Investigation of Durability of MEAs at Higher Temperature

PEFCs recently attract much more attention owing to the commercialization of fuel cell vehicles (FCVs). We are interested in the operation of PEFC at high temperature to improve overvoltage loss of reactions occurring in PEFC. In this study, extracting problems on catalytic degradation at higher temperature is particularly focused on, and then durability of MEAs at 105 °C is considered.  

MEAs were prepared using standard catalyst and electrolyte, Pt/Ketjen black catalyst (TEC10E50E, TKK Corp.) and Nafion. For obtained MEAs, current-voltage (IV) performance and durability were studied. Regarding to durability evaluation, two protocols^[1] recommended by Fuel Cell Commercialization of Japan (FCCJ), “load cycles” and “start-stop cycles”, were in use. Three different conditions of operation temperature and humidity (80 °C/RH100%, 105 °C/RH57%, and 80 °C/RH57%) were applied.

Regarding to durability analyses using “start-stop cycles”,^[2] the increased loss of IV performance was clearly observed at 105 °C/RH57%. To understand degradation mechanism, the structure change of cathode layers was evaluated using FIB-SEM. The cathode layers became thinner in all the cases, but the condition of 105 °C/RH57% led to the thinnest layer. By further three-dimensional evaluation of cathode layers, increased size of pores constructing the catalyst layer was observed. The condition of 105 °C/RH57% resulted in formation of large pores especially near the Nafion membrane. Therefore, we believe such large pore formation derived by carbon corrosion led to collapsing and finally thinning the cathode layer.

On evaluation of “load cycles” durability tests, the operation at 105 °C/RH57% resulted in lower durability than that at 80 °C/RH100%. However, the difference was much smaller comparing to the case in “stop-start cycles”. Regarding to Pt size evaluated by SEM (Figure 1), the size increase of Pt particles was more pronounced in the case of 80 °C/RH100%. The reason most likely comes from enhanced Pt growth under the higher water content. Thus, the decreased IV performance at 105 °C/RH57% involves additional factors such as Nafion degradation at the low humidity condition.

Possible degradation mechanism using two different durability protocols is discussed with macrostructure change in cathode layers.

Reference

[1] A. Ohma et al., ECS Trans, 41(2011) 755.

[2] M. Kitamura et al., ECS Trans, 64 (2014) 755.