In Situ Humidity Measurements at the CL Surface By MEMS-Based Sensors
Proton exchange membrane fuel cells (PEMFCs) are promising systems for mobile and vehicle applications because of its high power density and low operating temperature. However, this characteristic causes several problems such as water management. Too much water disturbs mass transport of oxygen and leads to current decrease, although proton exchange membrane (PEM) and ionomer in the catalyst layer (CL) need water to maintain sufficient proton conductivity. So, understanding water transport in PEMFCs is an important issue. Especially, it has recently been noticed that micro porous layer (MPL), which is often inserted between CL and MPL, controls the water transport and improves cell performance, but the role of MPL is not clarified yet.
For understanding water transport trough the MPL, temperature and relative humidity (RH) inside the PEMFC are important factors. There are some approaches to comprehend the RH inside PEMFCs. Nishida et al. measured the water vapor distributions between cathode GDL and the current collector by using water sensitive paper. They reported that the water concentration under the current collecting ribs becomes remarkably high in the cathode. Chi-Yuan Lee et al.conducted in-situ measurement of temperature and humidity at the GDL surface with developing the micro sensors. Thus, many researchers investigated about the water transport inside the PEMFCs, but it is still unclear because of absence of experimental data inside membrane electrode assembly (MEA).
So, we developed a thin film humidity sensor (TFHS) for measuring RH inside MEA. In our previous study, width of TFHS was 1.4mm, however, it is not enough narrow to distinguish the difference of under rib and channel, because the width of the rib and the channel is 1.0 mm. Furthermore, wider TFHS prevent the transports more. To solve these problems, we developed TFHS reducing the width of 0.35mm in this study.
2. Characteristic of humidity sensor
Capacitive type TFHS was developed with MEMS technology. Fig. 1 shows a schematic diagram and Fig. 2 shows a fabrication process of the THFS. The humidity sensor consists of Parylene® (SCS) and Au. Parylene was selected as an electrical insulating material and also sensing material to obtain a uniform and homogeneous film. 200 nm thick Au was deposited as electrodes and it is thin enough to obtain water vapor permeability to sensing material. The sensing area and width of the humidity sensor are 0.13 mm2and 0.35 mm, respectively.
Fig. 3 shows ex-situ calibration test of TFHS performed before using it. We measured capacitance and phase angle with an LCR meter (HIOKI 3522 LCR HiTESTER) changing supply gas RH ranging from 30% to 95%. Good linearity was observed between the RH and the capacitance, and the sensitivity was improved from the previous sensor by thinning the sensing material thickness.
3. Results and discussions
The cell performance with TFHS was examined with supplying RH60% Hydrogen/Air. Utilization ratios were 0.8/0.5 at 1.0A/cm2, and the separator temperature was maintained at 70 deg C. In this experiment, humidity sensor was inserted at the interface of CL-MPL under the rib in cathode downstream. Fig. 4 shows the capacitance change with current density. The black line shows the capacitance taken by new humidity sensor, whose width was 0.35 mm. The red line indicates was the value measured by previous 1.4mm width sensor. Capacitance change of the new sensor was steeply increased up to 0.2 A/cm2 and then increased gradually. This result implies that liquid water started to accumulate under the rib from the 0.2 A/cm2and new sensor was enough narrow to perceive the liquid water under the rib. On the other hand, the capacitance taken by the old sensor showed gradual increase over the whole current density range measured.
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