22
INTRODUCTIONINTRODUCTION
Each electrochemical reaction event results in the transfer of one or more electrons, the current produced by a fuel cell (number of electrons per time) depends on the rate of the electrochemical reaction (number of reactions per time).
Increasing the rate of the electrochemical reaction is therefore crucial to improve fuel cell performance.
- Catalysis;
- Electrode design….
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INTRODUCTIONINTRODUCTION
Electrochemical processes are heterogeneous.
Electrochemical reactions, like the HOR:
−+ +↔ eHH 222
take place at the interface between an electrode and an electrolyte.
44
INTRODUCTIONINTRODUCTION
Current expressed the rate of charge transfer
dt
dN
[ ]Adt
dNnF
dt
dQi ==
= the rate of the electrochemical reaction (mol/s)
Charge is the total amount of electricity produced
nFNQdti
t
==∫0
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INTRODUCTIONINTRODUCTION
Because electrochemical reactions only occur at interfaces, the current produced is usually directly proportional to the area at the interface. Therefore, current density (current per unit area) is more fundamental than current.
[ ]2cmA
A
ij =
The rate of the electrochemical reaction per unit area:
[ ]211 −−=== cmsmolnF
j
AnF
i
dt
dN
Av
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INTRODUCTIONINTRODUCTION
� ACTIVATION ENERGY
In order for reactants to be converted into products, they must first make it over the activation energy.
The probability that reactant species can make over this barrier determines the rate at which the reaction occurs.
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ACTIVATION ENERGYACTIVATION ENERGY
1) Mass transport of H2 gas to the electrode:
( ))(2)(2 electrodenearbulk HH →
−+ +↔ eHH 222
( )2)(2 HMMH electrodenear K→+
2) Adsorption of H2 onto the electrode surface:
3) Separation of the H2 molecule into two individually bound (chemisorbed) hydrogen atoms on the electrode surface:
( )HMMHM KK 22 →+
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ACTIVATION ENERGYACTIVATION ENERGY
4) Transfer of electrons from the chemisorbed hydrogen atoms to the electrode, releasing H+ ions into the electrolyte
( ) ( )[ ]+− ++→× electrodenearHeMHM K2
( ) ( )[ ]++ →× eelectrolytbulkelectrodenear HH2
5) Mass transport of H+ ions away from the electrode :
The overall reaction rate will be limited by the slowest step in the series.
Suppose that the overall reaction is limited by the electron transfer step between chemisorbed hydrogen and the metal electrode surface.
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ACTIVATION ENERGYACTIVATION ENERGY
1010
ACTIVATION ENERGYACTIVATION ENERGY
The slowest step can be represented as:
( ) +− ++↔ HeMHM K
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ACTIVATION ENERGYACTIVATION ENERGY
Curve 1: free energy of the reactant state as a function of the distance separation between the H atom and the metal surface
Curve 2: free energy of the product state as a function of the distance the H+ ion and the metal surface
Dark line = the minimum energy path for the conversion of [M…H] to [(M + e-) + H+]
a = the activated state
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ACTIVATION ENERGYACTIVATION ENERGY
Only species in the activated state can undergo the transition from reactant to product.
The probability of finding species in the activated state is exponentially dependent on the size of the activation barrier.
RTG
act eP*1∆−
=
The reaction rate in the forward direction (reactantsproducts)
( )RTG
R efcv*1
1
*
1
∆−=
cR* = the reactant surface concentration (mol/cm2)
f1 = the decay rate to products
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ACTIVATION ENERGYACTIVATION ENERGY
The net rate is given by the difference in rates between the forward and reverse reactions.
( )( ) +−
+−
++←
++→
HeMHM
HeMHM
K
K
The net reaction rate v is defined as:
*
2
*
1
2
*
1
*
21
*2
*1
GGG
efcefcv
vvv
rxn
RTG
P
RTG
R
∆−∆=∆
−=
−=
∆−∆−
� NET RATE OF A REACTION
Forward reaction
Reverse reaction
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ACTIVATION ENERGYACTIVATION ENERGY
The net rate of a reaction is given by the difference in rates between the forward and reverse reactions, both of which are exponentially dependent on an activation barrier, ∆G1
*.
( ) RTGG
P
RTG
Rrxnefcefcv
∆−∆−∆−−=
*1
*1
2
*
1
*
� NET RATE OF A REACTION
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ACTIVATION ENERGYACTIVATION ENERGY
( ) RTGG
P
RTG
R
rxnefcFnj
efcFnj
∆−∆−
∆−
=
=*1
*1
2
*
2
1
*
1
� EXCHANGE CURRENT DENSITY
At thermodynamic equilibrium, the forward and reverse current density must balance, there is no net current density (j=0).
021 jjj ==
j0 = exchange current density
Although at equilibrium the net reaction is zero, both forward and reverse reactions are taking place at a rate which is characterized by j0.
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ACTIVATION ENERGYACTIVATION ENERGY
� GALVANI POTENTIAL
Before the build-up of the interfacial potential (∆Φ), the forward rate was much faster than the reverse rate.
The build-up of an interfacial potential equalises the situation by increasing the forward activation barrier from ∆G1
* to ∆G* while decreasing the reverse reaction barrier from ∆G2
*to
∆G*.
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ACTIVATION ENERGYACTIVATION ENERGY
� BUTLER-VOLMER EQUATION
Electrochemical reactions = ability to manipulate the size of the activation barrier by varying the cell potential
If the Galvani potential across a reaction interface is reduced, the free energy of the forward reaction will be favoured over the reverse reaction. While the chemical energy system is the same as before, changing the electrical potential (b) upsets the balance between the forward and reverse activation barriers.
Reducing the Galvani potential by η reduces the forward activation barrier (∆G1
*<∆G*) and increases the reverse activation barrier (∆G2
*>∆G*).
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ACTIVATION ENERGYACTIVATION ENERGY
� BUTLER-VOLMER EQUATION
The forward activation barrier isdecreased by αnFη while the reverse activation barrier isincreased by (1-α)nFη.
The value of α depends on the symmetry of the activation barrier called the transfercoefficient.
For most electrochemicalreactions, α ranges from about 0,2 to 0,5.
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ACTIVATION ENERGYACTIVATION ENERGY
� BUTLER-VOLMER EQUATION
( )
( ) ( )
( ) ( ) ( )( )RTnFRTnF
RTnF
RTnF
eejj
ejj
ejj
ηαηα
ηα
ηα
−−
−−
−=
=
=
1
0
1
02
01
Butler-Volmer Equation
The current produced by an electrochemical reaction increases exponentially with activation overvoltage.
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ACTIVATION ENERGYACTIVATION ENERGY
� BUTLER-VOLMER EQUATION
Activation overvoltage ηact = voltage which is sacrificed (lost) to overcome the activation barrier associated with electrochemical reaction
The Butler-Volmerequation tells us that if you want more electricity (current) from our fuel cell, we must pay a price in terms of lost voltage.
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ACTIVATION ENERGYACTIVATION ENERGY
� BUTLER-VOLMER EQUATION
Having a high j0 is absolutely critical to good fuel cell performance. They are several ways to increase j0.
2222
HOW TO IMPROVE KINETIC PERFORMANCEHOW TO IMPROVE KINETIC PERFORMANCE
RTG
R efcFnj*1
1
*
0
∆−=
We have four ways to increase j0:
- Increase the reactant concentration CR*
- Decrease the activation barrier ∆G1*
- Increase the temperature T
- Increase the number of possible reaction sites (increase the reaction interface roughness)
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HOW TO IMPROVE KINETIC PERFORMANCEHOW TO IMPROVE KINETIC PERFORMANCE
� INCREASE REACTANT CONCENTRATION
The thermodynamic benefit is minor, due to the logarithmic form of the Nernst Equation.
In contrast, the kinetic benefit is significant, with a linear impact.
Kinetic reactant concentration effects generally work against us for several reasons:
-most fuel cells use air instead of pure oxygen at the cathode
-reactant concentrations tend to decrease at fuel cell electrodes during high-current-density operation (mass transport) : further kinetic penalties
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HOW TO IMPROVE KINETIC PERFORMANCEHOW TO IMPROVE KINETIC PERFORMANCE
� DECREASE ACTIVATION BARRIER
Highly catalytic electrode dramatically increases j0.
A catalytic electrode lowers the activation barrier. The free-energy curves depend on the nature of the electrode metal.
For the case of the hydrogen charge transfer:
-If the [M…H] bond is too weak, it is difficult for hydrogen to bond to electrode surface and to transfer charge from the hydrogen to the electrode;
-If the [M…H] bond is too strong, the hydrogen bonds too well to the electrode surface and it is difficult to liberate H+.
The optimal compromise between bonding and reactivity occurs for intermediate-strengh [M…H] bonds: Pt, Pd, Ir and Rh.
2525
10-9AcidPlatinumOxygen
10-3
10-4
AcidAlkaline
PlatinumHydrogen
j0 (A cm-2)MediumCatalyst
electrode
HOW TO IMPROVE KINETIC PERFORMANCEHOW TO IMPROVE KINETIC PERFORMANCE
� DECREASE ACTIVATION BARRIER
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HOW TO IMPROVE KINETIC PERFORMANCEHOW TO IMPROVE KINETIC PERFORMANCE
� INCREASE TEMPERATURE
Like changing the activation barrier, changing temperature has an exponential effect on j0.
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HOW TO IMPROVE KINETIC PERFORMANCEHOW TO IMPROVE KINETIC PERFORMANCE
� INCREASE REACTION SITES
Increasing the number of available reaction sites per unit area
If an electrode surface is extremely rough, the true electrode surface area can be orders of magnitude larger than the geometric (smooth) electrode area and provides many more sites for reaction.
'
'
00A
Ajj =
j0’ = the intrinsic exchange current density of a perfectly
smooth electrode surface
= an intrinsic property of an electrode for a specific electrochemical reaction
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SIMPLIFIED ACTIVATION KINETICSSIMPLIFIED ACTIVATION KINETICS
� POLARISATION RESISTANCE
When ηact Is Very Small
ηact<15mV
A Taylor series expansion of the exponential terms can be performed with powers higher than 1 neglected.
( ) ( ) ( )( )RTnFRTnFeejj
ηαηα −−−= 1
0
RT
nFjj actη0=
The current and overvoltage are linearly related for small deviations from equilibrium and are independent of α.
jRjnFj
RTtact ==
0
η
Rt = Polarisation resistance
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SIMPLIFIED ACTIVATION KINETICSSIMPLIFIED ACTIVATION KINETICS
� TAFEL EQUATION
When ηact Is Very Large
ηact> 50-100 mV
The second exponential term in the Butler-Volmer equation becomes negligible. On other words, the forward-reaction direction dominates, corresponding to a completely irreversible reaction process.
( )
jba
jnF
RTj
nF
RT
ejj
act
act
RTnF act
log
lnln 0
0
+=
+−=
=
η
ααη
ηα
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SIMPLIFIED ACTIVATION KINETICSSIMPLIFIED ACTIVATION KINETICS
� TAFEL EQUATION
3131
DIFFERENT FUEL CELL REACTIONS PRODUCE DIFFERENT DIFFERENT FUEL CELL REACTIONS PRODUCE DIFFERENT
KINETICSKINETICS
The HOR kinetics are extremely fast, while the ORR kinetics are extremely slow. Completion of the ORR requires many individual steps and significant molecular reorganization.
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DIFFERENT FUEL CELL REACTIONS PRODUCE DIFFERENT DIFFERENT FUEL CELL REACTIONS PRODUCE DIFFERENT
KINETICSKINETICS
V
3333
DIFFERENT DIFFERENT
FUEL CELL FUEL CELL
REACTIONS REACTIONS
PRODUCE PRODUCE
DIFFERENT DIFFERENT
KINETICSKINETICS
HORHOR
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DIFFERENT DIFFERENT
FUEL CELL FUEL CELL
REACTIONS REACTIONS
PRODUCE PRODUCE
DIFFERENT DIFFERENT
KINETICSKINETICS
ORRORR
3535
CATALYSTCATALYST--ELECTRODE DESIGNELECTRODE DESIGN
- Maximize reaction surface areaMaximize reaction surface area, highly porous, nanostructuredelectrodes to achieve intimate contact between gas phases pores, the electrically conductive electrode, and the ion-conductive electrolyte.
Reaction sites : triple phase zones or triple phase boundaries (TPBs)
Reaction can only occur where the three important phases : electrolyte, gas, and electrically connected catalyst regions are in contact.
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CATALYSTCATALYST--ELECTRODE DESIGNELECTRODE DESIGN
-- Optimal catalyst material:Optimal catalyst material:
- High mechanical strength
- High electrical conductivity
- Low corrosion
- High porosity
- Ease of manufacturability
- High catalytic activity (high j0)
For PEMFC: platinum is currently the best known catalyst
For higher temperature fuel cells, nickel- or ceramic-based catalysts are often used.
3737
CATALYSTCATALYST--ELECTRODE DESIGNELECTRODE DESIGN
Gas diffusion layer