PEM Fuel Cell Loss Mechanism

Activation Overpotential

In electrochemical systems, e.g. the oxygen reduction reaction on the cathode of a fuel cell, it is convenient to consider the limiting behavior of the Butler-Volmer equation (see Eq. 2.9) for large negative overpotential

j =−i₀exp

and in case for a slow oxidation reaction, the limiting behavior for large positive over-potential

2.2 Basic Principle of Electrochemistry in a PEM Fuel Cell

These relationships are often written in form of theTafel equation η = −RTln(10)

In case of a kinetic controlled reaction the polarization loss of the electrode under load is called activation overpotentialη =η_act.

Concentration Overpotential

The derivation of the Tafel approximation from the limiting behavior of the Butler-Volmer equation (Eqs. 2.17 and 2.18) was done with the assumption of a constant exchange current density (kinetic controlled), which implies a constant concentration of the educts c0,j. In general, a decline in concentration cj with increasing current occur due to mass transport limitation. To take this into account, Eqs. 2.17 and 2.18 can be written as

A comparison of Eq. 2.22 with Eq. 2.19 leads to the expression of the concentration overpotential

The intrinsic redox reaction can be hindered by chemical steps that must occur before the electron transfer reaction and thus determine the reaction rate. Such systems are known as ’preceding chemical reactions’

A ↔ Ox , (2.24)

Ox+ne⁻ ↔ Red . (2.25)

2 Fundamentals of a Polymer Electrolyte Membrane Fuel Cell

It is also possible that the product of a redox reaction has to react in a second chemi-cal step to clean the active sites, which is known as ’following chemichemi-cal reactions’

Ox+ne⁻ ↔ Red , (2.26)

A+Red ↔ P . (2.27)

If this first (reaction 2.24) and second step (reaction 2.27), respectively, is slow, it would determine the overall reaction rate. For a detail description of such systems, it is referred to Chapter 5, where the problematic is discussed in detail for a operating DMFC. These performance losses can be concluded as reaction overpotential η_r [2, 4].

Ohmic Overvoltage

Due to the finite conductivity of the fuel cell components (current collector, GDLs, CLs and membrane), as well as the non-negligible contact resistances between the layers, ohmic losses at high current densities has a strong attribute on fuel cell losses. These losses are called ohmic overvoltage and can be expressed by Ohm’s law

η_Ω =j R_Ω , (2.28)

whereRΩ is the specific resistance in (Ωcm²).

Crossover and Mixed Potentials

Due to the finite leak-tightness of the polymer membrane for the electrochemical ac-tive species, it is possible that passed oxidant reacts on the anode side and fuel on the cathode side, respectively. This leads to internal parasitic current that suppresses the electrode potential despite a nominal zero charge flux at the external circuit. This phenomenon is called mixed potential and is discussed in detail in Chapter 5.

Cell Voltage Under Load

Summarizing all loss mechanisms the cell voltage under load can be calculated as follows:

Figure 2.6 illustrates the potential distribution along a cross-section of a fuel cell.

Ohmic losses due to finite conductivity of the layers are visible by the potential

gra-2.2 Basic Principle of Electrochemistry in a PEM Fuel Cell

dients and the contact resistances by the steps between the interfaces. Activation, reaction and concentration overpotentials are integral parts of the reduced Galvani potential in the electrodes∆Φ_a/c. The cell voltage is the difference betweenΦ^e_c at the outer surface of the cathode bipolar plate andΦ^e_aon the anode bipolar plate. The pro-tonic potential is not measurable. Its value is determined by the cell current in such a way that the resulting overpotentials on both electrodes generate the same current which in term links the cathodic and anodic electronic potential.

Figure 2.6: Schematic potential distribution across the layers of a PEM fuel cell. On the cathode side the ohmic overvoltage due to contact resistance between the layers are labeled, on the anode side the ohmic losses due to finite conductivity are marked.

For analyzing the different losses within a fuel cell several characterization techniques are necessary which are discussed in the next chapter.

2 Fundamentals of a Polymer Electrolyte Membrane Fuel Cell

3

Characterization techniques

In this chapter a short overview of the applied characterization techniques and mea-suring devices used in the thesis is given. Depending on the question that is ad-dressed every experimental method has advantages or disadvantages. These are briefly discussed.

3.1 Cathode and Anode Polarization Curve

The most applied characterization technique for fuel cells is the measurement of steady-state voltage-current curves. Every operating point is held as long as equi-librium is reached and finally logged. This measurement technique is adequate if only the overall cell performance is of interest. The voltage-current curve can not pro-vide any information about the dynamic behavior of the cell since the recorded data are measured at steady-state. If not only the overall cell performance but rather the electrode performance is of interest, it is essential to distinguish between the anode and cathode polarization by means of a reference electrode. Embedding a reference electrode in a fuel cell is not state-of-the-art. It involves several technical difficulties and challenges, which have to be overcome in order to avoid systematic errors. A detailed discussion about this issue is given Chapter 4, where a realization of a test fuel cell with integrated reference electrodes is presented.

In this work a Solartron 1286 is used as electrochemical interface which measures the current for an applied cell voltage (potentiostatic mode) and the voltage for an applied current load (galvanostatic mode), respectively. The distinct potential differences be-tween working, counter and reference electrodes are measured with a datalogger from Agilent (HP 34970A).

3 Characterization techniques