Start-Stop Simulation

For all experiments reported in this chapter, BASF Celtec -based MEAs were used. These MEAs exhibit a thickness of approximately 820 pm including a membrane thickness of around 50-75 pm. They furthermoie cmisist of a highly H3P04-doped FBI membrane, electrodes with a symmetrical platinum loading of 1 mgp,cm with an active area of 45.15 cm and a carbon paper (Sects. 14.2 and 14.3) or carbon cloth (Sect. 14.4) gas diffusion layer. Furthermore, if it was not specifically noted otherwise, only dry gases were used for start/stop simulation and investigation. 

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. 

The two-dimensional, coupled kinetic and transport model can also be used to simulate start-stop processes. Figure 14 plots cathode potential and carbon corrosion current distribution at three instants when the H2/02 front passes through 10, 50, and 90% of anode flow path during the start process. As H2 displaces air in the anode flow-field, the size of the power source increases and the load size decreases accordingly. The balanced current density becomes larger, causing higher carbon corrosion current density. 

Click the Closed loop ATV bullet, start the simulation mnning, and click the Start test button. After several cycles, click the Pause button to stop the simulation and click the Finish test button at the bottom of the Tune window (see Fig. 3.79). The ultimate gain (3.73) and the ultimate period (4.8 min) are displayed, as shown in the left side of Figure 3.80. To calculate the controller tuning constants, click the Tuning parameters page tab on the Tune window and select either Ziegler-Nichols or Tyreus-Luyben. 

Fig. 10. Photon arrival time statistics of single emitters, (a) Schematic description of the temporal structure of single-emitter emission, (b) Simulated timetraces for different intersystem crossing rates as indicated, (c) Start-stop measurement yielding and anticorrelation, so called antibunching, at zero delay (the offset is due to different lengths of cables for both detectors), (d) Same measurement for pulsed excitation. Thick line Single emitter with missing peak at zero time delay. Thin line scattered laser light signal for comparison.

After a forgery. A stops simulating A and starts a dispute about the forgery, i.e., it inputs all the correct commands at the access points of the interest group and provides the correct connections at the connection-control access point. (And it makes no further inputs.) Before that, it may have to wait until previous transactions at those access points end availability of service guarantees that this happens. 

PLAY t Click to start model simulation or click during simulation to stop. 

Hartnig, C., and Schmidt, T.J. (2011) Simulated start-stop as a rapid aging tool for polymer electrolyte fuel cell electrodes. J. Power Sources, 196,... 

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

We are now ready to run the simulation. As shown at the top of Figure 4.33, there are a number of buttons that control the simulation. Clicking the first button to the right of the window showing Dynamic starts the simulation running in time. The mn can be paused (time stops changing and all variables are held at their current values) by clicking the fourth button. 

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. 

The effect of simulated start/stop cycling can not (Mily be measured electrochemically as shown before but also visualized by SEM imaging. In... 

Fig. 14.12 Relative perfomance decrease vs. beginning with 30 % Pt/Vulcan, 30 % Pt-alloy/Vulcan, and 20 % of life as a function of simulated start/stop cycles at 0.3 Pt-alloy/SC. Reproduced from [43] with permission of and 0.7 A extracted from I-E curves for cathodes Elsevier...

Figure 5.31b shows that the carbon corrosion current density in the R-domain is equal to the oxygen-limiting current density (2 A cm in this simulation. Table 5.10). Thus, during the start-stop cycle, carbon corrosion runs very fast and even short transients can severely damage the catalyst. The solution to this problem is in lowering the cell potential during the transient (Takeuchi and Fuller, 2(X)8). 

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. 

The Control Bar (Fig. 40) is used to start, pause, and stop simulations. Once the simulation has started, the simulation is locked, so that flie parameters within the simulation eannot be altered. 

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. 

Because of the very small bearing clearances in gas bearings, dust particles, moisture, and wear debris (from starting and stopping) should be kept to a minimum. Gas bearings have been used in precision spindles, gyroscopes, motor and turbine-driven circulators, compressors, fans, Brayton cycle turbomachinery, environmental simulation tables, and memory dmms. 

Qualitatively, the bilayer structures that result from DPD simulations are reasonable [65], In the simulation box, it is possible to find a stable bilayer in which the head groups shield the apolar core from the water phase. This means that the model effectively features a start-and-stop mechanism for... 

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