Electrode Potential and Kinetics in Electroanalytical Chemistry

1. 生物电分析化学Electroanalytical Chemsitry and Its Biological Applications主讲人：刘宏 教授2014年秋

2. Questions: • How an electrolytic reaction is influenced by electrode potential? • How is the electrode potential is established at equilibrium?

3. Electrode Potential/Kinetics (电极电位/反应动力学) • Chemicalthermodynamics & kinetics • (化学热力学与动力学) • Electrochemical thermodynamics • (基本电化学热力学) • Kinetics of electrode reactions • (电极反应动力学)

4. Basic Thermodynamics For a chemical reaction: dynamic equilibrium If the reaction is at equilibrium, Kc is the equilibrium constant (Keq) Standard Gibbs free energy:

5. Basic Thermodynamics If the reaction is not at equilibrium, the reaction quotient is ξ - extent of reaction The reaction will shift to the right The reaction will shift to the left

6. BasicThermodynamics For chemical reactions: For electrochemical reactions: So

7. Kinetics of Electrode Reactions At equilibrium, Exchange velocity: Transition state theory:

8. Kinetics of Electrode Reactions For an electrode reaction: The velocity of the forward reaction: The velocity of the backward reaction: Net velocity of the reaction: Faraday's law

9. Butler-Volmer Model of Electrode Kinetics

10. Butler-Volmer Model Because

11. Butler-Volmer Model Ifat equilibrium and So, The rate constant is called standard rate constant k0 So, Butler-Volmer formulation of electrode kinetics

12. Standard rate constant k0 A measure of the kinetic facility of a redox couple. A system with a large k0will achieve equilibrium on a short time scale, and requires small overpotential to drive the electrochemical reaction. but a system with a small k0will be sluggish, and requires large overpotential to drive the reaciton.

13. Kinetic equilibrium At equilibrium, i =0 So, After rearrangement, (Nerst equation) Even though the net current is 0 at equilibrium, we still can evision balanced faradaic activity, which can be expressed in terms of exchange current, i0 Becuase

14. Overpotential So, Current-overpotential equation

15. Overpotential

16. Overpotential If the solution is well stirred or current is very low, so that the surface concentration do not differ from bulk concentration, then which is historically known as Butler-Volmer equation

17. Overpotential

18. Overpotential If the overpotential is sufficiently small, then So the charge transfer resistance is: If the overpotential is sufficiently large, then Tafel behavior at large overpotential

19. Tafel Plot

20. Assignment 1. How an electrolytic reaction is influenced by electrode potential? 2. How is the electrode potential is established at equilibrium? 3.