Analysis of MPL Impact in PEFC with in-Situ Temperature Measurement in through-Plane Direction
Authors: Mizutani Chiaki , Matsumoto Tokuma , Nakajima Hironori , Kitahara Tatsumi , ITO Kohei 
Published: February 2020

Water management is a key issue to the practical operation of PEFC. Drying up in polymer electrolyte membrane (PEM) increase the ionic resistance and rises IR overvoltage. On the other hand, supplying excessive water causes flooding and increases the concentration overvoltage, and hence cell performance deteriorates. Therefore, appropriate water management is required.

Micro porous layer (MPL) has been developed to control the water and gas flow in cell. MPL is a hydrophobic component which consists of carbon black and PTFE on the surface of gas diffusion layer (GDL) by the CL side. MPL can control liquid water and gases transport, and keep high performance even when severe flooding or drying-up condition. Recently, MPLs are improved with adding hydrophilic layer for further robustness in PEFC operation.

However, the impact of MPL on cell performance is not sufficiently understood, and this situation becomes an obstacle in the development and improvement of MPL. Impacts of the structure and hydrophobicity in MPL are usually evaluated with IV characteristics. It is difficult to elucidate the change of water and gases transport only from IV characteristics. Local water behavior, temperature and current should be clarified in advance to efficiently optimize MPL.

Some methods have been reported to measure the internal state inside cell in-plane and through-plane direction. For example, neutron imaging discloses in-situ liquid water distribution in the cell, contributing to the understanding the relationship between operation parameters and local water behavior. Not only water behavior but also temperature may change with MPL, because thermal conductivity in MPL is different from cell components, such as GDL and CL. Temperature distribution especially in through-plane direction may change as MPL is employed and water behavior changes.

These backgrounds motivate us to challenge to measure the temperature distribution by fabricating ultrafine in-line thermocouples and placing them in a cell. The tailor-made thermocouples are accurately positioned under rib or channel and also placed in through-plane direction, so that temperature distribution measurement in the cell with and without MPL is conducted. Though the temperature difference between under the rib and channel did not vary from presence of MPL, the distribution in the through-plane direction changed considerably, suggesting that MPL impacts on water behavior significantly. This study discusses the impact of MPL on water behavior with the in-situ measured temperature distribution.

Ultrafine in-line thermocouples are fabricated in the following manner. Butt-welded K-type thermocouple wires in diameter 50 μm formed an in-line thermocouple. Electrical insulation is required to insert the thermocouple into the cell. For this purpose, the thermocouple was coated with polyimide.

The fabricated thermocouple is precisely placed into cell with the fixing tool which has a squire frame shape. In-line thermocouples were stretched across the tools, with an adjustment so that the thermocouple junctions are positioned under rib or channel. Four sets of the fixing frames are arrayed in piles, and temperature is measured along through-plane direction on the four points: anode GDL, anode CL, cathode GDL and cathode CL.

Air and hydrogen were supplied into the cathode and anode respectively with relative humidity of 20 % and the utilization ratio of 0.1 at 1A/cm2. The cell voltage difference between the cases with and without MPL was about 0.1 V. The temperature around CL with MPL was slightly higher than that without MPL. The distribution without MPL was homogeneous, however, the MPL case tended to have the maximum temperature at cathode CL; it might be caused by the low thermal conductivity of MPL.

Cell voltage in the case with MPL was 0.42 V higher than that without MPL, while cell was operated under relative humidity of 95 % and gas utilization ratio of 0.4. Although it is expected that flooding occurs at cathode CL and cell performance decreases in this condition, MPL promotes to drain the bulk liquid water. This is the reason that the performance with MPL became higher. Temperature with MPL was also higher than that in the case without MPL. The cell without MPL tends to accumulate water at CL, and water evaporates there. Thus, the latent heat with the evaporation decreases the temperature in the cell without MPL.

From the above, water behavior differences between with and without MPL is explained through the measured temperatures. It is suggested that the developed temperature measurement method can contribute to cell performance improvement with water management by MPL.