4.2.1 ACP-derived La0.6Ca0.4Co0.8Ru0.2O3
It is known that the type of carbons and their pretreatment methods play critical roles in their performances as catalyst supports. The corrseponding mechanism for the carbon-catalyst interaction differs contingent on the type of carbonaceous material and catalyst. In our case, the carbon materials of VXC72R, CNC, and BP2000 were first made into the GDEs to determine their intrinsic electrocatalytic abilities as shown in Fig. 4.10. The La0.6Ca0.4Co0.8Ru0.2O3, and carbonaceous materials were mixed together in order to determine their discharging I-V polarization curves. In addition, the BP2000-derived GDE demonstrated a moderately better performance than those derived from the carbon nanocapsules, and Vulcan XC72R. Table 4.4 summarizes some physic behavior of carbonaceous materials.
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supported on BP2000, VXC72R, and CNCs, respectively.
Table 4.4 Physical properties of carbonaceous materials
GDLs Loading
We selected BP2000 as the catalyst support because it exhibited a desirable surface area suitable for catalyst impregnation. For the ORR the carbon support with the largest available surface area is preferred provided that the catalyst dispersion can be prepared without excessive binders and dispersants [135]. Fig. 4.11 provides the
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ORR I-V profiles for the GDEs with electrocatalysts of La0.6Ca0.4Co0.8Ru0.2O3, La0.6Ca0.4Co0.8Ru0.2O3/BP2000, and La0.6Ca0.4CoO3/BP2000. Table 4.5 lists the voltage reading from selective current densities in the ORR I-V curves. These curves exhibited typical I-V characteristics where the potential values were found to decrease gradually from 1.6 V with increasing current densities. Among these samples, La0.6Ca0.4Co0.8Ru0.2O3/BP2000 exhibited the largest catalytic ability, delivering 1.039 and 0.907 V at 100 and 200 mAcm-2, respectively. In contrast, La0.6Ca0.4CoO3/BP2000 exhibited potentials of 0.983 and 0.648 V at identical current densities. This corresponds to a notable 0.259 V reduction in overpotential at 200 mAcm-2 for La0.6Ca0.4Co0.8Ru0.2O3/BP2000. The unsupported La0.6Ca0.4Co0.8Ru0.2O3
revealed an accelerated voltage deterioration with increasing current densities. This behavior was not unexpected because earlier literature had confirmed a synergistic effect for perovskite supported on a carbonaceous substrate [24,135].
Table 4.5 Voltages at selective current densities in unit of mAcm-2 in the ORR I-V curves for La0.6Ca0.4Co0.8Ru0.2O3/BP2000, La0.6Ca0.4CoO3/BP2000, and La0.6Ca0.4Co0.8Ru0.2O3.
50 100 150 200
La0.6Ca0.4Co0.8Ru0.2O3/BP2000 1.103 1.039 0.985 0.907 La0.6Ca0.4CoO3/BP2000 1.134 0.983 0.822 0.648 La0.6Ca0.4Co0.8Ru0.2O3 0.920 0.746 0.608 0.490
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As shown in the Fig. 4.2(b), for the ACP-derived La0.6Ca0.4CoO3 powders, there appears a minor phase of Co3O4. In contrast, both Co3O4 and La2O3 are present for the La0.6Ca0.4Co0.8Ru0.2O3 powders. To verify the ORR catalytic ability for Co3O4 and La2O3, we purchased both chemicals (La2O3: Aldrich, 99.9 wt % and Co3O4: Showa, 99.5 wt % Co3O4 + 5 wt % CoO) and fabricated the GDEs with identical catalyst loadings. The resulting ORR I-V curves are shown in Fig. 4.12. As shown, both Co3O4 and La2O3 revealed moderate catalytic abilities and their performances are considerably reduced. Apparently, there are marginal improvements as opposed to that of noncatalyzed GDE. According to Neburchilov et al., the ORR catalytic ability for La2O3 is limited because of its poor electrical conductivity [18]. Furthermore, Co3O4
is unstable during the ORR in alkaline electrolyte although it reveals a moderate
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Also shown is the noncatalyzed GDE for comparison purpose.
The impedance measurements were carried out on
La0.6Ca0.4Co0.8Ru0.2O3/BP2000 and La0.6Ca0.4CoO3/BP2000 to estimate the charge-transfer resistance (RCT) directly without interference from the electrolyte IR loss. Fig. 4.13 presents their impedance spectra in Nyquist plots at the open-circuit voltage as well as under polarizations of −100, −200, and −300 mV. As shown in Fig.
4.13(a), La0.6Ca0.4Co0.8Ru0.2O3/BP2000 exhibited a large Warburg impedance at the
49
open-circuit voltage, which was attributed to the double-layer capacitance of the cell.
Under cathodic polarizations, the impedance was reduced steadily with an increasing overpotential in conjunction with a distinct semi-circle at the high frequency regime.
This behavior is clearly shown in the inset of Fig. 4.13(a). It is understood that the intercept for the semi-circle at the highest frequency represented the electrolyte resistance and its diameter indicated the value for RCT. Apparently, the RCT became smaller at larger overpotentials. This behavior was consistent with the electrode reactions reported previously [136-138]. Identical behaviors were observed for La0.6Ca0.4CoO3/BP2000, as shown in Fig. 4.13(b). At the open-circuit voltage, we recorded a large Warburg impedance whose value was close to that of La0.6Ca0.4Co0.8Ru0.2O3/BP2000. This was likely because both La0.6Ca0.4Co0.8Ru0.2O3 and La0.6Ca0.4CoO3 probably due to similar specific surface area. Once the cathodic overpotential was imposed, the impedance spectra moved downward and the formation of semi-circle became apparent. An electrode with a smaller RCT is recognized for a facile faradaic reaction. Therefore, the equivalent circuit model for the fitting the impedance spectra in provided in Fig. 4.13(c). From these spectra, the RCT for both La0.6Ca0.4Co0.8Ru0.2O3/BP2000 and La0.6Ca0.4CoO3/BP2000 at –300 mV were 4.556 and 5.429 Ωcm2, respectively. This confirmed the better catalytic ability for the ORR of La0.6Ca0.4Co0.8Ru0.2O3/BP2000.
50
51
Figure 4.13 Impedance spectra for the (a) La0.6Ca0.4Co0.8Ru0.2O3/BP2000 and (b) La0.6Ca0.4CoO3/BP2000 at open-circuit voltage (□), and overpotentials of −100 mV (○), −200 mV (×), and −300 mV (☆), respectively. (c) is the equivalent circuit model used to fit the impedance spectra.
To verify the sustainable catalytic abilities for the ORR, we carried out galvanostatic measurements for La0.6Ca0.4Co0.8Ru0.2O3/BP2000 at selective current densities of 10, 50, 100, and 150 mAcm-2. Fig. 4.14 presents the resulting voltage profiles for 10 min. As shown, the voltage plateaus for most of the samples were relatively flat and their values were consistent with those reported in our earlier I-V polarization curves. At 150 mAcm-2, there was a slight fluctuation on the voltage response in conjunction with a slow decline, which was likely caused by a limitation on the O2 supply. Despite of this moderate performance degradation at 150 mAcm-2, catalytic behavior for La0.6Ca0.4Co0.8Ru0.2O3/BP2000was rather stable and sustainable provided that the current density was maintained below 100 mAcm-2.
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0 100 200 300 400 500 600
0.6 0.8 1.0 1.2 1.4 1.6
P o te n ti a l (V )
Time (s)
Figure 4.14 Galvanostatic ORR profiles for La0.6Ca0.4Co0.8Ru0.2O3/BP2000 at current densities of 10 (■), 50 (○), 100 (×), and 200 (☆) mAcm-2.
The life time performance for La0.6Ca0.4Co0.8Ru0.2O3/BP2000 was evaluated at 4.8 mgcm-2 loading (considering the weight of La0.6Ca0.4Co0.8Ru0.2O3 only) for a current density of 10 mAcm-2. The resulting voltage profile is presented in Fig. 4.15 in which the voltage curve experienced a gradual decline from 1.233 V at a slope of 0.063 mVh-1. At the 2000th h, La0.6Ca0.4Co0.8Ru0.2O3/BP2000 still maintained a respectable 1.108 V. Because the life time experiment was conducted with unfiltered ambient air, the residual CO2 engendered carbonation in the KOH electrolyte that not only affected the electrolyte conductivity but also altered the GDE microstructure. As a result, the voltage response expectedly decreased with the prolonged CO2 exposure.
At the 2622th h, we observed a sudden drop in voltage, which was believed to be due to the last stage of electrolyte flooding that inundated the catalytic surface entirely.
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This was likely because in our GDE, La0.6Ca0.4Co0.8Ru0.2O3/BP2000 was brush-painted on its surface as a thin layer.
0 500 1000 1500 2000 2500 3000 0.0
0.3 0.6 0.9 1.2 1.5 1.8
P o te n ti a l (V )
Time (h)
Figure 4.15 Life time performance for La0.6Ca0.4Co0.8Ru0.2O3/BP2000 (☆) at current density of 10 mAcm-2.
4.2.2 SSR-derived La0.6Ca0.4Co0.4Ru0.6O3
Fig. 4.16 shows the discharging I-V polarization curves of perovskite catalysts containing different amount of supported Ru on BP2000. It is worth mentioned that the perovskite catalysts exhibit much improvement for the doped one over the non-doped one. In addition, the composition of La0.6Ca0.4Co0.4Ru0.6O3/BP2000 GDE possesses the best catalytic ability.
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0 30 60 90 120 150 180 210 240 270 -1.0
-0.8 -0.6 -0.4 -0.2
P o ten ti al ( vo lt s vs. A g /A g C l)
Current density (mAcm-2)
La0.6Ca0.4CoO3/BP2000 La0.6Ca0.4Co0.8Ru0.2O3/BP2000 La0.6Ca0.4Co0.6Ru0.4O3/BP2000 La0.6Ca0.4Co0.4Ru0.6O3/BP2000 La0.6Ca0.4Co0.2Ru0.8O3/BP2000
Figure 4.16 The discharging I-V polarization curves of La0.6Ca0.4Co1-xRuxO3/BP2000.
Furthermore, the galvanostatic discharge curves for 10 min from current density of 10 mAcm-2 to 200 mAcm-2 are shown in Figs. 3.17(a)-(c). As clearly presented, the discharges curves are rather flat. In addition, the voltage readings are consistent with those recorded earlier in the discharging I-V polarization curves (Fig. 4.16).
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0 100 200 300 400 500 600 -0.9
-0.6 -0.3 0.0
P o ten ti al ( vo lt s vs. A g /A g C l)
Time (sec) (a)
0 100 200 300 400 500 600 -0.9
-0.6 -0.3 0.0
P o ten ti al ( vo lt s vs. A g /A g C l)
(b)
Time (sec)
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0 100 200 300 400 500 600 -0.9
-0.6 -0.3 0.0
P o ten ti al ( vo lt s vs. A g /A g C l)
(c)
Time (sec)
Figure 4.17 The galvanostatic discharge curves for the (a) La0.6Ca0.4Co0.8Ru0.2O3/BP2000 (b) La0.6Ca0.4Co0.6Ru0.4O3/BP2000 (c) La0.6Ca0.4Co0.2Ru0.8O3/BP2000 at current densities of 10 mAcm-2 (■), 50 mAcm-2 (△), 100 mAcm-2 (●), 150 mAcm-2 (×), and 200 mAcm-2 (☆).
Further comparisons with known electrocatalysts are illustrated in Fig. 4.18. The La0.6Ca0.4Co0.4Ru0.6O3/BP2000 GDE shows comparable performance with commercial product of EVT-Mn, EVT-MnCo, and home-made Ag/CNC. Indeed, La0.6Ca0.4Co0.4Ru0.6O3/BP2000 GDE possesses the best electrochemical performance.
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0 30 60 90 120 150 180 210 240 270 -1.2
-1.0 -0.8 -0.6 -0.4 -0.2 0.0
P o ten ti al ( vo lt s vs. A g /A g C l)
Current density (mAcm-2)
La 0.6 Ca 0.4 Co 0.4 Ru 0.6 O 3 /BP2000 Ag/CNC
EVT-Mn EVT-MnCo
Figure 4.18 The I-V polarization curves of the La0.6Ca0.4Co0.4Ru0.6O3/BP2000 GDE and the catalyzed GDEs with electrocatalysts of Ag/CNC, commercial EVT-Mn, and commercial EVT-MnCo.
Fig. 4.19 shows the charging I-V polarization curves of perovskite catalyses containing various Ru contents. As shown in the figure, the perovskites with Ru doping exhibit consistent performance enhancements over that of undoped one.
Furthermore, once the Ru content was increased, further improvement in the electrochemical performance was observed.
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0 30 60 90 120 150 180 210 240 270 0.3
0.6 0.9 1.2 1.5 1.8
E ( vo lt s vs. A g /A g C l)
Current density (mAcm-2)
La0.6Ca0.4CoO3/BP2000
La0.6Ca0.4Co0.8Ru0.2O3/BP2000 La0.6Ca0.4Co0.6Ru0.4O3/BP2000 La0.6Ca0.4Co0.4Ru0.6O3/BP2000 La0.6Ca0.4Co0.2Ru0.8O3/BP2000
Figure 4.19 The charging I-V polarization curves of La0.6Ca0.4Co1-xRuxO3/BP2000.
Again, Fig. 4.20 provides the galvanostatic charge curves for 10 min from 10 mAcm-2 to 200 mAcm-2. Clearly, the La0.6Ca0.4Co1-xRuxO3/BP2000 GDEs are consistently superior than the undoped one. Notably, these charging curves are rather flat for 10 min.
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0 100 200 300 400 500 600 0
1 2 3
P o ten ti al ( vo lt s vs. A g /A g C l)
(a)
Time (sec)
0 100 200 300 400 500 600 0.3
0.6 0.9 1.2 1.5
P o ten ti al ( vo lt s vs. A g /A g C l)
(b)
Time (sec)
60
0 100 200 300 400 500 600 0.3
0.6 0.9 1.2 1.5
P o ten ti al ( vo lt s vs. A g /A g C l)
(c)
Time (sec)
Figure 4.20 The galvanostatic charge curves for the (a) La0.6Ca0.4CoO3/BP2000(b) La0.6Ca0.4Co0.8Ru0.2O3/BP2000 (c) La0.6Ca0.4Co0.4Ru0.6O3/BP2000 at current densities of 10 mAcm-2 (■), 50 mAcm-2 (△), 100 mAcm-2 (●), 150 mAcm-2 (×), and 200 mAcm-2 (☆).
Finally, to confirm our Ru-doped perovskites are truly functional in practical cases, we conducted the test for 100-h discharge followed by 100-h charge at a constant current density of 30 mAcm-2 as shown in Fig. 4.21. Remarkably, the potential profiles are rather flat for the 200 h durations. Furthermore, the SEM images of electrode taken before and after the life time test shown in Fig. 4.22(a) and (b), respectively. Smooth carbon surfaces with a homogeneous distribution of the catalysts were observed in Fig. 4.22(a). However, the cracks on the surface of electrode were observed after the life time testing due to the oxygen pass through electrode as shown
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in Fig. 4.22(b). It is suggested that e result from the cracks were form under oxygen reduction and oxygen evolution reaction.
0 20 40 60 80 100
-0.6 -0.3 0.6 0.9 1.2
P o ten ti al ( vo lt s vs. A g /A g C l)
Charge Discharge
Time (h)
Figure 4.21 The life time testing of the La0.6Ca0.4Co0.4Ru0.6O3/BP2000 GDE for 100 h discharge followed by 100 h charge at a constant current density of 30 mAcm-2.
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Figure 4.22 The SEM images of La0.6Ca0.4Co0.4Ru0.6O3/BP2000 GDE (a) before and (b) after life time testing.
(a)
Catalyst
Carbon
(b)
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