The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

Electrochemical capacitors, also known as supercapacitors, are becoming increasingly important components in energy storage, although their widespread use has not been attained due to a high cost/performance ratio. Fundamental research is contributing to lowered costs through the engineering of new materials. Currently the most viable materials used in electrochemical capacitors are biomass-derived and polymer-derived activated carbons, although other carbon materials are useful research tools. Metal oxides could result in a step change for electrochemical capacitor technology and is an exciting area of research. The selection of an appropriate electrolyte and electrode structure is fundamental in determining device performance. Although there are still many uncertainties in understanding the underlying mechanisms involved in electrochemical capacitors, genuine progress continues to be made. It is argued that a large, collaborative international research programme is necessary to fully develop the potential of electrochemical capacitors.

Energy storage in electrochemical capacitors: designing functional materials to improve performance

We report a high-performance supercapacitor incorporating a poly(ionic liquid)-modified reduced graphene oxide (PIL:RG-O) electrode and an ionic liquid (IL) electrolyte (specifically, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide or EMIM-NTf2). PIL:RG-O provides enhanced compatibility with the IL electrolyte, thereby increasing the effective electrode surface area accessible to electrolyte ions. The supercapacitor assembled with PIL:RG-O electrode and EMIM-NTf2 electrolyte showed a stable electrochemical response up to 3.5 V operating voltage and was capable of yielding a maximum energy density of 6.5 W·h/kg with a power density of 2.4 kW/kg. These results demonstrate the potential of the PIL:RG-O material as an electrode in high-performance supercapacitors.

High-performance supercapacitors based on poly(ionic liquid)-modified graphene electrodes.

With the booming development of flexible and wearable electronics, their safety issues and operation stabilities have attracted worldwide attentions. Compared with traditional liquid electrolytes, gel polymer electrolytes (GPEs) are preferred due to their higher safety and adaptability to the design of flexible energy storage devices. This review summarizes the recent progress of GPEs with enhanced physicochemical properties and specified functionalities for the application in electrochemical energy storage. Functional GPEs that are capable to achieve unity lithium-ion transference number and offer additional pseudocapacitance to the overall capacitance are carefully discussed. The smart GPEs with self-protection, thermotolerant, and self-healing abilities are particularly highlighted. To close, the future directions and remaining challenges of the GPEs for application in electrochemical energy storages are summarized to provide clues for the following development.

Gel Polymer Electrolytes for Electrochemical Energy Storage

We demonstrate all-solid-state flexible supercapacitors with high physical flexibility, desirable electrochemical properties, and excellent mechanical integrity, which were realized by rationally exploiting unique properties of bacterial nanocellulose, carbon nanotubes, and ionic liquid based polymer gel electrolytes. This deliberate choice and design of main components led to excellent supercapacitor performance such as high tolerance against bending cycles and high capacitance retention over charge/discharge cycles. More specifically, the performance of our supercapacitors was highly retained through 200 bending cycles to a radius of 3 mm. In addition, the supercapacitors showed excellent cyclability with Csp (∼20 mF/cm2) reduction of only <0.5% over 5000 charge/discharge cycles at the current density of 10 A/g. Our demonstration could be an important basis for material design and development of flexible supercapacitors.

All-Solid-State Flexible Supercapacitors Fabricated with Bacterial Nanocellulose Papers, Carbon Nanotubes, and Triblock-Copolymer Ion Gels

Electrolytes are an important component in determining the performance and cycle life of ultracapacitors. Good electrolytes should have high ionic conductivity, large electrochemical windows, and excellent thermal and chemical stability. Ionic liquids are possibly ideal electrolytes to satisfy these requirements. In the present work, we developed easy-to-use methods to incorporate ionic liquids into a polymer matrix to synthesize gel polymer electrolytes (GPEs). Combining the advantages of both the ionic liquids and the conventional GPEs, the resultant solid-state ionic liquid-incorporated gel polymer electrolytes (ILGPEs) possess better properties than these two components considered individually. Our synthesized ILGPE films are flexible and mechanically strong and have a wide electrochemical window of 4 V, a high ionic conductivity up to 3.5 mS/cm (at room temperature), and a high thermal stability up to 350°C. These ILGPEs have been used to fabricate solid-state ultracapacitors that demonstrate improved performance and cycle life.

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Incorporating Ionic Liquid Electrolytes into Polymer Gels for Solid-State Ultracapacitors

CEAE Department, University of Colorado, Boulder, CO 80309, USA * Corresponding author. Email: wenl@adatech.com Electrolytes are an important component in determining the performance and cycle life of ultracapacitors. Good electrolytes should have high conductivity, large electrochemical windows, excellent thermal and chemical stability, and negligible electrolyte depletion. Ionic liquids are ideal electrolytes to satisfy these requirements. In the present work, we developed easy-to-use methods to incorporate ionic liquids into a polymer matrix to synthesize gel polymer electrolytes. The obtained solid-state ionic liquid-incorporated gel polymer electrolytes (ILGPEs) are mechanically strong, electrochemically and thermally stable, and highly conductive. We have used these electrolytes to fabricate solid-state ultracapacitors. Large cell voltage, high performance, and long cycle life have been realized for these ultracapacitors.

Craig Turchi

Papers

Incorporating Ionic Liquid Electrolytes into Polymer Gels for Solid-State Ultracapacitors

Ionic Liquid-Incorporated Gel Polymer Electrolytes for Ultracapacitors