Electrochemistry: Introduction Electrochemistry at your finger tips Part 3: Cyclic voltammetry: adsorption, surface confined redox species, electrocatalysis. Polarography, rotating disk electrode, potential step methods

Adsorption of non-electrochemically active species on an electrode + + - clean electrode capacitance current double-charged layer after adsorption I / mA - + background CVs + - adsorbed species E / V  dielectric constant is changed

Adsorption of redox species on an electrode Redox species adsorbing on the electrode capacitance current I / mA background CV E / V

Surface-confined redox species: a special kind of electrochemistry E0 I / mA Ep = 0 mV or very small E / V

Comparison of the diffusional and surface-confined electrochemical processes – they have quite different shapes in voltammograms http://unicorn.ps.uci.edu/243/handouts/Voltammetry.JPG

ip= n2F2GAv/4RT Q=nFAG e- Ip / mA v / (mV s-1) How can we recognize surface-confined electrochemical processes: ip= n2F2GAv/4RT Q=nFAG I / mA e- E / V Time / sec Ip / mA 1. No limiting current 2. Ip proportional to scan rate 3. Small peak-to-peak separation for the reversible processes v / (mV s-1)

Electrochemical kinetics for the surface-confined processes Laviron´s theory 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 E / mV Slope=-2.3RT/anF(cathodic peak) log( v / mV s-1) i=nFAks[G0exp{anF(E-E0’)RT-Grexp(1-a)nF(E-E0’)/RT}]

The peak area is used to calculate the surface concentration of the redox units. I / mA E / V Practical tips have to be discussed!

Real problems in recognizing diffusional or surface confined electrochemical processes: 1. If the range of the used potential scan rates is not large, the difference between v and v1/2 is not significant. 2. If the process is quasi-reversible or irreversible, the peak-to-peak separation could be big for both mechanisms. 3. If the peak is close to the background discharge, the peak looks like a shoulder and the limiting current is not easy to see.

Catalytic waves in cyclic voltammograms Surface-confined electrocatalyst http://www.chem.ox.ac.uk/icl/faagroup/pfv.html

Diffusional electrocatalyst http://www.edaq.com/teachapp3.html

Polarography is a linear sweep voltammetry performed on a mercury dropping electrode

http://elchem.kaist.ac.kr/jhkwak/AnalChem/07/05/01.gif

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)

Rotating disk electrode http://www.cheng.cam.ac.uk/research/groups/electrochem/JAVA/electrochemistry/ELEC/l7html/hydro.htm

Veniamin Grigorievich (Benjamin) Levich (1917-1987) Levich plot & equation

Potential step methods: Chronoamperometry Exponential decay Diffusional redox process Surface-confined redox process

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