such as increasing the mechanical flexibility, [3,4] sensitivity [5,6] and accuracy, [7] lowering down the operating voltage, [8,9] response speed, [1,10,11] etc. To realize the close-fitting to human bodies and the long-term monitoring, mechanical flexibility has become one of the most critical properties for wearable devices [12] and on-skin sensors. [13][14][15][16] Previously, conventional flexible devices usually applied thin polymer films as the substrates, such as poly(ethylene terephthalate), [17][18][19][20] poly(d imethylsiloxane), [21] and polyimide. [9] It is noted that some problems exist for such polymer-based substrates. For instance, the aforementioned polymers are nondegradable so that they would give rise to large amount of electronic waste. Besides, compared with the natural skin which serves as the transportation routine for nutrients and body fluids, most of the polymer-based substrates are neither biocompatible nor air/water permeable, suggesting they would damage the skin when continuously paste such polymer-based substrates on skin.In this regard, it is in great demand for developing advanced substrate materials which maintain good biocompatibility, tight adherence to biological tissues, and ideal air/water permeability. Recently, owing to the superior biocompatibility and relatively low cost, silk fibroin (SF) has become one of the most promising candidates for the future flexible substrate materials which display both high dielectric property and excellent mechanical compliance. [5,22,23] Moreover, the excellent air permeability of SF films (in the wet state), which is even comparable to that of natural human skin, also triggers their potential for application in artificial skin systems. [13,23] Despite the superior natural biodegradability and biocompatibility, the development of SF-based on-skin electronics is still at a preliminary stage due to the following reasons: First, the intrinsic brittleness and poor chemical stability of SF films prevent the fabrication of SF-based electronics through traditional techniques. Although the mechanical/chemical stability can be improved to a certain extent by doping polyurethane, [24] polyvinylalcohol (PVA), [25,26] glycerol, [27] or metal ions, [28] it is still far from the demand of real applications. Second, except for mechanical/chemical stability, the SF film served as the substrate is also supposed to be stretchable, so that they can be comfortably cling to skin. [29][30][31][32][33] Similarly, the conducting film on SF substrate should also be Due to the natural biodegradability and biocompatibility, silk fibroin (SF) is one of the ideal platforms for on-skin and implantable electronic devices. However, the development of SF-based electronics is still at a preliminary stage due to the SF film intrinsic brittleness as well as the solubility in water, which prevent the fabrication of SF-based electronics through traditional techniques. In this article, a flexible and stretchable silver nanofibers (Ag NFs)/SF based electrode is synthesized th...

Water electrolysis offers a promising green technology to tackle the global energy and environmental crisis, but its efficiency is greatly limited by the sluggish reaction kinetics of both the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER). In this work, by growing amorphous multi‐transition‐metal (cobalt and iron) oxide on two‐dimensional (2D) black phosphorus (BP), we develop a bifunctional electrocatalyst (CoFeO@BP), which is able to efficiently catalyze both HER and OER. The overpotentials for the hybrid CoFeO@BP catalyst to reach a current density of 10 mA cm−2 in 1 m KOH are 88 and 266 mV for HER and OER, respectively. Based on a series of ex‐situ and in situ investigations, the excellent catalytic performance of CoFeO@BP is found to result from the adaptive surface structure under reduction and oxidation potentials. CoFeO@BP can be transformed to CoFe phosphide under reduction potential, in situ generating the real active catalyst for HER.

Although challenging, fabrication of porous conducting polymeric materials with excellent electronic properties is crucial for many applications. We developed a fast in situ polymerization approach to pure polyaniline (PANI) hydrogels, with vanadium pentoxide hydrate nanowires as both the oxidant and sacrifice template. A network comprised of ultrathin PANI nanofibers was generated during the in situ polymerization, and the large aspect ratio of these PANI nanofibers allowed the formation of hydrogels at a low solid content of 1.03 wt %. Owing to the ultrathin fibril structure, PANI hydrogels functioning as a supercapacitor electrode display a high specific capacitance of 636 F g, a rate capability, and good cycling stability (∼83% capacitance retention after 10,000 cycles). This method was also extended to the preparation of polypyrrole and poly(3,4-ethylenedioxythiophene) hydrogels. This template polymerization method represents a rational strategy for design of conducing polymer networks, which can be readily integrated in high-performance devices or a further platform for functional composites.

Designing multi‐functional separators is one of the effective strategies for achieving high‐performance lithium–sulfur (Li–S) batteries. In this work, polyaniline (PANI) encapsulated amorphous vanadium pentoxide (V2O5) nanowires (general formula V2O5·nH2O and abbreviated as VOH) are synthesized by a facile in situ chemical oxidative polymerization method, and utilized as a basic building block for the preparation of functional interlayers on the commercial polypropylene (PP) separator, generating a VOH@PANI‐PP separator with multi‐functionalities. Compared to the crystalline V2O5, the amorphous V2O5 shows enhanced properties of polysulfide adsorption, catalytic activity, as well as ionic conductivity. Therefore, within the VOH@PANI‐PP separator, the amorphous V2O5 nanowire component contributes to the strong adsorption of polysulfides, the high catalytic activity for polysulfides conversion, and the high ionic conductivity. The PANI component further strengthens the above effects, improves the electrical conductivity, and enhances the flexibility of the modified separator. Benefiting from the synergistic effects, the VOH@PANI‐PP separator effectively suppresses polysulfide shuttling and improves the cycling stability of its composed Li–S batteries. This work provides a new research strategy for the development of efficient separators in rechargeable batteries by judiciously integrating the amorphous metal oxide with a conductive polymer.

Due to the obvious advantages of utilizing naturally abundant and low cost sodium resources, sodium ion batteries (SIBs) show great potential for large‐scale energy storage applications. And the high theoretical capacities of transition metal sulfides (TMSs) make them appealing anode materials for SIBs; however, structural collapse caused by the severe volume change during de/sodiation processes results in poor capacity retention and rate capabilities. Compared to the development of new materials and the improvement of their electrochemical performance, the studies on their reaction mechanisms are still rare, especially the operando characterizations. Herein, the synthesis, anode application, and the operando observation of the de/sodiation mechanism of a nitrogen‐doped porous carbon coated nickel cobalt bimetallic sulfide hollow nanocube ((Ni0.5Co0.5)9S8@NC) composite are reported. Such a material is synthesized via facile sulfidation of phenol formaldehyde coated Ni3[Co(CN)6]2 metal–organic framework precursors with Na2S followed by calcination. The nanocomposite displays a remarkable specific capacity of 752 mAh g−1 at 100 mA g−1 after 100 cycles and outstanding rate capability due to the synergistic effect of several appealing features. Particularly, the pseudocapacitive effect appears to substantially contribute to the sodium storage capability. Operando X‐ray diffraction reveals the conversion reaction mechanism of (Ni0.5Co0.5)9S8@NC, forming Ni, Co, Na2S, and Na2S5.