Ultralight and compressible carbon materials have promising applications in strain and pressure detection. However, it is still difficult to prepare carbon materials with supercompressibility, elasticity, stable strain-electrical signal response, and ultrasensitive detection limits, due to the challenge in structural regulation. Herein, a new strategy to prepare a reduced graphene oxide (rGO)-based lamellar carbon aerogels with unexpected and integrated performances by designing wave-shape rGO layers and enhancing the interaction among the rGO layers is demonstrated. Addition of cellulose nanocrystalline and low-molecular-weight carbon precursors enhances the interaction among rGO layers and thus produces an ultralight, flexible, and superstable structure. The as-prepared carbon aerogel displays a supercompressibility (undergoing an extreme strain of 99%) and elasticity (100% height retention after 10 000 cycles at a strain of 30%), as well as stable strain-current response (at least 10 000 cycles). Particularly, the carbon aerogel is ultrasensitive for detecting tiny change in strain (0.012%) and pressure (0.25 Pa), which are the lowest detection limits for compressible carbon materials reported in the literature. Moreover, the carbon aerogel exhibits excellent bendable performance and can detect an ultralow bending angle of 0.052°. Additionally, the carbon aerogel also demonstrates its promising application as wearable devices.

Mechanically stable and foldable air cathodes with exceptional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities are key components of wearable metal-air batteries. Herein, we report a directional freeze-casting and annealing approach for the construction of 3D honeycomb nanostructured, N, P-doped carbon aerogel incorporating in situ grown FeP/Fe 2 O 3 nanoparticles as the cathode for flexible Zn-air battery. The aqueous rechargeable Zn-air batteries assembled with this carbon aerogel exhibit a remarkable specific capacity of 648 mAh g -1 at current density of 20 mA cm -2 with good long-term durability, outperforming those assembled with commercial Pt/C+RuO 2 catalyst. Furthermore, such foldable carbon aerogel with directional channels can serve as freestanding air cathode for flexible solid state Zn-air battery without the use of carbon paper/cloth and additives, giving a specific capacity of 676 mA h g -1 and an energy density of 517 W

oxygen evolution reaction or OER) processes. [1][2][3][4][5] To avoid the use of costly noble metal catalysts, nitrogen-doped porous carbon materials are proposed as the electrode materials in these batteries since they can be derived from naturally abundant biomass. The performance of these porous carbon materials as electrodes depends on the chemistry that results in the generation of OER-active pyridinic N and ORR-active quaternary N groups in a high density as well as the porosity of the materials. [6] This is because these factors determine the extent of exposure of the active sites to the relevant chemical species such as O 2 , OH − , and H 2 O and help prevent the rapid clogging of the planar electrode surface. [7][8][9][10] SiO 2 [8,11,12] and Al 2 O 3[13] microbeads have been employed widely as templates for generating nanopores in carbon-based catalysts. However, this requires multiple steps such as the bottom-up synthesis of the catalysts from carbon precursors as well as etching and purification processes to remove the templates, which increases the cost for mass production. [14][15][16][17][18] In addition, porous carbon materials synthesized by bottom-up methods are generally in the powder form and are thus not self-supporting. Thus, the fabrication of air electrodes requires an additional process, wherein the powder carbon materials are electrosprayed onto carbon cloth/paper, which then lead to the inevitable decreases Porous carbon electrodes have emerged as important cathode materials for metal-air battery systems. However, most approaches for fabricating porous carbon electrodes from biomass are highly energy inefficient as they require the breaking down of the biomass and its subsequent reconstitution into powder-like carbon. Here, enzymes are explored to effectively hydrolyze the partial cellulose in bulk raw wood to form a large number of nanopores, which helps to maximally expose the inner parts of the raw wood to sufficiently dope nitrogen onto the carbon skeletons during the subsequent pyrolysis process. The resulting carbons exhibit excellent catalytic activity with respect to the oxygen reduction and oxygen evolution reactions. As-fabricated cellulosedigested, carbonized wood plates are mechanically strong, have high conductivity, and contain a crosslinked network and natural ion-transport channels and can be employed directly as metal-free electrodes without carbon paper, polymer binders, or carbon black. When used as metal-free cathodes in zincair batteries, they result in a specific capacity of 801 mA h g −1 and an energy density of 955 W h kg −1 with the long-term stability of the batteries being as high as 110 h. This work paves the way for the ready conversion of abundant biomass into high-value engineering products for energy-related applications.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201900341.Rechargeable Zn-air batteries have emerged as a promising technology for coping with future energy demands owin...

Lightweight and elastic carbon materials have attracted great interest in pressure sensing and energy storage for wearable devices and electronic skins. Wood is the most abundant renewable resource and offers green and sustainable raw materials for fabricating lightweight carbon materials. Herein, a facile and sustainable strategy is proposed to fabricate a wood-derived elastic carbon aerogel with tracheid-like texture from cellulose nanofibers (CNFs) and lignin. The flexible CNFs entangle and assemble into an interconnected framework, while lignin with high thermal stability and favorable stiffness prevents the framework from severe structural shrinkage during annealing. This strategy leads to an ordered tracheid-like structure and significantly reduces the thermal deformation of the CNFs network, producing a lightweight and elastic carbon aerogel. The wood-derived carbon aerogel exhibits excellent mechanical performance, including high compressibility (up to 95% strain) and fatigue resistance. It also reveals high sensitivity at a wide working pressure range of 0-16.89 kPa and can detect human biosignals accurately. Moreover, the carbon aerogel can be assembled into a flexible and free-standing all-solid-state symmetric supercapacitor that reveals satisfactory electrochemical performance and mechanical flexibility. These features make the wood-derived carbon aerogel highly attractive for pressure sensor and flexible electrode applications.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201910292. important applications in wearable sensors, electronic skins, and flexible energy storage devices. Although carbon aerogels with good mechanical performances can be achieved from nanocarbon unites, their carbon precursors are nonrenewable, and the synthesis process of CNT, graphene, or their aerogels is high-cost and complicated.Considering the natural abundance, renewability, environmental friendliness, and low cost, biomass has been regarded as a renewable and sustainable carbon precursor for fabricating carbon aerogels. Up to now, several biomass-derived carbon aerogels have been successfully developed from gelatin, [16] winter melon, [17] protein, [18] bacterial cellulose, [19] and raw cotton. [20] However, those carbon aerogels show poor compressibility, elasticity, and fatigue resistance owing to the intrinsic random porous architecture and severe volume shrinkage at annealing or carbonization. Wood, as one of the most abundant biomass resources, demonstrates hierarchical tracheid structure that is composed of CNFs and amorphous matrix (lignin and hemicelluloses). [21] Owing to the compact structure (large amounts of additives and various interaction among tracheids or CNFs), natural wood is rigid and the tracheids are hard to be compressed and easily collapsed. Therefore, fabricating compressible and elastic conductive carbon aerogel from original wood tracheids is challenging. To solve this problem, Hu et al. [22] put forward a "top-down" stra...

Compressible and elastic carbon aerogels (CECAs) hold great promise for applications in wearable electronics and electronic skins. MXenes, as new two-dimensional materials with extraordinary properties, are promising materials for piezoresistive sensors. However, the lack of sufficient interaction among MXene nanosheets makes it difficult to employ them to fabricate CECAs. Herein, a lightweight CECA is fabricated by using bacterial cellulose fiber as a nanobinder to connect MXene (Ti3C2) nanosheets into continuous and wave-shaped lamellae. The lamellae are highly flexible and elastic, and the oriented alignment of these lamellae results in a CECA with super compressibility and elasticity. Its ultrahigh structural stability can withstand an extremely high strain of 99% for more than 100 cycles and long-term compression at 50% strain for at least 100 000 cycles. Furthermore, it has a high sensitivity that demonstrates not only an ultrahigh linearity but also a broad working pressure range (0–10 kPa). In particular, the CECA has a high linear sensitivity in almost the entire workable strain range (0–95%). In addition, it has very low detection limits for tiny strain and pressure. These features enable the CECA-based sensor to be a flexible wearable device to monitor both subtle and large biosignals of the human body.