Supercapacitors 1. Principle of operation and physical models 2. Materials used in supercapacitors 3. Applications
Capacitors Electrical capacitance C = Q U U = D 0 E( x) dx Flat capacitor C = Sεε 0 D Separation of charge requires work stored in the form of potential energy 2 2 q Q CU W EL = Vdq = dq = = = C 2C 2 QU 2
History of capacitors Leyden jar
Electrolytic capacitors capacitance up to 1F small dimensions low price self discharge electrolyte degradation electrode degradation sensitive to polarization sensitive to temperature
Electrolytic capacitors Elektrody: - aluminiowe (warstwa Al 2 O 3 ) - tantalowe (warstwa Ta 2 O 5 ) - niobowe (warstwa Nb 2 O 5 ) Wymagania względem elektrolitu: + wysoka stała dielektryczna, + dobra przewodność, + wysokie napięcie dekompozycji. Najczęściej używane roztwory wodne kwasu borowego H 3 BO 3 lub soli sodowej kwasu borowego Na 2 B 4 O 7. Dodatek glikoli spowalnia proces parowania. Słaby kwas lub sole słabego kwasu + Rozpuszczalnik + Dodatki zagęszczające lub stabilizujące
N-n butylo isoquinoline Electrolytic capacitors OS-CON Organic Semiconductor Isoquinoline heterocyclic compund
Supercapacitors
Classification of supercapacitors
Principle of operation
Charging of a supercapacitor
Double layer 1 layer (absorptive) : adsorption on the surface 2 layer (diffusive) : coulombic forces
Models of double layer Helmholtz Guoy-Chapman Stern
DLC type capacitors The thickness of the layer depends on the kind of solvent, ion species, and their concentration up to 10 nm
R.A. Marcus Nobel in 1992 Model describes the rate of electron transfer between the molecules. It is based on redox type reactions without chemical bonds. Accumulation of charge related to redox processes is described as pseudocapacitance, and the proces is called physical
Charging of a pseudocapacitor Electrode Cations Anions Electrode
Materials: electrodes Requirements: - Large surface area - High conductivity - Chemical and thermal stability - Proper mechanical properties - Low manufacturing cost Nanomaterials
Materials: electrodes Small dimensions of the pores influence the properties of the solvation shell of the ion and allow the ion to approach the electrodes.
Carbon electrodes DLC
Pseudocapacitance: metal oxides Structures with large contact surface with the electrolyte. The topology must also provide diffusion in the electrolyte. Charge accumulation based on redox type reactions with physical adsorption RuO 2 TiO 2 VO 2 MoO 2, NiO 2 CoO 2 MnO 2 SnO 2 LiO 2 Cobalt oxide (Co 3 O 4 ) www.nanowerk.com
Electrodes: metal oxides Specific capacitance up to do 500 F/g Low operating voltage, expensive, toxic Cation Specific capacitance up to 150 F/g Voltage around 1V (2V in hybrid capacitors), low cost
Elektrodes: polymers Polyaniline, polypyrolle, polythiophene electronically conductive High specific capacitance, but low specific power due to slow diffusion processes. Layouts: - identical p-doped - p-doped and n-doped inorganic - polymer n and p-doped
Electrodes: polymers
Materials: electrolytes - High conductivity - High electrochemical stability (wide stability window voltage) - Good adhesion and wetting of electrodes - Sufficient safety Liquid: higher conductivity lower costs evaporation and freezing possibility of leakage voltages up to 1V Solid: 10 x lower conductivity high viscosity mechanically stable electrochemically stable safe
Polymer electrolytes C. Huang & P. S. Grant Scientific Reports 3, 2393 doi:10.1038/srep02393 Electrolyte based on Nafion membrane: 91 F/g, 3.3 kw/kg, 90% of capacitance after 2000 cycles Gel-type electrolytes: based on PEO, PMMA, PVA and polyaniline with inorganic acids or alkali groups.
Ionic liquids Lin et al., J. Phys. Chem. Lett., 2011, 2 (19), pp 2396 2401
Materials: separator - Prevents from short-circuiting - Enables ion exchange Polymer/celulose separator in organic electrolytes Ceramic separators in liquid electrolytes
Materials: current collectors Application of supercapacitors in high-power applications demands proper distribution of current. - aluminum - carbon (graphene, nanotubes) - platinum/gold
Simple electrical model
More realistic models Ions travel through porous structure. Reaching of inner parts of pores requires long charging time.
Electrical model The capacitance depends strongly on frequency.
Evaluation of capacitance (IEC) Class 1: Memory Klasa 2: Energy storage Klasa 3: Urządzenia dużej mocy Klasa 4, Impuls mocy I (ma) = 1 C (F) I (ma) = 0,4 C (F) V (V) I (ma) = 4 C (F) V (V) I (ma) = 40 C (F) V (V)
Operating voltage Water-based electrolytes: 1.2V / electrode = 2.4V Organic solvents: 1.8 V/ electrode = 3.6V Lithium conductors: up to 4V (minimal 2.2V) Energy and Power Engineering, 2010, 25-30
Internal resistance The internal resistance limits the maximum discharge current, as well as charging time. Full charging requires at least 5 time constants.
Properties of supercapacitors
Properties of supercapacitors
Properties: operating time The supercapacitors are sensitive to changes of voltage, especially to reverse voltage. Impulses of power may shorten operating period.
Self-discharge Redox processes (impurities): exponentialtype dependence. Diffusion of ions: voltage decreases proportionally to t 1/2
Applications of supercapacitors Non-volatile memory Mobile electronics and electric tools Energy Grids, renewable energy storage UPS systems Road and rail transportation Engine starting systems Hybrid and electric cars KERS systems Cranes and lifts Medicine (defibrillators etc.)
Applications: renewable energy
Applications: engine starters Supercapacitors allow to start the engine even with partially discharged battery.
Start/stop systems
Energy recovery systems
Energy recovery and fast charge Electric trams: Savings up to 30% of energy Some sections of track without supply wires Buses: Savings of fuel up to 15% - hybrid buses Fast charging electric buses Hybrid cars: - city: savings up to 20% (in respect to Liion only) - mixed cycle: savings up to 6%