Development of Pt and Pt-Alloy Electrocatalysts for the Next Generation PEFCs

Introduction

Development of highly active and durable electrocatalysts for the oxygen reduction reaction (ORR) are one of the most important subjects for polymer electrolyte fuel cells (PEFCs) as well as CO tolerant anode catalysts.¹ It is essential to achieve the reduction of Pt amounts from those currently used to less than 1/10 without loss of the performances in the commercialized systems. There are a couple of approaches to the goal, i.e., increases in Pt specific activity by alloying with the second metals etc. , in the specific surface area by nano-sizing the catalyst particles or in the effective use of them in the MEAs. The presentation introduces new findings which have been found with such approaches by us so far.

Results and Discussion

    We have first found noticeable enhancements by alloying of Pt with the second non-precious metals at ORR, which appear at Pt skin-layer formed on the alloys. ^2,3 It has been clarified that the enhanced dissociative adsorption of O₂ on Pt surface are the principal reason for the superior catalysis, by using new modern spectroscopic analyses combined with electrochemical measurements at the alloys and Pt single electrodes.⁴ So, our first topic of the presentation is the mechanistic discussion, which becomes a clue for the design of new catalysts.  While various binary and ternary Pt-based alloys supported on carbon black (CB) have been studied, the ORR activities of these supported catalysts are not always consistent with each other among them.  One reason is the absence of standardized evaluation skills or conditions for the prepared samples or their performances etc. Therefore, I will introduce detailed evaluation techniques involving a channel-flow double-electrode method which newly developed by us and others.  As the other reason of the inconsistences, uses of not well-defined sample catalysts, i.e., non-uniformity in the compositions, particle sizes or dispersion states at the supported catalyst particles, can be pointed out. In order to make clear such inconsistencies, I will show our very new results on the effects of composition as well as morphology of Pt_100-XCo_X alloys upon the ORR by using the single crystal alloys.⁵

In the approaching to the improvement of catalytic activity by nano-sizing of the particles, there has been an obstacle of the so-called “Particle-Size Effect”. We have demonstrated that there is no particle-size effect at least at the ORR for PEFCs in the corporative research with Wieckowski et al., ⁶ as well as PAFCs.⁷ The result clearly showed that an improved activity can be attained by the increase of the specific surface area which is inversely proportional to the mean particle size.

Recently, we have developed a “nanocapsule method” to prepare monodispersed pure Pt and Pt−M alloy particles supported on CB or graphitized CB (GC) with well-controlled composition and distinctively sharp size-distribution.⁸ In the presentation, I will also show the temperature dependencies of the real ORR activities and the superior activity at Nafion-coated Pt_XCo/CB (atomic ratio X = 1-3), evaluated by a channel flow double electrode method.⁹ We will also show an extraordinary high durability of the Pt/GC prepared by the nanocapsule method in comparison with commercial ones.¹⁰

Acknowledgement

The works were supported by MEXT and NEDO of Japan for “Leading Project” and “HiPer-FC Project”, respectively, for the next generation fuel cells.

References

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² T. Toda, et al., J. Electrochem. Soc. 146, 3750 (1999); ibid. 145, 4185(1998); J. Electroanal. Chem. 460, 285(1999).

³ N. Wakabayashi, et al., J. Phys. Chem. B 109, 5836 (2005)

⁴ M. Wakisaka, et al., J. Phys. Chem. C, 112, 2750 (2008); Langmuir, 25, 1897(2009); Phys.  Chem. Chem. Phys., 12, 4184(2010); Energy Environ. Sci., 4, 1662 (2011).

⁵ M. Wakisaka, et al., Electrochem. Commun., 18, No.1, 55 (2012).

⁶ H. Yano, et. Al., Phys. Chem. Chem. Phys, 8, 4932- (2006), E. Higuchi, et al., J. Electroanal. Chem., 583, 69 (2005).

⁷ M. Watanabe, et al., J. Electroanal. Chem., 261, 375- (1989).

⁸ H. Yano, et al., Langmuir 231, 6438 (2007) ; J. Phys. Chem. C 112, 8372 (2008).

⁹ K. Okaya, et al., ACS Appl. Mater. Interfaces, 2, 888 (2010) ; ibid., 4, 6982 (2012).

¹⁰ H. Yano, et al., Phys. Chem. Chem. Phys., 12, 3806 (2010) ; J. Electroanal. Chem., 688, 137 (2013).