Effect of Annealing Time on Various PtNi/C Binary Alloy Catalysts for ORR in PEFCs
Authors: Yamaguchi Kazuki , Garavdorj Batnyagt , Wakita Hidenobu , Ui Koichi , Takeguchi Tatsuya 
Published: December 2024

Introduction

PEFCs are attracting attention as a power source for automobiles, residential, and portable devices due to their low environmental impact and high energy efficiency 1-2). However, the cathode of PEFCs requires a large amount of platinum to accelerate the ORR reaction. The high cost of Pt has hindered the extensive development of PEFCs for large-scale commercialization. Therefore, the research that combines reduced Pt usage with the high activity of electrocatalysts is highly desirable. A commonly applied approach to develop highly efficient and low-cost electrocatalysts is to reduce the Pt loading in the catalysts supported on high specific surface area carbon and to alloy Pt with transition metals, which could lead to a significant improvement in the cell performance. In this study, the PtNi nanoparticle alloy catalysts were synthesized as the cathode catalysts using high specific surface area carbon, utilizing Ni, which is an inexpensive metal, and their catalytic activity was evaluated.

Experimental

A Pt/C catalyst was prepared with loadings of 35% and 50% weight by ethanol reduction method 3). Briefly, nickel nitrate precursor was precipitated into the Pt/C powder by controlling pH at 10. Drying black powder then annealed at 900 °C for 15, 30, 60, 90 min in H2/Ar flow. The resulting powder was noted as PtNi/C(BT). To obtain Pt rich surface, acid treatment was performed in 0.5 M H2SO4 at 80 °C. The resulting powder was noted as PtNi/C(AT). The physical characteristics were evaluated by using various techniques such as TEM, XRD, and XRF. CV was measured to calculate the ECSA in 0.1 M HClO4 at room temperature. LSV was performed to evaluate the ORR mass activity and initial activity of the PtNi/C catalyst. Finally, the I-V cell performance of the PtNi/C catalyst was evaluated using MEA as a cathode for ORR in PEFCs.

Results and Discussion

TEM images revealed that the PtNi/C catalyst particles were homogeneously dispersed. The average particle sizes increased with increasing annealing time. Fig.1 shows the XRD patterns obtained for the 39.7 ° peak on the Pt(111) plane was shifted to a wider angle, indicating the formation disordered fcc-phase of PtNi alloy. The peaks became sharper with increasing annealing time, suggesting higher crystallinity. The crystalline size was calculated by using the Scherrer equation (3.73 nm, 5.21 nm, 5.94 nm and 6.49 nm), respectively. The ECSA of all PtNi/C catalysts was higher compared to the commercial PtCo/C catalysts. Acid treatment would cause Ni leaching from the surface, leading to a Pt-enriched surface, which could result in higher ECSA. The ECSA values decreased with increasing the annealing time, possibly due to particle growth and larger particle size with longer annealing time. The ORR activity for all PtNi/C catalysts was improved and higher than commercial PtCo/C. This increase in ORR activity would be due to the internal alloying of Ni and the improved electronic structure resulting from annealing at 900 °C. Fig.2 shows similar I-V performance was observed for 60 and 90 min annealing time in PtNi/C catalysts. Instead, the PtNi/C catalyst annealed for 15 min had lower performance, which could be attributed to Ni leaching and particle agglomeration. After ADT, the cell voltage at high current density region (1.0 A cm−2) for 60 min catalysts of PtNi/C (AT) was lower degradation compared to the commercial PtCo/C catalyst, indicating improved durability. In this study, the improvement of ORR activity was investigated at different annealing times, and it was found that 60 minutes of annealing time showed better durability and activity.
This study was partly supported by NEDO, Japan.

References

(1) S. Maiti, et al., Energy Environ. Sci., 14, 3717 (2021).
(2) R. Borup, et al., Chem. Rev., 107, 3904 (2007).
(3) N. Narischat, et al.,
J. Phys. Chem. C,
118, 23003, (2014).

Figure 1