Investigation on Porosity of Mesoporous Carbon (MPC) Support and Evaluation of Single Cell Performance Using Pt/MPC Cathode Catalyst

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Introduction After 2030, 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 PEFCs used in fuel cell electric vehicles (FCEVs) in Japan [1]. To improve cell performance, we not only need to increase the oxygen reduction reaction (ORR) activity of Pt catalyst, but also new carbon supports that suppress ionomer poisoning and increase oxygen diffusivity [2]. Therefore, mesoporous carbon (MPC) has recently attracted attention as a carbon support for Pt and Pt-based catalysts [2-4]. It is important to know porosity of the MPC support because the ionomer poisoning and the oxygen diffusivity vary greatly depending on the porosity of the MPC support. In this study, Pt catalyst was synthesized by using a MPC support with mean size of 800 nm. Porosity of the MPC support was investigated by N2 gas adsorption, SEM and TEM techniques and I-V performance of MEA using the Pt/MPC cathode catalyst was evaluated. Experimental MPC (CNovel MH-18, primary particle size: ca. 800 nm, central mesopore diameter: 4 nm, SBET: 1,338 m2/g, TOYO TANSO Co. Ltd. [5]) was used as carbon support for the Pt catalyst. The Pt/MPC catalyst was synthesized by impregnation-thermal reduction method. The MPC support was impregnated with Pt(NO2)2(NH3)2 and thermally reduced at 400℃ in N2 gas atmosphere (Pt loading: 50 wt.%). Pt/KB-300J catalyst (TEC10E50E, Pt particle size: 2.5 nm, Pt loading; 47 wt.%, Tanaka Kikinzoku Kogyo Co. Ltd.) was used as a reference catalyst. Characterizations of the MPC support and the Pt/MPC catalyst were carried out by N2 gas adsorption, XRD, SEM, TEM and STEM-EDX techniques. Fuel cell performance was evaluated by using a single cell with an active area of 1 cm2. NafionTM NRE 211 (25 µm in thickness) was used as a membrane and NafionTM DE2020 (EW: 1,100) was used as an ionomer for the catalyst ink preparation (I/C ratio: 0.83 for Pt/KB-300J, 1.2 for Pt/MPC). Pt loading in the cathode catalyst layer was set to 0.16 mg/cm2. H2 and air were supplied to anode and 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 to 75% relative humidity. H2 utilization was 5% at a current density of 3.0 A/cm2. Results and Discussion Figure 1 shows pore size distribution of KB-300J and MPC supports. The KB-300J support does not have clear mesopores, but the MPC support has mesopores with central diameter of 4 nm. 3d TEM analysis revealed that the MPC support has a high density of highly interconnected mesopores of 2~6 nm. Cross-sectional SEM observation revealed that, in addition to the mesopores, large mesopores and macropores coexist in the MPC support as displayed in Figure 2. A cross-sectional TEM image of the Pt/MPC catalyst is depicted in Figure 3. The Pt catalyst NPs with a size of 2.5 nm are uniformly deposited throughout the MPC support.STEM-EDX compositional analysis of fluorine (F) and carbon (C) in the Pt/MPC cathode catalyst are shown in Figure 4. STEM-EDX line analysis revealed that the fluorine signal disappeared at 40 nm from the edge of the MPC support, meaning that the ionomer molecules penetrate only 40 nm from the edge of the MPC support. Figure 5 demonstrates I-V performance of MEAs using the Pt/KB-300J reference catalyst and the Pt/MPC cathode catalysts. The Pt/MPC catalyst showed higher cell voltage than the Pt/KB-300J catalyst in the high current density region over 1.0 A/cm2, which is thought to be because the MPC support has the porosity in which highly interconnected mesopores and macropores coexist. On the other hand, in the low current density region below 0.5 A/cm2, almost no difference was observed in the cell voltages of both catalysts. As demonstrated in the STEM-EDX compositional line analysis in Figure 4, the ionomer molecules penetrate only 40 nm from the edge of the MPC support. Therefore, in the low current density region where the amount of water produced is small, the Pt utilization decreased and the cell voltage did not increase sufficiently. At the conference, several attempts to increase the Pt utilization will be presented.This study was partly supported by NEDO, Japan.