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Proton Exchange membrane fuel cells (PEMFCs) for automotive applications are subject to hundreds of thousands of potential or load cycles during normal operation of the vehicle. Pt dissolves under such cycling and is a major contributing factor to the lowered durability of PEMFCs. This paper details the effect of various simplified potential-cycle profiles, the effect of operating conditions such as temperature, relative humidity, reactant gases etc., as well as different catalyst materials under normal fuel cell operating voltage ranges (0.95-0.60V) in fuel cells. Results on Pt dissolution utilizing complementary ex-situ methods are also presented. The fundamental and practical implications of varying the operating parameters as well as a possible hypothesis for Pt dissolution are reported.

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We studied oxygen reduction reaction (ORR) activities for outermost surfaces of 0.3 nm thick Co deposited on Pt(111) (Co0.3 nm/Pt(111)) bimetallic systems fabricated using molecular beam epitaxy at various Co deposition temperatures. Results show that Co0.3 nm/Pt(111) fabricated at temperatures lower than 393 K displays extra low-energy electron diffraction (LEED) spots outside the integer ones, indicating incoherent epitaxial growth of Co. A new IR band that is attributed to linearly bonded carbon monoxide (CO) on the Pt site influenced by neighboring Co atoms emerges at 2052 cm–1 for 333 K fabricated Co0.3 nm/Pt(111), in addition to the CO–Pt and CO–Co bands. With increasing fabrication temperature, the new band shifts to higher frequencies and reaches 2082 cm–1 for 773 K fabricated Co0.3 nm/Pt(111), which has a diffuse (1×1) LEED pattern. We evaluated the dependence of the deposition temperature on the lattice parameters of the Co0.3 nm/Pt(111) and ascribed the band at 2082 cm–1 to adsorbed CO on a Pt-enriched topmost surface having 6-fold symmetry. Although the incoherent epitaxial Co layer was unstable in 0.1 M HClO4 aqueous solution, the Pt-enriched topmost surface is rather stable and the ORR activity is 10 times higher than that for clean Pt(111). The activities for Pt0.3 nm,0.6 nm/Co0.3 nm/Pt(111) artificial sandwich (superlattice) surfaces were also evaluated. The obtained results indicate that the Co atoms located at the second atomic layer strongly modify the electrocatalysis of the topmost surface.

In order to examine the relationship between the oxygen reduction reaction (ORR) activity of a fuel cell catalyst and its structure and/or electronic state, carbon-supported Pt and Pt alloys having various structures, compositions, and morphologies were studied. Regardless of the atomic ordering or morphology (core−shell or random alloy) of the catalyst, the ORR activity was primarily dependent on the Pt−Pt bond distance. Among these materials, Pt 2 Co, having the shortest Pt−Pt distance, exhibited the highest ORR activity. The activities of this catalyst per unit surface area and per unit mass were approximately 10 times and 6 times higher than those of a commercially available carbon supported Pt electrocatalyst (Pt/C). This work also found a monotonic increase in catalytic activity with decreasing Pt−Pt distance.

A carbon-supported Pt-shell Au-core electrocatalyst (Pt/Au/C) was prepared by sequential deposition of Pt ions on the surface of Au nanoparticles supported on carbon. The area-specific activity of the oxygen reduction reaction (ORR) for the prepared Pt/Au/C in 0.1 M HClO4 aqueous solution was approximately 2 times higher than that for a commercial carbon-supported Pt electrocatalyst (Pt/C). The core–shell structure was confirmed using electrochemical methods and Pt and Au K-edge X-ray absorption fine structure (XAFS) analysis. XAFS analyses indicated that the Pt–Pt bond distance for the Pt/Au/C catalyst was shorter than that for Pt foil and the Pt/C catalyst. In addition, the Au–Au distance was much shorter than that for Au foil. The reason for the high ORR activity of Pt/Au/C is considered to be shorter Pt–Pt bond distance as compared to that of Pt/C.

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