Electrochemical Kinetics Flashcards
True or false: ‘Resistance is a form of parasitic loss’
True
Name the 5th category of variables that control an electrochemical cell.
1) Solution variables (e.g., electrolyte)
2) Electrical variables (e.g., current)
3) External variables (e.g., temperature)
4) Electrode variables (e.g., material)
5) Mass transfer variables (e.g., diffusion, convection, etc.)
Name the 3 main overpotentials/losses in a cell
1) Charge transfer or activation overpotential
2) Mass transfer (concentration) overpotential
3) Ohmic (voltage) losses (due to charge transport)
Why does charge transfer overpotential/resistance arise and at which part of the cell? By what name do we model this?
Charge distribution of ions at the electrode surface with electrolyte. This is modelled as a double layer.
What are we measuring across the double layer to assess the charge transfer overpotential/resistance? What does this tell us?
The difference in electrical potential between 2 points in a static electric field. This tells us the additional work needed to overcome/manipulate charge at the interface.
At equilibrium, which electrode reaction is promoted when the difference in electric potential is MORE than the equilibrium potential?
Anodic - oxidation - loss of electrons - increase in ionic charge
At equilibrium, which reaction is promoted when the difference in electric potential is LESS than the equilibrium potential?
Cathodic - reduction - gain of electrons - decrease in ionic charge
Define ‘activation energy’
The minimum amount of energy required to be provided for a chemical reaction
What is the cause of the activation barrier and how can these be modified?
The charged surface of the electrode. This is modified by taking the electrodes out of equilibrium (where both the forward and reverse reactions occur) and supplying work to the system by changing the electrode potential.
This additional work is called the activation overpotential.
What does the Butler-Volmer equation tell us?
The kinetics of how the cell behaves when taken out of equilibrium with an applied potential.
What does i0 depict in the B-V equation? If charge transfer is high, is this value small or large?
Equilibrium exchange current density.
For an electrode with high charge transfer (e.g. high surface overpotential), the value of i0 will be small (as if it is a rate of reaction). This is bad.
What does alpha represent in the B-V equation?
The symmetry in the cell, which is dependent on the number of electrons
How do you plot the B-V equation to determine the exchange current density? Sketch this.
Surface overpotential (x-axis) and current density (y-axis). The exchange current density is the distance between the anodic and cathodic lines.
What happens to the rate of reverse reaction and cathodic current density for high charge transfer overpotential/resistance?
The rate of the reverse reaction will get slower and the cathodic current density will approach 0.
How do you plot the Tafel equation?
Assume the condition that the surface overpotential is large and positive therefore, the anodic term dominates.
Plot the log of the current density on the x-axis and the surface overpotential on the y-axis.
