MEAs were prepared using a standard catalyst and two types of electrolytes, Pt/Ketjen black catalyst (TEC10E50E, TKK Corp.), and Nafion® and Aquivion®, respectively. Here, The Aquivion® has higher proton conductivity than Nafion®. The operating condition of the higher temperature with lower humidity, 105 °C-RH57%, was applied. Durability of MEAs was studied through the potential cycles between 1.0 and 1.5 V assuming “stop/start cycles” of FCVs.[1], [2]
Durability of two different MEAs (Nafion® MEA and Aquivion® MEA) was evaluated in terms of electrochemical and structural changes before and after the cycle test. Here, structural changes were analyzed using a FIB-SEM technique followed by 3D reconstruction of cathode catalyst layers. Although current–voltage curves were not very much different between two types of MEAs even after 2000 cycles, the macroporous structures of catalyst layers were different. Porosity was almost homogeneous through the catalyst layer in Aquivion® MEA, but larger pores intensely formed near the Nafion® membrane side in the case of Nafion® MEA. The possible degradation mechanism is illustrated in Fig.1 and explained in the following. The carbon oxidation reaction probably occurs mainly by reacting with H2O being formed by a fuel cell reaction if it is under the low humidity condition. If proton conductivity is high enough like Aquivion® ionomer, H2O is probably generated more uniformly and carbon oxidation also occurs uniformly throughout the cathode catalyst layers. On the other hand, when proton conductivity is low like Nafion® ionomer, H2O is mainly generated at the interface between the cathode layer and Nafion® membrane. Then, carbon oxidation intensely occurs at the interface between the cathode and the electrolyte membrane.
Reference
[1] A. Ohma et al., ECS Trans, 41 (2011) 755.
[2] H. Miyamoto et al., ECS Trans, 75 (2016) 329.