AChem Unit 4 - Electrochemistry

What is the nature of electrochemistry?

• charged particles; electrons and ions

• charged interfaces

• oxidation/reduction

• electrochemical cells

• interfacial reactions

Oxidation

loss of e (D → D+ + e-)

Reduction

gain of e (A + e → A-)

Charge is in units of 

C

Faraday’s Law

Q = nFN (n = num electrons, N = nom moles oxidized/reduced, F = Faraday’s constant)

Current is in units of

ampere (A = C/s)

Current is equivalent to _____ in electrochemistry

reaction rate

Potential is in units of

V (J/C)

E_cell develops bc of the

thermodynamic tendency of D to donate electrons

Electrode is a continuum of

energy levels, tunable source of sink/source for electrons

Molecule is

discrete energy levels (some occupied, some vacant)

If E_apl is negative, the electrode is the

reducing agent

If E_appl is positive, the electrode is the

oxidizing agent

Half-cell reaction

reaction of oxidized (e poor) species w e from electrode to give a reduced (e rich) species

Redox couple

oxidized and reduced forms of redox-active species considered together

Trends for O+ + ne- ←→ R

More negative potential means R is good electron donor and reducing agent

More positive means O is a good electron acceptor and oxidizing agent

Why does current flow through a conductor?

charge carriers move in an electric field

Ohm’s law

I = E/R

Resistance depends on

material and geometry

Conductivity and resistivity are intrinsic properties of

electrolyte

Conductance and resistance depend on

electrolyte and geometry

R =

rho*L/A, 1/kappa * L/A

Kappa is the symbol for

conductivity

Resistance in microdisk electrode

Resistance in microsphere electrode

Resistance for electrolyte in long tube

Resistance in parallel planar electrodes

Interphase

electrode surface and adjacent layer of ions (double layer); 1-100 nm thickness

Electrode and electrolyte are both

uncharged

When potential is applied to the electrode, the electrode develops a slight

surface charge and slight local excess of ions develops near the electrode to compensate the charge

Interphase acts as a _______ since it stores charge

capacitor

C = 

Q/E

Why is double layer capacitance important

current must flow to charge the double layer whenever the potential at an electrode is changed

Faradaic current is associated with

oxidation/reduction reactions

Non-Faradaic current is associated with

charging the double-layer capacitance

Non-Faradaic current is the _____ in electroanalysis

noise

Discrimination of Faradaic current from non-Faradaic current often sets the ______ in electroanalysis

detection limit

know this

reduction of O to R

Electron transfer

rate depends upon applied potential and molecular and electrode surface structure

Applied potential effects

modest negative potential is slow rate, extreme negative potential is fast rate

Electrode structure effects

Hg electrode, no surface-stabilized intermediates → slow rate

Pt electrode, has surface-stabilized intermediates → fast rate

Mass transfer is usually the factor that limits current when

ET is fast

Modes of mass transfer

diffusion

migration

convection

Diffusion

high to low conc

Migration

high to low potential

Convection

high to low pressure

Factors affecting MT

redox species structure and charge

medium viscosity

cell/electrode geometry

hydrodynamics (stirring)

Chemical reactions

bond breaking and making, involved in all but the simplest of electrode reactions

Mass transfer by diffusion

How do C0 and Cr near the electrode respond to the potential step at t = 0?

O is consumed near electrode surface → depletion layer of O grows over time

R is generated near electrode surface → accumulation layer of R grows over time

Flux

rate of MT; dN/Adt = -D(dC/dx)|x=0

D

diffusion coefficient, 10^-5 to 10^-7 cm²/s for small molecules in typical solvents

deltaC0

difference bt C0 at electrode surface and that in the bulk solution

xD

diffusion layer thickness, distance over which C0 and Cr are perturbed due to electrode reaction, 1-100 micrometers

I

-[nFAD/xD]*deltaC0

What about very small electrodes smaller than xD?

large electrode linear diffusion, small electrode radial diffusion

How much stirring is needed to affect MT to electrodes?

enough to shrink diffusion layer → stir solution, flow solution past electrode, rotate electrode

Equivalent circuits

representation of cell by group of electrical circuit elements

Double layer capacitance

electrode/solution interphase

Electrode

solution resistance

Rate of electron transfer

charge-transfer resistance

Amount of reaction

redox pseudocapacitance

In the case of no redox reactions

interphase is capacitor, resistor is electrolytes

What limits the timescale on which electrochemical measurements may be performed?

cell time constant

Case of redox reaction occurring at working electrode

Practical aspects of electrochemical experiments

reference and working electrodes

solvents and electrolytes

instrumentation

Electrochemical measurements and techniques

potentiometry (ion-selective electrodes)

potential step techniques (chronoamperometry)

potential sweep techniques (voltammetry)

combined sweep and step techniques

sinusoidal techniques (impedance)

Why are reference electrodes needed?

to establish a reliable basis for measuring/applying potentials

E_membrane depends on

membrane and ions in contact w membrane

Working electrodes

electrode where region of interest occurs

Desirable attributes of working electrodes

stable in air/water/solvents

wide potential window (electrode not easily oxidized/reduced, not catalytic for oxidation/reduction of solvent)

well-defined and reproducible surface chemistry

Most common materials for working electrodes

noble metals

other metals

carbon in many forms

Common working electrode configurations

embedded disk

embedded microwire

mercury drop

thin-layer flow cell

thin-film cells

high efficiency flow through cells

rotating disk/ring-disk electrodes

interdigitated electrodes

Non-aqueous solvents…why?

solubility/temperature/acid-base chemistry/water reactivity/chromatography/others

Issues in choosing solvent

liquid range

vapor pressure

polarity/dielectric constant

viscosity

reactivity

toxicity, cost, etc

Issues in choosing electrolyte

solubility in solvent

dissociation in solvent

conductivity

reactivity

toxicity, cost, etc

As you increase pH (become more basic), potential becomes more

negative

Does the pH trend also apply to organic solvents?

not necessarily

Amperometry procedure

apply potential to WE relative to RE and measure current at WE

What is the problem with two-electrode amperometry?

current flows through RE, which can change the composition and potential to change → original assumption is wrong

Potentiostat

allows current through AE and WE but not WE and RE, maintains applied voltage between RE and WE

Important note on potentiostats

• reference electrode must always be connected

• AE should be large

• all 3 electrodes should be physically close together

Amperometry techniques

• potential sweeps

• potential steps

• potential steps and sweeps combined

• sinusoidal potential variation

• signal vs time techniques

The reduction rate of cyclic voltammetry is faster as you get

more negative

For a potential sweep, peak current is proportional to

concentration, (scan rate)^.5

What two things can happen during a linear sweep voltammetry scan?

C0 at the electrode changes from C0* to 0

Depletion layer of C0* gradually becomes thicker

In cyclic voltammetry, can you measure just the Faradaic current?

no you have to measure Faradaic and non-Faradaic together

What does cyclic voltammetry tell you?

redox potentials and kinetics, coupled chemical reactions

CV is simple to

implement and understand

Why not use CV exclusively?

suffers from poor analytical detection limits (mostly from non-Faradaic current)

For a potential step in chronoamperometry, current is proportional to

concentration, (time)^-.5

In chronoamperometry, there is an increase in diffusion layer thickness over

charge

In chronoamperometry, the current decays exponentially with time, and this is usually much faster than that due to

diffusion limited mass transfer for Faradaic currents

Why use pulses instead of sweeps?

by selecting the pulse periods and delays properly, one can de-emphasize the non-Faradaic currents and emphasize Faradaic currents → improves detection limits

Differential pulse voltammetry

• charging currents further diminished

• response peak not sigmoid

• C_analyte proportional to deltaCurrent_peak

• Concentration detection limits near 10^-10 M

Stripping analysis

• two step process to analyze for trace metals

  • preconcentrate metal ions by reduction into Hg electrode

  • detection by voltametric oxidation of metals from Hg electrode

• detection limits as low as 10^-12 M

Why are AC methods used?

• studies of electrolyte properties

• fundamental studies of kinetics/dynamics of electrode reactions

  • Frequency = experimental timescale

• discrimination against non-Faradaic currents in electroanalysis

  • Frequency-selective detection filters noises

  • Phase-selective detection can minimize non-Faradaic currents and enhance Faradaic currents

  • Higher-harmonic detection can minimize contribution from background currents