Energy Storage Summary Questions Flashcards
Energy stored in capacitors
E = ½ CV^2
Why supercapacitors are “super”
Due to the very small Hemholtz layer the distance is very small. The formula for charge in a capacitor is C = e_0e_rA/d and a very small d gives a large C. The much larger C in a supercapacitor compared to conventional capacitors is the reason for the “super”
Why supercapacitors are also called double-layer
Due to the alignment of ions in the electrolyte that create a double layer on either side of the capacitor.
The solid electrode plate and the lined up ions (forming a liquid electrode wall) together create a capacitor with a potential difference across them. Hence the term “double-layer”
What is the Hemholtz layer?
Hemholtz layer is the name of the layer of ions that builds up to form a wall next to the solid electrode plate in a super capacitor.
Why are supercapacitor electrodes porous?
The porous electrodes increase surface allowing max area to store electric charge.
What is a common anode material?
Graphite
What is a common cathode material?
Lithium Cobalt Oxide
What are the advantages/disadvantages of silicon anode?
Silicon Nanowires
Advantages:
1. Ten times current capacity
Disadvantages:
1. Silicon films expand drastically when absorbing lithium ions during charging, and shrink when releasing ions during discharging leading to fracture/fatigue and short battery life.
How do we calculate stored energy in a battery?
Capacity is the term used to characterize battery performance which is equal to current (A) x Time (h)
What plot or diagram could we refer to for selecting a suitable electrode material?
Voltage vs. Capacity
Ragone plot
What is the driving energy for a chemical reaction?
Gibbs free energy
Standard chemical reaction: A + B = AB
Specifically the difference in the values of the standard Gibbs free energy of the
Products - Reactants
What is the difference between a chemical and an electrochemical reaction?
Key differences in a chemical reaction and an electrochemical reaction is the presence of an electrolyte and electrical path in the electrochemical reaction (battery). These do not exist in a chemical reaction.
A chemical reaction is the combination of A + B = AB
The electrochemical reaction is A < electrolyte > B with the connection made between A and B by an electrical path (wire).
What is Gibbs free energy as a function of temperature?
G = U + pV - TS
G = H - TS
How do you derive change of voltage in a battery and what is their reaction formula?
Derivation of change of voltage with respect to Temperature (dE/dT)
Delta G = Delta H - T*Delta S
Differentiating with respect to temperature: d(Delta G)/dT = -Delta S
Using the relation of voltage to Gibbs energy: E = -DeltaG/zF
We obtain: dE/dT = Delta S/zF
Describe 5 important parameters in a battery
Specific Energy: Energy per unit weight
Energy Density: Energy per unit volume
Operating Voltage: Voltage at which the battery operates during discharge - when supplying electrical energy and power - and charging
Energy quality: Ability of the form of energy to be converted to useful work
Charge Capacity
Stored Charge
Others
Why is the practical capacity lower than maximum theoretical capacity of an electrode material in a battery?
Presence of passive components
Effective utilization of active components is less than optimal
Active-turned-passive materials add to the weight and volume, but dont contribute
What is cyclic capacity and coulombic efficiency?
Cyclic capacity is when the capacity of the battery is reduced over time due to cycling (charging and discharging).
Coulombic efficiency is the fraction of prior charge capacity that is available during the following discharge.
What is self-discharge in a battery?
The decrease in capacity with time, even without use.
