Durability start/stop conditions

Hara, M., Lee, M., Liu, C.H., Chen, B.H., Yamashita, Y., Uchida, M., Uchida, H. Watanahe, M. Electrochemical and Raman spectroscopic evaluation of Pt/graphitized carbon black catalyst durability for the start/stop operating condition of polymer electrolyte fuel-cells. Electrochim. Acta 70 (2012), pp. 171-181. 

In this chapter, we attempt to evaluate state-of-the-art commercial conventional-carbon-support MEAs for their carbon corrosion kinetics, the relationship between cell voltage loss and carbon-support weight loss, and the fife projection of the catalyst support under automotive operating conditions. These operational conditions include steady-state operation, transient, start/stop, and unintended deviations from nominal run parameters. On the basis of these analyses, we elucidate (1) which operational conditions result in severe carbon corrosion, (2) whether current conventional-carbon-support MEAs are robust enough to meet automotive durability targets, and (3) if a state-of-the-art corrosion-resistant carbon-support MEA is absolutely required for improving automotive fuel cell durability. 

Carbon corrosion kinetics of commercial conventional-carbon-supported MEAs were studied at various potmtials and tempoalures. The lifetime projectiMi of OMventional-carbon-supported MEAs in the automotive fuel cell system was then analyzed using the kinetics shown in this chapto. It is found that these conventional-carbon-supported MEAs are not likely to meet automotive fuel cell durability targets under the severely dynamic automotive operational conditions. Automotive fuel cell systan start/stop and local anode starvation are beheved to be two of the rntgor contributors... 

Durability test conditions for start/stop cells (Fig. 13) CeU T = 80 C, humidifier T = 64/64°C (anode/cathode), RHs (anode/cathode) 50%/50% (anode/cathode), gas pressures 214kPa (abs), constant stoichiometric flows l.l. O (anode/cathode). Conditions for US06 drive-cycle testing were the same as those for ceU Dl, except that the RHs were 100%/ 00%. 

Assuming 10 s of hot time per stop, this necessitates catalyst support durability at 1.2 V of around 100 h with less than 30 mV drop in performance at 1.5 A cm l Similarly, it was estimated that the idle time of the stack at cathode potentials of approximately 0.9 V could amount to several thousand hours over the vehicle life. The data at 80 °C predict that standard Vulcan carbon supports do not meet automotive requirements with respect to start/stop, as well as prolonged idle conditions. 

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.

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