1. Review: Fermi level Electrochemical potential
Inner, outer, surface potential, work function
Inner potential difference, correct connection, absolute potential, relative potential (standard potential)

2. §2.2 Structure of Electrolyte/electrode surface 2.2.1 Surface charge
Review:
§2.2 Structure of Electrolyte/electrode surface
2.2.1 Surface charge
2) Transfer of charged species
1) Transfer of electrons
Cu2+(aq) Cu
Cu2+ e- Cu Zn
Zn2+ e-

3. Review: 3) Unequal dissolution / ionization+
AgI I¯ +
3) Unequal dissolution / ionization
4) specific adsorption of ions –  + +
Electron atmosphere
KCl
HCl H+ K+
Cl-
6) Liquid-liquid interfacial charge
5) orientation of dipole molecules

4. Electroneutrality: qm = -qs
Review:
2.2.2 Electric double layer – + Cu
Cu2+ e-
capacitor
Electroneutrality: qm = -qs
Holmholtz double layer (1853)

5. Review:
1) Ideal polarizable electrode E I E I

6. 2.2.4 Interfacial structure: experimental
Review:
2.2.4 Interfacial structure: experimental
1) Experimental methods:
(1) electrocapillary curve measurement
(2) differential capacitance measurement
Lippman equation

7. 2) Experiment equipment
Review:
2) Experiment equipment
When the composition of solution keeps constant

8. Electrocapillary curve
Review:
3) Experiment results
Electrocapillary curve
Zero charge potential: 0
(pzc: potential at which the electrode has zero charge)
Electrocapillary curves for mercury and different electrolytes at 18 oC.

9. 2.6.3 differential capacitance oscillograph 1) Measurement methodRs
Rct
Cdl
Cd = C()

10. 3) Experimental results Differential capacitance curves
Review:
3) Experimental results
Differential capacitance curves
NaF
Na2SO4 KI
0.0
-0.4
-0.8
-1.2
0.4
 / V
q / C·cm-2 4 8 12 -4 -8
-12 KF
K2SO4
KCl
KBr KI
0.4
0.8
1.2
1.6
0.0
 / V
Cd / F·cm-2 20 40 60
Dependence of differential capacitance on potential of different electrolytes.
Charge density on potential

11. Review: Potential-dependent Concentration-dependent
Minimum capacitance at potential of zero charge (Epzc)
36 F cm-2;
18 F cm-2;
differential capacitance curves for an Hg electrode in NaF aqueous solution

12. §2.3 Models for electric double layer
Review:
§2.3 Models for electric double layer
1) Helmholtz model (1853) d E

13. 2) Gouy-Chappman layer (1910, 1913)
Review:
2) Gouy-Chappman layer (1910, 1913) d E
Plane of shear

14. Review:
Gouy and Chapman quantitatively described the charge stored in the diffuse layer, qd (per unit area of electrode:)
Boltzmann distribution
Poisson equation  +  q qs c0

15. Review:

16. Review:
For a 1:1 electrolyte at 25 oC in water, the predicted capacitance from Gouy-Chapman Theory.
1) Minimum in capacitance at the potential of zero charge
2) dependence of Cd on concentration

17. Review: 3) Stern double layer (1924)
Combination of Helmholtz and Guoy-Chapman Models
The potential drop may be broken into 2:

18. At low c0 At high c0 Cd dominant Ci dominant Cd  Ct Ci  Ct
Inner layer diffuse layer
This may be seen as 2 capacitors in series: Ci Cd M S
Total capacitance (Ct) dominated by the smaller of the two.
At low c0
At high c0
Cd dominant
Ci dominant
Cd  Ct
Ci  Ct

19. Stern model: what have been solved, what have not?
Review:
experimental
calculation
Fitting result of Gouy-Chapman
Stern Fitting of mol·L-1 HCl
Stern model: what have been solved, what have not?

20. The progress of Model for electric double layer
Helmholtz model
Gouy-Chappman model
Stern model
what have been solved, what have not?
At higher negative polarization, the differential capacitance, approximately F·cm-2, is independent of the radius of cations. At higher positive polarization, differential capacitance approximates to be 36 F·cm-2.

21. 4) BDM model Bockris-Devanathan-Muller, 1963
Nom-electrostatic adsorption
Electrostatic adsorption

22. Specially adsorbed anion
Inner Helmholtz plane IHP 1
Outer Helmholtz plane, OHP, 2
Specially adsorbed anion
Solvated cation
Primary water layer
Secondary water layer
Weak Solvation and strong interaction let anions approach electrode and become specifically adsorbed.

23. Dielectric saturation
di i =5-6
do i =40
If the diameter of adsorbed water molecules was assumed as 2.7 m, i = 6, then
The theoretical estimation is close to the experimental results, F·cm-2, which suggests the reasonability of the BDM model.

24. What have been solved, what have not?
0.0
-0.4
-0.8
-1.2
0.4
M / C·cm-2 -2 -4 -6 2 4 6
0.8 K+ F
E-EPZC / V
What have been solved, what have not? d E
0.0
-0.4
-0.8
-1.2
0.4 -5
-10
-15 5 10 15
0.8 K+
Br
E-EPZC / V
M / C·cm-2

25. Surface excess curves For R.E. in equilibrium with cationKF
0.0
-0.4
-0.8
-1.2
0.4
 / V
q / C·cm-2 -2 -4 -6 2 4 6
KAc
KCl
KBr
Anion excess
cation excess
For R.E. in equilibrium with cation
For any electrolyte

26. 5) Gramham Model-specific adsorption
Normal adsorption due to electrostatic attraction of cations
 = 0  1  +
Specific adsorption due to chemical adsorption of anions + 
Overload adsorption

27. Triple layer Specifically adsorbed anionsd E
Triple layer
Specifically adsorbed anions
Helmholtz (inner / outer) plane

28. Summary: For electric double layer
1. A unambiguous physical image of electric double layer
2. The change of compact layer and diffusion layer with concentration
3. The fine structure of compact layer

29. 1 = 0 validate only at high concentration or larger polarization
§2.4 1 potential
1 = 0 validate only at high concentration or larger polarization 1 
-1 x
1 potential at outer Helmholtz plane

30. GCS model
When electrode bear negative charge
Discussion: When c0 and are very small
When c0 and are very large
Influential factors: concentration and potential

31. Dependence of 1 on c -0.1 -0.2 0.0 -0.5 -1.0 -1.5 1 / V  / V 0.001
0.01
0.1
1.0

32. Dependence of 1 on  IHP OHP  / V d / Å Hg in NaCl solution 0.4 0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
0.2
0.4
IHP
OHP
 / V
d / Å
Hg in NaCl solution

33. effect of 1 1. on concentration 2. on reaction ratex 1
-1
2. on reaction rate 
3. on polarization

34. Electrode/electrolyte interface: structure and properties
Chapter 2
Electrode/electrolyte interface: structure and properties

35. §2.3 Models for electric double layer
1) Helmholtz model (1853)
2) Gouy-Chappman model
Primary water layer
Secondary water layer
Inner Helmholtz plane IHP 1
Outer Helmholtz plane, OHP, 2
3) Stern double layer (1924)
4) BDM model
5) Gramham Model
§2.4 1 potential

36. 2.5 Potential at zero charge (PZC, PZC)
Definition: potential at which the electrode bears no charge.
2.4.1 Determination of PZC
1) Experimental method
(1) electrocapillary curve
(2) differential capacitance curve (most accurate )
(3) contact angle of gas bubble on the metal surface
(4) surface hardness
(5) wetting of surface

37. 0.5
0.5
1.0
 / Nm-1
0.3
0.4
q / Cm-2
0.3
E / V vs. SCE

38. 2) Some experimental results of PZC
Metals
Electrolyte
PZC Hg
NaF
-0.193
Bi (multicrystal)
KF (0.002)
-0.39
Bi (111 surface)
KF (0.01)
-0.42
Ag (111)
-0.46
Ag(100)
NaF (0.005)
-0.61
Ag (110)
-0.77 Cd
NaF (0.001)
-0.75
When the electrode potential is more positive than potential at zero charge, how is the electrode charged, positive or negative?

39. 3) Difficulties in measuring PZC
1) purification of electrolyte and metal (why do we usually 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

40. Differential capacitance curves of different crystal facets of Ag in 0
Differential capacitance curves of different crystal facets of Ag in 0.01 mol dm-1 NaF solution. 1. (100); 2. (100), 3. (111).
Different crystalline facet has different differential capacitance and thus different potential of zero charge Ag
(111)
0.001 moldm-3 KF
-0.46
(100)
0.005 moldm-3 NaF
-0.61
(110)
-0.77
(MC)
0.005 moldm-3 Na2SO4
-0.7 Au
+0.19
+0.50
+0.38 MC
+0.25
For multi-crystal, its differential capacitance is the sum of all the differential capacitance of the surface of single crystal times their fraction.

41. 4) Application of PZC Therefore:
Surface potential () still exists due to the specific adsorption, orientation of dipoles, polarization of surface atoms in metal electrode, etc.
Therefore:
PZC can not be taken as the absolute zero point for the interphase potential.

42. Potential standard: 1) potential versus reference electrode (0);
2) potential versus PZC (PZC)
Potentials refereed to PZC as zero point (E-EPZC) are named as rational potential standard.

43. 5) Relationship between PZC and We
For mercury-like metals:
-1.0
-0.5
0.0
4.0
4.5
5.0 Ti Cd In Ga Zn Ag Sn Bi Hg Sb Cu Au

44. Theoretical calculation of electrochemical potential
Vacuum
+  M
+ 
+ 
SHE

45. 2.6 Interface adsorption and Graham Model
The former four models for electric double layer are all electrostatic models without consideration of non-electrostatic interaction between species and electrode surface.
influential factors: 1) valence type; 2) concentration; 3) size of solvated ions; 4) potential related to 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.

46. 2.6.1 Some experimental phenomena
(1) Effect of ion on PZC
NaF
NaCl
KBr KI K+
Ta+
N(C3H7)4+
Special adsorption of cations:
Capillary curves of Hg in 0.01 mol dm-3 NaCl, NaBr and KI solution.
Dependence of PZC on anion and concentration
HS¯ > I¯ > Br¯ > Cl¯ > OH¯ > SO4¯ > F¯

47. (2) Effect of surface active agent on PZC
C- curve for n-pentanol at a dropping Hg electrode in 0.1 M KCl
Capillary curves of Hg in 0.01 mol dm-3 NaCl containing t-C5H11OH of different concentration.

48. (1) Adsorption of organic molecules
2.6.2 discussion
(1) Adsorption of organic molecules
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.

49. 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 integration capacitance
When adsorption/desorption occurs, d(Ci)/d becomes astonishingly large – false capacitance. The peak of false capacitance marks the adsorption/desorption of the surface active reagent.

50. (2) Degree of coverage
 can be used to characterize the formation of self-assembled monolayer, to evaluate the defect in polymeric coatings and determine the wetted area on substrate metal surface or water sorption of polymer materials.

51. S-Y ZHANG, et al., "Evaluation of thin defect-free epoxy coatings using electrochemical impedance spectroscopy", Journal of Applied Electrochemistry, 1998，28(11): 1277~1281

52. 2.6.3 Other ways to measure adsorption
Concentration change in solution;
Electrochemical oxidation or reduction of adsorbed species (coulomb);
Radioactive marks (radiation counter)
EQCM: Electrochemical quartz crystal micro-balance (gravimetric method)