INTRODUCTION TO ELECTROCHEMICAL CELLS AND BASIC ELECTROANALYTICAL MEASUREMENTS ANDREA MARDEGAN JAN 17th 2013

Although we would like to measure electrochemical observables (current, voltage, …) associated with a single working electrode, we can’t. We must always couple our working electrode to a second electrode in order to make a measurement. These two electrodes comprise an electrochemical cell.

1. It has a well defined and invariant potential. That is – no matter how much current we drow from this electrode, its potential must not vary 2. It has zero impedance. It imposes no resistive load on our cell. 3. It does not contaminate our solution. That is, it is not a source of undesidered ions in our electrochemical cell. (ideal) Reference electrode…

We wish to control the potential of this working electrode… … but to do that, we need a second electrode in the solution to complete the circuit THE PROBLEM

For example, let’s say both electrodes are platimum… At open circuit, no potential is applied between them… And we don’t know this potential

Now we apply +0.8 V to the WE The potential of both electrodes changes and NOT SYMMETRICALLY.

THE SOLUTION Substitute the platinum RE with a SCE

1. It has a well defined and invariant potential. That is – no matter how much current we drow from this electrode, its potential must not vary 2. It has zero impedance. That is, it imposes no resistive load on our cell. 3. It does not contaminate our solution. That is, it is not a source of undesidered ions in our electrochemical cell. Reference electrode…

3-electrode cell…

Let’s measure the current that flows as we chage the voltage of the platinum electrode

SUPPORTING ELECTROLYTE: an inert salt added to impart conductivity to the solution BACKGROUND LIMITS: the two limits at which the solvent + supporting electrolyte begin to react at WE POLARIZABLE ELECTRODE: an electrode operating within a potential range in which no Faradaic electrochemistry occurs.

A red-ox process that takes place at a WE can be considered as a sequence of 3 steps: -Mass transfer from the solution to the electrode surface -Reation on the electrode surface -Tranfer of the product from the electrode surface to the bulk solution. Polarization…

Diffusion: Diffusion: in order to minimize the differences in concentration (concentration gradients). Migration: Migration: movement of ionic species under an eletric field Convection : Convection : agitation, rotation, thermal gradients (external agents…) Mass transfer regimes…

The potential is scanned linearly from a value where no reaction occurs to a more negative potential: E - t t The current will increase when the potential is closed to E° (standard potential of a red-ox couple). Then a current decay (cottrellian decay) is caused by the lack of Ox species closed to the electrode surface. Linear sweep voltammetry

E - t t Red, generated in the first scan and being in the proximity of the electrode surface, is re- oxidixed to Ox. Cyclic voltammetry The potential is scanned linearly until a certain value (usually more than 90 mV after E peak) and then back to the starting potential

A B C D A: C ox = max; C Red = 0 B: C ox = C Red C: C ox = 0; C Red = max D: C ox = CRed Randles-Sevcik equation: Cyclic voltammetry

Redox Cycling through Inter Digitated electrodes Arrays(IDA) Two Working electrodes :Generator (Anode) and Collector (Cathode) electrodes in close proximity such that the adjacent regions with concentration gradients overlap The redox couple may redox cycle multiple times before they diffuse out into the bulk solution. This behavior results in an amplified signal thereby lowering the lower limit of detection (LOD) significantly (upto pM). Single mode: Cyclic Potential sweep across Generator, Collector not connected. Dual Mode: Constant reduction potential applied across collector while potential is swept linearly across generator Redox Cycling as electrode width and gap Redox Cycling as electrode height