(Invited) Development of an Innovative Flat-Metal Separator Integrating the Gdl Formed Gas-Flow Channels and the Analysis As Components of PEFCs
Authors: Watanabe Masahiro , Yanai Hiroshi , Nasu Mitsunori 
Published: February 2020

PEFCs are attracting enormous interest with the properties of cleanness and large power density in the wide application areas such as fuel cell vehicles (FCVs) or EneFarms, of which commercialization were already started in 2014 and 2008, respectively, in Japan. It is, however, still essential to reduce the cost of components, such as electrocatalysts, membranes, GDL and separator, by totally ca. 1/10, without losing the cell performances and durability. The cost of GDL plus separator, for example, is predicted to share more than 1/3 in FC systems as well as that of electrocatalysts, even at mass-production stage of the systems. Therefore, we have extensively challenged to develop a less-expensive and corrosion-resistive flat-metal separator coated with thin CB/Resin layer (FMS), and a less-expensive GDL, consist of short carbon fiber (CF) bound with conventional resin, formed gas-flow channels (FC+), named as GDLFC+, and also the integrated product of FMS and GDLFC+, named as FMSG+, in the last 8 years. This development is now promoted as 5 years national project supported by MEXT of Japan.

The project targets of FMSG+ for 2021; contact resistance < 10mΩ/cm2 (2020), corrosion resistance ≈ current Au or C-coated SUS or Ti separators and limiting current at cathode > current level×2 (4A/cm2 under ambient air). The cost target of FMSG+/piece is < 2$ (2030), which may bring an assembling cost reduction by the structure integrated above two parts. As these results, we can expect to reduce the cost to a half and improve the performance to double from the stat-of-the-art PEFCs.

Flat SUS 304 sheet has been tested to coat tightly with CB/Resin (20-40μm thick) applying various CB and resin. We have achieved 5.7mΩ/cm2 in the contact resistance for this type FMS, lower than 10 mΩ/cm2 of DOE target. No-corrosion were observed over 4,000hrs on optimized FMSs under the soaking in 0.1M H2SO4 solution at 90˚C, which has been used as our accelerated corrosion test on the FMSs since no common test protocol has been shown, serious enough compared to a practical operation condition of PEFCs. We are also testing their durability under the open circuit condition in a PEFC, fed pure H2 and O2 at 90˚C and 100% RH for the comparison with the above liquid-phase test.

Various combinations of CF and resin for the GDL have been examined to make balance among the pore size (gas-diffusivity), electric resistance and mechanical strength. Distinctive feature of our approach for the GDLFC+ is the forming of gas-channels in the GDL to make possible uniform distribution of reactant H2 and air over the whole catalyst layers and uniform removing of produced water by forced cross-flow of such reactants and products in GDL and by elimination of the shadow portion against the mass-transportation on the catalyst layer (CL) contacting with ribs of gas-flow channels in the conventional separators. Since this feature is strongly depends on the FC+ patterns, we have tried to create the effective pattern and procedures. Applying a developed GDLFC+, the limiting current of 3.3A/cm2 in the single cell has been achieved under the operation with H2 (UH2=80%) and air (Uair=40%) at 80oC, humidified at 100% RH.

For such optimization of GDLFC+ design, in-situ modern visualization methods, applied the phenomena such as the luminescence change on a sensing die depending on O2 concentration [1] or the absorption of neutron beam by H2O, have been used effectively to analyze the flows of the reactant gases or produced liquid water. The results will be shown in the presentation.

[1] J. Inukai, K. Miyatake, K. Takada, M. Watanabe, T. Hyakutake, H. Nishide, Y. Nagumo, M. Watanabe, M. Aoki, and H. Takano, Angew. Chem. Int. Ed, 47, 2792-2795 (2008).