Overvoltages fuel cell

Useful work (electrical energy) is obtained from a fuel cell only when a reasonable current is drawn, but the actual cell potential is decreased from its equilibrium potential because of irreversible losses as shown in Figure 2-2". Several sources contribute to irreversible losses in a practical fuel cell. The losses, which are often called polarization, overpotential, or overvoltage (ri), originate primarily from three sources (1) activation polarization (r act), (2) ohmic polarization (rjohm), and (3) concentration polarization (ricoiic)- These losses result in a cell voltage (V) for a fuel cell that is less than its ideal potential, E (V = E - Losses). 

This complex reduction reaction leads to a relatively high overvoltage - at least 0.3 V- thus decreasing the cell voltage of the fuel cell by the same quantity. Pt-X binary catalysts (with X = Cr, Ni, Fe,. ..) give some improvements in the electrocatalytic properties compared vdth pure Pt dispersed on Vulcan XC72 [17]. 

In addition to mass transport from the bulk of the electrolyte phase, electroactive material may also be supplied at the electrode surface by homogeneous or heterogeneous chemical reaction. For example, hydrogen ions required in an electrode process may be generated by the dissociation of a weak acid. As this is an uncommon mechanism so far as practical batteries are concerned (but not so for fuel cells), the theory of reaction overvoltage will not be further developed here. However, it may be noted that Tafel-like behaviour and the formation of limiting currents are possible in reaction controlled electrode processes. 

FIGURE 9.2 Current density vs. electrode potential curves for the H2-02 and the CH30H-02 fuel cells showing the reaction overvoltages T a and T c at different catalytic electrodes (Pt, Pt-Ru,...). 

Figure 3.3.7 Theoretical (dashed dotted) and real (solid) cell voltage (V) - current density (I) performance characteristics of a fuel cell. Overpotentials are responsible for the difference between theoretical and real performance and cause efficiency losses. They split into (i) activation polarization overpotentials at anode and cathode due to slow chemical kinetics, (ii) ohmic polarization overpotential due to ohmic voltage losses along the circuit, and (iii) concentration polarization overpotentials due to mass-transport limitations. The activation overpotentials of the cathode are typically the largest contribution to the total overvoltage.

Supported metal catalysts are the key to efficient fuel cell performance. Special techniques are required to ensure simultaneous gas/liquid/electrode contact, electrical conduction, and formulation to minimize overvoltage. 

Fuel cell, p. 779 Half-cell reaction, p. 761 Nernst equation, p. 772 Overvoltage, p. 787... 

However, these chemical thermodynamic considerations are important but not decisive for the technical exploitation of high temperature fuel cells. One practical important factor is the decrease in the electrokinetically determined overvoltage so that smaller losses occur in comparison with low temperature operation and, thus, higher efficiencies are obtained. Another difficulty is the direct utilization of the fuels which are produced at high temperatures... 

The theoretical tools discussed in this contribution address various optimization tasks in PEMFC research (i) highest system efficiencies and fuel cell power densities and, thus, minimum overvoltage losses in CCLs (ii) optimum catalyst utilization and, thus, minimal Pt loading (and minimal cost), and (iii) waterhandling capabilities of CCLs and their impact on the water balance of the complete fuel cell. Structural parameters, as well as operating and boundary conditions that control the complex interplay of processes enter at three major levels of the theory. 

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