0.3 0.25
jmean [A cm ]
0.2 0.15 Channel
Cell 0.1 0.05 0
Rib 0
50
100
150
200 time [s]
250
300
350
400
Figure 3. Current density distribution over a single repeating unit of channel and rib as function of time at -7 °C.
Figure 4. Integral current density and locally averaged current densities over the rib and over the channel as function of at -7 °C.
When focusing on the channel-rib scale, Figure 3 shows (exemplarily for -7 °C) the temporal development of the current density distribution. Initially the distribution shows little difference between the channel and land areas. However, under the rib the current starts to drop quickly while under the channel the peak current density seems to even increase slightly.
References
When the current is averaged over the rib and channel areas the same trend is observed. Figure 4 shows that when the current is averaged, also over the channel a monotonous decrease is observed. But still the current drops much quicker under the rib, here the current density decreases to zero in less than 60s. The difference between channel and rib can be explained by two trends (i) the lowest temperature in the cell after drawing current is under the rib (because of the heat capacity of the rib) and (ii) under the channel, the current density decreases slower, since more heat is produced and the heat capacity of the GDL only is low. Moreover, the short diffusion pathway under the channel may allow for super cooled water to be transported into the channel. Consequently, the channel domain is dominating the sub-zero performance. Temperature seems to influence the kinetic of ice formation, predominantly under the rib.
[3] E. Pinton, Y. Fourneron, S. Rosini, L. Antoni, J. Power Sources, 186, 80-88, (2009).
[1] J.P. Meyers, in Polymer electrolyte fuel cell durability, Eds. F.N. Büchi, M. Inaba and T.J. Schmidt, Springer, 369-382 (2009). [2] Y. Ishikawa, H. Hamad, M. Uehara, M. Shiozawa, J. Power Sources, 179, 547-552, (2008).
[4] S.A. Freunberger, M. Reum, J. Evertz, A. Wokaun, F.N. Büchi, J. Electrochem. Soc., 153, A2158-A2165, (2006). [5] M. Reum, S.A. Freunberger, A. Wokaun, F.N. Büchi, J. Electrochem. Soc., 156, B301-B310 (2009).
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