Patrice Simon, Yury GogotsiUniversite Paul Sabatier, 31062 Toulouse Cedex 4, FranceInstitut Universitaire de France, 75005 Paris, FranceDept of Materials Science and Engg, Drexel University, Philadelphia 19104, USA

Karthikeyan G09MS6021

Nature materials | Volume 7 | November 2008

Materials for electrochemical capacitors

Introduction

Lithium-ion batteries

Introduced by Sony in 1900

High energy densities – 180 Wh/kg

Slow power delivery or uptake

Supercapacitors

Charged and discharged in seconds

Low energy densities – 5 Wh/kg

High power delivery or uptake in short time

Supercapacitors / Ultracapacitors / Electrochemical capacitors

Types of Supercapacitors:

Elecrochemical double layer capacitors (EDLCs) – carbon basedactive materials

Pseudocapacitors / redox supercapacitors – fast and reversible surfaces for charge storage – Transition metal oxides, electricallyconducting polymers are used as active materials

Hybrid capacitors – combination of capacitive electrode with batteryelectrode

Electrochemical double layer capacitors (EDLCs)

Electrostatic storage of charges by absorption of ions of the electrolytic solution onto theactive material which has high accessible specific surface area.

Charge separation occurs on the electrode-electrolyte interface producing double layercapacitance C

C = εr A / d E = ½ CV2

This capacitance ranges between 5 and 20 μF/cm2 depending upon the electrolyte used.

Note: The capacitance achieved by the aqueous alkaline or acid solutions is generally higher than organic electrolytes. But organic electrolytes are widely used as they cansustain higher operating voltage.

Separator shocked in electrolyte

+ + + + ++ + + + + + ++ +

+ + + + ++ + + + + + ++ +- - - - -- - - - - - -- -

- - - - -- - - - - - -- -Activated carbon electrode

Electrolyte

Activated carbon electrode

Electrochemical double layer capacitors (EDLCs)

Advantages:No faradic reaction at the electrodes. In addition the surface storage mechanism allows very fast energy uptake and delivery and better power performance.

The absence of faradic reactions also eliminates swelling in the active materials that batteries shows during charging/discharging cycles. EDLC can survive millions of cycles whereas batteries can survive few thousands.

Finally solvent of the electrolyte is not involved in the charge storage mechanism. So itdoes not limit the choice of the solvents.

Disadvantages:Limiting energy densities.

Graphite carbon – high conductivity, open porosity, electrochemically stable.

Activated carbon, carbide derived carbons, carbon fabrics, nanotubes, fibers, onions, nanohorns are also tested for EDLC applications.

Activated carbon – carbonization of precursors in inert atmosphere with subsequent selectiveoxidation in CO

2, water vapor, KOH to increase the specific surface area and pore volume.

Precursors – coconut shells, wood, coal, polymers

Capacitance: 100-120 F/g (activated carbon – organic electrolyte) 150-300 F/g (activated carbon – aqueous electrolyte) 50-80 F/g (other carbon materials – organic electrolyte)

Pore size – pores under 1 nm are not accessible. There is no linear relation between surface area and capacitance.

Electrochemical double layer capacitors (EDLCs)

Electrochemical double layer capacitors (EDLCs)

SEM and TEM images for carbon based electrodes

Cyclic voltammetry recorded at room temp and scan rate of 20 mV/s

Electrochemical double layer capacitors (EDLCs)

a. Commercial spirally wound double layer capacitor

b. Assembled device weighing 500 g and ratedfor 2600 F

c. A small button cell – 1.6 mm height and stores 5 F

Redox based electrochemical capacitors

Fast reversible redox reaction at the surface of the active materials.

Metal oxides – RuO2, Fe

3O

4, MnO

2 and conducting polymers.

The specific capacitance exceeds that of carbon materials used in EDLCs but there isa lack of stability during the cycling.

Ruthenium oxide is widely studied and has the specific capacitance of more than 600 F/g.operating voltage 1 V

Conducting polymers – high pseudo capacitance in various non aqueous electrolytes atoperational voltages of about 3 V

Use of nanostructured redox active materials increases the capacitance.For eg: The specific capacitance of RuO

2 is increased by 1300 F/g

Redox based electrochemical capacitors

Hybrid systems

High energy densities by combination of battery like electrode (energy source) and capacitorlike electrode (power source) in the same cell.

– Pseudo capacitive metal oxides with a capacitive carbon electrode

In most of the hybrid systems, the faradic electrode led to an increase in the energy denisityat the cost of cyclability.

The use of positive MnO2 electrode and negative carbon (EDLC) electrode shows highcapacitance in neutral aqueous electrolyte with high cell voltage. The use of nanostructured MnO2 offers much more increase in capacitance

The use of Lithium ion capacitors increases the energy density more than 15 Wh/Kg at 3.8 V

Applications of Supercaps

Small devices (few farads) are widely used in power buffer applications, memory back-upin toys, cameras, mobile phones.

Cordless devices like screwdrivers, electric cutters using supercaps are already availablein market. This can be charged and discharged in 2 minutes.

EDLCs are used in emergency doors of Airbus A380.

In future we can see them in HEV, metro trains, tramways.

Conclusions

High energy delivery is achieved in lithium ion batteries with poor cyclability.

Energy densities in supercaps can be improved by using metal oxide and nanoparticles for pseudo capacitors.

The use of nanoporous carbons increases the capacitance. In future, ECs are expected to come close to present Li-ion batteries in energy densities,maintaining their high power density.

Flexible, printable, wearable ECs are likely to be integrated into smart clothing, sensors,wearable electronics and drug delivery systems.

Ultimately ECs serves as energy solutions where an extremely large number of cycles,long lifetime and fast power delivery are required.