*4.4. Failure Analysis*

Investigating the episodes resulting in the final failure, Figure 9 zooms in on the records of the four stacks in the range of hour 3581 to 3631. The frames show the voltages, the power generation, and the temperature from top to bottom. The figure shows a normal operation initially in which the power outputs from the stacks were all approximately 1 W, the voltages were controlled at 2.8 V, and the stack temperatures were rising above 55 ◦C as the ambient temperature rose.

The failure event started to develop at 3582 h into the experiment when the power generation and, thus, also the temperature, of the 4th stack on one side of the station, as shown in yellow, became lower than other stacks. The 4th stack was cut off and shut down because an abnormality was determined by the EMS at hour 3589 (➊) and its voltage peaked briefly and dropped gradually to zero followed by its temperature reaching 7 ◦C lower than others. It started to sink heat from the others. After hour 3591, the power of the 2nd stack, shown in red-orange, began to oscillate and become frail even though the ambient temperature fell back. At hour 3600 (➋), the 2nd stack got cut off, and stopped generating power causing temperature drops. Since it is sandwiched by the still normal and heat generating stacks 1 and 3, its temperature did not decrease as much as the 4th stack. The power of the 1st stack also became frail, as shown in brown, until it stopped at hour 3606 (➌). At this time, the power generation of the 3rd stack, shown in orange, briefly increased to compensate for the over-cooling temperature, and then finally diminished. At hour 3621 (➍), the researcher came to check and restarted the power station. Although the 1st and 2nd stacks could still be restarted, their powers quickly failed again. The EMS determined that the system could not operate anymore at hour 3628 (➎) and stopped. Based on this final episode, from the first stack power failure to the total shutdown took about 32 h. With multiple stack modules in the system, the redundancies enhanced durability and provided lead time to call for maintenances.

**Figure 9.** High-resolution experimental record from 3581 to 3631 h.

A forensic engineering study was carried out on the failed stack modules, and the final failure was caused by the aging of MEAs. The degradation of electrochemical efficiency leads to increasing fuel demand at the targeted operating condition beyond MEA capability and successively causing damage by flooding.
