PEFC Alloy Electrocatalysts Supported on SnO2: A Study on the Preparation Method
Authors: Matsumoto Shohei , Iwami Masahiro , Noda Zhiyun , Matsuda Junko , Hayashi Akari , Sasaki Kazunari 
Published: February 2020

Abstract

Pt/C is widely used as polymer electrolyte fuel cell (PEFC) electrocatalysts. However, under the high potential at the cathode, Pt detachment and aggregation due to carbon corrosion can occur. Therefore, Pt/C has a difficulty in durability. Electrocatalysts with tin oxide (SnO2) support are thus developed where vapor grown carbon fiber (VGCF-H) act as a conducting backbone, achieving both high activity and high durability [1-3]. However, their electrocatalytic activity should be further improved. For this reason, we are focusing on further activation by Pt-Co alloying (PtCox/Nb-SnO2/VGCF-H). Co is one of the relatively stable metals under the PEFC electrocatalyst environment and PtCox alloy catalysts are known to be capable of exhibiting higher activity over Pt catalysts [4]. The aim of this study is to develop Pt-Co alloy electrocatalysts on oxide support.
Sn0.98Nb0.02O2 was prepared on the VGCF-H (Sn0.98Nb0.02O2/VGCF-H) by the ammonia coprecipitation method or the homogeneous precipitation method. Pt(acac)2 and Co(acac)2 were applied to impregnate Pt and Co nanoparticles to obtain the electrocatalysts (acac method). In the acac method, two different procedures were applied: 1-step (simultaneously impregnating Pt and Co) and 2-step (after impregnating Pt, Co was impregnated). The Pt : Co ratio was adjusted to prepare Pt3Co alloy to obtain Pt3Co/Sn0.98Nb0.02O2/VGCF-H electrocatalyst.
In order to evaluate Pt and Co loadings, ICP analysis was performed. We also investigated Pt3Co alloying by XRD measurement for each different heat treatment condition. We then made half-cell tests to evaluate electrochemical activities of these electrocatalysts. Electrochemical surface area (ECSA) was measured by cyclic voltammetry (CV), and oxidation reduction reaction (ORR) activity was derived from kinetically controlled current density (ik
) in the rotating disk electrode (RDE) measurement.
From the ICP measurement results, we confirmed that we could impregnate both Pt and Co close to the initial ratio in the acac method. In XRD measurement, after 600℃ heat treatment in a short time, SnO2 was reduced and Pt-Sn alloy was formed. On the other hand, when 300℃ heat treatment was applied (2-step), Pt3­Co alloying proceeded. Figure 1 shows that, in the 2-step process, the XRD peak of the catalyst is shifted to a higher angle when compared with the XRD peak of pure Pt. Figure 2 shows EDS analysis by high-resolution STEM of Pt3Co/Sn0.98Nb0.02O2/VGCF-H prepared by the acac method (2-step, 300℃, 3h). These micrographs clearly show that Pt and Co are distributed in the same position to cover the SnO2 surface. We confirmed that Pt : Co was 7.23 : 2.73 (atomic ratio) by quantitative analysis of the catalyst particles. It is therefore concluded that Pt3­Co alloy catalysts impregnated on SnO2-based support can be prepared.

References

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Figure 1