Tags

, , , , ,

M.Sc (Chemistry)

ELECTRODE KINETICS (ELECTRODICS)-I

(BUTLER-VOLMER (BV) Equation: Elementary electrode Reaction)

00-INTRODUCTION

        It will be better if the student is able to recall the following topics for further study of Electro chemistry & Electrode kinetics (Electrodics)

Electrode kinetics (Electrodics): Kinetics of electrochemical reactions at an electrode.

Image-BV Plot             It will be better if the following concepts are kept in mind before entering into the field of electrodics.

  1. Electrode, Electrode-reactions
  2. Electrode potential (SRP)-Electro chemical series- Potential difference(EMF)
  3. Reference electrodes (Primary & Secondary- SHE; SCE, Quinhydrone, Glass…… electrodes)
  4. Electrochemical Cells (Galvanic & Electrolytic)
  5. Equilibrium electro chemistry-Nernst equation
  6. Electro chemical Quantities-Resistance (ohm), Conductance(S), Current (Ampere), Quantity of Current (coulomb), Potential & Potential difference, EMF- (volt).
  7. Electro analytical techniques :
  • Potentiometry (Potentiometric titrations): EMF vs conc.
  • Conductometry: conductance vs conc.
  • Amperometry: current vs conc.
  • Voltametry: current vs EMF
  • Cyclic voltametry: current vs emf-Cycle
  • Polarography (voltametry using DME as cathode):current vs emf

The following file in LINK-00 gives an idea for such SELF-STUDY

LINK-00:00-Elect Chem Intrd

         Ohm’s law (when there are no chemical reactions), V=IR. I vs V (applied potential) is linear passing through origin (Metallic conductors: slope = l / R).

          The extent of ohmic behavior (I vs V) of the electrode needs to be understood for electrode systems involving electrolytes.

         The equation for Ohm’s law V=IR may not be no longer valid for electrodes, which leads to the development of different kind of equation.       

01.BUTLER–VOLMER(BV) Equation for an Elementary Electrode Reaction

        The simplest electrode reaction is the one which involves (i) only one electron and (ii) only one step which is electro-chemical. The model of such a system, the current-potential equation for the same and these related concepts are explained in the file given in LINK-01 below with some illustrated problems at the end:

LINK-01: 01-BV Eq-Elementry

           Butler–Volmer Equation for an elementary electrode process

is  i = io[e(1-β)ηF/RT-e-βηF/RT].

The relationship between current and potential is not a ohmic behavior as seen in the case of a metallic system.

       The experimental variable is η (over potential) & the kinetic parameters are io , β. The current across the system depends mainly on η and β (depending on the system).

02-DISCUSSION OF BUTLER-VOLMER (BV) EQUATION

for different values of symmetry factor (βeta)

            Over potential across the electrode allows current across it.The electrode can pass anodic current or cathodic current depending on the value of β periodically when connected it to an AC in put. That is it can act as a rectifier if β is not equal to 0.5. The efficiency of the rectification depends on how much the electrode has it’s β value away from 0.5.

       The factor, β in the BUTLER-VOLMER equation: i = io[e(1-β)ηF/RT-e–βηF/RT] is called symmetry factor as it decides the nature of anodic & cathodic currents across an electrode for a given over-potential, η

       The file in the following link (LINK-2) explains the DISCUSSION OF BUTLER -VOLMER (BV) EQUATION for different values of β such as for β = 0.5 ; β < 0.5 and β > 0.5

LINK-02: 02-BV-Diss-Beta

03-DISCUSSION OF BUTLER-VOLMER(BV) EQUATION

For different values of over potential (eta)

             BUTLER-VOLMER equation may be deduced to different equations such as Nernst Equation; Ohm’s Law ; Tafel Equations ete., under different conditions of applied over potential, η as explained in the Link given below (LINK-03)

LINK-03: 03-BV-Diss-Eta

04-PROBLEMS & SOLUTION:Butler-Volmer (BV) Equation

  1. High field approximation (HFA) : │η│ > 0.1 V

Anodic current for high +ve η  ; i = ioexp{(1-β)ηF/RT}

                      ln i = ln io + (1-β)ηF/RT …………Tafel equation

Cathodic current for high magnitude of -ve    ; i = ioexp{-βηF/RT}

                       ln i = ln io – βηF/RT}   …………..Tafel equation

  1. Low field approximation (LFA) :│η│< 0.01 V

Ohm's Lawi=V/R ……………………Ohm’s Law

  1. For η 0.01 V – 0.1V

         The BV-equation must be applied as it is without any approximation.

NOTE:-

  1. BV equation can be used as it is under any circumstances.

           The file in the following link (LINK-4) contains some typical problems related to above form of Butler-Volmer (BV)Equation.

LINK-04:04-BV Eq-Probs & Solns

&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&

Please go to CONTENTS for all the Articles in PoC: Click below

https://dradchem.wordpress.com/2015/06/22/properties-of-chemistry-poc-contents-2/