Session T20: Focus Session: Computational Design of New Materials -- Energy

First principles studies of stability and reactivity of electro-catalysts for low-temperature fuel cells

Low-temperature fuel cells (FC) are promising clean source of electric power. However, Pt-based catalysts make FC unacceptably expensive. Low rate of the oxygen reduction reaction (ORR) on the Pt cathode, and poisoning of anode by CO also reduce efficiency of FC. New materials such as Ru nanoparticles with the Pt (Pt/Ru) and Se (Se/Ru) sub-monolayer coverage demonstrate improved electro-catalytic activity (reactivity) towards anodic hydrogen oxidation and the cathode ORR, respectively. Pd-Co, CrN structures are also considered catalysts for ORR. Although reactivity is a key characteristic of the catalysts, their stability is also very important issue. The goal of this talk is to show the efficiency of the first principles computational approach to the problems of stability and reactivity of the electro-catalysts for FC. The stability problem is illustrated with our calculation results obtained for the Pt/Ru and Se/Ru nano-structures. By analyzing energetic and geometry of Pt and Se adsorption on Ru substrates, we find that Se and Pt atoms behave absolutely different on Ru: the Pt tend to join into large 1D islands, while Se atoms prefer to stay apart from each other on the substrate. The technique recently developed by Norskov group [1] is used in this work to evaluate the onset cathode potential for ORR on the Pd-Co, CrN and Se/Ru surfaces. We find a good agreement of the calculation results with available experiment. The power of computational methods also illustrated with the results explaining high tolerance of Pt/Ru nano-structures towards CO poisoning of the FC anode.\\[4pt] [1] J. K. N{\o}rskov, et al., \textit{J. Phys. Chem. B} 108, 17886 (2004).   

Modifying carbon support for improving the stability of metal catalyst in fuel cell applications

In fuel cell technology, the development of efficient catalysts and method for catalyst deposition is crucial. Indeed, the efficiency of the catalyst will control the kinetics of the reaction by decreasing the activation energy. A catalyst widely used in fuel cell applications is platinum (Pt), which is responsible for the cost of the fuel cell system. The high surface area and enhanced electrocatalytic properties of carbon nanotubes if used as a template for Pt deposition would allow the reduction of the amount of Pt used in the fuel cells while improving its catalytic efficiency. In this paper we will present the influence of the support on the catalytic efficiency of Pt using density functional theory method. The presentation will describe the effect of dopants on the flat graphene sheet and single wall carbon nanotube on the Pt-support interactions and on the catalytic activity of Pt at the anode and at the cathode of a fuel cell.   

Identification of Descriptors for Oxygen Reduction Reaction on Solid Oxide Fuel Cell Cathodes

Perovskites are the major class of materials used for modern solid oxide fuel cell (SOFC) cathodes and have the ability to catalyze the oxygen reduction reaction (ORR) on their surfaces. However, difficulties in performing in-situ characterization of well-controlled samples means that the rate limiting steps and structure-property relationships underlying ORR on these materials are not understood. In particular, to date it has not been possible to find a simple set of descriptors that can be correlated to the ORR activity. A descriptor based approach has been very valuable in understanding many reactions, including the ORR, on metal catalysts (e.g. d-band center descriptor). In this talk we use an ab initio based approach to identify a descriptor for the ORR in perovskite SOFC cathodes. Energetics of key steps in the SOFC ORR are calculated for LaBO3 (B= Mn, Fe, Co, and Ni) systems and correlated with oxygen surface binding, oxygen surface vacancy formation, and oxygen band center. Reasonably good linear relationships suggest that these quantities could be effective descriptors for the ORR on SOFC perovskite cathodes.   

Exploration of the electronic structure properties of new tetrahedral oxides: zincites

The long standing chemical rules of isovalent species co-substitution allow, starting from adamantine semiconductors such as ZnS, to derive compounds such as the chalcopyrite-type CuGaS$_{2}$ in the same structural aristotype. These co-substitution rules when applied to tetrahedral oxides like ZnO, point to a field of compounds with underlying tetrahedral coordination, that is largely unsurveyed yet very promising for identifying new materials suited as transparent conducting oxides and perhaps photovoltaic adsorbers. We focus here on one such compound, derived from ZnO by a 3:1 co-substitution of Zn with Ag and V, Ag$_{3}$VO$_{4}$, as a paradigmatic component of this new compound field. The FLAPW method within the screened-exchange LDA scheme is employed to assess the band gap values, otherwise underestimated in simple LDA. The two main polymorphs of Ag$_{3}$VO$_{4}$ are considered here, i.e., $\alpha-$ and $\beta-$Ag$_{3}$VO$_{4}$, both {\em daughter} structures of the {\em parent} adamantine lattice. Both forms show Ag and V d-like valence and conduction band edges, respectively, with a band gap that depends on the coordination of the Ag site and is close to the visible range. Our investigation suggests Ag$_{3}$VO$_{4}$ to be a candidate co-substitution dopant of ZnO.   

First Principles Design of Functional Materials for Energy Applications

Materials design using first-principles techniques is one the ultimate goals in computational materials science. Due to the recent advancement in first-principles electronic structure theory and computer power, it is now possible to perform knowledge-based computational design of materials with unique optical, electrical, or magnetic properties that are tuned to specific energy applications. This vital tool, therefore, has the great potential to accelerate scientific discovery of energy materials. In this talk, selective recent works from my group will be discussed to illustrate how computational methods can be used to design functional materials. Some of the examples include (1) design PV absorber materials through cation atomic mutation; (2) design bipolar dopable transparent conducting oxides; (3) design nitride alloys for LED to fill the green gap; and (4) design oxides for hydrogen production through PEC water splitting.   

Strategies for optimal design for electrostatic energy storage in quantum multiwell heterostructures

The physical principles are studied for the optimal design of a quantum multiwell heterostructure working as an electrostatic energy storage device. We performed the search for an optimal multiwell trapping potential for electrons that results in the maximum static polarizability of the system. The response of the heterostructure is modeled quantum mechanically using nonlocal linear response theory. Three main design strategies are identified, which lead to the maximization of the stored energy. We found that the efficiency of each strategy crucially depends on the temperature and the broadening of electron levels. The energy density for optimized heterostructures can exceed the nonoptimized value by a factor more than 400. These findings provide a basis for the development of new nanoscale capacitors with high energy density storage capabilities.   

Ab-Initio Physics of Electrochemistry

We present a Joint Density Functional Theory (JDFT)\footnote{S. A. Petrosyan, A. A. Rigos, and T. A. Arias, J. Phys. Chem. B, 109, 15436-15444 (2005).}$^,$\footnote{J. Lischner and T. A. Arias, Phys. Rev. Lett. 101, 216401 (2008).} capturing the key electrostatic interactions between electronic systems and a fluid environment. This novel theory is relevant to the study of electrochemical systems and includes the dielectric properties of the fluid and charge screening due to the presence of ions in solution. We also demonstrate how DFT calculations can address the fundamental physical issues underlying electrochemistry, including the definition of a consistent reference potential, the treatment of charged surfaces under periodic boundary conditions, and the study of the solid-electrolyte interface as a function of the applied potential. Results for interfacial capacitances and potentials of zero charge calculated using these techniques will be compared to experimental values. Our theory allows simulation of a variety of materials, such as intermetallics and complex oxides, in contact with an ionic liquid environment. This method has a wide range of potential applications including catalysis in fuel cells, batteries, and photoelectrochemical cells.   

Density Functional Theory (DFT) study of Li adsorption on the CNT-C60 hybrid system

With a rapidly increasing demand of better electrochemical materials for high energy storage density, various types of Li intercalation materials have been exploited to promote Li adsorption. In this study, we investigate a new type of hybrid system consisting of carbon nanotubes (CNTs) and fullerenes (C60) using the first-principles computational methods such as quantum mechanics and molecular dynamics simulation, aiming at improving electrochemical characteristics such as adsorption capabilities and charge transfer. This hybrid system makes use of C60s as the electron acceptor from Li in the presence of CNT that act a role as a charge transport channel to electrode. We investigate adsorption energy as well as electronic properties such as band structure, density of states (DOS) and charge distribution through the density functional theory (DFT). The performance of the system during charging and discharging process will be discussed. From the calculation, we find that the adsorption energy of Li on the hybrid system is increased comparing with that on pristine CNT. We believe this complex not only improve Li adhesion but also utilize CNT as an electrode for better electron transport.   

Giant Magnetostriction of Fe-based alloys with an addition of 5d (Ir and Pt) element

Extraordinary large magnetostriction recently found in Fe-based alloys such as Fe1-xGax (Galfenol), continues to attract extensive research interest because of their potential for broad application. To provide clear understanding of the mechanism and also to make rational predictions, we performed full potential linearized argument plane-wave calculations for ternary alloys with the addition of 5d elements (Ir and Pt). In several hypothetical structures, these alloys may have giant tetragonal magnetostrictive coefficient because of the combination of strong magnetization of 3d atoms and the large spin-orbit coupling of 5d atoms. For example, we found that $\lambda $001(Fe75Pt6.25Ge18.75) is as large as -3553ppm. These predictions indicate that ternary alloys can be attractive smart materials and experimental verifications are underway. We will discuss the preferential distribution pattern of 5d and metalloid elements, spin density features, and inter-atomic hybridization in these alloys so as to provide insights for further experimental explorations.   

Calculating Magnetoelectric Susceptibilities from First Principles with an Applied Magnetic Field

First-principles methods are emerging as a valuable tool in the search for materials with strong magnetoelectric couplings. Here, we present a convenient route to computing magnetoelectric responses based on the introduction of a Zeeman response to an applied magnetic field. Prior successful approaches for computing the magnetoelectric response were largely based on the non-self-consistent application of an electric field to determine the field-induced distortion of the lattice from the effective charges of polar lattice modes. Subsequently, the electric-field-induced magnetization was computed, leading to the lattice-mediated part of the magnetoelectric response. In contrast, our approach in principle contains both the electric and lattice-mediated parts of the magnetoelectric response, and provides a more efficient method than the non-self-consistent application of an electric field. We compare both approaches using the example of Cr$_2$O$_3$, the prototypical magnetoelectric material.   

A highly parallel KKR method and its applications in the study of magnetic defects of Fe-based alloys

Korringa-Kohn-Rostoker (KKR) method is a multiple scattering theory based all-electron ab-initio approach to the electronic structure calculations for solids. Its capability for calculating the Green function provides unique opportunities for us to study disordered alloys, defects, electronic transport, etc., from the first principles. In this presentation, we show the latest progress we made in parallelizing the KKR method. By exploring the computational parallization of the energy contour integration, k-space integration, and the scattering matrix inverse while keeping the scattering matrix data highly distributed, we are able to scale the KKR method to the thousands of processes (cores or processors). We also show a new way of performing parallel 3-D FFT that is used in our code for solving the electrostatic potential. Unlike conventional approaches to the parallel 3-D FFT, our method does not require global data transpose operations, which involve all-to-all communications between processes. We demonstrate the application of the KKR method for large scale applications in the study of magnetic defects in Fe-based alloys.