Electrochemistry Lecture 4_notes
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Transcript of Electrochemistry Lecture 4_notes
16/08/2013
1
Practical details and
electrochemical techniques
Common techniques
• Cyclic Voltammetry
• Bulk Electrolysis
• Chronoamperometry, chronocoulometry
• Square wave, staircase voltammetry
• Differential pulse voltammetry
• Electrochemical Quartz Crystal
Microbalance
• Impedance spectroscopy
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Cyclic voltammetry (CV)
• The most widely used electrochemical
technique
• Simple to perform and extremely
informative
• Principles
• Practical considerations
• Examples
Edc = Einitial + ν t
Initial potential
Switching potential
Switching potentialA = B + e-
Sweep rate
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What does this actually represent?
Take the oxidation of species R to O
Point A: Only R is present in solution : still below the redox potential of solution species
Increase the electrode potential towards the redox potential : R is converted to O.
As R is converted to O a concentration gradient is setup at the electrode
Point B: R is instantaneously converted to O
After point B the current is dependent on the rate of mass transfer to the electrode surface
What does this actually represent?
Take the oxidation of species R to O
Point C: Reverse the scan directionR is still being converted to O
Decrease the electrode potential towards the redox potential
Species O which is present near the electrode surface is reduced back to R
Point D: a maximum is reached as in the forward sweep
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Classic example 1 electron oxidation process
Reversible process
The peak potential separation (Epa - Epc) is equal to 57 mV The peak current ratio (ipa/ipc) is equal to 1 for all scan ratesThe peak current increases linearly as a function of the square root of vThe peak current is proportional to concentration
-1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9
-60
-40
-20
0
20
40
I [µ
A]
E [V] vs Ag ref
Analysis
Oxidation of ferrocene
Reduction of cobaltocenium
Fe is in a 2+ oxidation state
Both one electron processes – what looks strange here?
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-1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9
-60
-40
-20
0
20
40
I [µ
A]
E [V] vs Ag ref
0.0 0.1 0.2 0.3 0.4 0.5 0.6
0.000000
0.000005
0.000010
0.000015
0.000020
0.000025
0.000030
i p [A
]
sweep rate [V s-1]1/2
ferrocene
Data1B
ip = 2.69 x 105 n3/2 A D1/2 v1/2 C
Randles Sevcik Equation
n : no of electrons D : diffusion coefficient C : concentrationA : electrode area v : sweep rate
Electrochemical cell
CE REF WE
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How does it really look?
Reference Electrode
Working Electrode
Counter Electrode
RMIT University 12
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Practical detailsNearly every experiment requires the presence of a supporting electrolyte – minimises solution resistance
For CV experiments we use a 3 electrode setup
WE : working electrode : process of interest occursTypically Pt, Au, carbon, ITO, boron doped diamond
CE : counter electrode : Pt wire/coil/mesh, graphite rod
REF : Reference electrode Dependent on solvent system
A potential is applied between WE and REF while current is recorded between WE and CE.
Therefore a stable REF electrode is essential
Reference electrodes
Reference electrode is an electrode which has a stable and well-known electrode potential
Therefore to form a basis for comparison with all other electrode reactions, Hydrogen's standard electrode potential (E0) is declared to be zero at all temperatures.
Potentials of any other electrode is compared with that of the SHE at the same temperature.
Common reference electrodes : Ag/AgCl, saturated calomel electrode (SCE)
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Reference electrodes
Ag/AgCl (3M NaCl) is one of the most commonly used
Based on
AgCl(s) + e- = Ag(s) + Cl-(aq)
Ideal non-polarizable electrode
E°′ = 0.220 V vs SHEUnit activity at standard conditions
For Ag/AgCl (3M KCl)E = 0.196 V
Cell Design
– Electrodes (Working, Reference, Auxiliary)
• material
• geometry (available theory?)
• size
• location
– Quiescence- no adventitious stirring caused by
• Source of vibration - fumehoods
• gas flow through or over solution
• density gradients (electrochemically induced)
• temperature gradients
– Temperature Control
– Integrity (“air” tight; vacuum tight)
•
Solvent
Supporting Electrolyte (excess assumed)
Choose analyte concentration
selection and purification;
maximize relevant electro-
chemical window.
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Develop a protocol
Find a suitable solvent for your analyte
Find a suitable supporting electrolyte (SE)
Run a background - SE + whatever (e.g., buffer, ligand,
acid, base…..) with no analyte present
Run a simple CV with the analyte
Chosen a value of ν – typically 50 or 100 mVs-1
• Change voltage ranges within the voltage window
for the system
• See what happens when ncycles = 2, 3, 4………50
• Run CVs over a range of v consistent with
working electrode size & geometry
• Change the concentration of analyte
• Look at T-dependence
•Re-evaluate requirements and consider
– optimizing/modifying cell/electrodes
– using different solvent, SE, etc.
– variations addressing specific interests
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Some examples
Cyclic voltammetry is very powerful in determining reaction mechanisms
Seen previously a one electron transfer reaction
The peak potential separation (Epa - Epc) is equal to 57 mV
What if more than 1 electron is transferred
The peak potential separation (Epa - Epc) is equal to 57/n mV
However much more information can be gathered
Some examples
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Oxidation product unstableConsumed chemically to an electrochemically inactive species
⇒ Less oxidised product available for reduction
Example of an EC mechanism
EC mechanism
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EC mechanism
IncreaseSweep rate
ECE mechanism
IncreaseSweep rate
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ECE mechanism
Many mechanisms• Identify some “basic” mechanisms
E A + e = B
EE A + e = B; B + e = C;
EC A1 + e = B1; B1 = B2
EC’ A + e = B; B + P = A + Q
EC2 A + e = B; 2B = B2
CE Y = A; A + e = B
ECE A1 + e = B1; B1 = B2; B2 + e = C2;
• Use DigiSim or a simulator of choice to explore the behavior of selected basic mechanisms.
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Several Electrode Types
Macrodisk electrodes : typically 1 – 3 mm in diameter
Rotating disk electrodes : as above but rotated
Rotating ring disk electrodes
Microelectrodes : typically < 100 µm in diameter
Microelectrode arrays
Mercury drop electrode
Several ElectrodeTypes
Stationary Macrodisk Rotating Macrodisk Electrode
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Rotating Disk Electrode (RDE)
Levich Equation
iL = 0.62nFACD2/3νννν-1/6ωωωω1/2
ω= 2Πf : angular rotation rate of electrode (rad s-1)
ν= kinematic viscosity (cm2 s-1)viscosity (g cm-1 s-1 ) / density (g cm-3)
This equation applies if the current is limited by diffusion and not
electron transfer
Microelectrodes
a
0.0 0.1 0.2 0.3 0.4 0.5 0.6-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
ibulk
= 4nFDca
25 µµµµm Pt UME
1 mM ferrocene methanol + 0.1 M KNO3
10 mV s-1
I [n
A]
E [V] vs Ag/AgCl
10
Hemispherical diffusion of mediator to the microelectrode
Limiting current in bulk solution
Ibulk = 4nFDcan: no of electrons
F : Faraday’s constant (C mol-1)
D : diffusion coefficient (cm2 s-1)
C : concentration (mol cm-3)
A: radius of electrode (cm)
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Microelectrode Uses
Scanning electrochemical microscopy (SECM)
Dark regions ⇒ highly doped/conducting
SEM image Electrochemical map
Chronoamperometry
• In this technique the potential of the working electrode is stepped, and the resulting current from faradic processes occurring at the electrode (caused by the potential step) is monitored as a function of time.
• Remember the case in cyclic voltammetry for a reversible cyclic voltammogram
A = B + e-
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Chronoamperometry
• Stationary electrode
• Solution must be stationary and unstirred = mass transport by diffusion
• Constant potential
• Measure current vs time (t)
Theory
Assume A = B + e-
- Both A and B are soluble
- Reversible reaction (electrochemically)
- Potential (E) set so oxidation or reduction goes to completion at the electrode surface
E
t (time)0
E1
E2
Other processes occurring at the electrode can perturb the response from Cottrellian behaviour
Capacitive Current – charging current is exponential in nature
However, only influences the beginning of the transient as the capacitive current decreases more rapidly than Faradaic current so at longer times the ratio IFar/Icap is very large
Occurrence of a coupled chemical
reactions e.g. A → B + e- followed by
2B = C where C is electroactive and can also be oxidised
C → B + e-
These can affects the shape of the current-time curve and is a good first step in identifying coupled reactions
Chronoamperometry
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-0.5 0.0 0.5 1.0
-0.6
-0.4
-0.2
0.0
0.2
Cu
rren
t (x
10
-4 A
mp
s)
Potential (V)
Using chronoamperometry
• We can model electrodeposition process to determine mechanism of
growth, we do this by holding the potential at a certain value for a
specified period of time
-0.5 0.0 0.5 1.0
0.0
2.0
4.0
Curr
ent (x
10
-4 A
mp
s)
Potential (V)
Peak maximum shifts to earlier times
Can analyse i–t profile to elucidate mechanism
Hills and Scharifker model
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What do these mechanisms of growth mean for electrodeposition?
Progressive nucleation and growth
Increase deposition time
1.3 mM AgClO4 in CH3CN + 0.1 M LiClO4
Note: Still silver deposition but now in organic solvent – the growth is affected!
Analysis shows instantaneous nucleation
30 s
50 s
90 s
What do these mechanisms of growth mean for electrodeposition?
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The Randles-Sevcik equation can be used to determine the diffusion coefficient of a species in solution. Use this equation to determine the diffusion coefficient of a redox active species, with the transfer of one electron, from the information provided below.
Electrode area: 0.1963 cm2
Faraday’s constant: 96,485 C mol-1
Universal gas constant: 8.314 J mol-1 K-1
Concentration: 2.55 mM
For the following electrochemical reaction, B → C + e-, a rotating disk
electrode study was carried out to determine the diffusion co-efficient of B.
Illustrated here is a plot of the limiting current versus ω1/2.
What is the name of the equation used to plot this data?
From this plot calculate the diffusion co-efficient of B. The electrode used
had an area of 0.1963 cm2, [B] = 2.55 x 10-6 mol cm-3, kinematic viscosity of
solution = 0.00916 cm2 s-1 and Faraday’s constant = 96,485 C mol-1.