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MMS210801 - 3 CREDITSElectrochemistry

Department of Metallurgy and Materias EngineeringUniversity of Indonesia

[email protected] [email protected]

Ir. Yunita Sadeli, Msc & Ahmad Ivan Karayan, ST, MEng

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I:

A science studying the relationship betweenchemical energy and electrical energy and the

rules of conversion of two energies.

II:

Electrochemistry is the study of solutions ofelectrolytes and of phenomena occurring atelectrodes immersed in these solutions.

Definition of Electrochemistry


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Based on conduction and conductivity,materials can be classified as:

Material Classification


Conductor

Semiconductors

Insulator (dielectrics)

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Conduction in Conductors

Electronic Conductor

Conduction in Conductor


Depending on the type of charges involved

Ionic Conductor

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Electronic Conduction

Found in all metals and in certain othersubstances: carbon materials (graphite,carbon black), oxides, and other inorganic

compounds (e.g. tungsten carbide)

Electronic Conduction


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The main topic of electrochemistry isinvestigation of the properties of ionicconductors and of electric circuitscontaining ionic conductors, andinvestigation of phenomena occurringduring passage of an electric current

through such circuits.

Ionic Conductors

Ionic Conductors


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Ionic Conductors

Also known as electrolytes.

Can be solid electrolytes (which isnaturally conductive) and othersubstances which are not conducting butproduce ionic conduction after beingdissolved in water or other solvent (e.g.

electrolyte solution and weak electrolyte)

Ionic Conductor


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The crystallographic structure in solids with fixed positions foratoms or ions restricts the free mobility of the ions. The ionicconductivity observed in solid electrolytes is based on defectsand disorder in the crystallographic structure. One candistinguish

(i) Crystal defectsdefects are formed during crystal growth.(ii) Intrinsic disorderdefects are formed according totemperature and partial pressure and can be described bythermodynamics.(iii) Extrinsic disorderdefects are formed by variation of thecomposition of a mixed phase or doping.(iv) Disorder in sub-lattices (quasi molten sub-lattices)thesetypes of materials are called super ionic conductors or fast ionicconductors. The material develops a structure of conduction

channels within a network

Ionic ConductorsIonic Conductance in Solids


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Intrinsic Disorder


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Yttria Stabilzed Zirconia in SOFC


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Electrolyte - A substance that conductselectricity through the movement of ions.

Most electrolytes are solutions or molten salts, butsome electrolytes are solids and some of thoseare crystalline solids. Different names are givento such materials:

Solid Electrolyte Fast Ion Conductor

Superionic Conductor

Solid Electrolytes


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So, what do electronic conductor of metal and solidelectrolytehave in common? And whats the differencebetween them?

They exist in solid stateMetals

Conductivity Range = 10 S/cm < s < 105 S/cm

Electrons carry the current

Conductivity Increases linearly as temperature decreases (phonon scattering

decreases as T )

Solid Electrolytes

Conductivity Range = 10-3 S/cm < s < 10 S/cm

Ions carry the current

Conductivity decreases exponentially as temperature decreases (activated transport)

Electronic Conductor and Solid Electrolyte


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Application of Solid Electrolyte


There are numerous practical applications, all based on electochemicalcells, where ionic conductivity is needed and it isadvantageous/necessary to use solids for all components.

Batteries Fuel Cells

Gas Sensors

In such cells ionic conductors are needed for either the electrodes, theelectrolyte or both.

Electrolyte (Material needs to be an electrical insulator toprevent short circuit) Electrode (Mixed ionic and electronic conductivity is needed to

avoid open circuit)

Electrolyte

Anode Cathode

Useful

Powere-

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Solid Electrolyte - Defects

In order for an ion to move through a crystal it must hop from anoccupied site to a vacant site. Thus ionic conductivity can onlyoccur if defects are present. The two simplest types of point

defects are Schottky and Frenkel defects.

Schottky Defect(i.e. NaCl)

Na+ + Cl- Vna + VCl

Frenkel Defect(i.e. AgCl)

Ag+ VAg+ Ag+interstitial


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Ionic Conductors

The most common ionic conductors: aqueoussolution of acids, bases, and salts.

Ionic Conductor


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Acids, bases, and salts (i.e., electrolytes in the second sense of theword) dissociate into ions when dissolved in water (or in other solvents).

This dissociation can be complete or partial. The fraction of the original molecules that have dissociated is known as

the degree of dissociation, .

Substances that exhibit a low degree of dissociation in solutionare called weak electrolytes, whereas when the value of comes close to unity we speak of strong electrolytes.

Ions in Electrolyte Solution


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Ionic halides - After melting, alkali halides consist exclusively of metal cations andhalide anions. These complexes are exclusively ionic. NaCl.

Ionic Melts

Ionic glasses - Glasses are amorphous substances consisting of a network former likeSiO2,B2O3, and P2O5, which have a strong tendency to form solid polymeric networks belowthe glass temperature Tg instead of crystalline structures. In addition they usually contain a

network modifier like Li2O or Ag2O and can contain other oxides like Al2O3. Theconductivity is achieved by the addition of a doping salt, e.g., alkali halides. ionicconducting glasses: 0.4 mole Ca(NO3)2 + 0.6 mole KNO3

Separation of the ions in an ionic crystal is also possible by thermal energy. At higher

temperatures the crystals melt which gives the ions a certain amount of free mobility.While pure salts have relatively high melting temperatures, mixtures of different salts form

eutectic systems with considerably lower melting points.


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Ionic liquids - A new class of ionic melts. They consist of organic ions or a mixture oforganic and inorganic ions. An ionic liquid is a salt in which the ions are poorly coordinated,which results in these solvents being liquid below 100C, or even at room temperature (roomtemperature ionic liquids, RTIL's). At least one ion has a delocalized charge and onecomponent is organic, which prevents the formation of a stable crystal lattice.

They are liquid at temperatures between room temperature and 100 C.

They are inflammable and have high thermal stability.

Newly developed ionic liquids also have hydrophobic properties that allow handling in air.

They have a negligible vapor pressure.

Their large electrochemical potential window (Figure 1.6) allows electrochemical

processes that are not possible in aqueous electrolytes such as deposition of metals that

are not deposable in aqueous electrolytes.

Find an example of ionic liquid and give a concise description of it !!

Ionic Melts


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By mixing a polymer with a salt, one can form ionic-conductingpolymers. The polymer can react with the ionic componentspartially dissociating the salt (polymer electrolytes)

Ionic Conductance in Polymers

An ionic-conducting polymer can also be prepared by mixing asolution of an alkali salt in an organic solvent (gel formingcomponent/plasticizer) with a polymer, which forms a gelpolymer electrolyte.

The polymer itself can contain ionic groups in which case onlythe counter ion is mobile and the polymer has ionic exchange

properties(ion exchangers).

The application of these materials in lithium batteries has led tointensive interest in this class of materials.


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Some polymers with hetero-atomscarrying free electron pairs havesolvating power to dissolve metal salts,e.g., alkali halides. An example is LiClO4dissolved in polyethylene oxide (PEO,Figure 1.8). The polymer has a helix likestructure with oxygen atomscoordinated to Li cations. The anionsstabilize the chain structure. Thepolymer and the LiClO4 can formdifferent complexes. The polymer hascrystalline and amorphous regions. Theconductivity increases with an increasein volume of the amorphous regions. A

reference point is the glass transitiontemperature Tg (transition fromcrystalline structure to amorphousstructure): the lower the glasstransition temperature, the higher theconductivity at a given temperature.

Polymer Electrolytes


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The addition ofgel-forming components(plasticizers) to polymer electrolytes above)produces gel like structures. Therefore, thistype of ion-conducting polymers can also bedescribed as gel polymer electrolytes.

Gel polymer electrolytes can also beprepared, if a solution of a salt in an organicsolvent is added to a polymer matrix(polyvinyl chloride, polyvinyl fluoride).

The solvent dissolves in the polymer matrixand forms a gel like structure. Theconductivity as well as the current densityand rate of diffusion, etc., are determinedby the mobility of the solvated ions in thepolymer matrix.

The transport constants are againproportional to the free volume in thepolymer.

Gel Polymer Electrolytes

Sulphur Lithium Ion Battery

Dye-Sensitized Solar Cell


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The representative substance of this type of polymer ionic conductorsis nafion.

This polymer consists of a porous perfluorated carbon matrix withsulfonic acid groups. While the anion is fixed, the material shows highcation conductivity and is of high chemical stability.

These materials are applied, for example, in membrane chlorine alkaline

electrolysis and in polymer electrolyte membrane (PEM) fuel cells. Other types of fixed anions are carboxylic acid groups (weakly acid). Examples for ion exchange polymer electrolytes with fixed cations and

anion conductivity are polymers with ternary amino groups (weaklybasic) or quaternary amino groups (strongly basic).

Ion Exchangers

Nafion is a copolymer of tetrafluoroethylene (Teflon) and perfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid. Like Teflon, Nafion is highly resistant to chemicalattack, but the presence of its exposed sulfonic acid groups confers unusualproperties. Sulfonic acid has a very high water-of-hydration, absorbing 13molecules of water for every sulfonic acid group in the polymer. Consequently,Nafion absorbs 22% by weight of water.


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Solids are mixed conductors that means electronic and ioniccharge carriers show mobility in the lattice. One speaks ofpreferential ionic conductivity if the electronic transferencenumber is te < 0.01.

The electronic conductivity increases exponentially with thetemperature and, for oxides, depends on the partial pressure ofoxygen.

Materials with preferential ionic conductivity can be found onlyin a certain temperature and pressure region.

Materials with comparable ionic as well as electronicconductivity are called MIECs (mixed ionic electronicconductors). These materials have become especially interestingfor applications. E.g Ce0.8Gd0.2O1.9.

Mixed Ionic and Electronic Conductance


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Faradays Law

Michael Faraday, a British chemist and physicist, studied the

decomposition of solutions of salts, acids and bases by electric current.


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Faradays Law

Faraday's law of electrolysis states that theamount of any substance that is deposited ordissolved in electrolysis is directly proportional tothe total passed electric charge.

It can be expressed as

mis the mass of the substance liberated at an electrode in gramsQis the total electric charge passed through the substanceF= 96,485 C mol1 is the Faraday constantMis the molar mass of the substancezis the valency number of ions of the substance (electrons transferred per ion).Note that M / zis the same as the equivalent weight of the substance altered..

http://en.wikipedia.org/wiki/Faraday_constanthttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Equivalent_weighthttp://en.wikipedia.org/wiki/Equivalent_weighthttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Faraday_constant

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Coulometer

The silver coulometer and copper coulometer arecommonly used ones.

If one mole of silver, 107.870 g, is deposited on

the cathode of a silver coulometer, we will knowthat the quantity of electricity passed throughthe coulometer is 1 coulomb.


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In solids, current is carried by electrons In solutions, current is carried by ions The definition of transport numbertis that current

carried by ion B divided by the sum of the current of all theions in solution, which is also called the transferencenumber of ion B.

cnI QtI I Q Q n

v

v v

Transport Numbers

anI Q

tI I Q Q n

v

v v

nc and na are the amount of substance of positive ionsmigrate out from anodic region and that of negativeions migrate out from cathodic region respectively.


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Suppose the boundary moves a distant xfrom AA to BB for the passage of Qcoulombs. All the ions, H+, passedthrough the boundary AA.The amount of substances transported is

thenQ/F

, of whicht

+Q/F

are carried bythe positive ion. If the volume betweenthe boundaries AA and BB is V, and theconcentration of HCl is c, then

/t Q F Vc

FVct

Q

Measurement of Transport Numbers byHittorf Method


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An electrolyte is a substance thatproduces ions when dissolved in water

AB A+ + B-

The presence of these ions enhances

the electrical conductivity of thesolution

Electrolytes


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Strong electrolytes ionize completelywhen dissolved in water

Addition of strong electrolyte leads to alarge increase in the electricalconductivity of the solution

Examples: HNO3, HCl, NaOH and saltsproduced by neutralization of a strongacid and strong base

Strong Electrolytes


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Incomplete ionization when dissolved inwater

K

AB(aq) A+(aq) + B-(aq)

Addition of weak electrolyte in solution

leads to some increase in conductivity Examples: weak acids and bases such as

CH3COOH

Weak and Non-Electrolytes


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Soluble in water but do not dissociate

Addition of non-electrolyte lead to noincrease in conductivity

Examples: ethanol (C2H6OH) andSucrose (C12H22O11)

Non-Elecrolytes


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Conductivity of Electrolyte Solution

NaCl


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Conductance and Conductivity

Conductance of a solution is the inverse of its resistance.Resistance is measured in ohms, W. Conductance is expressedas the reciprocal of ohms and is called the siemens, S (1S =1W-1)

Resistance l A

where lis the distance between the electrodes and Ais thearea of the electrodes

Resistance = l where is resistivity A

l

A


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Conductance G in S

in equation is specificconductivity. In S m-1.

Molar conductivity (in S m2 mol-1)

where Cis the molar concentration

1G

R

sA

Gl

/m c

Conductance and Conductivity


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Charge is carried in the solution by bothcations moving towards the cathode andanions moving to the anode.

The molar conductivity is therefore a

combination of the molar conductivities of thecation and anion: m = ++ + -- + and - are respectively the ionic

conductivities of the cations and anions + and - are the number of cations and anionsproduced per mole of salt

Conductivity


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An alternating voltage isapplied sequentially toattract and repel ionsfrom theelectrode/solution

interface. This allowsmeasurement of theresistance of thesolution withoutchanging the nature ofthe ions

Conductivity can bedetermined if the cellconstant is known

Measurement of Conductivity

Electrode B

+

-

+

Half cycle 1

Half cycle 2

Half cycle 3

V

Time

Electrode A

V/Volts

--

+

+

+

+

+

+

-

-

-

-

- - -

---

---

Electrode A Electrode B

H.C.1

H.C.2

H.C.3


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Measurement of Conductivity

31

4

RR

R R

31

4

RR

R R

3

1 4

1 R

G R R R

cell

s

lG G K

A

cell

s

lKA


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Concentration Dependence ofMolar Concentration

For strong electrolytes molarconductivity decreasesslightly with increasingconcentration

For weak electrolytes, molarconductivity is small atmoderate concentrations buthigh at very lowconcentrations. This isbecause at low

concentrations theproportion of dissociatedmolecules is high.


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The conductivity and molar conductivity ofa saturated aqueous solution of silver

chloride are 3.4110-4Sm-1 and

138.2610-4Sm2mol-1 respectively at25. The conductivity of the water used to

make the solution is 1.6010-4Sm-1 at

the same temperature. Calculate thesolubility of silver chloride in water at 25

Exercise


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Model for SIngle IonMolar Conductivities

Z+-+

V Volts; l metres

Electrical attraction

Viscous drag

The two forces acting on the ion are:

Electrical force = zeV

l

Viscous force = 6a

where is the velocity and is the viscosity


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The retarding force increases as the ion movesfaster. The ion in solution will reach as steadyvelocity, then the forces of attraction andrepulsion balance each other

zeV =6al

The molar conductivity of the ion is

ze

(V/l) 6a conductivity should be larger for : small ions, highly

charged ions and solvents of low viscosity



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When considering the size of the ion the size of the hydrationsphere should be taken into account

Ions of same charge but smaller size will have higher electricfield at the surface, leading to a larger hydrations sphere and

thus larger effective size. Potassium ion has higher conductivity in solution thanlithium ion but has a larger gas phase radii than lithium

Li+ K+

Hydration

Sphere



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H+ and OH- ions are very small but theyhave anomalously high conductivities,this is because their conduction

mechanism is different to other ions

Other ions are pulled to the electrode

by coulombic attraction and move bypushing aside solvent molecules

Ion Migration


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H+ and OH- ions take advantage of thehydrogen bonded nature of water. They caninduce re-organisation of hydrogen bonding to

affect rapid motion

Ion Migration

H + O - H| | | | | O - H | | | | | O - H | | | | | O - H| | | |

H H H H

H - O - H| | | | | O - H | | | | | O - H | | | | | O H+| | | |

H H H H

O - H| | | | | O | | | | | H - O | | | | | H O-| | | |

H H H H

O- H -O | | | | | H - O | | | | | H - O| | | |

H H H H


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Fro strong electrolytes, Kohlrausch observed that

m decreased with concentration according to

the expression

m m

A c

Concentration Dependence ofMolar Concentration


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Kohlrausch discovered relations between the values offor different electrolytes. For example

The difference in for pairs of salts having common ion isalways approximately constant.

2 -1(KCl) 0.01499 S m mol

m

2 -1(LiCl) 0.01150 S m mol

m

2 -1

3(KNO ) 0.01450 S m mol

m

2 -13

(LiNO ) 0.01101S m molm

Law of the Independent Migration of Ions


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This behavior indicates that ions in anextremely dilute solution migrateindependently. There is no interaction betweendifferent ions. Therefore

2 -1

3 3(KCl) (LiCl) (KNO ) (LiNO ) 0.00349 S m mol

m m m m

2 -1

3 3(KCl) (KNO ) (LiCl) (LiNO ) 0.00049 S m mol

m m m m

, ,m m m

Law of the Independent Migration of Ions


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At 25

(NaAc) = 91.010-4 Sm2mol1

(HCl)=426.2

10-4 Sm2mol

1

(NaCl)=126.510-4 Sm2mol1

What is the molar conductivity of HAc at

25

?

Exercise

2nd course

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