Poly (3, 4-ethylenedioxythiophene) (denoted PEDOT) already has a brief history of being used as an active material in supercapacitors. It has many advantages such as low-cost, flexibility, and good electrical conductivity and pseudocapacitance. However, the major drawback is low stability, which means an obvious capacitance drop after a certain number of charge–discharge cycles. Another disadvantage is its limited capacitance and this becomes an issue for industrial applications. To solve these problems, there are several approaches including the addition of conducting nanofillers to increase conductivity, and mixing or depositing metal oxide to enhance capacitance. Furthermore, expanding the surface area of PEDOT is one of the main methods to improve its performance in energy storage applications through special processes; for example using a three-dimensional substrate or preparing PEDOT aerogel through freeze drying. This paper reviews recent techniques and outcomes of PEDOT based composites for supercapacitors, as well as detailed calculations about capacitances. Finally, this paper outlines the new direction and recent challenges of PEDOT based composites for supercapacitor applications.
Supercapacitors
Figure. An interdigital microsupercapacitor with electrodes made of carbon onions. Image provided by Vadym Mochalin, Majid Beidaghi, and Yury Gogotsi, Drexel University.
Guest Editors
Liming Dai, Case Western Reserve University, USA
Yury Gogotsi, Drexel University, USA
Husnu Emrah Unalan, Middle East Technical University, Turkey
Scope
The rapid increase in the global energy consumption and the environmental impact of traditional energy resources has led to tremendously increased research activities on clean and renewable energy sources (e.g., solar, wind, water splitting) during the last decade. As these new energy forms are intermittent (e.g., solar and wind) or regionally limited (e.g., water), there is a pressing need to develop advanced energy storage systems, such as supercapacitors, for efficient storage of electrical energy at all scales ranging from solar through wind farms to wearable electronics.
Although the efficiency of energy storage devices depends on a variety of factors, their overall performance strongly relies on the structure and properties of the component materials. The recent development in nanotechnology has opened up new frontiers by creating new materials (e.g., carbon nanotubes, graphene, hexagonal carbides and carbonitrides – MXenes, and perovskites) and structures (e.g., 0D nanoparticles, 1D nanowires, 2D nanosheets, and 3D nanofoams) for efficient energy storage. This Focus Collection is aimed at providing a collection of contributions that describe recent progresses on the development of supercapacitors by nanomaterials design, structural/device engineering, and fundamental understanding of capacitive behaviors at molecular and nano scales to improve the device performance for a large variety of potential applications, including consumer electronics, wearable devices, hybrid electric vehicles, stationary and industrial systems, to name a few.
Topics of interests include, but are not limited to:
Fundamentals of Supercapacitors a) Atomistic and multiscale modelling that enables evaluation/selection and even design of, new materials, new architectures, new processing methods and new energy storage concepts.
b) Fundamental understanding of atomic- and molecular-level processes that govern operation, performance, failure and degradation of the current energy storage materials and devices and charge transfer and storage at electrochemical interfaces at the nanoscale.
c) In situ and in operando studies that include double layer charge storage, pseudocapacitance, asymmetric supercapacitors, nanoscale processes and the development and use of new techniques
Electrode and Electrolyte Materials a) Carbon-based nanomaterials including graphene, carbon nanotubes, nanodiamonds, carbon dots, carbon nitride and nitrogen-doped carbon, BCN, activated carbon as well as other carbon allotropes (onions, nanohorns, porous carbon etc.)
b) Organic and biomaterials including graphene, carbon nanotubes and nanodiamonds
c) Inorganic nanomaterials including metal oxide nanoparticles, nanowires, nanosheets, quantum dots, 2D carbides and carbonitrides - MXenes, carbides and nitrides
d) Organic-inorganic nanohybrids including perovskites, metal organic frameworks (MOFs) and nanocomposites
e) Electrolytes including aqueous electrolytes, organic electrolytes, gel polymer electrolytes and ionic liquids
Structures
Including 0D nanoparticles, 1D nanowires, 2D nanosheets, 3D nanofoams and networks, multi-dimensional assemblies, bio-inspired structures
Applications
Including transport, stationary, flexible, wearable and micro-supercapacitors
Editorial
Topical reviews
Energy conversion and storage devices play an important role in industry and society with the rapid growth of energy consumption. Supercapacitors are very attractive due to their superior power density, fast charge/discharge rates and long cycle lifetime. Graphene fiber (GF), a fascinating material, has drawn considerable attention and shown great potential as an active material in the field of supercapacitors owing to its unique and tunable nanostructure, high electrical conductivity, excellent mechanical flexibility, light weight, and ease of functionalization. This review focuses on the recent significant advances in the fabrication and application of graphene-based fiber as electrode material in supercapacitors. The synthetic strategies and application in the supercapacitor are presented, accompanied with the summary and outlook for the future development of GFs.
Papers
We report a simple and eco-friendly method for the fabrication of a titanium dioxide/functionalized multiwalled carbon nanotube (TiO2/FMWCNT) composite electrode for use in supercapacitors. The nanocomposite electrodes were formed by depositing titanium dioxide onto FMWCNTs using reactive magnetron sputtering, thus providing a green roue for the formation of the binder-free composite electrode. It is shown that the electrochemical performance of the fabricated electrodes can be altered by tuning the thickness of the titanium dioxide overlayer. The integrated nanocomposite electrode showed an improved specific capacitance of 90 Fg−1 in two-electrode configuration.
We report metallic cobalt pyrite (CoS2) nanowires (NWs) prepared directly on current collecting electrodes, e.g., carbon cloth or graphite disc, for high-performance supercapacitors. These CoS2 NWs have a variety of advantages for supercapacitor applications. Because the metallic CoS2 NWs are synthesized directly on the current collector, the good electrical connection enables efficient charge transfer between the active CoS2 materials and the current collector. In addition, the open spaces between the sea urchin structure NWs lead to a large accessible surface area and afford rapid mass transport. Moreover, the robust CoS2 NW structure results in high stability of the active materials during long-term operation. Electrochemical characterization reveals that the CoS2 NWs enable large specific capacitance (828.2 F g−1 at a scan rate of 0.01 V s−1) and excellent long term cycling stability (0–2.5% capacity loss after 4250 cycles at 5 A g−1) for pseudocapacitors. This example of metallic CoS2 NWs for supercapacitor applications expands the opportunities for transition metal sulfide-based nanostructures in emerging energy storage applications.
We designed a nickel-assisted process to obtain graphene with sheet resistance as low as 80 Ω square−1 from silicon carbide films on Si wafers with highly enhanced surface area. The silicon carbide film acts as both a template and source of graphitic carbon, while, simultaneously, the nickel induces porosity on the surface of the film by forming silicides during the annealing process which are subsequently removed. As stand-alone electrodes in supercapacitors, these transfer-free graphene-on-chip samples show a typical double-layer supercapacitive behaviour with gravimetric capacitance of up to 65 F g−1. This work is the first attempt to produce graphene with high surface area from silicon carbide thin films for energy storage at the wafer-level and may open numerous opportunities for on-chip integrated energy storage applications.
Inexpensive MnO2 is a promising material for supercapacitors (SCs), but its application is limited by poor electrical conductivity and low specific surface area. We design and fabricate hierarchical MnO2-based ternary composite nanostructures showing superior electrochemical performance via doping with electrochemically active Fe3O4 in the interior and electrically conductive SnO2 nanoparticles in the surface layer. Optimization composition results in a MnO2–Fe3O4–SnO2 composite electrode material with 5.9 wt.% Fe3O4 and 5.3 wt.% SnO2, leading to a high specific areal capacitance of 1.12 F cm−2 at a scan rate of 5 mV s−1. This is two to three times the values for MnO2-based binary nanostructures at the same scan rate. The low amount of SnO2 almost doubles the capacitance of porous MnO2–Fe3O4 (before SnO2 addition), which is attributed to an improved conductivity and remaining porosity. In addition, the optimal ternary composite has a good rate capability and an excellent cycling performance with stable capacitance retention of ∼90% after 5000 charge/discharge cycles at 7.5 mA cm−2. All-solid-state SCs are assembled with such electrodes using polyvinyl alcohol/Na2SO4 electrolyte. An integrated device made by connecting two identical SCs in series can power a light-emitting diode indicator for more than 10 min.
The growing demand for lightweight and flexible supercapacitor devices necessitates innovation in electrode materials and electrode configuration. We have developed a new type of three-dimensional (3D) flexible nanohybrid electrode by incorporating nanoporous polyaniline (PANI) into layer-by-layer ionic liquid (IL) functionalized carbon nanotube (CNT)–graphene paper (GP), and explored its practical application as a freestanding flexible electrode in a supercapacitor. Our results have demonstrated that the surface modification of graphene nanosheets and CNTs by hydrophilic IL molecules makes graphene and CNTs well-dispersed in aqueous solution, and also improves the hydrophility of the assembled graphene-based paper. Furthermore, the integration of highly conductive one-dimensional (1D) CNTs with two-dimensional (2D) graphene nanosheets leads to 3D sandwich-structured nanohybrid paper with abundant interconnected pores, which is preferred for fast mass and electron transport kinetics. For in situ electropolymerization of PANI on paper electrodes, the IL functionalized CNT–GP (IL–CNT–GP) offers large surface area and interlayer spacing and the unique π surface of graphene and CNTs for efficient and stable loading of PANI. A key finding is that the structural integration of multiple components in this 3D freestanding flexible sheet electrode gives rise to a synergic effect, leading to a high capacitance of 725.6 F g−1 at a current density of 1 A g−1 and good cycling stability by retaining 90% of the initial specific capacitance after 5000 cycles.
A chitosan (CS) based nitrogen doped carbon cryogel with a high specific surface area (SSA) has been directly synthesized via a combined process of freeze-drying and high-temperature carbonization without adding any activation agents. The as-made carbon cryogel demonstrates an SSA up to 1025 m2 g−1 and a high nitrogen content of 5.98 wt%, while its counterpart derived from CS powder only shows an SSA of 26 m2 g−1. Freeze-drying is a determining factor for the formation of carbon cryogel with a high SSA, where the CS powder with a size of ca. 200 μm is transformed into the sheet-shaped cryogel with a thickness of 5–8 μm. The as-made carbon cryogel keeps the sheet-shaped structure and the abundant pores are formed in situ and decorated inside the sheets during carbonization. The carbon cryogel shows significantly enhanced performance as supercapacitor and lithium ion battery electrodes in terms of capacity and rate capability due to its quasi two-dimensional (2D) structure with reduced thickness. The proposed method may provide a simple approach to configure 2D biomass-derived advanced carbon materials for energy storage devices.
We prepared hierarchical Cu2O/CuO/Co3O4 core–shell nanowires (NWs) via a facile chemical deposition method followed by calcination for use as the electrode of supercapacitors. The Cu2O/CuO/Co3O4 electrode showed a specific capacitance of 318 F g−1 at a current density of 0.5 A g−1. 80% of the original specific capacitance was retained after 3000 cycles at a current density of 5 A g−1. An asymmetric supercapacitor cell using Cu2O/CuO/Co3O4 NWs as the positive electrode and activated graphene as the negative electrode exhibited a maximum energy density of 12 Wh kg−1. The electrochemical properties of the electrode were strongly related to the hierarchical nanostructure, which not only provided rich active sites but also shortened ion transport pathways.
Activated carbon (AC) was prepared via carbonizing melaleuca bark in an argon atmosphere at 600 °C followed with KOH activation for high-rate supercapacitors. This AC electrode has a high capacitance of 233 F g−1 at a scan rate of 2 mV s−1 and an excellent rate capability of ∼80% when increasing the sweep rate from 2 to 500 mV s−1. The symmetric supercapacitor assembled by the above electrode can deliver a high energy density of 4.2 Wh kg−1 with a power density of 1500 W kg−1 when operated in the voltage range of 0–1 V in 1 M H2SO4 aqueous electrolyte while maintaining great cycling stability (less than 5% capacitance loss after 10 000 cycles at sweep rate of 100 mV s−1). All the outstanding electrochemical performances make this AC electrode a promising candidate for potential energy storage application.
Anodic composite deposition is demonstrated to be a unique method for fabricating a ternary ruthenium dioxide/reduced graphene oxide/carbon nanotube (RuO2 · xH2O/rGO/CNT, denoted as RGC) nanocomposite onto Ti as an advanced electrode material for supercapacitors. The rGO/CNT composite in RGCs acts as a conductive backbone to facilitate the electron transport between current collector and RuO2 · xH2O nanoparticles (NPs), revealed by the high total specific capacitance (CS,T = 808 F g−1) of RGC without annealing. The contact resistance among RuO2 · xH2O NPs is improved by low-temperature annealing at 150 °C (RGC-150), which renders slight sintering and enhances the specific capacitance of RuO2 · xH2O to achieve 1200 F g−1. The desirable nanocomposite microstructure of RGC-150 builds up the smooth pathways of both protons and electrons to access the active oxy-ruthenium species. This nanocomposite exhibits an extremely high CS,T of 973 F g−1 at 25 mV s−1 (much higher than 435 F g−1 of an annealed RuO2 · xH2O deposit) and good capacitance retention (60.5% with scan rate varying from 5 to 500 mV s−1), revealing an advanced electrode material for high-performance supercapacitors.
Graphene is being widely investigated as a material to replace activated carbon in supercapacitor (electrochemical capacitor) electrodes. Supercapacitors have much higher energy density, but are typically slow devices (∼0.1 Hz) compared to other types of capacitors. Here, top-down semiconductor processing has been applied to graphene-based electrodes in order to fabricate ordered arrays of holes through the graphene electrodes. This is demonstrated to increase the speed of the electrodes by reducing the ionic impedance through the electrode thickness. This approach may also be applicable to speeding up other types of devices, such as batteries and sensors, that use porous electrodes.