Surface Chemistry of Ruthenium Dioxide in Heterogeneous Catalysis and Electrocatalysis: From Fundamental to Applied Research

Spectroscopic insights into the nature of active sites in iron–nitrogen–carbon electrocatalysts for oxygen reduction in acid

Solid oxide cell (SOC) based energy conversion systems have the potential to become the cleanest and most efficient systems for reversible conversion between electricity and chemical fuels due to their high efficiency, low emission, and excellent fuel flexibility. Broad implementation of this technology is however hindered by the lack of high-performance electrode materials. While many perovskite-based materials have shown remarkable promise as electrodes for SOCs, cation enrichment or segregation near the surface or interfaces is often observed, which greatly impacts not only electrode kinetics but also their durability and operational lifespan. Since the chemical and structural variations associated with surface enrichment or segregation are typically confined to the nanoscale, advanced experimental and computational tools are required to probe the detailed composition, structure, and nanostructure of these near-surface regions in real time with high spatial and temporal resolutions. In this review article, an overview of the recent progress made in this area is presented, highlighting the thermodynamic driving forces, kinetics, and various configurations of surface enrichment and segregation in several widely studied perovskite-based material systems. A profound understanding of the correlation between the surface nanostructure and the electro-catalytic activity and stability of the electrodes is then emphasized, which is vital to achieving the rational design of more efficient SOC electrode materials with excellent durability. Furthermore, the methodology and mechanistic understanding of the surface processes are applicable to other materials systems in a wide range of applications, including thermo-chemical photo-assisted splitting of H2O/CO2 and metal–air batteries.

Controlling cation segregation in perovskite-based electrodes for high electro-catalytic activity and durability

Selenium nanomaterials: An overview of recent developments in synthesis, properties and potential applications

Synthesis of three-dimensional flowerlike nitrogen-doped carbons by a copyrolysis route and the effect of nitrogen species on the electrocatalytic activity in oxygen reduction reaction

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

We investigated the relative contribution of the native surface and of the grain boundaries intersecting the surface to the chemical surface exchange of La{sub 0.7}Sr{sub 0.3}MnO{sub 3} (LSM) thin films. The chemical surface exchange coefficients, k{sub chem}, were determined using electrical conductivity relaxation (ECR). Two k{sub chem} values were obtained, indicating two surface exchange processes occurred in parallel. These two k{sub chem} values had different activation energies (Ea), which were interpreted as belonging to (1) the native surface response of individual grains/variants and (2) the variant/grain boundaries of the textured films. The first process has an activation energy of E{sub a,1} {approx} 1.57 eV, while E{sub a,2} {approx} 0.87 eV. The k{sub chem,2} values are almost 3 orders of magnitude higher than the k{sub chem,1} values at low temperatures. At higher temperatures, the data can be fit with one k{sub chem} that has an intermediate value of Ea, indicative of a transition region.

Microstructural Effects on the Oxygen Exchange Kinetics of La0.7Sr0.3MnO3 Thin Films

Stability and Dissolution of the Platinum Single Crystal Surfaces in Perchloric Acid

We demonstrate the feasibility of using Ultra Small Angle X-ray scattering to study cathode structures in solid oxide fuel cells (SOFC) under operating conditions. Cathode thickness has been optimized and symmetric SOFC test structures have been characterized. We also demonstrate that we can follow the evolution of microstructure on a La0.6Sr0.4Co0.2Fe0.8 infiltrated yttria stablized zirconia backbone structure under annealing, showing the viability of characterizing multi-scale cathode structures in real time.

Kee-Chul Chang

Papers

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

Microstructural Effects on the Oxygen Exchange Kinetics of La0.7Sr0.3MnO3 Thin Films

Stability and Dissolution of the Platinum Single Crystal Surfaces in Perchloric Acid

Ultra Small Angle X-ray Scattering Studies of Solid Oxide Fuel Cell Cathode Powders