Fuel cell performance constant

In air-breathing PEM fuel cells, Jeong et al. [113] were able to demonstrate that with high PTFE content, the fuel cell performed poorly at high current densities because the high amount of PTFE lowered the porosity of the DL, as discussed previously. They concluded that cathode DLs (Toray CFPs) with 5-10 wt% PTFE performed the best (the PTFE content of the anode DL was kept constant at 20 wt% PTFE). 

However, after implementing the water balance measurements, they were not able to observe a significant difference on the net water drag coefficient for a fuel cell with a cathode MPL and an anode without an MPL compared to a cell without any MPLs. It is important to note that they were able to observe that the MPL does in fact improve the fuel cell performance and stability when operating at constant conditions (i.e., the voltage fluctuations are significantly reduced when the cathode DL has an MPL). These results do not correlate with the observations presented earlier thus, more experimental work is necessary to investigate the process behind how the MPL helps the performance of the fuel cell. 

Figure 5.34. Electric equivalent circuit for the impedance spectra in Figure 5.37. Ref. ohmic resistance Rct charge-transfer resistance CPE constant phase element IV Warburg element. The subscripts a and c denote anode and cathode, respectively [36]. (Modified from Boillot M, Bonnet C, Jatroudakis N, Carre P, Didierjean S, Lapicque F. Effect of gas dilution on PEM fuel cell performance and impedance response. Fuel Cells 2006 6 31-7. 2006 John Wiley Sons Limited. Reproduced with permission, and with the permission of the authors.)...

The fuel cell performance drop due to the change in internal resistance between two temperatures at a constant current density can also be estimated roughly based on Equation 6.20 ... 

To prolong the life of MCFC, the amount of electrolyte in the matrix must be maintained at an appropriate level over long-term operation. The growth of particles of L1A102 as an electrolyte retention material in molten carbonates leads to a decrease in the electrolyte retention ability. These phenomena result in a decrease of the fuel cell performance. It was found that zirconia powder added to lithium aluminate keeps the electrolyte retention ability constant for over 7000 hr in Li/Na carbonates and pCOx = 0.1. ... 

When a fuel cell is operated during 5,000 h at an air stoichiometry of 2, the accumulated number of contaminant molecules present in a concentration of 1 ppmv outnumbers the amount of platinum surface atoms by a factor of 300. Molecules that adsorb irreversibly on platinum will easily poison the platinum surface to such an extent that fuel cell performance is bound to approach zero in the course of 5,000 h. While the surface coverage of the contaminant slowly builds up, the available sites for oxygen reduction decrease. As two adjacent platinum sites are needed for oxygen desorption, the oxygen reduction activity at constant potential is proportional to (1 - 0c), in which 0c stands for the degree of coverage by the contaminant. 

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 ). 

The contamination impact of SO2 was studied by Fenning et al. [10] through exposing a fuel cell to 1 ppm S02/air for 100 hours at 70°C with a constant-current discharge at 0.5 A cm. From an initial value of 0.68 V (Figure 3.1), the cell performance fell to 0.44 V, a 35% decrease. Clearly, even a trace amount of SO2 can cause a significant degradation in PEM fuel cell performance. 

Figure 17.17 presents selected points of polarization curves as function of contact pressure for MEAs from three HT-PEM MEA suppliers operating with H2 and air. From Fig. 17.17a, b, the fuel cells behavior of Celtec P2100 MEAs exhibit a strong voltage drop at OCV and low current densities with increasing contact pressure. This is caused by an increase in hydrogen crossover and electrical short-circuit as shown in Fig. 17.16b, d. From Fig. 17.17c can be deduced that the Dapozol -G55 MEA fuel cell performance is nearly immune against contact pressure increase. The OCV is nearly constant in the whole range of contact pressure as hydrogen crossover has also been (Fig. 17.16c). At higher...

Cho et al. (2003) were the first to report on freeze-thaw testing. They investigated fuel cell performance under thermal cycling between 80 °C and -10 °C using H2/O2 as fuel and oxidant respectively. After four thermal cycles, the cell performance decreased by about 100 mV and impedance measmements revealed an increase in ohmic and charge transfer resistance. Ohmic resistance was increased from -0.3 to 0.7 Q cm at 0.7 V and 0.9 Q cm at 0.8 V applied potentials, while charge transfer resistance was increased from -0.2 to 0.3 2 cm at 0.7 V and -0.6 to 0.9 Q cm at 0.8 V. Since membrane proton conductivity remained constant throughout the experiment, it was concluded that the increase in ohmic resistance was due to the enhancement of contact resistance between the electrodes, the membrane and the flow fields. 

The presence of NO2 did not poison platinum electrocatalyst as no surface species were detected in cychc voltammograms however, fuel cell performance was degraded by 50% compared to pure air performance and it was completely recovered after 24 h imder constant flow of neat air (Mohtadi et al, 2004). 

From Equation 10.1 it is evident that water vapor pressure increases exponentially with rising temperature. When gas enters the fuel cell stack, the gas temperature will increase. Under certain circumstances, if a constant inlet pressure and RH are maintained, the water vapor intake will increase and that the amount of reactive gas will decrease, which may cause fuel cell material deficiencies as well as flooding phenomena the latter will lead to a decline in fuel cell performance. 

FIGURE 8.12 PEM fuel cell performance at 120 °C and 1.0 atm backpressure with different RHs. Nafion -112-membrane-based MEA with an active area of 4.4 cm hydrogen and air flow rates were kept at 0.75 and 1.0 L min", respectively the anode and cathode inlet RHs were kept constant [14]. (For color version of this figure, the reader is referred to the online version of this book.)... 

Fuel cell performance of the PPA produced m-PBI membranes is shown in Fig. 16. Although the fuel cell performance was measured under constant high flow-rate conditions, the membranes were operated reliably at high temperatures and dry gases. Additional work also showed that the fuel cells could run on a synthetic reformate containing 2000 ppm of carbon monoxide. 

Fig. 18 Fuel cell performance for the p-PBI membranes from the sol-gel process. Polarization curves of fuel cells under H2/air (squares) and H2/O2 (circles)) without any feed gas humidification. The membrane PA doping level was approximately 32 mol PA/PRU. The catalyst loading in both electrodes was l.Omgcm" Pt, and the cell was operated at 160 °C at constant stoichiometry of 1.2 stoic and 2.5 stoic at the anode and the cathode, respectively...

---