2D interfacial heterostructures have found an unassailable status in energy storage systems, particularly in supercapacitors citing the intriguing structural and electrochemical characteristics. Exactly a decade ago, MXene, a promising 2D transition metal carbide/nitride/carbonitride was found to possess excellent conductivity, hydrophilicity, laudable charge storage opportunities, and enriched surface functionalities conducive for supercapacitors with inherent challenging shortcomings. To substantially improve, assembled 2D/2D MXene heterostructures exhibit commendable performance backed by the fact of swift increase in research interest. In this review, state‐of‐the‐art research progress in material design and electrochemical performance of 2D/2D MXene heterostructures for supercapacitors are investigated. Discussion is initially on MXene fundamentals including synthesis and energy storage governing properties. Particularly, different preparation including electrostatic assembly, in situ growth, hydrothermal treatment, and objective specific strategies and its implications are elaborated. Especially, the electrochemical interface science, electrode–electrolyte interaction and ion/electron dynamics and synergistic enhancement of MXene/rGO, MXene/LDH, MXene/metal sulfides and timely investigations on other 2D MXene architectures are provided for its compatibility from solid‐state to microsupercapacitors for commerciality. To conclude, a well‐comprehended outlook, key challenges, and prospective research guidelines stretching from fundamental mechanism investigations to material and electrolyte optimizations are presented to encourage advanced 2D MXene architectures for future generation supercapacitors.

Insights into 2D/2D MXene Heterostructures for Improved Synergy in Structure toward Next‐Generation Supercapacitors: A Review

Flexible zinc-ion hybrid capacitors (ZIHCs) based on hydrogel electrolytes are an up-and-coming and highly promising candidate for potential large-scale energy storage due to their combined complementary advantages of zinc batteries and capacitors. However, the freezing induces a sharp drop in conductivity and mechanical property with tremendous compromise of the interfacial adhesion, thereby severely impeding the low-temperature application of such flexible ZIHCs. To achieve the flexible ZIHCs with excellent low-temperature adaptability, an antifreezing and self-adhesive polyzwitterionic hydrogel electrolyte (PZHE) is engineered via a self-catalytic nano-reinforced strategy, affording unparalleled conductivity and robust interfacial adhesion, together with superhigh mechanical strength over a broad temperature ranging from 25 to -60 °C. Meanwhile, the water-in-salt-type PZHE filled with ZnCl2 can provide ion migration channels to enhance the reversibility of Zn metal electrodes, thus greatly reducing side reactions and extending the cycling life. With distinctive integrated merits of the water-in-salt type PZHE, the as-built ZIHCs deliver a high-level energy density of 80.5 Wh kg-1, a desired specific capacity of 81.5 mAh g-1, along with a long-duration cycling lifespan (100 000 cycles) with 84.6% capacity retention at -40 °C, even outperforming the state-of-the-art ZIHCs at room temperature. More encouragingly, the extraordinary temperature-adaptability for both electrochemical and mechanical performance under severe mechanical challenges is achieved for the flexible ZIHCs at extremely low temperature. Noticeably, the ZIHC is also capable of operating in an ice-water bath and vacuum. It is believed that this strategy makes contributions to inspire the design and application of high-performance PZHEs in fields of flexible and wearable electronics that can work in extremely cold environments.

Engineering Self-Adhesive Polyzwitterionic Hydrogel Electrolytes for Flexible Zinc-Ion Hybrid Capacitors with Superior Low-Temperature Adaptability

Ionic Conductive Organohydrogels with Dynamic Pattern Behavior and Multi‐Environmental Stability

Electrochemical supercapacitors process ultra–high power density and long lifetime, but the relatively low energy density hinder the wide application. Therefore, supercapacitors with high energy density and high power density (“dual high”) have attracted great attentions. New active materials with high pseudocapacitance beyond electronic double layer capacitance and novel devices with high working voltage, are two approaches toward “dual high” supercapacitors. In this review, we briefly introduce the history, classification, basic mechanism, and device architecture of supercapacitors. Then, the key issues and design strategies for “dual high” supercapacitors will be proposed. The recent methodical advances will also be highlighted on the material with outstanding specific capacitance and the corresponding electrolyte systems with expanded operating voltage. Besides, the recent progresses of current collectors and separators for supercapacitors are also included. Finally, we will provide our insights regarding the major challenges and prospective solutions in this highly exciting field. We believe such a review will significantly expand the horizons in ever-deepening understanding toward “dual high” supercapacitors 

Materials design and preparation for high energy density and high power density electrochemical supercapacitors

One of the most exciting new developments in energy storage technology is Zn‐ion hybrid supercapacitors (ZHSCs). ZHSCs combine Zn‐ion batteries with supercapacitors (SCs) to address the energy and power needs of portable devices and electric automobiles. Low energy density and the development of cathode material are significant issues for ZHSCs. This review provides an in‐depth investigation of charge storage mechanisms from SCs to ZHSCs. The advantages/disadvantages of ZHSCs, the recent development of cathode materials, and the new design for device fabrications are critically summarized. New cathode materials should be developed to achieve high energy density while preserving the inherent power capability and stability. People increasingly engage with smart electronic and hybrid gadgets, demanding flexible, resilient, and highly safe energy storage devices. ZHSC has emerged as a complete alternative to risky sodium‐ion/lithium‐ion technologies. An overview of all reported carbon‐based, biomass‐derived carbons, metal oxides, MOFs, COFs, MXenes, graphene, and composite materials employed for ZHSCs is comprehensively provided. Furthermore, cathode materials for flexible, micro, wire‐shaped, printed, and photo‐rechargeable ZHSCs are also examined with their practical challenges. This review is anticipated to offer valuable recommendations for designing and manipulating cathode materials for high‐performance ZHSCs to achieve real‐world applications.

Fundamentals and Scientific Challenges in Structural Design of Cathode Materials for Zinc‐Ion Hybrid Supercapacitors

Functional bioelectronic implants require energy storage units as power sources. Current energy storage implants face challenges of balancing factors including high-performance, biocompatibility, conformal adhesion, and mechanical compatibility with soft tissues. An all-hydrogel micro-supercapacitor is presented that is lightweight, thin, stretchable, and wet-adhesive with a high areal capacitance (45.62 F g−1) and energy density (333 μWh cm−2, 4.68 Wh kg−1). The all-hydrogel micro-supercapacitor is composed of polyaniline@reduced graphene oxide/Mxenes gel electrodes and a hydrogel electrolyte, with its interfaces robustly crosslinked, contributing to efficient and stable electrochemical performance. The in vitro and in vivo biocompatibility of the all-hydrogel micro-supercapacitor is evaluated by cardiomyocytes and mice models. The latter is systematically conducted by performing histological, immunostaining, and immunofluorescence analysis after adhering the all-hydrogel micro-supercapacitor implants onto hearts of mice for two weeks. These investigations offer promising energy storage modules for bioelectronics and shed light on future bio-integration of electronic systems.

Biocompatible, High-Performance, Wet-Adhesive, Stretchable All-Hydrogel Supercapacitor Implant Based on PANI@rGO/Mxenes Electrode and Hydrogel Electrolyte

Miniaturized droplet reactors hold great promise for the development of green and sustainable chemistry. However, handling liquids with small volumes, especially viscous ones, in a convenient and loss‐free manner remains a challenge. Here, by electrically controlling the coalescence and mixing of particle‐coated droplets, also known as liquid marbles, an effective microreactor is demonstrated for miniaturized chemical reactions involving viscous reagents. By applying an electric voltage to marbles, the induced electromixing of marble microreactors promotes the reaction rate and the product yield. The advantages of electromixed marble reactors are manifested by a series of chemical reactions between aldehydes and 2‐methylindole in viscous glycerol solution. The electrically‐controlled coalescence and mixing establish liquid marbles as microreactors for rapid, efficient, and miniaturized chemical reactions.

Electrocontrolled Liquid Marbles for Rapid Miniaturized Organic Reactions

Tailoring the electronic conductivity of high-loading cathode electrodes for practical sulfide-based all-solid-state batteries

The emerging human–machine interfacing applications have driven the developments of soft electronic devices that can seamlessly bridge electronics and tissues. Despite impressive progress, mismatches between current bioelectronic devices and biological tissues in both mechanical properties and charge transportation still exist, which could cause severe pain and damage to tissues. Herein, we present an ultra-stretchable all-hydrogel device with custom-designed microchannel patterns perfused with ionic liquids. Hydrophobic ionic liquids (ILs) are sufficiently conductive and remain stably separated from aqueous surroundings both in air and underwater, representing a perfect soft conductor for stretchable electronics. The potential of such designer IL-perfused all-hydrogel devices is demonstrated as soft conductors for triggering light-emitting diodes and as soft sensors for skin-conformed sensing under large stretches. We envision that such soft ionic devices may be useful for stimulating neurons and muscles in vivo with reduced immune responses.

Soft ionic devices by perfusable all-hydrogel microfluidics

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Biocompatible, High-Performance, Wet-Adhesive, Stretchable All-Hydrogel Supercapacitor Implant Based on PANI@rGO/Mxenes Electrode and Hydrogel Electrolyte

Electrocontrolled Liquid Marbles for Rapid Miniaturized Organic Reactions

Tailoring the electronic conductivity of high-loading cathode electrodes for practical sulfide-based all-solid-state batteries

Soft ionic devices by perfusable all-hydrogel microfluidics