(6dt) First Principles Studies of Energy Materials

The development of novel energy materials depends on the ability to exploit their microscopic chemical and physical properties. My research focuses on attaining a fundamental understanding of the electronic, geometric, and magnetic characteristics of heterogeneous catalysts and battery cathode materials that govern their efficiency and effectiveness. In this poster I will discuss my ongoing research efforts on heterogeneous catalysts and Li metal oxide battery materials.

Oxide Supported 1B Metal Nanoclusters

While Au is known to be chemically inert in its bulk form, nanometer size Au particles dispersed on TiO₂ have been found to exhibit high activities for a variety of catalytic oxidation processes at or below room temperature [1]. Au atoms weakly interact with the TiO₂ surface and become unstable toward sintering in response to changes in the gaseous environment even at moderate temperatures [2-5]. Oxygen adspecies can greatly alter TiO₂ surface properties and Au-TiO₂ interfacial interactions, which may in turn directly influence the nucleation, growth, and sintering of Au particles [6-8].  In this poster, I will present the results of density functional theory calculations on the interactions between oxygen species and small Au particles on TiO₂ (110), with a focus on i) the dynamics of oxygen species on Au/TiO2 and ii) the effect of oxygen species on Au particle nucleation, growth, and sintering.

The Effect of Contaminants on Pt Alloy Catalysts

A critical issue in proton exchange membrane fuel cells (PEMFCs) is the slow rate of oxygen reduction reaction (ORR) on the Pt cathode electrocatalyst. Recent results have shown that alloyed systems such as Pt₃Ni have a higher activity than Pt metals towards the ORR which improves the overall efficiency of the fuel cell [9]. We have analyzed the electronic and geometric properties of Pt, Ni and Pt₃Ni (111) surfaces using first principles calculations. In addition, we have investigated the effect of sulfur contaminants on the chemical reactivity of these systems. Preliminary calculations have shown that S atoms can reduce the adsorption energy and even block adsorption sites between oxygen and hydrogen adsorbates and the (111) metal surfaces. Current results indicate that a single S atom can affect 13 neighboring Pt surface adsorption sites which is in good agreement with experimental data [10]. This effect is due to electrostatic interactions as well as a change in the electronic properties of these surfaces upon S adsorption. I will discuss (i) electronic and geometric properties of the metal surfaces, (ii) the adsorption geometry and bonding mechanism of oxygen and hydrogen gaseous species and (iii) the affect of S contaminants on oxygen and hydrogen surface kinetics.

Improving Phase Stability of Li Metal Oxide Battery Materials

Many properties crucial to good battery electrode performance can be accurately predicted using density functional theory (DFT) methods [11]. New materials can therefore be analyzed computationally and those with potentially superior qualities can be identified prior to synthesis and testing in the laboratory.  This can drastically reduce the amount of time and money spent on battery development, while simultaneously providing valuable microscopic-level insight into the physics and chemistry of electrode processes.

Stability against distortion in battery electrodes indicates a longer lifetime and a lower waste of energy during charging and discharging cycles. Preliminary experiments indicate that the addition of Al to LiCoO₂ is advantageous in terms of stability [12].  Here we determine computationally the optimal concentration of Al and Mg dopants by calculating the intercalation voltage of LiCoO₂. Additionally, we analyze the electronic and geometric changes that these dopants induce, and correlate this to the intercalation voltage. Other results on various Li 3d transition metal oxide materials as well as dopants will be discussed.

References

[1]        M. Haruta, Catal. Today, Size- and support-dependency in the catalysis of gold, 36, (1997), 153-166.

[2]        D. Pillay, Y. Wang, et al., Korean J. Chem. Eng., A comparative theoretical study of Au, Ag and Cu adsorption on TiO₂(110) rutile surfaces, 21, (2004), 537-547.

[3]        D. Pillay, G.S. Hwang, J. Molec. Struct., Structure of Au, Ag, and Cu Clusters on the TiO₂ (110) surface, (2005).

[4]        D. Pillay, G.S. Hwang, Phys. Rev. B, Growth and structure of small gold particles on rutile TiO₂(110), 72, (2005).

[5]        D. Pillay, Y. Wang, et al., Catal. Today, Nucleation and growth of 1B metal clusters on rutile TiO₂(110): Atomic level understanding from first principles studies, 105, (2005), 78-84.

[6]        D. Pillay, Y. Wang, et al., J. Am. Chem. Soc., Prediction of tetraoxygen formation on rutile TiO₂(110), 128, (2006), 14000-14001.

[7]        D. Pillay, G.S. Hwang, J. Chem. Phys., O₂-coverage-dependent CO oxidation on reduced TiO₂(110): A first principles study, 125, (2006).

[8]        Y. Wang, D. Pillay, et al., Phys. Rev. B, Dynamics of oxygen species on reduced TiO₂(110) rutile, 70, (2004), 193410.

[9]        V.R. Stamenkovic, B. Fowler, et al., Science, Improved oxygen reduction activity on Pt₃Ni(111) via increased surface site availability, 315, (2007), 493-497.

[10]      D. Pillay, M.D. Johannes, J. Phys. Chem. B, Sulfur Poisoning of Pt(111) and Pt₃Ni(111) Surfaces: A First Principles Study, (In Preparation).

[11]      G. Ceder, M.K. Aydinol, et al., Comp. Mat. Sci., Application of first-principles calculations to the design of rechargeable Li-batteries, 8, (1997), 161-169.

[12]      G. Ceder, Y.M. Chiang, et al., Nature, Identification of cathode materials for lithium batteries guided by first-principles calculations, 392, (1998), 694-696.