Paolo Malacrida

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.

Mass-selected nanoparticles of PtxY as model catalysts for oxygen electroreduction

Low-temperature fuel cells are limited by the oxygen reduction reaction, and their widespread implementation in automotive vehicles is hindered by the cost of platinum, currently the best-known catalyst for reducing oxygen in terms of both activity and stability. One solution is to decrease the amount of platinum required, for example by alloying, but without detrimentally affecting its properties. The alloy PtxY is known to be active and stable, but its synthesis in nanoparticulate form has proved challenging, which limits its further study. Herein we demonstrate the synthesis, characterization and catalyst testing of model PtxY nanoparticles prepared through the gas-aggregation technique. The catalysts reported here are highly active, with a mass activity of up to 3.05 A mgPt(-1) at 0.9 V versus a reversible hydrogen electrode. Using a variety of characterization techniques, we show that the enhanced activity of PtxY over elemental platinum results exclusively from a compressive strain exerted on the platinum surface atoms by the alloy core.

Enabling direct H2O2 production through rational electrocatalyst design.

Future generations require more efficient and localized processes for energy conversion and chemical synthesis. The continuous on-site production of hydrogen peroxide would provide an attractive alternative to the present state-of-the-art, which is based on the complex anthraquinone process. The electrochemical reduction of oxygen to hydrogen peroxide is a particularly promising means of achieving this aim. However, it would require active, selective and stable materials to catalyse the reaction. Although progress has been made in this respect, further improvements through the development of new electrocatalysts are needed. Using density functional theory calculations, we identify Pt-Hg as a promising candidate. Electrochemical measurements on Pt-Hg nanoparticles show more than an order of magnitude improvement in mass activity, that is, A g(-1) precious metal, for H2O2 production, over the best performing catalysts in the literature.

Pt5Gd as a highly active and stable catalyst for oxygen electroreduction.

The activity and stability of Pt(5)Gd for the oxygen reduction reaction (ORR) have been studied, using a combination of electrochemical measurements, angle-resolved X-ray photoelectron spectroscopy (AR-XPS), and density functional theory calculations. Sputter-cleaned, polycrystalline Pt(5)Gd shows a 5-fold increase in ORR activity, relative to pure Pt at 0.9 V, approaching the most active in the literature for catalysts prepared in this way. AR-XPS profiles after electrochemical measurements in 0.1 M HClO(4) show the formation of a thick Pt overlayer on the bulk Pt(5)Gd, and the enhanced ORR activity can be explained by means of compressive strain effects. Furthermore, these novel bimetallic electrocatalysts are highly stable, which, in combination with their enhanced activity, makes them very promising for the development of new cathode catalysts for fuel cells.

Trends in the electrochemical synthesis of H2O2: enhancing activity and selectivity by electrocatalytic site engineering.

The direct electrochemical synthesis of hydrogen peroxide is a promising alternative to currently used batch synthesis methods. Its industrial viability is dependent on the effective catalysis of the reduction of oxygen at the cathode. Herein, we study the factors controlling activity and selectivity for H2O2 production on metal surfaces. Using this approach, we discover two new catalysts for the reaction, Ag-Hg and Pd-Hg, with unique electrocatalytic properties both of which exhibit performance that far exceeds the current state-of-the art.

Iron-Treated NiO as a Highly Transparent p-Type Protection Layer for Efficient Si-Based Photoanodes

Sputter deposition of 50 nm thick NiO films on p(+)-n-Si and subsequent treatment in an Fe-containing electrolyte yielded highly transparent photoanodes capable of water oxidation (OER) in alkaline media (1 M KOH) with high efficiency and stability. The Fe treatment of NiO thin films enabled Si-based photoanode assemblies to obtain a current density of 10 mA/cm(2) (requirement for >10% efficient devices) at 1.15 V versus RHE (reversible hydrogen electrode) under red-light (38.6 mW/cm(2)) irradiation. Thus, the photoanodes were harvesting ∼80 mV of free energy (voltage), which places them among the best-performing Si-based photoanodes in alkaline media. The stability was proven by chronoamperometry at 1.3 V versus RHE for 300 h. Furthermore, measurements with electrochemical quartz crystal microbalances coupled with ICP-MS showed minor corrosion under dark operation. Extrapolation of the corrosion rate showed stability for more than 2000 days of continuous operation. Therefore, protection by Fe-treated NiO films is a promising strategy to achieve highly efficient and stable photoanodes.

MoS2—an integrated protective and active layer on n+p-Si for solar H2 evolution

A new MoS2 protected n(+)p-junction Si photocathode for the renewable H2 evolution is presented here. MoS2 acts as both a protective and an electrocatalytic layer, allowing H2 evolution at 0 V vs. RHE for more than 5 days. Using a MoSx surface layer decreases the overpotential for H2 evolution by 200 mV.

Enhanced activity and stability of Pt–La and Pt–Ce alloys for oxygen electroreduction: the elucidation of the active surface phase

Three different Pt–lanthanide metal alloys (Pt5La, Pt5Ce and Pt3La) have been studied as oxygen reduction reaction (ORR) electrocatalysts. Sputter-cleaned polycrystalline Pt5La and Pt5Ce exhibit more than a 3-fold activity enhancement compared to polycrystalline Pt at 0.9 V, while Pt3La heavily corrodes in 0.1 M HClO4 electrolyte. Angle Resolved X-ray Photoelectron Spectroscopy (AR-XPS) and Low Energy Ion Scattering (LEIS) have been extensively combined with electrochemical techniques to follow the chemical and structural changes at the surface. The highly reactive lanthanide atoms are not stable in the presence of oxygen and readily oxidize. The surface oxides are completely dissolved in the electrolyte. In Pt5La and Pt5Ce the so formed Pt overlayer provides kinetic stability against the further oxidation and dissolution. At the same time, it ensures a very high stability during ORR potential cycling, suggesting that these alloys hold promise as cathode catalysts in Proton Exchange Membrane Fuel Cells (PEMFCs).

New Platinum Alloy Catalysts for Oxygen Electroreduction Based on Alkaline Earth Metals

AbstractThe energy efficiency of polymer electrolyte membrane fuel cells is mainly limited by overpotentials related to the oxygen reduction reaction (ORR). In this paper, we present new platinum alloys which are active for the ORR and based on alloying Pt with very abundant elements, such as Ca. Theoretical calculations suggested that Pt5Ca and Pt5Sr should be active for the ORR. Electrochemical measurements show that the activity of sputter-cleaned polycrystalline Pt5Ca and Pt5Sr electrodes is enhanced by a factor of 5–7 relative to polycrystalline Pt. Accelerated stability testing shows that after 10,000 electrochemical cycles, the alloys still retain over half their activity. The stability is thus not quite on par with the similar Pt-lanthanide alloys, possibly due to the somewhat lower heat of formation. Graphical AbstractLeft: The structure of Pt5Ca with a compressed Pt overlayer. The compression increases activity for the Oxygen Reduction Reaction (ORR). Right: The measured ORR activity of the best Platinum alloys.

Determination of Core–Shell Structures in Pd-Hg Nanoparticles by STEM-EDX†

The structural and elemental configuration of a high‐performing Pd‐Hg electrocatalyst for oxygen reduction to hydrogen peroxide has been studied by means of high‐resolution scanning transmission electron microscopy. Pd‐Hg nanoparticles are shown to have a crystalline core–shell structure, with a Pd core and a Pd‐Hg ordered alloy shell. The ordered shell is responsible for the high oxygen reduction selectivity to H2O2.