Fuel cell performance ohmic losses

The current density and impurities also affect the fuel cell performance. In the initial stages, activation polarisation decreases the cell voltage. When current densities increase, then the concentration losses are predominant and a sharp decrease in cell performance is observed. During normal fuel cell application, ohmic losses are observed due to internal resistances of the fuel cell. This section deals with the effect of different variables on fuel cell performance and efficiency. The different fuel cells and their performances are discussed one by one. 

Prasarma et al. [185] were also able to observe an optimum thickness of DLs for fuel cells experimentally. They demonstrated that the thicker DLs experience severe flooding at intermediate current densities (i.e., ohmic region) due to low gas permeation and to possible condensation of water in the pores as the thickness of the DL increases. On the other hand, as the thickness of the DL decreases, the mass transport losses, contact resistance, and mechanical weakness increase significantly [113,185]. Through the use of mathematical modeling, different research groups have reported similar conclusions regarding the effect of DL thickness on fuel cell performance [186-189]. 

Most of the models show that fuel-cell performance is a balance among the various losses shown in Figure 3, in particular, ohmic losses and mass-transport limitations, which both increase with current. The reason for this is that the kinetic losses are hard to mitigate without significantly changing op-... 

Carbon supports strongly affect fuel cell performance. They may influence the intrinsic catalytic activity and catalyst utilization, but also affect mass transport and ohmic losses. This makes analyses of the role of carbon materials rather complicated. Although numerous studies have been devoted to the carbon support improvement, only a few have attempted to establish relationships between the substructural characteristics of carbon support materials and cell performance. The influence of carbon supports on the intrinsic catalytic activity is the subject of Section 12.6.1. In Section 12.6.2 we consider the influence of support on macrokinetic parameters such as the catalyst utilization, mass transport, and ohmic losses. In Section 12.6.3 we review briefly recent data obtained upon utilization of novel carbon materials as supports for fuel cell electrocatalysts. 

For optimum fuel cell performance it is crucial to keep the membrane fully hiunidified at all times, since the conductivity depends directly on water content [19]. The thickness of the membrane is also important, since a thinner membrane reduces the ohmic losses in a cell. However, if the membrane is too thin, hydrogen, which is much more diffusive than oxygen, will be allowed to cross-over to the cathode side and recombine with the oxygen without providing electrons for the external circuit. Typically, the thickness of a membrane is in the range of 5-300 jM [20]. 

Figure 15.1 indicates that mass transfer, the three-phase boundary, ohmic losses, and so on, are of relevance for the overall fuel cell performance and have to be optimized. However, it also becomes clear that the most demanding challenge... 

Fig. 7.25 (a) H2/air fuel cell performances including ohmic resistance voltage losses and (b) ohmic resistances of SPESK X30Y8 (lEC = 1.62 meq/g) at 100°C with humidification at 53 and 30% RH for both electrodes (Reprinted from [64] with permission from Wiley Interscience)... 

Another way to consider the impact of membrane conductivity on fuel cell performance is shown in Fig. 17.5. Figure 17.5a shows the conductivity of a few different EW membranes as a functiOTi of temperature with the atmosphere inside the conductivity cell held at a fixed dew point of 80°C [17]. When the conductivity cell is at 80°C, the %RH is 100%. As the temperature of the cell increases, the %RH at a fixed dew point decreases, causing a decrease in the membrane conductivity. This is similar to the situation in some PEMFC applications where the cell temperature may rise while the humidity level of the incoming gases remains constant. The graph in Fig. 17.5b uses the same data. Here the conductivity is used to calculate the resistance of a 25 pm membrane, and using Ohms law, that resistance is used to calculate the voltage loss (ohmic loss) one would see in a fuel cell at a 0.6 A/cm current density [17]. This represents the fuel cell performance loss due to the loss of membrane conductivity (certainly not the only performance loss under these conditions ). 

Halseid et al. [24] have tested the ohmic resistance of the cell while introducing 1 and 10 ppm NH3 into the anode stream. It was foimd that the increase in cell resistance after the cell was exposed to NH3 was about 20% in most cases, and made just 5% (1 ppm) and 15% (10 ppm) contribution to the performance losses. Ton exchange" may provide a reasonable explanation for the quick and severe impact of NH3, i.e., NHj would react with protons in the membrane, thus staying in the membrane and causing the decrease in the protonic conductivity of the membrane. In addition, the water content in the membrane phase decreases linearly with increasing NH4 fraction [22]. For PFSA membrane, dehydration will cause catastrophic consequences to the fuel cell performance. 

Extensive efforts have been directed toward the search for oxide materials that allow fabrication of an SOFC with low activation and ohmic losses. To determine the role of new materiak in the overall fuel cell performance, electrochemical impedance spectroscopy (EIS) has been used to measure the activation and/ or ohmic resktance contributed by the specific new material. An impedance spectrum k obtained by applying a periodic change in voltage and monitoring the current response at varying frequencies. The impedance spectra obtained... 

If all the losses that we have looked at, activation, ohmic and concentration, are combined then the actual operational graph of a fuel cell, the J-V curve, is produced. The current is usually expressed as current density J, i.e. the quotient of the current divided by the geometrical surface of the electrodes. Thus from the j-V curve, we can get the short-circuit current density Jsc, the open-circuit voltage Vqc, and the fuel cell power density, i.e., the J x V product All of these parameters are very important in the evaluation of the photocatal)dic fuel cell performance. 

As shown in Fig. 1.4 of Chapter 1, under a load, PEM fuel cell performance is determined by four voltage losses the voltage loss caused by mixed potential and hydrogen crossover, which is related to the Pt catalyst status and the membrane properties the activation loss, which is related to the electrode kinetics the ohmic loss, which is determined by ohmic resistance and the voltage loss caused by mass transfer, which is affected by the characteristics of the gas diffusion layer and catalyst layer. The voltage loss caused by mixed potential and hydrogen crossover will be discussed in detail in Chapter 7. The activation loss, ohmic loss, and mass transfer loss can be calculated from the charge transfer resistance, ohmic resistance, and mass transfer resistance, which can be determined by EIS measurement and simulation. 

In Chapter 1, Figure 1.4 shows a typical polarization curve of a PEM fuel cell. The voltage loss of a cell is determined by its OCV, electrode kinetics, ohmic resistance (dominated by the membrane resistance), and mass transfer property. In experiments, the OCV can be measured directly. If the ohmic resistance (Rm). kinetic resistance (Rt, also known as charge transfer resistance), and mass transfer resistance (Rmt) are known, the fuel cell performance is easily simulated. As described in Chapter 3, electrochemical impedance spectroscopy (EIS) has been introduced as a powerfiil diagnostic technique to obtain these resistances. By using the equivalent circuit shown in Figure 3.3, Rm, Rt, and R t can be simulated based on EIS data. 

The j-E curve can be used not only to quantitatively describe the overall fuel cell performance but also to identify and quantify the activation loss, ohmic loss, and the mass transfer limited current density. At low current density, the ohmic loss is negligible and hence the activation loss can be directly obtained from the j-E curve at low current density. The semi-log plot of the/-E curve is linear for low current density and it can be fit to a Tafel equation (Equation 5.83) as shown in Figure 8.1 at low current density. Using the line fit to the Tafel equation. 

Similarly, the fuel cell performance at operating current densities increases with increasing temperature owing to reduced mass transfer polarizations and ohmic losses. The increased temperature also yields higher-quality rejected heat. However, temperatures at which the various fuel cells can... 

A model of MCFC performance has been developed based on the dominating losses in the system, ohmic resistance, and anode and cathode kinetic losses. The fuel cell performance model from ref. [32] is given in Eqs. (7.5)-(7.8) ... 

Addition of hydroscopic metal oxides such as silica, zirconia, or titania to a proton-conducting polymer is the most obvious way to improve water retention at elevated temperatures (Aparicio et al., 2003). Unfortunately, due to the negligible proton conductivity of these oxides, an increase in the overall resistance of the composite membrane is observed, especially at low temperatures. However, as the temperature is increased, the conductivity gain due to better hydration offsets the loss due to the excluded conducting volume, and the net fuel cell performance is improved, as compared to an unmodified membrane (Adjemian et al., 2002a,b). It should be stressed that there are limits to the water sorption capability of the oxides. While these membranes retain more water than traditional PEM materials such as Nafion at high temperature and low RH, water uptake is insufficient and ohmic losses are still unacceptably high for PEMFC applications. 

Lifetime performance degradation is a key performance parameter in a fuel cell system, but the causes of this degradation are not fully understood. The sources of voltage decay are kinetic or activation loss, ohmic or resistive loss, loss of mass transport, or loss of reformate tolerance (17). 

Figure 2-1 shows that the reversible cell potential for a fuel cell consuming H2 and O2 decreases by 0.27 mV/°C under standard conditions where the reaction product is water vapor. However, as is the case in PAFC s, an increase in temperature improves cell performance because activation polarization, mass transfer polarization, and ohmic losses are reduced. 

The sources of polarization in PAFCs (with cathode and anode Pt loadings of 0.5 mg Pt/cm, 180°C, 1 atm, 100% H3PO4) have been discussed in Section 2 and were illustrated as half cell performances in Figure 2-3. From Figure 2-3, it is clear that the major polarization occurs at the cathode, and furthermore, the polarization is greater with air (560 mV at 300 mA/cm ) than with pure oxygen (480 mV at 300 mA/cm ) because of dilution of the reactant. The anode exhibits very low polarization (-4 mV/100 mA/cm ) on pure H2, and increases when CO is present in the fuel gas. The ohmic (iR) loss in PAFCs is also relatively small, amounting to about 12 m at 100 mA/cm. ... 

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