ELECTROCHEMISTRY

CHEM 4700

CHAPTER 2

DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences

Clayton state university

CHAPTER 2

ELECTRODE REACTIONS&

INTERFACIAL PROPERTIES

- Involves linear scanning of potential of a stationary electrode using a triangular waveform

- Solution is unstirred

- The most widely used technique for quantitative analysis of redox reactions

Provides information on- the thermodynamics of redox processes

- the kinetics of heterogeneous electron transfer reactions- the kinetics of coupled reactions

CYCLIC VOLTAMMETRY

- The current resulting from an applied potential is measured during a potential sweep

- Current-potential plot results and is known as cyclic voltammogram (CV)

CYCLIC VOLTAMMETRY

CYCLIC VOLTAMMOGRAM (CV)

Triangular waveform (left) and CV (right) of ferricyanide

- Assume only O is present initially

- A negative potential sweep results in the reduction of O to R(starting from a value where no reduction of O initially occurs)

- As potential approaches Eo for the redox process, a cathodic current is observed until a peak is reached

- The direction of potential sweep is reversed after going beyond the region where reduction is observed

- This region is at least 90/n mV beyond the peak

CYCLIC VOLTAMMETRY

- R molecules generated and near the electrode surface are reoxidized to O during the reverse (positive) scan

- Results in an anodic peak current

- The characteristic peak is a result of the formation of a diffusion layer near the electrode surface

- The forward and reverse currents have the same shape

CYCLIC VOLTAMMETRY

- Increase in peak current corresponds to achievement of diffusion control

- Decrease in current (beyond the peak) does not depend on the applied potential but on t-1/2

Characteristic Parameters- Anodic peak current (ipa)

- Cathodic peak current (ipc)- Anodic peak potential (Epa)

- Cathodic peak potential (Epc)

CYCLIC VOLTAMMETRY

Reversible Systems

- Peak current for a reversible couple is given by the Randles-Sevcik equation (at 25 oC)

n = number of electronsA = electrode area (cm2)

C = concentration (mol/cm3)D = diffusion coefficient (cm2/s)

ν = potential scan rate (V/s)

1/21/23/25p νACDn10x2.69i

CYCLIC VOLTAMMETRY

Reversible Systems

ip is proportional to C

ip is proportional to ν1/2

- Implies electrode reaction is controlled by mass transport

ip/ic ≈ 1 for simple reversible couple

- For a redox couple

2

EEE pcpao

CYCLIC VOLTAMMETRY

Reversible Systems

- The separation between peak potentials

Vn

0.059EEΔE pcpap

- Used to determine the number of electrons transferred

- For a fast one electron transfer ∆Ep = 59 mV

- Epa and Epc are independent of the scan rate

CYCLIC VOLTAMMETRY

Reversible Systems

- The half peak potential

- E1/2 is called the polarographic half-wave potential

Multielectron Reversible Systems- The CV consists of several distinct peaks if the Eo values for

the individual steps are well separated(reduction of fullerenes)

Vn

0.028EE 1/2p/2

CYCLIC VOLTAMMETRY

Irreversible Systems

- Systems with sluggish electron transfer

- Individual peaks are reduced in size and are widely separated

- Characterized by shift of the peak potential with scan rate

1/2

a1/2

o

a

op RT

Fναnln

D

kln0.78

Fαn

RTEE

1/21/21/2a

5p νACDαnn10x2.99i

CYCLIC VOLTAMMETRY

Irreversible Systems

α = transfer coefficientna = number of electrons involved in a charge transfer step

ko = standard heterogeneous rate constant (cm/s)

- ip is proportional to C but lower depending on the value of α

For α = 0.5ip,reversible/ip,irreversible = 1.27

- That is irreversible peak current is ~ 80% of reversible ip

CYCLIC VOLTAMMETRY

Quasi-reversible Systems

- Current is controlled by both charge transfer and mass transport

- Voltammograms are more drawn out

- Exhibit larger separation in peak potentials compared to reversible systems

- Shape depends on heterogeneous rate constant and scan rate

- Exhibits irreversible behavior at very fast scan rates

CYCLIC VOLTAMMETRY

Applications

1. Study of Reaction Mechanisms

E = redox step and C = chemical step

E- Only redox step

O + ne- ↔ R

CYCLIC VOLTAMMETRY

Applications

E = redox step and C = chemical step

EC- Redox step followed by chemical step

O + ne- ↔ R + A → Z

- R reacts chemically to produce Z- Z is electroinactive

- Reverse peak is smaller since R is chemically removedipa/ipc < 1

- All of R can be converted to Z for very fast chemical reactions

CYCLIC VOLTAMMETRY

Applications

E = redox step and C = chemical step

EC- Redox step followed by chemical step

O + ne- ↔ R + A → Z

Examples - Ligand exchange reactions as in iron porphyrin complexes

- Oxidation of chlorpromazine to produce a radical cation and subsequent reaction with water to produce sulfoxide

CYCLIC VOLTAMMETRY

Applications

E = redox step and C = chemical step

EC- Catalytic regeneration of O during a chemical step

O + ne- ↔ R + A ↔ O

- Peak ratio is unity

Example- Oxidation of dopamine in the presence of ascorbic acid

CYCLIC VOLTAMMETRY

Applications

E = redox step and C = chemical step

CE- Slow chemical reaction precedes the electron transfer step

Z → O + ne- ↔ R

ipa/ipc > 1 (approaches 1 as scan rate decreases)ipa is affected by the chemical step

ipc is not proportional to ν1/2

CYCLIC VOLTAMMETRY

Applications

E = redox step and C = chemical step

ECE- Chemical step interposed between redox steps

O1 + ne- ↔ R1 → O2 + ne- → R2

- The two redox couples are observed separately- The system behaves as EE mechanism for very fast

chemical reactions

- Electrochemical oxidation of aniline

CYCLIC VOLTAMMETRY

Applications

2. Study of Adsorption Processes

- For studying the interfacial behavior of electroactive compounds

Symmetric CV∆Ep = 0

- Observed for surface-confined nonreacting species

- Ideal Nernstian behavior

CYCLIC VOLTAMMETRY

Applications

Symmetric CV- Peak current is directly proportional to surface coverage (Γ)

and scan rate (ν)

4RT

ΓAνFni

22

p

Holds for relatively- slow scan rates

- slow electron transfer- no intermolecular attractions within the adsorbed layer

CYCLIC VOLTAMMETRY

Applications

Symmetric CV

CYCLIC VOLTAMMETRY

volts

current

∆Ep,1/2

Q (area under peak)

nFAΓQ

Applications

Symmetric CV

- The surface coverage can be determined from the area under the peak (Q)

Q = quantity of charge consumed

nFA

QΓ

CYCLIC VOLTAMMETRY

nFAΓQ or

Applications

3. Quantitative Determination

- Based on the measurement of peak current

CYCLIC VOLTAMMETRY

- Simultaneous measurement of spectral and electrochemical signals

- Coupling of optical and electrochemical methods

- Employs optically transparent electrode (OTE) that allows light to pass through the surface and adjacent solution

Examples Indium tin oxide (ITO), platinum, gold, silver, nickel

deposited on optically transparent glass or quartz substrate

SPECTROELECTROCHEMISTRY

-2.5

-1.5

-0.5

0.5

1.5

2.5

-0.4-0.200.20.40.60.8Volts vs Ag/AgCl

Curre

nt (M

illia

mps

)

Epa

Epc

ipa

ipc

0

200

400

600

800

1000

0 100 200 300 400 500 600

Time (Seconds)In

ten

sity

Io

I

ipa = anodic peak current ipc = cathodic peak current

Modulated Absorbance

Am = -log(I/Io)

SPECTROELECTROCHEMISTRY

Applications

- Useful for elucidation of reaction kinetics and mechanisms(for probing adsorption and desorption processes)

- Thin layer SE methods for measuring Eo and n (Nernst equation)

- Infrared SE methods for providing structural information

- UV-Vis spectroscopic procedures

- Vibration spectroscopic investigations

- Luminescence reflectance and scattering studies

SPECTROELECTROCHEMISTRY

- Technique for studying electrogenerated radicals that emit light

- Involves electrochemical generation of light-emitting excited-state species

- Usually carried out in deoxygenated nonaqueous media

Examples of SpeciesRu(bpy)3

2+

Nitro compoundsPolycyclic hydrocarbons

Luminol

ELECTROCHEMILUMINESCENCE (ECL)

- Used to acquire high resolution data of surface properties

- Achieved by sensing the interactions between a probe tip and the target surface as the tip scans across the surface

Examples- Scanning Tunneling Microscopy (STM)

- Atomic Force Microscopy (AFM)- Scanning Electrochemical Microscopy (SECM)

SCANNING PROBE MICROSCOPY

- Direct imaging of surfaces on the atomic scale

- Very sharp atomic tip moves over the sample surface with a ceramic piezoelectric translator

- The basic operation is the electron tunneling between the metal tip and the sample surface

- Tunneling current is measured as potential is applied between the tip and the sample

- Measured current at each point is based on sample-tip separation

SCANNING TUNNELING MICROSCOPY (STM)

- High resolution imaging of the topography of surfaces (surface structure)

- Allows for nanoscopic surface features while the electrode is under potential control

- Measures the force between the probe and the sample

- The probe has a sharp tip made from silicon or silicon nitride attached to a force-sensitive cantilever

- Useful for exploring both insulating and conducting regions

ATOMIC FORCE MICROSCOPY (AFM)

- Faradaic currents at a microelectrode tip are measured while the tip is moved close to the substrate surface immersed in a

solution containing an electroactive species

- The tip currents are a function of the conductivity and chemical nature of the substrate as well as the tip-substrate distance

Images obtained give information on - electrochemical activity

- chemical activity- surface topography

SCANNING ELECTROCHEMICAL MICROSCOPY (SECM)

- Cannot be used for obtaining atomic resolution

Used to investigate- Ionic flux through the skin or membranes

- Localized biological activity (biosensors)

- Heterogeneous reaction kinetics

SCANNING ELECTROCHEMICAL MICROSCOPY (SECM)

- For elucidating interfacial reactions based on simultaneous measurement of electrochemical parameters and mass

changes at the electrode surface

- Uses a quartz crystal wafer sandwiched between two electrodes which induces electric field

- The electric field produces a mechanical oscillation in the bulk of the wafer

ELECTROCHEMICAL QUARTZ CRYSTAL MICROBALANCE (EQCM)

- The frequency change (∆f) relates to the mass change (∆m) according to the Sauerbrey equation

ELECTROCHEMICAL QUARTZ CRYSTAL MICROBALANCE (EQCM)

μρA

mnf2Δf

2o

n = overtone numberfo = base resonant frequency of the crystal (prior to mass change)

A = area (cm2)μ = shear modulus of quartz (2.95 x 1011 gcm-1s-1)

ρ = density of quartz (2.65 g/cm3)

- Decrease in mass corresponds to increase in frequency

Useful for probing - processes that occur uniformly across the surface

- deposition or dissolution of surface layers- ion-exchange reactions at polymer films

- study of polymeric films

- Cannot be used for molecular level characterization of surfaces

ELECTROCHEMICAL QUARTZ CRYSTAL MICROBALANCE (EQCM)

- For probing the features of chemically-modified electrodes

- For understanding electrochemical reactions

- For electron transfer kinetics and diffusional characteristic studies

Impedance- Complex resistance encountered when a current flows

through a circuit made of combinations of resistors, capacitors, or inductors

IMPEDANCE SPECTROSCOPY

- Plots of faradaic impedance spectrum is known as Nyquist plot

Consists of - a semicircle portion at high frequencies

(corresponds to the electron-transfer-limited process)

and

- a straight line portion at low frequencies(coreesponds to the diffusion-limited process)

IMPEDANCE SPECTROSCOPY

- The impedance spectrum has only the linear portion for very fast electron transfer processes

- Very slow electron transfer processes are characterized by a large semicircle region

- Diameter of the semicircle equals the electron transfer resistance

IMPEDANCE SPECTROSCOPY