(585f) Optimization of Electrodes Based On Polymer Properties of Hydrocarbon Polyelectrolyte for PEFCs

Polymer electrolyte fuel cells (PEFCs) are promising future devices for power generation, portable and automotive applications. A membrane electrode assembly (MEA) is one of the key components in PEFC system. The MEA consists of polymer electrolyte membrane and electrodes which made up of a catalyst layer, a diffusion layer, and a carbon paper. Perfluorosulfonic acid (PFSA) such as Nafion has been used as an ionomer in the catalyst layer, and the Nafion-based electrodes have been optimized in a great many studies. Several problems still remain to restrict the application of PFSA to PEFCs, for instance, the high cost, the high environmental load and the difficultly in structure control. On the other hand, hydrocarbon polyelectrolytes have been developed as an alternative to PFSA because of the unique benefits including the low cost and the easiness of structure control. Therefore, the adjustment of the alternative ionomer to the electrode is fundamentally significant. However, none of index could be formed to optimize the structure of catalyst layer up till now. Furthermore, the performance of MEA containing hydrocarbon ionomer in the catalyst layer has been reported to decrease at comparatively-low current density, below 1,000 mA/cm², compared with that of Nafion. The followings are considered as main causes of the performance decrement: (a) the inhibition of oxygen diffusion by blocking a secondary pore, which is a pore outside agglomerated catalyst supported carbons (i.e. secondary particles), (b) the resistance of oxygen permeation from the secondary pore to a reaction site through the ionomer layer. In our research, polymer physical properties were investigated to clarify the main cause of performance decrement. The oxygen permeability and the swelling of ionomer were focused on as polymer physical properties since these properties are considered to relate to the above rate limiting steps.

The sulfonated poly(allylene ethel sulfone) (SPES) was synthesized to investigate polymer properties of hydrocarbon polyelectrolyte and to be used to the catalyst layer as the ionomer. Two types of electrodes were fabricated and the introduction weight ratio of Pt supported carbon (Pt/C): ionomer: and polytetrafluoroethylene (PTFE) was set to 60: 25: 15 (SPES-25, Nafion-25) and 80: 5: 15 (SPES-5) at the dry state, respectively. MEAs were fabricated with Nafion NR-212 membrane along with SPES-25 or SPES-5 electrode, respectively (MEA-SPES-25, MEA-SPES-5). The oxygen permeability coefficient [P_O2] was measured and calculated from the amount of permeated oxygen through the membrane. The swelling ratio [δ] was estimated from the volume change of the membrane.

P_O2 of SPES was lower, while δ of SPES was higher than these of Nafion. Firstly, to discuss the effect of ionomer swelling, the porosity of catalyst layer was estimated by considering the volumes of entire catalyst layer, Pt/C, ionomer, and PTFE. The porosity of SPES-25 dropped significantly compared with that of Nafion-based electrode. Besides, the effect of ionomer swelling decreased with decreasing RH. This means that, if the blocking of secondary pore caused by the SPES swelling is the main influence factor, the performance would improve with decreasing RH. Secondly, to discuss a difficulty of the oxygen permeation through the ionomer layer, the resistance of oxygen permeation was calculated from P_O2, δ, and the density. The oxygen permeation through the SPES ionomer layer of the SPES-25 is much more difficult than that through the Nafion ionomer layer of the Nafion-25. Furthermore, the effect of resistance increased with decreasing RH. This means that, if the oxygen permeation through the ionomer layer is the main cause of performance decrement, the performance would become lower with decreasing RH.

Therefore, the humidity dependence of performance of MEAs was investigated. The performance of MEA-SPES-25 improved with decreasing RH. These results indicate that the main cause of performance decrement is not the resistance of oxygen permeation but the inhibition of oxygen diffusion by blocking a secondary pore caused by ionomer swelling. Additionally, the performance of MEA-SPES-5 with thin layer of SPES ionomer was high and hardly changed with changing RH. This is because the thinner layer of SPES ionomer is not affected by the inhibition of oxygen diffusion by blocking. The results with MEA-SPES-5 also suggest that the broadened secondary pore enables to maintain the performance of MEA with the hydrocarbon ionomer in the catalyst layer. Therefore, the optimized structure of the catalyst layer will be fabricated by not only using the ionomer with low swelling ratio but also extending the secondary pore of the catalyst layer.