electrode/electrolyte interface: ----structure and...

Chapter 2

Electrode/electrolyte interface:

----Structure and properties

Extensive reading:

Bard, Electrochemical methods. Fundamentals and applications:

pp. 54-63

2.2 A MORE DETAILED VIEW OF INTERFACIAL POTENTIAL

DIFFERENCES

2.2.1 The Physics of Phase Potentials

2.2.3 Measurement of Potential Differences

2.2.4 Electrochemical Potentials

2.2.5 Fermi Level and Absolute Potential

2.5 Structure of solid/electrolyte interface

The electrocapillar curve of mercury. A remark on the

work on the homogeneous K. Bennwitz and A. Dejannis.

作者: Frumkin, A; Obrutschewa, AZEITSCHRIFT FUR PHYSIKALISCHE CHEMIE--

STOCHIOMETRIE UND

VERWANDTSCHAFTSLEHRE 卷: 138 期: 3/4 页: 246-250 出版年: NOV 1928

On the temperature dependency of the electrocapillary

curve of mercury - A contribution to the matter of the

absolute value of electrochemical potentials

作者: Koenig, O; Lange, EZEITSCHRIFT FUR ELEKTROCHEMIE UND

ANGEWANDTE PHYSIKALISCHE CHEMIE 卷: 35 页: 686-695 出版年: 1929

Remarks on the electro-capillary curve of mercury.

作者: Bennewitz, K; Kuchler, KZEITSCHRIFT FUR PHYSIKALISCHE CHEMIE-

ABTEILUNG A-CHEMISCHE THERMODYNAMIK

KINETIK ELEKTROCHEMIE EIGENSCHAFTSLEHRE 卷: 153 期: 5/6 页: 443-450 出版年: MAR 1931

2.5 structure of solid/electrolyte interface

2.5.1 Methods for studying solid electrode

Works on liquid gallium

ELECTROCAPILLARY CURVES OF LIQUID

GALLIUM

作者: FRUMKIN, AN; POLYANOVSKAYA, NS; GRIGOREV, NB

DOKLADY AKADEMII NAUK SSSR 卷: 157 期: 6 页: 1455-& 出版年: 196

Electrocapillary curve of gallium II

作者: Murtazajew, A; Gorodetzkaja, AACTA PHYSICOCHIMICA URSS 卷: 4 期: 1 页: B75-B84 出版年: 1936

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The drop-weight method for the determination of

electrocapillary curves

作者: Craxford, SR; Mckay, HACJOURNAL OF PHYSICAL CHEMISTRY 卷: 39 期: 4 页: 545-550 出版年: APR 193



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Merits of mercury:

1. High hydrogen evolution overpotential

2. Facile measurement of surface tension

3. Easy purification

4. No residual tension.

Other solid metals:

1. Metals with High hydrogen overpotential

2. Oxidation of metal surface

3. Embedment of impurity

4. Different crystal facets

5. History of thermal treatment



ELECTROCAPILLARY CURVES FOR GOLD

作者: LIN, KF; BECK, TRJOURNAL OF THE ELECTROCHEMICAL SOCIETY 卷: 122 期: 3 页: C110-C110 出版年: 1975

ELECTROCAPILLARY CURVES FOR GOLD

作者: LIN, KF; BECK, TRJOURNAL OF THE ELECTROCHEMICAL SOCIETY 卷: 124, 期: 7 页: C239-C239 出版年: 1977

ELECTROCAPILLARY CURVES OF SOLID COPPER

IN SODIUM TETRABORATE MELT

作者: POPEL, SI; DERYABIN, YA; PETROV, VVSOVIET ELECTROCHEMISTRY 卷: 14 期: 5 页: 595-598 出版年: 1978

ELECTROCAPILLARY CURVES OF SOLID METALS MEASURED BY EXTENSOMETER INSTRUMENT

作者: BECK, TRJOURNAL OF PHYSICAL CHEMISTRY 卷: 73 期: 2 页: 466-& 出版年: 1969



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ELECTROCAPILLARY CURVE ON SOLID METAL OBTAINED BY HOLOGRAPHIC-INTERFEROMETRY

作者: PANGAROV, N; KOLAROV, GJOURNAL OF ELECTROANALYTICAL CHEMISTRY 卷: 91 期: 2 页: 281-285 出版年: 1978

PIEZOELECTRIC MEASUREMENT OF ELECTROCAPILLARY CURVES

作者: MALPAS, RE; FREDLEIN, RA; BARD, AJJOURNAL OF ELECTROANALYTICAL CHEMISTRY 卷: 98 期: 2 页: 339-343 出版年: 1979



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A new very sensitive method was developed for obtaining

the “electrocapillary” curve of a solid metal. The method is

based on the measurement of small elastic deformations of a

strip caused by the changes of the surface tension forces. For

the precise measurement of the strip bending (the radius of

curvature) holographic interferometry was applied. It is

shown that a change of the surface tension ±0.1 mN

m−1 can be registered. The “electrocapillary” curve of

platinum in 0.05 M H2SO4 solution was obtained. It was

found that the zero charge potential is +0.25 V versus normal

hydrogen electrode. The double layer capacity was

evaluated.



(1) electrocapillary curve

(2) weight of mercury drop;

(3) contact angle of gas bubble

(4) wetting of surface

(5) differential capacitance curve

(6) surface hardness

(7) piezoelectric measurement;

(8) holography (全息摄影)



2.5.2 Some important results


2.5.2 Some important results


Differential capacitance curve of Cd electrode.Differential capacitance curve of Au(210) electrode.

2.6 Potential at zero charge

2.6.1 Determination of PZC

2.6 potential at zero charge

(1) Definition: potential at which the electrode bears no charge.

Differential capacitance curves of different

crystal facets of Ag in 0.01 mol dm-1 NaF

solution. 1. (100); 2. (100), 3. (111).

,multi ,singled i dC C=



Difficulties in measuring PZC

1) purification of electrolyte and metal (Why do we use mercury? )

2) specific adsorption (includes adsorption of hydrogen)

Hg-like metal: Cd, Sn, Pb, As, Sb, Bi; Ga, In, Tl

Pt-like metal: Ni, Pt, Pd; Co; Rh, Ir; Ru, Os

3) crystal facet and multi-crystal



Different crystalline facet has different differential capacitance and thus different potential of

zero charge

,multi ,singled i dC C=

For multi-crystal, its differential capacitance is the sum of all the differential capacitance of the

surface of single crystal multiplied with their fraction.

Ag

(111) 0.001 moldm-3 KF -0.46

(100) 0.005 moldm-3 NaF -0.61

(110) 0.005 moldm-3 NaF -0.77

(MC) 0.005 moldm-3 Na2SO4 -0.7

Au

(111) 0.005 moldm-3 NaF +0.50

(100) 0.005 moldm-3 NaF +0.38

(110) 0.005 moldm-3 NaF +0.19

MC 0.005 moldm-3 NaF +0.25

2.6.2 Some experimental results of PZC


d,multi d,iC C=

Capillary curves of Hg in 0.01 mol dm-3

NaCl, NaBr and KI solution.

Effect of ion on PZCHS¯> I¯> Br¯> Cl¯> OH¯> SO4

¯> F¯

Special adsorption of cations:



Dependence of PZC on anion and concentration



2.6.3 Relationship between PZC and We

PZC,vs SCEeW C− = +

For mercury-like metals:


PZC

We

Theoretical calculation of electrochemical potential

PZC,vs SCEeW C− = +

2.6.3 Relationship between PZC and We


2.6.4 significance and Application of PZC

PZC 0; ( Δ ) 0M S

q == =

Surface potential () still exists due to the specific adsorption, orientation of

dipoles, polarization of surface atoms in metal electrode, etc.

Therefore:0 0 0( ) [( ) ( ) ] 0

M S M S M S

q q q = = == +

PZC can not be taken as the absolute zero point for the interphase

potential. M S

?M S =

0( ) 0M S

q =


Potential standard:

Potentials refereed to PZC as zero point (-PZC) are named as rational

potential standard.

1) potential versus reference electrode

(−0);

2) Potential versus reversible potential;

3) potential versus PZC (−PZC)

2.6.4 significance and Application of PZC


Electrocapillary curve and differential capacitance curve in electrolytes

with same valence type and concentration should be similar and neutral

molecules have little effect on the curves.

Influential factors: 1) valence type; 2) concentration; 3) size of solvated ions;

4) potential related to PZC

The former four models for electric double layer are all electrostatic models

without consideration of non-electrostatic interaction between species and

electrode surface.

summary


2.7 Adsorption at electrode/solution interface

Za, p. 56 §Surface adsorption on electrodeBard, p. 557, 13.4.1 Study at solid electrodes

2.7.1 methods for studying interface adsorption


C- curve for n-pentanol at a dropping Hg electrode in 0.1 M KCl



At PZC, surface tension decrease

dramatically, but at higher polarization, no

significant change can be observed.

Effect of potential on surface adsorption:

around PZC, the adsorption attain maximum.

At high potential, water may replace organic molecules already adsorbed on the

electrode surface. And the arrangement of water molecules on the electrode surface

may change accordingly.



As concentration of surface active reagent

increases, the surface tension decreases, and

finally attains a limiting value.

Adsorption peaks appearing in differential

capacitance curve

Where Ci is integrated capacitance

When adsorption/desorption occurs, d(Ci)/d becomes astonishingly large – falsecapacitance. The peak of false capacitance marks the adsorption/desorption of the surface

active reagent.

( ) ( )i id i

d C d CdqC C

d d d

= = = +



(3) Degree of coverage

1 0(1 )q q q = == + −

0 1

1 0

1 0 0 1

(1 ) ( )

(1 ) ( )

d

dq dq dq dC q q

d d d d

dC C q q

d

= =

= =

= = = =

= = + − − −

= + − − −

can be used to characterize the formation of self-assembled monolayer, to evaluate thedefect in polymeric coatings and determine the wetted area on substrate metal surface or

water sorption of polymer materials.

0(1 )adC C C = + − 0

0 ad

C C

C C

−=

−



1) Concentration change in solution;

2) Electrochemical oxidation or reduction

of adsorbed species (coulomb);

3) Radioactive marks (radiation counter)

4) EQCM: Electrochemical quartz crystal

micro-balance (gravimetric method)

(4) Other ways to measure adsorption

Electrochemistry of LB film



Since the first observation of SERS spectra in

1974, various physical and chemical

enhancement mechanisms have been proposed

to understand the SERS mechanisms.

Electromagnetic enhancement (EM) and

chemical enhancement (CE) are the two most

widely accepted mechanisms. It is well

accepted that the EM mechanism contributes

dominantly to the total SERS enhancement,

showing enhancements from 4 to 11 orders of

magnitude. Although CE is only 10–100 times,

it can significantly modify the SERS features.



Figure 12. Label-free SERS detection of protein. (a)

Normal Raman (green) and SERS spectra (red) of

avidin from the Ag IMNPs colloid. (b) FEM simulation

of the electric field distribution of a 50 nm nanocube

and SERS spectra of cytochrome c obtained on Ag

nanocubes by using 532 nm (black) and 638 nm (red)

excitation wavelengths. The concentration of

cytochrome c is 0.1 nM. The Raman signal of a 1 mM

cytochrome c solution is also given for comparison

(blue).



(1) Hydrophobic interaction (intermolecular reaction—non-covalent

interaction, surface activity);

(2) Electrostatic and chemical interaction (coordination);

(3) Interaction between adsorbate molecules; (aggregate, island, crystallization)

(4) Replacement of adsorbed water molecules

Different from gaseous adsorption

sl 2 ad ad 2 slRH H O RH H On n+ === +

ad ad adG H T S = −

2.7.2 Interaction between adsorbate and metal

Chapter 2 Electrode/electrolyte interface

1) Difference between adsorption on

gas/solid interface and solution/electrode interface

Nearly all dissoluble organic compounds has more or less surface activity

at the solution/electrode interface, i.e., can adsorb on the interface. At

solution/electrode interface, adsorbed species have to replace the formerly

adsorbed water molecules and become contact physically or chemically

with the electrode surface.

RHslon + nH2Oad RHad + nH2Osoln⎯⎯→⎯⎯

2.7.2 Interaction between adsorbate and metal


Adsorption isotherms:

lnso

i

ads

i =

lnln,0,0 lnln soiso

i

ads

i

ads

i aRTaRT +=+

describe the relationship between the activity (ai) of a species in solution and the

amount of material adsorbed per unit area on electrode surface (i). The isotherms

are developed from the condition that the electrochemical potential for the adsorbed

and solution species are equal:

2.7.3 adsorption isotherms of electrode/electrolyte interface


ln,0,00 so

i

ads

iG −=

−=

RT

Gaa isoi

ads

i

0ln exp

i

so

i

ads

i aa ln=

The general form of an isotherm.



Langmuir isotherm 1

kbc

bc =

+

1) gaseous adsorption

Assumptions:

1) no interaction between species; i.e., Gad

is not a function of coverage.

2) No heterogeneity of surface, all surface

sites are equal.

3) there is a maximum coverage,

(monolayer)

ad

des

kb

k=

1

sl

i

sl

i

bc

bc =

+

Langmuir isotherm



Langmuir isotherm: plot of fractional coverage versus concentration

Langmuir isotherm



Interactions between adsorbed species complicate the problem by making the

energy of adsorption a function of surface coverage.

Temkin isotherm

Frumkin isotherm

0

ads m ads mΔ Δ (1 )H H = − ln( )a k bp=



( )i i iln2

bRT ag

= （0.2<

RH,ad(1 )

Xn

=

+ −2H O,ad

(1 )

(1 )

nX

n

−=

+ −

2

2

RH,ad H O,sl

RH,sl H O,ad

n

ad n

X XK

X X=

1

RH,sl

(1 )

(1 )

n

n n

nBc

n

−+ −=

−

Bockris-Swinkels adsorption isotherm

When n = 11

bc

bc =

+

sl 2 ad ad 2 slRH H O ===RH H On n+ +



Adsorption isotherms 1) Langmuir, 2) Temkin, 3) Frumkin



Adsorbate desorbs at higher

polarization.

Why?

1) Effect of potential

0( ) ( )expad adE z

K K nZkT

− = −

Bockris: water dipole, E strength, intermolecular interaction, n replacement number.

N NZ

N N

− =

+

2.7.4 Factors on interface adsorption


2) Nature of metal

Desorption capacitance peak of n-pentanol

in 0.1 M electrolyte on different metal.

n-pentanol leaves metal’s surface when q = -13.10.6 C/cm2.



For organic molecules with aromatic ring, there are two adsorbed states, lying or

standing.

Because surfactant can affect the surface state of electrode significantly, therefore,

the purification of solution used for electrochemical study is very important.

Deionized water triple distilled water



electrode/electrolyte interface: ----structure and...

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