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

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

Carbon materials for electrochemical capacitors

In pursuing higher energy density with no sacrifice of power density, a supercapacitor-battery hybrid energy storage device—combining an electrochemical double layer capacitance (EDLC) type positive electrode with a Li-ion battery type negative electrode—has been designed and fabricated. Graphene is introduced to both electrodes: an Fe3O4/graphene (Fe3O4/G) nanocomposite with high specific capacity as negative electrode material, and a graphene-based three-dimensional porous carbon material (3DGraphene) with high surface area (∼3355 m2 g−1) as positive electrode material. The Fe3O4/G nanocomposite shows a high reversible specific capacity exceeding 1000 mA h g−1 at 90 mA g−1 and remaining at 704 mA h g−1 at 2700 mA g−1, as well as excellent rate capability and improved cycle stability. Meanwhile the 3DGraphene positive electrode also displays great electrochemical performance. With these two graphene-enhanced electrode materials and using the best recommended industry evaluation method, the hybrid supercapacitor Fe3O4/G//3DGraphene demonstrates an ultrahigh energy density of 147 W h kg−1 (power density of 150 W kg−1), which also remains of 86 W h kg−1 even at high power density of 2587 W kg−1, so far the highest value of the reported hybrid supercapacitors. Furthermore, the energy density of the hybrid supercapacitor is comparable to lithium ion batteries, and the power density also reaches that of symmetric supercapacitors, indicating that the hybrid supercapacitor could be a very promising novel energy storage system for fast and efficient energy storage in the future.

A high-performance supercapacitor-battery hybrid energy storage device based on graphene-enhanced electrode materials with ultrahigh energy density

The demand for advanced energy storage devices such as supercapacitors and lithium-ion batteries has been increasing to meet the application requirements of hybrid vehicles and renewable energy systems. A major limitation of state-of-art supercapacitors lies in their relatively low energy density compared with lithium batteries although they have superior power density and cycle life. Here, we report an additive-free, nano-architectured nickel hydroxide/carbon nanotube (Ni(OH)2/CNT) electrode for high energy density supercapacitors prepared by a facile two-step fabrication method. This Ni(OH)2/CNT electrode consists of a thick layer of conformable Ni(OH)2 nano-flakes on CNT bundles directly grown on Ni foams (NFs) with a very high areal mass loading of 4.85 mg cm−2 for Ni(OH)2. Our Ni(OH)2/CNT/NF electrode demonstrates the highest specific capacitance of 3300 F g−1 and highest areal capacitance of 16 F cm−2, to the best of our knowledge. An asymmetric supercapacitor using the Ni(OH)2/CNT/NF electrode as the anode assembled with an activated carbon (AC) cathode can achieve a high cell voltage of 1.8 V and an energy density up to 50.6 Wh/kg, over 10 times higher than that of traditional electrochemical double-layer capacitors (EDLCs).

John Gural

Papers

Characteristics and performance of 500 F asymmetric hybrid advanced supercapacitor prototypes