Introduction to electrochemistry by t. hara

Introduction to Electrochemistry
1. Definition of electrochemistry
1.1. Conversion of chemical energy into electrical energy
1.2. Conversion of electrical energy into chemical energy
1.3. Secondary battery (Rechargeable battery)
1.4. Corrosion
1.5. Conversion of photon energy into electrical energy
via electrochemical reaction
1
Electrical
Energy
Chemical
Energy

1. Definition of Electrochemistry
Electrochemistry deals with
- the conversion of chemical energy
into electrical energy.
- the conversion of electrical energy
into chemical energy.
2
Electrical
Energy
Chemical
Energy

3
Energy?
To be unstable is
to have an ability to work,
to influence others,
or to change something.
Electrical
Energy
Chemical
Energy

4
Chemical
Energy?
* Fermi level, EF
[eV]=[1.6*10-19C·V]=[1.6*10-19C·J/C]
=[1.6*10-19J]
* Standard reduction potential
[V] =[J/C]
Electrical
Energy?
Energy [J]
= Potential [V=J/C]*Charge [C]
= Potential [V=J/C]*Current [A=C/s]
*Time [s]

Note:
Fermi level, EF, is equal to internal chemical potential at 0
K, μ0. But in electrochemistry, we have to take some effects
into consideration: solvation, counter ion, viscosity of
solution etc. can be listed as possible causes that can have
an influence on the chemical potential. In electrochemistry,
established theories such as Debye-Huckel limiting law
(logγ+-=-Bz+|z-|I1/2, γ is the activity coefficient, B is the
constant, z is the valency of ion, I is the ionic strength that
is caluculated as I=(1/2)Σcizi
2).
Works only for dilute solutions (typically <= 0.01 M) of
strong electrolytes that can be completely ionized.

6
Electrochemical reaction
= Redox reaction
= Reduction and Oxidation
= Reactions in which transfers of electrons take place.
Reduction
= the process in which electrons are gained by a reactant.
Oxidation
= the process in which electrons are donated by a reactant.

1.1. Conversion of chemical energy
into electrical energy.
One of the applications is primary battery (Galvanic cell).
Galvanic cell is one in which electrical energy is
spontaneously produced by chemical reactions.
7
Electrical
Energy
Chemical
Energy

This is a cartoon of
a primary cell, which is
called Daniell cell, which
was invented in the
1830’s by the British
chemist Daniell.
A zinc bar is placed
into a ZnSO4 solution,
a copper bar is placed
into a CuSO4 solution.
8
Zn
Cu
wire
Salt bridge
Cu(II) sulfateZn(II) sulfate
electrons

The zinc bar and the copper bar are connected with
conducting wire. Free mobile electrons flow from the
anode to cathode through the wire (and external
circuits, such as a flush light.).
9
The zinc sulfate solution
and the copper sulfate
solution are connected via
salt bridge. Ions in the
electrolyte transfer and
balance the charge via salt
bridge.
Zn
Cu
wire
Salt bridge
electrons

Because of the potential difference
between zinc and copper,
electrons are going to flow spontaneously
through the conducting wire,
resulting in the oxidation of zinc metal to zinc cation
and the reduction of copper cation to copper metal.
What does it mean?
10

Potential ? Potential difference?
… Thermodynamic driving forth!
11
A battery can have electromotive force (emf), the same
as electric potential difference between two
electrodes.
If two electrodes have the different potentials, the
battery can give us electricity until the two electrodes
have the same potential = Gibbs free energy difference
goes to zero = the Fermi levels are aligned.

12
An equilibrium electrochemical potential is described as
a standard reduction potential.
Zn2+(aq) + 2e- → Zn(s) -0.76 V
Cu2+(aq) + 2e- → Cu(s) +0.34 V

13
Zn2+(aq) + 2e- → Zn(s) -0.76 V
Standard reduction potential?
A measure of the tendency of a chemical species to
acquire electrons.
Cu2+(aq) + 2e- → Cu(s) +0.34 V
The more negative the potential, the greater the
species' ability to donate electrons and tendency to be
oxidized.

14
Cu2+(aq) + 2e- → Cu(s) +0.34 V
Zn2+(aq) + 2e- → Zn(s) -0.76 V
SRP
E=1.10 V
Thanks to the potential difference, you will gain an
emf, E (or total cell potential) of 1.10 V.
It means this reaction occurs spontaneously.

15
+0.34 V
-0.76 V
SRP
E=1.10 V
ΔG=-212 kJ/mole
Energy has a unit, Joule [J]
[J]=[V·C]=[V·A·s]
ΔG=-nFE (F=96485 C/mole)
[J/mole]=[C/mole][V]
n is the number of electrons
involved in the reaction.
Negative free energy change
(ΔG<0 because E>0)
is identified as defining a
spontaneous process.

16
Note:
The output potential (potential
difference between an anode and
cathode) of a primary cell gives the
maximum potential at zero current
flow.
Once electrons are allowed to flow
through the circuit, the actual output
potential changes with time, because
the driving force for the reaction
decreases as the system approaches
equilibrium.
Secondary battery can be recharged
and regain its electromotive force.
Potential
More negative potential:
It push electrons
through the external
circuit.

17
Note:
- All thermodynamic measurements are of differences
between states. There is no absolute value for any
thermodynamic property, except for entropy.
In order to quantify thermodynamic values (in
electrochemistry),
(1) a temperature is chosen at 298 K (25 °C),
(2) a pressure is chosen at 1 atm (105 Pa),
(3) a concentration is chosen at 1 mol/L.
These conditions are called standard conditions.

18
When cell is not at standard conditions, use Nernst
equation:
E=E0-(RT/nF)lnK
where, E0 is the total cell potential. So far, we have
omitted subscript zero, because we have taken standard
conditions for granted. R is gas constant 8.315 J/Kmol, T
is temperatures in Kelvins, and K is reaction quotient. In
Daniell cell, K=[Zn2+]/[Cu2+]. Note that K is the ratio of
[product]the number of moles of the product to [reactant]the number of
moles of the reactant. As E declines with reactants converting
products, E eventually reaches zero. Zero potential means
reactions is at equlibrium, namely, battery is dead.

19
Reduction:
Cu2+(aq) + 2e- → Cu(s)
Oxidation:
Zn(s) → Zn2+(aq) + 2e-
Total Cell Reaction:
Zn(s)+Cu2+(aq) → Zn2+(aq)+Cu(s)
Electrochemical reaction is composed of two half
reactions, namely, oxidation and reduction reactions.
Copper cation is
being reduced
(its oxidation
number is going
from +2 to 0).
Zinc is being
oxidized (its
oxidation number
is going from 0 to
+2).
Zn
Cu
wire
Salt bridge
electrons

20
Oxidation number represents the number of electrons
required to produce the charge on a species: (1) to be
oxidized is to lose electrons (e.g., Zn(s) → Zn2+(aq) + 2e-);
(2) to be reduced means to gain electrons (Cu2+(aq) + 2e-
→ Cu(s)). Here, (s) stands for the solid state, (aq) stands
for aqueous ion.
The oxidation number...
(1) for any elemental substance is zero.
(2) for an ion is its charge (e.g., Zn2+ has +2).

In electrochemical terminology, an electrode at which an
oxidation reaction occurs is called an anode. An electrode
at which a reduction reaction occurs is called a cathode.
21
Cathode Reaction:
Cu2+(aq) + 2e- → Cu(s)
Anode reaction:
Zn(s) → Zn2+(aq) + 2e-
Oxidation Reduction
Zn
Cu
wire
Salt bridge
electrons

Zn/Zn2+ pair has a more negative standard reduction
potential than Cu/Cu2+ has (the former is going to be
oxidized, the latter is going to be reduced.).
22
Cathode Reaction:
Cu2+(aq) + 2e- → Cu(s)
Anode reaction:
Zn(s) → Zn2+(aq) + 2e-
Cu2+(aq) + 2e- → Cu(s) +0.34 V
Zn2+(aq) + 2e- → Zn(s) -0.76 V
SRP
E=1.10 V

23
+0.34 V 4.78 eV
-0.76 V 3.68 eV
SRP EF
We can also use Fermi level,
that is used in solid state physics
and semiconductor physics.
After understanding the
relationship between the
different branches of science,
you will be able to use more
resources of knowledge.

24
Zn2+(aq) + 2e- → Zn(s) 3.68 eV
Fermi level?
A minimum energy to remove electron from a material.
Cu2+(aq) + 2e- → Cu(s) 4.78 eV
The more positive the level, the greater the species'
ability to donate electrons and tendency to be
oxidized.

25
+0.34 V 4.78 eV
-0.76 V 3.68 eV
SRP EF
Fermi level, EF
[eV]=[1.6*10-19C·V]
=[1.6*10-19C·J/C]
=[1.6*10-19J]

26
SRP
0 eV-4.44 V
0.00 V 4.44 eV
Standard Hydrogen Electrode
2H++2e-->H2
+1.23 V 5.63 eV
Standard Oxygen Electrode
O2(g) + 4H+(aq) + 4e- → 2H2O(l)
In electrochemistry, the
standard hydrogen
electrode (SHE) potential is
taken as a reference point.
EF

27
SRP
0 eV-4.44 V
0.00 V 4.44 eV
+1.23 V 5.63 eV
This is the vacuum level
(Evac) at which electrons
are at rest in vacuo, just
outside the surface of the
electrode.
It is taken as a reference
point in solid state
physics.
The closer an electron is
to the vacuum level, the
weaker it is bound to the
solid.
EF

28
In electrochemical reactions, electrons just change
Places. Charge is conserved.
In addition, a properly balanced redox reaction means
both mass and charge are conserved.
Cathode Reaction:
Cu2+(aq) + 2e- → Cu(s)
Anode reaction:
Zn(s) → Zn2+(aq) + 2e-

29
During discharging a primary battery, free mobile
electrons flow from the anode to cathode through the
wire, ions in the electrolyte transfer and balance the
charge.
Zn
Cu
wire
Salt bridge
electrons

30
Mass? Charge balance?
Zn2+(aq)
e-
Zn(s)
e-
Cu(s)
Cu2+(aq)
Cathode Reaction:
Cu2+(aq) + 2e- → Cu(s)
Anode reaction:
Zn(s) → Zn2+(aq) + 2e-
Zn
Cu
wire
Salt bridge
electrons

31
Zn2+(aq)
increased.
e-
Zn(s)
e-
Cu(s)
Cu2+(aq)
depleted.
Cathode Reaction:
Cu2+(aq) + 2e- → Cu(s)
Anode reaction:
Zn(s) → Zn2+(aq) + 2e-

32
Zn2+(aq)
increased.
e-
Zn(s)
e-
Cu(s)
Cathode Reaction:
Cu2+(aq) + 2e- → Cu(s)
Anode reaction:
Zn(s) → Zn2+(aq) + 2e-
Salt bridge. K+(aq) etc.

33
Salt bridge?
A tube or membrane packed
with a solution of salt
composed of ions not involved
in the cell reaction. The ions just
permit exchange of charge in
order to balance the charge.
Zn
Cu
wire
Salt bridge
electrons

34
Shorthand notation of cell.
cathode | catholite || anolite | anode
Zn
Cu
wire
Salt bridge
electrons
Shorthand notation of Daniell cell.
Zn(s) | Zn2+(aq, 1M) ||Cu2+(aq, 1M)| Cu(s)

Note:
Assume we have m g (N mole) of Zn with A.M.=65 g/mol,
the required I·t (t is the duration time [s] ) is calculated as
follows:
N=m/A.M.,
N=s·I·t/(n·F),
I·t=N·n·F/s=(m/65)·2·F/1,
where s is the stoichiometric coefficient of the species.
In the following reaction, the s of Zn(s) is one.
35

36
The LeClanché cell, often called dry cell (but it is not “dry,”
it uses gel electrolyte, and it can leak.), is commercially
available primary battery. The reactions involved are:
anode: Zn (s)->Zn2++2e–
cathode: MnO2(s)+H2O+NH4+e-->Mn(OH)3(s)+NH3(aq)
The output voltage is 1.55 V.
Alkaline battery uses the same reactant as above, but
under basic (alkaline) conditions.

SUMMARY of 1.1.
chemical energy -> electrical energy
We have seen how to construct a primary cell that is
capable of generating a spontaneous flow of electrons.
The flow of electrons (current) can be used to perform
work on electronic appliances.
A spontaneous flow of electrons is induced by
electromotive force (emf), potential difference between
an anode and a cathode.
Recall that Potential [V]=Energy [J]/Charge (C).
37

1.2. Conversion of electrical energy
into chemical energy.
One of the applications is electroplating.
38
Electrical
Energy
Chemical
Energy

This is the world oldest
electroplating equipment
that found in an ancient
tomb in Bagdad in 1936. It
consists of a 14-centimeter-
high egg-shaped clay jar with
an asphalt stopper. An iron
rod protruding out of the
asphalt is the anode, which is
surrounded by a copper
cylinder used as the cathode.
Filled with vinegar as an
electrolytic solution.
39

Electroplating of sacrificial anode (nonspontaneous)
40
Zn
Metal
e.g.,
steel
Zn(II) sulfate
electrons
Zn(s) deposition
Zn(s)
Zn2+(aq)
e-
Spontaneous reaction
Zn(s)+Fe2+(aq) → Zn2+(aq)+Fe(s) E=0.32 V

Control of electrochemical potentials of electrodes allows
the reaction to be controlled (even if the reaction cannot
be occur spontaneously).
41
Potential
Zn
Metal
Zn(II) sulfate
electrons
Zn deposition
0.76 V
>0.76 V
opposite to
spontaneous reaction

42
Energy [J]=[V·C]=[V·A·s]
ΔG [J/mole]=-nFE [C/mole][V]
Positive free energy change (ΔG>0 because E<0)
is identified as defining a nonspontaneous process.
Zn
Metal
Zn(II) sulfate
electrons
Zn(s) deposition
Zn(s)
Zn2+(aq)
e-

Note:
To electroplate m g (N mol) of metal with F.W.=M g/mol,
the required I·t (t is the duration time [s] ) is calculated as
follows:
N=m/M,
N=s·I·t/(n·F),
I·t=N·n·F/s=(m/M)·n·F/s,
where s is the stoichiometric coefficient of the species.
In the following reaction, the s of Zn(s) is one.
Cu(s)+Zn2+(aq)->Cu2+(aq)+Zn(s)
43

44
Primary Battery
Chemical Energy
-> Electrical Energy
ΔG<0 , spontaneous
Electroplating
Electrical Energy
-> Chemical Energy
ΔG>0 , nonspontaneous
SUMMARY of 1.2.
electrical energy -> chemical energy
With an external voltage in the opposite direction of
spontaneous reaction, electrical energy is converted into
chemical energy.

1.3. Secondary battery
(Rechargeable battery)
45
Electrical
Energy
Chemical
Energy

(1) During charging, electrical energy is converted into
chemical energy.
The charging is conducted by applying an external
voltage of opposite polarity to that of discharging in order
to gain higher total cell potential (=higher energy).
(2) During discharging, chemical energy is converted into
electrical energy.
So we can use the stored energy in the similar way as
primary batteries.
46

Charged
47
Potential
Discharged Charged
Basic concept

Open
circuit
48
Potential
Discharged
Practical charging/discharging strategy
Charged Charged

Lead acid battery
Discharging: (A) Pb+HSO4¯→PbSO4+2e-+H+
(C) PbO2+HSO4¯+3H++2e-→PbSO4+2H2O
(T) Pb+PbO2+2H2SO4→2PbSO4+2H2O E=2.1 V
Charging: opposite direction
49
HSO4
-
Pb
e-
PbO2
PbSO4

50
Lithium ion battery
Discharging: LiC6+2Li0.5CoO2 -> C+2LiCoO2 E=3.6 V
Charging: C+2LiCoO2 -> LiC6+2Li0.5CoO2
e-
LiC6 Li0.5CoO2

51
Lithium ion battery
C+LiCoO2 ↔ LiC6+Li0.5CoO2 E=3.6 V
Capacity/cell:
e.g., 2.2 [Ah]=2.2 [C/s]*3600 [s]
=7920 [C]
Energy density/cell:
e.g., 3.6 [V]*2.2 [Ah]=7.92 [Wh]
=3.6 [J/C]*7920 [C]=102643.2 [J]≈103 [kJ]
One Ah is defined as one ampere that is passed for one hour.

1.4. Electrochemical (Galvanic) corrosion
Corrosion is the gradual destruction of materials
(usually metals) by chemical reaction.
52

Corrosion can be the negative aspect of
spontaneous electrochemical reaction.
53

Electrochemical corrosion occurs
between two “electrodes”
which have electrical contact with each other
and are immersed in a common electrolyte.
54

The metal surfaces (except for Au) are covered with oxide
films in oxidative atmosphere (e.g., air). Some oxide films
are brittle and easily peeled off the metal surfaces. If a
metal with a surface oxide film and a bare metal surface
coexist and they have electrical contact, corrosion occurs.
55
Steel (Fe)
Fe2O3
Seawater

When the oxide-free surface of a metal becomes exposed
to the solution, positively charged metal ions tend to pass
from the metal into the solution, leaving electrons behind
on the metal.
56
Steel (Fe)
Fe2O3
Seawater
Anode reaction:
Fe(s) → Fe2+(aq) + 2e-
Fe2+
e-

The concentration of dissolved oxygen in air saturated
aqueous solutions at ambient temperature is about 2
mM. Oxygen that reaches at the surface of the steel gains
electrons from steel, and is reduced to hydroxide anion.
57
Steel (Fe)
Fe2O3
Seawater
Fe2+
e-
O2
OH-
O2
O2
O2
Cathode Reaction:
O2 + 2H2O + 4e- → 4OH-
O2

In other words. the accumulation of negative charge on
the steel surface due to the residual electrons leads to an
increase in the potential difference between the metal
and the solution. This change in the potential encourage
the deposition of dissolved metal ions from the solution
onto the metal.
58
Steel (Fe)
Fe2O3
Seawater
Fe2+O2
OH-
Fe2++2OH- ->Fe(OH)2
e-
FexOy·nH2O

59
Cathode Reaction:
O2 + 2H2O + 4e- → 4OH-
Anode reaction:
Fe(s) → Fe2+(aq) + 2e-
+0.40 V 4.84 eV
-0.44 V 4.00 eV
SRP
The system has an emf of 0.84 V.
This reaction occurs spontaneously.
EF

The concentration of dissolved oxygen is low and so the
rate of transport of oxygen often limits the cathodic
reduction current and the corrosion rate. Under these
conditions the corrosion rates depend only on the rate of
reduction of the cathodic reactant and the corrosion is
said to be under cathodic control.
Note that in almost all the elctrochemical reaction the
rate determining step is not the electron propagation but
the mass transfer, i.e., molecular or ion diffusion.
60

1.5. Conversion of photon energy
into electrical energy
via electrochemical reaction
61

The recent years dye sensitized solar cells or Grätzel cells
have attracted considerable attention worldwide due to
their mechanism that is different from “conventional”
semiconductor-based (Si, GaAs etc.) solar cells.
62

Energy conversion in a Grätzel cell is based on the
injection of an electron from a photoexcited state of the
sensitizer dye into the conduction band of a
nanocrystalline oxide semiconductor (anatase TiO2 etc.).
63
Measured .
StandardReductionPotential(V)
-0.5
0
0.5
1.0
SRP EF
TiO2 (-0.44 V) 4.0 eV
Dye* ( 0.74 V) 3.7 eV
Dye ( 1.06 V) 5.5 eV
I3
-/I- 0.55 V (3.9 eV)
e-
h
Conducting glass
electrode

The oxidized dye is reduced and regenerated to its
ground state by a liquid electrolyte redox couple (I3
-/I-
etc.). Regeneration of iodide ions to tri-iodide is achieved
at a counter electrode.
64
Measured .
StandardReductionPotential(V)
-0.5
0
0.5
1.0
SRP EF
TiO2 (-0.44 V) 4.0 eV
Dye* ( 0.74 V) 3.7 eV
Dye ( 1.06 V) 5.5 eV
I3
-/I- 0.55 V (3.9 eV)
e-
h
Conducting glass
electrode

Light absorption is accomplished by a monolayer of
photoactive dye adsorbed chemically at the TiO2 surface
and excited by interaction with an incident photon of
light.
65
Conducting glass
electrode
TiO2
h

Introduction to electrochemistry by t. hara

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