Kee-Chul Chang

Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring Li+-Ni(OH)2-Pt Interfaces

Combining two different types of catalysts accelerated the hydrogen-generation step in water electrolysis. Improving the sluggish kinetics for the electrochemical reduction of water to molecular hydrogen in alkaline environments is one key to reducing the high overpotentials and associated energy losses in water-alkali and chlor-alkali electrolyzers. We found that a controlled arrangement of nanometer-scale Ni(OH)2 clusters on platinum electrode surfaces manifests a factor of 8 activity increase in catalyzing the hydrogen evolution reaction relative to state-of-the-art metal and metal-oxide catalysts. In a bifunctional effect, the edges of the Ni(OH)2 clusters promoted the dissociation of water and the production of hydrogen intermediates that then adsorbed on the nearby Pt surfaces and recombined into molecular hydrogen. The generation of these hydrogen intermediates could be further enhanced via Li+-induced destabilization of the HO–H bond, resulting in a factor of 10 total increase in activity.

Design and synthesis of bimetallic electrocatalyst with multilayered Pt-skin surfaces.

Advancement in heterogeneous catalysis relies on the capability of altering material structures at the nanoscale, and that is particularly important for the development of highly active electrocatalysts with uncompromised durability. Here, we report the design and synthesis of a Pt-bimetallic catalyst with multilayered Pt-skin surface, which shows superior electrocatalytic performance for the oxygen reduction reaction (ORR). This novel structure was first established on thin film extended surfaces with tailored composition profiles and then implemented in nanocatalysts by organic solution synthesis. Electrochemical studies for the ORR demonstrated that after prolonged exposure to reaction conditions, the Pt-bimetallic catalyst with multilayered Pt-skin surface exhibited an improvement factor of more than 1 order of magnitude in activity versus conventional Pt catalysts. The substantially enhanced catalytic activity and durability indicate great potential for improving the material properties by fine-tuning of the nanoscale architecture.

Enhancing the Alkaline Hydrogen Evolution Reaction Activity through the Bifunctionality of Ni(OH)2/Metal Catalysts

Active in alkaline environment: The activity of nickel, silver, and copper catalysts for the electrochemical transformation of water to molecular hydrogen in alkaline solutions was enhanced by modification of the metal surfaces by Ni(OH)(2) (see picture; I = current density and η = overpotential). The hydrogen evolution reaction rate on a Ni electrode modified by Ni(OH)(2) nanoclusters is about four times higher than on a bare Ni surface.

Unique Activity of Platinum Adislands in the CO Electrooxidation Reaction

The development of electrocatalytic materials of enhanced activity and efficiency through careful manipulation, at the atomic scale, of the catalyst surface structure has long been a goal of electrochemists. To accomplish this ambitious objective, it would be necessary both to obtain a thorough understanding of the relationship between the atomic-level surface structure and the catalytic properties and to develop techniques to synthesize and stabilize desired active sites. In this contribution, we present a combined experimental and theoretical study in which we demonstrate how this approach can be used to develop novel, platinum-based electrocatalysts for the CO electrooxidation reaction in CO(g)-saturated solution; the catalysts show activities superior to any pure-metal catalysts previously known. We use a broad spectrum of electrochemical surface science techniques to synthesize and rigorously characterize the catalysts, which are composed of adisland-covered platinum surfaces, and we show that highly undercoordinated atoms on the adislands themselves are responsible for the remarkable activity of these materials.

Monodisperse Pt3Co nanoparticles as electrocatalyst: the effects of particle size and pretreatment on electrocatalytic reduction of oxygen

Monodisperse Pt(3)Co nanoparticles have been synthesized with size control via an organic solvothermal approach. The obtained nanoparticles were incorporated into a carbon matrix and applied as electrocatalysts for the oxygen reduction reaction to investigate the effects of particle size and pretreatment on their catalytic performance. It has been found that the optimal conditions for maximum mass activity were with particles of approximately 4.5 nm and a mild annealing temperature of about 500 degrees C. While the particle size effect can be correlated to the average surface coordination number, Monte Carlo simulations have been introduced to depict the nanoparticle structure and segregation profile, which revealed that the annealing temperature has a direct influence on the particle surface relaxation, segregation and adsorption/catalytic properties. The obtained fundamental understanding of activity enhancement in Pt-bimetallic alloy catalysts could be utilized to guide the development of advanced nanomaterials for catalytic applications.

Shape-Dependent Activity of Platinum Array Catalyst

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.

Functional links between stability and reactivity of strontium ruthenate single crystals during oxygen evolution

In developing cost-effective complex oxide materials for the oxygen evolution reaction, it is critical to establish the missing links between structure and function at the atomic level. The fundamental and practical implications of the relationship on any oxide surface are prerequisite to the design of new stable and active materials. Here we report an intimate relationship between the stability and reactivity of oxide catalysts in exploring the reaction on strontium ruthenate single-crystal thin films in alkaline environments. We determine that for strontium ruthenate films with the same conductance, the degree of stability, decreasing in the order (001)>(110)>(111), is inversely proportional to the activity. Both stability and reactivity are governed by the potential-induced transformation of stable Ru(4+) to unstable Ru(n>4+). This ordered(Ru(4+))-to-disordered(Ru(n>4+)) transition and the development of active sites for the reaction are determined by a synergy between electronic and morphological effects.

Surface X-Ray Speckles: Coherent Surface Diffraction from Au(001)

We present coherent speckled x-ray diffraction patterns obtained from a monolayer of surface atoms. We measured both the specular anti-Bragg reflection and the off-specular hexagonal reconstruction peak for the Au(001) surface reconstruction. We observed fluctuations of the speckle patterns even when the integrated intensity appears static. By autocorrelating the speckle patterns, we were able to identify two qualitatively different surface dynamic behaviors of the hex reconstruction depending on the sample temperature.

In situ x-ray studies of oxygen surface exchange behavior in thin film La0.6Sr0.4Co0.2Fe0.8O3−δ

In situ synchrotron x-ray techniques were used to investigate oxygen surface exchange behavior in thin film La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF)/Gd2O3-doped CeO2/Y2O3-stabilized ZrO2 heterostructures. Applying electrical potentials across the heterostructures results in significant expansion or contraction of the out-of-plane LSCF lattice parameter, indicating changes in the LSCF oxygen vacancy concentration. Oxygen transport across the LSCF/atmosphere interface is found to be rate limiting under both cathodic and anodic conditions.

Persistent oscillations of x-ray speckles: Pt (001) step flow

We observed well-defined oscillations of speckle intensities from Pt (001) surfaces at high temperatures, persisting for tens of minutes. We used a model of hex-reconstructed terraces to show that the coherent x-rays reflected from the terraces retain their phases relative to the illumination boundary and the observed oscillations come from surface dynamics due to “step-flow” motion. Our results demonstrate a possibility that x-ray speckles can be applied to monitor the real-time evolution of surfaces.