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INTRODUCTION TO ELECTROCHEMICAL CELLS AND BASIC ELECTROANALYTICAL MEASUREMENTS

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 .

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INTRODUCTION TO ELECTROCHEMICAL CELLS AND BASIC ELECTROANALYTICAL MEASUREMENTS

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  1. INTRODUCTION TO ELECTROCHEMICALCELLS AND BASIC ELECTROANALYTICALMEASUREMENTS ANDREA MARDEGAN JAN 17th 2013

  2. Althoughwewouldlike to measureelectrochemicalobservables (current, voltage, …) associated with a single workingelectrode, wecan’t. We must alwayscoupleourworkingelectrode to a secondelectrode in order to make a measurement. Thesetwoelectrodescomprise an electrochemical cell.

  3. (ideal) Reference electrode… 1. Ithas a welldefined and invariantpotential. Thatis – no matterhowmuchcurrentwedrow from thiselectrode, itspotential must notvary 2. Ithas zero impedance. Itimposes no resistive load on our cell. 3. Itdoesnot contaminate oursolution. Thatis , itisnot a source of undesideredions in ourelectrochemical cell.

  4. THE PROBLEM Wewish to control the potential of thisworkingelectrode… … but to do that, weneed a secondelectrode in the solution to complete the circuit

  5. For example, let’ssaybothelectrodes are platimum… At open circuit, no potentialisappliedbetweenthem… And wedon’tknowthispotential

  6. Nowweapply +0.8 V to the WE The potential of bothelectrodeschanges and NOTSYMMETRICALLY.

  7. THE SOLUTION Substitute the platinum RE with a SCE

  8. Reference electrode… 1. Ithas a welldefined and invariantpotential. Thatis – no matterhowmuchcurrentwedrow from thiselectrode, itspotential must notvary 2. Ithas zero impedance. Thatis, itimposes no resistive load on our cell. 3. Itdoesnot contaminate oursolution. Thatis , itisnot a source of undesideredions in ourelectrochemical cell.

  9. 3-electrode cell…

  10. Let’smeasure the currentthatflowsaswechage the voltage of the platinumelectrode

  11. SUPPORTINGELECTROLYTE: an inertsaltadded to impartconductivity to the solution BACKGROUND LIMITS: the twolimitsatwhich the solvent + supportingelectrolytebegin to reactatWE POLARIZABLEELECTRODE: an electrodeoperatingwithin a potential range in which no Faradaicelectrochemistryoccurs.

  12. Polarization… A red-oxprocessthattakesplaceat a WE can be consideredas a sequence of 3 steps: -Mass transfer from the solution to the electrodesurface -Reation on the electrodesurface -Tranfer of the product from the electrodesurface to the bulk solution.

  13. Mass transfer regimes… • Diffusion: in order to minimize the differences in concentration (concentrationgradients). • Migration:movement of ionicspecies under an eletricfield • Convection: agitation, rotation, thermalgradients(external agents…)

  14. Linear sweepvoltammetry The potentialisscannedlinearly from a valuewhere no reactionoccurs to a more negative potential: E - t The currentwillincreasewhen the potentialisclosed to E° (standard potential of a red-oxcouple). Then a currentdecay (cottrelliandecay) iscaused by the lack of Oxspeciesclosed to the electrodesurface.

  15. Cyclicvoltammetry E - The potentialisscannedlinearlyuntil a certainvalue (usually more than 90 mVafter E peak) and then back to the startingpotential t Red, generated in the first scan and being in the proximity of the electrodesurface, is re-oxidixed to Ox.

  16. Cyclicvoltammetry B C A D A: Cox=max; CRed= 0 B: Cox=CRed C: Cox= 0; CRed=max D: Cox=CRed Randles-Sevcikequation:

  17. Redox Cycling through Inter Digitated electrodes Arrays(IDA) • 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 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 (uptopM) . Redox Cycling as electrode width and gap Redox Cycling as electrode height

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