| Popis: |
Improvement in power generation performance of PEFC (Polymer Electrolyte Fuel Cell) is required for its market supremacy in automotive applications. In particular, it is important to suppress concentration overpotential in high current density operation. The microstructure of CCL (cathode catalyst layer) is known to have a great effect on the power generation performance of PEFC. Also, the requirement for the species transport characteristics should vary with the thickness-wise position in CCL. Therefore, the purpose of this study is to design the microstructure of CCL by using multi-layered cathode catalyst layer and to evaluate oxygen transport properties. In this research, we fabricated three kinds of multi-layered CCLs which have the difference in I/C (ionomer to carbon) ratio of GDL (gas diffusion layer)-side layer (0.3, 0.5, 0.9). And, we investigated oxygen transport-reaction phenomena by evaluating polarization characteristics, ECA (electrochemical surface area) and RCL (oxygen transport resistance in the CCL) using limiting current measurements. As I/C ratio is decreased from 0.9 to 0.5, RCL is decreased 49%. On the contrary, as I/C ratio is decreased from 0.5 to 0.3, RCL is slightly increased. ECA is monotonically decreased as I/C ratio is decreased from 0.9 to 0.3. These results show that the species transport characteristics alone have optimum condition in the GDL-side CCL at around I/C = 0.50. In addition, we fabricated four kinds of multi-layered CCLs which have the difference in I/C ratio of GDL-side layer (0.5, 0.9) and mass fraction of platinum (10 wt%, 46 wt%.) Oxygen transport resistance is evaluated in CCL by separating the resistance to two components, Rmacro (by Knudsen diffusion) and Rlocal (by dissolution diffusion in ionomer,) by applying the ladder resistance model in CCL. As I/C ratio is decreased from 0.9 to 0.5, Rmacro is decreased and Rlocal is increased regardless of mass fraction of Pt. These results show that the ionomer amount of micropore in CCL is decreased, the micropore diameter is increased, and dissolution surface area near platinum is decreased as I/C ratio is decreased 0.9 to 0.5. These results strongly suggest that there is an optimization strategy of I/C ratio of GDL-side layer in CCL. |
