The performance of polymer electrolyte fuel cell (PEFC) has a great dependency on the flow field pattern, which remarkably affects the distribution of reactant concentration, pressure drop, local current density, etc., and is supposed to be a solution of the PEFCs’ grand-scale commercialization. The gas diffusion layer (GDL) is utilized in a cell to provide a uniform in-plane reactant concentration distribution and electron conduction pathway. On the other hand, the local oxygen concentration under the rib tends to be relatively low, which negatively affects the cell performance. To promote the gas transfer in both of in-plane and through-plane directions, the flow field with narrowed structure was considered, as employed in Toyota 2nd-generation MIRAI, which slightly increases the pressure drop across the whole flow filed but force a portion of the reactant gas to transport through the GDL to the catalytic layer under the gas channels and ribs. However, a brief and clear understanding of the effect of the gas transfer promoted by the narrowed structure is still lack of report. In this study, the effects of flow fields with different geometrical shapes of the narrowed structure on the cell performance were directly measured by experiments and analyzed by theoretical model. The results are potentially helpful for cell design.
Experimental
The membrane electrode assembly (MEA) with 10 mm thick catalytic layer and Nafion® ionomer and membrane (DuPont NR-212) was set up in a JARI-type cell. The polarization curves were measured with 4 kinds of gas channels: parallel (PN0), parallel with staggered 1-mm-long narrowed structures (PSN1), parallel with neat-arranged 1-mm-long and 3-mm-long narrowed structures (PNN1, PNN3), whose geometries are shown in Fig. 1. The manifolds of gas channels have wider size than the separated channels in order to ensure an equal flow rate in each separated channel. 200 sccm H2 and 100 sccm O2 + 400 sccm N2 (20 °C, 1 atm) were supplied to anode and cathode respectively. The dew points of the humidifiers were 65 °C. The cell was operated at 80 °C and 1 atm.
Results and Discussion
The simulation was proceeded in isothermal conditions, and the anisotropies of gas permeability and diffusivity in GDL were considered. The reaction kinetics was obtained by experiments. The experimental polarization curves exhibit a good agreement with the simulation results, as shown in Fig. 2. The cell performance of PSN1=PNN1>PNN3>PN0. The gas channels with narrowed structures show better performance since a part of reactant gas flows into the GDL so that the mass transfer is enhanced. Fig. 3 shows the current density distribution of each gas channels obtained by calculation. Comparing to the PN0, higher current density generates where the narrowed structures are placed, and the average oxygen partial pressure on the whole catalytic surface increased 17.5%, 17.3% and 10.8% in case of PSN1, PNN1 and PNN3, as shown in Fig. 4.
The narrowed structure increases the gas convection in through-plane and in-plane directions. In case of the PSN1, the staggered narrowed structures cause the pressure gradient between gas channels, so that 5.4 % of gas in the odd-numbered gas channels flows to the even-numbered gas channels, shown as Fig. 5. However, in case of the PNN1, although nearly no pressure gradient exists between the gas channels, the performance is similar to the PSN1, which indicates the enhancement of gas transfer in the in-plane direction is unobvious.
On the other hand, the effective field, where the narrowed structures enhance the gas transfer, is wider than the geometries of narrowed structures itself. The distances of higher current density in the gas flow directions comparing to the PN0 are longer than the length of narrowed structures, shown as Fig. 6. It indicates that the PNN3 has narrower effective gas transfer enhanced field than the PNN1, so that the PNN3 has lower cell performance than the PNN1.
Conclusions
The narrowed structures in the parallel channel heighten the gas transfer to the catalytic layer surface, so that the cell performance is improved. This effect mainly exists in the through-plane direction, in case of the presented experimental conditions. The width of the narrowed structure should not be too large which may cause the waste of the effective gas transfer enhanced field. In addition, the non-linear relationship between cell performance improvement and pressure drop is going to be further discussed.
Acknowledgment
This work was supported by the FC Platform Program: Development of design-for-purpose numerical simulators for attaining long life and high performance project (FY 2020–FY 2024) conducted by the New Energy and Industrial Technology Development Organization (NEDO), Japan.