The Development of Uniform-Sized Pt-Based Binary Alloy Electrocatalysts for ORR in Polymer Electrolyte Fuel Cells
Authors: Garavdorj Batnyagt , Galindev Oyunbileg , Wakita Hidenobu , Ui Koichi , Takeguchi Tatsuya 
Published: December 2024

For the large-scale commercialization of fuel cell vehicles (FCVs) in Japan after 2030, NEDO is aiming to reach a target cell performance of 0.84 V at 0.2 A cm－
2 for polymer electrolyte fuel cells (PEFCs).1 It is desirable to achieve the target performance using a catalyst with a reduced amount of platinum (Pt) and high durability. Alloy-core and Pt-skin structured Pt-based alloy catalysts offer many benefits for reducing the platinum amount and raising the durability. 2-4 Therefore, this study presents the structural characteristics and electrocatalytic activities of the alloy-core and Pt-skin PtM/C (M = Co, Ni, and Fe) binary alloy catalysts with uniform particle sizes and large electrochemical surface areas for the cathodic oxygen reduction reaction (ORR) in PEFCs.

The PtM/C (Pt: 35 wt.%, M= Fe, Ni, and Co) alloy catalysts supported on a commercial carbon (KB-600JD, SBET: 1335 m2 g－
1) were prepared by an impregnation method. Briefly, Pt(NO2)2(NH3)2 and transition metal precursor (Mx(NO3)y) solutions were first impregnated into the carbon suspension. Subsequently, a dry powder was annealed at 880 ºC for 1 h under 5% H2/Ar atmosphere. Finally, the alloy catalyst was leached by 1 M HNO3 at 80 ºC for 12 h, followed by the treatment at 500 ºC in 5% H2/Ar atmosphere. The structure and composition were analyzed using TEM, XRF, TGA, and XRD. The electrocatalytic activity was evaluated by the RDE method. CV and LSV curves were recorded in N2- and O2-saturated electrolyte (0.1 M HClO4), respectively. The CO stripping experiments were carried out by absorbing CO gas at 0.05 V. A single-cell test was performed on MEA (1cm2). The MEA was fabricated by sandwiching a membrane electrolyte (NRE211) with the anode and cathode containing the catalyst layers (Pt loading: 0.17 mg cm－
2 and I/C: 1) on the carbon papers (MB30). The I-V measurement was conducted on the cell at 80 ºC supplied by H2 (418 nccm, 75% RH) and air (998 nccm, 75% RH) under a backpressure of 50 kPaG. The catalyst durability was evaluated by applying square wave cycling according to a revised protocol of NEDO.5

TEM images reveal a uniform dispersion and a narrow particle size distribution of PtM/C alloy nanoparticles. The average particle sizes of PtFe/C, PtNi/C, and PtCo/C (~400 particles) were 2.95, 3.22, and 2.94 nm, respectively, indicating the formation of relatively small-sized nanoparticles. As shown in Figure 1, PtNi/C and PtCo/C catalysts have a disordered fcc-structure (SG: Fm3̅m, PDF#03-065-9445, PDF#01-072-9178). The PtFe/C catalyst exists in an ordered fct-phase (SG: P4mmm, PDF#01-073-2622). Metal ratios in the alloy nanoparticles of PtFe/C (2.9:1), PtNi/C (3.7:1), and PtCo/C (3.8:1) were calculated by Vegard’s law from a Pt (111) plane. The ECSAHUPD values for PtFe/C, PtNi/C, and PtCo/C catalysts were lower (89.7 m2 g Pt
－
1, 75.7 m2 g Pt
－
1, and 71.1 m2 g Pt
－
1) than ECSA COads. values (102 m2 g Pt
－
1, 96 m2 g Pt
－
1, and 93 m2 g Pt
－
1). The mass activity (MA) of ORR shows an increasing order of 0.94 A mg－
1
Pt (PtNi/C) > 0.97 A mg－
1
Pt (PtCo/C) > 1.42 A mg－
1
Pt (PtFe/C) and surpasses that of commercial PtCo/C (0.62 A mg－
1
Pt). Figure 2 shows the I-V (H2-Air) polarization curves of PtM/C and commercial PtCo/C catalysts measured at 80 ºC of cell temperature under 50 kPaG. The PtM/C catalysts show higher cell performances at low current density (0.2 A cm－
2) such as 0.770 V (PtFe/C), 0.772 V (PtNi/C), and 0.772 V (PtCo/C), compared with the value (0.762 V) of the commercial PtCo/C catalyst. In addition, the ECSAs for all PtM/C catalysts were maintained after 10000 cycles in ADT (RDE), indicating that the ionomer could transfer from the catalyst surface into the carbon pores, thereby increasing the number of active sites.7 The MA of PtM/C catalysts has surpassed that of the commercial PtCo/C catalyst, showing their improved durability. These findings suggest that the PtM/C catalysts with the low Pt amount could still be promising candidates for ORR in PEFCs.

This study was partly supported by NEDO, Japan.

References

NEDO roadmap, https://www.nedo.go.jp/content/100973008.pdf (in Japanese)

Liu et al., J. Am. Chem. Soc.,
146, 2033 (2024).

Zhao et al.,
ACS Catal.,
10, 10637 (2020).

T. Y. Yoo et al.,
Energy Environ. Sci.,
16, 1146 (2023).

NEDO, PEFC evaluation protocol, https://www.nedo.go.jp/content/100963953.pdf (in Japanese)

Catalyst activity evaluation protocol of FC-Cubic (in Japanese).

Kobayashi et al.,
ACS Appl. Energy Mater., 4, 2307 (2021).

Figure 1