Polymer electrolyte fuel cell (PEFC) is beginning to be put into practical use due to its high efficiency. However, reduction of cost is necessary for the further spread of PEFC. Generally, platinum is used for the catalyst of PEFC electrode. Since platinum is scarce and expensive, it is required to reduce the amount of Pt in the catalyst. Core-shell catalyst using Cu-UPD (under potential deposition) followed by SLRR (surface limited redox replacement) has been developed as a low-platinum catalyst [1]. In order to use this catalyst for practical PEFC, preparation method suitable for mass production has been studied [2]. In this research, we propose a novel preparation method of core-shell catalyst by directly using Cu-UPD to the catalyst layer of core material. This method is expected to enable continuous production of Pt core-shell catalyst layer. Since this process treats catalyst layer of which the thickness is several tens of micrometers, it is necessary to consider factors different from conventional method, especially mass transfer. In this study, Pd@Pt/C catalyst layer was prepared by using Cu-UPD and SLRR directly to the catalyst layer of Pd/C, and the physical properties and the performance was evaluated.

Pd/C (30.7 wt%-Pd, Ishifuku Kogyo Co., Ltd.) was used as core material. The catalyst ink was prepared by mixing Pd/C with distilled water, 1-propanol, and Nafion® solution. The ink was coated on a carbon paper (CP) with a microporous layer (MPL). The fabricated catalyst layer was immersed in deaerated 0.5 M H₂SO₄ solution. After cleaning the electrode by CV, the catalyst layer was immersed in 50 mM CuSO₄ + 0.5 M H₂SO₄ solution, and Cu was deposited on Pd particles by chronoamperometry (CA) at 0.1 V versus Ag/AgCl for 30 minutes. The catalyst layer was then immersed in 5 mM K₂PtCl₄ + 0.5 M H₂SO₄ solution to obtain Pd@Pt/C catalyst layer. This process was repeated for 1-3 times. The obtained catalyst layer and Pt/C carbon paper were hot-pressed onto each side of the Nafion membrane to fabricate MEA. Terminal voltage vs. current density of the MEA was measured at 80˚C.

IR-free polarization curves of MEA using Pd@Pt/C catalyst layer (0.04 mg-Pt/cm²) and MEA using Pt/C (0.30 mg-Pt/cm²) are shown in Fig. 1. Since the loading amount of Pt was not the same, mass activity of each MEA calculated from the current density at the IR-free terminal voltage of 800 mV are shown in Table 1. The mass activity of Pd@Pt/C 2SLRR was 1.8 times higher than that of Pt/C. This result is similar to the mass activity reported by Inaba et al. for MEA using Pd @Pt/C prepared by “modified Cu-UPD method”. Therefore, it is considered that this novel method is effective to produce core-shell catalyst layers with performance comparable to that prepared by the conventional method.

Acknowledgement

References

1. J. Zhang, et al., Angew. Chem., Int. Ed., 44, 2132 (2005)
2. M. Inaba, et al., J. Jpn. Petrol. Inst., 58, 55 (2015)