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

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

Electrode Potential/Kinetics ( 电极电位 / 反应动力学 ) 1.Chemical thermodynamics & kinetics ( 化学热力学与动力学 ) 2.Electrochemical thermodynamics ( 基本电化学热力学 ) 3.Kinetics of electrode reactions ( 电极反应动力学 )

Basic Thermodynamics For a chemical reaction: If the reaction is at equilibrium, K c is the equilibrium constant (K eq ) Standard Gibbs free energy: dynamic equilibrium

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

Basic Thermodynamics For electrochemical reactions: For chemical reactions: So

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

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

Butler-Volmer Model of Electrode Kinetics

Butler-Volmer Model Because

Butler-Volmer Model If at equilibrium and So, The rate constant is called standard rate constant k 0 So, Butler-Volmer formulation of electrode kinetics

Standard rate constant k 0 A measure of the kinetic facility of a redox couple. A system with a large k 0 will achieve equilibrium on a short time scale, and requires small overpotential to drive the electrochemical reaction. but a system with a small k 0 will be sluggish, and requires large overpotential to drive the reaciton.

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, i 0 Becuase

Overpotential So, Current-overpotential equation

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

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

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

Tafel Plot

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