Fuel crossover losses

For a cell area of 10 cm, the current density becomes 2.7 mA/cm. The fuel crossover losses can be summarised as 

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Another important factor that needs to be considered while considering fuel cell efficiency is the fact that excess fuel is generally supplied in order to offset for any unwanted consumptions such as fuel crossover loss through electrolyte, incomplete and undesirable reactions, and leakage loss through cell components and to sustain the electrochemical reaction across the entire active surface area. Any unconsumed fuel will exit the cell as an element exhaust gas mixture. The fuel utilization factor or stoichiometric factor is defined as a measure of the excess fuel supplied as... 

Generally, in a fuel cell, the electrolyte ohmic overpotential is the dominant component of the ohmic overpotential owing to the lower ionic conductivity value as compared to the electronic conductivity of electrodes and interconnect materials. Research effort to improve ohmic loss in a fuel cell is, therefore, focused on the improvement of the electrolyte in terms of higher ionic conductivity and lower thickness. Use of a thinner electrol5de is limited by a number of factors such as structural integrity, manufacturability and defects, increased parasitic loss owing to fuel crossover loss, and dielectric limit of the electrolyte. 

The fuel crossover losses are due to the wastage of fuel passing through the electrolyte. Fuel crossover losses are prominent in the fuel cell operation at low temperatures. The activation losses are due to the slow reaction kinetics on the surface of the electrodes. Ohmic losses are due to the resistance offered to the flow of electrons and ions through electrodes and electrolytes, respectively. The concentration losses are due to the change in concentration of the reactants at the surface of electrodes. 

FIGURE 27.11 (See color insert following page S88.) H2 permeability as a function of temperature and RH. Upper limit (solid line) defined by crossover losses (assuming no contribution from O2 crossover), lower Umit (dotted Une) defined by electrode ionomer film-transport requirements, and data are for wet and dry Nafion 1100 EW-based membranes. (Reproduced from Gasteiger, H.A. and Mathias, M. F., in Proceedings of the Symposium on Proton Conducting Membrane Fuel Cells III, 2003. The Electrochemical Society of America. With permission from The Electrochemical Society, Inc.)... 

Fuel crossover is the amount of fuel that crosses the membrane from the anode to the cathode without being oxidized at the anode catalyst layer, which results in a loss of fuel. Internal current is the flow of electron from the anode to the cathode through the membrane instead of going through the external circuit. The combination of these two losses is typically... 

Cell degradation results in performance loss of the fuel cell. Appropriate measurement techniques are required in order to characterize the cell and determine the prevaihng degradation process. Cyclic voltammetry (CV) is a common diagnostic tool for the characterization of electrochemical cells [18, 58). With respect to PEM-FCs, it provides information about the electrochemical active area, the double-layer characteristics, and the hydrogen fuel crossover through the membrane. 

The total electrical current is the sum of external (useful) current and current losses due to fuel crossover and internal currents ... 

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... 

There is another voltage loss, which is caused by fuel erossover and internal currents. Basically, fuel crossover and internal currents have very marked effeets on the OCV, Eg y. For more information on this voltage loss, please see Seetion 1.2.4. 

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.

In a paper by Wasterlain et al. (2011) a new instrument developed in-lab is proposed to satisfy the requirements of electrochemical impedance studies to be led on large fuel cell plants made of numerous individual cells. Moreover, new voltammetry protocols dedicated to PEMFC stack analysis are described. They enable, for example, the study of membrane permeability and loss of platinum activity inside complete PEMFC assemblies. In the first part, a new electrochemical impedance spectrometer that makes testing large FC stacks possible has been presented. To validate this acquisition system and to demonstrate some of its capabilities, some experiments were conducted on a 20-cell PEMFC stack. In the second part, voltanunetry experiments were conducted on short FC stacks with a commercial potentiostat. The fuel crossover phenomenon in a three-cell PEMFC stack was analyzed using the linear sweep voltametric (LSV) method. The crossover rates were determined for each individual cell inside the complete assembly and for the entire stack as well. 

Fuel crossover and internal currents. This energy loss results from the waste of fuel passing through the electrolyte, and, to a lesser extent, from electron conduction through the electrolyte. The electrolyte should only transport ions through the ceU, as in Figures 1.3 and 1.4. However, a certain amount of fuel diffusion and electton flow will always be possible. Except in the case of direct methanol cells the fuel loss and current is small, and its effect is usually not very important. However, it does have a marked effect on the OCV of low-temperatuie cells, as we shall see in Section 3.5. 

So, in this case the losses correspond to a current / of 1.40 x 10 X 2 X 9.65 X 10 = 27 mA. The cell area is 10 cm, so this corresponds to a current density of 2.7mAcm . This current density gives the total of the current density equivalent of fuel lost because of fuel crossover and the actual internal current density. 

Figure 3.5 Graph showing the fuel cell voltage modelled using activation and fuel crossover/in-temal current losses only.

However, there are problems with this approach. The first is that all catalysts that do not promote the fuel oxidation tend only to very slowly promote the reaction of oxygen with the H+ ions. Thus, the activation losses on the cathode are made even worse than normal, and there is no increase in performance. Another problem is that although the mixed-potential problem may be solved, the fuel is still crossing over, and while it may not be reacting on the cathode, it will probably just evaporate instead. Thus, it will still be wasted. So, although it may be possible in the future to find selective cathode catalysts that amehorate the fuel crossover problem, this approach does not offer a complete solution. 

Theoretically, the exchange current density Jq can be obtained by measuring i versus q for a low range of q. Unfortunately, this measurement is not practical because of large experimental errors introduced by other fuel cell losses arising from ohmic resistances, mass transport effects, and reactant and product crossover effects. These losses are discussed in the next section. 

The fuels crossover and internal currents are equivalent that is, they both contribute voltage loss owing to a small equivalent cell current. However, fuel crossover and the internal cmrents have a different physical effect on fuel cell. In the internal current, the oxidation reaction has already taken place and the electrons are short-circuited through electrolyte. In case of fuel crossover such as hydrogen permeation from the anode to the cathode, first the fuel crosses over from the anode to the cathode and then oxidation and reduction reactions occur near the cathode. With reactant crossover and internal currents, a small amount of current is lost. In both cases, the current losses are similar to activation losses, and hence as an approximation, the current and potential behavior can be represented by the Tafel law. 

In the previous section, each of the fuel cell losses, cathode and anode activation losses, ohmic losses, mass transfer losses, and losses owing to short circuit and reactant crossover was discussed, and expression for each loses overpotential or the polarizations were obtained. Now, we have net fuel cell overpotential from Equation 5.149... 

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