To Polarize or Not to Polarize: Practical Advice on How …
113
Mass Transfer
Mass transfer polarization is also called concentration polarization. The polarization
arises from the change in concentration of a species at the electrode surface as
compared to the bulk electrolyte. The equilibrium potential for zinc reduction can
be calculated using the well-known Nernst equation (Eq. 4):
E rev = E
◦ −
RT
n F
ln
a Zn
a Zn(II)
(4)
where E° is the standard state reduction potential, n is the number of electrons transferred in the reaction, F is Faraday’s constant, and a Zn and a Zn(II) are the activities
of zinc metal and zinc (II) ion in solution, respectively. As current flows, the concentration of Zn (II) at the electrode surface decreases because the movement of Zn
(II) is controlled by the mass transfer of the Zn(II) ion across the Nernst boundary
layer. This results in a lower concentration of Zn (II) at the surface than the bulk
concentration. This lower Zn (II) concentration causes a shift in potential creating
an apparent overpotential. The mass transfer overpotential can be calculated based
on the amount of current flowing relative to the limiting current density, i L , (Eq. 5):
η MT =
RT
n F
ln
1 +
i
i L
(5)
Reaction
Reaction polarization is similar to mass transfer polarization except instead of the surface concentration of the reaction species being affected by mass transport through the
Nernst boundary layer, a chemical reaction is slowly producing the reducible specie.
This mechanism is not thought to be important at the cathode in zinc electrowinning.
Crystallization
Once a zinc atom is reduced on the cathode surface, it may or may not be at an
energetically favorable site. The atom will attempt to surface diffuse to a site within
an existing crystal lattice. If this process (surface diffusion and/or entry into the crystal
lattice) hinders the overall deposition kinetics, then crystallization overpotential is
generated. When zinc grains first start to form on an aluminum substrate, we call the
crystallization overpotential—the activation [3] or nucleation [4] overpotential.
113
Mass Transfer
Mass transfer polarization is also called concentration polarization. The polarization
arises from the change in concentration of a species at the electrode surface as
compared to the bulk electrolyte. The equilibrium potential for zinc reduction can
be calculated using the well-known Nernst equation (Eq. 4):
E rev = E
◦ −
RT
n F
ln
a Zn
a Zn(II)
(4)
where E° is the standard state reduction potential, n is the number of electrons transferred in the reaction, F is Faraday’s constant, and a Zn and a Zn(II) are the activities
of zinc metal and zinc (II) ion in solution, respectively. As current flows, the concentration of Zn (II) at the electrode surface decreases because the movement of Zn
(II) is controlled by the mass transfer of the Zn(II) ion across the Nernst boundary
layer. This results in a lower concentration of Zn (II) at the surface than the bulk
concentration. This lower Zn (II) concentration causes a shift in potential creating
an apparent overpotential. The mass transfer overpotential can be calculated based
on the amount of current flowing relative to the limiting current density, i L , (Eq. 5):
η MT =
RT
n F
ln
1 +
i
i L
(5)
Reaction
Reaction polarization is similar to mass transfer polarization except instead of the surface concentration of the reaction species being affected by mass transport through the
Nernst boundary layer, a chemical reaction is slowly producing the reducible specie.
This mechanism is not thought to be important at the cathode in zinc electrowinning.
Crystallization
Once a zinc atom is reduced on the cathode surface, it may or may not be at an
energetically favorable site. The atom will attempt to surface diffuse to a site within
an existing crystal lattice. If this process (surface diffusion and/or entry into the crystal
lattice) hinders the overall deposition kinetics, then crystallization overpotential is
generated. When zinc grains first start to form on an aluminum substrate, we call the
crystallization overpotential—the activation [3] or nucleation [4] overpotential.