Understanding the freezing mechanism of liquid water within polymer electrolyte fuel cells (PEFC) is crucial for its commercialization for mass market. Although various publications investigated the cold start capability of PEFCs, contradictory behaviors during cold starts at freezing temperatures have been published in literature. Interestingly, differences can be mainly identified between large and small size fuel cells: the latter have a significantly higher cold start capability than large size fuel cells. In order to understand the influence of different active areas, we present measurements of cold starts performed with cells sizes of 1 and 50 cm2 using the same materials, including a large count of experiments (>200) on the 1 cm2 cells to perform statistical analysis. The cold start capability is strongly reduced with an increasing size of the active area, and the operating times changes from stochastic values for small cells to reproducible values for large cells. Both effects can be explained by the higher probability of the aggregate state transition from super-cooled water to ice in large cells. Consequently this paper highlights the implications of downscaling PEFCs to draw conclusions for technical relevant cold-starts.
Starting PEFCs from sub-zero temperatures can be challenging as generated product water runs the risk to freeze, causing cell failure by blocking the gas pathways for the reactant gases in the porous layers. Hence a fundamental understanding of the cell failure mechanisms is essential to enable robust system functionalities even at temperatures far below 0 • C. In this work we set a focus on analysis of water transport processes during isothermal startup at temperatures between −10 and −2.5 • C. Neutron radiography was applied in order to analyze spatial heterogeneities of water production during a cold start and also to verify phase transitions from water to ice. The latter was facilitated by a recently developed dual spectrum neutron radiography method, which was applied for the first time to a 50 cm 2 test cell. Our results reveal that at −5 • C and above freezing can occur in a limited region while the rest of the cell continues generating liquid product water. But as temperature is shifted downwards, water distribution tends to be more uniform and freezing mechanisms seem to proceed more homogeneously over the cell plane.
This work focuses on water transport behavior in PEFC, particularly in the area situated between flowfield and sealing gasket. An in situ investigation of fundamental water and gas transport effects in this edge region was performed by means of neutron radiography. Five different 50 cm 2 test cell setups were operated, basically differing in the applied sealing solution, whereby it was accounted for the requirements posed by metallic bipolar plates. Main influencing factors on the water transport were identified, namely channel geometry, gas inlet humidification, temperature and gas pressure under static and dynamic conditions. It was found that the design of the edge region can have a significant influence on water transport properties and water accumulation in the cell. Depending on its gas accessibility, the edge region can act as a reservoir for water accumulation. Time constants for water transport can be shifted to the range of hours, which is crucial for an effective water removal as it is essential for cold starts. In addition, relatively wide edge channels, as required for sub-gasket based cell setups, can enable strong bypass flows around the flowfield and thus lower the effective stoichiometry, which can have a significant impact on the water content in the flowfield.
Isothermal cold starts of polymer electrolyte fuel cells were performed at sub‐zero temperatures and analyzed by means of neutron radiography in order to unravel the relation between the preconditioning of the cell and the cold start capability. It was found out that the initial humidification state of the membrane (determined by its resistance) has a clear correlation with the duration of the initial phase of the cold start, but not with the total duration of startup until cell failure. In the experimental setup the impact of realistic and commercial sealing solutions was taken into account by adding an edge channel. The impact of water accumulations in this region on the cold start capability was assessed. Liquid water located in the outer perimeter of a cell could be directly verified to freeze during cell cool down by a novel dual spectrum neutron radiography method. The amount of water accumulated in the outer cell perimeter showed some correlation with the total duration of the cold start. It was found out that residual water located in the edge channel can initiate freezing of a substantial part of the active area nearby.
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