Notably, many significant breakthroughs for a new generation of supercapacitors have been reported in recent years, related to theoretical understanding, material synthesis and device designs. Herein, we summarize the state-of-the-art progress toward mechanisms, new materials, and novel device designs for supercapacitors. Firstly, fundamental understanding of the mechanism is mainly focused on the relationship between the structural properties of electrode materials and their electrochemical performances based on some in situ characterization techniques and simulations. Secondly, some emerging electrode materials are discussed, including metal–organic frameworks (MOFs), covalent organic frameworks (COFs), MXenes, metal nitrides, black phosphorus, LaMnO3, and RbAg4I5/graphite. Thirdly, the device innovations for the next generation of supercapacitors are provided successively, mainly emphasizing flow supercapacitors, alternating current (AC) line-filtering supercapacitors, redox electrolyte enhanced supercapacitors, metal ion hybrid supercapacitors, micro-supercapacitors (fiber, plane and three-dimensional) and multifunctional supercapacitors including electrochromic supercapacitors, self-healing supercapacitors, piezoelectric supercapacitors, shape-memory supercapacitors, thermal self-protective supercapacitors, thermal self-charging supercapacitors, and photo self-charging supercapacitors. Finally, the future developments and key technical challenges are highlighted regarding further research in this thriving field.

Latest advances in supercapacitors: from new electrode materials to novel device designs.

Flexible solid-state supercapacitors (FSSCs) are frontrunners in energy storage device technology and have attracted extensive attention owing to recent significant breakthroughs in modern wearable electronics In this study, we review the state-of-the-art advancements in FSSCs to provide new insights on mechanisms, emerging electrode materials, flexible gel electrolytes and novel cell designs The review begins with a brief introduction on the fundamental understanding of charge storage mechanisms based on the structural properties of electrode materials The next sections briefly summarise the latest progress in flexible electrodes (ie, freestanding and substrate-supported, including textile, paper, metal foil/wire and polymer-based substrates) and flexible gel electrolytes (ie, aqueous, organic, ionic liquids and redox-active gels) Subsequently, a comprehensive summary of FSSC cell designs introduces some emerging electrode materials, including MXenes, metal nitrides, metal–organic frameworks (MOFs), polyoxometalates (POMs) and black phosphorus Some potential practical applications, such as the development of piezoelectric, photo-, shape-memory, self-healing, electrochromic and integrated sensor-supercapacitors are also discussed The final section highlights current challenges and future perspectives on research in this thriving field

Towards flexible solid-state supercapacitors for smart and wearable electronics

Supercapacitors have gained a lot of attention due to their unique features like high power, long cycle life and environment-friendly nature. They act as a link for energy-power difference between a traditional capacitor (having high power) and fuel cells/batteries (having high energy storage). In this perspective, a worldwide research has been reported to address this and rapid progress has been achieved in the advancement of fundamental as well as the applied aspects of supercapacitors. Here, a concise description of technologies and working principles of different materials utilized for supercapacitors has been provided. The main focus has been on materials like carbon-based nanomaterials, metal oxides, conducting polymers and their nanocomposites along with some novel materials like metal-organic frameworks, MXenes, metal nitrides, covalent organic frameworks and black phosphorus. The performance of nanocomposites has been analysed by parameters like energy, capacitance, power, cyclic performance and rate capability. Some of the latest supercapacitors such as electrochromic supercapacitor, battery-supercapacitor hybrid device, electrochemical flow capacitor, alternating current line filtering capacitor, micro-supercapacitor, photo-supercapacitor, thermally chargeable supercapacitor, self-healing supercapacitor, piezoelectric and shape memory supercapacitor have also been discussed. This review covers the up-to-date progress achieved in novel materials for supercapacitor electrodes. The latest fabricated symmetric/asymmetric supercapacitors have also been reported.

Review of supercapacitors: Materials and devices

Materials research has driven the development of modern nano-electronic devices. In particular, research in magnetic thin films has revolutionized the development of spintronic devices1,2 because identifying new magnetic materials is key to better device performance and design. Van der Waals crystals retain their chemical stability and structural integrity down to the monolayer and, being atomically thin, are readily tuned by various kinds of gate modulation3,4. Recent experiments have demonstrated that it is possible to obtain two-dimensional ferromagnetic order in insulating Cr2Ge2Te6 (ref. 5) and CrI3 (ref. 6) at low temperatures. Here we develop a device fabrication technique and isolate monolayers from the layered metallic magnet Fe3GeTe2 to study magnetotransport. We find that the itinerant ferromagnetism persists in Fe3GeTe2 down to the monolayer with an out-of-plane magnetocrystalline anisotropy. The ferromagnetic transition temperature, Tc, is suppressed relative to the bulk Tc of 205 kelvin in pristine Fe3GeTe2 thin flakes. An ionic gate, however, raises Tc to room temperature, much higher than the bulk Tc. The gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2 opens up opportunities for potential voltage-controlled magnetoelectronics7-11 based on atomically thin van der Waals crystals.

Gate-tunable Room-temperature Ferromagnetism in Two-dimensional Fe 3 GeTe 2

Silk fibroin biomaterials for tissue regenerations.

Conducting polymer micro-supercapacitors for flexible energy storage and Ac line-filtering

The recent discovery of nanoelectronics memristor devices has opened up a new wave of enthusiasm and optimism in revolutionizing electronic circuit design, marking the beginning of new era for the advancement of neuromorphic, high-density logic and memory applications. Here a highly non-linear dynamic response of a bio-memristor is demonstrated using natural silk cocoon fibroin protein of silkworm, Bombyx mori. A film that is transparent across most of the visible spectrum is obtained with the electronic-grade silk fibroin aqueous solution of ca. 2% (wt/v). Bipolar memristive switching is demonstrated; the switching mechanism is confirmed to be the filamentary switching as observed by probing local conduction behavior at nanoscale using scanning tunneling microscopy. The memristive transition is elucidated by a physical model based on the carrier trapping or detrapping in silk fibroin films and this appears to be due to oxidation and reduction procedures, as evidenced from cyclic voltammetry measurements. Hence, silk fibroin protein could be used as a biomaterial for bio-memristor devices for applications in advanced bio-inspired very large scale integration circuit design as well as in biologically inspired synapse links for energy-efficient neuromorphic computing.

A Natural Silk Fibroin Protein-Based Transparent Bio-Memristor

Natural silk fibroin protein has been used for the fabrication of bio-memristors The origin of the resistive switching characteristics is investigated in detail The conduction mechanism in silk protein is found to be controlled by multi-charge transport mechanisms along with filamentary type conduction process The redox process via hopping mechanism is found to be responsible for charge transport in silk protein The low-frequency noise (LFN) study reveals that the fluctuation in the current level is due to the traps existing near the conducting filaments (CFs) It is shown for the first time that the amide I region is mainly responsible for the conductivity change in silk fibroin protein-based bio-memristors under appropriate bias (polarity and magnitude) condition

Resistive switching in natural silk fibroin protein‐based bio‐memristors

The first successful fabrication of microsupercapacitors (μ-SCs) using fractal electrode designs is reported. Using sputtered anhydrous RuO2 thin-film electrodes as prototypes, μ-SCs are fabricated using Hilbert, Peano, and Moore fractal designs, and their performance is compared to conventional interdigital electrode structures. Microsupercapacitor performance, including energy density, areal and volumetric capacitances, changes with fractal electrode geometry. Specifically, the μ-SCs based on the Moore design show a 32% enhancement in energy density compared to conventional interdigital structures, when compared at the same power density and using the same thin-film RuO2 electrodes. The energy density of the Moore design is 23.2 mWh cm−3 at a volumetric power density of 769 mW cm−3. In contrast, the interdigital design shows an energy density of only 17.5 mWh cm−3 at the same power density. We show that active electrode surface area cannot alone explain the increase in capacitance and energy density. We propose that the increase in electrical lines of force, due to edging effects in the fractal electrodes, also contribute to the higher capacitance. This study shows that electrode fractal design is a viable strategy for improving the performance of integrated μ-SCs that use thin-film electrodes at no extra processing or fabrication cost.

Mrinal K. Hota

Papers

Conducting polymer micro-supercapacitors for flexible energy storage and Ac line-filtering

A Natural Silk Fibroin Protein-Based Transparent Bio-Memristor

Resistive switching in natural silk fibroin protein‐based bio‐memristors

Fractal Electrochemical Microsupercapacitors

P-Type SnO Thin Film Phototransistor with Perovskite-Mediated Photogating