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Electrochemical CharacterizationDaniele Procaccio
Nernst Equations
Redox potentials (E0) are defined at standard
conditions (ex: 1M concentrations, 1 bar, 1 atm)
Nernst equation relates equilibrium potentials
at standard conditions to real system conditions
Takes into account changes in activites,
concentrations, partial pressures, etc..
Ex: OER depends on pOH, partial pressure of
O2, and water activity (a0)πΈππ = πΈ0 β
π π
4πΉlπ
ππ»πβ4
ππ2
πΈπππ‘ = πΈ0 +π π
4πΉlπ ππ»πβ
4 π π»22
Nernst Equations
Redox potentials (E0) are defined at standard
conditions (ex: 1M concentrations, 1 bar, 1 atm)
Nernst equation relates equilibrium potentials
at standard conditions to real system conditions
Takes into account changes in activites,
concentrations, partial pressures, etc..
Ex: OER depends on pOH, partial pressure of
O2, and water activity (a0)πΈππ = πΈ0 β
π π
4πΉlπ
ππ»πβ4
ππ2
πΈπππ‘ = πΈ0 +π π
4πΉlπ ππ»πβ
4 π π»22
Anode 4 π»πβ β 2 π»2π + π2(π) + 4 πβ
Cathode 4 π»2π + 4 πβ β 4 π»πβ + 2 π»2(π)
Three main contributions
Ohmic Drop
Mainly due to to the resistance of ion conduction and
electron transfer through the MEA structure. It increase
linearly with current following Ohmβs law.
Ξ·π = π’ ππ
Overpotentials
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0 0.2 0.4 0.6 0.8 1
Ove
rpo
ten
tia
ls( Ξ·
) /
V
Current density ( j ) / A cm-2
Ohmic
ππ = πΈ β π β logπ
π0β ππ + π β log 1 β
π
ππππ
Three main contributions
Mass transport
Caused by the transport of the reactant species from
the bulk solution toward the electrodes.
Ξ· π΄π» = π π₯π¨π π βπ
ππππ
Overpotentials
0
0.01
0.02
0.03
0.04
0.05
0.06
0 0.2 0.4 0.6 0.8 1
Ove
rpo
ten
tia
ls( Ξ·
) /
V
Current density ( j ) / A cm-2
Transport
ππ = πΈ β π β logπ
π0β ππ + π β log 1 β
π
ππππ
Three main contributions
Kinetic or Activation
Generally due to the electron-transfer from the
electrode to the reactant species.
Overpotentials
0
0.05
0.1
0.15
0.2
0.25
0 0.2 0.4 0.6 0.8 1
Ove
rpo
ten
tia
ls( Ξ·
) /
V
Current density ( j ) / A cm-2
Kinetic
ππ = πΈ β π β logπ
π0β ππ + π β log 1 β
π
ππππ
ππ = πΈ β π β logπ
π0β ππ + π β log 1 β
π
ππππ
1.50
1.55
1.60
1.65
1.70
1.75
1.80
1.85
1.90
1.95
0 200 400 600 800 1000
Ove
rpo
ten
tia
ls( Ξ·
) /
V
Current density ( j ) / A cm-2
Ohmic drop
Three main contributions shown in a polarization curve
ET MT
MTOhmic dropET
ππ = πΈ β π β logπ
π0β ππ + π β log 1 β
π
ππππ
1.50
1.55
1.60
1.65
1.70
1.75
1.80
1.85
1.90
1.95
0 200 400 600 800 1000
Ove
rpo
ten
tia
ls( Ξ·
) /
V
Current density ( j ) / A cm-2
Ohmic drop
Three main contributions shown in a polarization curve
ET MT
MTOhmic dropET
Butler-Volmer
j = π0 πΞ±ππΉπ π Ξ· β π 1βΞ±
ππΉπ π Ξ·
The model is valid for a one step process, with a single electron transfer
O + πβ β πΉ
Low overpotentials, so mass transfer phenomena can be
neglected
The electrode material itself is inert and it is not undergoing
any chemical reactions (Outer sphere mechanism)
j = π0 πΞ±ππΉπ π
Ξ· β π 1βΞ±ππΉπ π
Ξ·
j : current density [mA cm-2]
F : Faraday constant = 96485.3 [C moleβ1]
j0 : exchange current density [mA cm-2]
T : working temperature [K]
n : number of electrons exchanged [mole- mol-1]
π : overvoltage [V]
πΌ : transfer coefficient
R : Universal gas constant = 8.314 [J-1 K-1 mol-1]
Butler-Volmer equation
Bard, Allen; Faulkner, Larry (2000). Electrochemical Methods: Fundamentals and Applications. J. Wiley and Sons.
j = π0 πΞ±ππΉπ π
Ξ· β π 1βΞ±ππΉπ π
Ξ·
j : current density [mA cm-2]
F : Faraday constant = 96485.3 [C moleβ1]
j0 : exchange current density [mA cm-2]
T : working temperature [K]
n : number of electrons exchanged [mole- mol-1]
π : overvoltage [V]
πΌ : transfer coefficient
R : Universal gas constant = 8.314 [J-1 K-1 mol-1]
Butler-Volmer equation
Bard, Allen; Faulkner, Larry (2000). Electrochemical Methods: Fundamentals and Applications. J. Wiley and Sons.
j = π0 πΞ±ππΉπ π
Ξ· β π 1βΞ±ππΉπ π
Ξ·
j : current density [mA cm-2]
F : Faraday constant = 96485.3 [C moleβ1]
j0 : exchange current density [mA cm-2]
T : working temperature [K]
n : number of electrons exchanged [mole- mol-1]
π : overvoltage [V]
πΌ : transfer coefficient
R : Universal gas constant = 8.314 [J-1 K-1 mol-1]
Butler-Volmer equation
Bard, Allen; Faulkner, Larry (2000). Electrochemical Methods: Fundamentals and Applications. J. Wiley and Sons.
Tafel approximation
Tafel slope change in according to the reaction
mechanism
At large value of overpotentials (generally >120 mV)
only one part of the current dominate, so the other
one can be neglected
Ξ· = Β±π log10π
π0
Catalyst characterization
Rotating Disk Electrode (RDE)
β’ The rotation let to control the thickness of the limit diffusive layer toward the electrode surface, minimizing the Mass Transport contribution. Controlled Hydrodinamic conditions.
β’ The rotation acts as a pump, bringing the reactant species from the bulk solution towardthe electrode surface.
β’ Laminar flow over the eletrode surface
Catalyst characterization
Hydrogen Evolution Reaction (HER)
INK preparation:
HER Catalyst was suspended in H2O/EtOH and 1:100 anionic-exchange ionomer related to catalyst mass
Low loadings ( 5-15 Β΅g cm-2 ) were used with a GC TIP for RDE
Half cell: Pt counter electrode, Hg/HgO REF and H2 saturated solution 0.2 M KOH
Bio-logic VMP3 multichannel potentiostat with EIS
iR correction by 90% of Re(Z) at 100 kHz.
Techniques:
1) Cyclic Voltametry (CV) Record at different sweeprates in order to confirm reversibility of HER/HOR and calculate ElectroChemical Surface Area (ECSA) from hydride adsorption/desorption peaks.
β’ Reversible hydride adsorption/desorption peaks;
β’ Constant specific ECSA of ca. 10 m2/g Pt;
β’ Lower ECSA at higher catalyst loadings
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
-0.77 -0.67 -0.57 -0.47 -0.37
j / m
A c
m-2
E vs Hg/HgO / V
Benchmark Pt/C
* Area normalized for the GC electrode
Techniques:
1) Cyclic Voltametry (CV) Record at different sweeprates in order to confirm reversibility of HER/HOR and calculate ElectroChemical Surface Area (ECSA) from hydride adsorption/desorption peaks.
2) Staircase Voltametry (SV)RDE experiments at 400-2500 RPM, 180s per point on the following ranges:
a) 0.025Γ·0.025V vs RHE for Butler-Volmer;b) 0.5V for KouteckΓ½βLevich;c) -0.05Γ· -0.12V vs RHE for Tafelapproximation.
Benchmark Pt/C
y = -0.1234x - 0.0711RΒ² = 0.998
-0.12
-0.11
-0.1
-0.09
-0.08
-0.07
-0.06
-0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30
E R
HE
/ V
log j / mA cm-2
Main Issues:
Bubble formation toward the electrode that can lead to partial coverage of the active area
Anionic-exchange ionomer (instead of Nafion), in order to have a double-layer similar to the workingcell
Ionomer to catalyst ratio was selected as best trade-off between stability and ionomersegregation, leading to lower ECSA;
Low loadings of the catalyst layer led to reproducible ECSA and rotation-speed constantdata after mass-transport correction.
Bubble formation on GC with catalyst loading > 40 Β΅g cm-2
Several studies of HER on Pt in acid using RDE report Tafel slopes around 30 mV/dec and exchange current densities, which are almost two orders of magnitude lower than the results obtained both from other techniques that removes the mass transport contributions. (i.e H2 pump or UME).
Mass transport overvoltages usually not corrected for HER in alkaline due to the βsluggishβ kineticscompared to the acidic media;
Two approaches have been compared:
1) Tafel approximation at high overvoltages;
2) KouteckΓ½-Levich correction for HOR:
πβπ = ππβπ + π©πβπ.π
*Levich, V. G. (1962). Physical Chemical Hydrodynamics. N.J.:Prentice-Hall.
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
-0.03 -0.02 -0.01 0.00 0.01 0.02 0.03
Log
j /
mA
E vs RHE / V
Uncorrected
Irreversiblecorrected
Comparable results exchange current densities Tafel and KouteckΓ½ Levich correction;
Both approaches led to values closer to the H2 pump experiments* due to the sluggish kinetic of Pt in alkaline media, compared to acidic media.
Analysis has been repeated for the HER using Tafel approximation at different temperature deriving the Ea from an Arrhenius Plot: 43.77 kJ mol-1
This data will be used for estimate the HER overvoltage at operating temperature and catalystloading in the single cell.
*J. Electrochem. Soc. 2015 volume 162, issue 14, F1470-F1481
y = -4.7899x + 12.728RΒ² = 0.9225
-5.00
-4.50
-4.00
-3.50
-3.00
-2.50
3.2 3.25 3.3 3.35 3.4 3.45 3.5 3.55 3.6
ln (
ma
ss a
ctiv
ity)
/ A
mg
-1ca
t
1000 / T
Ξ± Tafel slope / mv dec-1 j0 / mA cm-2
Tafel Approximation - 130 3.34
Irreversible correction HOR 0.56 131 3.20
Catalyst characterization
Oxygen Evolution Reaction (OER)
Ink preparation:
Ink prepared using Enapter OER catalyst suspended in EtOH/H2O and 1:500 anionic-exchange ionomer to catalyst ratio;
Low loadings (5-40 Β΅g/cm2) were deposited on a GC RDE tip;
Half cell: O2 saturated 0.2M KOH solution at room temperature and 35Β°C, using graphite rod and Hg/HgO ascounter and reference electrodes. Bio-logic VMP3 multichannel potentiostat with EIS;
iR correction by 90% of Re(Z) at 100 kHz.
Techniques:
1) ChronoAmperometry (CA) - Activation at 0.7 V vs Hg/HgO until stable current (30β-1h)
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1 2 3 4 5 6 7 8 9 10 11 12 13
j / m
A c
m-2
Time / min
Techniques:
1) ChronoAmperometry (CA) - Activation at 0.7 V vs Hg/HgO until stable current (30β-1h)
2) Staircase Voltammetry (SV)
RDE experiments at 1600 RPM, 180s per point between1.33Γ·1.42 V vs RHE for Tafel approximation.
y = 0.0409x + 1.5086RΒ² = 0.9983
1.37
1.38
1.39
1.40
1.41
1.42
1.43
-3.0 -2.9 -2.8 -2.7 -2.6 -2.5 -2.4 -2.3 -2.2 -2.1
E vs
RH
E /
V
Log|j| / mA cm-2
Techniques:
1) ChronoAmperometry (CA) - Activation at 0.7 V vs Hg/HgO until stable current (30β-1h)
2) Staircase Voltammetry (SV)
RDE experiments at 1600 RPM, 180s per point between1.33Γ·1.42 V vs RHE for Tafel approximation.
y = 0.0409x + 1.5086RΒ² = 0.9983
1.37
1.38
1.39
1.40
1.41
1.42
1.43
-3.0 -2.9 -2.8 -2.7 -2.6 -2.5 -2.4 -2.3 -2.2 -2.1
E vs
RH
E /
V
Log|j| / mA cm-2
Main Issues:
Similar to the HER (low loading, anionic-exchange ionomer, optimization Ionomer/catalyst ratio);
Catalyst βactivationβ at 0.7 V vs Hg/HgO required to obtain stable data.
* No variation changing catalyst loading or rotation speed
At potential higher than 1.39V, Tafel slope for OER of ca. 41 mV/dec*;
Exchange current density (j0) proportional to catalyst loading;
y = -17.757x + 47.407RΒ² = 0.979
-18
-16
-14
-12
-10
-8
3.20 3.25 3.30 3.35 3.40 3.45 3.50 3.55 3.60 3.65
ln (
ma
ss a
ctiv
ity)
/ A
mg
-1ca
t
1000 / T
Different test performed using catalyst loading 5-40 Β΅g cm-2
and different temperature between 5-40 Β°C
Mass activity @ 25Β°C 7.071 E-05 A mg-1
J0 @25 Β°C 4.124 E-04 mA cm-2
Activation Energy 147.64 KJ mol-1
This data will be used for estimate the OER overvoltageat operating temperature and catalyst loading in the single cell.
Thank you for your kind attention!