Abstract

The large capacity loss and low initial Coulombic efficiency (ICE) of a conventional SnO₂-based anode for Li ion batteries are originated largely from the limited reversibility of the conversion reaction associated with the anode. Often, the reversibility of the lithiation/delithiation of SnO₂ (with a high ICE value of ∼82%) declines with Sn coarsening in the Sn/Li₂O mixture during cycling, leading to gradual capacity decay. Here we demonstrate that the application of super-elastic films of NiTi alloy could accommodate the internal stress and volume change of lithiated nano-SnO₂ layer in a tri-layer NiTi/SnO₂/NiTi sandwich anode, effectively suppressing Sn coarsening. This unique electrode configuration has helped to retain the high reversibility of the SnO₂ layer with reversible capacity more than 800 mAh/g (based on SnO₂) for over 300 cycles, demonstrating stable charge capacities of ∼400 mAh/g in the potential ranges of 0.01–1.0 V and 1.0–2.0 V(vs. Li/Li⁺), respectively. Insitu spectroscopic and exsitu diffraction analyses corroborate the highly reversible electrochemical cycling, confirming that the reversibility and cyclability of SnO₂ anodes can be dramatically enhanced by preserving the nanostructure of Sn/Li₂O mixture, which facilitates the reversible conversion reaction.

Abstract

Abstract

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Abstract

Thin films of La_0.85Sr_0.15MnO₃ (LSM) are deposited on (1 0 0) silicon wafer and YSZ (yttria-stabilized zircornia) electrolyte substrates by magnetron sputtering using a single-phase LSM target. The conditions for sputtering are systematically studied, including substrate temperature (from room temperature to 600 °C), the argon background pressure (from 1.2 × 10⁻² to 3.0 × 10⁻² mbar), and deposition time. Results show that the optimal conditions for producing a dense, uniform, and crack-free LSM film include a substrate temperature of 600 °C and an argon pressure of 1.9 × 10⁻² mbar. Further, a testing cell with a dense LSM film, an YSZ electrolyte membrane, and a porous LSM counter electrode is prepared and the electrochemical properties of the dense LSM film on YSZ substrate are studied. It was found that the thickness, morphology, and microstructure of LSM films critically influence the electrochemical properties.

Abstract

Cleaning Solid Oxide Fuel Cells

Solid oxide fuel cells, which operate between 500° and 1000°C, transport oxygen through a ceramic material. At these temperatures, metals that catalyze hydrocarbon reforming reactions can also be incorporated so that conventional fuels such as methane can power the cell. One problem, however, has been rapid deactivation by sulfur impurities and carbon buildup. Yang et al. (p. 126; see the Perspective by Selman) report that doping of a barium zirconate-cerate with the rare-earths Y and Yb creates a material that transports both protons and oxygen ions at 750°C. This material, when used with nickel at the fuel cell anode, resists deactivation even when traces of hydrogen sulfide are present, apparently through enhanced ability to supply or remove water during surface reactions.

Bessere Kathodenmaterialien für Festoxidbrennstoffzellen können mit quantenchemischen Rechnungen zur Reduktion von Sauerstoff auf der Oberfläche von Sr-dotiertem LaMnO₃ identifiziert werden (La_0.5Sr_0.5MnO₃=LSM0.5). Die Reduktion verläuft über Superoxo- (La-super und Mn-super) und Peroxo-Intermediate (Mn-per), eine Dissoziation und den Einbau in den Festkörper (La-diss und Mn-diss) sowie Diffusion zu einer günstigeren Position (Produkt). YSZ=Y-dotiertes Zirconiumoxid.

Designing better cathode materials for solid oxide fuel cells can be aided by quantum-chemical calculations on oxygen reduction on Sr-doped LaMnO₃ surfaces (La_0.5Sr_0.5MnO₃=LSM0.5), which show that the reaction (see energy profile [eV]) proceeds via superoxo- (La-super and Mn-super) and peroxo-like (Mn-per) intermediates, dissociation and incorporation into the bulk (La-diss and Mn-diss), and diffusion to a more stable site (Product). YSZ=yttria-stabilized zirconia.

The applicability of the proton conductor Ba(Zr_0.1Ce_0.7Y_0.2)O_3–δ (BZCY7) as an electrolyte material for solid-oxide fuel cells (SOFCs) is investigated. The electrolyte material exhibits ionic conductivities (see figure) among the highest known for electrolytes viable for SOFC applications, and the chemical and thermal stability of BZCY7 appear adequate under a range of SOFC operating conditions. Several BZCY7-based single cells are fabricated using a modified dry-pressing method and show encouraging performance characteristics.

Abstract

The mechanisms of interaction between H₂S and Ni- or Cu-based anode surfaces in a solid oxide fuel cell were elucidated by density functional slab model calculations. Two reaction pathways via molecular and dissociative adsorption processes were mapped out following minimum energy paths. The energy for H₂S adsorption at the atop site of Ni(1 1 1) lying parallel to the surface is predicted to be −0.55 eV, while that for the dissociative adsorption is −1.75 eV. In contrast, the formation of initial molecular complexes on a Cu surface is energetically unfavorable (E_ad ∼ 0.0 eV), suggesting that Cu is more sulfur-tolerant than Ni.

Interactions between O₂ and CeO₂ are examined experimentally using in situ Raman spectroscopy and theoretically using density-functional slab-model calculations. Two distinct oxygen bands appear at 825 and 1131 cm⁻¹, corresponding to peroxo- and superoxo-like species, respectively, when partially reduced CeO₂ is exposed to 10 % O₂. Periodic density-functional theory (DFT) calculations aid the interpretation of spectroscopic observations and provide energetic and geometric information for the dioxygen species adsorbed on CeO₂. The O₂ adsorption energies on unreduced CeO₂ surfaces are endothermic (0.91<ΔE_ads<0.98 eV), while those on reduced surfaces are exothermic (−4. 0<ΔE_ads<−0.9 eV), depending on other relevant surface processes such as chemisorption and diffusion into the bulk. Partial reduction of surface Ce⁴⁺ to Ce³⁺ (together with formation of oxygen vacancies) alters geometrical parameters and, accordingly, leads to a shift in the vibrational frequencies of adsorbed oxygen species compared to those on unreduced CeO₂. Moreover, the location of oxygen vacancies affects the formation and subsequent dissociation of oxygen species on the surfaces. DFT predictions of the energetics support the experimental observation that the reduced surfaces are energetically more favorable than the unreduced surfaces for oxygen adsorption and reduction.

Porous nanostructured mixed-conducting materials with dual-scale porosities are fabricated using foam-like templates. The porous structures, as shown in the Figure, contain large pores (0.8–1.5 μm) for rapid gas transport and small pores (≈ 2.5 and 35 nm) for fast electrochemical reactions that require large surface areas. Thus, these electrode materials are ideally suited for electrochemical and catalytic applications, such as for solid oxide fuel cells.

Three-dimensional (3D) foam structure of a Cu₆Sn₅ alloy was fabricated via an electrochemical deposition process. The walls of the foam structure are highly porous and consist of numerous small grains. When used as a negative electrode for a rechargeable lithium battery, the Cu₆Sn₅ samples delivered a reversible capacity of about 400 mA h g^–1 up to 30 cycles. Further, these materials exhibit superior rate capability, attributed primarily to the unique porous structure and the large surface area for fast mass transport and rapid surface reactions. For instance, at a current drain of 10 mA cm^–2 (20C rate), the obtainable capacity (220 mA h g^–1) was more than 50 % of the capacity at 0.5 mA cm^–2 (1C rate).

Tin dioxide (SnO₂) box beams, or tubes with square or rectangular cross-sections, are synthesized on quartz substrates using a combustion chemical vapor deposition (CVD) method in an open atmosphere at 850 °C to 1150 °C. The cross-sectional width of the as-synthesized SnO₂ tubules is tunable from 50 nm to sub-micrometer depending on synthesis temperature. Each tubule is found to be a single crystal of rutile structure with four {110} peripheral surfaces and &&num;60;001&&num;62; growth direction. Although several growth patterns are observed for different samples, the basic growth mechanism is believed to be a self-catalyzed, direct vapor–solid (VS) process, where most new material is incorporated into the bottom parts of the existing SnO₂ tubules through surface diffusion. The tubes are readily aligned in the direction perpendicular to the substrate surface to form tube arrays. These well-aligned SnO₂ tubule arrays with tunable tube size could be the building blocks or templates for fabrication of functional nanodevices, especially those relevant to energy storage and conversion such as nanobatteries, nanofuel cells, and nanosensors. A gas sensor based on a single SnO₂ nanotubes demonstrated extremely high sensitivity to ethanol vapor.