Neal P. Sullivan

Polarization Characteristics and Chemistry in Reversible Tubular Solid-Oxide Cells Operating on Mixtures of H2, CO, H2O, and CO2

This paper reports the results of combined experimental and modeling studies of reversible solid-oxide cells. The tubular cells are fabricated using a Ni-YSZ (yttria-stabilized zirconia) fuel-electrode support, a dense YSZ electrolyte membrane, and a strontium-doped lanthanum manganate-YSZ composite air electrode. Experiments are designed to systematically vary gas-phase species partial pressures and operating temperatures. The fuels are mixtures of H 2 , CO, H 2 O, CO 2 , and Ar. Performance is measured under anodic (fuel cell) and cathodic (electrolysis) polarization. The models consider reactive porous-media transport within the composite electrodes, thermal chemistry on Ni and YSZ surfaces, and charge-transfer chemistry. All chemistry is modeled with elementary reversible reactions. Close coupling between experimental measurements and model-based interpretation provides a basis for establishing reaction pathways and rates. In addition to advancing fundamental understanding, the resulting detailed reaction mechanisms are valuable for incorporation into predictive models that can be used for design and optimization of fuel-cell and electrolysis systems.

Surface modification of polypropylene film using N2O-containing flames

Contact-angle measurements and X-ray photoelectron spectroscopy (XPS or ESCA) were used to characterize polypropylene (PP) films that were exposed to laminar premixed air: natural gas flames containing small quantities of nitrous oxide. During combustion, the nitrous oxide generates gas-phase nitrogen oxides that lead to the affixation of nitrogen-containing functional groups to the PP surfaces. Treatment of PP in nitrous oxide-containing flames also leads to an increase in surface oxidation and markedly improves wettability when compared with standard flame treatments. The chemical form of the nitrogen affixed to the PP surface is strongly dependent on the flame equivalence ratio. Fuel-lean flames tend to affix highly oxidized forms of nitrogen such as nitrate and nitro groups, while fuel-rich flames tend to affix less-oxidized nitrogen groups such as nitroso, oxime, amide, and amine. A computational model, SPIN, was used to elucidate the chemistry of the flame as it impinges upon the cooled PP surface. The SPIN modeling indicates that the principal reactive gas-phase species at or near the PP surface are O2, OH, H, NO, NO2, HNO, and N2O. A number of possible reactions between these species and the PP can account for the formation of the various nitrogen functional groups observed.

Preparation of dense mixed electron- and proton-conducting ceramic composite materials using solid-state reactive sintering: BaCe0.8Y0.1M0.1O3−δ–Ce0.8Y0.1M0.1O2−δ (M=Y, Yb, Er, Eu)

Mixed electronic and protonic conductor materials were prepared using BaCe0.8Y0.1M0.1O3−δ (BCYM) as the protonic conductive phase and Ce0.8Y0.1M0.1O2−δ (MYDC) as the electronic conductive phase (in reducing atmosphere), with M=Y, Yb, Er, Eu. Dense specimens of these ceramic/ceramic composite materials (cercers) were prepared by solid-state reactive sintering: all the precursors for BCYM and MYDC were mixed, pelletized, and fired without any pre-calcination step of the individual ceramic phases. The X-ray diffraction patterns revealed the presence of the two desired phases. The study of the lattice parameters showed that the Y and M co-dopants were fairly well distributed between the perovskite phase BCYM and the fluorite phase MYDC. This interesting discovery is of importance for the preparation of two-phase ceramic materials. In addition to the structural study, the samples were analyzed by scanning electron microscopy and were found to be free of any undesirable phases. The two ceramic phases could easily be distinguished using the back-scattered electron mode, with grains between 10 and 30 microns. Energy dispersive X-ray spectroscopy confirmed the distribution of the co-dopant between the two phases.

Biogas fuel reforming for solid oxide fuel cells

In this paper, strategies for biogas reforming and their ensuing effects on solid oxide fuel cell (SOFC) performance are explored. Synthesized biogas (65% CH4 + 35% CO2) fuel streams are reformed over a rhodium catalyst supported on a porous α-alumina foam. Reforming approaches include steam reforming and catalytic partial oxidation (CPOX) utilizing either air or pure oxygen as the oxidant. A computational model is developed and utilized to guide the specification of reforming conditions that maximize both CH4 and CO2 conversions. Model predictions are validated with experimental measurements over a wide range of biogas-reforming conditions. Higher reforming temperatures are shown to activate the biogas-borne CO2 to enable significant methane dry-reforming chemistry. Dry reforming minimizes the oxidant-addition needs for effective biogas conversion, potentially decreasing the thermal requirements for reactant heating and improving system efficiency. Such high-temperature reforming conditions are prevalent...

Highly durable, coking and sulfur tolerant, fuel-flexible protonic ceramic fuel cells

Protonic ceramic fuel cells, like their higher-temperature solid-oxide fuel cell counterparts, can directly use both hydrogen and hydrocarbon fuels to produce electricity at potentially more than 50 per cent efficiency1,2. Most previous direct-hydrocarbon fuel cell research has focused on solid-oxide fuel cells based on oxygen-ion-conducting electrolytes, but carbon deposition (coking) and sulfur poisoning typically occur when such fuel cells are directly operated on hydrocarbon- and/or sulfur-containing fuels, resulting in severe performance degradation over time3–6. Despite studies suggesting good performance and anti-coking resistance in hydrocarbon-fuelled protonic ceramic fuel cells2,7,8, there have been no systematic studies of long-term durability. Here we present results from long-term testing of protonic ceramic fuel cells using a total of 11 different fuels (hydrogen, methane, domestic natural gas (with and without hydrogen sulfide), propane, n-butane, i-butane, iso-octane, methanol, ethanol and ammonia) at temperatures between 500 and 600 degrees Celsius. Several cells have been tested for over 6,000 hours, and we demonstrate excellent performance and exceptional durability (less than 1.5 per cent degradation per 1,000 hours in most cases) across all fuels without any modifications in the cell composition or architecture. Large fluctuations in temperature are tolerated, and coking is not observed even after thousands of hours of continuous operation. Finally, sulfur, a notorious poison for both low-temperature and high-temperature fuel cells, does not seem to affect the performance of protonic ceramic fuel cells when supplied at levels consistent with commercial fuels. The fuel flexibility and long-term durability demonstrated by the protonic ceramic fuel cell devices highlight the promise of this technology and its potential for commercial application.Tests on a versatile protonic ceramic fuel cell resistant to carbon deposition and sulfur poisoning show that its durability and the wide range of fuels it can accept make it suitable for use in industry in the near future.

Solid Oxide Electrochemical Reactor Science

Solid-oxide electrochemical cells are an exciting new technology. Development of solid-oxide cells (SOCs) has advanced considerable in recent years and continues to progress rapidly. This thesis studies several aspects of SOCs and contributes useful information to their continued development. This LDRD involved a collaboration between Sandia and the Colorado School of Mines (CSM) ins solid-oxide electrochemical reactors targeted at solid oxide electrolyzer cells (SOEC), which are the reverse of solid-oxide fuel cells (SOFC). SOECs complement Sandias efforts in thermochemical production of alternative fuels. An SOEC technology would co-electrolyze carbon dioxide (CO{sub 2}) with steam at temperatures around 800 C to form synthesis gas (H{sub 2} and CO), which forms the building blocks for a petrochemical substitutes that can be used to power vehicles or in distributed energy platforms. The effort described here concentrates on research concerning catalytic chemistry, charge-transfer chemistry, and optimal cell-architecture. technical scope included computational modeling, materials development, and experimental evaluation. The project engaged the Colorado Fuel Cell Center at CSM through the support of a graduate student (Connor Moyer) at CSM and his advisors (Profs. Robert Kee and Neal Sullivan) in collaboration with Sandia.

The formation of ultrathin silicon oxide films using H2/N2O mixtures

This paper describes a process called N 2 O in situ steam generation, which uses a combustion-like reaction of lean hydrogen in N 2 O to form ultrathin (≈10 ) silicon oxide films on silicon. An important application is the formation of high-integrity gate dielectrics for integrated circuits. The atomic oxygen created through homogeneous reaction plays an important role in growing high-quality oxide films. This paper presents measured oxide-thickness profiles on 200 mm wafers under several processing conditions. Stirred-reactor and boundary-layer models are used to explain and interpret the data. An elementary chemical-reaction mechanism, which is drawn from the combustion literature, provides an excellent representation of this advanced materials process.

Exploring ISSG process space [Si oxidation]

This paper describes a computational-modeling effort that investigates silicon oxidation using In-Situ Steam Generation (ISSG). Using a fluid-mechanical boundary-layer model, we have simulated the chemically reacting flow in the Applied Materials Radiance RTP reactor. The model incorporates an elementary gas-phase chemical reaction mechanism that describes the essential free-radical chemistry that is responsible for ISSG. Comparing measurements of oxide thickness and uniformity with modeled flow fields over numerous process conditions for both 200 and 300 mm wafers, we observe a strong correlation between oxide physical characteristics and atomic-oxygen number density. Through this correlation, we find that ideal ISSG process conditions are those that result in a weak, diffuse reaction zone that spans the diameter of the heated wafer. We then expand the modeled process space to conditions outside of common ISSG practice. Varying hydrogen concentration, reactor pressure, reactant flow rate, and wafer temperature, we have extensively mapped the process space and identified process conditions that are robust to process variations.