Computational high-throughput screening of electrocatalytic materials for hydrogen evolution

Solar water splitting: progress using hematite (α-Fe(2) O(3) ) photoelectrodes

Photoelectrochemical (PEC) cells offer the ability to convert electromagnetic energy from our largest renewable source, the Sun, to stored chemical energy through the splitting of water into molecular oxygen and hydrogen. Hematite (α-Fe(2)O(3)) has emerged as a promising photo-electrode material due to its significant light absorption, chemical stability in aqueous environments, and ample abundance. However, its performance as a water-oxidizing photoanode has been crucially limited by poor optoelectronic properties that lead to both low light harvesting efficiencies and a large requisite overpotential for photoassisted water oxidation. Recently, the application of nanostructuring techniques and advanced interfacial engineering has afforded landmark improvements in the performance of hematite photoanodes. In this review, new insights into the basic material properties, the attractive aspects, and the challenges in using hematite for photoelectrochemical (PEC) water splitting are first examined. Next, recent progress enhancing the photocurrent by precise morphology control and reducing the overpotential with surface treatments are critically detailed and compared. The latest efforts using advanced characterization techniques, particularly electrochemical impedance spectroscopy, are finally presented. These methods help to define the obstacles that remain to be surmounted in order to fully exploit the potential of this promising material for solar energy conversion.

Synergetic effect of surface and subsurface Ni species at Pt-Ni bimetallic catalysts for CO oxidation

Various well-defined Ni-Pt(111) model catalysts are constructed at atomic-level precision under ultra-high-vacuum conditions and characterized by X-ray photoelectron spectroscopy and scanning tunneling microscopy. Subsequent studies of CO oxidation over the surfaces show that a sandwich surface (NiO(1-x)/Pt/Ni/Pt(111)) consisting of both surface Ni oxide nanoislands and subsurface Ni atoms at a Pt(111) surface presents the highest reactivity. A similar sandwich structure has been obtained in supported Pt-Ni nanoparticles via activation in H(2) at an intermediate temperature and established by techniques including acid leaching, inductively coupled plasma, and X-ray adsorption near-edge structure. Among the supported Pt-Ni catalysts studied, the sandwich bimetallic catalysts demonstrate the highest activity to CO oxidation, where 100% CO conversion occurs near room temperature. Both surface science studies of model catalysts and catalytic reaction experiments on supported catalysts illustrate the synergetic effect of the surface and subsurface Ni species on the CO oxidation, in which the surface Ni oxide nanoislands activate O(2), producing atomic O species, while the subsurface Ni atoms further enhance the elementary reaction of CO oxidation with O.

Density functional theory in surface chemistry and catalysis

Recent advances in the understanding of reactivity trends for chemistry at transition-metal surfaces have enabled in silico design of heterogeneous catalysts in a few cases. The current status of the field is discussed with an emphasis on the role of coupling theory and experiment and future challenges.

Proton transfer to charged platinum electrodes. A molecular dynamics trajectory study

A recently developed empirical valence bond (EVB) model for proton transfer on Pt(111) electrodes (Wilhelm et al 2008 J. Phys. Chem. C 112 10814) has been applied in molecular dynamics (MD) simulations of a water film in contact with a charged Pt surface. A total of seven negative surface charge densities σ between -7.5 and -18.9 µC cm(-2) were investigated. For each value of σ, between 30 and 84 initial conditions of a solvated proton within a water slab were sampled, and the trajectories were integrated until discharge of a proton occurred on the charged surfaces. We have calculated the mean rates for discharge and for adsorption of solvated protons within the adsorbed water layer in contact with the metal electrode as a function of surface charge density. For the less negative values of σ we observe a Tafel-like exponential increase of discharge rate with decreasing σ. At the more negative values this exponential increase levels off and the discharge process is apparently transport limited. Mechanistically, the Tafel regime corresponds to a stepwise proton transfer: first, a proton is transferred from the bulk into the contact water layer, which is followed by transfer of a proton to the charged surface and concomitant discharge. At the more negative surface charge densities the proton transfer into the contact water layer and the transfer of another proton to the surface and its discharge occur almost simultaneously.

The role of chemistry in the energy challenge

Chemistry with its key targets of providing materials and processes for conversion of matter is at the center stage of the energy challenge. Most energy conversion systems work on (bio)chemical energy carriers and require for their use suitable process and material solutions. The enormous scale of their application demands optimization beyond the incremental improvement of empirical discoveries. Knowledge-based systematic approaches are mandatory to arrive at scalable and sustainable solutions. Chemistry for energy, "ENERCHEM" contributes in many ways already today to the use of fossil energy carriers. Optimization of these processes exemplified by catalysis for fuels and chemicals production or by solid-state lightning can contribute in the near future substantially to the dual challenge of energy use and climate protection being in fact two sides of the same challenge. The paper focuses on the even greater role that ENERCHEM will have to play in the era of renewable energy systems where the storage of solar energy in chemical carries and batteries is a key requirement. A multidisciplinary and diversified approach is suggested to arrive at a stable and sustainable system of energy conversion processes. The timescales for transformation of the present energy scenario will be decades and the resources will be of global economic dimensions. ENERCHEM will have to provide the reliable basis for such technologies based on deep functional understanding.

Pt-decorated PdFe nanoparticles as methanol-tolerant oxygen reduction electrocatalyst

The activity and selectivity of carbon-supported Pt-decorated PdFe nanoparticles in the oxygen reduction reaction (ORR) were investigated in the presence and absence of methanol. The Pt-decorated PdFe nanoparticles, which consist of a PdPt surface and a PdFe interior, were prepared by the galvanic reaction between PdFe/C alloy nanoparticles and PtCl4(2-) in aqueous solution. The presence of a Pt-enriched surface after the replacement reaction was independently confirmed by several microstructural characterization techniques and cyclic voltammetry. The catalyst with such heterogeneous architecture is catalytically more active than a bulk PdFePt alloy catalyst with the same overall composition. The observed enhancements in catalyst performance can be attributed to the lattice strain effect between the shell and core components. The Pt-decorated PdFe (PdFe@PdPt/C) catalyst also compares favorably with a commercial Pt/C catalyst with four times as much Pt in terms of ORR activity, cost, and methanol tolerance.

A comprehensive survey of M(2)AX phase elastic properties

M(2)AX phases are a family of nanolaminate, ternary alloys that are composed of slabs of transition metal carbide or nitride (M(2)X) separated by single atomic layers of a main group element. In this combination, they manifest many of the beneficial properties of both ceramic and metallic compounds, making them attractive for many technological applications. We report here the results of a large scale computational survey of the elastic properties of all 240 elemental combinations using first-principles density functional theory calculations. We found correlations revealing the governing role of the A element and its interaction with the M element on the c axis compressibility and shearability of the material. The role of the X element is relatively minor, with the strongest effect seen in the in-plane constants C(11) and C(12). We identify several elemental compositions with extremal properties such as W(2)SnC, which has by far the lowest value of C(44), suggesting potential applications as a high-temperature dry lubricant.

Hydrogen evolution on nano-particulate transition metal sulfides

The hydrogen evolution reaction (HER) on carbon supported MoS2 nanoparticles is investigated and compared to findings with previously published work on Au(111) supported MoS2. An investigation into MoS2 oxidation is presented and used to quantify the surface concentration of MoS2. Other metal sulfides with morphologies similar to MoS2 such as WS2, cobalt-promoted WS2, and cobalt-promoted MoS2 were also investigated in the search for improved HER activity. Experimental findings are compared to density functional theory (DFT) calculated values for the hydrogen binding energies (deltaGH) on each system.

Charge transfer reactions at nanostructured Au(111) surfaces: influence of the substrate material on electrocatalytic activity

Nanostructured electrodes can be used as model catalysts in order to gain a basic understanding of electrocatalytic properties. In particular, the influence of particle size and particle dispersion of noble metal catalysts and a possible influence of the support material can be studied in detail. Electrocatalytic reactions such as the hydrogen oxidation reaction (HOR), the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR) are important for technical applications. Hence, palladium and platinum as typical catalysts were investigated on Au(111) substrates regarding the HOR, HER and ORR. A significant increase in catalytic activity was found for Pd and Pt deposited on Au(111) where, with a decreasing amount of deposited metal, an increase of specific activity is observed which is contrary to expectations. A different behaviour was found for the ORR, where, according to expectations, the reactivity increases with increasing amounts of Pt. Parameters influencing the electrocatalytic activity of nanostructured surfaces, such as strain of the overlayers induced by the support and a possible direct involvement of the Au(111) surface in the mechanism of HER, are discussed.