Single Cell Performance and Durability of Pt and Pt-Based Catalysts Supported on Mesoporous Carbon

Abstract

Introduction In Japan, an extremely high cell performance, i.e., 0.84 V at 0.2 A/cm2 and 0.70 V at 3.0 A/cm2, is required for polymer electrolyte fuel cells (PEFCs) used in fuel cell electric vehicles after 2030 [1]. In addition to high oxygen reduction reaction (ORR) activity of the Pt and the Pt-based cathode catalysts for increasing cell voltage in the low current density (LCD) region, high oxygen diffusivity is crucial for achieving the cell voltage in the high current density (HCD) region [2]. Therefore, it is considered that carbon supports for the catalyst NPs would play an important role for realizing the high cell performance. In this study, a mesoporous carbon (MPC) was selected as the support for Pt and Pd@Pt core-shell structured catalyst NPs and cell performance of the catalysts were compared with that of the catalyst supported on a conventional Ketjen Black (KB). Furthermore, durability of the MPC support against a high potential range was investigated. Experimental KB-600JD (SBET: 1,335 m2/g, LION) and MPC (CNovel MH-18, SBET 1,334 m2/g, TOYO TANSO [3]) were used as carbon supports. The Pt/MPC catalyst, Pt diameter of 2.5 nm and metal loading of 50 wt.%, was synthesized through impregnation of Pt(NO2)2(NH3)2 followed by thermal reduction in N2 at 400oC. Pt/Pd/KB and Pt/Pd/MPC core-shell catalysts were synthesized by a direct displacement reaction (DDR) method [4]. Diameter of Pd@Pt NPs was 5.8 nm and Pt and Pd metal loading were 20 wt.% and 30 wt.%, respectively. Cyclic voltammogram was recorded in Ar-saturated 0.1 M HClO4 at 25oC with a scan rate of 50 mV/s and ORR activity was evaluated in O2-saturated 0.1 M HClO4 at 25oC with a positive potential scan rate of 10 mV/s at 1,600 rpm. Fuel cell performance was evaluated by using a single cell with an active area of 1 cm2. Nafion DE2020 was used as an ionomer for the catalyst ink preparation (I/C ratio was set to 0.83-1.2) and Nafion NRE 211 with 25 µm in thickness was used as a membrane. Pt loading in the cathode catalyst layer was set to 0.1 mg/cm2. H2 and air were supplied to the anode and the cathode by 418 NmL/min. and 988 NmL/min., respectively, and pressurized to 150 kPa at gas outlets. The cell temperature was set to 80oC and humidified from 35% to 95% relative humidity (RH). H2 utilization was 5% at a current density of 3.0 A/cm2. Durability of the carbon supports was evaluated by using the single cell. H2 and N2 were fed to anode and cathode, respectively, and cathode potential was swept from 1.0 to 1.5 V vs. RHE with a scan rate of 500 mV/s for 1,000 cycles. Results and Discussion Pore size distribution of the carbon supports is shown in Figure 1. Both KB-600JD and MPC (CNovel MH-18) carbon supports have mesopores, and the MPC exhibits a narrower pore size distribution centered at 4 nm. Figure 2 demonstrates I-V performance of Pd@Pt core-shell catalysts, indicating that Pt/Pd/MPC catalyst shows a higher cell voltage in HCD region (> 2 A/cm2) than that of Pt/Pd/KB-600JD catalyst. Interconnectivity of the mesopores analyzed by 3D-TEM observation of the carbon supports is depicted in Figure 3. It is clear that density of interconnected mesopores is much higher in the MPC support, which is considered to contribute to the higher cell voltage in HCD region of the Pt/Pd/MPC catalyst. Figure 4 displays cross-sectional SEM image of the Pt/Pd/MPC catalyst. In addition to the well-interconnected mesopores (Figure 3), macropores coexist in the MPC support, which is also considered to contribute to the higher cell voltage in HCD region.A high potential durability test was performed by using potential cycling of 1.0-1.5 V vs. RHE for 1,000 cycles at 80oC. Thickness of Pt/MPC cathode catalyst layer was decreased by 36% after the durability test, while the decrease was mitigated to 14% with a heat treatment of the MPC support at 2,200oC in Ar. (Figures 5). As is well known, it was demonstrated that the high temperature heat treatment has a positive effect for durability improvement of the carbon support against the high potential range.This study was partly supported by NEDO, Japan.