Research Area Summary:
We investigate the underlying physical and chemical principles that facilitate efficient energy storage and creation. Through computationally driven insight into the relationships between structure, composition, and performance, we evaluate materials for their usefulness as battery cathodes and anodes or as fuel cell catalysts.
Rechargeable Battery Electrodes
The voltage, capacity, and power of a battery electrode are mainly determined by the materials properties of the electrode compounds. Neglecting heat exchange with the environment brought on by interaction with the electrolyte, these quantities can all be determined computationally. Because computational determination of the electrochemical performance of materials is accurate and efficient, it is possible to compare the properties of different battery electrodes materials without the necessity of expensive synthesis, characterization, and testing in the laboratory. The current program at NRL is focused on several aspects of improvement for rechargeable Li-ion battery cathodes.
A primary characteristic of interest is the battery capacity, that is, the number of Li ions that can be exchanged per formula unit repeatedly during battery operation. By developing materials that are stable throughout a large range of Li contents, we hope to increase this number, ideally to a figure greater than one for the cathode and greater than three for the anode. Accomplishing this requires identifying compounds that can contain a high number of Li ions but also verifying that the Li can be extracted and re-intercalated repeatedly without structural, electronic or chemical breakdown. Recently, it has been shown that many theoretically high energy materials suffer such breakdowns when synthesized in the bulk, but perform substantially better with nanoscale structure. Our program aims to understand the underlying source of improvement in stability and to search for optimal nanoscale morphologies for stable, ultra-high energy electrode materials.
Principal Investigator: Michelle Johannes
Fuel Cell Catalyst Materials
The oxygen reduction reaction (ORR) is considered to be the bottleneck reaction of the PEM fuel cell catalytic cycle. This reaction proceeds as follows: O2 + 4e- + 2H2 -> 2H2O. For this reaction to occur, the catalyst cathode material must bind incoming O2 molecules strongly enough to cause dissociation, but weakly enough to allow subsequent reduction (by OH molecules) and release. This double constraint gives severely limits the range of binding energies for O2 and OH that are acceptable for efficient fuel cell operation. This is an example of the Sabatier principle, often demonstrated by a "volcano plot" of the activity vs. the binding energy, wherein the resulting shape has a peak at some specific energy and falls away on the one side due to overbinding and on the other due to underbinding.
There are two approaches to improving catalytic activity toward the ORR. One is to optimize the binding energy toward the peak of the volcano plot. This approach has been successfully exploited using density functional theory to determine the optimal properties of alloyed Pt catalyst surfaces. The second approach is to search for "bi-functional" materials - those in which the binding and dissociation of O2 takes place on one material, which the recombination of O and H into water occurs on another material. Both these approaches are actively being pursued in this Research Area.
Another barrier to good fuel cell operation is contamination of the fuel cell surface. Under laboratory conditions, this contamination usually takes the form of over-oxidation of the surface which prevents further ORR activity. There are many studies concerning oxidation and its prevention. However, real fuel cells operate not under laboratory conditions, but in environments where pollutants not related to the catalytic process are present. The development of catalyst materials that are tolerant to such "poisons" is very important if fuel cells are to operate efficiently in non-sterile arenas. A computational analysis of how such contaminants bind to and affect catalyst surfaces and of how easily they may be removed through voltage cycling or hydrogenation and/or oxidation processes is a large part of this project.
Principal Investigator: Michelle Johannes