Faradaic Rxns: More General Situations--occur when DF WE/Soln reaches value adequate to achieve electron transfer across interface---either reduction or oxidation

Faradaic current tells us about the rate of an electro- chemical reaction. Since faradaic current is related to Faraday’s law of :

N = q/nF = moles electrolyzed at WE rate of this electrochemical reaction-- dN/dt = (dq/dt)/nF = i/nF =Velocity = moles/sec

However electrode reactions are always hetero- geneous--not in bulk solution; therefore rate of reaction is usually normalized to surface are of electrode:

V = mol/sec-cm2 = i/nFA = J/nF

J= = i/A = amps/cm2 i-E curves used to obtain information about faradaic reaction.

Departure of electrochemical cell from its equilibrium potential--Eeq--upon passage of a faradaic current is termed polarization.

The extent of polarization is measured by the overpotential for the cell --- = E - Eeq Current- curves for electrochemical cells obtained under steady-state conditions are often termed polarization curves

IPEs-show very large change in potential for passage of little or no faradaic current.

A depolarizer is an electroactive species that causes the potential of an electrode to be nearer its equilib. value by being oxidized or reduced at the surface of the electrode.

An ideally depolarized (or nonpolarizable) electrode is one whose potential does not change upon passage of faradaic current. i i IPE IDE

E Eappl appl

A saturated calomel electrode---prepared with a large surface area pool---would represent and IDE------See figure below:

wire

glass KCl soln tube

Hg2Cl2(s)

Hg If you have large surface area for reference elect. then the rate of mass transfer of active species controlling the potential of that electrode is faster than consumption per unit area---i.e., low J values for Calomel reference electrode:

- o - 1/2 Hg2Cl2(s) + 1 e <------> Hg + Cl If electrode was , then Cl- ion would be consumed----but if current density is low--rate of electrode chemical reaction per unit area is low compared to rate that Cl- can diffuse to electrode surface--- hence: [Cl-]x=o = [Cl-]bulk

We say there is no concentration polarization of the electrode (h conc =0 and Eelect = Eeq)

0 x=0 x=o Eelect = E + 0.059/n + log[Ox] / [Red]

o bulk bulk Eeq = E + 0.059/n + log [Ox] / [Red] Types of Overpotentials:

1. Concentration overpotential (h conc): difference in potential due to the difference in concentration between the bulk soln. and the solution adjacent to the electrode surface (x=0)

2. Activation Polarization or Charge Transfer

Overpotential (h ct): difference in potential vs. Eeq due to slow charge transfer or slow kinetics of electron transfer at interface of electrode.

0 E tells you about Keq for given redox reaction--- but does not tell you anything about the rates of oxidation or reduction ----tells you about ratio

of rates! Keq =kf/kr

In ---the equivalent of kf and kr (rate constants) are the kc and ka --which are the heterogeneous rate constants for cathodic (reduction) and anodic (oxidation) reactions!

3. Overpotentials due to coupled chemical rxns---

(h rxn )---complexation of Ox or Red, adsorption, C-V or i-E curves are essentially polarization curves: and generally 4 processes will govern shape of the curves---

1. Mass Transport: how fast electroactive species gets to the surface of the electrode to undergo oxidation or reduction reaction---3 ways for species to get to electrode---we will see shortly!

2. Rate of Electron Transfer reaction---if kc and ka are fast, then reaction is said to be reversible and shape of i-E curves not dependent on this!

3. Coupled Chemical Reactions: Reactions either preceding or following electron transfer rxn-- --catalytic decomposition of product, dimerization

4. Surface effects ---adsorption, desorption, change in apparent electrode area, fouling of electrode, etc. --- (can also be considered as #3)

For given current density, J, there will be a certain overpotential which is the sum of all the overpotentials in the cell---

Ref h ct h mt h rxn

Rct Rmt Rrxn Rsoln i

Eappl

Eappl = Eeq + h - iRsoln

Eeq is equilibrium potential predicted from !

i = + for cathodic reaction; - for anodic reaction

iRsoln = ohmic drop in solution of cell due to current flow consider the following cell---2 ideally depolarized electrodes---

Hg/Hg2Cl2(s), KCl (1M), Hg2Cl2(s)/Hg i Eappl h mt or h conc i

+ Eappl, -

there is no overpotential is this cell--until you get to very high + or - Eappl --since electrodes are IDE---therefore Eappl = Eeq -iRsoln so since Eeq = 0 V (symmetric cell)---once Eappl is > or < than 0, current will flow---but slope of line is 1/Rsoln

Eappl = iRsoln and I = Eappl/Rsoln slope = conductance of soln = W-1