Session Q38: Focus Session: Nanomaterials for Energy Applications II

Nanostructured Multimetallic Catalysts in Fuel Cells

There are two major driving forces for the global interests in research and development of fuel cells: the reality that fossil fuels are running out and the increasing environmental concern over pollution from using fossil fuels. Fuel cells utilizing hydrogen as fuels represent an important form of energy because hydrogen is a highly-efficient fuel and it is environmentally clean. Fuel cells such as proton exchange membrane fuel cell and direct methanol fuel cell are attractive because of their high conversion efficiency, low pollution, lightweight, and high power density. However, one of the important challenges for fuel cell commercialization is the preparation of active, robust and low-cost catalyst, which is key component in fuel cells counting for $\sim $30{\%} of the cost in manufacturing fuel cells. The durability of the catalysts can also be compromised by sintering and dissolution, especially at high electrode potentials or under load-cycling. We have been developing nanotechnological approaches and investigating nanostructured materials to address some of the fundamental issues in terms of catalyst activity, stability and cost. This presentation discusses recent findings of our investigations of the synthesis and processing for nanostructured catalysts with controlled size, composition, and surface properties by highlighting a few examples of bimetallic/trimetallic nanoparticles and supported catalysts. The results from the characterization of the nanoparticles and catalysts using an array of techniques and computational modeling will be discussed. The synergistic properties of the nanostructured materials in fuel cell reactions, including electrocatalytic methanol oxidation reaction and oxygen reduction reaction, will also be discussed, along with current challenges and opportunities.   

Reticular chemistry for clean energy

Linking molecular building blocks by strong bonds to make networks (Reticular Chemistry) has yielded a number of new classes of materials such as metal-organic frameworks, zeolitic imidazolate frameworks and covalent organic frameworks. These are new classes of porous materials in which inorganic 'joints' are linked by organic 'struts' to give extended structures with surface areas greater than 5000 m2/g. Their ultra-high surface area is useful in storing hydrogen and natural gas, and for capturing carbon dioxide. Recently we have shown that MOFs can be quite effective as air purification and capture of harmful gases. This presentation will highlight the milestones and future prospects of this new field   

Chemical tools for creating energy-relevant nanomaterials

An important pre-requisite for using nanoscale materials in energy-related applications is the ability to make them on-demand and to rigid and pre-determined standards. For example, creating nanoscale solids with controllable composition, crystal structure, size, morphology, and surface chemistry is necessary for optimizing and fine-tuning their properties, as well as spatially organizing them and interfacing them with other components in a device. This talk will summarize our efforts to controllably and rationally synthesize shape-controlled nanocrystals of complex multi-element metallic and semiconducting materials. Collectively, these results provide reliable and predictable guidelines for designing and synthesizing complex nanomaterials of solids that are typically viewed as challenging to make. The focus will be on applying these ideas to energy-relevant nanomaterials, including nanostructured superconductors with high critical fields, metal hydrides for hydrogen storage applications, nanoparticle catalysts relevant to fuel cells, and metal-based compounds for thermoelectric, battery, and photovoltaic applications.   

Structural and electrochemical properties of V$_{2}$O$_{5}$ and Ag$_{x}$V$_{2}$O$_{5}$ nanowries prepared by template assisted method

Vanadium pentaoxide and silver vanadium oxides are promising cathode materials for lithium ion battery as it allows easy intercalation/deintercalation of Li+ due its open layered structure. During Li+ intercalation energy is stored as chemical potential and during deintercalation the energy is released as electricity. Because of the large surface area nanostructured materials have enhanced energy storage capacity. We have prepared V$_{2}$O$_{5}$ and Ag$_{x}$V$_{2}$O$_{5}$ (x= 0.1, 0.5) nanowires by template assisted method using radiation track etched hydrophilic PC membrane. The nanowires were grown on ITO coated glass substrates for optical analysis and on stainless steel substrate for XRD, SEM, Raman and electrochemical measurements. The effects of Ag doping on the electrochemical properties of V$_{2}$O$_{5}$ nanowires were investigated using a three electrode cell with nanowries as working electrode and Li as counter and reference electrode and lithium perchlorate in propylene carbonate as the electrolyte. The electrochemical characteristics of V$_{2}$O$_{5}$ and Ag$_{x}$V$_{2}$O$_{5}$ nanowries such as lithium intercalation capacity, cyclic stability and diffusion coefficient will be presented.   

Stable Room Temperature Hydrogen Storage in Titainum-Doped Silica.

The optimum conditions for viable room temperature hydrogen storage require materials that possess isoteric heats of adsorption in between that of standard physisorbers and chemisorbers, typically in the $\sim $20-30~kJ/mol regime. It has been theoretically predicted that transition metal atoms incorporated onto high surface area materials could enable significant room temperature storage; herein we demonstrate a possible experimental proof of these predictions. Titanium(III) complexes are grafted onto porous silica hosts, then activated to generate sites for dissociative adsorption of hydrogen gas. Using a combination of sorption measurements and inelastic neutron scattering, we show that the activated titanium provides strong hydrogen binding sites at room temperature and that adsorbed hydrogen is stable for long periods of time at ambient conditions. Further, the hydrogen can be desorbed under mild processing conditions. Neutron vibrational spectra agree well with theoretically predicted vibrational modes of the Ti---H complex. These results represent an important step towards reversible room temperature hydrogen storage.   

Universal Behavior of Core-Shell Preferences in Transition-Metal Nanoparticles

Transition-metal, core-shell nanoparticles are becoming ubiquitous from (electro-) catalysis to biomedical applications, due to control of size, performance, biocompatibility, and cost. We investigate 66 binary core-shell nanoparticle systems (groups 8 to 11 in the periodic table) using density functional theory (DFT) and systematically explore their segregation energies to determine core-shell preferences. We find that core-shell preferences are described by two simple factors: (1) cohesive energy (related to vapor pressure) and (2) atomic size (quantified by Wigner-Seitz radius). Core-shell preferences determined from DFT segregation energies agree with all available observations, and predict others, which can be used for design purposes. We then provide a universal description of core-shell preference via tight-binding band-energy differences that (i) quantitatively reproduces the DFT segregation energies and (ii) confirms the electronic origins for core-shell behavior.   

Catalytic Gold Nanoparticles on an Iron Oxide Surface: A Scanning Tunneling Microscopy/Spectroscopy Study

We present a Scanning Tunneling Microscopy/Spectroscopy study of a model catalyst system consisting of supported gold nanoparticles on a reduced Fe$_{3}$O$_{4}$(111) surface in ultrahigh vacuum. Gold forms two electrically distinct types of nanoparticles on an iron oxide surface upon annealing a multilayer Au/Fe$_{3}$O$_{4}$(111). STS measurements show that large nanoparticles ($\sim $8nm) are metallic while single gold adatoms are bonded to the oxygen sites on the Fe$_{3}$O$_{4}$(111) surface. Site-specific adsorption at oxygen surface atoms and the size sensitive nature of the electronic structure (Coulomb blockade) suggest that Au adatoms are positively charged. When this Au/Fe$_{3}$O$_{4}$(111) catalyst system is dosed with CO at 260K, there is evidence for CO adsorption at gold adatom sites. These observations are consistent with the proposal that nonmetallic, positively charged, ``invisible'' Au particles are the catalytically active species for the water-gas-shift reaction on Au/metal oxide surfaces. http://clippercontrols.com/info/dielectric\_constants.html   

Intercalation dynamics in rechargeable batteries

We consider the ion intercalation of rechargeable battery electrode particles during charging (or discharging). We have developed a general phase-field model which incorporates entropic, enthalpic and elastic effects within the particle as well as the nonlinear chemical reactions at the particle- electrolyte interface. It is shown through linear stability analysis and numerical simulations that particle size and elastic effects will decrease or even eliminate both the spinodal region and the miscibility gap in the phase diagram.   

Nanowire-based solar cell fabricated by nanosphere lithography

Nanowire (NW) structures have been predicted to provide performance enhancement for solar cells due to improved light absorption [1] and (for radial $p-n$ junction geometry) improved carrier collection [2]. We report the development of NW-based solar cells fabricated using nanosphere lithography. This method provides a simple, scalable, low cost and high throughput technique to define large scale NW structures. The fabricated NW solar cells (0.25 $\mu $m diameter and 1.3 $\mu $m tall) on a $p$-Si (100) substrate show $\sim $30 {\%} higher short-circuit current and $\sim $4 {\%} higher open circuit voltage compared to the control cells (without any NWs) with baseline efficiency of 6.2 {\%}. The reflectance and quantum efficiency spectra reveal some advantages and shortcomings of the NW-based solar cell. This work marks some progress in the development of a scalable nanowire-based solar cell and highlights some key issues such as conformal-junction formation, surface passivation, and contact formation. [1] L. Hu and G. Chen, Nano Lett. 7, 3249 (2007). [2] B. M. Kayes \textit{et. al. }, J. Appl. Phys. 97, 114302 (2005).