Electrochemical Reaction Kinetics: Transport, Overpotential, and Temperature Sensitivity

Electrochemical Transport Stages and Limiting Current Density

Electrochemical processes involve several sequential stages:

Stages of Electrochemical Processes

1. Transfer of material (reagent) from the solution bulk to the electrode-solution interface (Mass Transport).
2. Charge transfer at the interface, leading to product formation.
3. Transfer of product material from the electrode interface to the solution bulk.

Dependence on Applied Overpotential

Depending on the overpotential applied, the following situations are distinguished:

1. Charge Transfer Control: The electronic transfer is the only rate-determining stage. This predominates at low overpotentials. The current density does not depend on the mass transport mechanism.
2. Mixed Control: The mass transfer rate is comparable to the electronic or charge transfer rate. This occurs with increasing overpotential. Reaction control is mixed, and the current density is partly dependent on mass transport mechanisms.
3. Mass Transport Control: At higher overpotentials, the charge transfer speed is so fast that the stage controlling the reaction rate is mass transfer. The current density depends on the mass transport mechanisms and is independent of overpotential. The concentration of the reagent on the electrode surface is zero.

Limiting Current Density (i_L)

A current density corresponding to the region in which the electrochemical process is controlled solely by mass transfer is called the limiting current density (i_L). In this case, the reagent is consumed as fast as it reaches the electrode as a result of charge transfer.

It is crucial to avoid working at current densities above the limiting current density because:

• Efficiency decreases and energy consumption increases.
• There is an increased probability of changes in the composition of the solution near the electrode-solution interface.

Temperature Effects on Reactor Reaction Rate

The reaction rate increases with temperature according to the Arrhenius Law (K = K₀ * e^{-(E_a / RT)}).

If we assume the concentration C_A is constant and represent the natural logarithm of the rate, Ln(rate), versus the inverse of the absolute temperature, 1/T, the slope of the resulting plot will be equal to -E_a/R.

The slope provides insight into the reaction's sensitivity to temperature:

• If the slope obtained is steep (high magnitude), the activation energy (E_a) is very high, meaning the reaction is highly sensitive to temperature.
• If the slope is gentle (low magnitude), the activation energy is low, and the reaction is not very sensitive to temperature.