1 4 electrochemistry(30) 101013

7/31/2019 1 4 Electrochemistry(30) 101013

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Introduction

One phase contains electrons. Other phase contains ions.

e-

- - - - - - - - - - - - > +

Fundamental act in electrochemistry

Passage of Current Caused by Chemical Changes

Broad Field: sensors, electro-analysis, electro-synthesis,

electrodeposition, corrosion, and

dye-sensitized solar cell (DSC), fuel cell, and battery.

Electrochemistry is the Study of Phenomena atthe Electrode/ElectrolyteInterfaces.

Electrochemistry Seunghoon http://bp.snu.ac.kr 2


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Introduction

Electrode reactions such as (1), (2), and (3)can only happen at the interface.

_______ 2

Cu2+

+ 2e

Cu (2) _______ _____

FeCp2 FeCp2+ + e (3)

ReactantProduct

composition in the nearby solution,

different from that further away.

There is a tendency for the system to transport

reactants from remote points and the depleted

zone widens as the reaction proceeds (Fig.1(b)). ______

Faulkner, Journal of Chem., Education, 1983

Just as the reactants is depleted near the interface,

the product accumulates nearby as shown in Fig.


.


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Potential is an Expression of Electron Energy.

The energy required to add or subtract an electron

an ar po en a : ox a on re uc on poss e.

potential.

Any electrode process is characterized by its ownreducedstandard potential E0.

On the positive side ofE0, the oxidized form of the

,

stable on the potential more negative than E0.

In the zone ~100 mV wide centered on E0, statistical

Fe/Pt = 0.27 oxidized

equilibrium of electrons on the electrode and species

in the solution permits mixtures of oxidized and

reduced forms with appreciable amounts (transition.

Faulkner ournal o Chem. Education 1983

Po ten t i a l i s an exp r ess ion o f elect r on energy .



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Work Function (not real)

s cro , o a e ys cs

__

Electrochemistry BP http://bp.snu.ac.kr 5


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Double Layer Work Function

Ashcroft

Solid State Physics



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Work Function

th. , ,



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Electron Transfer at the Interface

LUMOMagnitude of the potential controls the direction

and rate of charge transfer.

HOMO

As a potential is moved negative, the species will

be reduced first with the least negative E0.a speciesA in solution

LUMO

. . ar , ec roc em ca e o s, e on

HOMO

_______________________________________



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Standard Electrode Potential

NHE (= SHE)

Normal Hydrogen Electrode- Neglect any orientational dependence

- 298 K

- Acid solution with

A. J. Bard, Electrochemical Methods, 2ndedition

an activity of H+ = 1 mol/liter (pH 0)

eV

-


- 2010-10-04

2 e =


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Band Structure Determined by Cyclic Voltammetry for CdSe Nanocrystals

Valence band

maximum

_

Conduction bandminimum

________ ________________

______________

Thomas Nanns Group, Journal of Chemical Physics (2003)

Freiburg Materials Research Center (Germany)

Electrochemistry Yejun (The contents of page 10 are the same as those of page 27.) http://bp.snu.ac.kr 10


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Current is an Expression of Rate.

Suppose ferrocene is being oxidized.

Usually we focus attention on a singleelectrode called the working electrode

FeCp2 FeCp2+ + e (3)

Each molecule that is oxidized ives u electrons to

________

the working electrode.

The electron will pass through the external circuit,

Fe/Pt = 0.27

an ac nto t e so ut on at a secon e ectro e

called the counter electrode.

Electrons at either electrode can flow into or out of____________

the electrode by way of external circuit which can be

oxidizing or reducing current_________

.e ow o e ectrons s rect y proport ona to t e rateof reaction there. So, t h e cu r r en t i s an exp ression o f ra t e .

Faulkner, Journal of Chem., Education, 1983



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Reference Electrode

Potentials actually mean the voltage differences

between a working electrode and a reference electrode.

an ar y r ogen ec r o e =

The reference electrode is arranged

so that it does not pass current,and is in equilibrium.

The reference electrode is constructed so that it

Fe/Pt = 0.27

conta ns ot forms of re ox coup e

(e.g., H+/H2, Hg/Hg2SO4, or Ag/AgCl)at fixed composition.

Electron energies in the metallic part of the electrode

are fixed at a value near the E0.1. Platinized platinum electrode.

2. H dro en as.

(image from Wikipedia)

3. Acid solution with an activity of H+ = 1 mol/liter.

4. Hydroseal for prevention of oxygen interference.

5. Reservoir via which the second half-element of

e ga van c ce s ou e a ac e .



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Adrian W. Botts Group, Current Separations 14:2 (1995)

Bioanalytical Systems West LafayetteReference Electrode

Vycor porous glass

- modest leak rate

- poss e con am na on o so u on

- ionic conducting pathway

Electrochemistry Yejun http://bp.snu.ac.kr 13


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Counter Electrode >> Working Electrode

e e

Power generated

or supplied

e e

Power generated

or supplied

measured measured- - - - - - - - - - - - - - - - - - - - - - - - - - -

A+

B-

ee A+

B-

ee- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

RE: Reference electrodeWorking electrode Counter electrodeWorking electrode Counter electrode

RE: Reference electrode

- Surface area:Working electrode


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Counter Electrode ~ Working Electrode

e e

Power generatedor supplied

e e

Power generatedor supplied

measured measured- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

A+

B-

ee A+

B-

ee

Working electrode Counter electrode Working electrode Counter electrodeRE: Reference electrode RE: Reference electrode

- Surface area:Working electrode Counter electrode

- For different potential at working electrode,

different potential at counter electrode

http://bp.snu.ac.kr 15Electrochemistry Yejun


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Electrochemical Cells and Reactions

Fe/Pt = 0.27

A potential difference between two electrodes

. . . ar , ec roc em ca e o s



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Electrochemical Cells and Reactions

The potential that develops in a cell is a measure of the tendency for a reaction to proceedtoward equilibrium.

Standard oxidation/reduction reactions:

All relative to the H2

/H+ reaction, 298 K,

, .




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Double Layer


q meta = -q so ut on charge neutrality!

Compact layer = inner and outer Helmholtz planes

Diffuse layer = gradient of charge accumulation

metal = q metal/area (C/cm2)

- e excess c arge on a me a s con ne o e near sur ace reg on.

- However, the balancing charge on the solution side of the interface extends

out into the solution with some thickness (ionic zones in solution).



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.

to specifically adsorbed anions (inner).

2. Solvated ions are distributed in diffuse layer(~100 ), because of thermal agitation in thesolution (diffuse).

.and specifically adsorbed anions in inner layer isequal to the charge from solvated ions in diffuselayer.

metal

+ inner

= - diffuse

Bard, A. J., and Faulkner, L. R.Electrochemical Methods

I n n e r He lm h o l t z La y erD if f u s e La y er ( ~ 1 0 0 )

ey, ew or . , ap.



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Ch a r g e Pr o f i le i n t h e Vi ci n i t y o f I n t e r f ace

I nne r H el m ho l t z Lay er Di f f u se La y er ( ~ 1 0 0 )

-

x +

- Ther e a re e lect r on cloud s sp i l t over

,

i n t he v o l t age p r o f i l e nea r t he m e ta l su r f ace .

a r g e p r o e on e so u on s e s ow snega t i ve cha rg es near t he su r f ace b u t

d i f f us es aw ay w i t h i n inn e r lay er .META



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Vo l t a g e Pr o f i le in t h e Vi ci n i t y o f I n t e r f ace

I nne r H el m ho l t z Lay er D if f us e Lay er ( ~ 100 )

x



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+Schem at i cs o f Doub le Layer i n So lu t ion

- +

Nega t i ve l y

Charged

+ +e a - + Bard, A. J. and Faulkner, L. R.Electrochemical Methods

Wiiey, New York. 1980, Chap. 1.

Hamann, C. H., ElectrochemistryWiley-VCH, 1998, Chap. 3.

(x)

(x)Electrochemistry Seunghoon http://bp.snu.ac.kr 25


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Double Layer

The CRC Handbook of Solid State ElectrochemistryH. J. M. Bouwmeester, p. 27

Charge distribution

o en a s r u on

Metal Diffuse layer



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Helmholtz Layer and Diffuse Layer

nd. . , ,

_____

_____



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Potential Profile Across the Double-Layer

. . ar , ec roc em ca e o s, e on



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______________ ______________

PL

CV



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Band Structure Determined by Cyclic Voltammetry for CdSe Nanocrystals

Valence band

maximum_

Conduction bandminimum

________ ________________

______________

Thomas Nanns Group, Journal of Chemical Physics (2003)

Freiburg Materials Research Center (Germany)

Electrochemistry Yejun (The contents of page 10 are the same as those of page 27.) http://bp.snu.ac.kr 27


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Experiments and Variables in Electrochemical Cell

1. Mass transfer of reactant/product

to and away from the electrode

n er ace.

2. Electron transfer at the interface.

3. Chemical reactions.

4. Surface processes

(adsorption/desorption)




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A. J. Bard, Electrochemical Methods, 2ndeditionMass Transport

o es o ass ranspor :

Migration movement of ions by an electric field

Diffusion

by a concentration gradientConvection

by stirring or hydrodynamic transport


Th S f El t h i t


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The Scope of Electrochemistry

1. Investi ation of chemical henomena associated with

a charge-transfer reaction.

2. To assure electro-neutrality,

two or more half reactions take lace in o osite directions

(oxidation and reduction).

3. If the change of total Gibbs free energy is negative,

.


- 2010-10-13

1 4 electrochemistry(30) 101013

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