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# Overpotential - PowerPoint PPT Presentation

Overpotential. When the cell is producing current, the electrode potential changes from its zero-current value, E, to a new value, E’. The difference between E and E’ is the electrode’s overpotential , η . η = E’ – E The ∆ Φ = η + E, Expressing current density in terms of η

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## PowerPoint Slideshow about ' Overpotential' - jorn

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Presentation Transcript

• When the cell is producing current, the electrode potential changes from its zero-current value, E, to a new value, E’.

• The difference between E and E’ is the electrode’s overpotential, η.

η = E’ – E

• The ∆Φ = η + E,

• Expressing current density in terms of η

ja = j0e(1-a)fη and jc = j0e-afη

where jo is called the exchange current density, when ja = jc

• The overpotential ηis very small, i.e. fη <<1

• When x is small, ex = 1 + x + …

• Therefore ja = j0[1 + (1-a) fη]

jc = j0[1 + (-a fη)]

• Then j = ja - jc = j0[1 + (1-a) fη] - j0[1 + (-a fη)]

= j0fη

• The above equation illustrates that at low overpotential limit, the current density is proportional to the overpotential.

• It is important to know how the overpotential determines the property of the current.

• Example: The exchange current density of a Pt(s)|H2(g)|H+(aq) electrode at 298K is 0.79 mAcm-2. Calculate the current density when the over potential is +5.0mV.

Solution: j0 =0.79 mAcm-2

η = 6.0mV

f = F/RT =

j = j0fη

• The overpotential ηis large, but could be positive or negative!!!

• When η is large and positive

j0e-afη= j0/eafη becomes very small in comparison

to ja

Therefore j ≈ ja = j0e(1-a)fη

ln(j) = ln(j0e(1-a)fη ) = ln(j0) + (1-a)fη

• When η is large but negative

ja is much smaller than jc

then j ≈ jc = j0e-afη

ln(j) = ln(j0e-afη ) = ln(j0) – afη

• Tafel plot: the plot of logarithm of the current density against the over potential.

• The following data are the anodic current through a platinum electrode of area 2.0 cm2 in contact with an Fe3+, Fe2+ aqueous solution at 298K. Calculate the exchange current density and the transfer coefficient for the process.

η/mV 50 100 150 200 250

I/mA 8.8 25 58 131 298

Solution: calculate j0 and a

Note that I needs to be converted to J

• Voltammetry: the current is monitored as the potential of the lectrode is changed.

• Chronopotentiometry: the potential is monitored as the current density is changed.

• Voltammetry may also be used to identify species and determine their concentration in solution.

• Non-polarizable electrode: their potential only slightly changes when a current passes through them. Such as calomel and H2/Pt electrodes

• Polarizable electrodes: those with strongly current-dependent potentials.

• Concentration polarization: The consumption of electroactive species close to the electrode results in a concentration gradient and diffusion of the species towards the electrode from the bulk may become rate-determining. Therefore, a large overpotential is needed to produce a given current.

• Eqns 29.42 to 29.53 will be discussed in class

• Example 29.3: Estimate the limiting current density at 298K for an electrode in a 0.10M Cu2+(aq) unstirred solution in which the thickness of the diffusion layer is about 0.3mm.

• Cell potential: the sum of the overpotentials at the two electrodes and the ohmic drop due to the current through the electrolyte (IRs).

• Electrolysis: To induce current to flow through an electrochemical cell and force a non-spontaneous cell reaction to occur.

• Estimating the relative rates of electrolysis.