introduction to electrochemistry by t. hara

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Introduction to Electrochemistry

1. Definition of electrochemistry

1.1. Conversion of chemical energy into electrical energy1.2. Conversion of electrical energy into chemical energy1.3. Secondary battery (Rechargeable battery)1.4. Corrosion1.5. Conversion of photon energy into electrical energy via electrochemical reaction

Electrical Energy

Chemical Energy

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1. Definition of Electrochemistry

Electrochemistry deals with - the conversion of chemical energy into electrical energy.- the conversion of electrical energy into chemical energy.

Electrical Energy

Chemical Energy

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Energy?

To be unstable is to have an ability to work, to influence others, or to change something.

Electrical Energy

Chemical Energy

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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.

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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.

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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.

Electrical Energy

Chemical Energy

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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.

Zn Cu

wire

Salt bridge

Cu(II) sulfate Zn(II) sulfate

electrons

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Introduction to ElectrochemistryThe 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.).

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

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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?

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Introduction to ElectrochemistryPotential ? Potential difference? … Thermodynamic driving forth!

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.


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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

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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.


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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.


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+0.34 V

-0.76 V

SRP

E=1.10 VΔG=-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.


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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.

Pote

ntial

More negative potential:It push electrons

through the external circuit.


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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.


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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/Kmol, 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.


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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


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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).

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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.

Cathode Reaction:Cu2+(aq) + 2e- → Cu(s)

Anode reaction:Zn(s) → Zn2+(aq) + 2e-

Oxidation ReductionZn

Cu

wire

Salt bridge


electrons

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Introduction to ElectrochemistryZn/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.).



Cu2+(aq) + 2e- → Cu(s) +0.34 V

Zn2+(aq) + 2e- → Zn(s) -0.76 V

SRP

E=1.10 V


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+0.34 V 4.78 eV

-0.76 V 3.68 eV

SRP EFWe 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.

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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.


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+0.34 V 4.78 eV

-0.76 V 3.68 eV

SRP EFFermi level, EF

[eV]=[1.6*10-19C·V] =[1.6*10-19C·J/C] =[1.6*10-19J]


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SRP

0 eV-4.44 V

0.00 V 4.44 eVStandard Hydrogen Electrode

2H++2e-->H2

+1.23 V 5.63 eVStandard Oxygen Electrode

O2(g) + 4H+(aq) + 4e- → 2H2O(l)

In electrochemistry, the standard hydrogen electrode (SHE) potential is taken as a reference point.

EF


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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


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In electrochemical reactions, electrons just changePlaces. Charge is conserved.

In addition, a properly balanced redox reaction means both mass and charge are conserved.





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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


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Mass? Charge balance?

Zn2+(aq)

e-

Zn(s)

e-

Cu(s)

Cu2+(aq)



Zn Cu

wire

Salt bridge


electrons


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Zn2+(aq) increased.

e-

Zn(s)

e-

Cu(s)

Cu2+(aq) depleted.




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Zn2+(aq) increased.

e-

Zn(s)

e-

Cu(s)



Salt bridge. K+(aq) etc.


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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


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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)

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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.

Zn(s)+Cu2+(aq) → Zn2+(aq)+Cu(s)


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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.

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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).

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1.2. Conversion of electrical energy into chemical energy.

One of the applications is electroplating.

Electrical Energy

Chemical Energy

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Introduction to ElectrochemistryThis 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.

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Electroplating of sacrificial anode (nonspontaneous)

Zn

Metale.g.,steel

Zn(II) sulfate

electrons

Zn(s) deposition

Zn(s)Zn2+(aq)

e-

Spontaneous reactionZn(s)+Fe2+(aq) → Zn2+(aq)+Fe(s) E=0.32 V

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Introduction to ElectrochemistryControl of electrochemical potentials of electrodes allows the reaction to be controlled (even if the reaction cannot be occur spontaneously).

Pote

ntial

Zn Metal

Zn(II) sulfate

electrons

Zn deposition

0.76 V

>0.76 Vopposite to

spontaneous reaction

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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-

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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)

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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.

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1.3. Secondary battery (Rechargeable battery)

Electrical Energy

Chemical Energy

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(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.

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Charged

Pote

ntial

Discharged Charged

Basic concept

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Opencircuit

Pote

ntial

Discharged

Practical charging/discharging strategy

Charged Charged

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Introduction to ElectrochemistryLead 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

HSO4-

Pb

e-

PbO2

PbSO4

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Introduction to ElectrochemistryLithium ion battery

Discharging: LiC6+2Li0.5CoO2 -> C+2LiCoO2 E=3.6 V

Charging: C+2LiCoO2 -> LiC6+2Li0.5CoO2

e-

LiC6 Li0.5CoO2

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Introduction to ElectrochemistryLithium ion batteryC+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.

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1.4. Electrochemical (Galvanic) corrosion

Corrosion is the gradual destruction of materials (usually metals) by chemical reaction.

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Introduction to ElectrochemistryCorrosion can be the negative aspect of spontaneous electrochemical reaction.

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Electrochemical corrosion occurs between two “electrodes”

which have electrical contact with each other and are immersed in a common electrolyte.

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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.

Steel (Fe)

Fe2O3

Seawater

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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.

Steel (Fe)

Fe2O3

Seawater

Anode reaction:Fe(s) → Fe2+(aq) + 2e-

Fe2+

e-

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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.

Steel (Fe)

Fe2O3

SeawaterFe2+

e-

O2

OH-

O2O2

O2

Cathode Reaction:O2 + 2H2O + 4e- → 4OH-

O2

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Introduction to ElectrochemistryIn 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.

Steel (Fe)

Fe2O3

SeawaterFe2+ O2

OH-Fe2++2OH- ->Fe(OH)2

e-FexOy·nH2O

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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

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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.

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1.5. Conversion of photon energy into electrical energy

via electrochemical reaction

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Introduction to ElectrochemistryThe 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.

Introduction to ElectrochemistryEnergy 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.).

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Measured .

Stan

dard

Red

uctio

n Po

tenti

al (V

)

-0.5

0

0.5

1.0

SRP EF

TiO2 (-0.44 V) 4.0 eV

Dye* ( 0.74 V) 3.7 eVDye ( 1.06 V) 5.5 eVI3

-/I- 0.55 V (3.9 eV)

e-

h

Conducting glass electrode

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Introduction to ElectrochemistryThe 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.

Measured .

Stan

dard

Red

uctio

n Po

tenti

al (V

)

-0.5

0

0.5

1.0

SRP EF

TiO2 (-0.44 V) 4.0 eV

Dye* ( 0.74 V) 3.7 eVDye ( 1.06 V) 5.5 eVI3

-/I- 0.55 V (3.9 eV)

e-

h

Conducting glass electrode

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Introduction to ElectrochemistryLight 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. Conducting glass

electrode

TiO2h

introduction to electrochemistry by t. hara

Education

aq zn2

aq e zns e cus cu2

aq cathode reaction

e zns e cus cathode

zns zn2

e cus anode reaction

cus zn2

conversion of chemical