(Invited) Design of Electrocatalysts with Ad-Atoms, Single Crystals and Supported Nano-Particles for the Applications to Fuel Cells
Authors: Watanabe Masahiro 
Published: February 2020

For the wide applications of any types of fuel cells, the performance, durability and cost-reduction are essential. We have worked on developments of the materials, structures and the relating mechanisms for DMFC, PAFC, SOFC and PEFC. In this presentation, the author will briefly review first some major discoveries noted topics 1 and 2 below, but mainly discuss on the recent results relating to topic 3, obtained by e. g. “HiPer-FC Project”, targeted the development of high performance & durable electrocatalysts and hydrocarbon type PEMs based on mechanistic studies of the enhancements or degradations.

Development of catalysts for direct methanol fuel cells (DMFCs)

DMFCs are attractive by the high volumetric power-density, but they were accompanied with a large potential-loss at the anode even at Pt with the highest activity among pure catalysts. We discovered that in the classification of alloying components into relatively active A-metals (Pt, Ir, Pd, Rh etc.) and inert B-metals (Ru, Os, Re, etc.), AB alloys exhibit a noticeable enhancement and its optimum composition of about 1/1 for the methanol oxidation and AA alloys showed just a dilution effect. We proposed the “Bi-Functional Mechanism” on the catalysis, where each A and B atom contributes to the enhancement by playing a different role in the reaction scheme and generalized the theory by extending to several types “Electrocatalysis by Ad-Atoms” as convenient tools for new catalyst developments. The preparation of nano-sized Pt-Ru supported on carbon black (CB), invented by us, and the performance have widely been referred and the same catalyst has been uniquely applied to both of commercialized DMFCs and polymer electrolyte fuel cells (PEFCs) in residential co-generation systems.

Development of less-expensive Pt-alloy catalysts for anode/cathode

Much tolerance to the poisoning by CO remained in reformed H2 fuel is needed to the anode Pt catalyst for commercialized PEFCs. On the other hand, a management of the improvement of performance and the reduction of Pt loading at the cathode are essential because O2 reduction reaction (ORR) is much slower than that of H2 oxidation reaction (HOR). We discovered that a superior tolerance even to 100ppm CO for HOR can be achieved by alloys between non-precious transient-metal groups such as Fe, Co, Ni etc. with the other precious metal groups such as Pt, Pd, Rh etc. by the lowered CO adsorbing strength and coverage on their surfaces. The alloys exhibit a positive core-level shift in Pt4f binding energy, which is inversely proportional to Pt-CO bonding energy, vice vasa at non-CO tolerant alloys. Thus, a guiding principle for the development of CO tolerant anode catalysts is established. On the other hand, we also discovered an enhanced ORR at Pt alloyed with Co etc, which is more than one order of magnitude at their optimum compositions of 30-50%, compared with pure Pt. By efficient and unique applications of the electrochemical method (EC) together with modern experimental systems, we have clarified that the alloys are covered with a “Pt skin” consist of 1-2 atomic-layer modified in the electronic structure by that of underlying alloy, resulting in the noticeable enhancement of ORR as mentioned above.

Development of highly dispersed catalysts and the performances and durability

Reduction of Pt loading to less than 1/10 from the presently used level, corresponding to the amounts for the treatment of exhausting-gas from ICE cars, i.e., 10gPt/100kW, is essential for the wide penetration to fuel cell vehicles (FCVs) market. We aimed to overcome the difficult hurdle, i.e., the increase of the mass activity MA which is proportional to the specific surface area S and the specific area activity J, by nano-sizing (to ca. d=2nm in diameter) and alloying (as mentioned above) of the advanced catalysts, respectively. We proposed an original “Territory Theory” and demonstrated experimentally there is no reduction in the MA caused by the so-called “Particle Size Effect” even at d=2nm as long as the inter-crystallite distance is kept larger than ca. 15nm on the supporting substrates, resulting in the MA increased proportionally to increased S-value. Based on the above results, we invented a “Nano-Capsule Method” for the preparation of nano-sized pure and alloy catalysts, which are controlled strictly in the size, composition and dispersion state on the supporting materials and has been demonstrating their extremely high performance and durability in comparison with widely commercial catalysts and the same level durability as that of larger-size ones, independent of the particle sizes.
We have developed HC-type PEMs and already demonstrated single PEFC operations for 5,000 hrs with these advanced PEMs.