Proton exchange membrane fuel cells (PEMFCs) are useful systems because they operate at lower temperatures than other types of fuel cells. This characteristic causes water management issues. To elucidate effects of water on PEMFC performance, we developed a temperature sensor using micro-electro-mechanical systems (MEMS) techniques. This sensor was placed between the catalyst layer (CL) and the microporous layer (MPL) at the cathode. Slight effects on total cell performance were observed with the insertion of the sensor. This sensor can be applicable to typical PEMFCs without any special equipment such as a transparent separator. In-situ measurement with a temperature sensor showed that the maximum temperature rise at the cathode CL (CCL) was about 9 °C at 1.1 A/cm2. The temperature sensor also showed temperature gradients between the ribs and channels. In addition, we developed a capacitance-type humidity sensor and inserted it in the channel. In-situ measurement with a humidity sensor showed a relative humidity (RH) change in the channel. This sensor can detect not only RH but also accumulated water in the channel. Liquid water appeared in the channel at 0.7 A/cm2.

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