Electrochemistry will play a vital role in creating sustainable energy solutions in the future, particularly for the conversion and storage of electrical into chemical energy in electrolysis cells, and the reverse conversion and utilization of the stored energy in galvanic cells. The common challenge in both processes is the development of-preferably abundant-nanostructured materials that can catalyze the electrochemical reactions of interest with a high rate over a sufficiently long period of time. An overall understanding of the related processes and mechanisms occurring under the operation conditions is a necessity for the rational design of materials that meet these requirements. A promising strategy to develop such an understanding is the investigation of the impact of material properties on reaction activity/selectivity and on catalyst stability under the conditions of operation, as well as the application of complementary in situ techniques for the investigation of catalyst structure and composition.

Oxygen electrochemistry as a cornerstone for sustainable energy conversion

Synthesis and catalytic properties of metal nanoparticles: Size, shape, support, composition, and oxidation state effects

Compared with the conventional deposition techniques used for the epitaxial growth of metallic structures on a bulk substrate, wet-chemical synthesis based on the dispersible template offers several advantages, including relatively low cost, high throughput, and the capability to prepare metal nanostructures with controllable size and morphology. Here we demonstrate that the solution-processable two-dimensional MoS(2) nanosheet can be used to direct the epitaxial growth of Pd, Pt and Ag nanostructures at ambient conditions. These nanostructures show the major (111) and (101) orientations on the MoS(2)(001) surface. Importantly, the Pt-MoS(2) hybrid nanomaterials exhibit much higher electrocatalytic activity towards the hydrogen evolution reaction compared with the commercial Pt catalysts with the same Pt loading. We believe that nanosheet-templated epitaxial growth of nanostructures via wet-chemical reaction is a promising strategy towards the facile and high-yield production of novel functional materials.

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Solution-phase epitaxial growth of noble metal nanostructures on dispersible single-layer molybdenum disulfide nanosheets

Fuel cell technology is currently shifting very fast from fundamental research to real development. In addition to other aspects, this transition is possible because of the important improvements achieved in the field of electrocatalysis in the past decade. This perspective will give a focused overview summarizing the most outstanding contributions in the last 10 years in terms of activity and durability of the catalyst materials for ethanol oxidation and oxygen reduction reaction, respectively. In addition, it provides an outlook about new catalyst support materials with improved performance/stability, advanced characterization techniques, and fundamental studies of reaction mechanisms and degradation processes. All the studies referred to in this perspective significantly contribute to reaching the technical targets for PEFC commercialization.

Electrocatalysis for Polymer Electrolyte Fuel Cells: Recent Achievements and Future Challenges

This paper reports a systematical study on the electrochemical properties of MnO2-based nanostructures as low-cost catalysts for oxygen reduction reaction (ORR) in alkaline media The results show 

MnO2-Based Nanostructures as Catalysts for Electrochemical Oxygen Reduction in Alkaline Media†

We produced millions of morphologically identical platinum catalyst nanoparticles in the form of ordered arrays epitaxially grown on (111), (100), and (110) strontium titanate substrates using electron beam lithography. The ability to design, produce, and characterize the catalyst nanoparticles allowed us to relate microscopic morphologies with macroscopic catalytic reactivities. We evaluated the activity of three different arrays containing different ratios of (111) and (100) facets for an oxygen-reduction reaction, the most important reaction for fuel cells. Increased catalytic activity of the arrays points to a possible cooperative interplay between facets with different affinities to oxygen. We suggest that the surface area of (100) facets is one of the key factors governing catalyst performance in the electrochemical reduction of oxygen molecules.

Shape-dependent activity of platinum array catalyst.

Presented herein is the experimental observation of the long-range ordered (√19 × √19)R23.4°-13CO structure on Pt(111) in aqueous electrolytes by in situ surface X-ray scattering. The results confirmed the presence of two mirrored domains suggested earlier on the basis of scanning tunneling microscopy and infrared measurements. Based on the weak intensity of the second order adlattice reflections and earlier results obtained by other techniques, a further refinement of the (√19 × √19) structure with tilted CO molecules is proposed. The hystereses observed in transitions between (2 × 2 )- 3 CO and (√19 × √19)R23.4°-13CO phases, as well as in CO adsorption and stripping, with change in electrode potential are discussed.

In situ surface X-ray scattering observation of long-range ordered (√19 × √19)R23.4°-13CO structure on Pt(111) in aqueous electrolytes

We present a novel model system for nanoparticle electrocatalysts. A surface consisting of alternating (100) and (111) facets, several nanometers across and nearly 1 μm long, were self-assembled by annealing Pt single crystal surfaces initially cut at the midpoint between [111] and [100] directions, i.e., Pt(1+√3 1 1). The formation of these self-assembled arrays of nanofacets was monitored by in-situ surface X-ray scattering. These surfaces were further characterized with scanning probe microscopy and cyclic voltammetry. We found that the Pt(1+√3 1 1) surface is flat with less than 1 nm rms roughness when it was annealed in argon/hydrogen atmosphere. Then the surface forms nanofacets when it is annealed in pure air. This nanofaceting transition was completely reversible and reproducible. We investigated effects of CO adsorption on the voltammetric characteristics of both hydrogen-annealed and air-annealed surfaces. We found that CO-adsorption/desorption cycles in CO containing electrolyte solution result...

Nanofaceted platinum surfaces: a new model system for nanoparticle catalysts.

We used in situ Se K-edge X-ray spectroscopy to characterize Ru nanoparticles chemically modified with submonolayers of selenium (Se/Ru) [Cao et al. J. Electrochem. Soc. 2006, 153, A869]. X-ray powder diffraction verified that the Se/Ru catalyst had metallic Ru cores. The in situ X-ray absorption near edge structure taken at the open circuit potential showed that there were both elemental and oxidized selenium on the as-prepared Se/Ru samples. All selenium oxide was reduced to the elemental form of selenium by applying negative potentials. By applying positive potentials, selenium was subsequently reoxidized. The analysis of the extended X-ray absorption fine structure shows the appearance of selenium hydration (Se-OH{sub 2}) in a deaerated solution, which was not observed during the oxygen reduction reaction. We present evidence that Se-free Ru atoms play an important role in the ORR activity of the Se/Ru catalyst studied in this paper.

In situ synchrotron x-ray spectroscopy of ruthenium nanoparticles modified with selenium for an oxygen reduction reaction

The influence of temperature changes in water-based electrolytes on the atomic structure at the electrochemical interface has been studied using in situ surface X-ray scattering (SXS) in combination with cyclic voltammetry. Results are presented for the potential-dependent surface reconstruction of Au(100), the adsorption and ordering of bromide anions on the Au(100) surface, and the adsorption and oxidation of CO on Pt(111) in pure HClO4 and in the presence of anions. These systems represent a range of structural phenomena, namely metal surface restructuring and ordering transitions in both nonreactive spectator species and reactive adsorbate layers. The key effect of temperature appears to be in controlling the kinetics of the surface reactions that involve oxygenated species, such as hydroxyl adsorption and oxide formation. The results indicate that temperature effects should be considered in the determination of structure−function relationships in many important electrochemical systems.

Andreas Menzel

Papers

Shape-dependent activity of platinum array catalyst.

In situ surface X-ray scattering observation of long-range ordered (√19 × √19)R23.4°-13CO structure on Pt(111) in aqueous electrolytes

Nanofaceted platinum surfaces: a new model system for nanoparticle catalysts.

In situ synchrotron x-ray spectroscopy of ruthenium nanoparticles modified with selenium for an oxygen reduction reaction

Temperature-induced ordering of metal/adsorbate structures at electrochemical interfaces.