Proper water management is usually recognized as a key topic in the proton exchange membrane fuel cell (PEMFC) technology. Either too dry or too wet operating conditions can have a direct impact on cell performance, in particular limiting the maximal power density. Reducing these detrimental ežects to their minimum is therefore a requisite to maximize the power which can be obtained out of a deœned fuel cell size, or inversely, reduce the cell size-and subsequently its cost-for a given target maximal power. Besides the direct ežect on power density, the topic of water management is highly relevant in terms of improving fuel cells durability. On one side, the durability of a fuel cell can be ažected by the distribution of water (as a liquid or as vapor) in dižerent ways. še chemical degradation of Naœon, a widely used polymer electrolyte, has been reported to be much faster in low-humidity conditions (Endoh et al., 2004; Sethuraman et al., 2008; Chen and Fuller, 2009). Dry conditions combined with sudden load changes can lead to localized temperature elevations in the membrane leading to the formation of pinholes (Schneider et al., 2008). Excess of water can also have negative impact on durability, as ›ooding can induce reactant starvation, which in turn favors degradation processes (Yousœ-Steiner et al., 2009). še amount of residual water present a§er cell shutdown is also critical. Carbon corrosion induced by

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start/stop cycles is reported to have a clear dependence on the relative humidity (Linse et al., 2009). When residual water is present in nonoperating fuel cell in regions subject to cold climatic conditions, damage can be induced by freeze/thaw cycles (Kim and Mench, 2007; Kim et al., 2008). On the other side, some degradation processes occurring in a fuel cell may disturb the water management, as, for example, a change in wetting properties of the porous media. šus, part of the power losses of a degraded cell can for example have its origin in increased ›ooding. Proper water management should therefore be designed not only to be ežective in the pristine cell, but also a§er several thousands of hours of operation. šis later issue has not been addressed yet in the literature in the frame of visualization studies. Visualization techniques have an obvious interest in the context of water management studies, either with the idea of directly drawing conclusions from the observation of liquid water distribution, or with the aim of using liquid water distribution measurements in order to reœne models of water transport. šerefore, these techniques have received an increasing attention during the last decade. Several dižerent methods have been reported for the observation of liquid water in operating PEMFCs. šese methods include optical imaging on transparent fuel cells (Zhang, F. Y. et  al., 2006), nuclear magnetic resonance imaging (Tsushima et al., 2005; Minard et al., 2006; Dunbar and Masel, 2008; Zhang et al., 2008), neutron imaging (cf. Section 12.5), synchrotron x-ray imaging (Manke et al., 2007b; Büchi et al., 2008; Hartnig et al., 2008b), and Raman spectroscopy (Matic et al., 2005). šis chapter, dealing with neutron imaging, is intended to explain the basics of this unique method, to demonstrate the results obtained out of it, and to motivate for further studies using this tool. In the examples and in the review being part of this chapter, the theme of durability will only scarcely be explicitly mentioned. But one should keep in mind, as cited above, that proper water management is in all cases relevant for fuel cell durability.