Characterization of Pt Catalysts Supported on Ta-SnO2 with Fused Aggregated Network Structure

Polymer electrolyte fuel cells (PEFCs) are attractive power generation systems for electric vehicles and residential co-generation systems. Platinum-based catalysts supported on carbon black (CB) have electronically conductive paths with gas transport paths and are suitable for catalysts for PEFCs. Cathode catalysts reach potentials of over 1.5 V during startup/shutdown, where the carbon undergoes degradation due to its intrinsic thermodynamic instability [1,2]. Such degradation problems should somehow be overcome to achieve long-term durability of PEFCs.

 SnO₂ and TiO_x are promising candidates for cathode catalyst supports with long-term durability at high potentials [3,4]. We have recently characterized Pt catalysts supported on Nb-SnO₂ (Pt/Nb-SnO₂), which had a CB-like fused aggregated network structure and distinctive stability with respect to high potential exposure, by half-cell and membrane electrode assembly (MEA) measurements [5-9]. The ceramic supports with these structures were able to supply both electronically conductive paths and gas diffusion paths in the catalyst layers, resulting in high cathodic activity with durability for the PEFCs. In the present study, we found that Pt supported on Ta-SnO₂ (Pt/Ta-SnO₂) has even higher electrical conductivity than that of Pt/Nb-SnO₂, by half cell and MEA measurements.

We synthesized the Pt catalysts supported on the Ta-SnO₂ support by the flame combustion method and colloidal method; details of the methods and characterization were described in earlier papers [5-9].The Ta-SnO₂ support powders (BET surface 45 m² g^-1; crystallite size 16.7 nm) were determined to be single-phase rutile-type SnO₂ from the X-ray diffraction analysis. A transmission electron microscopic (TEM) image (Fig. 1) showed that the support was composed of fused aggregated network structures, and that the Pt nanoparticles (particle sizes 3.1 ± 0.6 nm; interparticle distances 9.4 nm; loaded amount 15.8 wt%) were uniformly dispersed on the support. The electrical conductivity of the Pt/Ta-SnO₂ was ca. 4 times higher than that of Pt/Nb-SnO₂. The electrochemically active surface area (ECSA) of Pt/Ta-SnO₂, which was estimated from the cyclic voltammograms (CVs, Fig. 2), was 66.4 m² g^-1, similar to that of Pt/Nb-SnO₂ (65.6 m² g^-1). The kinetically controlled current density and mass activity at 0.85V of Pt/Ta-SnO₂ (Fig. 3) were approximately equal to those of Pt/Nb-SnO₂ and were higher than that of a commercial Pt/CB catalyst (TEC10E50E, Tanaka Kikinzoku Kogyo Co.). Changes in the ECSA as a function of the logarithm of the number of potential step cycles (0.9-1.3 V, 30 s, Fig. 4) showed that the stability of Pt/Ta-SnO₂ was the same as that of Pt/Nb-SnO₂ and was superior to those of both a commercial Pt/CB and commercial Pt catalyst supported on graphitized carbon black (Pt/GCB, TEC10EA50E, Tanaka Kikinzoku Kogyo Co.). We also found by MEA evaluation that the ohmic resistance using the Pt/Ta-SnO₂ cathode was lower than that using Pt/Nb-SnO₂ (Fig. 5), and that the performance using a Pt/Ta-SnO₂ cathode was superior to those using either cathodes of Pt/Nb-SnO₂ or Pt/CB (Fig. 6). We consider that both the low resistivity and the high Pt utilization of Pt/Ta-SnO₂ led to both the lowering of the ohmic resistance and the improvement of catalytic activity in the catalyst layer. We conclude that the Pt/Ta-SnO₂catalyst is an attractive candidate for PEFC cathodes for fuel cell vehicles.

 

Acknowledgement

This research was supported by funds for the “Research on Nanotechnology for High Performance Fuel Cells” (HiPer-FC) project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, and the JSPS KAKENHI grant Number B24350093.

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