Cathode contamination cell voltage

Hui et al. [29] conducted toluene contamination tests at different toluene concentrations. They also studied the effects of different operational conditions on toluene contamination, including the effects of fuel cell relative humidity (RH), of Pt loading in the cathode catalyst layer, of back pressure, and of air stoichiometry [30]. Figure 3.8 shows a set of representative results of contamination tests at 1.0 A cm with various levels of toluene concentration in the air. It can be seen that the cell voltage starts to decline immediately after the introduction of toluene, and then reaches a plateau (steady state). These plateau voltages indicate the saturated nature of the toluene contamination. For example, the cell voltage drops from 0.645 V to 0.522 V at 1.0 A cm-2 within 30 min of the cathode... 

Figure 6.5 shows the transient cell performance behaviors at different current densities and with different toluene inlet concentrations. On the one hand, the effect of toluene contamination becomes more severe with a higher toluene concentration at the same cell current density for example. Figure 6.5(d) indicates that at the same current density of 1.0 Acm, the cell voltage drops due to toluene concentrations of 250, 500, and 750 ppb are 37, 42, and 48 mV, respectively. On the other hand, the toluene contamination increases steadily with increasing cell current density for example, the voltage drops in response to 750 ppb toluene in the cathode flow channel are 9,16, 27, and 48 mV, corresponding to cell current densities of 0.5, 0.75, and 1.0 AcmV respectively, as shown in Figure 6.5. Furthermore, the time required for the cell voltage to reach steady state is also affected by both toluene concentration and current density, i.e., a larger toluene concentration and a lower current density result in a longer time before cell performance reaches steady state.

Baturina et al. (Baturina and Swider-Lyons, 2009) found that the poisoning effect of airborne SOj on PEMFC performance depends on the operating cell voltage. Figure 8.3 shows the current density versus time obtained during the contamination of a FC cathode with 1 ppm SOj at 0.5,0.6, and 0.7 V. 

FIGURE 8.3 Current density versus time during the contamination of a PEM FC cathode with 1 ppm SOj at cell voltage of 0.5,0.6 and 0.7 V. Cell running at 80°C and 100% RH. (Reprinted from Baturina, O. A. and Swider-Lyons, K. E. 2009. Journal of the Electrochemical Society 156 B1423-B1430. With permission from The Electrochemical Society, Inc.)... 

FIGURE 8.16 Cell voltage loss during exposure of a PEM fuel cell cathode to a gas mixture containing 1 ppm SOj + 0.8 ppm NO2 + 0.2 ppm NO for 100 h while operating the cell at a current density of 0.5 A cm". (Reprinted from Journal of Power Sources, 166, Jing, F. N. et al. The effect of ambient contamination on PEMFC performance, 172-176, Copyright (2007), with permission from Elsevier.)... 

Low fuel crossover and low gas permeability are also important for high-performance HEMs. In fuel cells, when the fuel crosses over through the electrolyte from anode to cathode (or the oxidant permeates from cathode to anode), the cathode (or anode) potential is contaminated by fuel oxidation (or oxidant reduction), which lowers overall cell voltage. Additionally, HEMs must show low electrical conductivity to minimize internal short-circuiting that occurs when electrons pass directly through the membrane from anode to cathode without going through the external circuit. 

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