Over the last decades many efforts have been devoted to optimizing the membrane electrode assemblies (MEAs) and operation conditions of the PEMFC in order to improve their performance and durability. Among the operating conditions, the MEA activation is a procedure that typically has critical influence on the efficiency and long-term performance.231,232 The
activation protocol directly affects: the hydration of the polymer membrane and catalyst, removal of contaminants from the catalyst and electrode, generation of pathways for the reactants, and reaching a stable performance after the activation step. Many of the reported in- situ activation protocols are divided in two main groups: constant-potential and constant-low- current procedures. Besides, most of these protocols have been developed and implemented for the activation of platinum-based catalysts. The obtained activity results, post-activation procedure, have shown significant improvement of the catalyst activity but the times required to carry out such protocols vary from 2 to 100 hours, and this has a negative implication for the mass production of PEMFC, where even short activation times can cause production delays.
Although Fe-N-doped porous carbon materials have emerged as the most promising alternative to replace platinum-based catalysts, challenges in the low performance and durability in fuel cells still need to be addressed.233,234 As investigated for platinum-based catalysts, the activation
protocols have shown an improved catalytic activity in PEMFC. In this thesis we present for the first time a comparative study of break-in or activation protocols for Fe-N-doped OMC catalysts as outlined in Table 11, and their catalytic activity for ORR in PEMFC.
A large number of 5 cm2 MEAs were manufactured using the same catalyst ink batch and
deposition procedure and the activation protocols were performed on a different individual MEA sample for each evaluated protocol. All the MEAs have the same low content of Fe-N- doped OMC catalysts (i.e. 1.2 mg cm-2) explaining the comparably low catalytic conversion
efficiency. The Fe-N-doped OMC material was prepared following the synthesis procedure using FeCl3·6H2O salt as presented in Paper 5. The experiments were carried out using the same
fuel cell (5 cm2 TP5 Research cell, GreenLight Innovation), at 80 °C, 100 % RH and
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Table 11. Protocols evaluated for activation of Fe-N-doped OMC in PEMFC performance tests.
Figure 23 shows the polarization curves under H2/air and H2/O2, respectively comparing the
influence of the preceding activation protocols. Overall, no major difference is observed in the catalytic performance under H2/air condition between the different catalyst activation protocols.
On the other hand, a more noticeable difference is observed under H2/O2 conditions, wherein
the 0.3V-holding activated catalyst shows somewhat better performance along the current range compared to non-activated and 0.4V- and 0.5V-holding activated catalysts. In addition, the square wave potential protocol seems to give the best performance of all samples at the highest current densities. However, the differences are still comparably small between the samples and statistical significance should be confirmed before firm conclusions are drawn from these results.
Figure 23. Polarization plots of activated MEAs by different activation protocols run with a) H2/air and b) H2/O2.
Activation Description Note
No activation
Increase temperature up to 80 °C under H2/Air Polarization plot is
measured after desired temperature is reached
Potential holding @ 0.3 V
Increase temperature to 80 °C under H2/Air with
0.3 V potential holding
Polarization plot is measured after a stable
current is reached
Potential holding @ 0.4 V
Increase temperature to 80 °C under H2/Air with
0.4 V potential holding
Polarization plot is measured after a stable
current is reached
Potential holding @ 0.5 V
Increase temperature to 80 °C under H2/Air with
0.5 V potential holding
Polarization plot is measured after a stable
current is reached
Square wave potential loop 0.3 - 0.6 V
Increase temperature to 80 °C under H2/Air with
cycles of 0.3 V for 15 min → 0.6 V for 15 min → 0.3 V for 15 min
Polarization plot is measured after a stable
current is reached no significant change in current
is observed for two successive equal potentials
Stepwise potential loop
Increase temperature to 80 °C under H2/Air with
0.3 – 0.8 V – 0.3 V by 0.1 V step with 5 min holding
Polarization plot is measured after the loop is
completed
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Three additional activation protocols were evaluated to understand the influence of the pre- heating step and the effect of the gas composition during the activation. The protocols are outlined in Table 12. The polarization curves for the activated catalysts under H2/air and H2/O2
are shown in Figure 24. A significant improvement in performance compared to the best previous protocol of the 0.3V activated sample was found for the activation protocols preheated-N2 activated and preheated without gases, whereas the 0.3V-holding-O2 activated
catalysts was found to perform worse than the 0.3V activated under H2/air conditions. The
improved performance of the first two samples shows the importance of evaluating activation protocols as their performance at 0.5 V is around 50-70% higher than the previously best protocol. It thus appears beneficial to avoid flow of current through the MEA before the sample has been heat treated and even more beneficial to flush the fuel cell with nitrogen during this heating before operating the fuel cell. The detailed mechanism for this improvement remains to be completely understood.
Table 12. Protocols evaluated for activation of Fe-N-doped OMC in PEMFC performance tests.
Activation Description Note
Preheating with N2
1. Increase temperature to 80 °Cunder N2/N2 condition 2. Change to H2/air after the
temperature reaches 80 ℃ 3. Voltage holding at 0.3 V under
H2/air condition
Polarization plot is measured after a stable current is reached
Preheating with no gases
1. Increase temperature to 80 °Cwithout gas flowing
2. Change to H2/air after the temperature reaches 80 ℃ 3. Voltage holding at 0.3 V under
H2/air condition
Polarization plot is measured after a stable current is reached
Potential holding @ 0.3 V-O2 Increase temperature to 80 °C under
H2/O2 with 0.3 V potential holding
Polarization plot is measured after a stable current is reached
The lower catalytic performance of the 0.3V-holding-O2 activated catalyst may be related to
accelerated oxidation of the catalyst due to presence of pure oxygen. It is known for other Fe containing catalysts that such oxidation can be caused by corrosion of the carbon matrix and destruction of the F-N moieties due to the presence of peroxide radicals formed through the Fenton reaction that is catalyzed by Fe.233 However, this mechanism for the Fe-N-OMC catalyst
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Figure 24. Polarization plots of activated MEAs by different activation protocols run with a) H2/air and b) H2/O2.
Although the type of gas condition in the pre-heating step seems to influence the catalytic performance of the MEAs after the activation, it seems not to maintain this higher performance for more than a short period of around 40 hours in durability tests under H2/air at 0.4 V constant
potential as shown in Figure 25. At the beginning of the durability test, the activated catalysts show starting currents following the trend in Figure 24 but thereafter the current decreases to around 0.3 A after 36 hours of test for the three activated catalysts. This suggests that the activation protocol needs to be complemented by suitable operating conditions to preserve the long-term performance of the catalyst. The performance degradation decay is on average 55 % for the three activated catalysts within the 40 hours durability test. Likewise, the durability tests for the activated catalysts by the activation protocols in Table 11 exhibited 50 %-degradation decay (not shown here).
Figure 25. Stability test measured at 0.4 V under H2/air condition for activated Fe-N-doped OMC catalysts with activation
protocols: preheating N2 (black triangles), preheating with no gases (red circles) and potential holding at 0.3 V under H2/O2
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From the results presented above, it can be concluded that from the investigated activation protocols in PEMFC, the pre-heating step under different gas conditions influences the performance in the short-term to a large extent, but the advantage is lost during durability tests. This indicates that the Fe-N-doped OMC catalysts requires a different and simpler activation protocol compared to platinum-based catalysts. Certainly, this could bring an economical benefit for mass production of PEMFC, simplifying the catalyst activation step. However, the large degradation decay observed for all the activated MEAs, needs to be better understood and prevented in order to meet long term performance requirements in applications.
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