Skin-inspired thermoelectric nanocoating for temperature sensing and fire safety

The promotion mechanism of different nitrogen doping types on the catalytic activity of activated carbon electro-Fenton cathode: Simultaneous promotion of H2O2 generation and phenol degradation ability

Doping with nitrogen atoms can improve the catalytic activity of activated carbon cathodes in electro-Fenton systems, but currently there is a lack of understanding of the catalytic mechanism, which limits the further development of high-performance activated carbon cathodes. Here, a multi-scale exploration was conducted using density functional theory and experimental methods to investigate the mechanism of different nitrogen doping types promoting the redox performance of activated carbon cathodes and the degradation of phenol. The density functional theory results indicate that the introduction of nitrogen atoms enhances the binding ability between carbon substrates and oxygen-containing substances, promotes the localization of surrounding electrons, and makes it easier for O2 to bind with protons and catalyze the hydrogenation reaction of *OOH. Due to its weak binding ability with oxygen-containing substances, AC is difficult to form H2O2, resulting in a tendency towards the 4e-ORR pathway. The binding energy between graphite-N carbon substrate and pyridine-N carbon substrate with *OOH is closer to the volcano top, so graphite n and pyridine n can better promote the selectivity of activated carbon for 2e-ORR. In addition, the calculation results also indicate that pyrrole-N and graphite-N are more capable of catalyzing the reaction energy barrier between ·OH and phenol. Finally, the simulation results were used to guide the modification of nitrogen doped activated carbon and experimental verification was carried out. The degradation results of phenol confirmed the efficient synergistic effect between different types of nitrogen doping, and the NAC-800 electrode exhibited efficient and stable characteristics. This work provides a guiding strategy for further developing stable and highly selective activated carbon cathode materials.

Ultrafast Response and Threshold Adjustable Intelligent Thermoelectric Systems for Next-Generation Self-Powered Remote IoT Fire Warning

Fire warning is vital to human life, economy and ecology. However, the development of effective warning systems faces great challenges of fast response, adjustable threshold and remote detecting. Here, we propose an intelligent self-powered remote IoT fire warning system, by employing single-walled carbon nanotube/titanium carbide thermoelectric composite films. The flexible films, prepared by a convenient solution mixing, display p-type characteristic with excellent high-temperature stability, flame retardancy and TE (power factor of 239.7 ± 15.8 μW m-1 K-2) performances. The comprehensive morphology and structural analyses shed light on the underlying mechanisms. And the assembled TE devices (TEDs) exhibit fast fire warning with adjustable warning threshold voltages (1-10 mV). Excitingly, an ultrafast fire warning response time of ~ 0.1 s at 1 mV threshold voltage is achieved, rivaling many state-of-the-art systems. Furthermore, TE fire warning systems reveal outstanding stability after 50 repeated cycles and desired durability even undergoing 180 days of air exposure. Finally, a TED-based wireless intelligent fire warning system has been developed by coupling an amplifier, analog-to-digital converter and Bluetooth module. By combining TE characteristics, high-temperature stability and flame retardancy with wireless IoT signal transmission, TE-based hybrid system developed here is promising for next-generation self-powered remote IoT fire warning applications.

Thermoelectrics and thermocells for fire warning applications

Historically, fire disasters have killed numerous human lives, and caused tremendous property loss. Fire warning systems play a vital role in predicting fire risks, and are strongly desired to effectively prevent the disaster occurrence and significantly reduce the loss. Among the developed fire warning systems, thermoelectrics (TEs) and thermocells (TECs)-based fire warning materials are extremely important and indispensable in future research, owing to their unique capability of direct conversion between heat and electricity. Here, we present this review of the recent progress of TEs and TECs in fire warning field. Firstly, a brief introduction of existing fire warning systems is provided, including the mechanisms and features of various types. Then, the mechanisms of electronic TE (eTE), ionic TE (iTE) and TEC are elucidated. Next, the basic principles for the material preparation and device fabrication are discussed in their dimension sequence. Subsequently, some important advances or examples of TE fire warnings are highlighted in details. Finally, the challenges and prospects are outlooked.

Temperature-Arousing Self-Powered Fire Warning E-Textile Based on p-n Segment Coaxial Aerogel Fibers for Active Fire Protection in Firefighting Clothing

Firefighting protective clothing is a crucial protective equipment for firefighters to minimize skin burn and ensure safety firefighting operation and rescue mission. A recent increasing concern is to develop self-powered fire warning materials that can be incorporated into the firefighting clothing to achieve active fire protection for firefighters before the protective clothing catches fire on fireground. However, it is still a challenge to facilely design and manufacture thermoelectric (TE) textile (TET)-based fire warning electronics with dynamic surface conformability and breathability. Here, we develop an alternate coaxial wet-spinning strategy to continuously produce alternating p/n-type TE aerogel fibers involving n-type Ti3C2Tx MXene and p-type MXene/SWCNT-COOH as core materials, and tough aramid nanofiber as protective shell, which simultaneously ensure the flexibility and high-efficiency TE power generation. With such alternating p/n-type TE fibers, TET-based self-powered fire warning sensors with high mechanical stability and wearability are successfully fabricated through stitching the alternating p-n segment TE fibers into aramid fabric. The results indicate that TET-based fire warning electronics containing 50 p-n pairs produce the open-circuit voltage of 7.5 mV with a power density of 119.79 nW cm-2 at a temperature difference of 300 °C. The output voltage signal is then calculated as corresponding surface temperature of firefighting clothing based on a linear relationship between TE voltage and temperature. The fire alarm response time and flame-retardant properties are further displayed. Such self-powered fire warning electronics are true textiles that offer breathability and compatibility with body movement, demonstrating their potential application in firefighting clothing.

Intelligent cyclic fire warning sensor based on hybrid PBO nanofiber and montmorillonite nanocomposite papers decorated with phenyltriethoxysilane

An abundance of early warning graphene-based nano-materials and sensors have been developed to avoid and prevent the critical fire risk of combustible materials. However, there are still some limitations that should be addressed, such as the black color, high-cost and single fire warning response of graphene-based fire warning materials. Herein, we report an unexpected montmorillonite (MMT)-based intelligent fire warning materials that have excellent fire cyclic warning performance and reliable flame retardancy. Combining phenyltriethoxysilane (PTES) molecules, poly(p-phenylene benzobisoxazole) nanofiber (PBONF), and layers of MMT to form a silane crosslinked 3D nanonetwork system, the homologous PTES decorated MMT-PBONF nanocomposites are designed and fabricated via a sol-gel process and low temperature self-assembly method. The optimized nanocomposite paper shows good mechanical flexibility (good recovery after kneading or bending process), high tensile strength of ∼81 MPa and good water resistance. Furthermore, the nanocomposite paper exhibits high-temperature flame resistance (almost unchanged structure and size after 120 s combustion), sensitive flame alarm response (∼0.3 s response once exposure onto a flame), cyclic fire warning performance (>40 cycles), and adaptability to complex fire situations (several fire attack and evacuation scenarios), showing promising applications for monitoring the critical fire risk of combustible materials. Therefore, this work paves a rational way for design and fabrication of MMT-based smart fire warning materials that combine excellent flame shielding and sensitive fire alarm functions.

Biomimetic, Mechanically Strong Supramolecular Nanosystem Enabling Solvent Resistance, Reliable Fire Protection and Ultralong Fire Warning

A graphene oxide (GO)-based smart fire alarm sensor (FAS) has gained rapidly increasing research interest in fire safety fields recently. However, it still remains a huge challenge to obtain desirable GO-based FAS materials with integrated performances of mechanical flexibility/robustness, harsh environment-tolerance, high-temperature resistance, and reliable fire warning and protection. In this work, based on bionic design, the supermolecule melamine diborate (M·2B) was combined with GO nanosheets to form supramolecular cross-linking nanosystems, and the corresponding GO-M·2B (GO/MB) hybrid papers with a nacre-like micro/nano structure were successfully fabricated via a gel-dry method. The optimized GO/MB paper exhibits enhanced mechanical properties, e.g., tensile strength and toughness up to ∼122 MPa and ∼1.72 MJ/m³, respectively, which is ∼3.5 and ∼6.6 times higher than those of the GO paper. Besides, it also shows excellent structural stability even under acid/alkaline solution immersion and water bath ultrasonication conditions. Furthermore, due to the presence of promoting reduction effect and atom doping reactions in GO network, the resulting GO/MB network displays exceptional high-temperature resistance, sensitive fire alarm response (∼0.72 s), and ultralong alarming time (>1200 s), showing promising fire safety and protection application prospects as desirable FAS and fire shielding material with excellent comprehensive performances. Therefore, this work provides inspiration for the design and fabrication of high-performance GO-based smart materials that combine fire shielding and alarm functions.

Recent Advances on Early-Stage Fire-Warning Systems: Mechanism, Performance, and Perspective

Early-stage fire-warning systems (EFWSs) have attracted significant attention owing to their superiority in detecting fire situations occurring in the pre-combustion process. Substantial progress on EFWSs has been achieved recently, and they have presented a considerable possibility for more evacuation time to control constant unintentional fire hazards in our daily life. This review mainly makes a comprehensive summary of the current EFWSs, including the working mechanisms and their performance. According to the different working mechanisms, fire alarms can be classified into graphene oxide-based fire alarms, semiconductor-based fire alarms, thermoelectric-based fire alarms, and fire alarms on other working mechanisms. Finally, the challenge and prospect for EFWSs are briefly provided by comparing the art of state of fire alarms. This work can propose a more comprehensive understanding of EFWSs and a guideline for the cutting-edge development direction of EFWSs for readers.

Facile Fabrication of Graphene Oxide Nanoribbon-Based Nanocomposite Papers with Different Oxidation Degrees and Morphologies for Tunable Fire-Warning Response

Smart fire-warning sensors based on graphene oxide (GO) nanomaterials, via monitoring their temperature-responsive resistance transition, have attracted considerable interest for several years. However, an important question remains as to whether or not different oxidation degrees of the GO network can produce different impacts on fire-warning responses. In this study, we synthesized three types of GO nanoribbons (GONRs) with different oxidation degrees and morphologies, and thus prepared flame retardant polyethylene glycol (PEG)/GONR/montmorillonite (MMT) nanocomposite papers via a facile, solvent free, and low-temperature evaporation-induced assembly approach. The results showed that the presence of the GONRs in the PEG/MMT promoted the formation of an interconnected nacre-like layered structure, and that appropriate oxidation of the GONRs provided better reinforcing efficiency and lower creep deformation. Furthermore, the different oxidation degrees of the GONRs produced a tunable flame-detection response, and an ideal fire-warning signal in pre-combustion (e.g., 3, 18, and 33 s at 300 °C for the three PEG/GONR/MMT nanocomposite papers), superior to the previous GONR-based fire-warning materials. Clearly, this work provides a novel strategy for the design and development of smart fire-warning sensors.

Fire Intumescent, High-Temperature Resistant, Mechanically Flexible Graphene Oxide Network for Exceptional Fire Shielding and Ultra-Fast Fire Warning

Smart fire alarm sensor (FAS) materials with mechanically robust, excellent flame retardancy as well as ultra-sensitive temperature-responsive capability are highly attractive platforms for fire safety application. However, most reported FAS materials can hardly provide sensitive, continuous and reliable alarm signal output due to their undesirable temperature-responsive, flame-resistant and mechanical performances. To overcome these hurdles, herein, we utilize the multi-amino molecule, named HCPA, that can serve as triple-roles including cross-linker, fire retardant and reducing agent for decorating graphene oxide (GO) sheets and obtaining the GO/HCPA hybrid networks. Benefiting from the formation of multi-interactions in hybrid network, the optimized GO/HCPA network exhibits significant increment in mechanical strength, e.g., tensile strength and toughness increase of ~ 2.3 and ~ 5.7 times, respectively, compared to the control one. More importantly, based on P and N doping and promoting thermal reduction effect on GO network, the excellent flame retardancy (withstanding ~ 1200 °C flame attack), ultra-fast fire alarm response time (~ 0.6 s) and ultra-long alarming period (> 600 s) are obtained, representing the best comprehensive performance of GO-based FAS counterparts. Furthermore, based on GO/HCPA network, the fireproof coating is constructed and applied in polymer foam and exhibited exceptional fire shielding performance. This work provides a new idea for designing and fabricating desirable FAS materials and fireproof coatings.

An Ultralight Self-Powered Fire Alarm e-Textile Based on Conductive Aerogel Fiber with Repeatable Temperature Monitoring Performance Used in Firefighting Clothing

Firefighting protective clothing is an essential equipment that can protect firefighters from burn injuries during the firefighting process. However, it is still a challenge to detect the damage of firefighting protective clothing at an early stage when firefighters are exposed to excessively high temperature in fire cases. Herein, an ultralight self-powered fire alarm electronic textile (SFA e-textile) based on conductive aerogel fiber that comprises calcium alginate (CA), Fe₃O₄ nanoparticles (Fe₃O₄ NPs), and silver nanowires (Ag NWs) was developed, which achieved ultrasensitive temperature monitoring and energy harvesting in firefighting clothing. The resulting SFA e-textile was integrated into firefighting protective clothing to realize wide-range temperature sensing at 100-400 °C and repeatable fire warning capability, which could timely transmit an alarm signal to the wearer before the firefighting protective clothing malfunctioned in extreme fire environments. In addition, a self-powered fire self-rescue location system was further established based on the SFA e-textile that can help rescuers search and rescue trapped firefighters in fire cases. The power in the self-powered fire location system was offered by an SFA e-textile-based triboelectric nanogenerator (TENG). This work provided a useful design strategy for the preparation of ultralight wearable temperature-monitoring SFA e-textile used in firefighting protective clothing.