Graphene derivatives, such as graphene oxide (GO) and reduced graphene oxide (RGO), have been widely used as promising two-dimensional nanoscale building blocks due to their fascinating properties, cost-effective production, and good processability. Understanding the intrinsic self-assembling, colloidal, and rheological features of graphene derivatives is of critical importance to establish the formation-structure-property relationship of graphene-based materials. This article reviews recent progresses in our studies of these interesting properties of graphene derivatives for developing high performance supercapacitors. The content is organized to include characteristics of the dispersions of graphene derivatives, self-assembly of nanosheets from liquid medium, colloidal behavior, rheological properties of the dispersions, processing methods based on the properties, and performance of the fabricated supercapacitors. GO and RGO nanosheets are proved to form different types of assembled structures with unique morphologies, such as ultrathin layer-by-layer films, porous aggregates, and nanoscrolls. The unique rheological properties of GO dispersions and hydrogels, feasible for both the traditional wet-processing and newly-developed technology like three-dimensional printing, are highlighted for their potential in structural manipulation and scalable fabrication of graphene-based devices. The research devoted to up-grading the performance of supercapacitors is presented in some details, which could be applicable for fabricating other graphene-based energy storage devices. Some challenges and perspectives in our point of view are given in the last part of this feature article.

PEDOT, or poly(3,4-ethylenedioxythiophene), is among the most successful conducting polymer products because of its stable conductivity, colloidal processability, and rich assembly behavior. Since the very first patents on PEDOT filed in 1988, the material has been widely explored for decades in many applications. In this review, a comprehensive summary on the synthesis, processing and post-treatment of PEDOT will be presented for the sake of the discussion on PEDOT and its nanocomposites for energy storage. Knowing what PEDOT lends itself to the electrode materials is of importance to the rational design of energy storage devices that maximize the real-world performance. Based on these discussions, a roadmap for the development of PEDOT as promising multifunctional electrode component is presented.

In principle, conjugated polymers can work as electron donors and thus as low-cost p-type organic semiconductors to transport holes in photovoltaic devices. With the booming interests in high-efficiency and low-cost solar cells to tackle global climate change and energy shortage, hole transporting materials (HTMs) based on conjugated polymers have received increasing attention in the past decade. In this perspective, recent advances in HTMs for a range of photovoltaic devices including dye-sensitized solar cells (DSSCs), perovskite solar cells (PSCs), and silicon (Si)/organic heterojunction solar cells (HSCs) are summarized and perspectives on their future development are also presented.

Natural biopolymers feature natural abundance, diverse chemical compositions, tunable properties, easy processability, excellent biocompatibility and biodegradability, as well as nontoxicity, providing new opportunities for the development of flexible sensing and energy devices. Generally, biopolymers are utilized as the passive and active building blocks to endow the flexible devices with mechanical robustness and good biocompatibility. This review aims to provide a comprehensive review on natural biopolymer-based sensing and energy devices. The diverse structures and fabrication processes of three typical biopolymers, including silk, cellulose, and chitin/chitosan, are presented. We review their utilities as the supporting substrates/matrix, active middle layers, separators, electrolytes, and active components of flexible sensing devices (sensors, actuators, transistors) and energy devices (batteries, supercapacitors, triboelectric nanogenerators). Finally, the remaining challenges and future research opportunities are discussed.

Ionic liquids (ILs) have appeared as the most promising electrolytes for lithium-ion batteries, owing to their unique high ionic conductivity, chemical stability and thermal stability properties. Poly(ionic liquid)s (PILs) with both IL-like characteristic and polymer structure are emerging as an alternative of traditional electrolyte. In this review, recent progresses on the applications of IL/PIL-based semi-solid state electrolytes, including gel electrolytes, ionic plastic crystal electrolytes, hybrid electrolytes and single-ion conducting electrolytes for lithium-ion batteries are discussed.

Aqueous zinc-ion batteries, especially Zn-MnO₂ battery, have attracted intensive attention owing to their unique features of high capacity, environmental friendliness, and safety. However, the problem of Mn dissolution hinders the development of zinc-ion batteries with long-term usage and high-rate performance. In this work, a novel preparation method for the polyaniline (PANI)-coated composite aerogel of MnO₂ and rGO (MnO₂/rGO/PANI) electrode is reported. The obtained composite possesses high electrical conductivity, and also effectively suppresses the dissolution of Mn. The fabricated MnO₂/rGO/PANI//Zn battery exhibits a high capacity of 241.1 mAh·g⁻¹ at 0.1 A·g⁻¹, and an excellent capacity retention of 82.7% after 600 charge/discharge cycles. In addition, the rapid diffusion coefficient of the MnO₂/rGO/PANI electrode was further examined by galvanostatic intermittent titration technique. This work provides new insights into the development of high-performance Zn-MnO₂ battery with a better understanding of its diffusion kinetics.

Redox-active organic electrode materials are highly desirable in realizing next-generation all-in-one bendable electronic systems. Herein, a novel flexible supercapacitors (SCs) electrode is fabricated from poly(anthraquinonyl sulfide) (PAQS) and single-walled carbon nanotubes (SWCNTs) suspension by a simple vacuum filtration and named as PAQS-SWCNTs. The PAQS-SWCNTs electrode offered an initial capacitance of 223 F·g⁻¹ and outstanding capacitance retention up to 78.4% after 3 × 10⁴ charge-discharge cycles at 0.5 A·g⁻¹ current density. In a high potential range (0−3 V) and aprotic electrolyte, the PAQS-SWCNTs electrodes in coin cell exhibited an outstanding energy density of 69 Wh·kg⁻¹ at a power density of 90.6 W·kg⁻¹, whereas in the fabricated flexible SCs it retained 63.2 Wh·kg⁻¹. The PAQS-SWCNTs electrodes also showed extraordinary performance at a higher current density (20 A·g⁻¹) and maintained a specific capacitance of 55 and 47 F·g⁻¹ for coin and flexible SCs, respectively. Moreover, the flexible SC is further verified to be able to illuminate up multiple LEDs. These futuristic findings showed that the SCs assembled with flexible PAQS-SWCNTs electrodes have potential application in energy-storage devices and make them highly appealing for future redox supercapacitors.

The development of lightweight, flexible, and stretchable energy storage systems is essential for state-of-the-art electronic devices. We propose a new and broad strategy to fabricate a stretchable and conductive GO/CNTs-TPU fiber electrode by direct wet spinning, from which a flexible fibrous supercapacitor is fabricated. The fibrous electrode exhibits a high strength of 11.68 MPa, high conductivity of 342 S/cm, and high specific capacitances (21.8 mF/cm, 36.45 F/cm³, and 95 F/g). The specific capacitance of the assembled all-solid-state hybrid fiber-shaped supercapacitor reaches 14.3 F/cm³. After 5000 charge-discharge cycles, 97% of the capacitance of the hybrid supercapacitor is maintained. These high-strength electrochemical electrode materials could be potential candidates for applications in practical and large-scale energy storage systems and textile clothes.

An acidic solution based method towards flexible lithium ion battery (LIB) cathodes is developed in this work with perylene diimide (PDI) as the electroactive component and carbon cloth (CC) as the current collector. In this approach, PDI is firstly dispersed in concentrated sulfuric acid (H₂SO₄) and then deposited on CC substrate after the dilution of H₂SO₄, which provides an organic solvent-free strategy to construct integrated LIB cathodes. The acdic solution based fabrication process also allows the facile adjusting of loading amounts of PDI in the cathodes, which can effectively influence the battery performances of the PDI/CC cathodes. As the result, the acidic solution processed PDI/CC cathode can deliver a high specific capacity of ~ 36 mAh·g⁻¹ at the current density of 50 mA·g⁻¹ in both half cell with lithium foil as anode and full cell with pre-lithiated CC as anode. In both types of the batteries, the PDI/CC cathodes show good cycling stabilities by retaining ~ 84% of the initial capacities after 300 charge-discharge cycles at 500 mA·g⁻¹. Additionally, the excellent mechanical stability of the PDI/CC cathode enables the LIBs in pouch cell to maintain the electrochemical performances under various bending states, demonstrating their potentials for flexible LIBs.

Two reported three-dimensional covalent organic frameworks (3D-COFs), COF-300 and COF-301, which have hierarchical porous structures and large pore volumes, were synthesized and employed as host materials for lithium-sulfur batteries. Owing to possessing excellent porosities as well as abundant hydroxyl groups in the pore walls, COF-301 can not only trap lithium polysulfides (PSs) via physical adsorption inside the pores, but also capture PSs by chemical interactions to relieve the shuttle effect. Interestingly, it is the first time that 3D-COFs were utilized as host materials for lithium-sulfur batteries as well as hydroxyl groups were introduced into COFs for improving the battery performance. As a result, COF-301@S as cathode material could reserve the capacity of 411.6 mA·h·g^–1 after 500 cycles with only 0.081% fading per cycle at 0.5 C, exhibiting better battery performance compared with COF-300@S. This study not only expands the applications of 3D-COFs but also provides a new route for designing lithium-sulfur batteries.

An electrochemically stable two-dimensional covalent organic framework, PI-COF, has been synthesized by a scalable solvothermal method. PI-COF possesses a highly crystalline structure, well-defined pores, high specific surface area, and cluster macrostructure. Thanks to these features, PI-COF can work as electrode materials in organic supercapacitors, exhibiting a specific capacitance of 163 F/g at 0.5 A/g over a wide potential window of 0−2.5 V. Moreover, PI-COF shows excellent rate performance, which can deliver 96 F/g even at a high current density of 40 A/g. Because of the high capacitance and wide potential window, PI-COF has achieved a superior energy density of 35.7 W·h/kg at a power density of 250 W/kg. Most importantly, due to the remarkable electrochemical stability, the PI-COF based device shows outstanding cycling stability with 84.1% capacitance maintained (137 F/g) after 3.0 × 10⁴ charged/discharged cycles at 1 A/g. This work should shed light on designing new COF-based electrode materials for supercapacitors and other electrochemical devices.