Durability Estimation for Pt/Carbon-Sphere with the Reduced Graphene Oxide Wall Using the Start/Stop and the Load Fluctuation Tests in Acidic Electrolyte

Polymer electrolyte fuel cells (PEFCs) are attracting attention as a next-generation power source with low environmental impact. However, PEFCs have significant issues for widespread commercialization, which are high cost and short service life. Increasing Pt utilization in the catalyst layer will reduce the PEFCs cost. We reported new catalyst support with increasing Pt utilization compared to traditional carbon support [1]. Our carbon sphere (CS) support is composed of a SiO 2 bead core and a reduced graphene-oxide wall. And it was successful that Pt nanoparticles supported on a CS. The Pt utilization of Pt/CS was ~20% higher than that of commercial Pt/KB. Pt/CS is expected to decrease the cost of electrocatalyst of PEFCs. However, the durability of Pt/CS was not estimated, although the reduced graphene oxide was reported to have a high tolerance for carbon corrosion [2]. In this study, the durability of Pt/CS was estimated by two methods which were start/stop and load fluctuation tests proposed by the Fuel Cell Commercialization Conference of Japan. The preparation method of Pt/CS was followed by our previous study [1]. Durability tests were conducted using a three-electrode cell which was composed of a working electrode, Pt counter electrode, Ag/AgCl reference electrode, and 0.1 M HClO 4 electrolyte. This study converted the potentials from Ag/AgCl reference to RHE reference. Conditions of the start/stop test were 10,000 cycling between 1.0 V to 1.5 V with 2 s/cycle of the triangular potential waveform. The load fluctuation test was conducted by the potential step at 3 seconds at 0.6 V and 1.0 V with 10,000 cycles. Both test temperatures were 60°C. The Oxygen reduction-reaction (ORR) activity and the electrochemical active surface area (ECSA) were evaluated before and after durability tests. The initial state of Pt/CS showed high ORR activity and almost the same ECSA in comparison with commercial Pt/KB (TEC10E50E). These results agreed with our previous study [1]. After the start/stop test, commercial Pt/KB drastically degraded that ECSA dropped ~80% of the initial state, and the potential at half the value of a linear sweep voltammogram ( E 1/2 ) negative shifted to 0.35 V. In contrast, Pt/CS demonstrated high durability for the start/stop test. Its ECSA was kept at ~84% of the initial state, and the difference of E 1/2 between before and after the start/stop test was 0.06 V. Compared with SEM images after the start/stop test, the shape of carbon support in commercial Pt/KB was collapsed by the carbon corrosion reaction, although that of CS support in Pt/CS was maintained. The difference in durability between Pt/CS and Pt/KB was probably attributed to the number of functional groups at the support surface. Especially number of hydroxyl groups on the surface may affect the stability of the carbon corrosion. In the load fluctuation test, Pt/CS also demonstrated higher stability than commercial Pt/KB. ECSA retention of Pt/CS was ~67%, and it was about five times higher than that of commercial Pt/KB. Pt/CS was kept ~85% of the initial E 1/2 when commercial Pt/KB decreased to 55% of the initial E 1/2 . After the load fluctuation test, the Pt particle size immensely differed between Pt/CS and Pt/KB. In Pt/CS, the increase of Pt particle size was limited from 6.7 nm to 8.6 nm. In contrast, the Pt particle size of commercial Pt/KB increased from ~5 nm to 10.4 nm. The results of the XPS analysis indicated the chemical state of Pt in Pt/CS was the difference in Pt/KB, and it probably was affected by a CS as the catalyst support. The difference in catalyst stability of Pt/CS and commercial Pt/KB may be attributed to the strength of the interaction of Pt and the catalyst support. This study estimated the durability of Pt/CS, which had spherical catalyst support with the reduced graphene oxide wall, compared with that of commercial Pt/KB. Pt/CS demonstrated better initial ORR activity, and higher durability about the start/stop and the load fluctuation test than Pt/KB. The above results indicated that the carbon sphere was most likely one of the useful catalyst supports for PEFCs cathode catalysts. [1] T. Saida, et al., Energy & Fuels , 36 , 1027-1033 (2022). [2] P. Zhang, et al. Int. J. Electrochem. Sci ., 11 , 10763-10778 (2016).