Atomic structure observations and reaction dynamics simulations on triple phase boundaries in solid-oxide fuel cells

Accurate Force Fields for Atomistic Simulations of Oxides, Hydroxides, and Organic Hybrid Materials up to the Micrometer Scale

The simulation of metals, oxides, and hydroxides can accelerate the design of therapeutics, alloys, catalysts, cement-based materials, ceramics, bioinspired composites, and glasses. Here we introduce the INTERFACE force field (IFF) and surface models for α-Al2O3, α-Cr2O3, α-Fe2O3, NiO, CaO, MgO, β-Ca(OH)2, β-Mg(OH)2, and β-Ni(OH)2. The force field parameters are nonbonded, including atomic charges for Coulomb interactions, Lennard-Jones (LJ) potentials for van der Waals interactions with 12-6 and 9-6 options, and harmonic bond stretching for hydroxide ions. The models outperform DFT calculations and earlier atomistic models (Pedone, ReaxFF, UFF, CLAYFF) up to 2 orders of magnitude in reliability, compatibility, and interpretability due to a quantitative representation of chemical bonding consistent with other compounds across the periodic table and curated experimental data for validation. The IFF models exhibit average deviations of 0.2% in lattice parameters, <10% in surface energies (to the extent known), and 6% in bulk moduli relative to experiments. The parameters and models can be used with existing parameters for solvents, inorganic compounds, organic compounds, biomolecules, and polymers in IFF, CHARMM, CVFF, AMBER, OPLS-AA, PCFF, and COMPASS, to simulate bulk oxides, hydroxides, electrolyte interfaces, and multiphase, biological, and organic hybrid materials at length scales from atoms to micrometers. The nonbonded character of the models also enables the analysis of mixed oxides, glasses, and certain chemical reactions, and well-performing nonbonded models for silica phases, SiO2, are introduced. Automated model building is available in the CHARMM-GUI Nanomaterial Modeler. We illustrate applications of the models to predict the structure of mixed oxides, and energy barriers of ion migration, as well as binding energies of water and organic molecules in outstanding agreement with experimental data and calculations at the CCSD(T) level. Examples of model building for hydrated, pH-sensitive oxide surfaces to simulate solid-electrolyte interfaces are discussed.

Microstructure Evolution in a Solid Oxide Fuel Cell Stack Quantified with Interfacial Free Energy

Degradation of electrode microstructure is one of the key factors affecting long term performance of Solid Oxide Fuel Cell systems. Evolution of a multiphase system can be described quantitatively by the change in its interfacial energy. In this paper, we discuss free energy of a microstructure to showcase the anisotropy of its evolution during a long-term performance experiment involving an SOFC stack. Ginzburg Landau type functional is used to compute the free energy, using diffuse phase distributions based on Focused Ion Beam Scanning Electron Microscopy images of samples taken from nine different sites within the stack. It is shown that the rate of microstructure evolution differs depending on the position within the stack, similar to phase anisotropy. However, the computed spatial relation does not correlate with the observed distribution of temperature.

Parametric sensitivity analysis for a natural gas fueled high temperature tubular solid oxide fuel cell

For more than the last two decades, there has been research going on to develop advanced energy technologies involving minimum environmental pollution, to replace the conventional fossil energy systems. Solid oxide fuel cells (SOFCs) are one of the most promising, eco-friendly and efficient means for the generation of electricity to meet the future energy requirements. This research work focuses on the parametric sensitivity analysis for natural gas fueled high temperature tubular SOFC. Firstly, for the tubular SOFC, a one-dimensional radially symmetrical model has been developed and solved using the finite-difference method. Then, the effect of the variation of important operational and design parameters on its performance has been analyzed. The parameters typically include composition, inlet fuel temperature, pressure, length of SOFC tube and thicknesses of its components. The composition is expressed as steam to methane ratio and it has been observed that the voltage and power density developed by the SOFC diminishes as the ratio increases. Further, a change in the inlet fuel pressure of the tubular SOFC has no pronounced influence on the voltage and power density developed. On the other hand, with an increase in the inlet fuel temperature, a small improvement in these performance characteristics is exhibited. The axial length of the tubular SOFC does conspicuously influence its performance characteristics but solely at current densities greater than 4000A/m². An increase in the thickness of its components results in a reduction in its voltage and power density developed. The largest decline in these performance characteristics with the increase in thickness is observed for electrolyte followed by cathode and anode respectively.