Review:

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

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

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

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

5. I I 0 0 E E Review: 1) Ideal polarizable electrode

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

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

8. 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. Cdl Rs Rct 2.6.3 differential capacitance oscillograph 1) Measurement method Cd = C()

10. KI -12 60 Cd / F·cm-2 KBr q/ C·cm-2 -8 KCl -4 40 K2SO4 0 NaF 4 20 KI KF 8 Na2SO4 12 0.0 0.4 1.6 1.2 0.8  / V 0.4 0.0 -1.2 -0.8 -0.4  / V Review: 3) Experimental results Differential capacitance curves 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. E 0 d Review: §2.3 Models for electric double layer 1) Helmholtz model (1853)

13. Plane of shear E 0 d Review: 2) Gouy-Chappman layer (1910, 1913)

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

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. Ci Cd M S Inner layer + diffuse layer This may be seen as 2 capacitors in series: Total capacitance (Ct) dominated by the smaller of the two.

19. experimental calculation Review: Fitting result of Gouy-Chapman Stern Fitting of 0.0001 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 18-20 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. 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 10-10 m, i = 6, then The theoretical estimation is close to the experimental results, 18-20 F·cm-2, which suggests the reasonability of the BDM model.

24. What have been solved, what have not? K+ K+ 15 6 M/ C·cm-2 M/ C·cm-2 4 10 F Br 2 5 0 0 E -2 -5 -4 -10 0 -6 -15 d -0.4 -0.4 0.4 0.0 -1.2 0.4 0.0 -1.2 -0.8 -0.8 0.8 0.8 E-EPZC / V E-EPZC / V

25. KBr 6 cation excess KCl q/ C·cm-2 4 KAc 2 KF 0 Anion excess -2 KF -4 KCl KBr -6 KAc 0.4 0.0 -1.2 -0.8 -0.4  / V Surface excess curves For R.E. in equilibrium with cation For any electrolyte

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

27. E 0 d 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. §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. -0.2 1 / V -0.1 0.0 -1.5 -0.5 -1.0 / V Dependence of 1 on c 0.001 0.01 0.1 1.0

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

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

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

35. Inner Helmholtz plane IHP1 Outer Helmholtz plane, OHP, 2 Primary water layer Secondary water layer §2.3 Models for electric double layer 1) Helmholtz model (1853) 2) Gouy-Chappman model 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. 0.4 0.3  / Nm-1 q / Cm-2 0.3 0.3 0.5 1.0 0.5 0 E / V vs. SCE

38. 2) Some experimental results of PZC 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 0.01 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 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 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. 5.0 Au Cu Sb 4.5 Hg Sn Zn Ga Bi Ag Cd 4.0 In Ti 0.0 -1.0 -0.5 5) Relationship between PZC and We For mercury-like metals:

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. NaF NaCl KBr Ta+ K+ KI N(C3H7)4+ 2.6.1 Some experimental phenomena (1) Effect of ion on PZC 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. 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.