Pd@Pt Core-Shell Structured Catalyst Supported on Mesoporous Carbon for Achieving a High Cell Performance

Abstract

1. Introduction An extremely high performance is required for polymer electrolyte fuel cells (PEFCs) installed in fuel cell electric vehicles after 2030 as shown in Figure 1 [1] and a high oxygen accessibility [2] in addition to a high ORR activity and a durability is crucial for Pt-based cathode catalysts in PEFCs. In order to achieve the cell performance, it is of great importance to suppress cell voltage drop especially in a high current density (HCD) region and we consider that carbon support for the Pt-based catalysts would play an important role. In this study, mesoporous carbon (MPC) with a large specific surface area was selected for the support of Pd@Pt core-shell structured catalyst and cell performance of the catalyst supported on the MPC was compared with the catalyst supported on a conventional carbon support of Ketjen Black (KB). 2. Experimental KB-600JD (SBET: 1,335 m2/g) and MPC (CNovel MH-18, SBET 1,278 m2/g, TOYO TANSO [3]) were used as carbon supports for synthesis of Pd@Pt core-shell structured catalyst. The Pd@Pt core-shell structured catalyst was synthesized by a direct displacement reaction method [4]. The metal loadings of the catalyst were 20 wt.% for Pt and 30 wt.% for Pd. CV was recorded at 25oC in Ar-saturated 0.1 M HClO4 and ORR activity was evaluated by RDE method performed in O2-saturated 0.1 M HClO4 at 25oC with a rotation speed of 1,600 rpm and a positive potential scan rate of 10 mV/s. Fuel cell performance was evaluated by using a single cell with an active area of 1 cm2 (5 serpentine flow paths). Nafion® DE2020 was used as an ionomer for the catalyst ink preparation (I/C was set to 0.83) and Nafion® NRE 211 (25 µm in thickness) was used as a membrane. The Pt loading in cathode catalyst layer was set to 0.1 mg/cm2, and H2 and air were supplied to anode and cathode by 418 NmL/min. and 988 NmL/min., respectively (utilization was 5% at current density of 3.0 A/cm2). The cell temperature was set to 80oC and humidified by 95% RH. 3. Results and Discussion Figure 1 shows cell performances of a reference Pt catalyst supported on KB-300J (TEC10E50E, mean diameter: 2.8 nm, metal loading: 48 wt.%, TKK) and Pd@Pt core-shell structured catalyst supported on KB-600JD evaluated with 1.0 atm at gas outlet. Although the cell performance of the Pt/Pd/KB-600JD catalyst overcame the performance of the reference Pt/KB-300J catalyst, cell voltage significantly dropped in HCD region (> 2.0 A/cm2) compared to cell voltage required in 2030. In order to suppress the voltage drop, gas diffusion layer (GDL) was altered to a high current use (22BB, SGL Carbon Inc.) from a stational use (28BC, SGL Carbon Inc.) and the cell performances are overwritten on Figure 1. It is clear that the voltage drop in HCD region was well suppressed by using the GDL 22BB, which arose from a three-fold higher gas permeability of the GDL 22BB compared to the GDL BC28.Next, CNovel MH-18 was used as a carbon support and pore size distribution is demonstrated in Figure 2. The KB-600JD possesses mesopores broadly ranging from 2 to 20 nm in diameter, while the CNovel MH-18 has mesopores with a narrower size distribution centered at 4 nm in diameter. Figures 3 and 4 summarize cell performances using the Pt/KB-300J, the Pt/Pd/KB-600JD and the Pt/Pd/CNovel MH-18 cathode catalysts. When the cell performance was evaluated with 1.0 atm at gas outlet, the Pt/Pd/KB-600JD catalyst exhibited a superior cell performance to that of the Pt/Pd/CNovel MH-18 catalyst. On the contrary, when the cell performance was evaluated with 1.5 atm at gas outlet, the performance using the Pt/Pd/CNovel MH-18 catalyst was much improved and exhibited a higher cell voltage in a low current density region (0.01-1.0 A/cm2) as shown in Figure 5. Since more than 60% of the Pd@Pd NPs were deposited at interior part of the CNovel MH-18 MPC support, it is considered that the cell performance using the Pt/Pd/CNovel MH-18 catalyst exhibited the significant improvement under the pressurized condition at 1.5 atm and the higher cell voltage in the low current density region due to suppression of a direct ionomer adsorption onto the catalyst surface.This study was partly supported by NEDO, Japan.