Introduction The reduction of Pt-loading particularly in PEFC cathode layers is essential for cutting the cost of PEFCs and further spreading PEFC technology. Nowadays, several approaches of reducing Pt-loading have been reported, and then the importance of optimizing the microstructure of cathode layers including carbon supports has been confirmed.^1-2 Therefore, the objective of our research is to develop low Pt-loading PEFCs with high efficiency by optimizing the mount of both Pt and carbon. As a first step, MEAs with varied Pt-loading in cathode layers were prepared with a standard catalyst, TEC10E50E, and a common spray-printing method. Then, the minimum amount of Pt, which is necessary for cathode layers, was evaluated though the detailed electrochemical characterization.

Experimental MEAs with varied Pt-loading in cathode layers, such as 0.05, 0.10, 0.15, 0.20, 0.30, and 0.40 mg/cm², were prepared, while the amount of Pt in anode layers were kept to 0.30 mg/cm². Then, their performances were evaluated by measuring the IV response, impedance, and H_upd electric charge. Also, the cross-sections of cathode layers were observed by FIB-SEM.

Results and discussion From IR-corrected IV curves, activation overvoltage was separately analyzed. Activation overvoltage was clearly proportional to Pt-loading in the region of 0.05 - 0.15 (0.20) mg Pt/cm², but did not change much in 0.15 (0.20) - 0.40 mg Pt/cm². Therefore, further with ECSA analyses, the minimum amount of Pt needed for the cathode layer was determined to 0.15 - 0.20 mg Pt/cm² in this experimental condition. Interestingly, concentration overvoltage increased with decrease in the amount of Pt. In this case, since the amount of carbon decreased, the thickness of cathode layers resulted in decreasing. Possible reasons for this phenomenon can be explained in Figure 1. Although the shorter pathway for fuel diffusion is expected to reduce concentration overvoltage, the blockage of the pathway by water produced during the fuel cell reaction should also be considered. If the same amount of water is produced in catalyst layers with the different thickness, the path is more easily blocked in the thinner layer, leading to higher concentration overvoltage. Therefore, optimization of the cathode layer thickness is also an important factor to design better PEFCs. Accordingly, we will report the approach to develop highly efficient MEAs using the minimum amount of Pt and also reducing concentration overvoltage through controlling the amount of carbon.

References

(1) M. Lee, M. Uchida, D. A. Tryk, H. Uchida, M. Watanabe, Electrochim, Acta, 56 4783 (2011).

(2) K. Takahashi, K. Kakinuma, M. Uchida, J. Electrochem. Soc. 163.(10) F1182 (2016)