(Poster Award - 2nd Place) Enhancement of ORR Activity of Pt Catalyst for PEFCs By Modification with Hydrogen-Bonded Organic Framework Composed of Melamine and Cyanuric Acid

Abstract

Introduction Improvement in ORR activity of Pt and Pt-based catalysts used in PEFCs for FCEVs is crucial for cost reduction and widespread commercialization [1]. We have reported that surface modification of Pt and Pt-based catalysts with melamine and its derivatives enhance ORR activity [2, 3]. However, it is concerned that melamine is easily washed out by water generated at the cathode in PEFCs because melamine is soluble in water (3.1 g/L). In this study, a hydrogen-bonded organic framework (HOF) composed of melamine (MEL) and cyanuric acid (CA) [4] was prepared to suppress the washing-out problem and investigated as a modifier of Pt catalyst loaded on mesoporous carbon (MPC) support. Experimental MPC (CNovel MH-18, SBET 1,334 m2/g, TOYO TANSO) was used as a carbon support for Pt catalyst. Pt/MPC (50 wt%) was prepared by an impregnation-thermal decomposition method using Pt(NO2)2(NH3)2 as a Pt source. Pt/MPC was modified as the following procedure. First MEL was impregnated within the Pt/MPC by ultrasonically dispersing 100 mg Pt/MPC catalyst and 10 mg MEL (10 wt.% to the catalyst) in 100 mL pure water, followed by drying using a rotary evaporator (MEL-Pt/MPC). Then MEL-Pt/MPC (110 mg) was dispersed in 100 mL water containing 10 mg CA and dried to obtain HOF-Pt/MPC. The catalyst was loaded on a glassy carbon (GC) RDE. CV of the catalyst was recorded in Ar-saturated 0.1 M HClO4 at 25oC at a scan rate of 50 mV/s and ORR activity was evaluated by LSV performed at 25oC in O2-saturated 0.1 M HClO4 at a scan rate of 10 mV/s on the positive sweep while rotating the GC disk electrode at 1,600 rpm. To investigate the stability of MEL and HOF on the electrode, the GC electrode after the initial measurements was immersed in 0.1 M HClO4 at 80oC for 1 h, followed by washing with pure water, and CV and LSV were measured in the same manner at 25oC. Results and Discussion Figure 1 shows the molecular structure of MEL-CA HOF [4]. Figures 2 and 3 show CVs and LSVs, respectively, for bare, MEL-, and HOF-Pt/MPC before the stability tests. For bare Pt/MPC in Fig. 2, peaks were observed around 0.05−0.4 V and 0.6−1.0 V, which are assigned to the adsorption/desorption of underpotential deposited hydrogen (Hupd) and Pt-oxide species formation/reduction, respectively. For MEL- and HOF-Pt/MPC, the HUPD peak area decreased slightly, while the onset potential for Pt-oxide formation was shifted positively in Fig. 2. As a result, ORR activity of MEL- and HOF-Pt/MPC was improved remarkably as shown in Fig. 3 (mass activity: 894 and 829 A/g, respectively, at 0.9 V). These results indicate that HOF has an enhancement effect for ORR activity similar to that of MEL modification. Figure 4 shows CVs for bare, MEL-, and HOF-Pt/MPC after the stability tests at 80oC for 1 h. In Fig. 4, the waves of Hupd and Pt-oxide of MEL-Pt/MPC became similar to those of bare Pt/MPC after the stability test, which indicated that MEL was washed out in the electrolyte solution. On the contrary, the HOF-Pt/MPC catalyst substantially retained the waves shapes before the stability test shown in Fig. 2. These results indicate that the HOF has higher stability against washing-out than MEL owing to a much lower solubility at 80°C. Figure 5 shows LSVs after the stability tests. The loss of activity of HOF-Pt/MPC (639 A/g) was smaller than MEL-Pt/MPC (553 A/g) owing to the suppression of washing-out, which is consistent with the results in Fig. 4. The results of single cell tests using bare Pt/MPC, MEL-Pt/MPC and HOF-Pt/MPC cathode catalysts will be presented at the meeting.This study was partly supported by NEDO, Japan.