Enhancement of Liquid Water and Oxygen Transport By Partially Hydrophilic Mpls in a PEFC
Authors: Tanaka Yuya , Aoyama Yusuke , Uemura Suguru , Tabe Yutaka 
Published: December 2024

Management of produced water is extremely important to increase the output of PEFCs. Several methods have been investigated to improve the ability to manage liquid water transport by partially adding hydrophilicity to hydrophobic MPLs. In this study, we focused on MPLs coated with a hydrophilic layer(1). By separating the oxygen transport resistance inside and outside the catalyst layer (CL), we analyzed the effect of condensed water in the cell on the oxygen transport phenomenon, and aimed to provide guidelines for the production of MPLs for further power enhancement.

A mixture of carbon black: Li-400 (made by DENKA CO.) and Nafion dispersion liquid in a solvent consisting of pure water and ethanol was coated on the MPL side of the gas diffusion layer: 28BC (made by SGL CO.) and allowed to dry to produce the hydrophilic layer. The ratio of carbon black and Nafion in the hydrophilic layer was 10 : 1, and the thickness was about 5 μm.

In this study, active area of the cell was 1.8 cm2 (0.9 cm × 2.0 cm). The MPL on the anode side was 28BC and the cathode side was 28BC with a hydrophilic layer applied. The anode side was supplied with hydrogen, and the cathode side was supplied with a mixed gas of oxygen and nitrogen. The gas flow rates were 500 sccm for the anode and 4000 sccm for the cathode. Experiments were conducted at a cell temperature of 35°C with supplied gases of 81% RH. The catalyst coated membrane was produced by Japan Gore-Tex Inc.

The oxygen transport resistances were measured by using the limiting current method(2). Inside the CL (pressure independence), it was assumed that the water production rate dominates the water accumulation, which is proportional to current densities. Outside the CL (pressure dependence), it was assumed that the product of total pressure and current density determined the component of oxygen transport resistance. Based on this assumption, the division of oxygen transport resistance was determined by a least square error calculation of actual total oxygen transport resistances and theoretical values(2). This experiment was conducted with different pressure (100 kPa, 140 kPa, 180 kPa), and different oxygen concentration (1%, 2%, 4%, 6%, 8%, 12%, 15%, 18%, 21%).

Figure 1 shows the results of oxygen transport resistance in each inside and outside CL. Here, I
Lim is limiting current density, and R
P (Figure 1 (a)) and R
NP (Figure 1 (b)) are pressure-dependent or independent oxygen transport resistances, respectively. These corresponds to the oxygen transport resistance outside and inside the CL. The MPL-1 is 28BC and the MPL-2 is 28BC with a hydrophilic layer applied. In Figure 1(a), the R
P of MPL-2 is larger by the hydrophilic layer, and the tendency of change is similar: the R
P increases from the low current density region. This shows that the effect of the hydrophilic layer on condensed water is negligible. In Figure 1(b), the R
NP of MPL-2 is kept smaller in the low current density region, while it is slightly smaller than that of MPL-1 in the medium and high current density regions. This suggests that condensed water is efficiently sucked out of the CL in the low current density region, but the ability of water removal is insufficient in the medium and high current density regions. The condensed water may be retained in the hydrophilic layer due to its strong hydrophilicity.

In this study, the oxygen transport resistance was separated based on the pressure dependence, allowing quantitative investigation of mass transport phenomena inside the cell. The analysis suggested the improvement of water removal ability from the CL by adding the hydrophilic layer to the MPL. We try to apply adding carbon nanotubes to the MPL for further improvement of the ability of water management.

Reference

(1) T. Kitahara, et al., Transactions of the JSME (B) (in Japanese), 78-794, 1849 (2012).

(2) Y. Iizuka, et al, J. Electrochem. Soc., 169-12, 124510 (2022).

Figure 1