(Plenary) Development of PFSA Ionomers and Their Use in Fuel Cells
Authors: Kinoshita Shinji , Shimohira Tetsuji , Watakabe Atsushi , Hommura Satoru , Saito Susumu , Tanuma Toshihiro , Yamada Kohta 
Published: February 2020

Widespread of fuel cells are strongly desired from the point of reducing the GHG and for clean environment. In order to achieve these, reducing the cost of the fuel cell vehicle is needed and from this point, higher operating temperatures of PEFC can contribute for designing compact and simple cooling system. We have been developing variety of new Perfluorosulfonic acid (PFSA) ionomers having high ion exchange capacity (IEC) and other properties required for each the membrane and the electrolyte in the electrodes. The ionomer with high proton conductivity was tested by copolymerization of tetrafluoroethylene with a new bifunctional fluorosulfonyl monomer 1. The ion exchange capacity was 1.58 meq/g and the water uptake after immersion in hot water at 80 °C for 16 hours was much lower compared to that of conventional ionomer.

Reduction in Pt loading in the cathode is another requirement for PEFC to reduce the cost. Especially at higher temperature and lower RH conditions under which the flux of oxygen to the Pt surface through the ionomer in the electrode could be the rate limiting factor for the oxygen reduction reaction (ORR). In order to increase the oxygen flux to Pt surface, the use of ionomers having higher oxygen permeability can be a solution. We have designed and synthesized various ionomers based on a general rule that the lower the density of a fluorinated polymer, the higher the oxygen permeability. One of the newly prepared ionomer having lower density showed more than 2 times higher the oxygen permeability 2 at low RH conditions. The use of this high oxygen permeable ionomer at cathode showed the possibility of achieving the same cell voltage with half the amount of Pt loading.

We see that the ionomer plays an important row in developing PEFC achieving the requirements from the market. The ionomer can also be used in the micro porous layers (MPLs) between the catalyst layers and the Gas diffusion substrate to create hydrophilic MPLs. Conventionally, hydrophobic MPLs were applied between the GDM and the catalyst layer to prevent flooding due to excess water in the electrode. But the hydrophobicity of the MPL is not favorable when the cell is operating under a dry condition. We have been reporting the use of hydrophilic MPL between the GDM and the catalyst layer 3. It can control the water balance in the MEA, achieving higher performance under low RH conditions without facing the low performance due to mass transfer issue at wet conditions. This hydrophilic MPL was used in the cathode in combination with a high proton conductive membrane, and the MEA showed excellent performance under a very dry condition.

We are continuously making efforts developing PEFC materials getting into the next stage.

References

1. A.Watakabe, H Yamamoto, M Iwaya, S Saito, K Yamada, S Hommura and T Shimohira, Fluoropolymer (2012)

2. K. Yamada, S. Hommura, and T. Shimohira, ECS Trans., 50 (2), 1495 (2013).

3. T. Tanuma and S. Kinoshita, J. Electrochem. Soc., 161 (1), F94 (2014).