Carbon corrosion start/stop conditions

Under normal fuel cell operating conditions, the highest oxidative potentials in the cathode range between 0.6 V (vs. RHE) at high-current density and 0.95 V (vs. RHE) at open circuit (the anode potential remains always near 0 V vs. RHE), so that carbon-support corrosion is negligible. However, under start/stop conditions or in the case of localized hydrogen starvation, the cathode potential significantly exceeds 1 V versus RHE and the associated rapid carbon-support corrosion leads to... 

Studies [16-18] have shown that these carbon corrosion processes can be accelerated in PEM fuel cells by harsh operating conditions, such as high voltage, high temperature, low humidity, and start/stop cycles. 

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. 

Within the 5,500 operational hours of automotive PEMFC systems, the stack could sit for a long time at idle condition, i.e., when no power is supplied to the propulsion system, and the only power drawn from the fuel cell stack is that required to support the fuel cell system andllaries (e.g., cooling pumps). This results in cathode potentials of approximately 0.90V (Mathias et al. 2005). This idle time could total over several thousand hours depending on the particular usage profile. The stack could also be at open circuit voltage (OCV) condition, where no current is drawn from the stack and the eathode potential increases to approximately 0.95 V. This can occur just before fuel ceU system shutdown, immediately after startup, as weU as in the case of battery/fuel ceU hybrid systems in which the battery provides the electrical load under low power or idle conditions. The accumulated time at OCV condition for 38,500 start/stop cycles could be over lOOh (38,500 cycles x 10 s per cycle). Figure 8 shows the predicted carbon weight loss as a function of time at these two potentials, based on the carbon corrosion... 

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... 

Fundamental model analyses incorporating the measured carbon corrosion kinetics were developed for conditions of start/stop or local starvation. The combination... 

On the right axis in Fig. 7, the percentage of carbon consumption during one complete simulated start/stop cycle is plotted. As is obvious, e.g., at 160°C, one complete start/stop cycle under the experimental conditions applied results in corrosion of approximately 0.4% of the carbon present in the catalyst layer, allowing an estimation of approximately 250 cycles for the complete oxidation of the calhode carbon under the experimental conditions applied. 

In fact, this potential driven corrosion of carbon can be quite severe, causing substantial loss of the electrochemically active surface area as the electrode degrades with the loss of the catalyst support. Enhanced fuel cell degradation can occur under the additional stress conditions associated with cold start and hot stopping [62]. 

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