Electrochemistry 1 the basic of the basic

1. Electrochemistry 1 1 The Basic of the basic 1. Interface ; 2. Thermodynamics & Kinetics; 3. Overpotential.

2. 2 Electrochemistry 1 2 1. Interface Electrochemistry is the study of reactions in which charged particles (ions and/or electrons) cross the interface between two phases of matter, such as the interface between a solid and a liquid (=electrode /electrolyte).

3. 3 Electrochemistry 1 3 1. Interface

4. 4 Electrochemistry 1 4 1. Interface Electrode processes take place within an electric double layer. Electric double layer that is a transition region between two phases consists of (1) an inner monomolecular layer and (2) an outer diffuse region. Between the inner molecular layer and outer diffuse layer, and (3 ) a layer intermediate between inner molecular layer and outer diffuse layer exists. (1) (3) (2)

5. 5 Electrochemistry 1 5 1. Interface (1) an inner monomolecular layer of adsorbed molecules or ions in which a very large potential gradient is produced (e.g. 1 volt across 1 angstrom that corresponds to 100 MV/cm ). (1) (3) (2)

6. 6 Electrochemistry 1 6 1. Interface (2) an outer diffuse region that compensates for any local charge unbalance that gradually merges into the completely random arrangement of the bulk solution (charge unbalance, namely, the violation of electronuetrality , can be temporarily/locally produced but it will be nuetralized /compensated.). (1) (3) (2)

7. 7 Electrochemistry 1 7 1. Interface (3) a layer intermediate between inner molecular layer and outer diffuse layer exists. In this intermediate layer, excess charges are solvated or weakly bonded with counter ions. (1) (3) (2)

8. 8 Electrochemistry 1 8 2. Electrode thermodynamics and kinetics Thermodynamics: Thermodynamic equation ∆G=- nFE refers to the movement of n moles of charge across the cell potential E. The value of ∆G expresses the maximum useful energy that a system can give the surroundings. This quantity can only be perfectly extracted from the system under the limiting conditions of a reversible change, which implies zero current. The more rapidly the cell operates, the less electrical energy it can supply.

9. 9 Electrochemistry 1 9 2. Electrode thermodynamics and kinetics Kinetics: (1) If the redox reaction steps of an electrode reaction are rapid enough, then its potential is equal to the equlibrium potential (the electrode will be non-polarizable). (2) If, on the other hand, an equlibrium is established only slowly due to a kinetic inhibition of steps involved in an electrode reaction, then the electrode will be polarizable: in order to induce the reaction to proceed in a given direction, the kinetic inhibition of the reaction must be overcome by applying a high overpotential.

10. 10 Electrochemistry 1 10 3. Overpotential Within the theory of thermodynamic, the Nernst equation should predict what electrode reaction will take place. According to the Nernst equation, the hydrogen evolution potential as a function of the concentration of proton [H+] is E=0.000-0.059*log(1/[H + ]). At pH=7, E=-0.414 V. Therefore, only metals whose reduction potentials are less negative than -0.41 V should be reduced and plate out at the cathode in the electrolysis of aqueous solution of electrolytes. This means that it should not be possible to reduce metal ions such as Zn 2+ (E =-0.76 V) from aqueous solution. However, some such metals including Zn do plate out of aqueous solution of electrolytes.

11. 11 Electrochemistry 1 11 3. Overpotential The Nernst equation is a thermodynamic equation that tells nothing about kinetics. For example, the evolution of H 2 at some cathode surfaces in some aqueous solutions of electrolytes are too slow to occur at the potentials given by the Nernst equation and only take place at higher voltages (it needs overpotentials ). Activation overpotentials for the evolution of H 2 on Zn, graphite, and glassy carbon electrodes are -0.77 V, -0.62 V, and more negative than -0.62 V, respectively. Carbon fibers are also sp 2 carbons, and its activation overpotentials are usually more negative than that of graphite but less negative than those of glassy carbons.

12. 12 Electrochemistry 1 12 3. Overpotential The cell overpotential is considered to be composed of a number of independent contributions : (1) ohmic drop; (2) activation overpotential; and (3) diffusion overpotential.

13. 13 Electrochemistry 1 13 3. Overpotential (1) Ohmic drop between electrodes results from the fact that the electrolyte solution has a finite conductivity;

14. 14 Electrochemistry 1 14 3. Overpotential (2) activation overpotential at one or both electrodes arising from kinetic inhibition of one of the steps involved in the electrode reaction ( desolvation of the reactive ion, chemisorption of the reaction product, etc.);

15. 15 Electrochemistry 1 15 3. Overpotential (3) diffusion overpotential at one or both electrodes due to the presence of concentration gradients in the vicinity of the electrode surface. As a result of electrochemical reaction, the concentration at the electrode surface no longer have their equilibrium values .

16. 16 Electrochemistry 1 16 3. Overpotential (3) (diffusion overpotential. continued) If migration through the electric double layer is very rapid, then diffusion from the bulk of the solution towards the electrode will be unable to replenish the ions at the double layer quickly enough and a concentration gradient will result.

17. 17 Electrochemistry 1 17 3. Overpotential

18. 18 Electrochemistry 1 18 3. Overpotential Detailed information ( 1) Ohmic (IR) drop Polarization measurements include a so-called ohmic potential drop through a portion of the electrolyte surrounding the electrode, through a metal- reaction product film on the surface, or both. An ohmic potential drop always occurs between the working electrode and the reference electrode. This contribution to polarization is equal to IR , where I is the current density, and R is the resistance .

19. 19 Electrochemistry 1 19 3. Overpotential Detailed information Ohmic (IR) drop If copper is made cathode in a solution of dilute CuSO 4 in which the activity of cupric ion is represented by α (Cu +2 ), then the potential φ 1 , in absence of external current, is given by the Nernst equation, φ 1 =0.34+(0.059/2)*log[ α (Cu +2 )].

20. 20 Electrochemistry 1 20 3. Overpotential Detailed information (2) Activation overpotential Activation polarization is caused by a slow electrode reaction. The reaction at the electrode requires an activation energy in order to proceed. The most important example is that of hydrogen ion reduction at a cathode, H + +e − →0.5H 2 . For this reaction, the polarization is called hydrogen overpotential. Overpotential is defined as the polarization (= potential change) of an equilibrium electrode that results from current flow across the electrode/solution interface. Hydrogen overpotential can vary with metal, current density, etc.

21. 21 Electrochemistry 1 21 3. Overpotential Detailed information (3) Diffusion overpotential When current flows, copper is deposited on the electrode, thereby decreasing surface concentration of copper ions to an activity α (Cu +2 ) s . The potential φ 2 of the electrode becomes, φ 2 =0.34+(0.059/2)*log[ α (Cu +2 ) s ]. Since α (Cu +2 ) s is less than α (Cu +2 ), the potential of the polarized cathode is less positive than in the absence of external current. The difference of potential, φ 2 −φ 1 , is the concentration polarization, equal to φ 1 -φ 2 =(0.059/2)*log[ α (Cu +2 ) s / α (Cu +2 )].

22. 22 Electrochemistry 1 22 3. Overpotential Detailed information (3) Diffusion overpotential The larger the current, the smaller the surface concentration of copper ion, or the smaller the value of α (Cu +2 ) s , thus the larger the corresponding polarization.

23. 23 Electrochemistry 1 23 3. Overpotential Note The product, IR, decays simultaneously with shutting off the current, whereas concentration polarization and activation polarization usually decay at measurable rates. Concentration polarization decreases with stirring, whereas activation polarization and IR drop are not affected significantly with stirring.