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Electrochemistry Kinetics Flashcards

1
Q

What is Overpotential in fuel cells?

A

The overptential (Greek eta) of a cell is defined as the difference between the actual potential and the reversible potential E_N, the Nernst potential.

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2
Q

What are the different overpentials called?

A

Activation overpotential
Ohmic overpotential
Concentration overpotential

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3
Q

What is the activation overpotential (eta_A)?

A

The first relatively steep drop in the polarisation curve under load is due to the activation overpotential. The potential difference between the reversible potential and the onset of a “visible current” is referred to as activation overpotential. This comes about from the fact that the rate of charge transfer at the electrochemical interface is finite – limited. It can be overcome by a higher voltage difference to the reversible potential – a strongly nonlinear kinetic mechanism, which can be understood as a resistance sets in when a current starts to flow.

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4
Q

What is the ohmic overpotential (eta_ohm)?

A

The second linear decay of the polarisation curve under load is addressed as ohmic overpotential originating from ohmic losses in the cell or stack materials. This is caused by the materials’ resistance and it is ohmic, hence linear. When the linear curve is extrapolated to the Y-axis the difference in voltage of the interception point and the Nernst potential provides the activation potential. The slope of the ohmic overpotential in the E-I curve represents the resistance of the cell, which is determined by specific resistances of the materials and the materials’ thickness.

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5
Q

What is the concentration overpotential (eta_c)?

A

The steep drop at high current densities is due to insufficient supply of fuel gas or atmospheric oxygen and is therefore called concentration overpotential. The designation should indicate that the low concentration of the gases involved is the cause of the overvoltage.

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6
Q

Where is the working area of a cell described in a polarisation curve under load?

A

The working area shown essentially moves in the ohmic range, i.e., with fully formed activation overvoltage and before the onset of concentration overvoltage.

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7
Q

What is the problem with operating a cell under activation or concentration overpotential?

A

Operating a cell under such “starving” conditions may lead to irreversible cell damage, whereby degree and damage mechanisms differ depending on the cell type.

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8
Q

At what voltages is electrical energy generated?

A

The voltage range from zero Volt to the cell voltage (E_Z) generates electrical energy. The voltage range between the cell voltage and the heating voltage represents losses due to resistances, which lead to heat production in the cell.

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9
Q

What is the formula for the total Energy lost in a FC?

A

W_loss,total = nF(E°_H - E_Z)

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10
Q

What is the formula for the electrical energy produced in a FC?

A

W_el = nFE_Z

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11
Q

What is the formula for the efficiency (eta_cell) of a cell?

A

Eta_cell = E_Z/E°_H

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12
Q

What is activation energy (W_eta) and how is it connected to the activation overpotential?

A

It is the minimum quantity of energy which the reacting species must possess in order to undergo a specified reaction.
W_eta = (Int.) eta_A*I dt

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13
Q

What does the activation energy (E_a) represent?

A

It cannot be used to generate electricity and represents a heat loss.

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14
Q

What is the role of catalysts in the reaction?

A

The activation energy and dG_R are separate and the catalyst only affects the former. The E_a can be though of as a hump and the role of catalysts is to find a reaction pathway through the “energetic hump” and not over it. This is done by intermediary reactions through interaction with the catalytic surface, which would not occur without that surface. The catalyst has no impact on the start and end point of the reaction since these are thermodynamically determined. The catalyst also remains in the same state, other than degradation effects.

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15
Q

What is the relation between reaction kinetics and temperature?

A

Reaction kinetics is thermally activated, hence it (exponentially) increases with increasing temperature. This is notable between fuel cell types, within a fuel cell type and even for a certain stack design under varying operating conditions. The problem with high temperatures is the materials‘ limitations and heat losses.

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16
Q

What is the difference between Nernst potential and Open Circuit Voltage (OCV)

A

Nernst potential is thermodynamic, OCV is electrical and can be measured.

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17
Q

What are the reasons for the deviation of the OCV from the Nernst potential?

A
  • Internal short circuits can originate from electronic conductivity through the electrolyte.
  • Gas crossover (permeation or diffusion) of a reactant through the membrane with subsequent reaction at the counter electrode can occur
  • Significant leakage of a gas to the outside, e.g. reduction of partial pressure in a gas chamber can occur
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18
Q

Why can there be different Nernst potentials along the gas channel Doping on the location?

A

Due to the drop in partial pressure of the gases owing to the consumption of the reacting gases along the channel.

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19
Q

Why must the concentration of a reacting gas be prevented from getting too low and why does it happen?

A

If it gets to low, concentration polarisation sets in, which is perilous for the fuel cell longevity. This can occur due to the reactant reacting with the electrodes.

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20
Q

What is reaction overpotential?

A

It may occur owing to sluggish reaction rates and can – other than the activation potential – occur over the whole range of operating current densities.
It particularly occurs with more complicated multi-step reactions. Like in direct methanol fuel cells or even more in direct ethanol fuel cells.

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21
Q

What is the reason behind activation overpotential?

A

• Limited velocity of transport at the phase boundary
• Occurs in every electrochemical reaction.

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22
Q

What is the reason behind ohmic overpotential?

A

Material conductivity.

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23
Q

What is the reason behind concentration overpotential?

A

Slow mass transport or absence of reactants at the electrodes

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24
Q

What is the reason behind reaction overpotential?

A

Insufficient rate of chemical reactions

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25
Q

What are the influencing factors in activation overpotential?

A

• Reactants
• Electrolyte
• Electrodes
• Temperature

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26
Q

What are the influencing factors in ohmic overpotential?

A

Ohmic resistances - Electrolyte, electrodes, interconnectors (BiP), if applicable sealing.

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27
Q

What are the influencing factors in concentration overpotential?

A

• Too low reactants concentration
• Too low porosities of electrodes
• Too high current density

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28
Q

What are the influencing factors in reaction overpotential?

A

• Electrodes
• Temperature

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29
Q

What are the possible technical solutions for activation overpotential?

A

• Higher operation temperature
• Electrode material
• Electrode structure.

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30
Q

What are the possible technical solutions for ohmic overpotential?

A

• Materials with higher conductivity
• Higher temperature (ceramics, ionic conductor, molten salt).

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31
Q

What are the possible technical solutions for concentration overpotential?

A

• Higher porosity or thinner electrodes
• Lower current density to avoid reactant shortage
• Lower depletion of the fuel gas.

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32
Q

What are the possible technical solutions for reaction overpotential?

A

• Electrode material and structure
• Higher operation temperature

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33
Q

What is the regime of activation overpotential?

A

• Low current density
• Non linear with current density.

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34
Q

What is the regime of ohmic overpotential?

A

• Average current density
• Linear with current density

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35
Q

What is the regime of concentration overpotential?

A

• High current density
• Non linear with current density

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36
Q

What is the regime of reaction overpotential?

A

• All current densities
• Non linear with current density.

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37
Q

What is the formula for Area Specific Resistance ASR?

A

𝐴𝑆𝑅 = 𝜌 ⋅ 𝑙 = 𝑅 ⋅ 𝐴 = Δ𝐸/Δj in (Ω*𝑐𝑚^2)
Where j is the current density [mA/cm^2]
ASR can be found as the slope of the curve if j is x axis instead of I

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38
Q

What is the type of resistance in anode and cathode?

A

Metallic resistance

39
Q

What is the type of resistance in Bipolar plate?

A

Metallic resistance

40
Q

What is the type of resistance between IC and Anode and Cathode?

A

Ohmic contact resistance

41
Q

What is the type of resistance in electrolyte?

A

Ionic resistance

42
Q

What is the resistance between the electrolyte and the electrodes?

A

R_üohm + non-ohmic polarisation

43
Q

What are the largest resistances in a FC?

A

Ohmic in electrolyte and then polarisation

44
Q

What is Area Specific resistance?

A

The area specific resistance represents a technical measure of the resistance of the stack components. In this size, the thickness of the components is taken into account according to the above formula. By specifying the ASR, different stack concepts can be compared with regard to their ohmic resistance.

45
Q

What are some reasons for surface resistances?

A
  • sometimes very thin – oxidation layers
  • too little contact pressures that the surfaces of the different components of a stack are not in completely in touch with each other
  • as a consequence from deviations of the designed shapes of the components -mostly from a deviation from an ideal flat shape.
46
Q

What are the solutions to the surface resistances?

A

To reduce these resistances in fuel cell stacks materials forming no or well conducting oxidation scales and coatings can be used. Pressure to get the components’ surfaces in close touch or flexible layers like felts or woven sheets of materials like carbon or nickel, depending on the fuel cell types may apply.

47
Q

What is the reaction at anode during standby mode?

A

H2 ⇌ 2H+ + 2e-

48
Q

What is the reaction at cathode during standby mode?

A

1⁄2O2 + 2H+ +2e- ⇌ H2O

49
Q

What are the characteristics of standby mode?

A

• Both half-reactions occur in both sides continuously.
• Forward and backward reactions are at thermodynamic equilibrium.
• Net current is equal to zero.
• The intensity of the reactions depend on the quality of catalyst and its layer.

50
Q

What is the potential drop directly at the electrode in relation with the electrical bilayer called?

A

Surface potential

51
Q

What is the region of almost constant potential that corresponds to the Helmholtz layer in relation with the electrical bilayer called?

A

Volta potential

52
Q

What is the potential in the electrode in relation with the electrical bilayer called?

A

Galvanic potential

53
Q

What is j_0?

A

j0 is in the exchange current density, which cannot be measured directly, but can be determined from the course of the current against the overpotential.

54
Q

How does j_0 relate with other parameters?

A

• j_0 = f (concentration of oxidizing and reducing species, temperature and type of electrode material)
• j_0 ~ the active surface of the three-phase boundary; this is a measure of the quality of the catalyst layer
• j_0 ~ the reaction constant; this is a measure of the quality of the catalyst material
• The low j_0 is, the better is the performance of the electrode

55
Q

How to improve j_0?

A

•Optimize catalyst
- More catalyst (helps to to a certain level)
- Higher specific area
- More active composition
- Improved surface structure
• Temperature increase

56
Q

Give the Tafel equation.

A

j = j_0exp(eta_A/b)
Log(j) = log(j_0) + (eta_A/b)
Where 1/b is ((alphazF)/(RT))
j_0 is the exchange current density at standby mode
Eta_A = phi - phi_0 where phi_0 is the potential at standby mode and phi is the one under current flow
Moreover the Tafel equation is only for high activation potentials, otherwise the Butler-Volmer equation is used

57
Q

What is the relation between the concentration c and j?

A

When c=0, j=0

58
Q

What is standby mode?

A

No current flow

59
Q

What is the transfer coefficient alpha?

A

Transfer coefficient indicates the impact of overpotential on the anode and cathode half- reactions.

60
Q

What does alpha tell about the steepness of the curve?

A

Alpha = 0,5 ; anodic and cathodic branches are symmetrical
Alpha > 0,5 ; anodic branch steeper than cathodic branch
Alpha < 0,5 ; cathodic branch steeper than anodic branch

61
Q

What is the three-phase boundary?

A

The three-phase boundary is especially important for the good functioning of the electrode layer. In the three-phase boundary, the three reactants come together. For example the oxygen in the gas phase reacts precisely where electronic conductivity is provided by the electrode and ionic conductivity by the electrolyte.

62
Q

What is the problem with the three-phase boundary?

A

The area is infinitesimally small. Thus, high current peaks would occur in such a range, which would also lead to high local temperatures and damage the structure.

63
Q

What is a possible solution to the three-phase boundary being very small?

A

The three-phase boundary is stretched as far as possible. Such an extended three- phase boundary can be achieved by making the electrode mixed. In the case of the SOFC, this is achieved by doping the electrode. Now, the reaction cannot only take place at the true three-phase boundary, but also away from it on the surface of the electrode.

64
Q

What is the problem with doping the electrolyte in relation with three-phase boundary?

A

It has also been attempted to make the electrolyte mixed-conductive at the interface by means of doping. This has, in principle, the same effect, but has not been successful, as the doping components diffused too deeply into the electrolyte and have made it mixed conductive. Although a mixed conductivity at the phase boundary is desirable, in the electrolyte itself it leads to an electronic shunt and so adversely affects the function of the electrolyte.

65
Q

What are the essential steps in the process of oxygen reduction in an electrode?

A
  1. Ion transport
  2. Adsorption / Desorption (Equilibrium) 3. Dissociation (O2 → 2 {O})
  3. Reduction ( {O} + 2e- → O2-)
  4. Crystal lattice formation
  5. Ion transport
66
Q

How does the three-phase boundary work with liquid electrolytes?

A

At the liquid level, which forms at the edge of the electrodes with the liquid, the electrolyte (solution) is applied. It wets the electrode material with a thin layer through which the gas phase can easily diffuse to the electrode.

67
Q

Why is the PEFC the most complicated case of the three-phase boundary?

A

There are no sufficiently mixed conductive electrolyte materials and no mixed conductive electrode materials.

68
Q

What is the solution to the three-phase boundary problem in PEFC?

A

A mixture of the different materials in as fine a form as possible must be resorted to, in which the electrolyte is introduced into threads in the electrode structure. At the same time, the electrode consists of carbon particles (soot), which is provided with the finest platinum particles (2-4 nm). The actual reaction can only take place at those points where there is a platinum particle and soot bound to the electrolyte and the carbon black. All platinum particles that do not have these contact points are inactive and thus lead to the electrode having higher costs without further uses.

69
Q

How is the gas provided in the PEFC cell?

A

It is provided by the porosity of the electrode. Through these pores, the gaseous reactants diffuse.

70
Q

What must be done to ensure the diffusion of gases in PEFC cell through the electrode pores?

A

In order to ensure diffusion, parts of the pores must be rendered hydrophobic to prevent water from entering and “clogging” them. The hydrophobization is usually carried out with PTFE (teflon). Especially important is the hydrophobization of the cathode, as the water is produced there.

71
Q

What must be done to ensure the diffusion of gases through electrode pores in PEFC?

A

In order to ensure diffusion, parts of the pores must be rendered hydrophobic to prevent water from entering and “clogging” them. The hydrophobization is usually carried out with PTFE (teflon). Especially important is the hydrophobization of the cathode, as the water is produced there.

72
Q

For an SOFC (e.g.), why is the cathode towards the outside coarser and finer towards the electrolyte?

A

In the functional layer (towards electrolyte), it is important that the highest possible internal surface be created. This is only possible with a highly fine-porous structure. The actual cathode layer, on the other hand, must also provide for gas transport by means of its porosity. Therefore, a higher porosity and coarser structure is required.

73
Q

What are the essential characteristics for the design of the three-phase boundary?

A

Large three-phase interface
• Extremely fine microstructure
• Sufficient porosity
• Use of mixtures
• Mixture of ionic conductors and electron conductors

High catalytic activity
• Precious metals (Pt, PtRu, Raney-Nickel, Ni, Perovskite)
• Selection and deposition technique on carrier substances

Aqueous systems
• Mixture of hydrophobic and hydrophilic substances so that pores contain partly liquid phase and partly gas phase

74
Q

What is sheet/surface resistance?

A

The sheet resistance characterizes the resistance of a real cell or real stack. With sheet resistance, the height of the components, in contrast to the resistivity, is included. Such a definition is not only useful if the material characteristics are to be compared, but also the design aspect should be taken into consideration. With the specific conductivity, the length (i.e., stack height, cell thickness) is multiplied. The current density is not included.

75
Q

Formula for specific resistance rho?

A

Rho = (RA)/L (ohmm, ohm*cm)

76
Q

Formula for specific conductivity small sigma

A

Small sigma= 1/rho (1/(ohmcm), 1/(ohmm))

77
Q

Formula for specific conductivity (small sigma) of species i

A

Small sigma_i = z_imu_ic_i*e
Where z_i= valence electron, mu_i = Lańdungsträgerbeweglichkeit, c= Ladungsträgerkonzentration, e= elementarladung

78
Q

Formula for current density (j) of species i in the electron field E

A

J_i= — small sigma_i*E

79
Q

Formula for total specific conductivity of a material

A

Add all small sigmas of species

80
Q

Formula for transfer number of species i

A

t_i = (small sigma_i)/ (small sigma for the whole material)

81
Q

Formula for sheet/ surface resistance

A

ASR = rhol=RA (ohm*cm^2)

82
Q

What is the relation between temperature and the polarisation curve?

A

As the temperature decreases, overvoltages and internal resistance increase. Therefore, the curves are worse. Thermodynamically, the open cell voltages would have to increase with decreasing temperature, but this is usually not visible on real characteristics, as the drop is too strong due to the activation polarization.

83
Q

What is energy and anergy?

A

Exergy is defined as the maximum amount of work that can be produced by a stream or system. Anergy is the part of an energy which cannot be converted into work.

84
Q

What can be done with the waste heat produced by FC?

A

Heat can be both exergy and anergy, depending on the temperature of the waste heat. In particular, in downstream gas and steam turbine processes, the waste heat of the fuel cell can be further converted into exergy in the form of electricity.

85
Q

How to find the operating point/ power point of the FC?

A

The performance of the fuel cell is described by P_FC = U_KI. The power of the load is described by P_v = I^2R_a. This determines the power point of the fuel cell and results in IR_V = U_K. This power point is the intersection of the power curve of the fuel cell with that of the load.
U_K is the clamping voltage and is = U_N - IR_i (where IR_i can be the total polarisation).
Since I= U_K/R_V, another equation for U_K after substituting is U_K= (U_N)/(1+(R_i/R_V)) = U_Neta (efficiency).
U_N the Nernst voltage is conveyed by the gas composition.

86
Q

What is the operating principle of a current controller?

A

It absorbs the given power and releases it (with losses) with a different voltage level.

87
Q

Why is a current controller needed?

A

There are two current-/voltage operation points resulting in the same power output. The difference between these two points is the lower efficiency when the current is higher due to higher polarisation.

88
Q

Formula for average current density.

A

J = I/A with a fixed current

89
Q

What is the local current density?

A

The current density of the fuel cell is locally dependent. As fuel gas is consumed and thus the composition of the fuel gas changes as it flows across the cell, the Nernst voltage also changes locally across the cell. As the channel length increases, the Nernst voltage decreases and the curve becomes steeper. This sets both of the different clamping voltages, as well as the different current densities, during operation.

90
Q

Formula for local current density in a fuel cell with equivalent electrodes (cell voltage stays the same)

A

J(x) = ((E_N(x))/ASR) - (U_z/ASR) where x is the length in the fuel cell
The second part can be taken as a constant since ASR is assumed to be a constant factor, but only for linear polarisation curves)

91
Q

What is the relation between Nernst voltage and local current density?

A

The higher the Nernst voltage, the higher the local current density.

92
Q

What is the reason for cross current in stacks?

A

Modeling has shown that cross-flows do not occur in principle, except when the fuel gas is depleted (inadequate flow, clogged). These cross-currents occur when the local Nernst voltage becomes smaller than the clamping voltage. In extreme cases, the cell can even be reversed, with local electrolysis then taking place in the cell. The condition for this is that the Nernst voltage plus the electrolysis overvoltages are smaller than the clamp voltage.

93
Q

Why must cross currents be avoided?

A

This process must be avoided at all costs, as it not only leads to efficiency losses due to high cross-currents, but also to irreversible damage in most cases.

GTB (14 decks)

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