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The search for wearable electronics has been attracted great efforts, and there is an ever-growing demand for all-solid-state flexible energy storage devices. However, it is a challenge to obtain both positive and negative electrodes with excellent mechanical strength and match positive and negative charges to achieve high energy densities and operate voltages to satisfy practical application requirements. Here, flexible MXene (Ti3C2Tx)/cellulose nanofiber (CNF) composite film negative electrodes (MCNF) were fabricated with a vacuum filtration method, as well as positive electrodes (CP) by combining polyaniline (PANI) with carbon cloth (CC) using an in-situ polymerization method. Both positive and negative free-standing electrodes exhibited excellent electrochemical behavior and bendable/foldable flexibility. As a result, the all-pseudocapacitance asymmetric device of MCNF//CP assembled with charge-matched between anode and cathode achieves an extended voltage window of 1.5 V, high energy density of 30.6 Wh·kg−1 (1211 W·kg−1), and 86% capacitance retention after 5000 cycles, and the device maintains excellent bendability, simultaneously. This work will pave the way for the development of all-pseudocapacitive asymmetric supercapacitors (ASC) with simultaneously preeminent mechanical properties, high energy density, and wide operating voltage window. [ABSTRACT FROM AUTHOR]

• The single-step synthesis from wheat straw to a porous carbon only required 5 min. • The wheat straw-derived char acted as a good microwave absorber. • The obtained porous carbon possessed a balanced pore distribution. • A high specific capacitance and an excellent rate capability were both achieved. Single-step synthesis of porous carbon (PC) from biomass is a challenge via microwave heating, because biomass rarely absorbs the microwave energy. Herein, wheat-straw-derived char, as a good microwave absorber, was used to achieve rapidly single-step synthesis of PC from an agricultural waste (wheat straw). KOH was used to generate abundant micropores in the PCs. High heating rate caused by microwave heating combined with the pyrolysis gases resulted in the formation of meso-/macropores. A series of post-oxidation reactions between active sites in the PCs and oxygen in the air led to the doping of oxygen-containing chemical groups. Consequently, the obtained PC possessed a high specific surface area of 1905 m2 g−1, a balanced pore distribution with abundant micropores (0.62 cm3 g−1), considerable content of meso-/macropores (0.53 cm3 g−1), and an oxygen-enriched structure (oxygen content up to 21.6%). These characteristics not only contributed to the achievement of a high specific capacitance of 268.5 F g−1 at 0.5 A g−1 for the resultant supercapacitor, but also resulted in an excellent rate capability with a high capacitance retention of 81.2% at 10 A g−1 in a gel electrolyte (polyvinyl alcohol/LiCl). This supercapacitor can extract a high energy density of 21.5 W h kg−1 at 0.5 A g−1 and a high power density of 7.2 kW kg−1 at 10 A g−1. [ABSTRACT FROM AUTHOR]

• Porous carbon was prepared from hydrogel bead for a short duration of 10 min. • Porous carbon was prepared at an air-free atmosphere. • Porous carbon possessed hierarchical and O, N co-doped structure. • Excellent rate capability with a capacitance retention of 87.5% was achieved. As ZnCl 2 is a good microwave absorber, it was usually used as an porogen to produce porous carbon from biomass by microwave heating. In this study, a facile microwave self-activation with the absence of ZnCl 2 was proposed to produce chitosan-hydrogel-bead-based porous carbons (CPC-Air) for a short duration of 10 min. KOH was used as a substitute for the typically used NaOH to form chitosan-based hydrogen beads. During microwave irradiation, KOH acted as a chemical porogen to generate abundant micropores with a pore volume of 0.39 cm3 g−1 in CPC-Air, while water wrapped in hydrogel beads was transformed into steam, serving as a physical porogen to form a considerable content of mesopores (41.8%) and creating a air-free atmosphere. Microwave irradiation led to fast pyrolysis of chitosan, generating meso-/macropores. Combined with self-activation process, it resulted in a high defects degree in CPC-Air. Nitrogen with a high content of 3.4% remained in CPC-Air after self-activation. In addition, a significantly high oxygen content of 17.5% was introduced by the post-oxidation in air atmosphere. Consenquently, the proposed approach can produce a hierarchical and O, N co-doped CPC material with a high defects degree at air-free atmosphere, and with comparable electrochemical properties but with much simpler process and less production duration than the conventional method using ZnCl 2. Particularly, owing to the combined effects of those characteristics in CPCs, an excellent rate capability was achieved in the prepared electrode, demonstrating an ultra-high capacitance retention of 87.5% when the current load was increased from 0.5–10 A g−1. The supercapacitor prepared by this electrode can deliver the energy densities of 17.3–20.6 W h kg−1 and the power densities of 0.5–9.0 kW kg−1. [ABSTRACT FROM AUTHOR]

The demerits of the carbonized eggshell membrane (EM), such as high cost, high brittleness, immutable shape and size, greatly limit its application in demanding supercapacitors as free-standing electrode. Herein, the reconstituted EM (REM) with good flexibility and excellent size-customizability is developed, which is due to their fibrous structure and abundant surface polar groups. Ti 3 C 2 nanosheet (a typical MXene) with ultra-high electrical conductivity and good electrochemical activity is then coated on REM surface, and undergoes a low-temperature carbonization (350 °C) to prepare CREM/T. Multi-functions of Ti 3 C 2 are exhibited: (1) constructing a conductive network on REM surface by randomly stacking to yield a high electrical conductivity of 78.1 S cm−1, (2) being as a protective mold to remain the inherent flexibility and porosity of REM during carbonization, (3) creating nanopores by inducing self-activation, and (4) yielding a large capacitance of 1729 mF cm−2 at 0.5 mA cm−2 and a high rate capability of 82 % after increasing the current density by 50 folds. Furthermore, an all-EM-based supercapacitor is fabricated with REM as the separator and CREM/T as the electrode. It delivers a high energy density of 16.1 μW h cm−2 at 1301 μW cm−2, and shows stable capacitive behaviors during bending. • The reconstitution strategy endows good size-customizability to eggshell membrane. • MXene remains the inherent structure of eggshell membrane during carbonization. • MXene induces a self-activation on eggshell membrane during carbonization. • An all eggshell-membrane-based supercapacitor is fabricated. [ABSTRACT FROM AUTHOR]

Lignocellulose including cellulose, lignin, and hemicellulose could be extracted from wood, and has been used to prepare carbon electrode. However, complicated extraction greatly increases preparation cost. To achieve maximum utilization of lignocellulose and avoid complicated extraction, wood with porous structure and good mechanical strength is used as carbon precursor. Additionally, chemical activation is commonly used to create micropores to provide high capacitance, but it brings in natural structure destruction, and generation of wastewater during pickling. Moreover, to achieve desirable energy density, multi-step strategy with long duration is required for loading active materials on carbonized lignocellulose (CL). Herein, a one-step method is developed to prepare a free-standing hybrid CL electrode (CLE) by using Lewis acid in three aspects: (1) as structure protection agent, (2) as activating agent, (3) as active materials donor, which bypasses pickling and further avoids the generation of wastewater. Additionally, natural vessels in wood can not only provide large space for active materials loading, but also act as rapid ions diffusion way, simultaneously confining active materials detachment. Benefiting from the synergistic effect of porous structure and Lewis acid, this work not only makes full utilization of lignocellulose, but also makes CLE exhibit excellent performance in hybrid oxide supercapacitor. [ABSTRACT FROM AUTHOR]

[Display omitted] • This facile strategy boosts the volumetric capacitance of the electrode by 9 times. • This strategy endows the electrode with stable capacitive behaviors during bending. • This strategy is more advanced as a substitute for designing thick electrodes. Wood-based thick electrodes (>400 μm) always exhibit low volumetric loading of active material and poor flexibility. Herein, a "thick to thin" strategy including densification and low-temperature heating treatment is innovatively proposed and then applied in a MXene (Ti 3 C 2 T x)/wood thick electrode (500 μm) to simultaneously realize large volumetric loading and excellent flexibility. It is found that the densification can significantly increase the volumetric mass loading of Ti 3 C 2 T x by 10 times owing to the great reduction of the electrode thickness (50 μm), and endow the electrode with excellent flexibility by changing porous microstructure to compact microstructure. Furthermore, the low-temperature heating treatment greatly improves the electrochemical performances of the electrode by generating meso -/macropores for ion transport and exposing more active sites to the electrolyte. After conducting our proposed strategy, the optimized electrode exhibits 9 times higher volumetric capacitance (reaching 187F cm−3) than the control electrode, and still has a high rate capability of 77 % when the current density increases by 100 folds. Additionally, the optimized free-standing electrode is used to fabricate a quasi-solid-state supercapacitor which demonstrates stable capacitive behaviors during bending and a large volumetric energy density of 3 mWh cm−3 at 30.6 mW cm−3. [ABSTRACT FROM AUTHOR]

• A highly conductive net is constructed in MXene/PC film by introducing CNT. • The flexibility of MXene/PC film is improved without sacrificing its conductivity. • The flexible supercapacitor can withstand an ultrahigh san rate of 1 V s−1. Porous carbon (PC) can effectively alleviate the typical self-stacking phenomenon of 2D MXene-based films as a spacer, and can easily customize their porous structure. Nevertheless, the contact between 3D PC and 2D MXene flakes is generally presented at a point-to-point form owing to the irregular shape of PC, leading to a low efficiency on electron delivery and stress concentration with a fragile characteristic in the resulting films. Herein, 1D carbon nanotube (CNT) was introduced to construct a highly conductive net structure, tightly anchoring PC on MXene flakes, thus ensuring fast electron delivery by increasing the contact area between MXene and PC. Additionally, the interwoven CNTs bridge the horizontal MXene flakes, making internal structure more integral, thereby enhancing the flexibility. Consequently, the Ti 3 C 2 T x (a typical MXene)/CNT/PC (TCP) film has an ability to bear a large scan rate of 1 V s−1 and shows a high areal specific capacitance of 364.8 mF cm−2 at 0.5 mA cm−2 which remains above 80% even at a high current density of 50 mA cm−2. Furthermore, the fabricated flexible quasi-solid-state supercapacitor (SC) demonstrates a large areal energy density of 10.5 μ Wh cm−2 at 29.8 μ W cm−2. This study provides a promising approach to overcome the poor flexibility of MXene/PC film without sacrificing the conductivity, meanwhile paving a way for the development of flexible SCs with large charge storage capacity and high rate capability. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel coating method with quickness and high uniformity is developed. • A high MXene loading capacity of 6.1 mg cm−2 is achieved in the composite. • A supercapacitor consisting of all clean wipers is fabricated. • The prepared all-solid-state supercapacitor is flexible and stretchable. MXene-loaded textiles can be used as free-standing electrodes for multi-functional supercapacitors (SCs), due to their good conductivity, high electrochemical activity, excellent flexibility, and stretchability. However, the typically self-restacking phenomenon would emerge with the increase in MXene amount, resulting in a significant reduction of the ion-accessible surface areas and active sites of MXene. Herein, a dot-matrix drop-casting method with the advantages of quickness and high uniformity is developed to load high amount (6.1 mg cm−2) of Ti 3 C 2 (a typical MXene) on a clean wiper (CW, a typical textile), and simultaneously to alleviate the resultantly self-restacking phenomenon of Ti 3 C 2. The results show that the obtained composite (CWM) has a high conductivity of 2.3 S cm−1 and large specific capacitance of 282 mF cm−2 at 1 mA cm−2, whereas it remains a high tensile strength of 20.1 MPa and developed porous structure (33.4 m2 g−1). These attractive properties make CWM an ideal free-standing electrode. Further, CW is used as a separator to fabricate an all-CW-derived SC (CWM//CW//CWM), because its porous structure provides abundant routes for facile ion transport. Such a SC has an excellent rate capability with a high capacitance retention of 78.2% from 1 to 20 mA cm−2, large specific capacitance of 118 mF cm−2 at 1 mA cm−2, and high energy density of 10.1 μWh cm−2 at 389.9 μW cm−2. Moreover, an all-solid-state SC (CWM//CWM) is fabricated, and shows an excellent stability on capacitive behaviors during bending (0–120°) or stretching (10–40% strains), manifesting its great potential on wearable and portable electronics. [ABSTRACT FROM AUTHOR]