(363y) Carbon-Free, Core-Shell Connected Nanonetwork Electrocatalysts with Enhanced Oxygen Reduction Activity and Durability for Polymer Electrolyte Fuel Cells

1. Introduction

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) are highly promising as an alternative sustainable energy generating device, due to their high efficiency in clean power production. 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.

To tackle these issues, our group has developed carbon-free, connected Pt-alloy catalysts. 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] However, the formation of interconnected Pt-alloy nanoparticles requires high-temperature annealing (700 °C), resulting in particle size growth. Consequently, connected Pt-alloy catalysts demonstrate insufficient ORR mass activity (ORR activity per Pt mass) due to a reduced surface area resulting from nano-network enlargement.^[2]

This work introduces a novel synthesis method that enables the development of connected core-shell nanoparticle catalysts without any high-temperature treatment, yielding Pd cores enveloped by connected Pt atomic shells, as illustrated in Fig. 1. Additionally, we explore the control of core-shell structures in connected catalysts and investigate their structural effects on ORR activity.

2. Methods

Connected core-shell catalysts with Pd nanoparticles as core and Pt as atomic shell were synthesized through a novel one-pot polyol process (~100 °C), yielding stable nanonetworks with a high surface area without requiring high-temperature annealing (Fig. 1). At first, Pd nanoparticles were deposited on a polymer functionalized silica template (Pd/SiO₂). Then, Pt shell was formed on Pd/SiO₂ using a controlled polyol method to obtain carbon-free, connected Pd@Pt nanonetwork. The structure of Pt shell on core metals was precisely controlled by adjusting factors like reaction temperature, time and incorporation method of Pt precursor in the Pt shell formation reaction. Catalyst structures were evaluated by XRD, ICP, TEM and STEM-EDX line-mapping techniques.

Electrochemical analysis was carried out using a rotating disk electrode technique in 0.1 M HClO₄ electrolyte solution to estimate the electrochemical surface areas per Pt mass (ECSAs) and ORR activities of the catalysts. Furthermore, the catalyst's durability was assessed through an accelerated durability test, which involved 10,000 load cycles using square-wave potential cycling (0.6 V for 3 sec ↔ 1.0 V for 3 sec) at 60 °C.

3. Results and Discussion

This work successfully developed Pd-core@Pt-shell structures on connected nanoparticle catalysts as shown in Fig. 2 of the STEM–EDX area mapping and line mapping result. The formation of Pd@Pt core-shell nanonetworks (< 10 nm) consisting of connected nanoparticles formed by Pt atomic shells was confirmed by TEM and STEM–EDX analysis. The Pt-shell thickness was controlled in the range of 1–6 atomic layers.

The electrochemical measurements show that the ORR mass activity was notably enhanced in the catalyst with shell thickness ranging from 2.5 to 3.5 Pt atomic layers. The electrochemical performance of the connected Pd@Pt catalyst with 3.5 Pt atomic layers and the reference data are summarized in Table 1. The higher ECSA of the developed Pd@Pt catalyst is attributed to the new synthesis method in Fig. 1 that does not use high temperature annealing. The The d-band center position significantly influences the oxygen binding energy of the Pt surface, resulting in a change in ORR specific activity.³ The improved ECSA and ORR specific activity of the connected Pd@Pt core-shell catalyst led to three-time higher ORR mass activity than a commercial Pt/C.

Furthermore, high durability of the connected core-shell catalyst with high ORR activity retention against the load cycles at 60 °C was also demonstrated. The structural analysis after 10000 load cycles indicated a highly stable nanonetwork connected by Pt atomic shell.

4. Conclusion

Herein, we have successfully demonstrated an innovative synthesis method for the formation of carbon-free connected catalysts through low temperature Pt atomic shell deposition. This approach effectively tackles several challenges encountered in previous connected catalysts, including significant grain growth and low surface area. The demonstrated durability and superior performance of these catalysts highlight their potential for carbon-free electrocatalysis. Further improvement will contribute to the realization of advanced ORR catalysts for fuel cell technology.

Acknowledgement

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)

[3] V. Stamenkovic et al., Angew. Chem.45, 2897 (2006).