2. Transient Behaviors of CO Poisoning in the Anode Catalyst
2.3 Results and Discussion
The 4th order Runge Kutta algorithm were applied to solved the coverage of H2 and CO. The governing equations must be solved simultaneous for the dependent variables. The steady-state condition is defined as the relative error reaches:
8
2.3 Results and Discussion
To examine the transient behaviors of the poisoning process, various CO concentrations are employed to simulate a wide range of hydrogen fuel from the
reformer. Several physical parameters are considered to analyze the reactant gas distribution, coverage, current density, and the time response needed to reach the steady state condition after a the start-up operation. Table 2.1 presents the parameters used in this work.
Transient evolutions of the hydrogen and CO distributions across the anode catalyst layer with 100 ppm CO are shown in Fig. 2.2. Because of the fast kinetics of hydrogen, the current density was provided by hydrogen electro-oxidation resulting much lower concentration profiles. In contrast, the concentration distribution of CO was only depleted slightly across the anode catalyst layer. As a result, both the hydrogen and the CO concentrations take 541 s to reach the steady-state condition after a start-up operation.
The transient distributions of the hydrogen coverage across the anode catalyst layer for 100 ppm CO are indicated in Fig.2.3. It is clearly seen that the hydrogen coverage, θH, decreases with the time due to the CO adsorption on the catalyst site. Owing to the high affinity between CO and Pt catalyst, large anode overpotential is needed to oxidize CO. As shown in Fig. 2.4, the catalyst site is adsorbed by the CO when the reacting time goes. This causes the hydrogen to diffuse deeply into the catalyst layer, which in turn, seeks for more catalyst sites. Without extremely high overpotential to make the CO oxidation, the accumulation of CO on the catalyst site is sustained and therefore reduces the hydrogen oxidation.
Figure 2.5 shows the unsteady variations of the hydrogen oxidation current density across the anode catalyst layer. It is observed in Fig. 2.5 that the hydrogen oxidation increases sharply after the start-up operation (within the first 2 μm). Comparison of the corresponding hydrogen concentration distributions in Fig. 2.2 indicates that the fast kinetics of hydrogen results in a significant increase in the hydrogen oxidation
much smaller than the hydrogen oxidation current. This implies that the CO oxidation current can be neglected.
Under various levels of the CO concentration from the reformer, the cell performance decreases with an increase in the CO concentration. Figure 2.7 shows the steady-state hydrogen coverage under various CO concentrations in the range between 10~100 ppm. It is seen that the fuel with a high CO level would reduce the hydrogen coverage on the catalyst sites which can reduce the cell current density significantly.
Otherwise, a significant rise in the θCO from 0.5 to 0.94 at the range from 10-100 ppm CO is found in Fig. 2.8.
Figure 2.9 presents the total cell current density distributions across the anode catalyst layer under various ppm CO. The predicted CO poisoning results are compared with the experimental data of Oetjen et al. [79]. The current density nearly 1 A cm-2 was obtained at η=0.01 in present result without CO contained. The experimental data were subjected to different CO concentration polarization curve at 0.6 V which corresponds to CO-free at current density 1 A cm-2. As shown in Fig 2.9, the results show a good agreement with experimental data. A careful examination of Fig. 2.9 discloses that the current density would decrease from 1. A cm-2 to, 0.487, 0.365 or 0.263 A cm-2 when the hydrogen is subjected to 25, 50 or 100 ppm CO, respectively. The predicted steady-state current density under different ppm CO is consistent with those of Oetjen [79].
As mentioned above, the hydrogen with various CO concentration not only drops the cell current density but also affects the response time interval tss. Figure 2.10 shows the effects of the ppm CO on the response time interval tss for different anode overpotential and gas porosity. An overall inspection of Fig. 2.10 indicates that the ppm CO has a significant impact on the response time interval. This is because that the hydrogen with a high CO concentration would increase the CO adsorption on the
catalyst sites, which in turn, cause a decrease in the response time interval tss. While the overpotential and gas porosity have a slight influence on the response time interval.
In addition, the response time interval decreases with an increase in the ppm CO.
Figure 2.11 presents the variations of current density with CO concentration under different anode overpotential and gas porosity. It is seen that with a higher anode overpotential or gas porosity at the catalyst layer can obtain a much greater current density, especially at low CO concentration. This can be made plausible by noting the fact that the catalyst layer with a high porosity, the hydrogen fuel can be easily fed into the catalyst layer and high anode overpotential can free up the catalyst reacting sites for hydrogen oxidation by bringing about the great CO oxidation. With 0.01 V anode overpotential and 10 ppm CO, the corresponding current density is 0.79 A cm-2 and 0.6 A cm-2 for the porosity being 0.5 and 0.3, respectively. But, when the CO concentration is increased, the effects of the gas porosity on the current density would become less significant. A similar trend can be obtained for the case with overpotential 0.005V at the same gas porosity under various CO concentrations.
Table 2.1 Values of parameters used in the present study [15,56].
T 353 K
P 3 atm
α 0.5
DH2 2.59×10-6 cm2 s-1
kfH0 100 A cm-2 atm
bfH 0.5
keH 4 A cm-2
kECO 10 A cm-2 atm
bfCO 1.51×10-9 atm
keCO 1×10-8 A cm-2
Fig. 2.1 Schematic diagram of a PEM fuel cell anode.
Anode
Membrane CL
GDL
Lc x
0 2 4 6 8 10
x (μm)
0 0.2 0.4 0.6 0.8 1
C / C
int = 1 sec t = 100 sec t = 300 sec t
ss= 541 sec . : H
2( : CO
Fig. 2.2 Hydrogen ( ) and carbon monoxide ( ) distributions at various time steps across anode catalyst layer for 100 ppm CO, εCL=0.4, η=0.01, and Lc=10μm.
0 2 4 6 8 10
x (μm)
0 0.2 0.4 0.6 0.8 1
θ
H2t = 1 sec t = 100 sec t = 300 sec t
ss= 541 sec
Fig. 2.3 Distributions of θH2 at various time steps across anode catalyst layer for 100 ppm CO, εCL=0.4, η=0.01, and Lc=10μm.
0 2 4 6 8 10
x (μm)
0 0.2 0.4 0.6 0.8 1
θ
COt = 1 sec t = 10 sec t = 100 sec t = 300 sec t
ss= 541 sec
Fig. 2.4 Distributions of θCO at various time steps across anode catalyst layer for 100 ppm CO, εCL=0.4, η=0.01, and Lc=10μm.
0 2 4 6 8 10
x (μm)
0 0.4 0.8 1.2
i
H2( A / cm
2)
t = 1 sec t = 100 sec t = 300 sec t
ss= 541 sec
Fig. 2.5 Distributions of hydrogen oxidation current at various time steps across anode catalyst layer for 100 ppm CO, εCL=0.4, η=0.01, and Lc=10μm.
0 2 4 6 8 10
x (μm)
0 2x10
-84x10
-86x10
-88x10
-8i
CO( A / cm
2)
t = 1 sec t = 100 sec t = 300 sec t
ss= 541 sec
Fig. 2.6 Distributions of CO oxidation current at various time steps across anode catalyst layer for 100 ppm CO, εCL=0.4, η=0.01, and Lc=10μm.
0 2 4 6 8 10
x (μm)
0 0.2 0.4 0.6 0.8 1
θ
H210 ppm CO 25 ppm CO 50 ppm CO 100 ppm CO
Fig. 2.7 Distributions of θH2 at steady state across anode catalyst layer for different CO concentration with εCL=0.4, η=0.01, and Lc=10μm.
0 2 4 6 8 10
x (μm)
0 0.2 0.4 0.6 0.8 1
θ
CO10 ppm CO 25 ppm CO 50 ppm CO 100 ppm CO
Fig. 2.8 Distributions of θCO at steady state across anode catalyst layer for different CO concentration with εCL=0.4, η=0.01, and Lc=10μm.
0 2 4 6 8 10
x (μm)
0 0.1 0.2 0.3 0.4 0.5
i ( A / c m
2)
present results 25 ppm CO 50 ppm CO 100 ppm CO
Oetjen et al. [79]25 ppm CO 50 ppm CO 100 ppm CO
&
* ,
Fig. 2.9 Distributions of current density at steady state across anode catalyst layer for different CO concentration with εCL=0.4, η=0.01, and Lc=10μm.
0 100 200 300 400 500
CO Concentration (ppm)
0 400 800 1200 1600
t
ss( sec)
η = 0.01V, εCL
= 0.5
η = 0.01V, εCL= 0.3
η = 0.005V, εCL= 0.5
η = 0.005V, εCL= 0.3
Fig. 2.10 Effects of the ppm CO concentration on the response time interval for different anode overpotential η and gas porosity εCL with Lc=10μm.
0 100 200 300 400 500
CO Concentration (ppm)
0 0.2 0.4 0.6 0.8
i ( A /cm
2)
η = 0.01V, εCL
= 0.5
η = 0.01V, εCL= 0.3
η = 0.005V, εCL= 0.5
η = 0.005V, εCL= 0.3
Fig. 2.11 Effects of the ppm CO concentration on the current density for different anode overpotential η and gas porosity εCL with Lc=10μm.