RRDE Analysis of Ethylene Glycol and Caprolactam Effects on the ORR

Abstract

Polymer electrolyte fuel cells (PEFCs) are very promising as clean energy sources for future automotive applications. However, the performance and durability of PEFCs can be significantly decreased by the presence of contaminants. Such species may adsorb on the Pt catalyst, penetrate into the ionomer and membrane, or compete with the main reactions, resulting in a decrease of the active surface area, ionic conductivity of the catalyst layer and membrane, and cell performance. Also, H2O2 is a side product of the oxygen reduction reaction (ORR) and its associated yield often increases in the presence of organic contaminants, such as acetonitrile, propene, naphthalene, and methyl methacrylate [1-3]. A larger H2O2 generation rate exerts an undetermined influence on the performance and durability of PEFCs. The formation of H2O2 has been linked to the radical attack decomposition of the Nafion® ionomer, which is accelerated by the presence of Fe2+ and Cu2+ ions [4,5]. The Nafion® ionomer is not only used as an electrolyte membrane, but is also added to the catalyst layer to exploit its binding property and improve catalyst utilization. Consequently, the decomposition of the Nafion® ionomer is a significant risk to PEFC performance and durability. Therefore, it is necessary to investigate the ORR, hydrogen oxidation reaction (HOR), and H2O2 yield in the presence of contaminants to understand their effects on PEFCs. Ethylene glycol (EG) is of interest because it is widely used as a coolant, antifreeze agent, and de-icing solution. De-icing of airport runways and airplanes is the primary source of EG in the environment. EG is also dispersed in the environment by the disposal of products that contain it. Caprolactam is another potential contaminant released by PEFC system materials either as a result of degradation or leaching. It has been detected in many leachates including from polyphthalamide materials that are being considered for use as balance-of-plant structural materials [6,7]. Both EG and caprolactam can poison ORR Pt/C catalysts [8-10], but the H2O2 yield has not been measured. In contrast to the slow ORR kinetics, the HOR kinetics on Pt/C catalysts is so fast that the cell voltage losses are negligible even for very low Pt loadings. However, attention has not been given to the poisoning effects of EG and caprolactam on the HOR. These considerations are important for prevention because tolerance levels are currently missing. The rotating ring/disk electrode (RRDE) is the equipment of choice to reveal the H2O2 yield and reaction pathway. In this presentation, the contaminant impacts of EG and caprolactam on the kinetics of ORR and HOR in acid media using RRDE are reported. Results will emphasize the changes in the electrochemical surface area, ORR and HOR kinetic currents, Tafel slope, H2O2 yield,and reaction pathway as a function of EG and caprolactam concentration. Acknowledgments Authors are grateful to the Office of Naval Research (award N00014-13-1-0463) and the Hawaiian Electric Company for their ongoing support to the operations of the Hawaii Sustainable Energy Research Facility.