112
M. Moats and T. Hymer
With experience, we believe we have come to learn how to navigate this treacherous
terrain. Our hope is that this paper will help operators and academics to understand polarization and use this knowledge to produce optimal deposit structures and
maximize the profitability of zinc refineries.
Polarization
Polarization is defined as the deviation of the electrode potential from the reversible
value when faradic current flows [1]. As current is applied to an electrolysis cell, the
electrode potential deviates from the equilibrium potential. This deviation is called
overpotential, η. Thus, overpotential is the difference between the actual electrode
potential, E, with current flowing and the reversible equilibrium potential, E rev (Eq. 1).
Polarization is, therefore, related to overpotential and the words are occasionally used
interchangeably.
η = |E − E rev |
(1)
Polarization would be much easier to understand if there was only one type of
overpotential. Unfortunately, there are five different types of polarization listed by
Khorüm [2] (1) charge transfer (CT), (2) mass transfer (MT), (3) reaction (Rxn), (4)
crystallization (Cry), and (5) resistance (). Therefore, an experimental measured
overpotential is really the sum of five individual overpotentials (Eq. 2)
η = η CT + η MT + η Rxn + η Cry + η Ω
(2)
Charge Transfer
Charge transfer overpotential is related to the energy needed to overcome the activation barrier between oxidized and reduced species. The relationship between charge
transfer overpotential and current flow (actually current density, i) is not linear. The
Butler-Volmer Equation [1] describes the mathematical relationship. If η CT > 0.1 V,
then the simplified Tafel Equation (Eq. 3) applies. The Tafel equation has two constants, a and b. The a constant contains the exchange current density, which indicates
how easy/hard the reaction is on the electrode.
η CT = a + b log i
(3)
The charger transfer overpotential is related to the electrodeposition on zinc and/or
hydrogen gas evolution on a metal surface when only the transfer of electrons from
the substrate to the reducing specie is considered.
M. Moats and T. Hymer
With experience, we believe we have come to learn how to navigate this treacherous
terrain. Our hope is that this paper will help operators and academics to understand polarization and use this knowledge to produce optimal deposit structures and
maximize the profitability of zinc refineries.
Polarization
Polarization is defined as the deviation of the electrode potential from the reversible
value when faradic current flows [1]. As current is applied to an electrolysis cell, the
electrode potential deviates from the equilibrium potential. This deviation is called
overpotential, η. Thus, overpotential is the difference between the actual electrode
potential, E, with current flowing and the reversible equilibrium potential, E rev (Eq. 1).
Polarization is, therefore, related to overpotential and the words are occasionally used
interchangeably.
η = |E − E rev |
(1)
Polarization would be much easier to understand if there was only one type of
overpotential. Unfortunately, there are five different types of polarization listed by
Khorüm [2] (1) charge transfer (CT), (2) mass transfer (MT), (3) reaction (Rxn), (4)
crystallization (Cry), and (5) resistance (). Therefore, an experimental measured
overpotential is really the sum of five individual overpotentials (Eq. 2)
η = η CT + η MT + η Rxn + η Cry + η Ω
(2)
Charge Transfer
Charge transfer overpotential is related to the energy needed to overcome the activation barrier between oxidized and reduced species. The relationship between charge
transfer overpotential and current flow (actually current density, i) is not linear. The
Butler-Volmer Equation [1] describes the mathematical relationship. If η CT > 0.1 V,
then the simplified Tafel Equation (Eq. 3) applies. The Tafel equation has two constants, a and b. The a constant contains the exchange current density, which indicates
how easy/hard the reaction is on the electrode.
η CT = a + b log i
(3)
The charger transfer overpotential is related to the electrodeposition on zinc and/or
hydrogen gas evolution on a metal surface when only the transfer of electrons from
the substrate to the reducing specie is considered.