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Chapter 1
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Reactions involving the
reactant – the electron
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• Broad Field : electroanalysis, sensors, energy storage
and conversion devices, corrosion, electrosynthesis, and
metal electroplating
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• These methods make possible the determination of a
particular oxidation state of an element
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Electrochemical Cells
Electrochemical cells consist of two electrodes: an anode
(the electrode at which the oxidation reaction occurs) and a
cathode (the electrode at which the reduction reaction
occurs)
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Electrochemical Cells
Conduction
1
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Solution (ion
migration)
3
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Electrochemical Potentials
The potential that develops in a cell is a measure of the
tendency for a reaction to proceed toward equilibrium
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Ref

Eo´

2
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Electrochemical Potentials
We use concentrations in the Nernst equation, but really
activities are the proper term
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e
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Fe+3 + e- ↔ Fe+2 FeCl+2, etc
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AgCl(s) + e- ↔ Ag(s) + ClE = Eo + (0
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Hg2Cl2(s) + 2e- ↔ 2Cl- + 2Hg(l)
E = Eo + (0
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303 RT/F = 0
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00)//CuSO4 (aCu+2 = 1
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Electrochemical Cells and Reactions
Electrode (conductor) – Electrolyte (ionic solution)
Electrode
solid

solution

Solution
e-

Ox
Red

Electrodes: Pt, Au, Pd, C, Hg
Ox + e-

Red

Electrolyte solutions (low ohmic resistance):
ionic solutions (NaCl), molten salts, and
ionic polymers (Nafion)
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Electrochemical Cells and Reactions
Two electrified interfaces but only one of interest
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Rate of oxidation = Rate of reduction
Reference electrode: AgBr + e-

Ag + Br-(aq) E0 = 0
...
NHE

Electrochemical Cells and Reactions

Magnitude
of
the
potential controls the
direction and rate of
charge transfer
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Electrochemical Cells and Reactions
• There are two types of current flow:
1
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Q = nFN

dQ = i = nF dN
dt
dt

2
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at the electrode-electrolyte
interface
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Electrochemical Cells and Reactions

Pt/H+, Br- (1M)/AgBr/Ag
(+) current = cathodic
(-) current = anodic
(+) potential = left
(-) potential = right

Br2 + 2e2H+ + 2eEcell = Ec - Ea

2BrH2

E0 = 1
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NHE
E0= 0
...
NHE

Electrochemical Cells and Reactions

Hg/H+, Br- (1M)/AgBr/Ag
Kinetically fast reactions have
significant faradaic current
flow near Eo, while sluggish
reactions have little current
flow except at large
overpotentials
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14 V vs
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00 V vs
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14 V vs
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00 V vs
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Ions move in and out of the interfacial
region in response to potential changes
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Excess electrons on one
plate and a deficiency on
the other
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However, the balancing charge on the solution side of the interface
extends out into the solution with some thickness
...
)

Electrified Interfaces
σmetal = -(σIHP + σOHP + σdiffuse)
Structure of the electric double layer has
a major effect on electrode reaction
kinetics! (Faradaic reaction rates)
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Φ2 – Φs is wasted!!
Φm – Φs is potential felt by analyte

⎛ dy ⎞
Field strength ⎜ dx ⎟ x = x 2 is critical!!
⎝ ⎠

Electrified Interfaces
The solution side of the interface consists of a compact
layer (inner and outer Helmholtz layers) plus a diffuse layer
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Ionic distribution influenced by ordering due to coulombic
forces and disorder caused by random thermal motion
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Mass transfer of
reactant/product to and away
from the electrode interface
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Electron transfer at interface
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Preceeding or follow-up
chemical reactions
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Surface processes
(adsorption/desorption)

Working Electrode (Indicator Electrode)

Electrochemical Experiment and Variables in
Electrochemical Cells
η = E - Eeq

dQ
(coulombs/s)
dt
Q (coulombs)
= N (moles electrolyzed)
nF (coulombs/mol)
i (amperes) =

Rate (mol/s) =

dN
dt

=

i
nF

Rate (mol/s-cm2) =

i
nFA

Mass Transport
Modes of Mass Transport
1
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2
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3
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Ji(x) = -Di

Ji(x) = flux of I (mol/s-cm2)
dΦ(x)
= potential gradient
dx

dCi(x)
dx

ziF
RT

dΦ(x)
DiCi
dx

D = diffusion coeff
...
(mol/cm3)

υ(x) = velocity (cm/s)

Mass Transport

l = (2Dt)1/2
Diffusion layer thickness

υmt = Do (dCo/dx)x=0
υmt = Do (C*o – Co(x=0))/δo
Do (C*o – Co(x=0))/δo = i/nFA = Dr(Cr(x=0) – C*r)/δr

Current-Voltage Curve Shapes



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