Fuel Cell Irreversibilities—Voltage Losses

Cell voltage characteristics for different exchange current densities. 

Potential loss owing to activation and ohmic losses. 

In the fuel cell, there are a series of steps involved in electrode reactions  

Fuel cell overpotential or polarization curve showing the activation, ohmic, concentration, and fuel and oxidant crossover and short circuit losses. 

one may express the overpotential as the sum of all the losses through an equation  

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The theoretical OCV has the same value as the reversible eell potential. However, even when no current is drawn from a fuel cell, there is irreversible voltage loss, which means that the actual values of the OCV are always lower than the theoretically expected values. To date, a quantitative explanation for such OCV behavior has not been clear in the literature. One explanation attributes this behavior to H2 crossover and/or internal current, as described in the fuel cell book written by Larminie and Dicks [26]. A mixed potential [121-124] has also been widely used to interpret the lower OCV. The combined effects of fuel crossover, internal short, and parasitic oxidation reactions occurring at the cathode are the source of the difference between the measured open circuit cell voltage and the theoretical cell potential. Therefore, the actual OCV is expressed as... 

In this case, the PEM operates like a linear ohmic resistance, with irreversible voltage losses t]pem = jolpEM/ p, where jo is the fuel cell current density. In reality, this behavior is only observed in the limit of small Jo- At normal current densities of fuel cell operation, y o l A cm , the electro-osmotic coupling between proton and water fluxes causes nonuniform water distributions, which lead to nonlinear effects in tipem. These deviations result in a critical current density Jpc, at which the increase in tipem incurs a dramatic decrease of the cell voltage. It is, thus, crucial to develop membrane models that could predict the value of Jpc on the basis of primary experimental data on structure and transport properties. 

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). 

Figure 21.2 shows a typical polarization curve (or current-voltage curve) of PEMFCs. This curve results from both the anodic HOR and the cathodic ORR reactions. The actual celt voltage is much lower than the ideal celt voltage and the theoretical cell voltage. When the current is drawn from a fuel cell, the actual cell voltage will drop from its ideal due to several types of irreversible losses, as shown in Figure 21.2. The drop is mainly caused by mixed potential and fuel crossover, activation overpotential, ohmic overpotential, as well as mass transfer (concentration) overpotential.

The actual fuel cell potential is decreased from its full potential, the Nemst potential, because of irreversible losses. Multiple phenomena contribute to irreversible losses in an actual fuel cell. Eor the hydrogen oxidation reaction, the func-tionahty of fuel cell voltage, E, is typically given by [42-44]... 

In spite of the progress, the ORR in the cathode still incurs about 40% of all irreversible energy losses in the cell, as well as a proportional fraction of voltage losses. Moreover, at the current mass loadings required for high cell performance, Pt is responsible for 30-70% of the total cost of a fuel cell stack, although it only amounts to about 0.1 % of the stack volume. The foremost challenge in PEFC research remains to maximize performance with a minimal amount of Pt. 

From thermodynamics, we can get the ideal voltage for a fuel cell. However, in practice, the actual cell voltage is less than the ideal thermodynamically predicted voltage due to irreversible losses, even when the open circuit voltage is measured. Normally, the losses can be broken up into three major types activation losses, ohmic losses and concentration losses. The real cell voltage can thus be written as below ... 

Using the methods described below in Section 3.10, it is possible to distinguish this particular irreversibility fi om the others. Using such techniques it is possible to show that this ohmic voltage loss is important in all types of cell, and especially important in the case of the solid oxide fuel cell (SOFC). Three ways of reducing the internal resistance of the cell are as follows ... 

A fuel cell performance may be expressed by considering different quantities such as thermod5mamic efficiency based on energy forms voltage efficiency based on operating voltage and all irreversible losses, and current efficiency based on excess fuel supplied. 

Irreversible losses cause a difference in the efficiency of reversible and real processes. These losses can be described and quantified by their irreversible entropy production. The consideration of the ohmic losses shows that the irreversible entropy production in a SOFC is smaller than in another low-temperature fuel cell. This is caused by the lower irreversible entropy production of the heat dissipated at a higher temperature. The effects of the irreversible mixing of reactants and products lead to an irreversible entropy production as well that reduce the cell voltage. The changes in the Nernst voltage can be understood by the analysis of the fuel utilisation. 

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