Electrochemical capacitors (i.e. supercapacitors) include electrochemical double-layer capacitors that depend on the charge storage of ion adsorption and pseudo-capacitors that are based on charge storage involving fast surface redox reactions. The energy storage capacities of supercapacitors are several orders of magnitude higher than those of conventional dielectric capacitors, but are much lower than those of secondary batteries. They typically have high power density, long cyclic stability and high safety, and thus can be considered as an alternative or complement to rechargeable batteries in applications that require high power delivery or fast energy harvesting. This article reviews the latest progress in supercapacitors in charge storage mechanisms, electrode materials, electrolyte materials, systems, characterization methods, and applications. In particular, the newly developed charge storage mechanism for intercalative pseudocapacitive behaviour, which bridges the gap between battery behaviour and conventional pseudocapacitive behaviour, is also clarified for comparison. Finally, the prospects and challenges associated with supercapacitors in practical applications are also discussed.

Electrochemical capacitors: mechanism, materials, systems, characterization and applications

Supercapacitors have drawn considerable attention in recent years due to their high specific power, long cycle life, and ability to bridge the power/energy gap between conventional capacitors and batteries/fuel cells. Nanostructured electrode materials have demonstrated superior electrochemical properties in producing high-performance supercapacitors. In this review article, we describe the recent progress and advances in designing nanostructured supercapacitor electrode materials based on various dimensions ranging from zero to three. We highlight the effect of nanostructures on the properties of supercapacitors including specific capacitance, rate capability and cycle stability, which may serve as a guideline for the next generation of supercapacitor electrode design.

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Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions

We developed two-step solution-phase reactions to form hybrid materials of Mn3O4 nanoparticles on reduced graphene oxide (RGO) sheets for lithium ion battery applications. Mn3O4 nanoparticles grown selectively on RGO sheets over free particle growth in solution allowed for the electrically insulating Mn3O4 nanoparticles wired up to a current collector through the underlying conducting graphene network. The Mn3O4 nanoparticles formed on RGO show a high specific capacity up to ~900mAh/g near its theoretical capacity with good rate capability and cycling stability, owing to the intimate interactions between the graphene substrates and the Mn3O4 nanoparticles grown atop. The Mn3O4/RGO hybrid could be a promising candidate material for high-capacity, low-cost, and environmentally friendly anode for lithium ion batteries. Our growth-on-graphene approach should offer a new technique for design and synthesis of battery electrodes based on highly insulating materials.

Mn3O4-Graphene Hybrid as a High Capacity Anode Material for Lithium Ion Batteries

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

This report demonstrates a simple, but efficient method to print highly ordered nanopillars without the use of sacrificial templates or any expensive equipment. The printed polymer structure is used as a scaffold to deposit electrode material (manganese dioxide) for making supercapacitors. The simplicity of the fabrication method together with superior power density and energy density make this supercapacitor electrode very attractive for the next-generation energy storage systems.

Highly ordered MnO₂ nanopillars for enhanced supercapacitor performance

DOI: 10.1002/adma.201400440 approach proposed by Simon and Gogotsi is depositing pseudocapacitive material (like MnO 2 ) onto a nanostructured current collector. [ 9 ] Despite some achievements that have been made by using similar methods, such as depositing MnO 2 onto nanowires, [ 10 ] nanotubes, [ 11 ] and nanopillars, [ 12 ] these arrays usually suffer from structural collapse, resulting in a decrease of useful surface area, reduction of deposited active materials and limited accessibility of electrolyte. Furthermore, these nanostructured current collectors are either derived from expensive and environment-unfriendly template methods or tedious fabrication processes. Therefore, it is still a challenge to readily and simply fabricate nanostructured electrodes with large area, template-free, and high aspect ratio arrays without nanostructures clumping together. Here we developed a large area, template-free, high aspect ratio, and freestanding CuO@AuPd@MnO 2 core-shell nanowhiskers (NWs) design. Our electrochemical measurements show that these CuO@AuPd@MnO 2 NWs exhibit remarkable properties including high specifi c capacitance, excellent reversible redox reactions, and fast charge-discharge ability. Moreover, a novel coaxial supercapacitor cable (CSC) design which combines electrical conduction and energy storage by modifying the copper core used for electrical conduction was demonstrated. For accomplishing large surface area necessary for high supercapacitor performance, we developed NWs on the outer surface of the electrical copper wire. The NWs structure is developed by just heating the inner core and therefore is practical to upscale the process to make extended lengths. An attractive advantage of using the coaxial design is that electricity can be conducted through the inner conductive metal wire and electrical energy can be stored in the nanostructured concentric layers added to this inner metal wire with an oxide layer in between. It is always a vital task for many applications including aviation to fi nd better methods to save weight and space, while maintaining the intended purpose. Therefore, integration of electrical cable and energy storage device into one unit offers a very promising opportunity to transmit electricity and store energy at the same time. In addition, CSC built from these NWs exhibits excellent fl exibility and bendability, superior long-term cycle stability, and high power and energy densities. The development of this innovative lightweight, fl exible, and space saving CSC will be very attractive for many applications including hybrid and allelectric vehicles, electric trains, heavy machineries, aircrafts and military. Fabrication of electrode involves three steps: (1) growth of CuO NWs from a pure copper wire by heat treatment; (2) deposition of a conducting metal layer by sputter-coating; (3) electrodeposition of active material onto the nanostructures ( Figure 1 a). Scanning electron microscope (SEM) image (Figure 1 b) clearly Currently, millions of miles of electrical cables have been used for providing electrical connections in machineries, equipment, buildings and other establishments. Energy storage devices are completely separated from these electrical cables if used. However, it will revolutionize energy storage applications if both electrical conduction and energy storage can be integrated into the same cable. Coaxial cable, also called coax, is one of the most common and basic cable designs that is used to carry electricity or signal. It has an inner conductor enclosed by a layer of electrical insulator, and covered by an outer tubular conducting shield (see Supporting Information, SI, Figure S1). The term “coaxial” is used because both the inner and outer conductors share the same geometric axis. In a coaxial design, it is possible to combine two different functional devices into one device that can still perform the original functions independently. Supercapacitors, also known as electrochemical capacitors, have become one of the most popular energy storage devices in recent years. Compared to other energy storage devices like batteries, supercapacitors have faster charge-discharge rates, higher power densities, and longer life times. [ 1 ] As a signature of their performance, safety, and reliability, they have recently been employed in the emergency doors of Airbus A380. Supercapacitors store energy by Faradaic or non-Faradaic reactions. Supercapacitors which utilize the Faradaic reactions are also called pseudocapacitors. Metal oxides/hydroxides like ruthenium oxide (RuO 2 ), [ 2 ] manganese dioxide (MnO 2 ), [ 3 ] cobalt oxide (Co 3 O 4 ), [ 4 ] nickel oxide (NiO), [ 5 ] and nickel hydroxide (Ni(OH) 2 ), [ 6 ] have been extensively studied as electrode materials in pseudocapacitors. Among them, MnO 2 has stood out due to its outstanding characteristics such as high theoretical specifi c capacitance (∼1,400 F g −1 ), natural abundance, and environmental friendliness. [ 7 ] However, the poor electrical conductivity of MnO 2 (∼10 −5 – 10 −6 S cm −1 ) is a limiting factor in achieving its theoretical specifi c capacitance. [ 7b , 8 ] One feasible

Energy Storing Electrical Cables: Integrating Energy Storage and Electrical Conduction

Functionalized graphene aerogel composites for high-performance asymmetric supercapacitors

We demonstrate the design and fabrication of a Ag/PEDOT:PSS/MnO2 layer by layer structure for high performance flexible supercapacitors (SCs). This is a unique design combining Ag-nanowires and PEDOT:PSS-nanopillars as current collectors in SCs. In this sandwich-like structure, electrons can be easily transferred through a networked structure of Ag-nanowires to PEDOT:PSS-nanopillars and finally to the MnO2 layer and vice versa. This scheme provides excellent specific capacitances of 862 F g−1 (based on MnO2) at a current density of 2.5 A g−1 and robust long-term cycling stability. Moreover, SCs fabricated based on this structure exhibit exceptional flexibility and bendability (less than 2% loss in specific capacitance even after 100 bends) and high energy and power densities. All wet processing methods of fabrication along with superior performance make these SCs very attractive for the next generation flexible energy storage systems.

Zenan Yu

Papers

Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions

Highly ordered MnO₂ nanopillars for enhanced supercapacitor performance

Energy Storing Electrical Cables: Integrating Energy Storage and Electrical Conduction

Functionalized graphene aerogel composites for high-performance asymmetric supercapacitors

Flexible, sandwich-like Ag-nanowire/PEDOT:PSS-nanopillar/MnO2 high performance supercapacitors