Carbon-Free, Connected Core-Shell Nanoparticle Catalysts with Advanced Oxygen-Reduction Performances for PEFCs
Authors: Chitra Sudheer Aparna , Gopinathan M. Anilkumar , Kuroki Hidenori , Yamaguchi Takeo 
Published: February 2024

As the world faces diminishing non-renewable energy sources and a surge in global pollution, establishing a hydrogen and low-carbon economy has become crucial. Polymer electrolyte fuel cells (PEFCs) hold great promise as a sustainable energy source with the potential for high efficiency, and clean power generation in both stationary and mobile applications. However, the conventional platinum nanoparticle catalysts on carbon support (Pt/C) used for the oxygen reduction reaction (ORR) at the cathode shows sluggish ORR activity, leading to high cost and inferior PEFC performance. Moreover, the carbon-supports used for the Pt catalysts are susceptible to degradation caused by carbon-corrosion during start-stop operations, which can severely compromise the overall performance of the PEFCs. Hence, in order to significantly enhance the commercial viability of PEFCs, considerable progress must be made in terms of cost, performance, and durability.
Previously our group has reported carbon-free, connected Pt-alloy catalysts to address the above issues. This catalyst is composed of connected Pt-alloy nanonetworks with high electronic conductivity and exhibits enhanced ORR activity, high start-stop durability, and thin catalyst layers. Specifically, this catalyst exhibited 9 times higher specific activity than that of the commercial Pt/C, making it an excellent candidate for cathode catalyst applications.[1,2] However, high-temperature annealing process is required to develop connected networks between the Pt-alloy nanoparticles, resulting in larger crystallite sizes and reduced surface areas. This limitation has prevented connected Pt-alloy catalysts from achieving significant improvement in mass activity.
In this work, we have proposed a novel synthesis method that enables the development of connected core-shell nanoparticle catalysts without any high-temperature treatment. This method eliminates the particle growth and exhibit a high surface area, making it highly effective for ORR applications. Here, a carbon-free catalyst with a core-shell nanonetwork structure having a non-Pt (Pd) core and Pt shell was developed. Additionally, the relationship between the thickness of the Pt-shell layer and ORR activity was investigated.
Connected core-shell catalysts with Pd nanoparticles as core and Pt as atomic shell (Pd@Pt) were synthesized in a one-pot polyol process that does not require high-temperature annealing as shown in Fig. 1. The formation of connected Pd@Pt core-shell nanonetworks (< 10 nm) was confirmed by TEM and STEM-EDX analysis. Also, the structure of Pt shell on core metals was successfully tuned by controlling the reaction temperature and time, precursor ratio, etc. Here, the number of the Pt layers on the surface of the connected Pd@Pt core-shell catalysts were varied from 1–6 atomic layers and the results show that the ORR mass activity was notably enhanced in the catalyst with a moderate shell thickness ranging from 2.5 to 3.5 Pt atomic layers. This suggests that connected Pd@Pt catalyst with a moderate Pt layer achieves both high surface area and ORR specific activity. The high surface area of the developed catalysts is attributed to a new synthesis method that does not use high temperature annealing and a low Pt core-shell structure. The improvement in ORR specific activity can be attributed to the more suitable d-band center position of the Pt shell, which is influenced by strain and ligand effects resulting from the Pd core. The ORR performance of the carbon-free connected Pd@Pt catalyst showed 2–4 times enhancement in electrochemical surface area (ECSA) and 1.5–3 times improvement in ORR mass activity compared to the connected Pt-alloy catalysts prepared using high-temperature annealing. Further, high durability of the connected core-shell catalyst against load cycles (0.6 ↔1.0 V) at 60 °C in acidic electrolyte solution was also demonstrated, indicating that a high stability nanonetwork connected by Pt atomic shell.
In summary, we have successfully demonstrated a new synthesis method for the formation of connected core-shell nanoparticles by Pt atomic shell without high-temperature annealing and prepared a carbon-free, connected Pd@Pt core-shell catalyst with advanced ORR performance, highlighting its great potential as a cathode catalyst.

Acknowledgement

The part of this presentation is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

References

[1] Tamaki, T., Yamaguchi, T. et al., Energy Environ. Sci., 8, 3545–3549 (2015)
[2] Kuroki, H., Yamaguchi, T. et al., ACS Appl. Nano Mater., 3, 9912–9923 (2020)

Figure 1