Week 3 - Electrodes

Potentiostat/Electrochemical Analyzer: controls the electrical potential between the working and reference electrodes at a preset value and forces whatever current is necessary to flow between the working and counter electrodes to keep the desired potential.

• Working Electrode (EC-biosensor): redox electrode where electrode processes occur; measured against suitable reference electrode.

• Reference Electrode: has well known and stable equilibrium electrode potentials and serves as a reference point against which the potential of other electrodes can be measured.

• Counter Electrode: serves to make an electrical connection to the electrolyte so that a current can be applied to the working electrode.

Working Electrode

• Metal

– solid • Pt, Au, Ag, C

– liquid • dropping mercury electrode (DME)

• Semiconductors

– Si, GaAs

– In-SnO2/glass (optically transparent)

Geometry

Disk– area: πr2

• wire (cylinder)– area: l(2 πr) πr2

• Mesh– optically transparent

Note: Geometric area < effective surface area 11 • Sheet

Pzc (potential of zero charge) 0.18 V– E negative of pzc ⇒ excess negative charge (electrostatic interactions possible)

Counter Electrode

The area must be greater than that of working, usually long Pt wire (straight or coiled) or Pt mesh (large surface area)

Can’t modify ~ don’t know why, but he kept saying it.

Reference Electrode

• Purpose: provide stable potential against which other potentials can be reliably measured • Criteria:– stable (time, temperature)– reproducible (you, me)– potential shouldn’t be altered by passage of small current = not polarizable– easily constructed– convenient for us

Standard hydrogen electrode (SHE)

Practical Reference Electrodes

Aqueous ~ Saturated calomel electrode (SCE) or Ag/AgCl

SCE: very dependent on temperature

Ag/AgCl:

Nonaqueous ~ Ag+/Ag pseudoreferences (Pt, Ag wires) and use ferrocene