Research Activities of the Wheeler Group

Dr. Wheeler and his research group are working to improve the performance of electrochemical systems and processes through understanding molecular behavior and mechanisms. Below is a list of current research areas.

High-power lithium batteries for vehicles

Lithium batteries are great energy storage devices. This is why they are used ubiquitously in mobile phones and laptop computers. However, there are a number of challenges to adapting these batteries for use in hybrid- and fully-electric vehicles. In particular, we need a cheap, abuse-tolerant, and long-life battery stack that can generate a large amount of specific power. Our work in this area is to engineer improved electrode morphologies in order to promote battery power and life. In addition, we perform experiments and computer simulations in order to obtain ionic and electronic transport properties in battery components. If you are interested in using one of our battery models, additional information is here. This work is supported by the Office of Naval Research.

Improved electrochemical deposition

There are many industrially important processes that are nevertheless poorly understood. The electrodeposition of copper to form microscale interconnections on computer chips is one of these processes. While it is a key process in a multibillion-dollar industry, not much is known about the mechanisms by which specific organic additives promote a good copper deposit from the electrolyte bath. We have developed a new computer simulation technique, called electrode charge dynamics, that allows us to perform computer simulations of the molecular processes at the metal/electrolyte interface. This allows us to investigate the dynamic organization of solution species as they compete for energetically favorable positions at the interface. Our progress in this area will help us understand the way additives work and hence enable better design of additives for future electrodeposition processes on smaller length scales. The tools we are developing will also permit study of other systems where molecular behavior at metal/solution interfaces is important. This work is partially supported by the Petroleum Research Fund of the American Chemical Society.

Nanowire formation

We are part of an interdisciplinary group (chemical engineering, chemistry, and physics) investigating ways to create patterned nanowires on insulating surfaces. By "nanowires" we mean metal lines of width 5-50 nm that can be used as electrical connections to individual molecules and small aggregates, such as quantum dots. We have devised and are testing several candidate technologies that utilize electrochemical processes to form the nanowires. For instance, we use the tip of an atomic force microscope to scribe a surface, which alters the structure of chemical bonds in a localized area. In the presence of the right reactants, the deposition of metal ions is promoted in the area of the scribing. This and other techniques may prove to be a viable way to "write" nanoscale metal lines on surfaces. This work is partially supported by the National Science Foundation.

Membranes for low-temperature fuel cells

The catalysts used in fuel cells to convert chemical energy into electrical energy work better at higher temperature. Unfortunately, low-temperature fuel cells that use membranes such as Dupont's Nafion, have trouble above 100 deg. C. This is due to the fact that the membrane must be hydrated in order for ions (protons) to move across the membrane efficiently. Moreover, at higher temperatures there is greater undesirable transport of gases across the membrane. We are beginning a new project to investigate, by molecular dynamics simulations, the mechanisms by which protons and other species are transported in partially and fully hydrated Nafion. We plan to implement a highly efficient simulation algorithm to account for proton transport by the Grotthuss (structural diffusion) mechanism. The work will permit a better understanding of structure/function relationships in the polymer membrane, which in turn could facilitate new designs exhibiting improved performance at higher temperatures.

Molecular simulation tools

The group has especial expertise in performing molecular-scale computer simulations, such as molecular dynamics, to elucidate the connections between microscopic and macroscopic behavior--that is, how the interactions of molecules influence thermodynamic and transport properties of materials. The molecular simulations serve as a means of estimating properties when experimental data is difficult to obtain. In adapting molecular simulations to the investigation of particular systems, we have developed new algorithms that can be helpful to other researchers using molecular simulations. These include accurate treatment of polarizable metal surfaces, simulating multicomponent mass transport in bulk systems, and more efficient ways to handle Coulombic interactions through the Ewald Sum. Additional details are available in our publications.

Below is shown a configuration snapshot from a molecular dynamics simulation of 1 molar LiPF₆ in propylene carbonate solvent, an electrolyte used in lithium batteries. The red balls are lithium ions, the blue “jacks” are PF₆^- anions.