Electrochemistry: Introduction Electrochemistry at your finger tips Part 2: Voltammetry (linear sweep voltammetry & cyclic voltammetry)

Historical preface: All modern electrochemistry starts from polarography technique developed by J. Heyrovsky The dropping mercury electrode (DME) was introduced by J. Heyrovsky in 1920s. J. Heyrovsky M. Shikata The first polarograph designed by J. Heyrovský and M. Shikata (1924)

Typical diffusional voltammogram Linear sweep voltammetry & cyclic voltammetry: Current Potential Faradaic current Final E A current corresponding to the reduction or oxidation of some chemicalsubstance. Capacitance current Diffusionally limited Faradaic current Initial E E0 Time Potential Typical diffusional voltammogram

E > E0 ; only capacitance current is observed Working electrode - + + + Oxidized redox species - - Electrolyte cations and anions Potential is negative shifted - + - + Oxidized redox species do not react - + Electrolyte cations and anions are re-organized near the electrode

- + - + - + - + - + - + E < E0 ; Faradaic current is observed Working electrode - + - + - + Potential is negative shifted Reduced redox species are generated e- - + - Oxidized redox species started to react + - +

- + - + - + - + - + - + E < E0 ; Faradaic current is observed Reduced redox species are generated e- - + - + - + Potential is negative shifted e- - + Reduced species diffuse from the electrode to the bulk solution - + - Oxidized species diffuse to the electrode from the bulk solution + Surface concentration of the oxidized species is very low

Diffusion is particularly significant in an electrolysis experiment since the conversion reaction only occurs at the electrode surface. Consequently there will be a lower reactant concentration at the electrode than in bulk solution. Similarly a higher concentration of product will exist near the electrode than further out into solution. http://www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/electrochemistry/ELEC/l2html/l2mt.html

Linear sweep voltammogram & the concentration profile: Distance from the electrode surface Before the electrochemical reaction Concentration of the oxidized species Upon reaching the diffusionally limited current Current Potential

Current development upon linear potential scan http://www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/electrochemistry/ELEC/l4html/cv.html

Concentration profile upon linear potential scan http://www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/electrochemistry/ELEC/l3html/ps.html

Voltammograms: S-shape or peak shape? electrode rotation Hydrodynamic electrode: 1. Rotating disk electrode 2. Mercury dropping electrode Current Stationary electrode Potential electrode growth

Cyclic voltammetry is composed of two linear potential sweeps performed in the opposite directions http://www-biol.paisley.ac.uk/marco/Enzyme_Electrode/Chapter1/Cyclic_Voltammetry1.htm

Comparison of a linear sweep voltammogram and a cyclic voltammogram A typical linear sweep voltammogram showing the important parameters. A typical cyclic voltammogram showing the important parameters. http://www.epsilon-web.net/Ec/manual/Techniques/CycVolt/cv.html

Animated Cyclic Voltammetry experiment This animation shows the cyclic voltammogram for the reversible oxidation (forward sweep) and reduction (reverse sweep) for hydroxy-ferrocene. In aqueous buffered electrolyte, hydroxy-ferrocene undergoes a simple outer sphere one-electron redox process according to the following scheme: http://www-biol.paisley.ac.uk/marco/Enzyme_Electrode/Chapter1/Ferrocene_animated_CV1.htm

e-  t1/2 Ip / mA v1/2 / (mV s-1)1/2 How we can recognize diffusional electrochemical processes: Randles-Sevcik equation (for reversible systems): Ip=(2.69x105)n3/2ACD1/2v1/2 e-  t1/2 diffusion Ip / mA I / mA E / V Time / sec v1/2 / (mV s-1)1/2

In order to prove the diffusional character of linear sweep or cyclic voltammogram, we need to perform the measurements with different potential scan rates. http://www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/electrochemistry/ELEC/l4html/cv.html

Experimental results should look like these plots: http://www-biol.paisley.ac.uk/marco/Enzyme_Electrode/Chapter1/Scanrate.htm

Reversible diffusional CV -1 Reduction Epc Cathodic current Cathodic current Current / mA Oxidation Anodic current Anodic current Epa Ep = 59/n mV +1 -1 Potential / V

E0 Nernst equation + Potential / V -

Reversible electrochemical process: The oxidized & reduced redox species are always at equilibrium with the electrode at any applied potential. The electron transfer rate constants should be high enough to maintain the equilibrium at the used potential scan rate. The reversible CV should be used to find E0, but the kinetics of the electrochemical process cannot be found. e- e-

How it might look like on your computer http://www.stolaf.edu/people/walters/pix/378CVscottnelsonresults.gif

Quasi-Reversible diffusional CV in the case of slow electron transfer process -1 E1/2 Epc I / mA The cathodic & anodic peaks might be shifted non-symmetrically: E1/2 = E0 Epa Ep > 59/n mV +1 -1 E / V

linear sweep voltammogram Effect of the electron transfer rate constant on the shape of linear sweep or cyclic voltammograms linear sweep voltammogram cyclic voltammogram http://www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/electrochemistry/ELEC/l4html/cv.html

R.S. Nicholson, I. Shain, Anal. Chem. 1964, 36, 706-723. To calculate the rate constant we need to know: 1. E as a function of the potential scan rate 2. Number of electrons per molecule 3. Diffusion coefficient and concentration of the redox species E / mV log( v / mV s-1) The most important theoretical paper about diffusionally limited cyclic voltammetry: R.S. Nicholson, I. Shain, Anal. Chem. 1964, 36, 706-723.

Q + e- Q- reversible electron transfer step Irreversible electrochemical processes with the following chemical steps: Q + e- Q- reversible electron transfer step Q- + H+ QH. irreversible chemical step (protonation) Problems: 1. No information about E0 2. No information about kinetics Useful for analytical purposes only!

A calibration plot can be still useful for analytical purposes. There is no information about the redox potential or kinetics of the irreversible electrochemical process. A calibration plot can be still useful for analytical purposes. current concentration http://www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/electrochemistry/ELEC/l5html/cvr.html

How to change irreversible (quasi-reversible) process to reversible: 1. If the process is limited by the slow electron transfer - make the scan rate slower. 2. If there is an irreversible chemical step after the fast electron transfer process: make the scan rate faster.

A multi-step electrochemical process http://www.wpi-europe.com/products/biosensing/pot500.htm

Recommended textbooks on electrochemistry: Electrochemical Methods : Fundamentals and Applications, by A.J. Bard and Larry R. Faulkner Analytical Electrochemistry, by Joseph Wang Broadening Electrochemical Horizons: Principles and Illustration of Voltammetric and Related Techniques, Edited by Alan Bond