Electrocatalysis will play a key role in future energy conversion and storage technologies, such as water electrolysers, fuel cells and metal-air batteries. Molecular interactions between chemical reactants and the catalytic surface control the activity and efficiency, and hence need to be optimized; however, generalized experimental strategies to do so are scarce. Here we show how lattice strain can be used experimentally to tune the catalytic activity of dealloyed bimetallic nanoparticles for the oxygen-reduction reaction, a key barrier to the application of fuel cells and metal-air batteries. We demonstrate the core-shell structure of the catalyst and clarify the mechanistic origin of its activity. The platinum-rich shell exhibits compressive strain, which results in a shift of the electronic band structure of platinum and weakening chemisorption of oxygenated species. We combine synthesis, measurements and an understanding of strain from theory to generate a reactivity-strain relationship that provides guidelines for tuning electrocatalytic activity.

No abstract

Anomalous small angle X-ray scattering (ASAXS) is shown to be an ideal technique to investigate the particle size and particle composition dynamics of carbon-supported alloy nanoparticle electrocatalysts at the atomic scale. In this technique, SAXS data are obtained at different X-ray energies close to a metal absorption edge, where the metal scattering strength changes, providing element specificity. ASAXS is used to, first, establish relationships between annealing temperature and the resulting particle size distribution for Pt25Cu75 alloy nanoparticle electrocatalyst precursors. The Pt specific ASAXS profiles were fitted with log-normal distributions. High annealing temperatures during alloy synthesis caused a significant shift in the alloy particle size distribution towards larger particle diameters. Second, ASAXS was used to characterize electrochemical Cu dissolution and dealloying processes of a carbon-supported Pt25Cu75 electrocatalyst precursor in acidic electrolytes. By performing ASAXS at both the Pt and Cu absorption edges, the unique power of this technique is demonstrated for probing composition dynamics at the atomic scale. These ASAXS measurements provided detailed information on the changes in the size distribution function of the Pt atoms and Cu atoms. A shift in the Cu scattering profile towards larger scattering vectors indicated the removal of Cu atoms from the alloy particle surface suggesting the formation of a Pt enriched Pt shell surrounding a Pt-Cu core. Together with XRD and TEM, ASAXS is proposed to play an increasingly important role in the mechanistic study of degradation phenomena of alloy nanoparticle electrocatalysts at the atomic scale.

Dealloyed Pt 25 Cu 75 bimetallic nanoparticle electrocatalysts exhibit up to six times higher oxygen reduction reaction activities than pure nanoparticle Pt catalysts at 0.9 V/ reversible hydrogen electrode ͑RHE͒. The active form of the catalyst is formed in situ from Pt-Cu precursor material using voltammetric dealloying. The effects of composition of precursors as well as effects of the annealing temperature and duration on the catalyst activity are studied. We vary the composition between Pt 25 Cu 75 and Pt 75 Cu 25 and change the annealing conditions from 600 to 950°C and for 7 and 14 h. X-ray diffraction and electrochemical analyses are used to obtain insight on the structural details of the catalyst samples. Information regarding the extent of alloying, atomic ordering, the Pt and Cu compositions, and distributions on the nanoparticles and particle ͑crystallite͒ sizes is correlated with the trends observed from mass and specific activities of the catalysts. It was found that an annealing duration of 14 h offers little or no benefit to catalytic activities compared to 7 h. Dealloyed Pt 25 Cu 75 annealed for 7 h, at 800°C yielded an optimal active material with respect to the extent of alloying and particle size growth and exhibited the highest Pt mass-based and favorable specific catalytic oxygen reduction reaction ͑ORR͒ activity. The occurrence and role of a noncubic Pt 50 Cu 50 Hongshiite phase is discussed.Pt-Cu catalytic systems have been the focus of research studies for more than a decade. Initial research involves understanding Cu surface segregation using single crystals to understand the relationships between such segregation with atomic arrangement and electronic structures. 1 Little was observed from that work, probably due to the extremely low Cu content in their initial samples ͑Ͻ3 atom %͒. Since then, building on the initial dealloying study by Erlebacher et al., 2 Cu richer Pt-Cu alloys have raised the interest of corrosion scientists for the ease of removing Cu from Pt-Cu alloys. The resultant porous platinum structure offered a significant potential as materials for high surface area electrodes in biomedical sensors. 3,4 In addition, due to the hydrogenation promoting property of PtCu, the system had spurred interests in a large number of different research fields as well as in a wide variety of synthesis methods to produce the most uniform and catalytically active material. Of these, the colloidal dispersion method had been used to produce polymerprotected Pt-Cu alloy clusters at low temperatures. 5 Their material was used for the hydration of acrylonitrile and hydrogenation of 1, 3-cyclooctadiene. Then, core-shell Pt-Cu nanoparticles were synthesized using the polyol-coreduction process for the heterogeneous NO x reduction reaction. 6 At the same time, the plausibility of using polyamidoamine dendrimer for the synthesis of Pt-Cu, with the ability of finely controlling particle sizes and particle morphology ͑well-mixed or core-shell͒ for the purpose of CO oxidation and toluene hydrogenation...

No abstract