380 research outputs found
Enhancing Ionic Conductivity of Bulk Single Crystal Yttria-Stabilized Zirconia by Tailoring Dopant Distribution
We present an ab-initio based kinetic Monte Carlo model for ionic
conductivity in single crystal yttria-stabilized zirconia. Ionic interactions
are taken into account by combining density functional theory calculations and
the cluster expansion method and are found to be essential in reproducing the
effective activation energy observed in experiments. The model predicts that
the effective energy barrier can be reduced by 0.15-0.25 eV by arranging the
dopant ions into a super-lattice.Comment: Submitted to Phys. Rev. Lett. on 8/3/2010 (in review
Incorporating finite temperature into materials by design for nonstoichiometric complex functional oxides
Enabled by dramatic advancements in computational capabilities and the tightening integration of theory and experiment, materials by design is rapidly becoming a leading paradigm in materials science. However, to most effectively accelerate the pace of materials design and discovery, first-principles calculations must move closer to experimental reality by taking into account the finite temperature effects corresponding to typical growth and/or operating conditions. Our work aims to develop capabilities to incorporate these finite temperature effects, which include atomic and magnetic disorder as well as the temperature dependence of the free energies of solids, into modern materials by design.
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Graphene-Based Nanostructures in Electrocatalytic Oxygen Reduction
Application of graphene-type materials in electrocatalysis is a topic of
growing scientific and technological interest. A tremendous amount of research
has been carried out in the field of oxygen electroreduction, particularly with
respect to potential applications in the fuel cell research also with use of
graphene-type catalytic components. This work addresses fundamental aspects and
potential applications of graphene structures in the oxygen reduction
electrocatalysis. Special attention will be paid to creation of catalytically
active sites by using non-metallic heteroatoms as dopants, formation of
hierarchical nanostructured electrocatalysts, their long-term stability, and
application as supports for dispersed metals (activating interactions)
An open-source toolbox for PEM fuel cell simulation
In this paper, an open-source toolbox that can be used to accurately predict the distribution of the major physical quantities that are transported within a proton exchange membrane (PEM) fuel cell is presented. The toolbox has been developed using the Open Source Field Operation and Manipulation (OpenFOAM) platform, which is an open-source computational fluid dynamics (CFD) code. The base case results for the distribution of velocity, pressure, chemical species, Nernst potential, current density, and temperature are as expected. The plotted polarization curve was compared to the results from a numerical model and experimental data taken from the literature. The conducted simulations have generated a significant amount of data and information about the transport processes that are involved in the operation of a PEM fuel cell. The key role played by the concentration constant in shaping the cell polarization curve has been explored. The development of the present toolbox is in line with the objectives outlined in the International Energy Agency (IEA, Paris, France) Advanced Fuel Cell Annex 37 that is devoted to developing open-source computational tools to facilitate fuel cell technologies. The work therefore serves as a basis for devising additional features that are not always feasible with a commercial cod
Solar thermochemical water splitting: Advances in materials and methods
Photoelectrochemical (PEC) water splitting, termed artificial photosynthesis, converts solar energy into hydrogen by harvesting a narrow spectrum of visible light using photovoltaics integrated with water-splitting electrocatalysts. While conceptually attractive, critical materials issues currently challenge technology development(1) and economic viability(2). Despite decades of active research, this approach has not been demonstrated at power levels above a few watts, or for more than a few days of operation.
High-temperature solar thermochemical (STCH) water splitting is an alternative approach that converts solar energy into hydrogen by using the deceptively simple metal oxide-based thermochemical cycle presented in figure 1. The STCH process requires very high temperatures, achieved by collecting and concentrating solar energy. Unlike PEC, two-step metal oxide water-splitting cycles have been demonstrated at the 100kW scale(3), and continuous operation at even higher power levels is nearing pre-commercial demonstration (HYDROSOL-3D). Nonetheless STCH, like PEC, faces critical materials issues that must be addressed for this technology to achieve commercial success.
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Size-dependent spinodal and miscibility gaps for intercalation in nano-particles
Using a recently-proposed mathematical model for intercalation dynamics in
phase-separating materials [Singh, Ceder, Bazant, Electrochimica Acta 53, 7599
(2008)], we show that the spinodal and miscibility gaps generally shrink as the
host particle size decreases to the nano-scale. Our work is motivated by recent
experiments on the high-rate Li-ion battery material LiFePO4; this serves as
the basis for our examples, but our analysis and conclusions apply to any
intercalation material. We describe two general mechanisms for the suppression
of phase separation in nano-particles: (i) a classical bulk effect, predicted
by the Cahn-Hilliard equation, in which the diffuse phase boundary becomes
confined by the particle geometry; and (ii) a novel surface effect, predicted
by chemical-potential-dependent reaction kinetics, in which
insertion/extraction reactions stabilize composition gradients near surfaces in
equilibrium with the local environment. Composition-dependent surface energy
and (especially) elastic strain can contribute to these effects but are not
required to predict decreased spinodal and miscibility gaps at the nano-scale
Frequency Dependent Dynamical Electromechanical Response of Mixed Ionic-Electronic Conductors
Frequency dependent dynamic electromechanical response of the mixed
ionic-electronic conductor film to a periodic electric bias is analyzed for
different electronic and ionic boundary conditions. Dynamic effects of mobile
ions concentration (stoichiometry contribution), charge state of acceptors
(donors), electron concentration (electron-phonon coupling via the deformation
potential) and flexoelectric effect contribution are discussed. A variety of
possible nonlinear dynamic electromechanical response of MIEC films including
quasi-elliptic curves, asymmetric hysteresis-like loops with pronounced memory
window and butterfly-like curves are calculated. The electromechanical response
of ionic semiconductor is predicted to be a powerful descriptor of local
valence states, band structure and electron-phonon correlations that can be
readily measured in the nanoscale volumes and in the presence of strong
electronic conductivity.Comment: 36 pages, 10 figures, accepted to J. Appl. Phy
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