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Capacitors > Super capacitors > Flashcards

Super capacitors Flashcards

1
Q

Principle of EDLC (compared to caps and batteries)

A

Cap with properties that are halfway between regular caps and batteries

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2
Q

Main differences between battery and EDLC

A
  • Battery stores E charge through chemical reaction (chemical energy)
  • EDLC stores charge by means of E double layer formed by ions adhering to the surface of an activated carbon electrode
  • Battery requires hours to charge (maximum rates of charge/discharge)
  • EDLC can be charged in seconds
  • Number of charge cycles in the battery is limited
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3
Q

Classification of supercapacitors (Thomas)

A
  • EDLC
  • Pseudocapacitors
  • Hybrid caps
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4
Q

3 Materials used in the electrodes of EDLC to increase surface area

A
  • Activated carbons
  • Carbon aerogels
  • Carbon nanotubes
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5
Q

2 Materials used as electrode of pseudocaps

A
  • Conducting polymers
  • Metal oxides
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6
Q

Classification of hybrid caps (3 types)

A
  • Composite hybrids
  • Asymmetric hybrids
  • battery-type hybrids
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7
Q

Draw the curve of charge/discharge of a battery and ultracap

A

Draw on paper and write (derive) the equation
- Q = V*I/(dV/dt)

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8
Q

Compare dis/charge time, energy and power density, dis/charge efficiency, recycle life, T range of ultracaps and batteries

A

Ultracaps|Battery
- dis/charge time: seconds|hours
- energy density (Wh/kg): 5-15|20-150
- power density (W/kg): 2k-10k|20-150
- efficiency: 98%|80%
- recycle life (cycles): 1 million|1000
- T range: -50+70°|-10+40°C

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9
Q

What is the reason (2) of very high C value of EDLC?

A
  • The very large surface area of activated carbon due to its porosity
  • distance is also minimized to increase C
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10
Q

Main components of EDLC

A
  • metallic I collectors
  • Electrodes (of activated carbon or Li-ion doped)
  • Electrolyte (forms the oxide layer)
  • Separator
  • Sealing
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11
Q

Draw the schematic of charge process of EDLC (Thomas pag 12)

A

Draw on paper

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12
Q

Main elements of double layer at electrode interface of EDLC (draw on paper) (Thomas pag 13)

A
  • Hydration sheat
  • Inner Helmholtz
  • Outer Helmholtz
  • Diffuse space charge
  • Unsolvated ion
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13
Q

Supercapacitor cell types (Thomas pag 14) and one application

A
  • Chip type: clock
  • Coin type: for standby power
  • Pouch type: energy harvesting
  • Cylindrical type: LED signs
  • Modules; UPS, wind power
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14
Q

3 Supercaps cell types characterized by ultra-low (<0.1F) and low (0.1-1F) C value

A
  • Chip type (utra-low)
  • Coin type (both)
  • Pouch type (low)
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15
Q

Supercaps cell types characterized by medium (1-100F) and high (>100F) C value

A
  • Cylindrical (medium)
  • Modules (both)
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16
Q

Which structure has even larger surface area than activated carbon?

A
  • Carbon nanotubes
  • Graphene
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17
Q

3 Main classification of electrolyte used in EDLC (Thomas pag 25)

A
  • Liquid electrolyte
  • Solid-state or quasi-solid-state
  • Redox-active
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18
Q

Main solvents and solute used in organic electrolyte (Thomas pag 25)

A

Organic electrolyte
- Solvent: Acetonitrile ACN
- Solute: TEABF4

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19
Q

3 main functions of separator in EDLC

A
  • To prevent from short circuit
  • Storage of electrolyte
  • Allow passage of (transport) ions during charging/discharging
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20
Q

Which characteristic of the separator affects power and energy density of cap? (Thomas pag 27)

A

Ionic conductivity (it influences ESR)

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21
Q

5 Main characteristics required in the separator

A
  • thermal stability
  • high mechanical strength
  • high porosity
  • High impregnation capability
  • HIgh ionic conductivity
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22
Q

Main problem of series connection of supercaps

A
  • V imbalance, can cause over-voltage of a cell
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23
Q

Methods to balance voltage of supercaps

A
  • parallel resistors
  • zener diodes across caps
  • FETs
  • dynamic stack monitoring
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24
Q

Rule of thumb to choose parallel R for V balancing

A
  • Measure leakage I of supercap
  • use parallel R that has 10x the I at nominal cell V
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24
Q

What are the reasons (2) of V imbalance in series-connected caps

A
  • Tolerance of the cells (i.e. 20%)
  • variation of leakage I
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25
Q

6 main effects of overvoltage, over T, and excessive mechanical stress in cap cells

A
  • capacitance loss
  • increase of ESR
  • venting
  • electrolyte decomposition
  • gas generation
  • reduced lifetime
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26
Q

How does FET balancing work? Draw the schematic of this solution

A
  • FETs will turn on slightly as the capacitor cell approaches the operating rated V
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27
Q

Main steps in production process of supercaps

A
  • Mixing
  • Coating
  • Slitting
  • Electrode making
  • Winding
  • Forming
  • Heat sealing
  • Filling and presealling
  • Formation
  • vacuum sealling
  • Test
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28
Q

Which process consumes a lot of water that has then to be dried?

A
  • Coating
  • water has to be dried afterwards
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29
Q

Mention a problem of replacing nuclear/hydraulic plants for distributed renewable generators

A
  • Nuclear/hydraulic have huge inertia that can help to stabilize the grid. Disturbances from the loads cause almost no effect (robustness)
  • Distributed renewable generators have small inertia.
  • i.e. spinning turbines of thermal plants connected to grid create kinetic energy (inertia) helping to keep the network at the right frequency
30
Q

In which application can supercapacitors replace Al El caps?

A

Supercaps can replace much larger banks of El caps in UPS, reducing cost per cycle, maintenance, downsizing the battery, and extending battery life

31
Q

5 General applications of supercaps

A
  • kinetic energy recovery in F1 cars or EVs: energy is used during deceleration and used to power electrical systems when motor is stopped
  • backup power to low-power equipment i.e. RAM or microcontrollers
  • to stabilize power supplies in laptops, PV systems, bikes
  • deliver power for camera flashes
  • help to extend lifecycle of vehicle batteries subjected to aggressive power profile
32
Q

at least 6 main general characteristics of supercaps

Includes 1 of assymetric

A
  • very high C value
  • low V ratings
  • very high power density
  • unlimited number of cycles
  • cannot withstand AC V (assymetric)
  • fast charge/discharge
  • low self-discharge
33
Q

write the equation of the time t that a supercap can deliver a constant I

A

t = C*(Vcharge-Vmin)/I
capacitor V decreases from Vcharge to Vmin

34
Q

write the equation of time t that a supercap can deliver constant power

A

t = C(Vcharge^2-Vmin^2)/(2P)
capacitor V decreases from Vcharge to Vmin

35
Q

What is the reason of using electrolyte in supercaps and Al EL caps?

Not comparison

A

Because it’s the best material to make contact on the huge surfaces of the electrodes

36
Q

Do ultracaps work with charged ions instead of polarized atoms? give a reason

A
  • They work with charged ions
  • Ions are stronger than polarized atoms (higher polarization)
37
Q

How much surface does a gram of activated carbon has?

A

The surface of a football stadium

38
Q

2 disadvantages of activated carbon (class notes)

A
  • cannot withstand high V
  • has small amounts of water which will result in electrolysis
39
Q

2 Reasons by which supercaps cannot withstand high V

A
  • activated carbon
  • electrolysis
40
Q

Where does the V drop occur in a supercap?

A

In the Helmholtz layers

41
Q

Is the activated carbon the dielectric in an EDLC and justify?

A

Activated carbon is not the dielectric because voltage drop does not occur here but in the Helmholtz layers

42
Q

Basic principle (4 ideas) of electrochemical cap (EC)

3 ideas related to dielectric layer

A
  • No separate dielectric is used
  • Dielectric is naturally formed as thin layer once electrolyte flows through a porous activated carbon electrode and after V is applied
  • Double Helmholtz layer acts as the dielectric
  • V rating limited by decomposition potential of electrolyte, so series parallel stacks of caps are used
43
Q

V rating of aqueous electrolyte and non-aqueous electrolyte

A
  • Aqueous: 0.8-1.6V
  • Non-aqueous: 3-5V
44
Q

How an EDLC is formed?

A
  • Two porous carbon electrodes are used
  • Electrolyte fills the remaining space
  • Separator is used between the 2 sections
  • 2 layers of dielectric are formed, a C at each surface
  • total C is the series combination of 2 caps, with increased operating V
45
Q

Classification of supercaps based on electrode type

A
  • Symmetrical caps
  • Asymmetrical caps
  • Li-ion caps
46
Q

Explain basics (5 ideas) of symmetrical caps

A
  • Both electrodes use same material (carbon)
  • Cell voltage rating 1.2V (aqueous) and 3.5V (non-aqueous)
  • don’t have polarity
  • can be discharged to zero volts
  • Lower ESR than asymmetric (usually)
47
Q

Explain basics of asymmetrical caps (6 ideas)

Compare to symmetrical caps (EDLCs)

A
  • Use different materials for the 2 electrodes (nickel hydroxide and carbon)
  • higher cell voltage than symmetric
  • can be discharged only to half the rated V
  • can store 4 to 5 times energy density than symmetrical
  • Electrodes undergo Faradaic reactions (like battery, + terminal) and capacitive behaviour (activated carbon, - terminal)
  • Polar as they don’t have balanced electrode design
48
Q

Explain basics (7 ideas) of Li-ion caps (Deshpande 213)

A
  • Activated carbon used as cathode
  • anode made of carbon material pre-doped with Li ions
  • pre-doping lower the potential at the anode and allows a high output V
  • output V of about 3.8-4V, so higher energy densities
  • Capacity of anode is much higher than that of cathode
  • Change of anode potential during charge/discharge is much smaller than change in cathode potential (V=Q/C)
  • Expensive
49
Q

6 Advantages of supercaps over a battery

A
  • Higher energy efficiency
  • Lighter (smaller)
  • Deliver more power
  • Almost unlimited life
  • Fast charge/discharge (no electrochemical process)
  • Work over large T range
50
Q

Explain the phenomena that happen in an EDLC when V is applied (5 ideas)

A
  • Positive and negative charge builds up on the 2 electrodes
  • Each electrode attracts ions of opposite charge
  • Ions in electrolyte remain in charge balance but are diffused to oppositely charge electrode
  • No chemical reactions occur
  • Ions form a very thin sheet (1nm). This electrical double layer can sustain up to 10MV/cm or more
51
Q

General construction of EDLC

A
  • Metal electrodes are coated with activated carbon
  • plates are immersed in electrolyte of + and - ions dissolved in a solvent (works as dielectric)
  • Separator filled with electrolyte isolates carbon electrodes and prevent electrical short
52
Q

Compare electrochemical with Al El caps (at least 4 differences)

A
  • Al El cap is more expensive per farad
  • EC has higher ESR
  • Al El can sustain higher V
  • EC has higher energy density
53
Q

at least 5 Advantages of supercaps (Desphande ultracaps)

A
  • Long life, little degradation
  • High power output
  • Fast charge/discharge
  • High efficiency 98%
  • Low ESR
  • Safer, no corrosive electrolyte low toxicity
  • No overcharging problem
54
Q

3 Disadvantages of supercaps (Deshpande ultracaps)

1 compared to battery

A
  • Lower stored energy per weight compared to batteries
  • Highest dielectric absorption
  • Low V rating
55
Q

2 Problems of using non-aqueous electrolytes

Related to manufacturing, not electrical properties or performance

A
  • Requires dry rooms (expensive)
  • Vacuum chambers (expensive)
56
Q

4 Factors that are being improved to increase C value of supercaps (energy density)

A
  • Make charge separation very thin (d)
  • Choice of chemicals to get high dielectric constant of separation barrier
  • Increase effective area
  • Use electrolyte with higher decomposition V (organic Electrolyte)
57
Q

What is the effect of having two caps connected in series in the EDLC in terms of V and C?

A
  • BV voltage is doubled
  • C is reduced by half
58
Q

One of the main factors that determine the operating V

A
  • Electrolyte
59
Q

Difference between faradaic and non-faradaic mechanisms

A
  • Faradaic: involve transfer of charge between electrode and electrolyte
  • Non-faradaic: does not use chemical mechanism, charges distributed on surfaces by physical processes without making or breaking chemical bonds
60
Q

3 Types of storing charge mechanism in supercaps

A
  • Faradaic
  • Non-faradaic
  • combination of the two
61
Q

Describe briefly pseudocaps

A
  • structure and characteristics similar to EDLC
  • use metal oxide electrodes instead of carbon
  • use oxidation-reduction reactions at an interface (like in batteries)
  • store charge faradaically by transfering charge between electrode and electrolyte, but ions simply stick without any chemical reaction to the atomic structure of an electrode.
62
Q

5 advantages of pseudocaps over EDLC

A
  • High surface area and fast faradaic reactions
  • Low-cost high performance material
  • Higher energy density
  • Similar fabrication process than EDLC
  • Larger C for same volume
63
Q

3 limitations of pseudocaps against EDLC

A
  • lower power densities (slower)
  • limited cycle life due to mechanical stress during reduction-oxidation reactions
  • Best metal-oxide electrodes are expensive and require aqueous electrolytes (low voltage)
64
Q

3 properties electrodes must have

A
  • high conductivity
  • high T stability
  • chemically inert and resistant against corrosion
65
Q

Main difference between supercaps and Al El caps regarding the function of electrolyte

A
  • In Al El caps electrolyte is the cathode
  • In EDLC is the conductive connection between electrodes
66
Q

Materials of separator, 2 functions, 3 factors affected by this component

A
  • Made of paper or polymers
  • separates electrodes and allows ions to diffuse across opposite electrode
  • Determines V capability and ESR
  • Impacts cap frequency response
67
Q

What are the two contributors to C in an EDLC?

Not the high surface area of activated carbon or small thickness

A
  • double layer capacitance
  • pseudocapacitance due to functional groups located on carbon edges on the surface
68
Q

why is coconut shell considered superior for fabricating activated carbon?

A

Because of small macrospores structures

69
Q

5 factors influenced by electrolyte in ultracaps?

Not V or frequency response

A
  • performance
  • safety
  • lifetime
  • ESR
  • Power
70
Q

4 factors involved in the conductance of electrolyte

A
  • concentration of free charge carriers (cations and anions)
  • ionic mobility
  • solubility of salt
  • viscosity of solvent (T dependent)
71
Q

2/3 Characteristics of aqueous electrolytes (ultracaps 133)

A
  • High ionic conductivity
  • Low V (1.2V)
  • over higher V the electrolyte is reduced or oxidized at an electrode
72
Q

1 Characteristic of organic (dry) electrolyte

A
  • Higher operating V due to higher decomposition limits
73
Q

Main properties of pseudocapacitors

A
  • structure and characteristics similar to EDLC
  • uses oxidation-reduction reactions at an interface (like battery)/use metal oxide electrodes (instead of carbon)
  • store charge faradaically transferring charge between electrode and electrolyte
  • It’s a type of electrochemical cap
  • Not same as Li-ion caps
  • Imporoved energy density
  • Need only 60% of volume of EDLC caps for same C

Capacitors (6 decks)

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