week 2 electrochemistry (full size)
DESCRIPTION
electrochemTRANSCRIPT
MTE4599
Materials for energy technologies
So, which technologies ?• Wind turbines and thermal solar power plants; most main components are made from well-known materials like concrete and steal. Crucial materials engineering in the wind-turbine blades made from reinforced polymers – please refer to e.g. MTE4572.
Mainly the emerging technologies *Photo-voltaic cells (power producing solar-cells)
* Materials for water splitting (hydrogen production)
* Materials for storing and converting energy
* Fuel-cell materials
* Need electrochemistry !
Electrochemistry 1.0
Electrode potential
(batteries, fuel-cells and corrosion)
Electrochemical reactions
(water-splitting, dye sensitized solar-cells and reversible batteries)
Electrochemical analytical methods
(Standard) Electrode potential
Equilibrium E° (volts)
-3.03
-2.92
-2.87
-2.71
-2.37
-1.66
-0.76
-0.44
-0.13
0
+0.34
+0.80
+1.50
Standard conditions ? - 1M
(Standard) Electrode potential
Equilibrium E° (volts)
-3.03
-2.92
-2.87
-2.71
-2.37
-1.66
-0.76
-0.44
-0.13
0
+0.34
+0.80
+1.50
Standard conditions ? - 1M
(Standard) Electrode potential- If you can write the equilibrium equation…
Standard conditions ? - 1M and 1atm
Non metalsCl2 + 2e- � 2Cl- (+1.36V)
O2 + 4e- + 4H+ � 2H2O (+1.23V)
SO42+ + 2e- + 4H+ � SO2 + 2H2O (+0.17V)
S + 2e- + 2H+ � H2S (+0.14V)
CO2 + 2e- + 2H+ � HCOOH (-0.20V)
Carbon (?)
(Standard) Electrode potential- If you can write the equilibrium equation…
Standard conditions ? - 1M and 1atm
Non metalsCl2 + 2e- � 2Cl- (+1.36V)
O2 + 4e- + 4H+ � 2H2O (+1.23V)
SO42+ + 2e- + 4H+ � SO2 + 2H2O (+0.17V)
S + 2e- + 2H+ � H2S (+0.14V)
CO2 + 2e- + 2H+ � HCOOH (-0.20V)
Carbon (?)
“Redox couples”Fe3+ + e- � Fe2+ (+0.77V)
I3- + 3e- � 3I- (+0.53V)
Hydroquinone
Etc.
Electrode potential =>
Battery cell
Using standard reduction potentials:
0.34V – (- 0.76V) = 1.1V (?)
V
Salt bridge
Standard conditions ? And if not, call a friend !
A friend called Nernst
The free energy of the electrode reaction:
ΔG° = –nFE°
F
F = 96,485 C/mol
A friend called Nernst
ΔG° = –nFE° (and ΔG = –nFE)
And
ΔG = ΔG° + RT ln Q (Q = the reaction quotient)
(http://en.wikipedia.org/wiki/Gibbs_free_energy)
Gives
–nFE = –nFE° + RT ln Q
A friend called Nernst
ΔG° = –nFE° (and ΔG = –nFE)
And
ΔG = ΔG° + RT ln Q (Q = the reaction quotient)
Gives
–nFE = –nFE° + RT ln Q
Rearranged it becomes the Nernst Equation:
or
(at 25°C)
R = Gas constant = 8.314 J/(K*mol) F = 96,485 C/mol
Nernst in the lab
What is the electrode potential of Ag+ + e- � Ag at
various concentrations ?
E = 0.80 – 0.059/1*log(1/[Ag+])
(at 25°C)
Nernst in the lab
What is the electrode potential of Ag+ + e- � Ag at
various concentrations ?
E = 0.80 – 0.059/1*log(1/[Ag+])
(at 25°C)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
-10 -8 -6 -4 -2 0 2
V v
s S
HE
Log[Ag+]
E(Ag/Ag+)
Nernst in the lab
What is the electrode potential of Ag+ + e- � Ag ?
E = 0.80 – 0.059/1*log(1/[Ag+])
(at 25°C)
Concentration
batteries !- Any problems in
practical
applications ?
V
Nernst in the lab
What is the electrode potential of Ag+ + e- � Ag ?
- At different temperatures (1mM solution) ?
E = 0.80 -8.314*T
* ln(1/0.001)1*96,485
This temperature dependence is the
underlying principle for the “thermo-cell”
And also the problem of using
reference electrodes at
different temperatures !
Nernst Nernst Nernst Nernst and Water
Oxidation: H2(g) → 2H+ + 2e–
(at 25°C)
E = E° - (.059/2) × log (PH2/[H+]2)
Reduction: O2(g) + 4H+ + 4e– → H2O
E = 1.23 – (.059/4) x 4pH =
1.23 – 0.059 pH
And then what ?
E = 0 - (.059/2) × 2 pH = – 0.059 pH
Overall: 2H2(g ) + O2(g) → 2H2O
E = E° - (.059/4) × log (1/([H+]4 x PO2))
Electrode potential => Fuel cell
A “continuous” battery…
Using standard reduction potentials:
1.23V – 0V = 1.23V
Splitting Water
Reduction: 2H+ + 2e– → H2(g)
E = – 0.059 pH
Oxidation H2O → O2(g) + 4 H+ + 4e–
E = 1.23 – 0.059 pH
Eh, what just happened ?
We went from measuring potentials on electrodes
to applying potentials to electrodes
– much more about that later under
Electrolytic cells and electrolysis(Next lecture or so)
The driving force in science:
What happens when…
Applying potential to
a half frog ?
Luigi Galvani, circa 1786,
electrochemistry and
electrophysiology began with
frogs' legs.
PART 2
Applying potential to an electrode
- in an electrolyte containing ions.
- Just a small potential though !
-
-
+
-
-
--
-
-
-
-
-
+
+
+
+
+ +
++
+
Applying potential to an electrode
- in an electrolyte containing ions.
- Just a small potential though !
Double layer capacity
-
-
+
-
-
--
-
--
-
- +
+
+ +
+
++
+
+
++
+
++
+
+
+++
+
+
-
-
+
-
-
--
-
-
-
-
-
+
+
+
+
+ +
++
+
Applying potential to an electrode- in an electrolyte containing ions.
- Just a small potential though !
Double layer capacity• Surface area
• Salt and concentration
• Potential (!)
• Characteristic charging/discharging
curve at constant |currents|-
-
+
-
-
--
-
--
-
- +
+
+ +
+
++
+
+
++
+
++
+
+
+++
+
+
V
Time
+I -I
Applying potential to an electrode- in an electrolyte containing ions.
Higher potentials ?
-
-
+
-
-
--
-
--
-
- +
+
+ +
+
++
+
+
++
+
++
+
+
+++
+
+
V
Time
+I -I
????
????
Applying potential to an electrode- in water containing ions.
-
-
+
-
-
--
-
--
-
- +
+
+ +
+
++
+
+
++
+
++
+
+
+++
+
+
V
Time
+I -I
????
????
Applying potential to an electrode- in water containing ions.
-
-
+
-
-
--
-
--
-
- +
+
+ +
+
++
+
+
++
+
++
+
+
+++
+
+
V
Time
1.23V (?)
Water splitting-
-
+
-
-
--
-
-
-
-
-+
+
+ +
+
++
+
+
--
-
--
-
---
-
+
+
Applying potential to an electrode- in water containing ions,
constant current.
-
-
+
-
-
--
-
--
-
- +
+
+ +
+
++
+
+
++
+
++
+
+
+++
+
+ V vs SHE
Time
> 1.23V (?)
Water splitting
-
-
+
-
-
--
-
-
-
-
-+
+
+ +
+
++
+
+
--
-
--
-
---
-
+
+
Oxidation: H2O → O2(g) + 4 H+ + 4e–
Reduction: 2H+ + 2e– → H2(g)
Applying potential to an electrode(- in water containing ions and ?)
What else could happen ?
ANY reaction where the equilibrium
(“standard“) electrode potential is within
the potential range where water is stable.
Metal deposition:
Mz+ + ze- � M
Metal dissolution:
M � Mz+ + ze-
Gas reactions
e.g. SO2 + 2H2O � SO42+
+ 2e- + 4H+ (+0.17V)
“Redox” couples e.g.
Ferocene/ferocenium and
Iodine/iodide
(Oxidative) polymerization
e.g. conducting polymers
Not water ?
- same logic, but different potentials possible
Applying potential to an electrode
Not water ?
- same logic, but different potentials possible
- “Electrochemical window”
E.g. Propylene carbonate
NB electrolyte salt (ions)
also need to stable !
> 4V
Applying potential to an electrode
How much product are we getting from
electrochemical reactions ?
- Meet our second friend: M. Faraday
Faraday’s 1st law links the amount (mass/mol) of
material produced with the charge used (Q).
At constant current the number of moles is:
Importance in batteries ?
Applying potential to an electrode
How much product are we getting ?
- Meet our second friend: M. Faraday
Please note that Faraday’s law is NOT taking the
potential needed to drive the reaction into account
=>
Faraday’s law can be used to estimate conversion
efficiency (“mole per coulomb”), but NOT to
evaluate ENERGY efficiency of the reaction.
(Remember: Power = Current x Voltage)
Applying potential to an electrode- in water containing ions,
constant current.
-
-
+
-
-
--
-
--
-
- +
+
+ +
+
++
+
+
++
+
++
+
+
+++
+
+ V vs SHE
Time
> 1.23V (?)
Water splitting
-
-
+
-
-
--
-
-
-
-
-+
+
+ +
+
++
+
+
--
-
--
-
---
-
+
+
Oxidation: H2O → O2(g) + 4 H+ + 4e–
Reduction: 2H+ + 2e– → H2(g)
A closer look;Equilibrium, over-potential and our next friends…
Cu2+ + 2e- � Cu
+0.34 V is the equilibrium potential.
This means that at this potential the reaction:
Cu2+ + 2e- ���� Cu (cathodic exchange current density ic)
(charge transfer coefficient: αc) and
Cu2+ + 2e- Cu (anodic exchange current density ia)
(charge transfer coefficient (symmetry factor): αa)
The overall electrode current i is zero,
but the exchange current density i0 is not and
i0 = ||||ic|||| = ia at equilibrium
(NB. In one-step, one-electron reactions αc + αa = 1)
A closer look;Equilibrium, over-potential and our next friends…
Defining the over-potential ηηηη = E - Eeq
The anodic and catodic exchange currents
can be written as a function of
• the exchange current density (io),
• the overpotential (η) and
• the symmetry factor (α)
ia = io[e(1-α)ηf] and
ic = io[-e(-α)ηf],
where f =nF/RT
We will not go into the
deeper reasoning
behind this math !
The main job for you
is to
accept/agree/imagine
that an exchange
current density exists !
A closer look;Equilibrium, over-potential and our next friends…
Defining the over-potential ηηηη = E - Eeq
i(η) = ia + ic = io[ e (1 - α) η f – e -α η f ], where f =nF/RT
Known as the Butler-Volmer equation
A closer look;Overall current depending on exchange current density i0
i = ia + ic = io[ e (1 - α) η f – e -α η f ], where f =nF/RTHere for one-electron process n=1. Also α = 0.5
over-potential ηηηη = E - Eeq
A closer look;Overall current depending on charge transfer coefficient αααα
i = ia + ic = io[ e (1 - α) η f – e -α η f ], where f =nF/RTHere for one-electron process n=1
over-potential ηηηη = E - Eeq
Can αααα be 1 ?At least close…
SO2 � SO42-
A good electro-catalyst ?
i = ia + ic = io[ e (1 - α) η f – e -α η f ], where f =nF/RT
High io for all steps in the reaction
e.g.
O2 + 4e- + 4H+ � 2H2O
Contains multiple steps
one for each electron transferred + adsorption and desorption
- And no α “against” you…
Take one of the currents in the Butler-Volmer equation
ln i = ln i0 + αηFn/RT Solving for η:
ηc = -b*log i0 + b*log i, (where b is 2.3RT/αnF)
This is the Tafel equation
Measuring io and α - meet our last friend: Julius Julius Julius Julius TafelTafelTafelTafel
i = ia + ic = io[ e (1 - α) η f – e -α η f ], where f =nF/RT
over-potential ηηηη = E - Eeq
Tafel Slope is 2.3RT/αnF
Intercept “gives” log i0
Overall two possibilities:
1) Controlling current and measuring potential
2) Controlling potential and measuring current
But we can do it in many different ways depending on what you
want to get out of the measurement
A couple of examples…
- Chronovoltammetry (1)
- Chronoamperometry (2)
- Cyclic Voltametry (CV) (2)
Electrochemical Measurements
PART 3
Test Cell set-up: Two or three electrodes ?
Where the 3rd electrode is a reference electrode
- Chronovoltammetry - Chronoamperometry - Cyclic Voltametry (CV)
Electrochemical MeasurementsPART 3
Counter electrode
Test Cell set-up: Two or three electrodes ?
Where the 3rd electrode is a reference electrode
- Chronovoltammetry -Chronoamperometry -Cyclic Voltametry (CV)
Electrochemical MeasurementsPART 3
Counter electrode
Ref CounterWorking
V
A
Potentiostat
Test Cell set-up: Two or three electrodes ?
Where the 3rd electrode is a reference electrode
- Chronovoltammetry -Chronoamperometry -Cyclic Voltametry (CV)
Electrochemical MeasurementsPART 3
Ref CounterWorking
V
A
Potentiostat
Applying a constant current and measuring the potential
Typical use: Battery charge and discharge experiments
Capacitor characterization
Fuel-cell characterization
Metal deposition
Electrochemical Measurements
Chronovoltammetry
PART 3
Applying a constant current and measuring the potential
Typical use: Battery charge and discharge experiments
Capacitor characterization
Fuel-cell characterization
Metal deposition
Electrochemical Measurements
Chronovoltammetry
PART 3
Applying a constant current and measuring the potential
Typical use: Battery charge and discharge experiments
Capacitor characterisation
Fuel-cell characterization
Metal deposition
Electrochemical Measurements
Chronovoltammetry (1)
PART 3
C-rate
1C: charge or discharge in one hour
10C: charge or discharge in six minutes
Applying a constant current and measuring the potential
Typical use: Battery charge and discharge experiments
Capacitor characterization
Fuel-cell characterization
Metal deposition
Electrochemical Measurements
Chronovoltammetry
PART 3
Applying a constant voltage and measuring the current
Typical use: Electro-catalytic measurements (over-potential)
Diffusion measurements (e.g. conducting polymers)
Stability test
Electro deposition
Electrochemical Measurements
Chronoamperometry
PART 3
-2.5
-2.0
-1.5
-1.0
-0.5
0
-600 -400 -200 0 200
Ewe (mV vs. SCE)
I (m
A/c
m2 )
Steady state values
(after 1 hour) for
oxygen reduction
on
PEDOT/Au/Goretex
cathode
Applying a potential sweep and measuring the current
Typical use: Redox activity on electrodes
Diffusion measurements (!)
Determining limiting reaction (surface or bulk)
De-convoluting capacity form resistance and catalysis
Solar-cell characterisation
With rotation disk
Determining reaction (number of electrons involved)
Electrochemical Measurements
Cyclic Voltametry (CV)
PART 3
Applying a potential sweep and measuring the current
Typical use: Redox activity on electrodes
Electrochemical Measurements
Sweep- and Cyclic Voltametry (CV)
PART 3
V
I
Resistor R
I = 1/R*V
Applying a potential sweep and measuring the current
Typical use: Redox activity on electrodes
Electrochemical Measurements
Sweep- and Cyclic Voltametry (CV)
PART 3
V
I
Capacitor C
Q = C × V
I x t =C x V
I = C x V/t
Applying a potential sweep and measuring the current
Typical use: Redox activity on electrodes (e.g. Fe2+ � Fe3++ e-)
Electrochemical Measurements
Sweep- and Cyclic Voltametry (CV)
PART 3
Why this current response ?
Applying a potential sweep and measuring the current
Typical use: Redox activity on electrodes
Electrochemical Measurements
Sweep- and Cyclic Voltametry (CV)
PART 3
Applying a potential sweep and measuring the current
Typical use: Redox activity on electrodes
Electrochemical Measurements
Sweep- and Cyclic Voltametry (CV)
PART 3
Why the peak
current ?
Electrochemistry 1.0
• This was just the fast introduction !
• USE IT in the rest of the course !
• Next week: Materials for Fuel-Cells
Applying potential to an electrode- in acidic water containing ions,
constant current.
V vs SHE
Time
> 1.23V (?)
Water splitting
Oxidation: H2O → O2(g) + 4 H+ + 4e–
Reduction: 4H+ + 4e– → 2H2(g)
How much product are we getting ?