Cold-starts of a polymer electrolyte fuel cell (PEFC), isothermally maintained at various subfreezing temperatures, were visualized using high resolution dynamic in-plane neutron imaging. The results obtained aim to bring new knowledge about the water accumulation mechanisms leading to the voltage drop usually observed in isothermal mode after a given working time, also called voltage failure. In particular, the data presented should be useful for comparison to simulation predictions provided by modeling studies. As main result, water in a condensed phase was observed to accumulate not only in the membrane-electrode-assembly (MEA), but also in the cathode gas diffusion layer (GDL) at −15 • C and even in the cathode gas channels at −10 • C. Moreover, approximately 400 cold-starts were realized without neutron imaging and revealed stochastic distributions of working times. The presence of water in super-cooled state is discussed and finally retained as only valid explanation of the results obtained. Additionally, the sudden freezing of super-cooled water is thought to cause the rapid water accumulation observed in the MEA during the voltage failure.

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A novel experimental method for the measurement of mass transport losses in a polymer electrolyte fuel cell (PEFC) was developed, based on the comparison of cell voltage during operation with air, helox, (79% He, 21% O 2 ) or pure O 2 . Using only short periods (2 seconds) of helox or pure O 2 operation, the disturbing artifacts (dry out or change of catalyst surface oxidation state) associated with continuous operation with these gases were avoided. High resolution neutron imaging was conducted simultaneously in order to study the relation between mass transport loss and liquid water accumulation. For the studied design, comparisons in steady state and dynamic experiments indicated a larger impact of water accumulation in the gas diffusion layers (GDL) under the flow channels, or in the portion of the channels near the GDL surface. Increased diffusion losses were observed, as expected, under high humidity conditions, but also at very low humidity conditions in absence of any liquid water. The latter was explained by a redistribution of current toward the rib region, having longer diffusion paths.Polymer Electrolyte Fuel Cells (PEFCs) are energy converters, bearing attractive characteristics for automotive applications in terms of efficiency and absence of pollutant emissions. The dimension of an automobile fuel cell system is primarily determined by the required peak power. In consequence, reaching the highest possible power density allows a reduction of the system size, weight, and cost. At high current density operation, losses introduced by the limitations in the diffusive transport of oxygen through the gas diffusion layers (GDLs) can become significant. The presence of liquid water in the gas diffusion layers reduces the effective diffusivity of such media and results in increased losses.The visualization of liquid water in operating PEFCs using different methods has been increasingly reported. A recent review on in situ visualization methods can be found in a publication by Tsushima et al. 1 Besides its high contrast for liquid water, neutron imaging 2-20 offers the advantages of good transparency of usual fuel cell materials (including metals), and of a negligible impact of the non-ionizing radiation on cell operation, making it an excellent non-invasive method. Water visualization in fuel cells has been a strong driving force for addressing the issue of the limited spatial resolution of neutron imaging. [19][20][21][22][23] The approach used at the Paul Scherrer Institut, based on optimized optical setup and scintillator screens, 24 combined with specific improvements for fuel cell imaging, 20,21 allowed reaching the combination of an effective spatial resolution of 20 μm with exposure times of 10 seconds. Thus, the measurement of liquid water content in different layers of a fuel cell is possible, not only in steady state, but also in transient operation. In order to assess the impact of liquid water on cell performance, a method for directly measuring diffusive transport losses is of high interest. For...

Recent work at PSI based on anisotropic resolution improvement allowed reaching a combination of 20 ȝm of effective spatial resolution with 10 s exposure times for neutron imaging. This new setup was applied to dynamic in plane imaging of PEFCs in different applications such as the characterization of bulk and interfacial water transport parameters of membranes using 1 H-2 H contrast imaging, the study of water/ice behavior during sub-zero startup, and the correlation between water accumulation and diffusive voltage loss by combining neutron imaging with transient Helox operation. Future studies will take advantage of a new setup allowing the simultaneous operation and neutron imaging of 6 small scale differential cells. IntroductionThe present paper describes the recent and future work at PSI in terms of application of neutron imaging to the study of water transport in polymer electrolyte fuel cells (PEFCs). Neutron imaging has been increasingly used for this purpose during the past decade (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19). The characteristics that make neutron imaging attractive for PEFC research in comparison to other imaging methods are: -The good transparency of usual fuel cell construction materials to neutrons -The high contrast of liquid water.-The negligible effects of neutron radiation (no heat absorbed, no radiation damage).-For specific experiments, the isotopic sensitivity of neutron imaging can be used. The first section of the paper will summarize PSI studies using neutron imaging that have recently been published or are in the process of publication. In the second section, the result of a yet unpublished study about the combination of neutron imaging with transient Helox operation will be presented in more detail. The possibilities opened by the latest improvements in detector and fuel cell test setup will be presented in a third section. Recent Work at PSI in Neutron Imaging of PEFCs High Resolution Time Resolved Neutron Imaging of PEFCsAlthough the recent record of publications (e.g. (14)(15)(16)(17)20)) show that valuable results are still to be obtained with "conventional" through plane imaging, the focus was placed ECS Transactions, 41 (1) 27-38 (2011) 10.1149/1.3635541 © The Electrochemical Society 27 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 137.99.43.51 Downloaded on 2015-06-21 to IPat PSI in the recent years on high resolution imaging of fuel cells in in plane configuration (membrane parallel to the beam axis). Until recently, no imaging setups with sufficient spatial resolution for effectively observing the water distribution across a state-of-the-art fuel cell gas diffusion layer were available. Application to fuel cell imaging has been one of the main driving forces for realizing high resolution neutron imaging setups. The approach used at PSI consists in a setup with optimized optics and detector materials (21), combined with specific improvements targeting the applic...

Cold-starts of a polymer electrolyte fuel cell (PEFC), isothermally maintained at various subfreezing temperatures, were visualized by dynamic in-plane neutron imaging. The results obtained strongly suggest that super-cooled water is present in the system and that its sudden freezing is the initiator of the final voltage drop occurring at the and of the cold-start.