Start-stop degradation

Consequently, since graphitized carbon-supports have lower carbon corrosion rates, the use of cathode catalysts with graphitized supports significantly reduces H2/air-front start-stop damage.12,22 Furthermore, if the ORR activity of the anode electrode is reduced by lowering anode Pt loading, H2/air-front start-stop degradation is decreased.22,23... 

Measured Start-Stop Degradation Rates at 1.2 A/cm2 Normalized to H2/02 Front Residence Time on a 50 cm2 Cell at 80 °C, 66% RHj, 150 kPa.lhs, Compared to the Modeled... 

Perry ML, Patterson TW, Reiser C (2006) Systems strategies to mitigate carbon corrosion in fuel cells durability— fuel starvation and start/stop degradation. ECS Trans 3 783-795... 

Fig. 14.3 Start/stop degradation mechanism including the main reaction pathways (a) and schematic polarization curves (b) within the different characteristic fuel...

Figure 6 shows examples of potential cycling test protocols that are used to simplify this test of carbon cathode durability when it is conducted on MEAs. These test protocols do not require the air supply system to the anode or the voltage-hmiting circuit shown in Fig. 3. As one example of their application. Fig. Vshows the results that were obtained when a potential-cycling test was conducted under the conditions in Fig. 6a. An analysis of the results indicates that CO was generated by potential cycling and that it was accompanied by a dechne in cell performance. This suggests that the test protocol is one effective method of evaluating start-stop degradation.

The principal cause of start-stop degradation induced by carbou corrosion is a sharp rise in the cathode potential therefore, controlling the potential is an effective way of improving stack durability. 

The stability of electrocatalysts for PEMFCs is increasingly a key topic as commercial applications become nearer. The DoE has set challenging near-term durability targets for fuel cell technology (automotive 5,000 h by 2010 stationary 40,000 h by 2011) and has detailed the contribution of the (cathode) catalyst to these. In particular, for automotive systems as well as steady-state stability, activity after simulated drive cycles and start-stop transients has been considered. In practice, both these treatments have been found to lead to severe degradation of the standard state-of-the-art Pt/C catalyst, as detailed next. 

Modeling of Membrane-Electrode-Assembly Degradation in Proton-Exchange-Membrane Fuel Cells - Local H2 Starvation and Start-Stop Induced Carbon-Support Corrosion... 

Ettingshausen, E, Kleemann, J., Marcu, A., Toth, G., Fuess, H., and Roth, C. (2011) Dissolution and migration of platinum in PEMFCs investigated for start/stop cycling and high potential degradation. Fuel Cells, 11, 238- 245. 

Meier JC et al (2012) Degradation mechanisms of Pt/C fuel cell catalysts under simulated start-stop conditions. ACS Catal 2 832-843... 

Design of HOR catalysts which have no activity for the ORR [14] and their integration into anode electrodes, which is an alternative approach to mitigating degradation caused by start/stop. 

Besides the already-mentioned system-based strategies for mitigating carbon oxidation, there is obviously also the possibility to improve the CL composite (Pt/carbon, binder, PA) regarding start-/stop-induced degradation. It was shown that Pt alloys in HT-PEMFC exhibit reduced electrode overpotentials subsequent to simulated start/stop cycling compared to pure Pt. It was furthermore found that changing the catalyst carbon support to stabilized (graphitized) carbon (SC) also positively affects fuel cell durability. 

Meier, J.C., Galeano, C., Katsounaros, I., Topalov, A.A., Kostka, A., Schiith, F. Mayrhofer, K.J.J. Degradation mechanisms of Pt/C fuel-cell catalysts under simulated start-stop conditions. ACS Catal. 2 (2012a), pp. 832-843. 

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