Abstract
1. Purpose
Further improvement in initial performance and durability is required for the widespread use of polymer electrolyte fuel cells. So far, many researchers reported that the position of Pt particles affects ORR activity and cell performance in a catalyst using carbon black (CB) as a support. In recent years, the focus has been on creating a catalyst structure and ionomer coating that maximizes Pt utilization by supplying protons and oxygen to the surface of Pt particles, but the guidelines for maximizing Pt utilization are not clear. In addition, since the durability test is performed by sweeping the potential under H2 / N2 conditions, the influence of generated water cannot be taken into consideration, and it is considered that the deterioration of the catalyst due to actual operation is different. In this study, the coating state of ionomer was considered from the results of N2 gas adsorption measurements, and the design guidelines for maximizing the Pt utilization rate were examined. Furthermore, an actual load cycling test was conducted under H2 / air conditions to investigate the effect of the catalyst structure on the deterioration of cell performance.
2. Experiment
Pt was supported on CB samples with specific surface areas of about 800 m2 / g (K8) and about 1300 m2 / g (K13) as supports to prepare the catalysts, Pt / K8 and Pt / K13. Ionomer was added to these catalysts and sprayed on the electrolyte membrane to prepare a cathode catalyst layer. An anode catalyst layer was prepared on the opposite surface and pressed together with the GDL by hot-pressing to form the MEA. N2 gas adsorption measurement of the cathode catalyst layer and evaluation of MEA power generation performance were performed. We also defined the hysteresis volume of P / P0 = 0.5 on the isotherm as the volume of the primary pores of the catalyst. The cell conditions were a relative humidity of 80% (cell temperature, 80 °C), a hydrogen utilization rate of 80%, and an oxygen utilization rate of 40%. Three types of catalysts including (Pt / K4) were used, including a catalyst prepared using CB with a specific surface area of about 400 m2 / g (K4) as a support in an actual load cycling test, in which each cycle consisted of OCV (60 s) and 0.53 A / cm2 (30 s), with a total cycle number of 10,000, under the same conditions.
3. Results and Discussion
As a result of the N2 gas adsorption measurements, it was found that Pt / K8 and Pt / K13 mainly have pores with a diameter of 2 to 10 nm. Figure 1(a) shows P / P0 = 0.5 hysteresis volume and (b) shows the current density at 0.6 V at the weight ratio (I / C) of each ionomer to CB. A sharp decrease in the hysteresis volume was observed at I / C 1.3 (Pt / K8) and I / C 1.7 (Pt / K13). On the other hand, since the current density was maximized at I / C values below these, it can be concluded that the rapid decrease in hysteresis is caused by the ionomer covering the pores of the catalysts. The cell performance deterioration may be caused by the Pt particles supported inside the pores becoming unavailable. In addition, Pt / K13, which has a small Pt particle size and a long interparticle distance, showed high cell performance and a high ECSA value after the durability test. Since the small Pt particle size was maintained even after the durability test, it is considered that the long distance between Pt particles suppressed the coarsening due to Ostwald ripening and aggregation, and thus showed high durability. Based on these results, we conclude that the cell performance and durability can be improved by coating the catalyst pores with ionomer so as not to cover them and increasing the distance between Pt particles.
Figure 1