The Experimental Repeatability - UNCERTANINTY ANALYSIS

CHAPTER 3 UNCERTANINTY ANALYSIS

3.4 The Experimental Repeatability

In general, the life of the commercial MEA is about 300hr, so the fuel cell performance will decline with an increase of test time. Because CO makes anode catalyst decaying and ageing in the poisoning test, the life of fuel cell is expected to be shorter. In order to improve the cell CO tolerance, it uses air-bleeding technique as the CO oxidant to remove the adsorbed CO from catalyst surface. However, the major amount of will react with to form water and resultant reaction heat is quite high.

This is the other factor that may cause anode catalyst decay. Therefore, it must complete experiment as quick as possible to reduce the time effect,

O

H

which influences the experiment results. Due to these factors, it is difficult to the perform repeatability test. In order to confirm the accuracy and confidence of the experiment, the cell performance must recover to the base performance before it carries out the next poisoning experiment. The present experimental works can be divided into three parts, therefore, there are three test samples available to perform tests.

The repeatability of baseline performances is shown in Table 3.9, 3.10 and 3.11, which correspond to each part’s sample. However, these data are too many to list, so it only shows the three average performances to determine the error.

CHAPTER 4 RESULT AND DISSCUSSION

The test results of air-bleeding experiments are given and discussed in this chapter. They include two scenarios, which one is to fix cell voltage and the other is to fix current density, to carry out CO poisoning tests, respectively, to identify which one has a better CO tolerance.

After that, it will discuss the timing effects of air bleeding into the CO poisoning tests under the case of fixed current density. They consist of the long- (30min) and short-duration (3min) poisonings, separately, and the results from theses two specific durations are compared with each other. Finally, the effects of using two different catalysts, Pt and Pt alloy, on CO poisoning tests with and without air-bleeding are discussed.

4.1 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

/ cm2

CO

H /

₂

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 /

₂

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

/ cm2

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

/ cm2

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 /

₂

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.

/ cm2

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.

/ cm2

4.2 Effect of Air-Bleeding Timing

In this case, it will investigate the effect of air-bleeding on CO tolerance improvement. The parameters are the specific air-bleeding timings, after which the air is injected into anode poisoned fuel stream, and the CO concentration. Two timings (3min. and 30min.) and 3 CO

concentrations (10.1, 25 and 52.7ppm) are selected. The current density is fixed at 800mA/ cm².

4.2.1 Long Duration Poisoning (30min)

In the present experimental work, it discusses the effect of CO concentrations on the performance of PEMFC. In order to determine the influence on CO tolerance by using air-bleeding technique, it should obtain the stable poisoned cell performance as the base line first without air-bleeding. In Fig. 4.6, three transient curves under fixed current density at 800 are obtained by performing the poisoning tests with three CO concentrations (52.7, 25 and 10.1ppm). Remind that the fuel streams are pure hydrogen at the first 5min. in all transient tests, after that, the streams are changed to ones with specified CO concentration.

/ cm2

CO H /

₂

In this figure, the initial cell voltages for all tests are the same (0.701V) at the first 5min due to no CO poisoning. In the transient experiments, the cell voltage decays with the time when CO is contained in the anode fuel stream. The transient curve declines to the steady potential of 0.417V with 52.7ppm CO that spends only 40min. With 25ppm of CO, it takes 85min to achieve the steady poisoned potential of 0.476V. It needs over 3.5hr to get the steady state potential of 0.588V when the anode fuel stream contains 10.1ppm of CO. It can be seen that the higher CO concentration, the faster poisoning and cell voltage decay rates. The higher CO concentration has a greater probability to attack the active site on the catalyst surface and cause more serious CO poisoning effect. In the curve of 52.7ppm CO, it shows a potential

oscillation after reaching the steady poisoned potential. The anode potential may be up to the onset potential of CO oxidation in this situation. Therefore, it can cause some CO desorption from catalyst surface and let cell voltage have a sudden recovery. The interaction between adsorption and desorption makes the cell to form an oscillation.

In the 25 and 10.1ppm CO curves, the anode potential can not reach up to the CO onset oxidation potential. Therefore, no cell voltage oscillation in the steady poisoned state is found. The anode poisoned fuel turns back to pure hydrogen when the transient curves decay to the steady state as shown in Fig. 4.6. After that, the cell voltage is recovered very quickly since CO is removed from catalyst surface by pure hydrogen. These cell performances recover to the value of about 0.675V after purging pure hydrogen for 30min.

In Fig. 4.7, it shows the steady polarization curves with different CO concentrations and the baseline performance curve, fed with pure hydrogen. It can be found that the higher CO poisoning concentration results in a lower cell power output. The cell power output declines to 0.22285 2

W at 0.51V with 52.7ppm of CO. It is only 20% of the performance of baseline value (1.1628 2

W ). In the 10ppm of CO condition, the power output (0.64111 2

W ) decays to the half of baseline value at 0.51V cell potential. Even the anode catalyst uses the Pt alloy catalyst, it still cannot reduce CO poisoning effect significantly in many cases of only very little CO concentration. The polarization curves, after purging pure hydrogen for 30min, recover to 80% of the baseline performance at 0.51V.

The results by air-bleeding in the long duration poisoning (30min) with 52.7ppm of CO are shown in Figs. 4.8a and 4.8b. The long duration transient poisoning test introduces air into anode fuel stream after 30min. of CO poisoning application. The initial cell potential is 0.701V with a current density 800mA/ cm².

Fig. 4.8a is the cell voltage versus time. It shows the different cell voltage recovery curves with different air-bleeding ratio in anode fuel stream. It indicates that the cell potential is recovered very quickly when the air is introduced into fuel stream at 30min. The oxygen is absorbed by the catalyst , then it can proceed the oxidation reaction with to form . Therefore, CO is depleted from catalyst surface and the hydrogen can obtain more active sites to carry out the oxidation reaction. This makes anode potential to drop and recovers the cell potential. Apparently, air-bleeding technique can improve the CO tolerance for PEM fuel cell. In this case, air-bleeding ratios change from 2% to 8% in the transient tests. It is found that the recovery rate of cell performance increases with an increase of air ratio. The cell potentials recover to 0.681, 0.684 and 0.687V as the air-bleeding ratios are 2, 3 and 4%, respectively. Moreover, it takes about 15min after the injection of air into anode fuel stream to recover the cell potential to a steady-state value. There is an interesting phenomenon occurred in the transient curves. It can no long achieve a better performance of CO tolerance as the air ratio is over 4%. Apparently, the optimum air-bleeding ratio is 4% with 52.7ppm of CO poisoning. It is because CO adsorption and desorption by oxygen reach a steady state as air-bleeding ratio above

) (

O

_ads

ads 2

CO CO

4%, and no more contribution can be made by adding more air.

In Fig. 4.8b, it continues changing air ratio until the air-bleeding condition gets to a steady state. The transient curve shows that the cell potential recovers to a constant value (0.687V) even the air ratio is raised to 8%. The major consumption of is used for oxidation reaction and just a little fraction is for CO oxidation reaction. Therefore, the excess of reacts with in the anode to produce or

. On the contrary, the air-bleeding ratio above 8% may cause a decline in cell performance. The excess of lessens the amount of , which can proceed oxidation with catalyst surface. On the other hand, the oxidation of with

O

generates heat in the catalyst surface.

The heat may destroy anode catalyst and cell membrane to cause a loss of cell performance. The air-bleeding technique is useful to improve the CO tolerance, however, a suitable air ratio is a more important factor.

O

H

₂

In Fig. 4.9, it shows the steady polarization behaviors as a function of air-bleeding ratio after long duration CO (52.7ppm) poisoning condition. From these polarization curves, it can be seen that an increase of air ratio from 2% to 4% increases the cell performance.

Comparing the recovered polarization curves using air-bleeding with the one without air-bleeding, it can be seen that a remarkable improvement of cell performance is achieved with this technique. The cell performance can recover to over 94% of baseline data at the same current density by using 4% of air-bleeding. Fig. 4.9 also shows the different performance

在文檔中空氣吹離法與時間因子對PEMFC CO容忍度之改善研究 (頁 37-0)