Direct observation of the dealloying process of a platinum-yttrium nanoparticle fuel cell cathode and its oxygenated species during the oxygen reduction reaction.

Size-selected 9 nm PtxY nanoparticles have recently shown an outstanding catalytic activity for the oxygen reduction reaction, representing a promising cathode catalyst for proton exchange membrane fuel cells (PEMFCs). Studying their electrochemical dealloying is a fundamental step towards the understanding of both their activity and stability. Herein, size-selected 9 nm PtxY nanoparticles have been deposited on the cathode side of a PEMFC specifically designed for in situ ambient pressure X-ray photoelectron spectroscopy (APXPS). The dealloying mechanism was followed in situ for the first time. It proceeds through the progressive oxidation of alloyed Y atoms, soon leading to the accumulation of Y(3+) cations at the cathode. Acid leaching with sulfuric acid is capable of accelerating the dealloying process and removing these Y(3+) cations which might cause long term degradation of the membrane. The use of APXPS under near operating conditions allowed observing the population of oxygenated surface species as a function of the electrochemical potential. Similar to the case of pure Pt nanoparticles, non-hydrated hydroxide plays a key role in the ORR catalytic process.

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.

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.

Exploring the phase space of time of flight mass selected Pt(x)Y nanoparticles.

Mass-selected nanoparticles can be conveniently produced using magnetron sputtering and aggregation techniques. However, numerous pitfalls can compromise the quality of the samples, e.g. double or triple mass production, dendritic structure formation or unpredicted particle composition. We stress the importance of transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) for verifying the morphology, size distribution and chemical composition of the nanoparticles. Furthermore, we correlate the morphology and the composition of the PtxY nanoparticles with their catalytic properties for the oxygen reduction reaction. Finally, we propose a completely general diagnostic method, which allows us to minimize the occurrence of undesired masses.

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.

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.

Importance of Surface IrOx in Stabilizing RuO2 for Oxygen Evolution.

The high precious metal loading and high overpotential of the oxygen evolution reaction (OER) prevents the widespread utilization of polymer electrolyte membrane (PEM) water electrolyzers. Herein we explore the OER activity and stability in acidic electrolyte of a combined IrOx/RuO2 system consisting of RuO2 thin films with submonolayer (1, 2, and 4 Å) amounts of IrOx deposited on top. Operando extended X-ray absorption fine structure (EXAFS) on the Ir L-3 edge revealed a rutile type IrO2 structure with some Ir sites occupied by Ru, IrOx being at the surface of the RuO2 thin film. We monitor corrosion on IrOx/RuO2 thin films by combining electrochemical quartz crystal microbalance (EQCM) with inductively coupled mass spectrometry (ICP-MS). We elucidate the importance of submonolayer surface IrOx in minimizing Ru dissolution. Our work shows that we can tune the surface properties of active OER catalysts, such as RuO2, aiming to achieve higher electrocatalytic stability in PEM electrolyzers.

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

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.

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.

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.