Investigation of a Correlation Between PEFC Operation Conditions and Electrocatalysts Degradation

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One of the strategies to further increase PEFC performance is elevation of the operation temperature, reducing the reaction overvoltage of oxygen reduction reactions. On the other hand, durability of materials used in PEFC is assumed to be lowered at the higher temperature condition. In this study, understanding degradation phenomena of electrocatalysts at the higher temperature is particularly focused on in order to develop durable electrocatalysts at the elevated temperature.MEAs were prepared using the standard catalyst and electrolyte, Pt/Ketjen black catalyst (TEC10E50E, TKK Corp.) and Nafion, respectively. Three different conditions of operation temperature and humidity (80 °C/RH100%, 105 °C/RH57%, and 80 °C/RH57%) were applied. Durability of MEAs in three conditions was studied through the potential cycles between 0.6 and 1.0 V assuming “load cycles” of FCVs.[1], [2] Since Pt dissolution/re-precipitation is assumed to occur during the cycles, the changes in activation overvoltage and ECSA were carefully examined under three conditions. After 20000 potential cycles, activation overvoltage and ECSA increased and decreased, respectively, the most at 105 °C/RH57%. On the other hand, such degradation was much suppressed under the operation at 80 °C/RH57%. In order to understand the difference in degradation phenomena, the amount of water molecules was considered. With the presence of a large amount of water molecules, the rate of reducing ionized Pt (Pt2+) into Pt metal is assumed to be smaller. Then, in the case of 80 °C/RH57%, the amount of water molecules was smallest among three conditions, leading to fast and reversible reduction of Pt2+to Pt metal. Therefore, Pt particle growth was believed to be suppressed. However, if two factors, the amount of water and temperature, are considered, the temperature factor most likely influences more on Pt particle growth. Reference[1] A. Ohma et al., ECS Trans, 41 (2011) 755.[2] M. Kitamura et al., ECS Trans, 69 (2015) 701.