When Size Matters: Active Area Dependence of PEFC Cold Start Capability

Abstract

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.

Statistical Analysis of Isothermal Cold Starts of PEFCs: Impact of Gas Diffusion Layer Properties

“…the water cluster volume, the hydrophobicity of the substrate as well as its microstructure. This theoretical study is in good agreement with recent experiments 12 of our group, where local variations of the material itself led to significant variations of the cold start capability. If these features that promote the freezing can be identified and removed, the cold start capability can be significantly improved for lab-scale as well as for technical fuel cells.…”

“…As has already been described in Ref. 12, no cell failures occur during the Voltage peaks.- Figure 3 shows the detailed temporal evolution of the cell voltage during three cold starts of type I at −10…”

“…The statistical investigation is based on more than 700 cold starts performed on five different cell configurations with at least two repetitions each (see Table I). As has been shown previously, 12 experiments with small scale fuel cells enable the disclosure of hidden effects inside technical fuel cells. In Table I.…”

“…Second, as these experiments were performed on lab-scale fuel cells of 50 cm 2 , cell failures occur in a repeatable manner with high probability after short operating times. 12 Based on these two differences, the freezing mechanism of larger labscale fuel cells operated under technical operating conditions appears to be different on the global scale even though the freezing mechanism is identical on the local scale.…”

“…47 Due to the beam time constraints, only one experiment was performed with imaging for each cell configuration.…”