Engineering topics

Rashkeev, Sergey N

Publications and source records attributed to Rashkeev, Sergey N.

7 records

DOE OSTI2012

Atomic-Scale Mechanisms of Oxygen Electrode Delamination in Solid Oxide Electrolyzer Cells

Materials used for different components (electrodes, electrolyte, steel interconnects, etc.) of solid oxide electrolyzer cell (SOEC) devices for hydrogen production have to function in aggressive, corrosive environments and in the presence of electric fields. This results in a number of degradation processes at interfaces between components. In this study, we used a combination of first-principles, density-functional-theory (DFT) calculations and thermodynamic modeling to elucidate the main processes that contribute into the oxygen delamination in typical SOEC device consisting of yttria-stabilized zirconia (YSZ) electrolyte and Sr-doped LaMnO3 (LSM) oxygen electrode. We found that high temperature inter-diffusion of different atoms across the LSM/YSZ interface significantly affects structural stability of the materials and their interface. In particular, we found that La and Sr substitutional defects positioned in ZrO2 oxide and near LSM/YSZ interface significantly change oxygen transport which may develop pressure buildup in the interfacial region and eventually develop delamination process. Simple models for estimating these effects are proposed, and different possibilities for inhibiting and/or mitigating undesirable delamination processes are discussed.

08 HYDROGEN↗

DOE OSTI2011

Interaction of gas molecules with crystalline polymer separation membranes: Atomic-scale modeling and first-principles calculations

Carbon dioxide (CO2)-induced plasticization can significantly decrease the gas separation performance of membranes in high-temperature or high pressure conditions, such as industrial methane (CH4) separations. In this paper, we investigated the crystalline phase of three polymers (polybenzimidazole (PBI), Bis(isobutylcarboxy)polybenzimidazole (PBI-Butyl), and KaptonTM) and interactions between gas molecules (CO2 and N2) and these polymers. A novel, molecular dynamics (MD) based, computational technique was employed to find unknown crystalline structures of these polymer materials. The interaction of CO2 and N2 gases with these crystals was studied by first-principles calculations and by classical MD simulations. The results showed that the packing structure and the interlayer coupling in polymer crystals determine the permeability and diffusivity of gas molecules. This methodology also allows prediction of plastic swelling in these materials caused by gas molecules absorbed in the polymer matrix.

03 NATURAL GAS↗

DOE OSTI2011

Control of Oxygen Delamination in Solid Oxide Electrolyzer Cells via Modifying Operational Regime

Possible modifications of operational regimes for solid oxide fuel cell (SOEC) devices for hydrogen production are discussed. It is shown that applying alternating current (AC) voltage pulses at a certain frequency range to SOECs could reduce oxygen delamination degradation in these devices and significantly increase their lifetime. This operational scheme provides wide possibilities to increase longevity of SOEC devices required for their use in commercial hydrogen production processes, without any significant modification of used materials and/or cell design. Developed simulation method possesses a broad generality and be employed in a number of other industrial processes.

08 HYDROGEN↗

DOE OSTI2011

Structural stability and catalytic activity of lanthanum-based perovskites

Perovskite-type oxide materials with a general formula La(1-x)A(x)Fe(1-y)Co(y)O(3-delta), where A is an alkaline earth metal Sr or Ba have been studied as cathode materials for catalytic reduction of oxygen in solid oxide fuel cells (SOFC), as well as combustion catalysts. In this study, we use a combination of temperature programmed reduction measurements, X-ray diffraction, carbon black catalytic oxidation measurements, and first-principles, density-functional-theory (DFT) calculations to elucidate the main processes that contribute into the structural stability and catalytic activity for soot oxidation of these materials. In particular, we investigate the dynamics of the structure reconstruction with oxygen loss during the regulated increase of the temperature. The calculations are in good qualitative agreement with catalytic experiments and allow identify special combinations of the perovskite chemical composition and local surface structures for which one could expect the highest catalytic activity for the soot oxidation process.

36 MATERIALS SCIENCE↗

DOE OSTI2010

Helium Behavior in Oxide Nuclear Fuels: First Principles Modeling

UO2 and (U, Pu)O2 solid solutions (the so-called MOX) nowadays are used as commercial nuclear fuels in many countries. One of the safety issues during the storage of these fuels is related to their self-irradiation that produces and accumulates point defects and helium therein. We present density functional theory (DFT) calculations for UO2, PuO2 and MOX containing He atoms in octahedral interstitial positions. In particular, we calculated basic MOX properties and He incorporation energies as functions of Pu concentration within the spin-polarized, generalized gradient approximation (GGA) DFT calculations. We also included the on-site electron correlation corrections using the Hubbard model (in the framework of the so-called DFT + U approach). We found that PuO2 remains semiconducting with He in the octahedral position while UO2 requires a specific lattice distortion. Both materials reveal a positive energy for He incorporation, which, therefore, is an exothermic process. The He incorporation energy increases with the Pu concentration in the MOX fuel.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2010

Modeling Degradation in Solid Oxide Electrolysis Cells

Idaho National Laboratory has an ongoing project to generate hydrogen from steam using solid oxide electrolysis cells (SOECs). To accomplish this, technical and degradation issues associated with the SOECs will need to be addressed. This report covers various approaches being pursued to model degradation issues in SOECs. An electrochemical model for degradation of SOECs is presented. The model is based on concepts in local thermodynamic equilibrium in systems otherwise in global thermodynamic no equilibrium. It is shown that electronic conduction through the electrolyte, however small, must be taken into account for determining local oxygen chemical potential, , within the electrolyte. The within the electrolyte may lie out of bounds in relation to values at the electrodes in the electrolyzer mode. Under certain conditions, high pressures can develop in the electrolyte just near the oxygen electrode/electrolyte interface, leading to oxygen electrode delamination. These predictions are in accordance with the reported literature on the subject. Development of high pressures may be avoided by introducing some electronic conduction in the electrolyte. By combining equilibrium thermodynamics, no equilibrium (diffusion) modeling, and first-principles, atomic scale calculations were performed to understand the degradation mechanisms and provide practical recommendations on how to inhibit and/or completely mitigate them.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2010

Beryllium Adsorption at Transition Aluminas: Implications for Environmental Science and Oxidation of Aluminum Alloys

It is demonstrated that?gamma- and?eta- aluminas (transition Al2O3 polytypes with defect spinel structure) can effectively capture beryllium atoms. Although the bulk crystal structures of these two oxides are characterized only by slight differences in cation vacancy distributions, the interaction of Be with the two polytypes are different. For gamma- Al2O3, the Be adsorption energy is high (~ 5 eV per atom), and all Be atoms are captured and trapped at the surface - all attempts to move Be in the subsurface region result in its expulsion back to the surface. On the other hand, for ?eta- alumina Be atoms can be captured both at the surface and in octahedrally-coordinated subsurface cation vacancies. This result implies that both alumina oxides could be successfully used for Be capture out of wastewater streams related to industrial processes of aluminum and alumina production. Also, the surface adsorption mechanism of Be at?gamma- Al2O3 explains why very small additions of Be (of the order of several ppm) to Al-Mg and Al-Mg-Si casting and wrought alloys prevent run-away oxidation of these materials in molten state, as well as ingot cracking. We also discuss possibilities to use other additives (e.g., Ca and Sr) yielding the same protective effect for aluminum alloys but which are less toxic than beryllium.

36 MATERIALS SCIENCE↗