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MXenes have shown great potential as an emerging two-dimensional (2D) material for micro-supercapacitors (MSCs) due to their high conductivity, rich surface chemistry, and high capacity. However, MXene sheets inherently tend to lay flat on the substrate during film formation to assemble into compact stacked structures, which hinders ion accessibility and prolongs ion transport paths, leading to highly dependent electrochemical properties on the thickness of the film. Here, we demonstrate a vertically aligned Ti3C2Tx MXene based micro-supercapacitor with an excellent electrochemical performance by a liquid nitrogen-assisted freeze-drying method. The vertical arrangement of the 2D MXene sheets allows for directional ion transport, enabling the vertical-MXene based MSCs to exhibit thickness-independent electrochemical properties even in thick films. In addition, the MSCs displayed a high areal capacitance of 87 mF cm−2 at 10 mV s−1 along with an excellent stability of ∼87.4% after 10 000 charge–discharge cycles. Furthermore, the vertical-MXene approach proposed here is scalable and can be extended to other systems involving directional transport. [ABSTRACT FROM AUTHOR]

Abstract: Approaching state‐of‐the‐art areal and volumetric capacitances while maintaining high‐power characteristic is a big challenge that promotes practical application of flexible solid‐state micro‐supercapacitors (MSCs), which have recently attracted great attention with the rapid development of flexible microelectronics. Herein, it is reported that freestanding extrahigh conductive Ti3C2Tx (MXene) films with excellent flexibility and effectively controlled thickness ranging from 1–21 µm performed as excellently scalable and flexible solid‐state MSCs owing to their ultrahigh underlying electrical conductivity (up to 1.25 × 105 S m−1) and self‐functionalized surfaces (O, OH, and F terminations). Amazingly, freestanding conductive Ti3C2Tx based flexible solid‐state MSCs with interdigital electrodes and polyvinyl alcohol/sulfuric acid (PVA/H2SO4) gel electrolyte display outstanding areal capacitances of 340 mF cm−2 at 0.25 mA cm−2 based on the two working electrodes. Moreover, the maximum corresponding volumetric capacitance and energy density of flexible solid‐state MSCs reach up to 183 F cm−3 and 12.4 mWh cm−3, which is on the topmost level among all the unconventional supercapacitors to date. Compared with materials currently used in MSCs, this freestanding conductive Ti3C2Tx shows potential and scalability in increasing overall micro‐supercapacitor performance, which evidently sheds light on promising application of freestanding conductive MXenes for next‐generation flexible, portable, and integrated MSCs. [ABSTRACT FROM AUTHOR]

The low‐cost processing and uniquely structured design of conducting polymer hydrogels (CPHs), with the advantages of both conducting polymers and 3D frameworks, are the keys to their wide commercial application in next‐generation flexible solid supercapacitors. Herein, an all‐sprayable hierarchically nanostructured polyaniline–phytic acid conducting polymer hydrogel (pp‐CPHs) for upgradable art patterns and massively manufactured supercapacitor electrodes is presented. Ascribing to fully anticipating the efficient electrode–electrolyte interface of 3D porous nanostructures constructed by interconnected nanorods, the pp‐CPH‐based flexible all‐solid‐state supercapacitors exhibit an excellent areal capacitance of 91 mF cm−2 at a current density of 1 mA cm−2 and a drastically improved cycling stability with 93.5% capacitance retention after 10 000 cycles at 5 mA cm−2. In addition, the electrochemical performance of this device can be constantly maintained under various mechanical loadings such as bending and twisting. Also, this device shows a remarkable integration ability from its universal tandem and parallel connections. Unambiguously, these aforementioned merits of pp‐CPH‐based flexible all‐solid‐state supercapacitors make it an ideal maneuver for the massive fabrication of flexible power supply. [ABSTRACT FROM AUTHOR]

The O-N-S co-doped hierarchical porous carbons are prepared by direct pyrolysis of kraft lignin which is the byproduct from papermaking black liquor. The proposed preparation method is extremely facile, green-environmental, and low-cost without any additional activating agents, additives or templates. The kraft lignin-derived carbon materials possess large specific surface areas (338−1307 m2 g−1), hierarchical porous structures and abundant multi-heteroatoms co-doping (9.84–19.91 wt%). Benefiting from above synergistic advantages, the as-fabricated symmetric supercapacitor in aqueous electrolyte delivers a high specific capacitance of 244.5 F g−1 at 0.2 A g−1, excellent rate-capability (81.8% retention of initial capacitance at 40.0 A g−1), and outstanding cycling stability (91.6% retention over 10000 cycles). Importantly, this device in aqueous electrolyte delivers an energy density of 8.5 W h kg−1 at a power density of 100 W kg−1. Furthermore, a remarkable energy density of 66.8 W h kg−1 at a power density of 1.75 kW kg−1 has been achieved and 32.2 W h kg−1 is still maintained even at an ultrahigh power density of 40.0 kW kg−1 when ionic liquid serves as electrolyte. This study demonstrates the successful conversion of low-valued natural biomass derivative into sustainable high-performance supercapacitor electrode materials with a simple, low-cost, and green-environmental production process. Image 1 [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • RCSSs are rationally synthesized by in-situ conversion with ex-situ assembly method. • RCSSs exhibit excellent rate capability and cycle stability. • High-power density and high energy density are simultaneously achieved. Abstract Rationally designing three-dimensional complex carbon superstructures for energy storage applications offers opportunities to tackle challenges driven by increasingly higher demand of portable energy devices. Herein we report in-situ controlled constructibility and ex-situ confined assembly strategies to enable novel raspberry-like carbon superstructures (RCSSs) for the enhancement of advanced supercapacitors. Through precise control of edge-to-surface ratio and optimization of inner pore structure, the resultant RCSSs show three-dimensional hierarchical porous framework, large specific surface area (SSA, ∼1000 m2 g−1) and high electrical conductivity (∼2700 S m−1), which facilitate ion diffusion and electron transfer. The predesigned RCSSs used as the electrode materials in symmetrical supercapacitors exhibit high rate capability, as indicated by 25,000% increment of the current density only leading to low capacitance degradation of 11.7%, and show long-term cycling stability (98.3% retention after 10,000 cycles) in 6 M KOH electrolyte. Moreover, the RCSSs simultaneously deliver high energy density of 46.5 Wh kg−1 and high power density of 52.5 kW kg−1 in ionic liquid electrolytes. We believe that the combination of in-situ and ex-situ approaches yielded carbon superstructures with complex microstructure and outstanding electrical properties show promising application for universally advanced energy devices with superior power and energy characteristics. [ABSTRACT FROM AUTHOR]

Nickel hydroxide has shown extraordinary promise for hybrid supercapacitors because of the high theoretical capacity and low cost, but is usually limited by its low power density and complex synthetic process. Here, we report the design of a self-assembly three-dimensional (3D) porous Ni(OH) 2 for high-performance energy storage through a simple, cost-effective, and environmentally friendly solvothermal-precipitation method. The 3D structure with interconnected network provides superior electron transportation properties, and the hierarchical porous architecture facilitates quick ion diffusion. The Ni(OH) 2 obtained at 250 °C delivers a high capacitance of 2110 F g −1  at 1.0 A g −1 and a moderate cycling stability with 53% of the maximum specific capacitance over 2000 cycles at 5.0 A g −1 . Additionally, on integration of the prepared Ni(OH) 2 with hierarchically porous carbons as negative electrode material, the hybrid supercapacitor device delivers a high capacitance of 115 F g −1 and a high energy density of 40.9 W h kg −1 at a power density of 405 W kg −1 . This study not only provides a simply controllable growth method of the unique self-assembly 3D porous Ni(OH) 2 with high performance but also opens up a new approach to prepare various metallic oxides/hydroxides with excellent performance for highly promising practical energy-storage systems. [ABSTRACT FROM AUTHOR]

Supercapacitors with ionic liquid (IL) electrolytes can reach high work voltage and accompanied high energy density, which are the critical parameters for supercapacitors’ rapid development. However, supercapacitors with IL electrolytes usually suffer from low power density due to low conductivity, large ionic size and high viscosity of the electrolytes. Herein we reported hierarchically porous sheet-like nanocarbons (HPSNCs) prepared by direct activation of graphene oxide and polytetrafluoroethylene (PTFE) polymer are promising electrode materials for high power supercapacitors with also high energy density. During the activation process, the PTFE particles as a spacer that can effectively hinder the restack of graphene oxide and simultaneously transformed into sheet-like nanocarbons at high temperatures. As a result, the as-prepared samples exhibit highest surface area of ∼2000 m 2 g −1 and largest pore volume of 1.90 cm 3 g −1 . Benefit from hierarchically porous structure from micro-to-macro-pores, which largely shorten the diffusion distance of electrolyte ions, the HPSNC electrodes show a high energy density of 51.7 Wh kg −1 at a power density of 35 kW kg −1 in symmetric supercapacitors with IL electrolyte. In addition, the HPSNC-based supercapacitors also possess an excellent cycling stability with 88% capacitance retention after 5000 cycles. Unambiguously, this work demonstrated the potential of HPSNCs for high power supercapacitors with high energy density and application in integrated energy management electronics. [ABSTRACT FROM AUTHOR]