An extra layer

Introducing a layer of gold alloy between the outer platinum shell and palladium core of a nanoparticle improves its activity as a catalyst for oxygen reduction.

Credit: © 2010 ACS

A polymer electrolyte membrane fuel cell works by oxidizing hydrogen molecules at an anode — liberating their electrons — and ferrying the resultant protons to a cathode where they can react with oxygen through the oxygen reduction reaction (ORR). Platinum is an active catalyst for the ORR but it is scarce, expensive and quite slow. Therefore the search is on for alternatives, and one possibility that has been the focus of much research recently is the use of core–shell nanoparticles with a platinum monolayer surface.

The deposition of a platinum layer on top of a metal substrate results in a geometrical lattice mismatch between layers that induces compressive or expansive strain on the platinum layer. This alters the electronic structure and thus bonding properties of the surface, which in turn affects catalytic activity. The effect is substrate-dependent, and now Radoslav Adzic, Yangchuan Xing and colleagues at Brookhaven National Laboratory have developed a method¹ for modifying the substrate, and therefore activity — the introduction of an alloy sublayer in between the core and shell of a Pd–Pt core–shell nanoparticle.

A technique known to restrict metal deposition to single monolayers was used to first deposit a layer of Pd–Au alloy onto the surface of a palladium nanoparticle and then to deposit a monolayer of platinum atoms. Nanoparticles with different sublayer compositions were made with 5%, 10% and 20% gold, and were all more active for ORR when compared with Pd–Pt core–shell nanoparticles. The improved activity, generated by the gold atoms, was attributed to lattice strain effects and also to their possible protection of the platinum surface from oxidation, which also improved the stability of the catalyst.