Super capacitors Flashcards
Principle of EDLC (compared to caps and batteries)
Cap with properties that are halfway between regular caps and batteries
Main differences between battery and EDLC
- 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
Classification of supercapacitors (Thomas)
- EDLC
- Pseudocapacitors
- Hybrid caps
3 Materials used in the electrodes of EDLC to increase surface area
- Activated carbons
- Carbon aerogels
- Carbon nanotubes
2 Materials used as electrode of pseudocaps
- Conducting polymers
- Metal oxides
Classification of hybrid caps (3 types)
- Composite hybrids
- Asymmetric hybrids
- battery-type hybrids
Draw the curve of charge/discharge of a battery and ultracap
Draw on paper and write (derive) the equation
- Q = V*I/(dV/dt)
Compare dis/charge time, energy and power density, dis/charge efficiency, recycle life, T range of ultracaps and batteries
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
What is the reason (2) of very high C value of EDLC?
- The very large surface area of activated carbon due to its porosity
- distance is also minimized to increase C
Main components of EDLC
- metallic I collectors
- Electrodes (of activated carbon or Li-ion doped)
- Electrolyte (forms the oxide layer)
- Separator
- Sealing
Draw the schematic of charge process of EDLC (Thomas pag 12)
Draw on paper
Main elements of double layer at electrode interface of EDLC (draw on paper) (Thomas pag 13)
- Hydration sheat
- Inner Helmholtz
- Outer Helmholtz
- Diffuse space charge
- Unsolvated ion
Supercapacitor cell types (Thomas pag 14) and one application
- Chip type: clock
- Coin type: for standby power
- Pouch type: energy harvesting
- Cylindrical type: LED signs
- Modules; UPS, wind power
3 Supercaps cell types characterized by ultra-low (<0.1F) and low (0.1-1F) C value
- Chip type (utra-low)
- Coin type (both)
- Pouch type (low)
Supercaps cell types characterized by medium (1-100F) and high (>100F) C value
- Cylindrical (medium)
- Modules (both)
Which structure has even larger surface area than activated carbon?
- Carbon nanotubes
- Graphene
3 Main classification of electrolyte used in EDLC (Thomas pag 25)
- Liquid electrolyte
- Solid-state or quasi-solid-state
- Redox-active
Main solvents and solute used in organic electrolyte (Thomas pag 25)
Organic electrolyte
- Solvent: Acetonitrile ACN
- Solute: TEABF4
3 main functions of separator in EDLC
- To prevent from short circuit
- Storage of electrolyte
- Allow passage of (transport) ions during charging/discharging
Which characteristic of the separator affects power and energy density of cap? (Thomas pag 27)
Ionic conductivity (it influences ESR)
5 Main characteristics required in the separator
- thermal stability
- high mechanical strength
- high porosity
- High impregnation capability
- HIgh ionic conductivity
Main problem of series connection of supercaps
- V imbalance, can cause over-voltage of a cell
Methods to balance voltage of supercaps
- parallel resistors
- zener diodes across caps
- FETs
- dynamic stack monitoring
Rule of thumb to choose parallel R for V balancing
- Measure leakage I of supercap
- use parallel R that has 10x the I at nominal cell V
What are the reasons (2) of V imbalance in series-connected caps
- Tolerance of the cells (i.e. 20%)
- variation of leakage I
6 main effects of overvoltage, over T, and excessive mechanical stress in cap cells
- capacitance loss
- increase of ESR
- venting
- electrolyte decomposition
- gas generation
- reduced lifetime
How does FET balancing work? Draw the schematic of this solution
- FETs will turn on slightly as the capacitor cell approaches the operating rated V
Main steps in production process of supercaps
- Mixing
- Coating
- Slitting
- Electrode making
- Winding
- Forming
- Heat sealing
- Filling and presealling
- Formation
- vacuum sealling
- Test
Which process consumes a lot of water that has then to be dried?
- Coating
- water has to be dried afterwards
Mention a problem of replacing nuclear/hydraulic plants for distributed renewable generators
- 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
In which application can supercapacitors replace Al El caps?
Supercaps can replace much larger banks of El caps in UPS, reducing cost per cycle, maintenance, downsizing the battery, and extending battery life
5 General applications of supercaps
- 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
at least 6 main general characteristics of supercaps
Includes 1 of assymetric
- 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
write the equation of the time t that a supercap can deliver a constant I
t = C*(Vcharge-Vmin)/I
capacitor V decreases from Vcharge to Vmin
write the equation of time t that a supercap can deliver constant power
t = C(Vcharge^2-Vmin^2)/(2P)
capacitor V decreases from Vcharge to Vmin
What is the reason of using electrolyte in supercaps and Al EL caps?
Not comparison
Because it’s the best material to make contact on the huge surfaces of the electrodes
Do ultracaps work with charged ions instead of polarized atoms? give a reason
- They work with charged ions
- Ions are stronger than polarized atoms (higher polarization)
How much surface does a gram of activated carbon has?
The surface of a football stadium
2 disadvantages of activated carbon (class notes)
- cannot withstand high V
- has small amounts of water which will result in electrolysis
2 Reasons by which supercaps cannot withstand high V
- activated carbon
- electrolysis
Where does the V drop occur in a supercap?
In the Helmholtz layers
Is the activated carbon the dielectric in an EDLC and justify?
Activated carbon is not the dielectric because voltage drop does not occur here but in the Helmholtz layers
Basic principle (4 ideas) of electrochemical cap (EC)
3 ideas related to dielectric layer
- 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
V rating of aqueous electrolyte and non-aqueous electrolyte
- Aqueous: 0.8-1.6V
- Non-aqueous: 3-5V
How an EDLC is formed?
- 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
Classification of supercaps based on electrode type
- Symmetrical caps
- Asymmetrical caps
- Li-ion caps
Explain basics (5 ideas) of symmetrical caps
- 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)
Explain basics of asymmetrical caps (6 ideas)
Compare to symmetrical caps (EDLCs)
- 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
Explain basics (7 ideas) of Li-ion caps (Deshpande 213)
- 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
6 Advantages of supercaps over a battery
- Higher energy efficiency
- Lighter (smaller)
- Deliver more power
- Almost unlimited life
- Fast charge/discharge (no electrochemical process)
- Work over large T range
Explain the phenomena that happen in an EDLC when V is applied (5 ideas)
- 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
General construction of EDLC
- 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
Compare electrochemical with Al El caps (at least 4 differences)
- Al El cap is more expensive per farad
- EC has higher ESR
- Al El can sustain higher V
- EC has higher energy density
at least 5 Advantages of supercaps (Desphande ultracaps)
- Long life, little degradation
- High power output
- Fast charge/discharge
- High efficiency 98%
- Low ESR
- Safer, no corrosive electrolyte low toxicity
- No overcharging problem
3 Disadvantages of supercaps (Deshpande ultracaps)
1 compared to battery
- Lower stored energy per weight compared to batteries
- Highest dielectric absorption
- Low V rating
2 Problems of using non-aqueous electrolytes
Related to manufacturing, not electrical properties or performance
- Requires dry rooms (expensive)
- Vacuum chambers (expensive)
4 Factors that are being improved to increase C value of supercaps (energy density)
- 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)
What is the effect of having two caps connected in series in the EDLC in terms of V and C?
- BV voltage is doubled
- C is reduced by half
One of the main factors that determine the operating V
- Electrolyte
Difference between faradaic and non-faradaic mechanisms
- 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
3 Types of storing charge mechanism in supercaps
- Faradaic
- Non-faradaic
- combination of the two
Describe briefly pseudocaps
- 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.
5 advantages of pseudocaps over EDLC
- 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
3 limitations of pseudocaps against EDLC
- 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)
3 properties electrodes must have
- high conductivity
- high T stability
- chemically inert and resistant against corrosion
Main difference between supercaps and Al El caps regarding the function of electrolyte
- In Al El caps electrolyte is the cathode
- In EDLC is the conductive connection between electrodes
Materials of separator, 2 functions, 3 factors affected by this component
- Made of paper or polymers
- separates electrodes and allows ions to diffuse across opposite electrode
- Determines V capability and ESR
- Impacts cap frequency response
What are the two contributors to C in an EDLC?
Not the high surface area of activated carbon or small thickness
- double layer capacitance
- pseudocapacitance due to functional groups located on carbon edges on the surface
why is coconut shell considered superior for fabricating activated carbon?
Because of small macrospores structures
5 factors influenced by electrolyte in ultracaps?
Not V or frequency response
- performance
- safety
- lifetime
- ESR
- Power
4 factors involved in the conductance of electrolyte
- concentration of free charge carriers (cations and anions)
- ionic mobility
- solubility of salt
- viscosity of solvent (T dependent)
2/3 Characteristics of aqueous electrolytes (ultracaps 133)
- High ionic conductivity
- Low V (1.2V)
- over higher V the electrolyte is reduced or oxidized at an electrode
1 Characteristic of organic (dry) electrolyte
- Higher operating V due to higher decomposition limits
Main properties of pseudocapacitors
- 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
