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Transcript
ELECTROCHEMISTRY
CHEM 4700
CHAPTER 2
DR. AUGUSTINE OFORI AGYEMAN
Assistant professor of chemistry
Department of natural sciences
Clayton state university
CHAPTER 2
ELECTRODE REACTIONS
&
INTERFACIAL PROPERTIES
CYCLIC VOLTAMMETRY
- 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 VOLTAMMOGRAM (CV)
Triangular waveform (left) and CV (right) of ferricyanide
CYCLIC VOLTAMMETRY
- 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)


i p  2.69 x 105 n 3/2ACD1/2 ν1/2
n = number of electrons
A = electrode area (cm2)
C = concentration (mol/cm3)
D = diffusion coefficient (cm2/s)
ν = potential scan rate (V/s)
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
E 
o
E pa  E pc
2
CYCLIC VOLTAMMETRY
Reversible Systems
- The separation between peak potentials
ΔE p  E pa  E pc 
0.059
V
n
- 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
E p/2
0.028
 E 1/2 
V
n
- 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)
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
o

αn
Fν
RT
k
 a  
o
Ep  E 
 
0.78  ln 1/2  ln 
αn a F 
D
 RT  
ip
 2.99 x 10 nαn 
1/2
5
a
ACD1/2 ν1/2
CYCLIC VOLTAMMETRY
Irreversible Systems
α = transfer coefficient
na = 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 = 0.5
ip,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 removed
ipa/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)
n 2 F 2 ΓAν
ip 
4RT
Holds for relatively
- slow scan rates
- slow electron transfer
- no intermolecular attractions within the adsorbed layer
CYCLIC VOLTAMMETRY
Applications
Symmetric CV
Q (area under peak)
Q  nFAΓ
current
∆Ep,1/2
volts
CYCLIC VOLTAMMETRY
Applications
Symmetric CV
- The surface coverage can be determined from the
area under the peak (Q)
Q = quantity of charge consumed
Q  nFAΓ
or
Q
Γ
nFA
CYCLIC VOLTAMMETRY
Applications
3. Quantitative Determination
- Based on the measurement of peak current
SPECTROELECTROCHEMISTRY
- 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
1000
Io
-1.5
800
ipc
-0.5
Intensity
Current (Milliamps)
Epc
0.5
600
I
400
ipa
1.5
200
Epa
2.5
0
0.8
0.6
0.4
0.2
Volts vs Ag/AgCl
0
-0.2
ipa = anodic peak current
ipc = cathodic peak current
-0.4
0
100
200
300
400
500
Time (Seconds)
Modulated Absorbance
Am = -log(I/Io)
600
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
ELECTROCHEMILUMINESCENCE (ECL)
- 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 Species
Ru(bpy)32+
Nitro compounds
Polycyclic hydrocarbons
Luminol
SCANNING PROBE MICROSCOPY
- 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 TUNNELING MICROSCOPY (STM)
- 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
ATOMIC FORCE MICROSCOPY (AFM)
- 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
SCANNING ELECTROCHEMICAL
MICROSCOPY (SECM)
- 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
ELECTROCHEMICAL QUARTZ CRYSTAL
MICROBALANCE (EQCM)
- 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
Δf 
 2mnf o2
A μρ
n = overtone number
fo = 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)
ELECTROCHEMICAL QUARTZ CRYSTAL
MICROBALANCE (EQCM)
- 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
IMPEDANCE SPECTROSCOPY
- 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