introduction to electrochemistry - linköping university · lecture outline • modern...
TRANSCRIPT
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Introduction to Electrochemistry
Mikhail VaginIFM, Linkoping University
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Lecture Outline• Modern Electrochemistry as Technology• Electrochemical ‘Kitchen’ (how it looks like?)
• Equilibrium at Interfaces• Electrochemical thermodynamics• Electrochemical kinetics• Mass transport• Electric double layer
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Modern Electrochemistry as Technology
(How we are affected by electrochemistry?)
• NiMH and Li-ion batteries
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Modern Electrochemistry as Technology
(How we are affected by electrochemistry?)
• Fuel Cells
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Modern Electrochemistry as Technology
(How we are affected by electrochemistry?)
• Sensors and biosensors
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Modern Electrochemistry as Technology
(How we are affected by electrochemistry?)
• Solar cells
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Modern Electrochemistry as Technology
(How we are affected by electrochemistry?)
• Corrosion protection
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Electrochemical ‘kitchen’ (How it looks like?)
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Electrochemical ‘kitchen’ (How it looks like?)
• Electrochemistry as a spectroscopic method
SYSTEMOF INTEREST RESPONSE
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Electrochemical ‘kitchen’ (How it looks like?)
• Standard three electrode set-up
The potentiostat is a feedback operational amplifier,Which controls the potential at the working electrode
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Electrochemical ‘kitchen’ (How it looks like?)
• Counter electrode• Currents flows between the
counter (auxiliary) and the working electrodes;
• Surface area larger than working electrode area so that the electron transfer at counter is not rate limiting;
• Should be inert to avoid dissolution.
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Electrochemical ‘kitchen’ (How it looks like?)
• Reference electrode• The potential difference is
measuring between the reference and working electrodes;
• The reference electrode holds the fixed interfacial potential;
• Standard hydrogen electrode;
• Ag/AgCl;• Saturated calomel electrode
Hg/Hg2 Cl2 .
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Electrochemical ‘kitchen’ (How it looks like?)
• Working electrode (‘business’ electrode)
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Equilibrium at interfaces
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Interfaces
Electron exchange
Ion exchange
Exchange of neutral species
phase phase
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Interfaces• Phases in electrochemistry
– Electronic conductors• metals• semiconductors
– Ionic conductors• electrolytes• ionic liquids• membranes
– Mixed conductors (electrons and ions)• conducting polymers• redox polymers
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Equilibrium at Interfaces
• Two metals in contact
electrons flowfrom
to
Metal Metal
Metal Metal ++++
----
Fermi level, i.e. energy
level with probability to find electron equal to 0.5
Contact potential difference
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Equilibrium at Interfaces
• Metal and redox couple in contact
electrons flowfrom
to redox couple
The probability to find the particle at this energy
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Equilibrium at Interfaces• Interfacial potential difference depends on
the concentrations (activities) of O and R and vice versa
][][ln0,
RO
nFRT
Nernst equation
Galvani potential difference
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Equilibrium at Interfaces• By convention standard reference electrode is
Standard Hydrogen Electrode (SHE)
HeH
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Equilibrium at Interfaces• Reference electrodes are used due to their fixed
interfacial potentiels
• Ag/AgCl, silver-silver chloride• Hg/Hg2 Cl2 , calomel electrodee• Hg/HgSO4 , mercury-mercurous sulphate
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Electrochemical thermodynamics
Will a molecule react at electrode spontaneously?
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Electrochemical thermodynamics (Will a molecule react at electrode spontaneously?)
• Control of electrochemical potential allows the reaction to be controlled
Changed due to changes in potential
by convention: reduction
by convention: oxidation
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Electrochemical thermodynamics (Will a molecule react at electrode spontaneously?)
][][ln0
RO
nFRTEE
nFEGKRTGnFE
OR
nFRTE
E
ln][][ln
0
00
0
Nernst equation:
if
• Nernst equation relates the applied potential difference to the concentration of oxidized and reduced species in solution;
• Small changes in aplied potential can lead to large changes in concentration of oxidized and reduced species.
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Electrochemical kinetics How does the molecule get to the electrode?
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Electrochemical kinetics (How does the molecule get to the electrode?)
• Observed electrochemical response is dominated by rate determining step;
• If the transport to/from surface is slowest process, then the reaction is ‘mass transport limited;
• If the electron transfer is slowest, the reaction is ‘electron transfer limited ’;
• Or mixed control
Total reaction
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Electrochemical kinetics Interfacial electron transfer
• Rates of electrode processes
• Rates and Current Densities
][ surfaceCkrate mol cm-2 s-1
Rate constant: cm s-1
Surface concentration:
mol cm-3
Fnratej **Current density: A cm-2
mol cm-2 s-1
Number of transferred electrons per one act Faraday’s constant
96485 C mol-1
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Electrochemical kinetics• Dynamic equilibrium • If E >E0
metal solutionwith redox particles
e-
Fe3+
Fe2+
0jjj
Exchange current density
e-
Fe3+
Fe2+
jjj
metal solutionwith redox particles
)(EfJ ???
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Electrochemical kinetics
100
32 FeFeCCnFkj
• The Butler Volmer equation
RTnF
RTnFjj exp)1(exp0
0EE where
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Electrochemical kinetics
• Steady-state current density vs overpotential curve
cathodic overpotentials
= < 0anodic overpotentials
> 0
anodic currents
cathodic currents
RTnFjj )1(exp0
RTnFjj
exp0
increase
RTnF
RTnFjj exp)1(exp0
Butler-Volmer equation:
less than 100mV
increase
Here: exp(x)=1+x,
0nFjRTRCT
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Electrochemical kinetics
RTnFjj )1(lnln 0
• Botler Volmer curve at high overpotentials
RTnFjj
0lnln
lnj0
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Electrochemical kinetics• Example: j0 for hydrogen evolution due to reduction
0HeH
Metal -log10 (j0, A cm-2)
Ru 2.1
Pt 3.6
Fe 6.0
Cr 7.4
Hg 12.5
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Electric double layer
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Electric double layer
Diffuse layer
Helmholtz layer
ncompositiopTDL E
qC,,
tCEi
Charging current
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Total currentTotal current = Faradaic current + Charging current + Ohmic drop
Applied by potentiostat due to electron
transfer;The most
informative part
We usually need to correct
How to split them?
potential or current stepscurrent spike
potential ramp
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Mass transport
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Mass transport• Why we do care about mass transport?
• Electrochemical reactions take place on electrodes (obviously!).
• Electrons can only tunnel a few Angstroms.• In the electrochemical cell, most reactants are not at the
electrode surface.• The rate of electron transfer (or measured current) can
be dependent on getting reactants to electrode surface.
Rate determining steps here, in this
chapter
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Mass transport• Mass transport in electrochemistry
• Diffusion is a movement of molecules along a concentration gradient, from a area of high concentration to the area of low concentration
• Migration is a movement of charged species under the influence of an electric field.
• Convection is a movement of species by hydrodynamic transport (e.g. natural thermal motion and/or stirring).
but it will help us later!
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Mass transport• Fick’s laws
xODJ
][
00
tODnFAi
][
1st law:
2nd law:
Quantification of movement of a species with respect to distance x from electrode with the flux J
2
2
0][][
xOD
tO How the surface concentration
changes as a function of time
Solving for planar geometry and
electrodereductionc OnFAki ][
Bulk concentration
Surface concentration
What does it mean?
Cottrell equation:
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• Fe3+ solution of low concentration in background electrolyte;
• Apply 0.5 V (E1) then jump to 0 V (E2):
0.5 V0 V
• Measure the change in current with time.
Mass transport
eFeFe 32
eFeFe 32
tODnFAi
][
• Chronoamperometry experiment‘Excitation’
Response
tODnFAi
][
DOAFnt
i bulk22222 ][
1 Straight line means
diffusion controltODnFAi
][
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Mass transport
eFeFe 32
• Diffusion layer
time
diffusion layeraround 500 nm Dt • If D = 10-10 m2s-1, after 1 s
is around 20m
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Voltammetry (the most popular electrochemical
technique)
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Voltammetry• Linear Sweep Voltammetry
• Instead of step (‘pulse’) lets sweep!
exponential increase with potential:kinetic control i
exp E
50/50
[O] depleted from the surface
Cottrell area:Diffusion controlI
1/(t)1/2
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Voltammetry• Effect of scan rate
Scan rate
• Since the current is proportional to flux, and the flux is proportional to gradient of concentration between bulk and surface, it should be evident that a higher scan rate give sharper gradient and higher current.
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Voltammetry
mVn
EEE cathodicpeak
anodicpeakpeaks
59
1cathodicpeak
anodicpeak
i
ica
peaki ,
• Cyclic voltammetry
If: • Fully reversible electrode reaction
• Randles-Sevcik equation:
DOFAnip ][2/3
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Voltammetry• Rotating Disk Electrode • Sometimes controlled
convection is not bad!
timet
no rotation: rotation is on:
ft
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0.1 0.2 0.3 0.40.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8 2400rpm
i, m
A c
m-2
E vs Ag/Ag+/CH3CN, V
0rpm
Voltammetry
xO
xOD
tO
x
][][][
2
2
0
• Rotating Disk Electrode
2nd Fick’s law:
convection
6/13/2][692.0 DOnFAi bulkL
Levich equation:
exponential increase with potential:kinetic control i
exp E
limiting current ilim f(t)area of mass transport control
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Voltammetry• Microelectrodes
0
0 ][][r
OnFADt
ODnFAitotal
Cottrell steady-statecurrent
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Voltammetry• Microelectrodes
Macro
MicroEnhanced diffusion at microelectrodes:•Shorter response time•Faster kinetics can be studied•Higher Signal-to-Noise ratios
(Faradaic vs charging currents)•Less ohmic drop•Insensitive to convection•Could be integrated into microsystems
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Thank you!