Effective Electrode Edge Protection for Proton Exchange Membrane Fuel Cell Drive Cycle Operation

Abstract

Roll-to-Roll (R2R) manufacturing techniques have been demonstrated to be a cost-effective approach to continuously fabricate polymer electrolyte membrane fuel cell (PEMFC) materials at a large scale.1 In practical R2R manufacturing, non-uniformities including electrode coating irregularities do appear during the fabrication of the PEMFC membrane electrode assembly (MEA). In-line diagnostic tools to monitor quality have been developed at the National Renewable Energy Laboratory (NREL) and have been used to successfully detect a variety of electrode coating irregularities in a R2R manufacturing setting.2-3 However, limited understanding exists regarding if and to what extent electrode irregularities impact PEMFC performance and lifetime. Previously, we have investigated how cathode irregularities (bare and thin spots) impact MEA lifetime using a quasi in-situ infrared thermography technique in combination with accelerated stress tests (AST).4 In this work, driving cycle (DC) tests based on the New European driving cycle (NEDC)5 were employed to study the impact of cathode irregularities on degradation of MEA performance over time. Previous degradation studies indicated that the edge of the electrode area was the dominant failure location of the MEAs used. This observation was attributed to the excessive mechanical stress that was created by the relative humidity (RH) cycling procedure of the utilized AST.6 Edge protection techniques have been developed by the community to lengthen MEA lifetime and to prevent the immediate failure in ASTs.4, 6-7 Driving cycle tests employ rapid load cycling which will result in rapidly changing local operating conditions and consequently high non-uniform mechanical stress at the electrode perimeter. In order to better investigate the impact of electrode irregularities on the long-term behavior of the cell, it is necessary to exclude the edge effects of the MEAs as a failure mode. Therefore, an effective electrode edge protection technique using thin protective gaskets and a hot-pressing procedure was developed which dramatically prolonged DC lifetime from 288 hours to 1296 hours and additionally resulted in MEA performance improvements. Open circuit voltage (OCV), electrochemically active surface area (ECSA), air polarization and hydrogen crossover limiting current (iH2) data were monitored during the driving cycle tests. With edge protection, the OCV decay and the iH2 growth of the MEA was slowed. For post-DC ex situ analysis, an in-house developed pinhole detection apparatus (PDA) was employed to analyze quantity, size, and location of the failure locations of MEAs with and without edge protection. Non-protected MEAs typically developed tears at the electrode perimeter, while the longer-lasting protected MEAs exhibited seemingly random pinhole development. To better understand the degradation behavior of protected MEAs during the DC tests, scanning electron microscopy with energy dispersive X-Ray spectroscopy (SEM/EDS), transmission electron microscopy (TEM), and X-ray diffraction (XRD) were employed to study the morphology and structure of the MEAs at the beginning of life (BOL) and the end of life (EOL). Results will be discussed in the presentation.