Guy Lalande

Recent developments in the applications of nanocrystalline materials to hydrogen technologies

The paper discusses the application of nanocrystalline alloys as hydrogen storage materials and as electrocatalysts for solid polymer electrolyte fuel cells. After reviewing some of the requirements of metal hydrides for hydrogen fueled vehicles, the paper presents new results on the structure and hydrogen sorption properties of high storage capacity ball-milled magnesium hydride. The great advantages of milling the hydride instead of the pure metals for producing novel nanostructures with high surface area and for improving hydrogen sorption kinetics are presented. In the second part of the paper, the same technique has been extended to the milling of carbides and chlorides and coupled to a lixiviation process to produce new electrocatalysts for polymer electrolyte fuel cells. This new technology offers the possibility of producing nanoparticles with metastable structures whose specific surface area is much larger than that of any nanocrystalline powders made by conventional ball milling techniques. Pt-based nanoparticles were fabricated and tested as anode in fuel cells under pure and CO-contaminated hydrogen feedstreams.

Structural and surface characterizations of nanocrystalline Pt–Ru alloys prepared by high-energy ball-milling

The bulk and surface characteristics of Pt–Ru alloys prepared by high-energy ball-milling were studied over the whole compositional range. These materials were prepared in their as-milled and Mg-dispersed forms. The latter form is obtained by milling Mg with nanocrystalline Pt–Ru alloy prepared in a first step and then leaching out Mg in an acidic solution. The structure of the various alloys was determined by X-ray diffraction. Quantitative structural information was extracted from the X-ray histograms through Rietveld refinement analysis. In the as-milled form, a nanocrystalline fcc or a hcp structure is obtained after milling (40 h) of an initial powder mixture whose composition falls into the Pt- or Ru-rich side of the compositional range, respectively. For mixtures whose composition falls into the immiscibility gap, a metastable amorphous phase is formed, which co-exists with a fcc or a hcp structure, depending on which side of the immiscibility gap the composition is. The structure of Mg-dispersed Pt–Ru alloys is identical to that of the as-milled materials. The chemical composition and structure of the alloy surface was determined by X-ray photoelectron spectroscopy. From measurements of the Ru 3p and Pt 4f core level peaks, it is shown that the surface composition of both as-milled and Mg-dispersed alloys closely follow their bulk composition, with only a slight enrichment in Pt. There is also no change in the surface composition resulting from the extra milling step with Mg and its subsequent leaching. The separation between the maximum of the Ru 3d5/2 and Pt 4f7/2 core level peaks varies with the composition of the alloy, indicating that a true surface alloy is formed between Pt and Ru.

Improvement of the high energy ball-milling preparation procedure of CO tolerant Pt and Ru containing catalysts for polymer electrolyte fuel cells

Ball-milling has been used to prepare performing CO tolerant polymer electrolyte fuel cell anode catalysts that contain Pt and Ru. The catalyst precursors are obtained by milling together Pt, Ru and a dispersing agent in the atomic ratio 0.5, 0.5 and 4.0. This precursor is not easily recovered after milling because it sticks to the walls of the vial and on the grinding balls. However, the precursor is recovered as a powder when a process control agent (PCA) is added during the milling step. Various PCAs have been used. The PCA should not interfere with the electrocatalytic activity of the catalysts obtained by leaching the precursor. The best preparation of catalyst precursors are obtained by milling: (i) Pt, Ru and Al (dispersing agent) in the atomic ratio 0.5, 0.5, 4.0 + 10 wt% NaF (PCA) or (ii) Pt , Ru and MgH2 in the 0.5, 0.5, 4.0 atomic or molecular ratio. In this case, MgH2 plays at the same time the role of a dispersing agent and that of a PCA. The catalysts are obtained by leaching Al and NaF in (i) or MgH2 in (ii). The CO tolerance of these catalysts is equivalent to that of Pt0.5Ru0.5 Black from Johnson Matthey. The ball-milled catalysts have a surface area comprised between 30 and 44 m2 g−1. As-prepared catalysts are mainly made of metallic Pt and metallic plus oxidized Ru. After fuel cell tests, Pt is completely metallic while the oxidized Ru content decreases but does not disappear. These catalysts are composed of particles with crystallites of two different sizes: in (i) nanocrystallites (∼4 nm) that contain essentially Pt alloyed with Al and perhaps some Ru, and larger (≥∼30 nm) crystallites that contain essentially Ru; in (ii) Pt nanocrystalline particles that may contain some Ru and larger particles that contain essentially either Ru or Pt.