-
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
2.5 structure of solid/electrolyte interface
2.5.1 Methods for studying solid electrode
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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
2.5 structure of solid/electrolyte interface
2.5.1 Methods for studying solid electrode
-
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
2.5 structure of solid/electrolyte interface
2.5.1 Methods for studying solid electrode
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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
2.5 structure of solid/electrolyte interface
2.5.1 Methods for studying solid electrode
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2.5 structure of solid/electrolyte interface
2.5.1 Methods for studying solid electrode
-
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.
2.5 structure of solid/electrolyte interface
2.5.1 Methods for studying solid electrode
-
(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 structure of solid/electrolyte interface
2.5.1 Methods for studying solid electrode
-
2.5.2 Some important results
2.5 structure of solid/electrolyte interface
-
2.5.2 Some important results
2.5 structure of solid/electrolyte interface
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=
2.6.1 Determination of PZC
2.6 potential at zero charge
-
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
2.6.1 Determination of PZC
2.6 potential at zero charge
-
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
2.6 potential at zero charge
d,multi d,iC C=
-
2.6.2 Some experimental results of PZC
2.6 potential at zero charge
-
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:
2.6 potential at zero charge
2.6.2 Some experimental results of PZC
-
Dependence of PZC on anion and concentration
2.6 potential at zero charge
2.6.2 Some experimental results of PZC
-
2.6.3 Relationship between PZC and We
PZC,vs SCEeW C− = +
For mercury-like metals:
2.6 potential at zero charge
PZC
We
-
Theoretical calculation of electrochemical potential
PZC,vs SCEeW C− = +
2.6.3 Relationship between PZC and We
2.6 potential at zero charge
-
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 =
2.6 potential at zero charge
-
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
2.6 potential at zero charge
-
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
2.6 potential at zero charge
-
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
2.7 Adsorption at electrode/solution interface
-
C- curve for n-pentanol at a dropping Hg electrode in 0.1 M
KCl
2.7.1 methods for studying interface adsorption
2.7 Adsorption at electrode/solution interface
-
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.
2.7.1 methods for studying interface adsorption
2.7 Adsorption at electrode/solution interface
-
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
= = = +
2.7.1 methods for studying interface adsorption
2.7 Adsorption at electrode/solution interface
-
(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
−=
−
2.7.1 methods for studying interface adsorption
2.7 Adsorption at electrode/solution interface
-
2.7.1 methods for studying interface adsorption
2.7 Adsorption at electrode/solution interface
-
2.7.1 methods for studying interface adsorption
2.7 Adsorption at electrode/solution interface
-
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
2.7.1 methods for studying interface adsorption
2.7 Adsorption at electrode/solution interface
-
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.
2.7.1 methods for studying interface adsorption
2.7 Adsorption at electrode/solution interface
-
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).
2.7.1 methods for studying interface adsorption
2.7 Adsorption at electrode/solution interface
-
(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
Chapter 2 Electrode/electrolyte interface
-
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
Chapter 2 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.
2.7.3 adsorption isotherms of electrode/electrolyte
interface
Chapter 2 Electrode/electrolyte interface
-
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
2.7.3 adsorption isotherms of electrode/electrolyte
interface
Chapter 2 Electrode/electrolyte interface
-
Langmuir isotherm: plot of fractional coverage versus
concentration
Langmuir isotherm
2.7.3 adsorption isotherms of electrode/electrolyte
interface
Chapter 2 Electrode/electrolyte interface
-
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=
2.7.3 adsorption isotherms of electrode/electrolyte
interface
Chapter 2 Electrode/electrolyte interface
( )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+ +
2.7.3 adsorption isotherms of electrode/electrolyte
interface
Chapter 2 Electrode/electrolyte interface
-
Adsorption isotherms 1) Langmuir, 2) Temkin, 3) Frumkin
2.7.3 adsorption isotherms of electrode/electrolyte
interface
Chapter 2 Electrode/electrolyte interface
-
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
Chapter 2 Electrode/electrolyte interface
-
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.
2.7.4 Factors on interface adsorption
Chapter 2 Electrode/electrolyte interface
-
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
2.7.4 Factors on interface adsorption
Chapter 2 Electrode/electrolyte interface