatly influenced by the dielectric constant of material. However, contrary to polymer composites, the dielectric constant cannot be substantially increased in pure polymers. This study shows that interfacial polarization in bilayer polymer films significantly enhances their dielectric constants, increasing the surface charge density and triboelectric performance. This is the first report of the introduction of interfacial polarization of dielectric materials for the enhancement of triboelectric output performance. The elastic bilayer films consisting of polydimethylsiloxane (PDMS) coated with poly (2,2,2-trifluoroethyl methacrylate) (PTF) and poly ( L -lysine) (PLL) as negative and positive triboelectric materials, respectively, are utilized in triboelectric devices, which show a dramatic increase in the dielectric constants (24.4 for PTF-PDMS and 15.2 for PLL-PDMS bilayers at 10 kHz) through interfacial polarization, resulting in significant enhancement of triboelectric performance when compared with hard single-layer films. For proofs of concept, we fabricated a temperature-sensitive triboelectric sensor and vibration-based triboelectric nanogenerator. The sensor showed the highest temperature sensitivities among triboelectric sensors whereas the nanogenerator provided vibration-induced sustainable power for LED operation without rectification, which was attributed to the charge accumulation in the bi-layered dielectric films.

Development of broadband photodetectors is of great importance for applications in high-capacity optical communication, night vision, and biomedical imaging systems. While heterostructured photodetectors can expand light detection range, fabrication of heterostructures via epitaxial growth or wafer bonding still faces significant challenges because of problems such as lattice and thermal mismatches. Here, a transfer printing technique is used for the heterogeneous integration of InGaAs nanomembranes on silicon semiconductors and thus the formation of van der Waals heterojunction photodiodes, which can enhance the spectral response and photoresponsivity of Si photodiodes. Transfer-printed InGaAs nanomembrane/Si heterojunction photodiode exhibits a high rectification ratio (7.73 × 104 at ±3 V) and low leakage current (7.44 × 10–5 A/cm2 at −3 V) in a dark state. In particular, the photodiode shows high photoresponsivities (7.52 and 2.2 A W–1 at a reverse bias of −3 V and zero bias, respectively) in the broad...

The development of flexible photodetectors has received great attention for future optoelectronic applications including flexible image sensors, biomedical imaging, and smart, wearable systems. Previously, omnidirectional photodetectors were only achievable by integration of a hemispherical microlens assembly on multiple photodetectors. Herein, a hierarchical photodiode design of ZnO nanowires (NWs) on honeycomb-structured Si (H-Si) membranes is demonstrated to exhibit excellent omnidirectional light-absorption ability and thus maintain high photocurrents over broad spectral ranges (365 to 1,100 nm) for wide incident angles (0° to 70°), which enabled broadband omnidirectional light detection in flexible photodetectors. Furthermore, the stress-relieving honeycomb pattern within the photodiode micromembranes provided photodetectors with excellent mechanical flexibility (10% decrease in photocurrent at a bending radius of 3 mm) and durability (minimal change in photocurrent over 10,000 bending cycles). When employed in semiconductor thin films, the hierarchical NW/honeycomb heterostructure design acts as an efficient platform for various optoelectronic devices requiring mechanical flexibility and broadband omnidirectional light detection.

Multiple-patterned nanostructures prepared by synergistically combining block-copolymer lithography with nano-imprinting lithography have been used as back reflectors for enhancing light absorption in organic optoelectronic devices. The multiple-patterned electrodes have significantly boosted the performance of organic photovoltaics and photo-transistors, owed to the highly effective light scattering and plasmonic effects, extending the range of their practical applications.

Bifunctional catalysts are prominent to attain high capacity, maximum energy efficiency and long cycle-life for aqueous rechargeable Na-air batteries. In this work, we report the synthesis of bi-functional noble metal free, Co3(PO4)2 nanostructures by facile precipitation technique and evaluated its electrocatalytic activity. Co3(PO4)2 nanostructure was investigated as a potential electrocatalyst for rechargeable aqueous Na-air battery for the first time. The synthesized Co3(PO4)2 grain-like nanostructures showed better oxygen evolution activity compared to Pt/C catalyst. The fabricated Na-air battery with the Co3(PO4)2 catalyst as air-cathode delivered low overpotential and its round trip energy efficiency reached up to 83%. The Na-air battery exhibited stable cycle performance up to 50 cycles.

Sodium (Na) ion batteries are interesting candidates for replacing lithium (Li) ion batteries, primarily due to the abundance of Na in the environment. However, the performance and energy density of a Na-ion battery are inferior to those of a Li-ion battery. Organic active materials can help overcome the drawbacks associated with Na-ion batteries because of their many advantages. However, such organic polymer electrodes are subjected to a high self-discharge and low practical capacity because the polymer electrode easily dissolves in an organic electrolyte and forms an insulating layer. Therefore, in this study, we have designed a unique organic electrode in which an active polymer is encapsulated into a carbon nanotube (CNT) to form an electrode with high polymer content. The CNT is able to retain the active polymer within the electrode structure, providing an effective electronic conduction path. Moreover, the CNT can contain large amounts of active polymer and therefore exhibits superior electrochemical properties without self-discharge, making it well suited for use as a cathode material in a Na-ion battery.

Plasmonic systems based on particle-film plasmonic couplings have recently attracted great attention because of the significantly enhanced electric field at the particle-film gaps. Here, we introduce a hybrid plasmonic architecture utilizing combined plasmonic effects of particle-film gap plasmons and silver film over nanosphere (AgFON) substrates. When gold nanoparticles (AuNPs) are assembled on AgFON substrates with controllable particle-film gap distances, the AuNP-AgFON system supports multiple plasmonic couplings from interparticle, particle-film, and crevice gaps, resulting in a huge surface-enhanced Raman spectroscopy (SERS) effect. We show that the periodicity of AgFON substrates and the particle-film gaps greatly affects the surface plasmon resonances, and thus, the SERS effects due to the interplay between multiple plasmonic couplings. The optimally designed AuNP-AgFON substrate shows a SERS enhancement of 233 times compared to the bare AgFON substrate. The ultrasensitive SERS sensing capability is also demonstrated by detecting glutathione, a neurochemical molecule that is an important antioxidant, down to the 10 pM level.

This work describes the effect of zirconium(IV) acetylacetonate (Zr(acac)4) on the self-healing and thermomechanical properties of urethane–acrylate copolymers, and which were compared with dibutyltin dilaurate (DBTDL). For prepolymer synthesis, glyceryl methacrylate (GlyMA) and 2-hydroxyethyl methacrylate (HEMA) were chosen and copolymerized with methyl methacrylate (MMA) and butyl acrylate (BA) at a concentration of 8 mol%. The prepolymers (G8 and H8) were then crosslinked with HDI trimer to prepare self-healing copolymers, and DBTDL or Zr(acac)4 was used as both crosslinking and self-healing catalysts. The self-healing performance of new carbonate bond formation by DBTDL and transesterification by Zr(acac)4 were studied and compared using single scratch test with atomic force microscopy (AFM). Owing to the secondary hydroxyl groups of GlyMA, DBTDL shows scratch-healing capability only in the GlyMA-based copolymer (G8) within 1 h at 55 and 75 °C. On the other hand, Zr(acac)4 shows scratch-healing capability in both copolymers (G8 and H8) within 1 h at 55 and 75 °C; however, transesterification may result in the changes of storage modulus and glass-transition temperature (Tg). This was confirmed from dynamic mechanical analysis (DMA) that a low temperature (∼55 °C) exhibited excellent self-healing performance with minimal hardness change by transesterification.

Self-powered multifunctional sensors have been receiving great attention for their potential use in sustainable wearable electronics. In this work, we demonstrate a facile approach for the development of self-powered multimodal pressure-temperature sensors based on an identical ferroelectric copolymer, poly (vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) for the contact pair materials by switching the triboelectric polarity via ferroelectric polarization. This approach enables the identical material-based triboelectric devices with remarkably enhanced triboelectric and pyroelectric output performances. The inversely-polarized P(VDF-TrFE) device exhibits ~106 and ~12 times higher triboelectric and pyroelectric currents, respectively, compared to those of non-polarized devices. Consequently, our triboelectric device provides the highest pressure sensitivity of 40 nA kPa−1 and 1.4 V kPa−1 with a broad pressure detection range (98 Pa–98 kPa) as well as competitive temperature sensitivity of 0.38 nA °C−1 and 0.27 nA °C−1 in cooling and heating states, respectively, compared to previous PVDF-based self-powered sensors. Furthermore, our self-powered sensor can discriminate multiple stimuli including pressure as well as temperature without signal interference. Our sensor can be utilized for the monitoring of weak pulse pressure as well as multimodal finger touch. This work provides a facile fabrication approach to realize triboelectric and pyroelectric multimodal devices with outstanding output performances.

Nylon-11, the most studied member of the odd-numbered nylon family, is a promising functional material for future electronic devices or energy storage systems. To utilize nylon-11 in those applications, it is necessary to control the formation of the applicable metastable crystal phases such as the piezoelectric γ-phase or ferroelectric δ’-phase. Herein, we describe a facile method of fabricating metastable γ-phase nylon-11 fibers via electrospinning with a tailored solvent system. By using a solvent mixture of 1,1,1,3,3,3-hexafluoro-2-propanol and trifluoroacetic acid [HFIP:TFA (75:25 mol%)], we could fully eliminate the formation of the most stable α-phase and create metastable γ-phase nylon-11 fibers. The electrospun γ-phase nylon-11 fibrous membrane displayed a typical piezoelectric response when it experienced a periodic external force. Furthermore, the γ-phase nylon-11 fibrous membrane exhibited higher thermal stability, electrolyte wettability, and ionic conductivity than the conventional Celgard separator. Consequently, it achieved decent performance when applied as a separator in a sodium metal half-cell. This study indicates that piezoelectric γ-phase nylon-11 fibrous membranes have great potential for further development in energy harvesting and storage, especially as piezo-separators in self-charging power cells.

A chemically defined patterned surface was created via a combined process of controlled evaporative self-assembly of concentric polymer stripes and the selective surface modification of polymer brush. The former process involved physical adsorption of poly (methyl methacrylate) (PMMA) segments into silicon oxide surface, thus forming ultrathin PMMA stripes, whereas the latter process was based on the brush treatment of silicon native oxide surface using a hydroxyl-terminated polystyrene (PS-OH). The resulting alternating PMMA- and PS-rich stripes provided energetically favorable regions for self-assembly of high &chi, polystyrene-block-polydimethylsiloxane (PS-b-PDMS) in a simple and facile manner, dispensing the need for conventional lithography techniques. Subsequently, deep reactive ion etching and oxygen plasma treatment enabled the transition of the PDMS blocks into oxidized groove-shaped nanostructures.

Flexible pressure sensors with a high sensitivity over a broad linear range can simplify wearable sensing systems without additional signal processing for the linear output, enabling device miniaturization and low power consumption. Here, we demonstrate a flexible ferroelectric sensor with ultrahigh pressure sensitivity and linear response over an exceptionally broad pressure range based on the material and structural design of ferroelectric composites with a multilayer interlocked microdome geometry. Due to the stress concentration between interlocked microdome arrays and increased contact area in the multilayer design, the flexible ferroelectric sensors could perceive static/dynamic pressure with high sensitivity (47.7 kPa–1, 1.3 Pa minimum detection). In addition, efficient stress distribution between stacked multilayers enables linear sensing over exceptionally broad pressure range (0.0013–353 kPa) with fast response time (20 ms) and high reliability over 5000 repetitive cycles even at an extremely hi...

The gradient stiffness between stiff epidermis and soft dermis with interlocked microridge structures in human skin induces effective stress transmission to underlying mechanoreceptors for enhanced tactile sensing. Inspired by skin structure and function, we fabricate hierarchical nanoporous and interlocked microridge structured polymers with gradient stiffness for spacer-free, ultrathin, and highly sensitive triboelectric sensors (TESs). The skin-inspired hierarchical polymers with gradient elastic modulus enhance the compressibility and contact areal differences due to effective transmission of the external stress from stiff to soft layers, resulting in highly sensitive TESs capable of detecting human vital signs and voice. In addition, the microridges in the interlocked polymers provide an effective variation of gap distance between interlocked layers without using the bulk spacer and thus facilitate the ultrathin and flexible design of TESs that could be worn on the body and detect a variety of pressing, bending, and twisting motions even in humid and underwater environments. Our TESs exhibit the highest power density (46.7 μW/cm

Nanomembranes and nanowires build tiny, transparent loudspeakers and sensitive, voice-recognition microphones that attach to skin., We demonstrate ultrathin, transparent, and conductive hybrid nanomembranes (NMs) with nanoscale thickness, consisting of an orthogonal silver nanowire array embedded in a polymer matrix. Hybrid NMs significantly enhance the electrical and mechanical properties of ultrathin polymer NMs, which can be intimately attached to human skin. As a proof of concept, we present a skin-attachable NM loudspeaker, which exhibits a significant enhancement in thermoacoustic capabilities without any significant heat loss from the substrate. We also present a wearable transparent NM microphone combined with a micropyramid-patterned polydimethylsiloxane film, which provides excellent acoustic sensing capabilities based on a triboelectric voltage signal. Furthermore, the NM microphone can be used to provide a user interface for a personal voice-based security system in that it can accurately recognize a user’s voice. This study addressed the NM-based conformal electronics required for acoustic device platforms, which could be further expanded for application to conformal wearable sensors and health care devices.

Integration of surface-enhanced Raman scattering (SERS) sensors onto transparent and flexible substrates enables lightweight and deformable SERS sensors which can be wrapped or swabbed on various nonplanar surfaces for the efficient collection and detection of analytes on various surfaces. However, the development of transparent and flexible SERS substrates with high sensitivity is still challenging. Here, we demonstrate a transparent and flexible SERS substrate with high sensitivity based on a polydimethylsiloxane (PDMS) film embedded with gold nanostar (GNS) assemblies. The flexible SERS substrates enable conformal coverage on arbitrary surfaces, and the optical transparency allows light interaction with the underlying contact surface, thereby providing highly sensitive detection of analytes adsorbed on arbitrary metallic and dielectric surfaces which otherwise do not provide any noticeable Raman signals of analytes. In particular, when the flexible SERS substrates are covered onto metallic surfaces, the SERS enhancement is greatly improved because of the additional plasmon couplings between GNS and metal film. We achieve the detection capability of a trace amount of benzenethiol (10

In insect eyes, ommatidia with hierarchical structured cornea play a critical role in amplifying and transferring visual signals to the brain through optic nerves, enabling the perception of various visual signals. Here, inspired by the structure and functions of insect ommatidia, a flexible photoimaging device is reported that can simultaneously detect and record incoming photonic signals by vertically stacking an organic photodiode and resistive memory device. A single-layered, hierarchical multiple-patterned back reflector that can exhibit various plasmonic effects is incorporated into the organic photodiode. The multiple-patterned flexible organic photodiodes exhibit greatly enhanced photoresponsivity due to the increased light absorption in comparison with the flat systems. Moreover, the flexible photoimaging device shows a well-resolved spatiotemporal mapping of optical signals with excellent operational and mechanical stabilities at low driving voltages below half of the flat systems. Theoretical calculation and scanning near-field optical microscopy analyses clearly reveal that multiple-patterned electrodes have much stronger surface plasmon coupling than flat and single-patterned systems. The developed methodology provides a versatile and effective route for realizing high-performance optoelectronic and photonic systems.

A microliter-scale solution process was used to fabricate large-area, uniform films of silver nanowires (AgNWs). These thin films with cross-AgNWs were deposited onto Au substrates by dragging the meniscus of a microliter drop of a coating solution trapped between two plates. The hot spot density was tuned by controlling simple experimental parameters, which changed the optical properties of the resulting films. The cross-AgNW films on the Au surface served as excellent substrates for surface-enhanced Raman spectroscopy, with substantial electromagnetic field enhancement and good reproducibility.

With the increasing demand of cost-effective and high-energy devices, sodium–air (Na–air) batteries have attracted immense interest due to the natural abundance of sodium in contrast to lithium. In particular, an aqueous Na–air battery has fundamental advantage over non-aqueous batteries due to the formation of highly water-soluble discharge product, which improve the overall performance of the system in terms of energy density, cyclic stability and round-trip efficiency. Despite these advantages, the rechargeability of aqueous Na–air batteries has not yet been demonstrated when using non-precious metal catalysts. In this work, we rationally synthesized a binder-free and robust electrode by directly growing urchin-shaped MnO2 nanowires on porous reduced graphene oxide-coated carbon microfiber (MGC) mats and fabricated an aqueous Na–air cell using the MGC as an air electrode to demonstrate the rechargeability of an aqueous Na–air battery. The fabricated aqueous Na–air cell exhibited excellent rechargeability and rate capability with a low overpotential gap (0.7 V) and high round-trip efficiency (81%). We believe that our approach opens a new avenue for synthesizing robust and binder-free electrodes that can be utilized to build not only metal–air batteries but also other energy systems such as supercapacitors, metal–ion batteries and fuel cells. Growing metal nanostructures on graphene-coated electrodes makes batteries that run on salt, water and air more practical to operate. Sodium's abundance gives batteries that use this metal a price advantage over lithium-based cells. Hyunhyub Ko and co-workers in South Korea have developed a way around aqueous sodium-air batteries' Achilles heel – a reliance on precious-metal catalysts for recharging. The team used gas bubbles, which form on a submerged carbon microfibre mat coated with graphene, as templates to transform inexpensive manganese oxide (MnO2) catalysts into ‘sea urchin’-shaped nanostructures. The ion-accessible morphology of the MnO2 urchins, coupled with the highly conductive substrate, boosted the rechargeability and efficiency of a prototype battery. Additionally, this technique's ability to generate catalytic electrodes without binder opens opportunities for devices such as fuel cells that require lightweight materials. We have rationally synthesized a binder-free and robust electrode by directly growing urchin-shaped α-MnO2 nanostructures on porous reduced graphene oxide-coated carbon microfiber (MGC) mats and fabricated an aqueous sodium–air cell using the MGC as an air electrode to demonstrate the rechargeability of an aqueous sodium–air battery. The fabricated aqueous Na–air cell exhibited excellent rechargeability, rate capability and round-trip efficiency.