Measurement of Through-Plane Humidity Distribution inside an MEA by Thin Film Sensor

Abstract

A Proton exchange membrane fuel cell (PEMFC) is attracting attention as a clean and new power source system. However, there are several problems for commercial uses. One of the problems is water management inside the PEMFC. Too much water inside the PEMFC disturbs mass transport and leads to lower power density, on the other hand, proton exchange membrane (PEM) and an ionomer in a catalyst layer (CL) need water to maintain proton conductivity. In addition, water vapor and liquid water flow coexist inside the PEMFC because the operating temperature is lower than 100 degrees. Therefore, understanding water transport inside the PEMFC is difficult and a key to improve the cell performance. Especially, it has recently been noticed that a micro porous layer (MPL), which is inserted between a CL and a gas diffusion layer (GDL), controls the water transport and improve cell performance, but the role of the MPL is not clarified yet. For understanding water transport through the membrane electrode assembly (MEA), relative humidity (RH) is an important factor. So, we developed a thin film humidity sensor (TFHS), which is capacitive type, for measuring RH inside the MEA. In our previous study[1], width of TFHS was 0.35mm. However, wider sensor prevents the mass transport more. In this study, we developed thinner TFHS whose width was 0.1mm.Capacitive type TFHS was developed with MEMS technology. Fig. 1 shows an overview and schematic cross section of TFHS. The humidity sensor consisted of Parylene, Ni and Au. Parylene, that is polymer material, was applied as an insulating material and a sensing material to obtain a uniform and homogeneous film. In addition, hysteresis of it was very small and a uniform and homogeneous film was evaporated. Ni was deposited as an electrode and it was thin enough to obtain water vapor permeability to sensing material. Au was deposited as reinforcement so as not to break the sensor when inserted inside the PEMFC. In situ measurements with TFHS was investigated with supplying RH20% and RH60% gas. Flow rate of gases was 200 and 500 NmL/min respectively, and the separator temperature was set to 60oC. In this experiment, TFHS was inserted at the CL/MPL interface under the rib. In situ measurements were conducted three times at the each condition, and reproducible results were obtained. The results implies that water is mainly transported as vapor phase near the MPL. In the RH60 condition, liquid water may start to accumulate under the rib from 1.0 A/cm2 because RH change is almost saturated near the RH100%. Reference [1]. J. Tsujikawa, R. Minami, and T. Araki, ECS transactions, 69, 471-476, (2015) Figure 1