(Poster Award - Honorable Mention) Ta2O5 Catalyst Support for Polymer Electrolyte Fuel Cells
Authors: Sanami Kojiro , Nishiizumi Ryosuke , Yasutake Masahiro , Noda Zhiyun , Lyth Stephen Matthew , Matsuda Junko , Hayashi Akari , Sasaki Kazunari 
Published: February 2024

Pt/C is widely used as polymer electrolyte fuel cells (PEFC) electrocatalysts. The cathodic environment of PEFC is prone to high acidity and high potential, which can degrade cell performance due to carbon corrosion 1. Therefore, electrocatalysts with oxide support such as SnO2 supported on carbon-based materials have been developed, achieving high durability. Further improvement of their catalytic performance is desirable, using a Pt-based alloy catalyst and a mesoporous carbon (MC) 2,3. However, studies using SnO2 as a support have suggested that the low melting point of Sn caused alloying of Pt and Sn and/or melting of metallic Sn, causing degradation of catalytic activity 3. From these reasons, here, we focus on tantalum (Ta) with much higher melting point to fabricate Ta2O5/MC electrocatalysts support, which can support Pt alloy catalysts to achieve both high durability and high activity.
Ta2O5 was prepared by a sol-gel method using Ta-ethoxide, and Pt-Co alloy nano-particles were deposited via the acetylacetonate method 4. The amount of Ta2O5 on the MC was set to be 30 and 60 wt.%, and the durability was evaluated by varying the amount of Ta2O5. The molar ratio of Pt to Co was set at 3:1. The microstructure of the electrocatalyst was observed by field-emission scanning electron microscopy (FESEM) and scanning transmission electron microscopy (STEM), and the alloy was evaluated by XRD measurements. Half-cell tests were made to evaluate electrochemical performance and durability (load cycle and start-stop cycle).
From the XRD measurements, the Pt peaks were shifted and the Ta2O5 peaks were confirmed, indicating that Pt-Co alloys were formed without forming the alloy of Pt and Ta. FESEM observation revealed that Ta2O5 particles were impregnated with a diameter of about 5 nm, and Pt-Co alloy particles were impregnated with a diameter of 2 to 3 nm. Some of catalyst particles were found to be impregnated on Ta2O5, but no significant selectivity to Ta2O5 was observed.
Both Pt effective surface area (ECSA) and oxygen reduction reaction (ORR) activity of the prepared catalysts were higher than of Pt/C, confirming the positive effect of Pt-Co alloy and MC on activity enhancement. Figure 1 shows that the load potential cycle durability was improved using MC, and the start-stop cycle durability was improved by using Ta2O5. In particular, the electrocatalysts with a lower loading of Ta2O5 exhibited higher durability under load potential cycling and subsequent higher ECSA compared to the Pt-Co/C electrocatalyst.

Acknowledgment

This paper is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

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Figure 1