Martin Hangaard Hansen

Tuning the activity of Pt alloy electrocatalysts by means of the lanthanide contraction

A lanthanide boost for platinum High loadings of precious platinum are needed for automotive fuel cells, because the kinetics of the oxygen reduction reaction (ORR) are relatively slow. Escudero-Escribano et al. studied a series of platinum alloys with lanthanides and alkaline earth elements. When the surfaces were leached to leave pure platinum, they developed compressive strain that boosted the ORR activity—up to a factor of 6 for terbium. Enthalpy effects helped to stabilize these alloys under operating conditions. Science, this issue p. 73 Alloying platinum with lanthanide elements compresses its surface layer and boosts its oxygen reduction activity. The high platinum loadings required to compensate for the slow kinetics of the oxygen reduction reaction (ORR) impede the widespread uptake of low-temperature fuel cells in automotive vehicles. We have studied the ORR on eight platinum (Pt)–lanthanide and Pt-alkaline earth electrodes, Pt5M, where M is lanthanum, cerium, samarium, gadolinium, terbium, dysprosium, thulium, or calcium. The materials are among the most active polycrystalline Pt-based catalysts reported, presenting activity enhancement by a factor of 3 to 6 over Pt. The active phase consists of a Pt overlayer formed by acid leaching. The ORR activity versus the bulk lattice parameter follows a high peaked “volcano” relation. We demonstrate how the lanthanide contraction can be used to control strain effects and tune the activity, stability, and reactivity of these materials.

Water at Interfaces

The interfaces of neat water and aqueous solutions play a prominent role in many technological processes and in the environment. Examples of aqueous interfaces are ultrathin water films that cover most hydrophilic surfaces under ambient relative humidities, the liquid/solid interface which drives many electrochemical reactions, and the liquid/vapor interface, which governs the uptake and release of trace gases by the oceans and cloud droplets. In this article we review some of the recent experimental and theoretical advances in our knowledge of the properties of aqueous interfaces and discuss open questions and gaps in our understanding.

Investigating the coverage dependent behaviour of CO on Gd/Pt(111)

Gd modified Pt(111) single crystals have been prepared in an ultra high vacuum (UHV). By vacuum deposition of ∼200 Å Gd on a sample heated to 800 °C, a Pt5Gd alloy terminated by a single atomic layer of Pt was formed. Subsequently the surfaces were characterized using low energy electron diffraction (LEED), showing that a highly ordered crystal structure had appeared. To study the molecular dynamics on this surface a detailed study of the CO adsorption on the surface was conducted using temperature programmed desorption (TPD) of CO. The TPD spectra show a desorption peak shifted down in temperature compared to those of pure Pt(111). The shape of the desorption peak and the desorption temperature were shown to be strongly dependent on the CO coverage of the surface. A systematic investigation of CO desorption temperature as a function of coverage was consequently performed. A simple simulation of the TPD spectra was carried out, based on adsorption energies from density functional theory (DFT). This simulation reproduces the shift and the narrowing of the desorption spectrum from the experiments and the DFT calculations suggest that the sharp TPD feature arises from cooperative adsorbate interactions, caused by subtle reconstructions occurring at coverages above 1/3 ML CO, whereas the overall temperature shift relative to pure Pt(111) comes from weaker CO binding due to the contraction of the Gd/Pt(111) surface.

Frontispiece: Active-Phase Formation and Stability of Gd/Pt(111) Electrocatalysts for Oxygen Reduction: An In Situ Grazing Incidence X-Ray Diffraction Study

Alloys of platinum and gadolinium present significant activity enhancement over pure Pt for the oxygen reduction reaction (ORR), both in the form of extended electrode surfaces and nanoparticulate catalysts. The active phase consists of a compressed Pt overlayer formed on Pt5 Gd electrodes upon exposure to the electrolyte by acid leaching. Here, we investigate the formation, strain and correlation lengths of the active Pt overlayer by using in situ synchrotron grazing incidence X-ray diffraction on Gd/Pt(111) single-crystalline electrodes. The overlayer forms upon exposure to electrolyte under open circuit conditions; the compressive strain relaxes slightly upon repeated electrochemical cycling in the potential range 0.6 to 1.0u2005V versus the reversible hydrogen electrode (RHE). In addition, the strain relaxes strongly when exposing the electrode to 1.2u2005V versus RHE, and the thickness of the crystalline portion of the overlayer increases with potential above 1.3u2005V versus RHE.

Photo-electro Catalytic Water Splitting over Pure and Modified Iron Oxide Thin Films

Photo-catalysis offers the possibility to utilize solar energy to split water, thereby extracting chemical energy in the form of hydrogen. The general challenge is to synthesize a material with the right electronic and chemical properties for efficiently driving the process, while being sustainable in terms of stability and costs. The focus of this bachelor thesis is photo-electrocatalytic splitting of water on modified iron oxide thin films. Through various surface treatments and modification of thin film compositions, this material has shown that its water splitting properties can be improved.