Session L36: Focus Session: New Energy IV

Study of excitonic energy transport in thin-film J-aggregates

The concentration and transfer of light through materials is one of the main current scientific goals in the ongoing quest for a new clean energy source. Molecular structures with optimal exciton transfer properties find widespread applications, ranging from solar cells and photonic devices to photographic and lithographic systems. J-aggregates of organic dye molecules are a good example of such structures where a strong interaction between molecular electronic transitions results in a partial exciton delocalization and a large exciton diffusion length. In this presentation I will discuss theoretical aspects of exciton dynamics in two dimensional films of J-aggregates with an emphasis on macroscopic transport properties that could be used for device modeling. As specific illustrations I will use results of our recent studies of exciton dynamics in aggregates of cyanine dyes.   

Control of exciton delocalization pathways using ZnTPP functionalization

Zn(II)-Tetraphenylporphyrin (ZnTPP) derivatives are attractive candidates for photoinduced electron-transfer mediators in dye sensitized solar cells. We have investigated the influence on solar cells efficiency, of the energy alignment and of the molecular adsorption geometry at the ZnTPP/metal-oxides interface. In this work, using x-ray, UV and inverse photoemission spectroscopies in conjunction with density functional theory (DFT) calculations, we have determined the energy alignment of molecular levels with respect to the substrate band edges for several ZnTPP derivatives adsorbed on ZnO(11-20) and TiO$_{2}$(110) surfaces. The ZnTPP derivatives were functionalized with COOH anchoring groups, to allow upright or flat adsorption on the surfaces. While the energy alignment is found similar for all of these systems, large differences in devices efficiencies are observed. We have thus explored the adsorption geometry of the same ZnTPPs at the surface of ZnO and TiO$_{2}$ using UV-visible absorption and NEXAFS spectroscopies and scanning tunnel microscopy. It is found that for ZnTPPs, upright adsorption opens deleterious exciton delocalization pathways, due to dipole/dipole interactions competing with electron transfer to the substrate. Choosing the adsorption geometry is thus critical for the electronic pathway control.   

Coupled polaronic and ion transport in nanocrystalline metal oxide electrodes

We report new computational methods and fundamental understanding in the dynamics of coupled charge and ion transport in nanoscale metal oxides. The methods attack the multi-scale problem of simulating the collective diffusivities of ions and charge compensating e-/h+ carriers in single crystal particles, across particle-particle grain boundaries, and through networks of grains for select systems. Methods include embedded quantum mechanical clusters at the DFT and MP2 levels of theory for atomic-scale polaronic and ion transport kinetics, classical DFT-based free energy calculations for grain-scale conductivity in the framework of the Poisson-Nernst-Planck formalism, and phase field simulation of charged particle diffusivity for conductivity at the grain network scale. This combination of approaches is one of a kind in terms of its multi-scale range, scaling, and computational efficiency. We are presently focused on coupled electron and Li+ ion transport in polymorphs of TiO2, and also in mixed valence spinel oxides, for electrode conductivity optimization and improving energy storage materials performance for Li+ batteries.   

Electrochemical properties of Pt-Co-Zr thin films on high surface area NSTF supports

Nanostructured thin film supports (NSTF) are a promising fuel cell (FC) technology demonstrated by 3M [1]. We have examined the electrochemical performance of Pt-Co-Zr films deposited onto NSTF supports by dc-magnetron sputtering. In this presentation, we will present results of microstructural, composition, and electrochemical properties, for NSTF supported (Pt$_{3}$Co)$_{100-x}$Zr$_{x}$ thin films, with 10 $<$ x $<$ 40 (At. {\%}). Electrochemical measurements show that the films are electrochemically stable, and active for the oxygen-reduction-reaction (ORR), with ORR kinetic current densities at 0.9 V (vs. NHE), up to 57X greater than those of Pt(111) films measured in the same cell. The composition dependence of the ORR, and relevant physical properties will be discussed. \\[4pt][1] M. K. Debe, A. J. Steinbach, G. D. Vernstrom et al, J. Electrochemical Soc. 158, B910 (2011). \\[4pt] Acknowledgements: The research in this presentation was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. We acknowledge support from the Department of Energy (DE-PS36-08GO98101). We thank our collaborators Dr. M. Debe, Dr. R. Atanasoski, and G. D. Vernstrom of 3M Corp.   

Enhanced Surfactant Adsorption on Activated Carbon through Manipulation of Surface Oxygen Groups

Passive energy storage is a necessary component for balancing the lifecycle budget with new forms of green energy. The work presented describes how surface oxygen groups (SOG) on granulated activated carbon have been manipulated using Nitric Acid in a controlled, stepwise fashion. The structure and surface functionality of the activated carbon samples were characterized using DRIFTS, Raman Spectroscopy and Porosimetry. Total surface area was found to increase proportionally with the removal of heteroatom material, exposing previously insulated active sites responsible for SOG attachment. Broad oxide peaks were deconvoluted and analyzed, allowing for absolute identification of evolving functionality at each oxidation stage. SOGs were maximized on the third oxidation cycle with the presence of conjugated aromatic, phenol, lactone, and carboxylic acid groups. FSN Zonyl nonionic was applied to all oxidized samples at various concentrations. Total adsorbed surfactant was quantified for each concentration / oxidation scheme using attenuated total reflection. The relative quantity and polarity of chemisorbed surfactant were qualitatively assessed for each equilibrium concentration.   

Coarse-grained potential models for structural prediction of carbon dioxide in confined environments

Geological storage of carbon dioxide is a promising option to reduce the level of carbon dioxide in the atmosphere and mitigate its effect on climate change. In geological storage, high-pressure carbon dioxide is injected into the underground porous rock formations, where it gets trapped inside the tiny nanometer size pores of the rocks. Thus, a good understanding of the microstructure of carbon dioxide inside nanoscale confinements is of great practical importance in developing an efficient technology for carbon dioxide storage. In this work, we discuss the systematic development of coarse-grained single-site (CGSS) potential models to study the structure of carbon dioxide in confined environments. These single-site potentials allow computationally efficient simulations, which are several orders of magnitude faster than all-atom MD (AAMD) simulations. The potential models are used in our earlier proposed multi-scale quasi-continuum theory, called EQT, and in coarse-grained MD (CGMD) to predict the equilibrium structure of carbon dioxide confined inside graphite slit pores. The results obtained from both EQT and CGMD are found be in good agreement with those obtained from computationally expensive AAMD simulations.   

Multiscale Simulation of Electrochemical{\_} Phenomena: Fuel Cells and Batteries

Results will be presented from multiscale simulations of two important systems from renewable energy technology, fuel cell proton membranes and electrochemical cells. In the first case, the solvation and transport of hydrated protons in proton exchange membranes (PEMs) such as Nafion$^{TM}$ will be described using a novel multi-state reactive molecular dynamics (MD) approach. The multi-state MD methodology allows for the treatment of explicit (Grotthuss) proton shuttling and charge defect delocalization which, in turn, can strongly influence the properties of the hydrated protons in various aqueous and complex environments. The role of PEM hydration level and morphology on these properties will be further described. A new multiscale computational methodology for describing the mesoscopic features of the proton transport will also be described, which can be coupled to the results from the molecular-scale simulations. On the second topic, a computationally efficient method will be presented for the treatment of electrostatic interactions between polarizable metallic electrodes held at a constant potential and separated by an electrolyte. The method combines a fluctuating uniform electrode charge with explicit image charges to account for the polarization of the electrode by the electrolyte, and a constant uniform charge added to the fluctuating uniform electrode charge to account for the constant potential condition. The method is used to calculate electron transport rates using electron transfer theory; these rates are incorporated in a multiscale approach to model oxidation/reduction reactions in an electrochemical cell efficiently.   

Collective Behavior of Water on Platinum

We present the results of molecular dynamics simulations of a interface between water and a platinum electrode. Using importance sampling techniques we probe a variety of collective phenomenon that emerge at the interface. We consider platinum electrodes with two different geometries and discuss how different behaviors result from a competition between geometrical frustration and favorable local interactions.   

Superstructured tungsten oxide photoanodes

Tungsten oxide is a robust and stable semiconductor for photoanodic applications in aqueous solutions. Typical deposition techniques like electroplating or sputtering produce granular films which increase interfacial recombination of minority carriers. This has a deleterious effect on the photovoltaic performance of these materials. Using a variety of templating methods, we explore multiscale structuring strategies for increasing the surface area of the photoanode while maintaining significant light absorption. We describe photoelectrochemical and reflectivity measurements on structured and templated tungsten oxide photoanodes and consider how these results guide future photosynthetic electrochemical device design.