Simultaneous Measurements of Liquid Water Distributions and Catalyst Layer Surface Temperature inside Operating PEMFC
Proper water management in PEMFC is critical to achieve a high power density and durability of construction materials. While high current density operation, the gas transport is blocked by liquid water remained in GDL and gas channel, and causing performance decrement with increase of concentration overpotential. Therefore, it is necessary to realize the water distribution inside the cell. The absolute temperature influences the water distribution, but the through-plane temperature gradient affects the water transport as well. Especially, a temperature gradient becomes large at high current density, then, the understanding of the relations between water distribution and a temperature gradient is needed to achieve a high current density.
Recently, several techniques have been developed for in-situ measurements of liquid water in PEMFC such as neutron radiography, X-ray computed tomography (CT), and magnetic resonance imaging (MRI). Tsushima et al. observed the liquid water transport in PEMFC with operating separator temperature by using soft X-ray radiography, and reported that a smaller amount of liquid water in the MPL and GDL was observed as increase the operating temperature. Hickner et al. investigated the effect of the separator temperature on water distribution in an operating PEMFC using neutron radiography, and reported water content decreased by temperature rise. In these visualizing studies only separator temperature were measured, however, the temperature at the cathode CL is normally highest and a temperature gradient exists in through-plane direction. So, it is necessary to measure local temperature inside the operating cell.
Some experiments have been conducted to measure the temperature inside a cell. Ito et al. fabricated an in-line thermocouple of 49µm diameter and measured a temperature in the through-plane direction in the cell. However, It is assumed that the thickness of the thermocouple influences the PEMFC components.
Then we developed an ultra thin film thermocouple as thin as 10um not to deform the PEMFC components, and local temperature at the boundary between CL and MPL were measured. In addition, we simultaneously performed the in situ observation of the liquid water distribution in the GDL by using X-ray CT to investigate the effects of through-plane temperature gradient on liquid water distribution in GDL.
A thin film thermocouple was developed using MEMS techniques. The sensor was placed between cathode CL and MPL as shown Fig. 1, and the liquid water distributions were visualized simultaneously by using laboratory-based X-ray CT (Yamato TDM1000H-II (2K)). The spatial resolution was 2.8µm and a whole reconstructed 3D image was taken in 15min by 1440 angle projection. The cathode separator temperature was also measured by sheath type thermocouple.
3. Result and discussion
Two operation modes were performed. In operation mode A, the current density was increased every 10min, and the current density was changed intermittently every 1min like 0.3 [A/cm2]→0.01 [A/cm2]→0.5 [A/cm2] in operation mode B．Hydrogen and air were used as the fuel at flow rates of 100 [mL/min]. The cell was operated at room temperature and the anode and cathode gas streams were unhumidified. Fig. 2 shows X-ray CT images of a GDL near the cathode separator at 0.9 [A/cm2] in each operation mode. It was found that in operation mode A, liquid water hardly visualized. On the other hand, liquid water observed under the rib in operation mode B. Fig. 3 shows the temperature rise from OCV at the interface of CL/MPL and separator in each operation mode, and Fig. 4 shows the temperature rise from low current density to 0.9 [A/cm2] in each operation mode. As shown in Fig. 3, The absolute temperature in operation mode A was higher, but the temperature gradient between CL and separator in mode B became larger than mode A. This is due to the temperature at the interface of CL/MPL showed a sharp rise in mode B in Fig. 4 shows. From these, it is considered that the difference of the amount of liquid water was affected by not only the absolute temperature but also the temperature gradient.
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