Poisoning Effects of Fixed Cell Voltage and Current Density

First, the test samples of fuel cell are fixed at two specific conditions to perform the transient CO poisoning experiments. One is to fix the voltage, the other is to fix the current density. In the former condition, the voltages are fixed at 0.5, 0.6, 0.7V, respectively, whereas the current densities are fixed at 600, 1000, 1200 , respectively, in the latter one. In these tests, the anode is fed by pure hydrogen in the first 5min., then, it is changed to , where the concentration is specified as 52.7ppm. The cell performance will be varied with time. The

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CO

H /

CO

poisoned polarization curves are determined as soon as the performance subjected to CO poisoning reaches a steady state.

The results of transient experiments, whose cell potentials are fixed at 0.5, 0.6, and 0.7V are shown in the Fig. 4.1. In general, it can be found when fuel is changed to , the cell performance of resultant current density decays very quickly. It is because that fuel cell operation temperature is always maintained between 65 and 85 , in this range, CO has a stronger adsorbability with Pt catalyst than that of . In other words, it will take over the active site of catalyst when CO presents in reaction chamber. Therefore, the less active site of catalyst is available for the hydrogen reaction that reduces the cell performance.

CO H /

οC

οC

H

In Fig. 4.1, the current density declines from 735 (pure ) to a stable poisoned current 370 after 65min in the case of 0.7V.

As the cell voltage fixes at 0.6V, the current density decreases from 1460 (pure ) to 530 after 40min. For 0.5V case, it declines from 2200 to 700 after 35min. From these observations, it is found that the performance decline rate becomes faster at the lower fixed cell voltage. The reason is that the lower cell voltage produces a higher current density, which requires higher fuel flow rate.

Consequently, it results in a higher supply amount of CO, consequently, the accumulation and adsorbed rate of CO becomes higher in the reaction chamber. Finally, the competition of adsorbed reaction with Pt alloy catalyst between hydrogen and CO reaches to a balance state, defined as the steady state. The corresponding times for each fixed voltage to reach

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steady state are mentioned above.

When the steady state is reached, the anode fuel is turned back to pure hydrogen and no CO exists in the fuel stream at all. Under this circumstance, the CO must be desorbed from catalyst surface by pure hydrogen or oxidized by the anode catalyst alloy. The recovery of cell performance almost simultaneously takes place as the fuel is turned back to pure hydrogen as shown in Fig 4.1. However, it can only recover to about 80 percentage of the original performance after 30min of purging pure hydrogen, indicating that there is a lot of CO still adsorbed on the catalyst and cannot be removed completely.

Figure 4.2 shows the baseline polarization curve, the poisoned and recovered polarization curves with different poisoning conditions (0.5, 0.6 and 0.7V). In this figure, it is significant to find that for a given concentration of CO (52.7ppm) the resultant steady state poisoning polarization curves are almost coincident no matter the applications of different fixed cell voltage in these transient tests, which the cell operations and fuel humid temperatures are the same. It implies that under a given CO concentration, the hydrogen and CO adsorption reactions to the Pt alloy catalyst have a fixed balance state, or a constant polarization behavior. The only difference is the duration to reach the steady state.

In Fig. 4.2, it also shows that the recovery rate after purging the pure hydrogen is faster for the case of lower cell voltage (V) in the transient tests. The similar reason for poisoning effect has been given in the discussion of Fig. 4.1. The cell at the lower voltage gains a higher current density, which requires the higher fuel flow rate. The higher rate

may accelerate CO desorption from catalyst. The other reason is that the fuel cell at low voltage can force CO to proceed oxidation reaction to remove itself from catalyst. Therefore, the transient experiment at a lower fixed voltage, such as 0.5V, can get a better recovered rate (more than 85%) in the Fig. 4.1. This explains why the different polarization behaviors show up in recovered performance with different transient poisoned conditions (0.5, 0.6 and 0.7V).

Next, the transient poisoning tests are performed at different fixed current densities. In Fig. 4.3, it shows three cell voltages transient curves, which the corresponding fixed current density are 600, 1000 and 1200 , respectively. In this case, the cell performance, expressed as cell voltage, decays very fast when (52.7ppm) fuel stream is introduced into anode. It can cause a rise of anode potential, relatively, a decrease in cell potential because CO adsorption reduces the catalyst active site. Finally, the performance subjected to CO poisoning reaches to a steady state as shown in Fig.4.3. The cell voltage for a fixed cell current density of 600 decays from 0.725V to a stable voltage 0.55V when anode fuel contains 52.7ppm CO after 50min. The one fixes at 1000 , the voltage decays from 0.662 to 0.410V after 45min and the one at 1200 decays from 0.632 to 0.355V after 30min. It can find that the higher fixed current density can result in a faster poisoned rate. The reason is same as that in Fig. 4.1.

There exists a different phenomenon between the fixed cell voltage case and current density one in the transient tests. In the fixed current density transient test, the cell voltage shows the oscillation sometimes

when the poisoning performance reaches a balance condition as shown in Fig.4.3, whereas no such phenomenon happens in Fig. 4.1. The CO adsorption can raise anode potential and the higher current density causes a higher anode potential. These effects quickly reach to CO onset oxidation potential and lead CO to be removed from catalyst locally, which results in a bit of cell voltage recovery. The CO adsorption and oxidation reaction form a repeated influence to each other and this interaction causes the voltage oscillation.

In Fig. 4.4, it shows the baseline polarization curve, the poisoned and recovered polarization curves with different poisoning conditions (600, 1000 and 1200 ). It can observe that a better CO poisoned tolerance performance can be obtained when cell is fixed at higher current density (1200 ) to perform the transient test. In this situation, the CO poisoned phenomenon is indicated by the decrease of cell voltage. The CO poisoning effect always makes anode potential rising and causes overall cell performance to decline. However, CO on the catalyst surface can perform oxidative reaction if the anode potential rises to the CO onset potential of oxidation. In the literature [10], it indicated that the onset of CO oxidation with Pt alloy catalyst occurs when anode potential is about 0.2V. The higher anode potential, greater than 0.2V, can raise CO oxidation rate in a specific range. In Fig. 4.3, it can calculate the rise of stable anode potential under 52.7ppm of CO in different current density conditions, such as 600, 1000 and 1200 . They are 0.175, 0.252 and 0.277V, respectively. The higher anode potential cause more CO removed from catalyst surface and obtain a better CO tolerance. Therefore, the transient condition with 1200

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has the best steady poisoned polarization performance as shown in Fig.

4.4. On the contrary, the transient poisoned condition of fixed cell voltage may limit the change of anode potential. Therefore, it cannot oxidize CO from catalyst surface in the stable poisoned state. The stable values of CO adsorption are the same for any transient conditions (0.5, 0.6 and 0.7V), and the steady poisoned polarization behaviors are almost coincident to each other.

In Fig. 4.5, it shows the anode polarization curves obtained from the poisoned polarization curves in Figs. 4.2 and 4.4. At first, it should describe the polarization behaviors with pure hydrogen fuel; no CO poisoning. The typical polarization behavior, fed with pure hydrogen, for fuel cell is shown in the baseline curve of Fig. 4.2 and 4.4. There are three main effects that cause cell potential to drop. They are the kinetic losses, ohmic losses and mass transport limitations [26]. The initial fall is associated with the poor electrode kinetics at a voltage close to the rest voltage. This sharply sudden drop is due to the sluggish kinetics of the oxygen reduction reaction. As the current density rises, the cell potential varies nearly linearly with current density. It is mainly due to ohmic and mass transport losses in solution between electrodes. The anode overpotential can be neglected when anode is fed with pure hydrogen. So, the cell potential drop in the typical polarization curve can be treated as the drop of cathode potential. The anode potential is calculated from the difference between the cell potential with pure hydrogen and the one with at the same current density. Then, the polarization curve is obtained from the collection of anode potentials

CO

H /

for all of current densities. In Fig. 4.5, it can observe that the rising rate is smaller with a higher current density in the transient poisoning test.

The higher fixed current density can force anode potential to rise to the value above onset one of CO oxidation and, then, to remove CO from catalyst surface. Therefore, it causes a better CO tolerance, which has a lower anode potential slope. On the other hand, in the transient condition of 600 , its anode potential slope is higher than the others (1000, 1200 ). It is because that the anode potential only rises to 0.175V in the stable poisoned state when the current density is fixed at 600 . This resultant potential is lower than the onset potential of CO oxidation, so it cannot improve CO tolerance.

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In the transient test of fixed cell voltage, the discrepancy among the stable poisoned polarization curves with different poisoning conditions is insignificant. The anode polarization curves are more or less the same in this case. Its potential slope is higher than the one in fixed current density case (1000, 1200 ). It can conclude that changing cell current density to a higher value can improve CO tolerance, whereas it cannot change the stable poisoned polarization behaviors in the transient experiment of fixed cell voltage.

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