Pt-Sn(Oxidized Shell)/C and Pt-Sn(Reduced)/C as Cathode Catalysts for the Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cells: Catalyst Performances and Characterization

Autor: Oki Sekizawa, Tomoya Uruga, Yoshiaki Imaizumi, Yasuhiro Iwasawa, Takashi Yamamoto, Shin-ichi Nagamatsu, Gabor Samjeské, Shinobu Takao, Kensaku Nagasawa
Rok vydání: 2013
Předmět:
Zdroj: ECS Transactions. 50:1651-1657
ISSN: 1938-6737
1938-5862
DOI: 10.1149/05002.1651ecst
Popis: Until now, pure Pt in the form of carbon-supported nanoparticles is the best performing and highest durable electrocatalyst at both anode and cathode in a polymer electrolyte fuel cell (PEFC). However, high price and limited resources may rule out a broader application of pure Pt catalysts. Additionally, the sluggish kinetics of the oxygen reduction reaction (ORR) at the fuel cell cathode is a main factor limiting performances. Therefore, replacing Pt partially or completely while further increasing performance and durability has been in focus of research in the last decade. One approach is to use carbon-supported bimetallic nanoparticle catalysts (Ptx-My) with supposed synergy function towards the ORR, as for example in the bifunctional mechanism. Here, we present the first results using Pt-Sn(oxidized shell)/C and Pt-Sn(reduced)/C as membrane assembly electrode (MEA) cathode catalysts in PEFC under operating conditions. Carbon-supported Pt-Sn nanoparticles were synthesized by an impregnation method similar to Roman-Martinez et al. and then exposed to O2 at elevated temperatures to obtain a Pt-Sn(oxidized shell)/C catalyst. The obtained Pt-Sn(oxidized shell)/C catalyst was subsequently reduced to obtain Pt-Sn(reduced)/C nanoparticles. 2 The catalyst powder with a typical Ptloading of about 40 wt% was used as cathode material to fabricate MEAs of 9 cm electrode area size and a load of 0.6 mgPt/cm (CHEMIX Co., Ltd, Japan). Typical operations were conducted at 80oC cell temperature, relative humidity between 92% and 63%, and flow rates of 100 sccm (anode) and 600 sccm (cathode), respectively. Conditioning of the MEA was done by a sequence of 150 conditioning cycles with every conditioning cycle consisting of the same sequence of step-wise increasing galvanostatic current-steps. After conditioning, the MEAs were tested using cyclic voltammetry (CV), frequency response analysis (FRA) and galvanostatic controlled load-cycles to obtain I-V curves and power-plots. A high resolution transmission-electron-microscope (TEM) image of Pt-Sn(oxidized shell)/C catalyst particles after 130 conditioning cycles is shown in Figure 1. The average particle size is about 6 nm. A more detailed transmission-electron-microscope reveals inhomogeneous Pt-Sn bimetallic nanoparticles on the carbon support. To study the performance and durability behavior, load-cycles under galvanostatic conditions with fixed current steps between open-circuit voltage (ocv) and a maximum current keeping the cell-voltage > 0.1 V were applied, and the I-V load curves of the first 20 loadcycles are shown in Figure 2. As can be clearly seen, we found a significant increase of performance. Such a large increase in the performance was not observed for the load-cycles in the case of Pt-Sn(reduced)/C and standard Pt/C (TEC10E50E, Tanaka Kikinzoku) cathode MEA catalysts investigated under the similar conditions. Therefore, we may ascribe the observed large increase of performance to a synergy effect of Pt nanoparticle and SnOx cluster formed on the surface of the Pt nanoparticle as will be further discussed. The white line intensities of the Pt L3-edge XANES spectra for Pt-Sn(oxidized shell)/C and Pt/C at OCV were almost identical, which indicates no electron transfer from Sn to Pt. The white line intensity of Pt-Sn(oxidized shell)/C at higher voltages was much lower than that of Pt/C, indicating that the Pt-Sn(oxidized shell)/C cathode catalyst was hard to be oxidized compared to Pt/C (Figure 3). Sn K-edge XANES spectra did not change significantly upon changing voltages, where the oxidation state of Sn species remained unchanged during the voltage operation. The great effect of the SnOx cluster addition to Pt/C was found in this study. The role of the SnOx clusters will be discussed.
Databáze: OpenAIRE
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