The realization of highly flexible and all-solid-state energy-storage devices strongly depends on both the electrical properties and mechanical integrity of the constitutive materials and the controlled assembly of electrode and solid electrolyte. Herein we report the preparation of all-solid-state flexible supercapacitors (SCs) through the easy assembly of functionalized reduced graphene oxide (f-RGO) thin films (as electrode) and solvent-cast Nafion electrolyte membranes (as electrolyte and separator). In particular, the f-RGO-based SCs (f-RGO-SCs) showed a 2-fold higher specific capacitance (118.5 F/g at 1 A/g) and rate capability (90% retention at 30 A/g) compared to those of all-solid-state graphene SCs (62.3 F/g at 1A/g and 48% retention at 30 A/g). As proven by the 4-fold faster relaxation of the f-RGO-SCs than that of the RGO-SCs and more capacitive behavior of the former at the low-frequency region, these results were attributed to the facilitated ionic transport at the electrical double layer by means of the interfacial engineering of RGO by Nafion. Moreover, the superiority of all-solid-state flexible f-RGO-SCs was demonstrated by the good performance durability under the 1000 cycles of charging and discharging due to the mechanical integrity as a consequence of the interconnected networking structures. Therefore, this research provides new insight into the rational design and fabrication of all-solid-state flexible energy-storage devices as well as the fundamental understanding of ion and charge transport at the interface.

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naming of energy storage devices after their own core materials: conventional batteries such as lead-acid batteries, which are named after the lead-based active material and the acidic electrolyte; nickelcadmium batteries named after the nickeland cadmium-based active materials; nickel-metal hybrid batteries named after the nickel-and metal hydride-based active materials; state-of-the-art lithium-ion batteries (LIBs), named after the lithiated host materials and lithium-ion carriers; and next-generation batteries such as sodiumion, lithium-sulfur, and lithium-air batteries, which are named after the lithium metal anode and sulfur or O 2 cathode. [4][5][6][7][8][9] In short, whenever energy storage materials made a breakthrough, new-generation energy storage devices appeared. Among electrochemical energy storage devices, supercapacitors (SCs), which can store charges at the surface, have advantages over LIB and conventional batteries in terms of high power, fast charging/ discharging rates, and long cyclability, as will be discussed in Section 2.1. However, the low energy density of SCs remains a critical challenge for emerging applications such as next-generation electronic systems, electrical vehicles (EVs), and renewable energy storage systems (ESSs). [10,11] Based on the long history of energy storage research, new and emerging materials are expected to provide solutions to this problem.Returning to the chronological development of SCs, SCs have been revolutionized by the emergence of new materials, as Two dimensional (2D) nanomaterials are very attractive due to their unique structural and surface features for energy storage applications. Motivated by the recent pioneering works demonstrating "the emergent pseudocapacitance of 2D nanomaterials," the energy storage and nanoscience communities could revisit bulk layered materials though state-of-the-art nanotechnology such as nanostructuring, nanoarchitecturing, and compositional control. However, no review has focused on the fundamentals, recent progress, and outlook on this new mechanism of 2D nanomaterials yet. In this study, the key aspects of emergent pseudocapacitors based on 2D nanomaterials are comprehensively reviewed, which covers the history, classification, thermodynamic and kinetic aspects, electrochemical characteristics, and design guidelines of materials for extrinsically surface redox and intercalation pseudocapacitors. The structural and compositional controls of graphene and other carbon nanosheets, transition metal oxides and hydroxides, transition metal dichalcogenides, and metal carbide/nitride on both microscopic and macroscopic levels will be particularly addressed, emphasizing the important results published since 2010. Finally, perspectives on the current impediments and future directions of this field are offered. Unlimited combinations and modifications of 2D nanomaterials can provide a rational strategy to overcome intrinsic limitations of existing materials, offering a new-generation energy storage materials toward a high and new p...

2D materials, their features and possible approaches to mitigating the challenges in Li–S batteries are discussed.