1. Introduction

Carbon supported Pt shell-Pd core structured catalyst (Pt/Pd/C) is a promising candidate for decreasing Pt usage owing to high Pt utilization and ORR activity [1]. We found that ORR specific activity of Pt/Pd/C catalyst was drastically enhanced with accelerated durability test (ADT) performed at 80°C although electrochemical surface area (ECSA) of the catalyst largely decreased, which moderately enhanced ORR mass activity [2]. Thus, we developed high activation protocol (HAP) to mitigate ECSA decay and largely enhance ORR mass activity [3]. Furthermore, we developed H₂-O₂ chemical activation treatment which mimics HAP performed on glassy carbon (GC) electrode and is suitable for mass-production of highly active catalyst [3].

Besides ORR activity enhancement, synthetic method of Pt/Pd/C catalyst suitable for mass-production is highly important. Although we demonstrated that Pt/Pd/C catalyst could be mass-produced by modified Cu-UPD/Pt displacement method [2, 4], the Cu-UPD method was complicated. Therefore, in this study, we synthesized Pt/Pd/C catalyst via direct displacement (DD) method in which Pd core was directly displaced with Pt precursors, which is much simpler synthetic method compared with Cu-UPD one. Durability and electrochemical properties of Pt/Pd/C catalyst synthesized via DD method were investigated.

2. Experimental

200 mL of H₂SO₄ aqueous solution (pH1) was deaerated with N₂ gas and 300 mg of carbon supported Pd core (Pd/C, mean diameter: 4.7 nm, metal loading: 33 wt.%, ISHIFUKU Metal Industry) was dispersed with stirring, and the solution temperature was controlled in a range of 5-70°C. Then, Pt precursors (K₂PtCl₄ or H₂PtCl₆) corresponding to Pt monolayer shell were added at the controlled temperature and stirred for 3 h. The obtained Pt/Pd/C catalyst was activated by HAP in which rectangular potential cycling of 0.4 V (300 s)-1.0 V (300 s) was applied to GC electrode in Ar saturated 0.1 M HClO₄ at 80°C for 60 cycles [3]. Pt/Pt/C catalyst was also activated with H₂-O₂ chemical treatment which mimics HAP performed on GC electrode and is suitable for mass-production. 300 mg of Pt/Pd/C catalyst was dispersed in 2 M H₂SO₄ aqueous solution at 80°C and H₂ and O₂ gasses were alternately introduced for every 6 minutes in which equilibrium potentials of H₂ (ca. 0.0 V) and O₂ (ca. 1.0 V) were alternately applied to the catalyst [3]. ADT was carried out by using rectangular potential cycling of 0.6 V (3 s)-1.0 V (3 s) in Ar saturated 0.1 M HClO₄ at 80°C for 10,000 cycles. Characterization of Pt/Pt/C catalyst was conducted by using XRF, TG, XRD, TEM, LEIS and CV techniques. ORR activity of the catalyst was evaluated by RDE method performed at 1,600 rpm with positive scan rate of 10 mV/s in O₂ saturated 0.1 M HClO₄ at 25°C.

3. Results and Discussion

TEM images of Pt/Pd/C catalyst synthesized via modified Cu-UPD/Pt displacement method and DD method are shown in Fig. 1. In DD method (H₂PtCl₆@70°C), fine catalyst particles disappeared and particle density decreased compared with Cu-UPD method, which improved durability of the catalyst as depicted in Fig. 2. This tendency became evident when H₂PtCl₆ was used as Pt precursor, which was considered to be due to higher Cl^- content in H₂PtCl₆ precursor than that in K₂PtCl₄ one.

ORR mass activity enhancement of Pt/Pd/C catalyst with HAP is summarized in Fig. 3. The highest initial ORR mass activity was obtained when K₂PtCl₄ was used as Pt precursor and displacement was performed at 70°C, and the activity increased to 876 A/g-Pt with HAP, which was 2.7-fold of a reference Pt/C catalyst (TEC1050E, TKK). ORR activity was influenced by Pt shell coverage of as-synthesized Pt/Pd/C catalyst. LEIS spectra of the catalysts displaced at 70°C using K₂PtCl₄ and H₂PtCl₆ precursors are demonstrated in Fig. 4. Pt/Pd signal ratio obviously increased in Pt/Pd/C catalyst displaced with K₂PtCl₄, indicating that the catalyst has higher Pt shell coverage. It was considered that the higher Pt shell coverage was obtained through one to one displacement reaction, i.e., Pd + Pt²⁺ = Pd²⁺ + Pt, when Pt²⁺ precursor (K₂PtCl₄) was used. In the meeting, a revised DD method improving Pt shell coverage and activation by H₂-O₂ chemical treatment will be presented.

Acknowledgement

We would like to thank to KOBELCO Research Institute for LEIS measurement. This study was supported by NEDO, Japan.

References

[1] J. Zhang et al., J. Phys. Chem. B, 108, 10955 (2004).

[2] N. Aoki et al., Electrocatal., DOI: 10.1007/s12678-017-0399-1 (2017).

[3] N. Aoki et al., Electrochim. Acta, 244, 146 (2017).

[4] N. Aoki et al., The 228^th Electrochemical Society Meeting, #1376, Phoenix, USA, October 2015.