Synthesis of Multicomponent Pt Alloy Catalysts for Oxygen Reduction Reaction and Durability Evaluation

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Introduction In order to popularize PEFCs, it is important to increase the oxygen reduction reaction (ORR) activity and reduce the amount of expensive Pt catalyst used. It is well known that the Pt-based alloy catalysts show high ORR activity, but the problem is that the 3d transition metals are oxidatively dissolved in the PEFC cathode environment, deteriorating the ORR activity. Studies using single crystal model electrodes have reported that a high-entropy alloy (HEA) catalyst (PtFeCoNiCu) exhibits higher ORR activity and durability compared to a PtCo binary alloy catalyst [1]. Chida et al. demonstrated that the coexistence of Mn and Cr in a model PtCoNiMnCrFe catalyst suppresses the thermal diffusion of the constituent elements [2]. Furthermore, it has been reported that the addition of melamine suppresses the dissolution of 3d transition metals and improves the durability of PtCo alloy catalyst [3]. In this study, we synthesized PtCo, PtCoMn and PtCoMnCr alloy catalysts and investigated the effects of Mn and Cr additions on ORR activity and durability, along with the effect of melamine addition. Experimental 500 mg mesoporous carbon (MPC) support (CNovel MH-18, primary particle size: 800 nm, central mesopore diameter: 4 nm, SBET: 1345 m2/g, TOYO TANSO) and Pt(NO2)2(NH3)2 equivalent to 500 mg of metallic Pt were added to a EtOH/H2O mixed solvent (37.5 mL/250 mL), and refluxed at 90℃ under N2 gas atmosphere for 4 h to synthesize Pt/MPC catalyst (Pt particle size: 2.5 nm, Pt loading: 50 wt.%). The obtained Pt/MPC catalyst (600 mg) was dispersed in 200 mL DI water and metal nitrates of Co, Mn and Cr were added in the following molar ratios; (i) Pt50Co50, (ii) Pt50Co37.5Mn12.5 and (iii) Pt50Co25Mn12.5Cr12.5. An aqueous NaBH4 solution (30 mL) corresponding to 100 times the equivalent of the metal nitrates was added dropwise to this solution over 2 h while stirring at 300 rpm at 30° C. The solution was filtered, dried at 60℃ in air, and the dried powder was heat treated at 800℃ for 2 h under 15% H2-Ar atmosphere to obtain PtCo/MPC, PtCoMn/MPC and PtCoMnCr/MPC alloy catalysts. Characterizations of the alloy catalysts were performed using XRD, XRF, TG-DTA and TEM. The initial CV and LSV of the catalysts were recorded in Ar/O2 saturated 0.1 M HClO4 at 25°C. The accelerated durability test (ADT) of the catalysts was conducted by using a square wave potential cycling of 0.6 V (3 s)-0.95 V (3 s) vs. RHE performed in Ar-saturated 0.1 M HClO4 at 80°C for 10 k cycles with and without addition of 10 μM of melamine to the electrolyte. Results and Discussion Figure 1 shows cross-sectional TEM images of the three Pt alloy/MPC catalysts thermally reduced at 800℃. The Pt alloy catalyst NPs were uniformly deposited inside the MPC support with a particle size of approximately 4.5 nm. In the outer part of the MPC support, large particles were observed due to a lack of cage effects of mesopores. The XRD analysis in Fig. 2 indicated that the Pt alloy catalysts partially transformed to the L10 ordered phase by the heat treatment at 800℃. LSV measurements showed that ORR mass activity (MA) at 0.9 V vs. RHE of the PtCo/MPC binary alloy catalyst increased from 1,430 to 1,700 A/g by the addition of Mn. Figure 3 summarizes MA of the three types of Pt alloy catalysts before and after ADT performed at 80°C for 10 k cycles. MA of all the Pt alloy catalysts decreased to ca 500 A/g after ADT. Figure 4 shows LSVs before and after ADT of PtCo/MPC and PtCoMnCr/MPC alloy catalysts with and without the addition of melamine. PtCo/MPC and PtCoMnCr/MPC alloy catalysts retained high MA of 1050 and 800 A/g, respectively, even after ADT due to the addition of melamine, and there was little difference in MA between the two alloy catalysts. From these results, it is concluded that the addition of Mn to the PtCo/MPC alloy catalyst improved the ORR activity, but the addition of Cr did not improve the durability of the PtCoMn/MPC alloy catalyst and that the addition of melamine significantly improved the durability of the Pt alloy catalysts. Further studies are required to investigate the effect of Mn and Cr addition on the durability of PtCo/MPC catalysts when the particle size increases (>5 nm). Reference [1] T. Chen et al., Science, 26, 105890 (2023).[2] Y. Chida et al., Nat. Commun., 14, 4492 (2023).[3] H. Daimon et al., ACS Catal., 12, 8976 (2022). Figure 1