Chapter 7 Electrochemistry - Shandong Universitycourse.sdu.edu.cn/G2S/eWebEditor/uploadfile/20171129221741237.pdf · Pourbaix diagram of iron-water system (1) Active dissolution:

Chapter 7 Electrochemistry

§7.12 Basic principal and application of electrolysis

Anodic

reaction

cathodic

reaction

Active dissolution

Passivation and conversion

Anodization

Inert

anode

Active

anode

Oxidation of species in

solution

reduction of species in solution

reduction of oxide/conversion layer

Electrolysis

reaction


For evolution of gas, the overpotential is relatively large,

therefore, the overpotential should be taken into

consideration.

Ag+, Cu2+, H+, and Pb2+ will liberates at 0.799 V; 0.337 V;

0.000 V; -0.126 V, respectively without consideration of

overpotential;

Overpotential of hydrogen liberation on Cu is 0.6 V, on Pb

is 1.56 V

0.337 V

⊖ Cu2+/Cu

-0.126

⊖ Pb2+/Pb

0.799 V

⊖Ag+/Ag

0.000

⊖ H+/H2

For liberation of metal, the overpotential is usually very

low, and the reversible potential can be used in stead of

irreversible potential.

1. Cathode reaction


a(Ag+) = 1.510-8

0.799 V

a(Cu2+) = 2.210-16

0.337 V

a(Pb2+) = 3.310-49

-0.126 V -1.56 V

The liberation order and the residual concentration of the ions upon negative

shift of potential of cathode

Potential sweep and residual concentration

1. Cathode reaction


2) Application

1) Separation of metal (Lanthanum)

2) Quantitative and qualitative analysis (polarography)

3) Electroplating of single metal and alloy

4) Electrolytic metallurgy (Al, Ti, Mn)

5) Electrorefining of metal (Cu)

6) Electrosynthesis (Aniline)

1. Cathode reaction


2. Anode reaction

When inert material such as Platinum and graphite

was used, the species in the solution discharge on

the electrode in the order of liberation potential.

F < Cl < Br < I

Henri Moissan

1906 Noble Prize

France

1852/09/28 ~ 1907/02/20

Investigation and isolation of the element fluorine

1) Reaction on inert anode


(1) Active dissolution;

(2) Anodic passivation

(3) Anodic oxidation

2) Reaction of active anode

Pourbaix diagram of iron-water system

(1) Active dissolution:

At pH=4 and low current density, active

dissolution occurs.

Fe Fe2+ + 2e

Fe2+

Fe2O3

Fe

pH

/ V

2 4 6 8 10 12 140

Fe3O4

Fe3+

FeO22

We usually judge the reaction based on

Porbaix diagram

2. Anode reaction


(2) Anodic passivation:

At pH= 12 and high potential, upon polarization,

compact thin layer of Fe3O4 forms and passivation

of iron takes place.

3Fe + 4H2O – 8e Fe3O4 + 8 H+

Passivation curve of iron

Active dissolution

passivation

Trans-passivation

2. Anode reaction


Anodic oxidation of aluminum at

constant current density(3) Anodic oxidation

t / hE

/ V

Barrier

layer

Porous

layer

Initiation

of pores

2. Anode reaction


top surface

Cross-section

§7.13 Corrosion and protection of metals

1) Corrosion:

Destruction of materials due to the

chemical, electrochemical and physical

attack of the media.

White marble of Jinshui Bridge, Beijing

Stone Sculpture before the Capitol, Washington D.C.

1. General introduction


metal

Surroundings

(1)Materials:

(2)Environment:

(3)Reaction;

(4)Uniformity;

(5)Other

1. General introduction


metal

film

Surroundings

Local corrosion:

1) uniformity of

metal

2) Surface film

3) Solution

2. Theoretical consideration

Zn + 2 HCl ZnCl2 + H2

Why does Zn of 99.5 % purity dissolve in dilute HCl in 1 min, while that of

99.999% purity does not dissolve even after 8 h?

anode reaction:

Zn Zn2+ + 2e

Cathode reaction:

2H+ + 2e H2

Conjugation reaction


Conjugation reaction

Corrosion current Corrosion / stable / mixed potential

2H+ + 2e H2

H2 2H+ + 2e

Zn Zn2+ + 2e

Zn2+ + 2e Zn

re Zn2+/Zn

re H+/H2

/

V

lg jlg jcorr

corr

2. Theoretical consideration


re Zn2+/Zn

re H+/H2

2H+ + 2e H2

H2 2H+ + 2e

Zn Zn2+ + 2e

Zn2+ + 2e Zn

lg jcorr

/

V

lg j

corr

metal

Surroundings

film

1) metal

2) Surface film

3) Solution

4) Electricity

3. Corrosion protection


§7.14 Chemical power sources

Galvanic cell / battery

Levine: p. 440

Section 14.11 batteries

1. Classification of batteries

1) The way to use

Primary battery: Dry battery

Secondary/rechargeable battery: Lead-acid battery

Fuel cell: direct methanol cell

Liquid-fluid cell:

2) Type of electrolyte

Acidic battery: Lead-acid battery

Neutral battery: Neutral dry battery

Basic battery: Basic dry battery, Cd-Ni battery


Energy conversion efficiency:

internal combustion: 20%25%

electrical power generation: 35%40%

fuel cells: 50%60% or more, depending on type

Advantages:

1) low noise; 2) high efficiency; 3)

small scale; 4) portable

G H T S

Electric energy Chemical energy Heat

100%G

H

Energy conversion efficiency


3. Parameters for chemical batteries

Working voltage:

1.2 V for dry battery, 3.0 V for lithium-ion battery

Nominal Capacity:

1200-1500 mAh for AA type nickel-hydride battery

Power density: (Wh/kg, or Wh/L)

lead-acid batter is much lower than lithium battery

Cycle life: for lead-acid battery > 500 charge-discharge cycles required.

Charge-discharge efficiency: the higher, the better

Self-discharge: the less, the better

Environment friendliness: no hazard materials was used.


Parameters Lithium-ion Ni-Cd Ni-MH

Mass power density (Wh/kg） 90 40 60

Volume power density (Wh/1） 210 100 140

Nominal voltage （V） 3.7 1.2 1.2

Cycle life 1000 1000 800

Self-discharge（％/month） 6 15 20

Parameters for lithium-ion, nickel-cadmium and nickel-metal

hydride batteries


First charge:

Before use, the newly purchased secondary battery must be charged to full.

Charging strategy:

Charge your battery with constant current at first and then with constant

voltage.

Memory effect:

Don’t charge your battery before it is used up.


Documents

Chapter 7 Electrochemistry - Shandong Universitycourse.sdu.edu.cn/G2S/eWebEditor/uploadfile/20171129221741237.pdf · Pourbaix diagram of iron-water system (1) Active dissolution: