- Email: [email protected]
HISTORY | Electrochemical Capacitors
Electrochemical Capacitors P Kurzweil, University of Applied Sciences, Amberg, Germany & 2009 Elsevier B.V. All rights reserved.
Introduction The terms ‘supercapacitor’ and ‘ultracapacitor’ were coined by Nippon Electric Company (NEC), Japan, and Pinnacle Research Institute, USA, respectively. In technical terminology, it is named as electrochemical double-layer capacitor (EDLC), in contrast to ‘electrolytic capacitor’. Electrochemical capacitors basically differ from ordinary electrostatic capacitors or antiquated electrical ‘condensers’. Historically, supercapacitors are divided into genuine double-layer capacitors that use the high surface area of carbon materials, and faradaic systems that additionally use redox reactions at metal oxides or conductive polymers.
The Discovery of Capacitance at Solution Interfaces Early Insights against a Water Capacitor In the nineteenth century, when Alessandro Volta invented the first battery, static electricity was generated by the rubbing of amber, preferably in the so-called Wimshurst machine (1883) – a rotating circular plate, containing inset sectors of amber-like material, rubbing against contact plates. The principle that electrical charge can be stored in a ‘Leyden jar’ was known since 1745. It was originally developed by Ewald von Kleist and independently invented by Pieter van Musschenbroek: A glass jar was wrapped with silver or tin foil (Figure 1); a salt solution or impure water guaranteed the electrical contact between a conductive metal wire immersed in the jar and the inner glass surface. However, the negligible double-layer capacitance at the metal–solution interface was not recognized at that time. Benjamin Franklin proved in 1749 that the electric charge was rather stored on the facing glass surfaces, and not in the water as was assumed initially. The Leyden jar was charged by connecting two wires – one from the inside electrode and one from the outside foil – to an electrostatic machine or a Volta’s pile. Franklin coined the name ‘battery’ for an electrical combination of Leyden jars, derived from an artillery battery. Later jar designs comprised a top electrode, connected by a metal chain to the inner glass surface, and an external tin foil wrapping; the liquid inside was replaced with a foil lining. The electrostatic capacitor was introduced, and the double-layer capacitor was still waiting to be discovered.
596
All capacitors subsequent to Volta’s ebonite plate capacitor, around the year 1780, consisted of two charge collector plates (electrodes), which were separated by a thin layer of a nonconducting material, the dielectric, in the form of air, a liquid, or a solid film. The term ‘dielectric’ was coined by Michael Faraday who had some of the first correct ideas about polarization in dielectrics and air capacitors. It was found that the dielectric material is essential to reduce the size of a capacitor and to prevent arcing between the plates. All trials to develop a stable ‘water capacitor’ failed owing to the intrinsic conductivity of water. No manufacturer did seriously intend to replace a dielectric by an electrolyte. Around 1850 natural mica, cut in slices, was introduced, although it started being produced commercially only since World War I. Wound-paper capacitors followed in 1876 and ceramic capacitors in 1900. Franklin measured the storage ‘capacity’ of his electrical equipment in ‘jars’, a traditional unit of capacitance equivalent to 1.1 nF. Until 1950, mainly paper capacitors in tube radios were labeled using the CGS units: 1 cm8f4pe0 gF ¼ 1:11 pF. The SI unit of capacitance, farad, named after the British physicist Michael Faraday (1791–1867), is defined as the ability to store 1 C of electric charge per volt of potential difference between the two conductors: 1 F ¼ 1 C V1 ¼ 1 A s V1. In conventional dielectric capacitors, capacitance is measured in microfarads, nanofarads, or sometimes even in picofarads. Electrochemical capacitors tap the full range of up to several thousand farads owing to their extraordinarily high energy density. The term ‘capacity’, used in battery technology, indicates the extent of charge storage Q , in units of coulombs or watt-hours. The Birth of the Electrochemical Capacitor Table 1 lists the chronology of important milestones leading to the birth of the electrochemical capacitor. Around 1840, C. F. Scho¨nbein and W. R. Grove discovered that electrochemical cells comprising two electrodes in an electrolyte show properties of a power source in the voltage range below the decomposition voltage, at which electrolysis takes place. The interface between electronic and ionic conductors has been studied since the nineteenth century, but the electrochemical phenomena occurring outside of batteries have not attracted much interest for electrotechnical applications for a long time. Hermann von Helmholtz formulated his concept of the electrolytic double layer in 1853. Between
History | Electrochemical Capacitors
597
Connection −
+
Conductor
Metal chain Porous graphite electrode Sat. aluminum sulfate solution
Glass Tin foil Electrodes Electrolyte
Filter paper or membrane US 2800616 (1957) H.I. Becker
Leyden jar
+
US 3288641 (1966) R. A. Rightmire
Porous Collector electrode layer
Casing
Porous separator Gasket Carbon electrode
Separator
EP 0712143 (1996) Japan Gore Tex
-
US 3700975 (1972) Bell Telephone Lab
Glassy carbon electrode
Sulfuric acid
Current collector Current collector Seal
Opposing electrode Electrode assembly
Separator
Electrode coating Substrate
Electrode US 3634736 (1972) SOHIO
US 4337501 (1982) Siemens
EP 0449145 (1992) Matsushita
Figure 1 Historical embodiments of electrochemical double-layer capacitors: The prototypes of Becker and Rightmire led to practical coin-shaped and spiral-wound devices. The Leyden jar marks the prototype of all capacitors.
1910 and 1950, the theory of the diffuse double layer and specific adsorption of ions was developed by G. Gouy, D. L. Chapman, O. Stern, A. Frumkin, and D. C. Grahame. Becker’s practical capacitor of 1957, disclosed in a patent placed by General Electric, employed porous carbon electrodes perfused with an aqueous electrolyte. The exceptionally high capacitance was thought to be due to the geometric surface of the carbon pores and the double layer
at the interface between carbon and the solution. The double-layer capacitor was introduced, although the appurtenant US patent title ‘low-voltage electrolytic capacitor’ is a bit misleading. The principle is based on the capacitance at the interface between the electronic conductor and the thereon adsorbed ions of the liquid solution, and does not bear on any electrostatic space charge layer between an oxide dielectric and a metal support.
598
History | Electrochemical Capacitors
Table 1 B1650 1745 1746 1749 1775/1882 1832/1833 1853 1906–1955 1957 1966 1972
1975–1990 1978 1983 1984–1995 1990 1991 1993 2001 2002 2003 2005
Chronology: Discovery and commercialization of the supercapacitor Otto von Guericke: Friction generator, a sulfur ball rotating on a shaft Ewald J. G. von Kleist: ‘Amplification bottle’, a glass jar wrapped with silver foil and filled with water, alcohol, or mercury and a piece of bare wire Pieter van Musschenbroek: ‘Leyden jar’, a glass bottle shrouded by tin foil and filled with a liquid Benjamin Franklin (1706–1790): Experiments with Leyden jars and air dielectric Alessandro Volta (1745–1827): Metal plate capacitor with a dielectric of ebonite (hard rubber made of natural latex and 20–40% sulfur) Michael Faraday (1791–1867): Faraday’s laws H. von Helmholtz: Concept and model of the electrolytic double layer Theory of double layer and electrocapillarity (see Figure 3) H. I. Becker (General Electric): Low-voltage electrolytic capacitor, US 2800616, 23 July 1957 R. A. Rightmire (Standard Oil Company, Cleveland): Electrical energy-storage apparatus, US 3288641, 29 November 1966 D. L. Boos and J. E. Metcalfe (Standard Oil Co. Ohio), US 3648126: Electrical capacitor employing paste electrodes and a separator saturated with electrolyte B. E. Hart and R. M. Peekema (IBM), US 3652902: Electrochemical double-layer capacitor: activated carbon plates separated by a porous inert spacer, impregnated with KOH or H2SO4 D. Butherus and K. R. Newby (Bell Telephone Lab), US 3700975: Double layer capacitor with carbon electrodes, propylene carbonate, lithium tetrafluoroborate (and others), ‘utilizing the space charge polarization effect’ Military projects, no market for low-voltage capacitors Double-layer capacitors for memory backup (Matsushita, NEC) D. Craig, EP 0078404: ‘Electric energy storage device’ based on ceramic oxide coated electrodes and pseudocapacitance Mixed metal oxides for military and automotive applications (Pinnacle Research Institute) ‘Ultra-high current power capacitor’ (Matsushita, Isuzu) Bonding fine metal filaments with carbon fibers (Maxwell Lab); ionomer membrane-bound metal oxide supercapacitor (Giner) Ruthenium oxide hydrate in practical supercapacitors (Dornier); polymer-bound activated carbon black electrodes in organic electrolyte (Alcatel); carbon aerogels and foams Supercapacitor in fuel cell vehicles (Honda ‘FCX-V3’); overvoltage-limiting thyristor diode in parallel to series-connected supercapacitors (ABB Research) Regenerative braking (Toyota) Supercapacitor in wind power plants (Enercon) Energy storage on board of hybrid electric vehicles and trolleybuses; supercapacitors in emergency doors of passenger aircrafts
The Standard Oil Company, Cleveland, Ohio, realized in the early 1960s, during their work on fuel cells, that the double layer at carbon particles behaves like a capacitor of relatively high specific capacity. R. A. Rightmire’s ‘energy storage apparatus’, disclosed in a US patent filed in June 1962, employed porous graphite electrodes in aluminum sulfate solution, and an ion permeable membrane as separator between the electrodes (incidentally, this is considered as most modern even today). At that time, 400 m2 g1 carbons were state of the art. Rightmire’s ‘electrical storage device’, disclosed in the British patent 1097615 of 1968, contained porous electrodes of charred polyacrylonitrile (PAN), graphite, or Raney nickel in any ‘molten salt electrolyte’, particularly alkali and alkaline earth metal halides, in which ‘‘the ions will transport their charge directly to the boundary surface throughout the porous electron conductor.’’ The electrodes with current collectors were separated by an asbestos spacer. The capacitor case had to be heated to provide the molten salt, obviously similar to the then molten salt batteries. SOHIO’s disk-shaped ‘electrokinetic capacitor’, described in a brochure of 1969, utilized porous carbon
paste electrodes in a nonaqueous electrolyte, which enabled it to be charged up to about 3 V. The ‘electrical capacitor’ in a 1972 patent by Daniel Boos and Joseph Metcalfe employed both an active carbon and a metal powder paste electrode. The powders were bound with the electrolyte to form a viscous paste and then compressed to tablets. The electrolyte was an aqueous potassium hydroxide or salt solution or, among others, quaternary ammonium salts in acetonitrile and propylene carbonate. Organic systems accommodate higher charge densities than aqueous systems, because the storable energy increases with the square of the voltage: W ¼ 12CU 2 . The elastomer gasket was stamped from a sheet of a copolymer of vinylidene fluoride and hexafluoropropylene (DuPont’s Viton). Owing to a lack of market demand, SOHIO stopped the development and licensed the technology to NEC later. Verbatim ‘electrochemical double-layer capacitors’ were described in the August issue of 1971 IBM Technical Disclosure Bulletin. The related 1972 patent illustrates a 0.3 F device of ‘‘two blocks of carbon one-half inch thick, separated by a sheet of facial tissue paper soaked in a saturated solution of table salt.’’ Petroleum coke-based
History | Electrochemical Capacitors
carbon, fabricated into wafers 0.5 mm thick and activated by heating in air at 500 1C, until a surface of 50 m2 cm3 was generated, was used for a 1.4 F/3 V two-cell capacitor utilizing 30% sulfuric acid in a microporous plastic separator. Moreover, nonaqueous solvents and solid electrolytes such as RbAg4I5 were proposed. In addition 1972, Bell Telephone Laboratory Inc. patented a double-layer capacitor ‘‘utilizing the space charge polarization effect.’’ The device was intended primarily for use in filter applications in transistor circuits. The electrode material was selected from the group consisting of carbon, nonstoichiometric electronically conductive metal oxides, transition metal nitrides, transition metal borides, and finely divided metallic platinum. The electrolyte was composed of an ‘ionically conducting nonaqueous aprotic solvent–solute system’. The solvent was selected from the group consisting of propylene carbonate, dimethyl sulfoxide, dimethyl sulfite, tetrahydrofuran, nitromethane, and butyrolacetone. The supporting electrolyte was a lithium salt with PF6 , BF4 , BH4 , or AlH4 anion. The first usable supercapacitors for electrical applications were developed in Japan in the late 1970s. NEC produced the first commercially successful double-layer capacitor under the name ‘supercapacitor’. In 1978, Matsushita–Panasonic developed the ‘gold capacitor’. Owing to its high internal resistance, this device was primarily designed for memory backup in computers and consumer appliances. ELNA’s ‘Dynacap’ followed not much later. In the early 1970s Trasatti and his coworkers searched for substitutes for the graphite in chlor-alkali electrolysis, and recognized that the electrochemical charging behavior of ruthenium dioxide films was similar to that of capacitors. D. Craig, in a Canadian patent of 1985, described novel electrochemical energy stores, which have high power-to-volume ratios achieved by the practical use of pseudocapacitance, electrodeposited species on surfaces, and the charge separation in double layers as a result of the reactions occurring between electrode and electrolyte. He distinguished between ‘kinetic reversibility’ (passage of approximately equal and substantial charge or discharge currents at about the same rate) and ‘coulombic reversibility’ (passage of substantially equal numbers of coulombs in the charging and discharging of the device). From 1975 onward, Conway and his coworkers, under contract with the then Continental Group Inc., followed the earlier concept of Craig and investigated the pseudocapacitance of ruthenium rods in aqueous sulfuric acid, and the so-called underpotential deposition of hydrogen, lead, bismuth, or copper at platinum or gold electrodes. The work on metal oxide ‘ultracapacitors’ was taken over by Pinnacle Research Corp. in 1982, mainly with military funding, and it focused on ruthenium dioxide.
599
Improvement and Commercialization of Supercapacitors Early supercapacitors since the 1970s have been designed for applications at low currents such as memory backup in computers and standby power in consumer electronics. Novel applications have established niche markets for bicycle lighting, flashlights, alarm systems, and battery substitute in toys. Military visions of laser weaponry, missile guidance, electric guns, all-electric tanks, and gigantic short-time pulse power sources in the late 1980s triggered worldwide research activities for supercapacitors and similar energystorage devices. An ultracapacitor development program was under way at Maxwell Laboratories, San Diego, which finally aimed at bipolar units capable of providing at least 5 Wh kg1 and 1000 W kg1. An impressive progression of energy densities was achieved from about 20 J kg1 (in 1962) to 30 MJ kg1 (around 1990) in experimental supercapacitors developed by the Maxwell– Auburn group. After the cold war, amazing applications in the former Soviet Union became generally known: Supercapacitors started diesel engines in tanks and locomotives in the cold Siberian winter. Some developments were commercialized later by ESMA in Troitsk (Russia) and ELIT in Kursk. The European aerospace industry in 1990, dominated by the DASA, a corporation of the later Daimler Group, was developing supercapacitors for the electrical power supply of future satellite antennae. High pulse power density, excellent cycle efficiency (reversibility), and little degradation over hundreds of thousands of cycles were required for this application. Since 1990, the Annual International Seminar on Double-Layer Capacitors and Similar Energy Storage Devices has been bringing together researchers and decision makers. A new market for power trains of electric vehicles came up, driven by the regulatory actions of the State of California requiring at least 15% of the automobiles without toxic gas emissions by the next decade. The US Department of Energy promoted projects in the context of hybrid electric vehicles from 1992 to 1998. The automotive industry launched extensive R&D programs. The objectives changed over the years, but worldwide research in the field of ‘zero-emission vehicles’ was kicked off. In the 1990s forward-looking applications came up in power electronics, automotive, and railway industry: bridge power, power factor correction, regenerative breaking, electric vehicle load leveling, cold-starting assistance, and catalyst preheat. However, supercapacitors with the required high operating voltages were not available at that time (Figure 2). Critical points of interest were the strong dependence of capacitance on
600
History | Electrochemical Capacitors
Supercapacitors for mobility and electronics
Low voltage
High voltage
Automotive Engineering • • • •
Automotive and Railway Systems
Cold-starting assistance Steer-by-wire Brake-by-wire Catalyst preheat
• Regeneration braking • Electric vehicle load-leveling • Electric trains
Consumer Electronics
Space Technology and Military
• Standby power • Memory backup • Toys
• Pulse power • Electric guns • Silent, vehicles ,
Electric and Aerospace Engineering • • • •
Pulse power supply Security systems Factory automation Robotics
Electrical Power Engineering • Bridge power • Uninterruptable power supply • Power factor correction
Replacement Substitute for batteries and common capacitors
Figure 2 The tree of applications, which was designed in the early 1990s, ramifies in a low-voltage and a high-voltage branch. The challenging goals have brought about the first low-voltage devices in the last decades.
frequency and temperature, and the voltage balancing of the individual cells in a series combination of capacitors. Hybrid vehicles, using a combination of batteries and supercapacitors, have been discussed with all technical consequences involving state-of-charge electronic control and switching equipment. By 1995 supercapacitor devices based on various technologies were being manufactured: a) Carbon – organic: Panasonic (3 V, 2.2 Wh kg1, 400 W kg1), Maxwell Laboratories (24 V, 6 Wh kg1, 2000 W kg1); b) Carbon aerogel – aqueous: Livermore National Lab (1 V, o2 Wh kg1, 1000 W kg1); c) Metal oxide – aqueous: Pinnacle Research Institute (28 V, 0.8 Wh kg1, 500 W kg1); d) Conductive polymers – aqueous: Los Alamos National Lab (0.75 V, o2 Wh kg1, 500 W kg1). Since the beginning of the twenty-first century, supercapacitors have been produced by a respectable number of companies all over the world: Ness Capacitor Co. (Korea); NEC-Tokin, Panasonic/Matsushita, Nippon Chemi-Con ( Japan); Cooper ‘PowerStor’, ELNA ‘Dynacap’, Jeol ‘Nanogate Capacitor’, Evans Inc., Maxwell ‘BoostCap’ (USA); Tavrima (Canada); WIMA (Germany); ESMA (Russia); and Cap-XX (Australia).
Meanwhile, hybrid vehicles have been powered by a combination of internal combustion engine, generator, batteries, and supercapacitors. In Japan, Toyota is already using a supercapacitor module for the energy backup of the breaking system of its ‘Prius’ hybrid vehicle. Honda presented its ‘FCX’ fuel cell vehicle in 2001, where a supercapacitor module was used in the 78 kW power-storage system. BMW in Germany supported the 4.4 l V8 internal combustion engine in a ‘SUV X 5’ vehicle by an electric drive train with a powerful supercapacitor module, which is able to provide an extra torque of 650 Nm at 1000 min1 and a 15% reduction in fuel consumption by regenerative breaking. MAN equipped a diesel electric bus, in the city of Nuremberg (Germany), with a module of 8 36 supercapacitors, which was able to store up to 300 kWh of braking energy and to deliver 90 kW of accelerating power for 13 s after each stop. During the 3-month test, fuel consumption was reduced by 20%, and low noise emission was an additional benefit. Since around 2003 supercapacitors have been tested for pitch control of wind power generators as maintenance-free energy backup for the emergency systems under extreme environmental conditions. Mounted inside the nacelle close to the blades, the capacitor bank of >30 capacitors connected in series is exposed to heavy
History | Electrochemical Capacitors
mechanical stress. Nevertheless, thousands of modules have proven their reliability in 24-h duty for >3 years. ¨ V (Technical Inspection Authority) has The German TU approved 220 F/28 V modules for starting large-volume diesel engines of emergency generators, trucks, and locomotives. A few companies produce integrated modules with voltage balancing circuitry. Many engineering aspects of the technology development of electrochemical capacitors, similar to those for the manufacture of batteries and electrolytic capacitors, have been considered in recent years, for example, packaging, electrode sealing, separator technology, and accommodation of electrolyte solutions. At the same time, specialized requirements have arisen, such as the preparation of low-resistive materials with defined poresize distribution corresponding to maximized areas per gram of active material that are accessible to charging at the electrolyte interface. Highly pure electrode materials and electrolyte solutions were made available in the last decade, although they cannot yet meet the cost requirements for the mass markets in automotive and electric industry. In 2006, supercapacitors with capacitance of 1000– 5000 F and usable energy densities between 3.5 and 4.9 Wh kg1 were commercially available from Maxwell, Ness, EPCOS, Nippon Chem-Con, and Power Systems. Low-cost supercapacitors might capture future automobile markets, although the small energy density, in contrast to 25 Wh kg1 of a typical lead–acid battery, has hampered the market success in power applications. EPCOS in Germany surprisingly stopped their production of ultracapacitors in summer 2006. Actual production of supercapacitors is restricted to a relatively small number of leading companies.
Ever-Young Carbon Technology Carbon electrodes have been widely used as anode and cathode material in electrochemical processes since the nineteenth century (see Figure 3). Carbon was found to be a more or less inert electrode material that comes close to an ideally polarizable electrode. However, its industrial use for electrolysis has been restricted since the 1970s, when its oxidation to organic acids and carbon dioxide was observed. Nevertheless, the multifaceted field of specialized carbon materials has flourished with respect to the development of fuel cells and lithium-ion batteries. For technical details, see Capacitors: Electrochemical Double-Layer Capacitors: Carbon Materials. From Charcoal to Continuous Electrodes The book by Hassler gives an impression of the state of the art on activated carbons in the early 1960s, when the first supercapacitors were developed. Becker’s prototype
601
employed tar-lump black in sulfuric acid. Rightmire used carbon pastes, pathbreaking for later developments in Japan. High specific surface area powders and carbon fibers have been employed in supercapacitors, in close relationship to fuel cell applications since the 1970s. Graphite has been generated in the form of high-area fibers and felts. Powders of amorphous carbon and carbon black (soot), which are also useful for adsorption purposes in chemistry, have been adapted. Matsushita explored polyvinylpyrrolidone and carboxymethyl cellulose as binders for graphite, carbon black, and active carbons. Around 1986 they produced activated carbon fibers from phenol resins, PAN, and rayon. Asahi Glass, Elna, Hitachi, and, later, Toyota were among the first companies that used PTFE-bound carbon in supercapacitors. A Japanese patent of 1989 (JP 1227417) reads: ‘‘Activated carbon powder, carbon black and PTFE are mixed at the 8:1:1 ratio by wet kneading and the mixture is rolled into sheets to make the polarization electrodes.’’ In the early paste electrodes of the 1970s, carbon powder was bound by paraffin or polymer binders. Activated carbon particles were later painted or rolled on aluminum foil collectors. The rock-like carbon pieces were originally bound by wires of PTFE and additional carbon black to facilitate conductivity between the activated carbon particles. By around 1990 (JP 2235320) Japanese manufacturers had command of the fabrication of continuous electrodes by pressing a mixture of carbon powder, fluoropolymer, and liquid lubricant between heated rolls. Active carbon band electrodes were presumably the most memorable milestone in the commercialization of spiral-wound double-layer capacitors. NEC’s thin-type electrodes of 1998 (EP 0867902) contained carbon fine grains of 10 mm diameter, dispersed in a thermoplastic binder resin mixture of polyvinyl butyrate, alcohol, and acetate. By using a doctor blade process, current collector and polarizable electrode sheets were formed, and then they were laminated by thermocompression. Similar polymer-bound carbon electrodes were suggested in patents by Matsushita (EP 0948005), Asahi Glass (DE 19849929), Nippon Valqua (JP 2000323363), Japan Gore Tex (US 6359769), Ness Capacitor Co. (EP 1256966), EPCOS (DE 10203143), and Bollore´ (US 6671166). In a recent patent (EP 1672652) of Japan Gore Tex Inc. and Hitachi Powdered Metals, the production of a continuous electrode is disclosed: An etched aluminum collector is coated with a mixture of natural flake-like graphite, acetylene black, and styrene–butadiene rubber, carboxymethyl cellulose, or a thermosetting polyimide. The polarized porous electrode is formed of a kneaded mixture of activated carbon and PTFE powders, and then rolled at a thickness of 0.3 mm. The separator is a drawn porous PTFE film, which was rendered hydrophilic.
602
History | Electrochemical Capacitors
Carbon
Metal oxide
1991 Panasonic
Polymer
1991 Dornier
Power capacitor
1992 Alcatel, Innovision (Danionics)
Double-layer capacitor 1990 Giner, USA
1990 Evans, USA
1985−1994 Pinnacle
RuO2 / Nafion
Capattery
Ultracapacitor
1985 Siemens
1978−1980 Matsushita
Raney nickel
Gold capacitor 1980 McHardy
1979 NEC
Iridium oxide Nafion
Supercapacitor
1975−1985 Craig, Conway
ab 1962 Maxwell 1974 Blackjack 1979 Shiva 1982 Scremp 1985 Checmate 1988 Army
1853, 1879 Helmholtz
1984 Gottesfeld
1906, 1910 Gouy
1962−1971 SOHIO
1957 Becker
1969 Electrokinetic capacitor
1913 Chapman
1924 Stern
Underpotentialdeposition
Low-voltage electrolytic capacitor
1947, 1955 Grahame
1971 Trasatti
Figure 3 The historical roots of supercapacitor technology. Most of the commercially available devices are based on the carbon technology.
Activated carbon materials have been fabricated by thermal carbonization of pitch, coal, coconut shells, or wood, or by pyrolysis of organic polymers, such as PAN. In the late 1990s it was clear that the accessible surface area at the electrode/electrolyte interface seems by far more important than the physical surface area measured by the Brunauer–Emmet–Teller absorption (BET) method. Activation procedures have been developed after the discovery that heat treatment in vacuum, nitrogen, hydrogen, steam, or carbon dioxide (at 1000–2800 1C) opens up microporous structures, changes surface oxygen functional groups, and modifies the aromatic character of graphite layers. The electrolytic oxidation of carbon fibers in potassium hydroxide to improve adhesion to resins was known in 1983 (e.g., JP 58104222). Around 1995, high-surface carbonaceous materials were developed by treating coconut shells, wood, flour, coal, or resin in an alkali hydroxide bath (US
5430606). Takeuchi et al. (JEOL Ltd) described in a 2004 patent (US 6721168) how petroleum-based needle coke is dry-distilled and activated with potassium hydroxide powder at 800 1C in a stream of nitrogen, washed, posttreated in a stream of hydrogen, vacuum-dried, kneaded in a mixture of carbon black and PTFE binder, and finally pressed in hot rollers between aluminum foils. Carbon Fibers Vapor-permeable sheet-like materials have been produced by Toray Industries in Japan since the 1970s, as stated in GB 1221372 and US 6489051. Such activated carbon papers have been widely known for their use in fuel cells. Fibrous carbon structures from pitch, PAN, or cellulose fibers have been modified by high-temperature programmed activation after the discovery that this leads to multiple splitting of the
History | Electrochemical Capacitors
fibers, producing an enhanced surface area and allowing higher charge/discharge currents. Novoloid fibers have been manufactured, e.g., by Nippon Kynol Inc., as three-dimensional phenolic-aldehyde fibers by acid-catalyzed cross-linking of melt-spun novolac resin with formaldehyde. Maxwell’s 2300 F/2.3 V double-layer capacitor patented in 1999 (US 5862035) comprised activated carbon fiber cloth electrodes, impregnated with aluminum, on a current collector foil; an electrolyte based on acetonitrile and tetraethylammonium tetrafluoroborate; and a porous polypropylene separator. Glassy Carbon, Foams, and Nanomaterials In the late 1970s, Siemens (DE 2842352) obtained glassy carbon electrodes by carbonizing three-dimensionally cross-linked phenol-formaldehyde or furan resins, and activation in boiling sulfuric acid. Glassy carbon is a most plane, chemically inert, highly conductive, and gas-tight material that can be polished by a corundum slurry. Fundamental research was done by the Paul Scherrer Institute in Switzerland around 2001. Despite worldwide R&D activities, double-layer capacitors based on glassy carbon, aerogels, and nanomaterials have not become commercially available so far, as these materials are far more expensive than activated carbon powders. Microcellular carbon foams (aerogels) were intensively investigated in the 1990s. Cooper Electronic Technologies (Boynton Beach, Florida) was the first to market aerogel supercapacitors. Derived by controlled pyrolysis and carbonization of polymers such as PAN, polymethacrylonitrile (PMMA), resorcinol-formaldehyde, divinylbenzene cross-linked methacrylonitrile, phenolics, and cellulose rayons, carbon aerogels provide high surface areas of about 400–1000 m2 g1, good electrical conductivity, and a controllable pore-size distribution. The extreme blackness is caused by internal scattering and absorption of light at the graphite particles. Aerogels can be spun into nanotube fibers with a strength greater than that of Kevlar. Aerogels were originally produced in 1931 from silicon dioxide, and they exhibit the lowest density, highest thermal insulation, lowest refractive index, and the highest surface area per unit volume for any solid (up to 99.9% empty space). The alternative to the expensive supercritical drying was found by Hoechst (Germany) and Yonsei University (South Korea), who developed an extremely hydrophobic coating on the silica wet gels to prevent shrinking while water is rapidly expelled. Carbon nanotubes were described in a 1987 patent of H. G. Tennent (US 4663230) as ‘cylindrical discrete carbon fibrils’. Hollow nanotubes were grown by catalytic decomposition of hydrocarbons or generated in an electric arc.
603
Discovery of Redox-Active Metal Oxides Ruthenium Dioxide
Since the early 1970s, ruthenium dioxide has been used in ‘dimensionally stable anodes’ (DSA) for the chlor-alkali electrolysis, which operates at about 95 1C in brine. According to Henri Beer in US 3632498 (1972), the DSA comprises a catalytic coating consisting of titanium–ruthenium oxide formed on a titanium base. A precursor material, for example, an alcoholic solution of ruthenium(II) chloride, (RuCl3) or (NH4)3RuCl6, is painted onto titanium substrates, sometimes with the addition of titanium isopropylate or titanium(III) chloride (TiCl3), and heated up to 350–550 1C. Platinum metal oxides form electronically and ionically conductive powders, which can be coated on titanium or nickel supports. In 1971, Trasatti recognized the pseudocapacitance of ruthenium dioxide films, thermally formed on titanium, in the almost rectangular, mirror image-like cyclic voltammograms. Ruthenium dioxide exhibits two or three overlapping, extremely reversible oxidation and reduction steps in charge or discharge over a potential range of 1.4 V above the reversible hydrogen potential, e.g., in aqueous sulfuric acid. By integration of current versus voltage in the voltammogram curves, Trasatti found that the electric charges for the oxidation and reduction processes tend to decrease with increasing temperature of calcination, whereas the increasing thickness of the ruthenium(IV) oxide (RuO2) films up to several micrometers was advantageous. Obviously, the fraction of the oxide film accessible to faradaic reactions was destroyed at high temperatures. Various researchers confirmed later that the pseudocapacitance of RuO2 cannot only be due to surface redox processes and double-layer charging, but some bulk-phase reactions must also take place. Single-crystal RuO2, except at defective regions at the surface, did not prove to be electrochemically active. Enhancement of electrocatalysis for anodic chlorine evolution at thermochemically prepared ruthenium and iridium dioxide was observed after extended potentiodynamic cycling, reflecting a progressive increase in accessible surface area. Nonstoichiometry of the oxide material proved to be responsible for the good electronic conductivity. In a 6-year project from 1975 onward, the Ottawa group around Conway, in collaboration with Continental Group Inc., tested various embodiments of RuO2 and mixed oxides with titanium oxide or tantalum peroxide (Ta2O5) for applications in double-layer capacitors. Bipolar stacked electrodes were used in 12-V devices. Thermochemically prepared films were found to be much less reversible than films obtained on ruthenium metal by continuous anodic cycling between 0.05 and 1.4 V. Thoughts of a bulk diffusion process involving H3O þ species were suggested by Gerischer in 1978. Platinum
604
History | Electrochemical Capacitors
metal oxides appear to have electron-hopping conductivity, and the protons from the electrolyte are able to access the quasi-three-dimensional electrode structure. Iridium dioxide shows its metal-like conductivity and pseudocapacitance over a more limited potential range (0.5–1.4 V RHE) and less reversibly than RuO2. Rajeshwar and coworkers published in 1991 the low activation energy of 4–5 kJ mol1 for the charge storage in RuO2 electrodes, and attributed this to a facile proton diffusion via a Grotthus-type hopping mechanism. In March 1993, Dornier, a company of the later Daimler Group, placed a German patent DE 4313474 of a supercapacitor utilizing ruthenium oxide hydrate. RuO2 xH2O was prepared by precipitation from RuCl3 solution with potassium hydroxide (which proved better than sodium hydroxide). The preparation of ultrafine particles by socalled sol–gel processes was in the air at that time. The obtained powders were filtrated, washed, and dried at 90– 105 1C to remove adhesive water, and then coated on nickel foil or carbon paper with the help of an oily binder, which had then also been used as a solvent in paint and finishing products. Titanium supports, employed in earlier devices, were soon discarded because of the growth of a badly conducting oxidation layer. The electrolyte was 6 mol L1 potassium hydroxide solution and 3 mol L1 sulfuric acid, respectively. In an automotive project aiming at hybrid vehicles, bipolar supercapacitors with rated voltages of up to 50 V were manufactured and tested successfully over several million charge–discharge cycles (Figure 4). The outstanding capacitance of up to 4 F cm2 and roughly 450 F g1 was explained by the high specific surface area of finely distributed oxide on porous carbon paper, the facile
conversion of ruthenium between several oxidation states, and the proton mobility between the oxide and hydroxyl sites in hydrated ruthenium oxide. In particular, high capacitance requires that ruthenium oxide be in the amorphous state with a high degree of hydration. The technology was taken over in 2000 by Hydra, the legal successor of the former AEG, who wanted double-layer capacitors for applications in electrical power engineering. In 1995, partially ‘hydrated’ metal oxides were described by a Russian group with reference to earlier work on DSAs. The authors stated that thermochemically prepared ruthenium oxide layers consist of both crystalline oxide, practically anhydrous, and amorphous hydrated oxide, their ratio being dependent on the calcination temperature and the complex species in the precursor solutions. Ruthenium oxide prepared at 450 1C is described as a crystalline phase, with rutil structure containing 30% of amorphous ‘RuO(OH)2’. Above 650 1C, hydroxy groups and chlorine traces are completely removed; crystallinity tends to 100%; and cyrstallite size increases. Hydrous ruthenium oxide was described in electrochemical literature later in more detail. Jow and coworkers studied the degree of surface hydration and noncrystallinity by X-ray diffractometry in 1995. US 5600535 recommends a pH of 3–6 for the sol–gel process, in which ruthenium(III) chloride is hydrolyzed by alkali metal alkoxides. C. Zheng and R. M. Franklin at Auburn University (US 6025020) reported in 2000 a specific capacitance of 1040 F g1 for highly dispersed ruthenium oxide hydrate, when they heated an aqueous solution of ruthenium chloride on carbon fibers, immobilized in a cellulose
Capacitor voltage (at 12 °C) 300 22 V 250
Current (A)
200 20 V 150 100 18 V 50 0 Motor starts 0
0.5
1.0 Time (s)
1.5
2.0
Figure 4 Current transients during cold starting of a Mercedes C220 with the help of a 60 F/30 V metal oxide hydrate supercapacitor on a fresh spring morning at Dornier in Friedrichshafen (Daimler Group, Germany), in 1997.
History | Electrochemical Capacitors
matrix, in a steam and oxygen atmosphere up to 420– 475 1C, and dried it afterward at 100–150 1C. Iridium Dioxide and Nickel Oxide Iridium dioxide has been investigated since the late 1970s for use in alkaline electrolysis. Iridium dioxide has not been as widely used as RuO2 owing to its higher price and the less reversible redox reactions, which can be studied with the aid of the cyclic voltammogram. Since around 2002, nanoparticles of platinum metal oxides have attracted attention for various electrotechnical applications that have not yet reached a commercial status. Nickel oxide has been intensively studied for use in batteries. Work on its use in supercapacitors by research groups in Asia and the United States has become known mainly since around 2002. For details of nickel in asymmetric supercapacitors, see Capacitors: Electrochemical Hybrid Capacitors. Perovskites and Other Classes Perovskites, such as alkaline earth ruthenates, were developed in the mid-1990s by the Center for Solar Energy and Hydrogen Research in Germany (DE 19640926) in cooperation with Dornier (Daimler Group). Specific capacitance of up to 28 F g1 was obtained for SrRuO3 in supercapacitor applications. Cobalt or aluminum mixed hydroxides/oxyhydroxides exhibit capacitance up to 360 F g1, but the potential window is very limited. A Brazilian research group reported in 2002 that Ir0.3Mn0.7O2, which was synthesized by annealing a mixture of indium(TV) oxide and manganese(IV) oxide between 400 and 450 1C, delivered a specific capacitance close to 550 F g1. Other oxide and nonoxide materials, such as WO3, Co3O4, MnO2, NiO2, WC, TiC, and molybdenum nitrides, have been studied by various researchers. Due to poor operating behavior and narrow voltage window (B0.7 V), none of these materials has found its way into stable supercapacitors so far. For technical details, see Capacitors: Electrochemical Metal Oxides Capacitors.
605
lithium or alkylammonium salts as electrolyte, as for solid polymer electrolyte lithium batteries. Solid polymer electrolytes have been extensively developed in the field of lithium batteries. Polyaniline, known as aniline black, was already described in 1862 by H. Letheby, as a more or less undefined oxidation product of aniline. ‘Metallically’ conducting polymers were discovered in the 1980s, when polyacetylenes were studied by MacDiarmid and other researchers. In addition, polypyrrole and polythiophene were intensively studied in battery technology. Conductive polymers combine the pseudocapacitance of redox storage mechanisms with a high surface area. In the early 1990s, the use of conductive polymers for supercapacitors was discussed by Gottesfeld and other researchers, based on the excellent reversibility of electrochemical charge and discharge over a potential range of about 0.8 V. Polymer-type supercapacitors were developed first by Alcatel Alsthom Recherche in France, the then Innovision in Denmark, and industrial research groups in Japan. However, polypyrrole, polyaniline, and other candidates did not prove stable over some hundred thousand cycles.
Hybrid Capacitors Evans hybrid capacitor of 1995, called ‘capattery’, combined an electrolytic-type tantalum anode, bearing a Ta2O5 barrier oxide layer, and a ruthenium oxide cathode, having a large capacitance, but a low voltage drop. Operating voltages of around 200 V were achieved with capacitances up to 0.1 F, as the stored energy of the capacitor was mainly associated with high voltage drop in the Ta2O5 dielectric. The energy storage capability was therefore higher than that in a regular RuO2 double-layer capacitor. The hybrid pooled 11 times higher volumetric and 2.5 times higher gravimetric energy density than a corresponding aluminum electrolytic capacitor. For details on development by Russian groups, see Capacitors: Electrochemical Hybrid Capacitors.
Solid-State and Polymer Technology Nomenclature Solid-state electrochemical capacitors based on RbAg4I, a silver ion conductor described in the late 1960s, were never realized. The critical factor was to achieve sufficient conductivity at ambient temperatures. Giner Inc. (USA), in the early 1990s, presented supercapacitors based on ruthenium oxide that was bonded by Nafion solution and coated on a Nafion membrane (US 5136474). The design avoided a liquid electrolyte and was analogous to membrane electrolyte fuel cells. Bollore´ (France) suggested carbon electrodes and thin films of polyethylene oxide as solvent, and
Symbols and Units C Q U W
capacitance charge storage voltage storable energy
Abbreviations and Acronyms BET EDLC PAN PMMA
Brunauer–Emmet–Teller adsorption electrochemical double-layer capacitor polyacrylonitrile polymethacrylonitrile
606 PP PTFE RHE
History | Electrochemical Capacitors polypropylene polytetrafluoroethylene reversible hydrogen electrode
See also: Capacitors: Electrochemical Double-Layer Capacitors: Carbon Materials; Electrochemical Hybrid Capacitors; Electrochemical Metal Oxides Capacitors; Electrochemical Polymer Capacitors.
Further Reading Boos DL and Argade SD (1991) Historical background and new perspectives for double-layer capacitors. Proceedings of 1st International Seminar on Double Layer Capacitors. 9–11 December. Deerfield Beach FL, USA: Florida Educational Seminars. Bullard GR, Sierra-Alcazar HB, Lee HK, and Morris JJ (1989) Operating principles of the ultracapacitor. IEEE Transactions on Magnetics 25(1): 102--106. Conway BE (1999) Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications. New York: Kluwer Academic/Plenum Publishers. Delnick FM, Ingersoll D, Andrieu X, and Naoi K (eds.) (1997) Electrochemical Capacitors II. Proceedings, Vol. 96–25, Pennington, NJ: The Electrochemical Society. Doblhofer K, Metikos M, Ogumi Z, and Gerischer H (1978) Electrochemical oxidation and reduction of RuO2/Ti electrode surface. Berichte der Bunsen-Gesellschaft fu¨r Physikalische Chemie 82: 1046. Grahame DC (1947) The electrical double layer and the theory of electrocapillarity. Chemical Reviews 41: 441--501. Guther TJ, Oesten R, and Garche J (1997) Development of Supercapacitor Materials Based on Perovskites, in: Delnick, pp. 16–25. Hassler JW (1963) Activated Carbon. New York: Chemical Publishing Co. Helmholtz Hv (1870) Studien u¨ber elecktrische Grenzschiten. Annalen der Physik 7: 337--382. Helmholtz Hv (1879) Annalen Der Physik, Leipzig 3: 223. (1853) 89: 21. Kurzweil P and Fischle H-J (2003) Proceedings of 13th International Seminar on Double Layer Capacitors. Deerfield Beach, USA, 8–10 December. Kurzweil P and Schmid O (1996) High performance metal oxide supercapacitors. Proceedings of 6th International Seminar on
Double Layer Capacitors and Similar Energy Storage Devices. Deerfield Beach, USA. Kurzweil P, Schmid O, and Schmid B (1994) Precipitated Ruthenium Oxide-Hydrate. German Pat. DE 4313474, filed on 24 April 1993; EP 0622815 (1994), US 5,550,706 (1996). Lee H, Bullard GL, Mason CG, and Kern K (1989) Improved pulse power sources with high-energy density capacitor. IEEE Transactions on Magnetics 25(1): 324--330. Letheby H (1892) Journal of the Chemical Society, London 15: 161. Nigrey PJ, MacInnes J, Nairns DP, MacDiarmid AG, and Heeger AJ (1981) Lightweight rechargeable storage batteries using polyacetylene, (CH)x, as the cathode-active material. Journal of the Electrochemical Society 128: 1651. Novak P, Mu¨ller K, Santhanam KSV, and Haas O (1997) Electrochemically active polymers for rechargeable batteries. Chemical Reviews 97: 207. Roginskaya YE and Morozova O (1995) The role of hydrated oxides in formation and structure of DSA-type oxide electrocatalysts. Electrochimica Acta 40(7): 817--822. Sanada K and Hosokawa M (1979) Electric double layer capacitor ‘super capacitor’. NEC Research and Development 21: 21--28. Sarangapani S, Lessner P, Forchione J, Griffith A, and Laconti AB (1990) Advanced double layer capacitors. Journal of Power Sources 29: 355--364. Schmickler W (ed.) (1995) Ladungsspeicherung in der Doppelschicht, Proceedings of 2nd Ulm Electrochemical Talks, 291–310; and literature cited there. Ulm: Universita¨tsverlag. Sekido S, Yoshina Y, Muranaka T, and Mori M (1980) Article on ‘Gold capacitor’ by Matsushita. Denki Kagaku 48: 40. Takasu Y, Onoue S, Kameyama K, Murakami Y, and Yahikozawa K (1994) Preparation of ultrafine RuO2–IrO2–TiO2 oxide particles by a sol–gel process. Electrochimica Acta 39(13): 1993--1997. Trasatti S and Buzzanca G (1971) Ruthenium dioxide: A new interesting electrode material, solid state structure and electrochemical behavior. Journal of Electroanalytical Chemistry 29: App: 1--5. Trasatti S and Kurzweil P (1994) Electrochemical supercapacitors as versatile energy stores. Platinum Metals Review 38(2): 46. Trasatti S and Lodi G (1980) Conductive Metal Oxides, Vols. A and B, Amsterdam: Elsevier. Tsai EW and Rajeshwar K (1991) Influence of temperature on the voltammetric response of thermal ruthenium oxide elctrodes. Electrochimica Acta 36: 27. Zheng JP, Cygan PJ, and Jow TR (1995) Hydrous ruthenium oxide as electrode material for electrochemical capacitors. Journal of Electrochemical Society 142: 2695. Zheng JP, Cygan PJ, and Jow TR (1995) Journal of Electrochemical Society 142(8): 2699. Zheng JP and Jow TR (1995) Journal of Electrochemical Society 142(1): L6.
Introduction The terms ‘supercapacitor’ and ‘ultracapacitor’ were coined by Nippon Electric Company (NEC), Japan, and Pinnacle Research Institute, USA, respectively. In technical terminology, it is named as electrochemical double-layer capacitor (EDLC), in contrast to ‘electrolytic capacitor’. Electrochemical capacitors basically differ from ordinary electrostatic capacitors or antiquated electrical ‘condensers’. Historically, supercapacitors are divided into genuine double-layer capacitors that use the high surface area of carbon materials, and faradaic systems that additionally use redox reactions at metal oxides or conductive polymers.
The Discovery of Capacitance at Solution Interfaces Early Insights against a Water Capacitor In the nineteenth century, when Alessandro Volta invented the first battery, static electricity was generated by the rubbing of amber, preferably in the so-called Wimshurst machine (1883) – a rotating circular plate, containing inset sectors of amber-like material, rubbing against contact plates. The principle that electrical charge can be stored in a ‘Leyden jar’ was known since 1745. It was originally developed by Ewald von Kleist and independently invented by Pieter van Musschenbroek: A glass jar was wrapped with silver or tin foil (Figure 1); a salt solution or impure water guaranteed the electrical contact between a conductive metal wire immersed in the jar and the inner glass surface. However, the negligible double-layer capacitance at the metal–solution interface was not recognized at that time. Benjamin Franklin proved in 1749 that the electric charge was rather stored on the facing glass surfaces, and not in the water as was assumed initially. The Leyden jar was charged by connecting two wires – one from the inside electrode and one from the outside foil – to an electrostatic machine or a Volta’s pile. Franklin coined the name ‘battery’ for an electrical combination of Leyden jars, derived from an artillery battery. Later jar designs comprised a top electrode, connected by a metal chain to the inner glass surface, and an external tin foil wrapping; the liquid inside was replaced with a foil lining. The electrostatic capacitor was introduced, and the double-layer capacitor was still waiting to be discovered.
596
All capacitors subsequent to Volta’s ebonite plate capacitor, around the year 1780, consisted of two charge collector plates (electrodes), which were separated by a thin layer of a nonconducting material, the dielectric, in the form of air, a liquid, or a solid film. The term ‘dielectric’ was coined by Michael Faraday who had some of the first correct ideas about polarization in dielectrics and air capacitors. It was found that the dielectric material is essential to reduce the size of a capacitor and to prevent arcing between the plates. All trials to develop a stable ‘water capacitor’ failed owing to the intrinsic conductivity of water. No manufacturer did seriously intend to replace a dielectric by an electrolyte. Around 1850 natural mica, cut in slices, was introduced, although it started being produced commercially only since World War I. Wound-paper capacitors followed in 1876 and ceramic capacitors in 1900. Franklin measured the storage ‘capacity’ of his electrical equipment in ‘jars’, a traditional unit of capacitance equivalent to 1.1 nF. Until 1950, mainly paper capacitors in tube radios were labeled using the CGS units: 1 cm8f4pe0 gF ¼ 1:11 pF. The SI unit of capacitance, farad, named after the British physicist Michael Faraday (1791–1867), is defined as the ability to store 1 C of electric charge per volt of potential difference between the two conductors: 1 F ¼ 1 C V1 ¼ 1 A s V1. In conventional dielectric capacitors, capacitance is measured in microfarads, nanofarads, or sometimes even in picofarads. Electrochemical capacitors tap the full range of up to several thousand farads owing to their extraordinarily high energy density. The term ‘capacity’, used in battery technology, indicates the extent of charge storage Q , in units of coulombs or watt-hours. The Birth of the Electrochemical Capacitor Table 1 lists the chronology of important milestones leading to the birth of the electrochemical capacitor. Around 1840, C. F. Scho¨nbein and W. R. Grove discovered that electrochemical cells comprising two electrodes in an electrolyte show properties of a power source in the voltage range below the decomposition voltage, at which electrolysis takes place. The interface between electronic and ionic conductors has been studied since the nineteenth century, but the electrochemical phenomena occurring outside of batteries have not attracted much interest for electrotechnical applications for a long time. Hermann von Helmholtz formulated his concept of the electrolytic double layer in 1853. Between
History | Electrochemical Capacitors
597
Connection −
+
Conductor
Metal chain Porous graphite electrode Sat. aluminum sulfate solution
Glass Tin foil Electrodes Electrolyte
Filter paper or membrane US 2800616 (1957) H.I. Becker
Leyden jar
+
US 3288641 (1966) R. A. Rightmire
Porous Collector electrode layer
Casing
Porous separator Gasket Carbon electrode
Separator
EP 0712143 (1996) Japan Gore Tex
-
US 3700975 (1972) Bell Telephone Lab
Glassy carbon electrode
Sulfuric acid
Current collector Current collector Seal
Opposing electrode Electrode assembly
Separator
Electrode coating Substrate
Electrode US 3634736 (1972) SOHIO
US 4337501 (1982) Siemens
EP 0449145 (1992) Matsushita
Figure 1 Historical embodiments of electrochemical double-layer capacitors: The prototypes of Becker and Rightmire led to practical coin-shaped and spiral-wound devices. The Leyden jar marks the prototype of all capacitors.
1910 and 1950, the theory of the diffuse double layer and specific adsorption of ions was developed by G. Gouy, D. L. Chapman, O. Stern, A. Frumkin, and D. C. Grahame. Becker’s practical capacitor of 1957, disclosed in a patent placed by General Electric, employed porous carbon electrodes perfused with an aqueous electrolyte. The exceptionally high capacitance was thought to be due to the geometric surface of the carbon pores and the double layer
at the interface between carbon and the solution. The double-layer capacitor was introduced, although the appurtenant US patent title ‘low-voltage electrolytic capacitor’ is a bit misleading. The principle is based on the capacitance at the interface between the electronic conductor and the thereon adsorbed ions of the liquid solution, and does not bear on any electrostatic space charge layer between an oxide dielectric and a metal support.
598
History | Electrochemical Capacitors
Table 1 B1650 1745 1746 1749 1775/1882 1832/1833 1853 1906–1955 1957 1966 1972
1975–1990 1978 1983 1984–1995 1990 1991 1993 2001 2002 2003 2005
Chronology: Discovery and commercialization of the supercapacitor Otto von Guericke: Friction generator, a sulfur ball rotating on a shaft Ewald J. G. von Kleist: ‘Amplification bottle’, a glass jar wrapped with silver foil and filled with water, alcohol, or mercury and a piece of bare wire Pieter van Musschenbroek: ‘Leyden jar’, a glass bottle shrouded by tin foil and filled with a liquid Benjamin Franklin (1706–1790): Experiments with Leyden jars and air dielectric Alessandro Volta (1745–1827): Metal plate capacitor with a dielectric of ebonite (hard rubber made of natural latex and 20–40% sulfur) Michael Faraday (1791–1867): Faraday’s laws H. von Helmholtz: Concept and model of the electrolytic double layer Theory of double layer and electrocapillarity (see Figure 3) H. I. Becker (General Electric): Low-voltage electrolytic capacitor, US 2800616, 23 July 1957 R. A. Rightmire (Standard Oil Company, Cleveland): Electrical energy-storage apparatus, US 3288641, 29 November 1966 D. L. Boos and J. E. Metcalfe (Standard Oil Co. Ohio), US 3648126: Electrical capacitor employing paste electrodes and a separator saturated with electrolyte B. E. Hart and R. M. Peekema (IBM), US 3652902: Electrochemical double-layer capacitor: activated carbon plates separated by a porous inert spacer, impregnated with KOH or H2SO4 D. Butherus and K. R. Newby (Bell Telephone Lab), US 3700975: Double layer capacitor with carbon electrodes, propylene carbonate, lithium tetrafluoroborate (and others), ‘utilizing the space charge polarization effect’ Military projects, no market for low-voltage capacitors Double-layer capacitors for memory backup (Matsushita, NEC) D. Craig, EP 0078404: ‘Electric energy storage device’ based on ceramic oxide coated electrodes and pseudocapacitance Mixed metal oxides for military and automotive applications (Pinnacle Research Institute) ‘Ultra-high current power capacitor’ (Matsushita, Isuzu) Bonding fine metal filaments with carbon fibers (Maxwell Lab); ionomer membrane-bound metal oxide supercapacitor (Giner) Ruthenium oxide hydrate in practical supercapacitors (Dornier); polymer-bound activated carbon black electrodes in organic electrolyte (Alcatel); carbon aerogels and foams Supercapacitor in fuel cell vehicles (Honda ‘FCX-V3’); overvoltage-limiting thyristor diode in parallel to series-connected supercapacitors (ABB Research) Regenerative braking (Toyota) Supercapacitor in wind power plants (Enercon) Energy storage on board of hybrid electric vehicles and trolleybuses; supercapacitors in emergency doors of passenger aircrafts
The Standard Oil Company, Cleveland, Ohio, realized in the early 1960s, during their work on fuel cells, that the double layer at carbon particles behaves like a capacitor of relatively high specific capacity. R. A. Rightmire’s ‘energy storage apparatus’, disclosed in a US patent filed in June 1962, employed porous graphite electrodes in aluminum sulfate solution, and an ion permeable membrane as separator between the electrodes (incidentally, this is considered as most modern even today). At that time, 400 m2 g1 carbons were state of the art. Rightmire’s ‘electrical storage device’, disclosed in the British patent 1097615 of 1968, contained porous electrodes of charred polyacrylonitrile (PAN), graphite, or Raney nickel in any ‘molten salt electrolyte’, particularly alkali and alkaline earth metal halides, in which ‘‘the ions will transport their charge directly to the boundary surface throughout the porous electron conductor.’’ The electrodes with current collectors were separated by an asbestos spacer. The capacitor case had to be heated to provide the molten salt, obviously similar to the then molten salt batteries. SOHIO’s disk-shaped ‘electrokinetic capacitor’, described in a brochure of 1969, utilized porous carbon
paste electrodes in a nonaqueous electrolyte, which enabled it to be charged up to about 3 V. The ‘electrical capacitor’ in a 1972 patent by Daniel Boos and Joseph Metcalfe employed both an active carbon and a metal powder paste electrode. The powders were bound with the electrolyte to form a viscous paste and then compressed to tablets. The electrolyte was an aqueous potassium hydroxide or salt solution or, among others, quaternary ammonium salts in acetonitrile and propylene carbonate. Organic systems accommodate higher charge densities than aqueous systems, because the storable energy increases with the square of the voltage: W ¼ 12CU 2 . The elastomer gasket was stamped from a sheet of a copolymer of vinylidene fluoride and hexafluoropropylene (DuPont’s Viton). Owing to a lack of market demand, SOHIO stopped the development and licensed the technology to NEC later. Verbatim ‘electrochemical double-layer capacitors’ were described in the August issue of 1971 IBM Technical Disclosure Bulletin. The related 1972 patent illustrates a 0.3 F device of ‘‘two blocks of carbon one-half inch thick, separated by a sheet of facial tissue paper soaked in a saturated solution of table salt.’’ Petroleum coke-based
History | Electrochemical Capacitors
carbon, fabricated into wafers 0.5 mm thick and activated by heating in air at 500 1C, until a surface of 50 m2 cm3 was generated, was used for a 1.4 F/3 V two-cell capacitor utilizing 30% sulfuric acid in a microporous plastic separator. Moreover, nonaqueous solvents and solid electrolytes such as RbAg4I5 were proposed. In addition 1972, Bell Telephone Laboratory Inc. patented a double-layer capacitor ‘‘utilizing the space charge polarization effect.’’ The device was intended primarily for use in filter applications in transistor circuits. The electrode material was selected from the group consisting of carbon, nonstoichiometric electronically conductive metal oxides, transition metal nitrides, transition metal borides, and finely divided metallic platinum. The electrolyte was composed of an ‘ionically conducting nonaqueous aprotic solvent–solute system’. The solvent was selected from the group consisting of propylene carbonate, dimethyl sulfoxide, dimethyl sulfite, tetrahydrofuran, nitromethane, and butyrolacetone. The supporting electrolyte was a lithium salt with PF6 , BF4 , BH4 , or AlH4 anion. The first usable supercapacitors for electrical applications were developed in Japan in the late 1970s. NEC produced the first commercially successful double-layer capacitor under the name ‘supercapacitor’. In 1978, Matsushita–Panasonic developed the ‘gold capacitor’. Owing to its high internal resistance, this device was primarily designed for memory backup in computers and consumer appliances. ELNA’s ‘Dynacap’ followed not much later. In the early 1970s Trasatti and his coworkers searched for substitutes for the graphite in chlor-alkali electrolysis, and recognized that the electrochemical charging behavior of ruthenium dioxide films was similar to that of capacitors. D. Craig, in a Canadian patent of 1985, described novel electrochemical energy stores, which have high power-to-volume ratios achieved by the practical use of pseudocapacitance, electrodeposited species on surfaces, and the charge separation in double layers as a result of the reactions occurring between electrode and electrolyte. He distinguished between ‘kinetic reversibility’ (passage of approximately equal and substantial charge or discharge currents at about the same rate) and ‘coulombic reversibility’ (passage of substantially equal numbers of coulombs in the charging and discharging of the device). From 1975 onward, Conway and his coworkers, under contract with the then Continental Group Inc., followed the earlier concept of Craig and investigated the pseudocapacitance of ruthenium rods in aqueous sulfuric acid, and the so-called underpotential deposition of hydrogen, lead, bismuth, or copper at platinum or gold electrodes. The work on metal oxide ‘ultracapacitors’ was taken over by Pinnacle Research Corp. in 1982, mainly with military funding, and it focused on ruthenium dioxide.
599
Improvement and Commercialization of Supercapacitors Early supercapacitors since the 1970s have been designed for applications at low currents such as memory backup in computers and standby power in consumer electronics. Novel applications have established niche markets for bicycle lighting, flashlights, alarm systems, and battery substitute in toys. Military visions of laser weaponry, missile guidance, electric guns, all-electric tanks, and gigantic short-time pulse power sources in the late 1980s triggered worldwide research activities for supercapacitors and similar energystorage devices. An ultracapacitor development program was under way at Maxwell Laboratories, San Diego, which finally aimed at bipolar units capable of providing at least 5 Wh kg1 and 1000 W kg1. An impressive progression of energy densities was achieved from about 20 J kg1 (in 1962) to 30 MJ kg1 (around 1990) in experimental supercapacitors developed by the Maxwell– Auburn group. After the cold war, amazing applications in the former Soviet Union became generally known: Supercapacitors started diesel engines in tanks and locomotives in the cold Siberian winter. Some developments were commercialized later by ESMA in Troitsk (Russia) and ELIT in Kursk. The European aerospace industry in 1990, dominated by the DASA, a corporation of the later Daimler Group, was developing supercapacitors for the electrical power supply of future satellite antennae. High pulse power density, excellent cycle efficiency (reversibility), and little degradation over hundreds of thousands of cycles were required for this application. Since 1990, the Annual International Seminar on Double-Layer Capacitors and Similar Energy Storage Devices has been bringing together researchers and decision makers. A new market for power trains of electric vehicles came up, driven by the regulatory actions of the State of California requiring at least 15% of the automobiles without toxic gas emissions by the next decade. The US Department of Energy promoted projects in the context of hybrid electric vehicles from 1992 to 1998. The automotive industry launched extensive R&D programs. The objectives changed over the years, but worldwide research in the field of ‘zero-emission vehicles’ was kicked off. In the 1990s forward-looking applications came up in power electronics, automotive, and railway industry: bridge power, power factor correction, regenerative breaking, electric vehicle load leveling, cold-starting assistance, and catalyst preheat. However, supercapacitors with the required high operating voltages were not available at that time (Figure 2). Critical points of interest were the strong dependence of capacitance on
600
History | Electrochemical Capacitors
Supercapacitors for mobility and electronics
Low voltage
High voltage
Automotive Engineering • • • •
Automotive and Railway Systems
Cold-starting assistance Steer-by-wire Brake-by-wire Catalyst preheat
• Regeneration braking • Electric vehicle load-leveling • Electric trains
Consumer Electronics
Space Technology and Military
• Standby power • Memory backup • Toys
• Pulse power • Electric guns • Silent, vehicles ,
Electric and Aerospace Engineering • • • •
Pulse power supply Security systems Factory automation Robotics
Electrical Power Engineering • Bridge power • Uninterruptable power supply • Power factor correction
Replacement Substitute for batteries and common capacitors
Figure 2 The tree of applications, which was designed in the early 1990s, ramifies in a low-voltage and a high-voltage branch. The challenging goals have brought about the first low-voltage devices in the last decades.
frequency and temperature, and the voltage balancing of the individual cells in a series combination of capacitors. Hybrid vehicles, using a combination of batteries and supercapacitors, have been discussed with all technical consequences involving state-of-charge electronic control and switching equipment. By 1995 supercapacitor devices based on various technologies were being manufactured: a) Carbon – organic: Panasonic (3 V, 2.2 Wh kg1, 400 W kg1), Maxwell Laboratories (24 V, 6 Wh kg1, 2000 W kg1); b) Carbon aerogel – aqueous: Livermore National Lab (1 V, o2 Wh kg1, 1000 W kg1); c) Metal oxide – aqueous: Pinnacle Research Institute (28 V, 0.8 Wh kg1, 500 W kg1); d) Conductive polymers – aqueous: Los Alamos National Lab (0.75 V, o2 Wh kg1, 500 W kg1). Since the beginning of the twenty-first century, supercapacitors have been produced by a respectable number of companies all over the world: Ness Capacitor Co. (Korea); NEC-Tokin, Panasonic/Matsushita, Nippon Chemi-Con ( Japan); Cooper ‘PowerStor’, ELNA ‘Dynacap’, Jeol ‘Nanogate Capacitor’, Evans Inc., Maxwell ‘BoostCap’ (USA); Tavrima (Canada); WIMA (Germany); ESMA (Russia); and Cap-XX (Australia).
Meanwhile, hybrid vehicles have been powered by a combination of internal combustion engine, generator, batteries, and supercapacitors. In Japan, Toyota is already using a supercapacitor module for the energy backup of the breaking system of its ‘Prius’ hybrid vehicle. Honda presented its ‘FCX’ fuel cell vehicle in 2001, where a supercapacitor module was used in the 78 kW power-storage system. BMW in Germany supported the 4.4 l V8 internal combustion engine in a ‘SUV X 5’ vehicle by an electric drive train with a powerful supercapacitor module, which is able to provide an extra torque of 650 Nm at 1000 min1 and a 15% reduction in fuel consumption by regenerative breaking. MAN equipped a diesel electric bus, in the city of Nuremberg (Germany), with a module of 8 36 supercapacitors, which was able to store up to 300 kWh of braking energy and to deliver 90 kW of accelerating power for 13 s after each stop. During the 3-month test, fuel consumption was reduced by 20%, and low noise emission was an additional benefit. Since around 2003 supercapacitors have been tested for pitch control of wind power generators as maintenance-free energy backup for the emergency systems under extreme environmental conditions. Mounted inside the nacelle close to the blades, the capacitor bank of >30 capacitors connected in series is exposed to heavy
History | Electrochemical Capacitors
mechanical stress. Nevertheless, thousands of modules have proven their reliability in 24-h duty for >3 years. ¨ V (Technical Inspection Authority) has The German TU approved 220 F/28 V modules for starting large-volume diesel engines of emergency generators, trucks, and locomotives. A few companies produce integrated modules with voltage balancing circuitry. Many engineering aspects of the technology development of electrochemical capacitors, similar to those for the manufacture of batteries and electrolytic capacitors, have been considered in recent years, for example, packaging, electrode sealing, separator technology, and accommodation of electrolyte solutions. At the same time, specialized requirements have arisen, such as the preparation of low-resistive materials with defined poresize distribution corresponding to maximized areas per gram of active material that are accessible to charging at the electrolyte interface. Highly pure electrode materials and electrolyte solutions were made available in the last decade, although they cannot yet meet the cost requirements for the mass markets in automotive and electric industry. In 2006, supercapacitors with capacitance of 1000– 5000 F and usable energy densities between 3.5 and 4.9 Wh kg1 were commercially available from Maxwell, Ness, EPCOS, Nippon Chem-Con, and Power Systems. Low-cost supercapacitors might capture future automobile markets, although the small energy density, in contrast to 25 Wh kg1 of a typical lead–acid battery, has hampered the market success in power applications. EPCOS in Germany surprisingly stopped their production of ultracapacitors in summer 2006. Actual production of supercapacitors is restricted to a relatively small number of leading companies.
Ever-Young Carbon Technology Carbon electrodes have been widely used as anode and cathode material in electrochemical processes since the nineteenth century (see Figure 3). Carbon was found to be a more or less inert electrode material that comes close to an ideally polarizable electrode. However, its industrial use for electrolysis has been restricted since the 1970s, when its oxidation to organic acids and carbon dioxide was observed. Nevertheless, the multifaceted field of specialized carbon materials has flourished with respect to the development of fuel cells and lithium-ion batteries. For technical details, see Capacitors: Electrochemical Double-Layer Capacitors: Carbon Materials. From Charcoal to Continuous Electrodes The book by Hassler gives an impression of the state of the art on activated carbons in the early 1960s, when the first supercapacitors were developed. Becker’s prototype
601
employed tar-lump black in sulfuric acid. Rightmire used carbon pastes, pathbreaking for later developments in Japan. High specific surface area powders and carbon fibers have been employed in supercapacitors, in close relationship to fuel cell applications since the 1970s. Graphite has been generated in the form of high-area fibers and felts. Powders of amorphous carbon and carbon black (soot), which are also useful for adsorption purposes in chemistry, have been adapted. Matsushita explored polyvinylpyrrolidone and carboxymethyl cellulose as binders for graphite, carbon black, and active carbons. Around 1986 they produced activated carbon fibers from phenol resins, PAN, and rayon. Asahi Glass, Elna, Hitachi, and, later, Toyota were among the first companies that used PTFE-bound carbon in supercapacitors. A Japanese patent of 1989 (JP 1227417) reads: ‘‘Activated carbon powder, carbon black and PTFE are mixed at the 8:1:1 ratio by wet kneading and the mixture is rolled into sheets to make the polarization electrodes.’’ In the early paste electrodes of the 1970s, carbon powder was bound by paraffin or polymer binders. Activated carbon particles were later painted or rolled on aluminum foil collectors. The rock-like carbon pieces were originally bound by wires of PTFE and additional carbon black to facilitate conductivity between the activated carbon particles. By around 1990 (JP 2235320) Japanese manufacturers had command of the fabrication of continuous electrodes by pressing a mixture of carbon powder, fluoropolymer, and liquid lubricant between heated rolls. Active carbon band electrodes were presumably the most memorable milestone in the commercialization of spiral-wound double-layer capacitors. NEC’s thin-type electrodes of 1998 (EP 0867902) contained carbon fine grains of 10 mm diameter, dispersed in a thermoplastic binder resin mixture of polyvinyl butyrate, alcohol, and acetate. By using a doctor blade process, current collector and polarizable electrode sheets were formed, and then they were laminated by thermocompression. Similar polymer-bound carbon electrodes were suggested in patents by Matsushita (EP 0948005), Asahi Glass (DE 19849929), Nippon Valqua (JP 2000323363), Japan Gore Tex (US 6359769), Ness Capacitor Co. (EP 1256966), EPCOS (DE 10203143), and Bollore´ (US 6671166). In a recent patent (EP 1672652) of Japan Gore Tex Inc. and Hitachi Powdered Metals, the production of a continuous electrode is disclosed: An etched aluminum collector is coated with a mixture of natural flake-like graphite, acetylene black, and styrene–butadiene rubber, carboxymethyl cellulose, or a thermosetting polyimide. The polarized porous electrode is formed of a kneaded mixture of activated carbon and PTFE powders, and then rolled at a thickness of 0.3 mm. The separator is a drawn porous PTFE film, which was rendered hydrophilic.
602
History | Electrochemical Capacitors
Carbon
Metal oxide
1991 Panasonic
Polymer
1991 Dornier
Power capacitor
1992 Alcatel, Innovision (Danionics)
Double-layer capacitor 1990 Giner, USA
1990 Evans, USA
1985−1994 Pinnacle
RuO2 / Nafion
Capattery
Ultracapacitor
1985 Siemens
1978−1980 Matsushita
Raney nickel
Gold capacitor 1980 McHardy
1979 NEC
Iridium oxide Nafion
Supercapacitor
1975−1985 Craig, Conway
ab 1962 Maxwell 1974 Blackjack 1979 Shiva 1982 Scremp 1985 Checmate 1988 Army
1853, 1879 Helmholtz
1984 Gottesfeld
1906, 1910 Gouy
1962−1971 SOHIO
1957 Becker
1969 Electrokinetic capacitor
1913 Chapman
1924 Stern
Underpotentialdeposition
Low-voltage electrolytic capacitor
1947, 1955 Grahame
1971 Trasatti
Figure 3 The historical roots of supercapacitor technology. Most of the commercially available devices are based on the carbon technology.
Activated carbon materials have been fabricated by thermal carbonization of pitch, coal, coconut shells, or wood, or by pyrolysis of organic polymers, such as PAN. In the late 1990s it was clear that the accessible surface area at the electrode/electrolyte interface seems by far more important than the physical surface area measured by the Brunauer–Emmet–Teller absorption (BET) method. Activation procedures have been developed after the discovery that heat treatment in vacuum, nitrogen, hydrogen, steam, or carbon dioxide (at 1000–2800 1C) opens up microporous structures, changes surface oxygen functional groups, and modifies the aromatic character of graphite layers. The electrolytic oxidation of carbon fibers in potassium hydroxide to improve adhesion to resins was known in 1983 (e.g., JP 58104222). Around 1995, high-surface carbonaceous materials were developed by treating coconut shells, wood, flour, coal, or resin in an alkali hydroxide bath (US
5430606). Takeuchi et al. (JEOL Ltd) described in a 2004 patent (US 6721168) how petroleum-based needle coke is dry-distilled and activated with potassium hydroxide powder at 800 1C in a stream of nitrogen, washed, posttreated in a stream of hydrogen, vacuum-dried, kneaded in a mixture of carbon black and PTFE binder, and finally pressed in hot rollers between aluminum foils. Carbon Fibers Vapor-permeable sheet-like materials have been produced by Toray Industries in Japan since the 1970s, as stated in GB 1221372 and US 6489051. Such activated carbon papers have been widely known for their use in fuel cells. Fibrous carbon structures from pitch, PAN, or cellulose fibers have been modified by high-temperature programmed activation after the discovery that this leads to multiple splitting of the
History | Electrochemical Capacitors
fibers, producing an enhanced surface area and allowing higher charge/discharge currents. Novoloid fibers have been manufactured, e.g., by Nippon Kynol Inc., as three-dimensional phenolic-aldehyde fibers by acid-catalyzed cross-linking of melt-spun novolac resin with formaldehyde. Maxwell’s 2300 F/2.3 V double-layer capacitor patented in 1999 (US 5862035) comprised activated carbon fiber cloth electrodes, impregnated with aluminum, on a current collector foil; an electrolyte based on acetonitrile and tetraethylammonium tetrafluoroborate; and a porous polypropylene separator. Glassy Carbon, Foams, and Nanomaterials In the late 1970s, Siemens (DE 2842352) obtained glassy carbon electrodes by carbonizing three-dimensionally cross-linked phenol-formaldehyde or furan resins, and activation in boiling sulfuric acid. Glassy carbon is a most plane, chemically inert, highly conductive, and gas-tight material that can be polished by a corundum slurry. Fundamental research was done by the Paul Scherrer Institute in Switzerland around 2001. Despite worldwide R&D activities, double-layer capacitors based on glassy carbon, aerogels, and nanomaterials have not become commercially available so far, as these materials are far more expensive than activated carbon powders. Microcellular carbon foams (aerogels) were intensively investigated in the 1990s. Cooper Electronic Technologies (Boynton Beach, Florida) was the first to market aerogel supercapacitors. Derived by controlled pyrolysis and carbonization of polymers such as PAN, polymethacrylonitrile (PMMA), resorcinol-formaldehyde, divinylbenzene cross-linked methacrylonitrile, phenolics, and cellulose rayons, carbon aerogels provide high surface areas of about 400–1000 m2 g1, good electrical conductivity, and a controllable pore-size distribution. The extreme blackness is caused by internal scattering and absorption of light at the graphite particles. Aerogels can be spun into nanotube fibers with a strength greater than that of Kevlar. Aerogels were originally produced in 1931 from silicon dioxide, and they exhibit the lowest density, highest thermal insulation, lowest refractive index, and the highest surface area per unit volume for any solid (up to 99.9% empty space). The alternative to the expensive supercritical drying was found by Hoechst (Germany) and Yonsei University (South Korea), who developed an extremely hydrophobic coating on the silica wet gels to prevent shrinking while water is rapidly expelled. Carbon nanotubes were described in a 1987 patent of H. G. Tennent (US 4663230) as ‘cylindrical discrete carbon fibrils’. Hollow nanotubes were grown by catalytic decomposition of hydrocarbons or generated in an electric arc.
603
Discovery of Redox-Active Metal Oxides Ruthenium Dioxide
Since the early 1970s, ruthenium dioxide has been used in ‘dimensionally stable anodes’ (DSA) for the chlor-alkali electrolysis, which operates at about 95 1C in brine. According to Henri Beer in US 3632498 (1972), the DSA comprises a catalytic coating consisting of titanium–ruthenium oxide formed on a titanium base. A precursor material, for example, an alcoholic solution of ruthenium(II) chloride, (RuCl3) or (NH4)3RuCl6, is painted onto titanium substrates, sometimes with the addition of titanium isopropylate or titanium(III) chloride (TiCl3), and heated up to 350–550 1C. Platinum metal oxides form electronically and ionically conductive powders, which can be coated on titanium or nickel supports. In 1971, Trasatti recognized the pseudocapacitance of ruthenium dioxide films, thermally formed on titanium, in the almost rectangular, mirror image-like cyclic voltammograms. Ruthenium dioxide exhibits two or three overlapping, extremely reversible oxidation and reduction steps in charge or discharge over a potential range of 1.4 V above the reversible hydrogen potential, e.g., in aqueous sulfuric acid. By integration of current versus voltage in the voltammogram curves, Trasatti found that the electric charges for the oxidation and reduction processes tend to decrease with increasing temperature of calcination, whereas the increasing thickness of the ruthenium(IV) oxide (RuO2) films up to several micrometers was advantageous. Obviously, the fraction of the oxide film accessible to faradaic reactions was destroyed at high temperatures. Various researchers confirmed later that the pseudocapacitance of RuO2 cannot only be due to surface redox processes and double-layer charging, but some bulk-phase reactions must also take place. Single-crystal RuO2, except at defective regions at the surface, did not prove to be electrochemically active. Enhancement of electrocatalysis for anodic chlorine evolution at thermochemically prepared ruthenium and iridium dioxide was observed after extended potentiodynamic cycling, reflecting a progressive increase in accessible surface area. Nonstoichiometry of the oxide material proved to be responsible for the good electronic conductivity. In a 6-year project from 1975 onward, the Ottawa group around Conway, in collaboration with Continental Group Inc., tested various embodiments of RuO2 and mixed oxides with titanium oxide or tantalum peroxide (Ta2O5) for applications in double-layer capacitors. Bipolar stacked electrodes were used in 12-V devices. Thermochemically prepared films were found to be much less reversible than films obtained on ruthenium metal by continuous anodic cycling between 0.05 and 1.4 V. Thoughts of a bulk diffusion process involving H3O þ species were suggested by Gerischer in 1978. Platinum
604
History | Electrochemical Capacitors
metal oxides appear to have electron-hopping conductivity, and the protons from the electrolyte are able to access the quasi-three-dimensional electrode structure. Iridium dioxide shows its metal-like conductivity and pseudocapacitance over a more limited potential range (0.5–1.4 V RHE) and less reversibly than RuO2. Rajeshwar and coworkers published in 1991 the low activation energy of 4–5 kJ mol1 for the charge storage in RuO2 electrodes, and attributed this to a facile proton diffusion via a Grotthus-type hopping mechanism. In March 1993, Dornier, a company of the later Daimler Group, placed a German patent DE 4313474 of a supercapacitor utilizing ruthenium oxide hydrate. RuO2 xH2O was prepared by precipitation from RuCl3 solution with potassium hydroxide (which proved better than sodium hydroxide). The preparation of ultrafine particles by socalled sol–gel processes was in the air at that time. The obtained powders were filtrated, washed, and dried at 90– 105 1C to remove adhesive water, and then coated on nickel foil or carbon paper with the help of an oily binder, which had then also been used as a solvent in paint and finishing products. Titanium supports, employed in earlier devices, were soon discarded because of the growth of a badly conducting oxidation layer. The electrolyte was 6 mol L1 potassium hydroxide solution and 3 mol L1 sulfuric acid, respectively. In an automotive project aiming at hybrid vehicles, bipolar supercapacitors with rated voltages of up to 50 V were manufactured and tested successfully over several million charge–discharge cycles (Figure 4). The outstanding capacitance of up to 4 F cm2 and roughly 450 F g1 was explained by the high specific surface area of finely distributed oxide on porous carbon paper, the facile
conversion of ruthenium between several oxidation states, and the proton mobility between the oxide and hydroxyl sites in hydrated ruthenium oxide. In particular, high capacitance requires that ruthenium oxide be in the amorphous state with a high degree of hydration. The technology was taken over in 2000 by Hydra, the legal successor of the former AEG, who wanted double-layer capacitors for applications in electrical power engineering. In 1995, partially ‘hydrated’ metal oxides were described by a Russian group with reference to earlier work on DSAs. The authors stated that thermochemically prepared ruthenium oxide layers consist of both crystalline oxide, practically anhydrous, and amorphous hydrated oxide, their ratio being dependent on the calcination temperature and the complex species in the precursor solutions. Ruthenium oxide prepared at 450 1C is described as a crystalline phase, with rutil structure containing 30% of amorphous ‘RuO(OH)2’. Above 650 1C, hydroxy groups and chlorine traces are completely removed; crystallinity tends to 100%; and cyrstallite size increases. Hydrous ruthenium oxide was described in electrochemical literature later in more detail. Jow and coworkers studied the degree of surface hydration and noncrystallinity by X-ray diffractometry in 1995. US 5600535 recommends a pH of 3–6 for the sol–gel process, in which ruthenium(III) chloride is hydrolyzed by alkali metal alkoxides. C. Zheng and R. M. Franklin at Auburn University (US 6025020) reported in 2000 a specific capacitance of 1040 F g1 for highly dispersed ruthenium oxide hydrate, when they heated an aqueous solution of ruthenium chloride on carbon fibers, immobilized in a cellulose
Capacitor voltage (at 12 °C) 300 22 V 250
Current (A)
200 20 V 150 100 18 V 50 0 Motor starts 0
0.5
1.0 Time (s)
1.5
2.0
Figure 4 Current transients during cold starting of a Mercedes C220 with the help of a 60 F/30 V metal oxide hydrate supercapacitor on a fresh spring morning at Dornier in Friedrichshafen (Daimler Group, Germany), in 1997.
History | Electrochemical Capacitors
matrix, in a steam and oxygen atmosphere up to 420– 475 1C, and dried it afterward at 100–150 1C. Iridium Dioxide and Nickel Oxide Iridium dioxide has been investigated since the late 1970s for use in alkaline electrolysis. Iridium dioxide has not been as widely used as RuO2 owing to its higher price and the less reversible redox reactions, which can be studied with the aid of the cyclic voltammogram. Since around 2002, nanoparticles of platinum metal oxides have attracted attention for various electrotechnical applications that have not yet reached a commercial status. Nickel oxide has been intensively studied for use in batteries. Work on its use in supercapacitors by research groups in Asia and the United States has become known mainly since around 2002. For details of nickel in asymmetric supercapacitors, see Capacitors: Electrochemical Hybrid Capacitors. Perovskites and Other Classes Perovskites, such as alkaline earth ruthenates, were developed in the mid-1990s by the Center for Solar Energy and Hydrogen Research in Germany (DE 19640926) in cooperation with Dornier (Daimler Group). Specific capacitance of up to 28 F g1 was obtained for SrRuO3 in supercapacitor applications. Cobalt or aluminum mixed hydroxides/oxyhydroxides exhibit capacitance up to 360 F g1, but the potential window is very limited. A Brazilian research group reported in 2002 that Ir0.3Mn0.7O2, which was synthesized by annealing a mixture of indium(TV) oxide and manganese(IV) oxide between 400 and 450 1C, delivered a specific capacitance close to 550 F g1. Other oxide and nonoxide materials, such as WO3, Co3O4, MnO2, NiO2, WC, TiC, and molybdenum nitrides, have been studied by various researchers. Due to poor operating behavior and narrow voltage window (B0.7 V), none of these materials has found its way into stable supercapacitors so far. For technical details, see Capacitors: Electrochemical Metal Oxides Capacitors.
605
lithium or alkylammonium salts as electrolyte, as for solid polymer electrolyte lithium batteries. Solid polymer electrolytes have been extensively developed in the field of lithium batteries. Polyaniline, known as aniline black, was already described in 1862 by H. Letheby, as a more or less undefined oxidation product of aniline. ‘Metallically’ conducting polymers were discovered in the 1980s, when polyacetylenes were studied by MacDiarmid and other researchers. In addition, polypyrrole and polythiophene were intensively studied in battery technology. Conductive polymers combine the pseudocapacitance of redox storage mechanisms with a high surface area. In the early 1990s, the use of conductive polymers for supercapacitors was discussed by Gottesfeld and other researchers, based on the excellent reversibility of electrochemical charge and discharge over a potential range of about 0.8 V. Polymer-type supercapacitors were developed first by Alcatel Alsthom Recherche in France, the then Innovision in Denmark, and industrial research groups in Japan. However, polypyrrole, polyaniline, and other candidates did not prove stable over some hundred thousand cycles.
Hybrid Capacitors Evans hybrid capacitor of 1995, called ‘capattery’, combined an electrolytic-type tantalum anode, bearing a Ta2O5 barrier oxide layer, and a ruthenium oxide cathode, having a large capacitance, but a low voltage drop. Operating voltages of around 200 V were achieved with capacitances up to 0.1 F, as the stored energy of the capacitor was mainly associated with high voltage drop in the Ta2O5 dielectric. The energy storage capability was therefore higher than that in a regular RuO2 double-layer capacitor. The hybrid pooled 11 times higher volumetric and 2.5 times higher gravimetric energy density than a corresponding aluminum electrolytic capacitor. For details on development by Russian groups, see Capacitors: Electrochemical Hybrid Capacitors.
Solid-State and Polymer Technology Nomenclature Solid-state electrochemical capacitors based on RbAg4I, a silver ion conductor described in the late 1960s, were never realized. The critical factor was to achieve sufficient conductivity at ambient temperatures. Giner Inc. (USA), in the early 1990s, presented supercapacitors based on ruthenium oxide that was bonded by Nafion solution and coated on a Nafion membrane (US 5136474). The design avoided a liquid electrolyte and was analogous to membrane electrolyte fuel cells. Bollore´ (France) suggested carbon electrodes and thin films of polyethylene oxide as solvent, and
Symbols and Units C Q U W
capacitance charge storage voltage storable energy
Abbreviations and Acronyms BET EDLC PAN PMMA
Brunauer–Emmet–Teller adsorption electrochemical double-layer capacitor polyacrylonitrile polymethacrylonitrile
606 PP PTFE RHE
History | Electrochemical Capacitors polypropylene polytetrafluoroethylene reversible hydrogen electrode
See also: Capacitors: Electrochemical Double-Layer Capacitors: Carbon Materials; Electrochemical Hybrid Capacitors; Electrochemical Metal Oxides Capacitors; Electrochemical Polymer Capacitors.
Further Reading Boos DL and Argade SD (1991) Historical background and new perspectives for double-layer capacitors. Proceedings of 1st International Seminar on Double Layer Capacitors. 9–11 December. Deerfield Beach FL, USA: Florida Educational Seminars. Bullard GR, Sierra-Alcazar HB, Lee HK, and Morris JJ (1989) Operating principles of the ultracapacitor. IEEE Transactions on Magnetics 25(1): 102--106. Conway BE (1999) Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications. New York: Kluwer Academic/Plenum Publishers. Delnick FM, Ingersoll D, Andrieu X, and Naoi K (eds.) (1997) Electrochemical Capacitors II. Proceedings, Vol. 96–25, Pennington, NJ: The Electrochemical Society. Doblhofer K, Metikos M, Ogumi Z, and Gerischer H (1978) Electrochemical oxidation and reduction of RuO2/Ti electrode surface. Berichte der Bunsen-Gesellschaft fu¨r Physikalische Chemie 82: 1046. Grahame DC (1947) The electrical double layer and the theory of electrocapillarity. Chemical Reviews 41: 441--501. Guther TJ, Oesten R, and Garche J (1997) Development of Supercapacitor Materials Based on Perovskites, in: Delnick, pp. 16–25. Hassler JW (1963) Activated Carbon. New York: Chemical Publishing Co. Helmholtz Hv (1870) Studien u¨ber elecktrische Grenzschiten. Annalen der Physik 7: 337--382. Helmholtz Hv (1879) Annalen Der Physik, Leipzig 3: 223. (1853) 89: 21. Kurzweil P and Fischle H-J (2003) Proceedings of 13th International Seminar on Double Layer Capacitors. Deerfield Beach, USA, 8–10 December. Kurzweil P and Schmid O (1996) High performance metal oxide supercapacitors. Proceedings of 6th International Seminar on
Double Layer Capacitors and Similar Energy Storage Devices. Deerfield Beach, USA. Kurzweil P, Schmid O, and Schmid B (1994) Precipitated Ruthenium Oxide-Hydrate. German Pat. DE 4313474, filed on 24 April 1993; EP 0622815 (1994), US 5,550,706 (1996). Lee H, Bullard GL, Mason CG, and Kern K (1989) Improved pulse power sources with high-energy density capacitor. IEEE Transactions on Magnetics 25(1): 324--330. Letheby H (1892) Journal of the Chemical Society, London 15: 161. Nigrey PJ, MacInnes J, Nairns DP, MacDiarmid AG, and Heeger AJ (1981) Lightweight rechargeable storage batteries using polyacetylene, (CH)x, as the cathode-active material. Journal of the Electrochemical Society 128: 1651. Novak P, Mu¨ller K, Santhanam KSV, and Haas O (1997) Electrochemically active polymers for rechargeable batteries. Chemical Reviews 97: 207. Roginskaya YE and Morozova O (1995) The role of hydrated oxides in formation and structure of DSA-type oxide electrocatalysts. Electrochimica Acta 40(7): 817--822. Sanada K and Hosokawa M (1979) Electric double layer capacitor ‘super capacitor’. NEC Research and Development 21: 21--28. Sarangapani S, Lessner P, Forchione J, Griffith A, and Laconti AB (1990) Advanced double layer capacitors. Journal of Power Sources 29: 355--364. Schmickler W (ed.) (1995) Ladungsspeicherung in der Doppelschicht, Proceedings of 2nd Ulm Electrochemical Talks, 291–310; and literature cited there. Ulm: Universita¨tsverlag. Sekido S, Yoshina Y, Muranaka T, and Mori M (1980) Article on ‘Gold capacitor’ by Matsushita. Denki Kagaku 48: 40. Takasu Y, Onoue S, Kameyama K, Murakami Y, and Yahikozawa K (1994) Preparation of ultrafine RuO2–IrO2–TiO2 oxide particles by a sol–gel process. Electrochimica Acta 39(13): 1993--1997. Trasatti S and Buzzanca G (1971) Ruthenium dioxide: A new interesting electrode material, solid state structure and electrochemical behavior. Journal of Electroanalytical Chemistry 29: App: 1--5. Trasatti S and Kurzweil P (1994) Electrochemical supercapacitors as versatile energy stores. Platinum Metals Review 38(2): 46. Trasatti S and Lodi G (1980) Conductive Metal Oxides, Vols. A and B, Amsterdam: Elsevier. Tsai EW and Rajeshwar K (1991) Influence of temperature on the voltammetric response of thermal ruthenium oxide elctrodes. Electrochimica Acta 36: 27. Zheng JP, Cygan PJ, and Jow TR (1995) Hydrous ruthenium oxide as electrode material for electrochemical capacitors. Journal of Electrochemical Society 142: 2695. Zheng JP, Cygan PJ, and Jow TR (1995) Journal of Electrochemical Society 142(8): 2699. Zheng JP and Jow TR (1995) Journal of Electrochemical Society 142(1): L6.