Synergistic Catalyst Design for Fuel Cell Reactions, Biofuel Production, and Other Energy Applications using Simulation Methods
In the synergistic approach, I will design various pieces of the catalytic system, such as molecular composition and support material, to work cooperatively in order to optimize activity. The goal of my research is to design efficient, effective, and inexpensive catalyst systems for alternative energy applications and emissions control. The synergistic approach goes beyond current computational design techniques by simultaneously including the complex chemical relationships between the external (e.g. fluid phase), support, and catalyst environments.
Solid Oxide and Direct Methanol Fuel Cells
Fuel cells use catalysis to convert chemical energy to electrical energy and are highly efficient. They are comprised of two electrodes (anode, cathode), a non-electrically conducting electrolyte, and an electrically conducting medium (Figure 1). Fuel is oxidized at the negatively charged anode, oxidizing compounds are reduced at the positively charged cathode, and the resulting electron difference is transferred between the two electrodes, generating an electric current.
Solid oxide fuel cells (SOFCs) are characterized by their solid oxide electrolytes, typically yttria stabilized zirconia (YSZ), and their abilities to utilize a variety of hydrocarbon fuels . They are typically employed on a large scale, for example, to power buildings. Oxygen gas is reduced to O2− at the cathode, and the anions diffuse through the electrolyte to the anode due to an oxygen pressure gradient (pcathode = 1 atm, panode = 10−20 atm) . Operating temperatures in the range 700 – 1000 K are necessary to drive O2− diffusion, but waste heat can be recovered by coupling with a steam turbine or heat exchanger, depending on the operating temperature . Electrolyte materials must maintain sufficient pressure gradient, conduct O2−, and insulate against electric conduction; and anode and cathode materials must catalyze their respective reactions (fuel oxidation and O2 dissociation), conduct O2− and electric current, and retain stability under operating conditions . I propose to develop design criteria for SOFCs based on operating temperature in order to provide a variety of design options for buildings that can benefit from SOFC heat integration. Specifically I will compute 1) O2− diffusion kinetics in YSZ and related electrolyte materials, 2) O2 dissociation thermodynamics and kinetics and O2− diffusion kinetics in candidate cathode materials (e.g. strontium doped lanthanum manganite and related materials), and 3) simple hydrocarbon (e.g. methane) decomposition kinetics and CO oxidation kinetics on candidate anode materials (e.g. nickel.) Prospective electrolyte, cathode, and anode materials will be combined based on optimal operating temperature, and, based on the cooperative chemical properties, new designs will be sought to minimize material costs and maximize efficiency.
Direct methanol fuel cells (DMFCs) operate nearer ambient conditions and are used on a smaller scale (small vehicles, portable devices). Optimal electrode materials should readily decompose methanol and O2 but prevent build up of site blocking species such as CO, O, OH, and H. Pt is the most common material, but it is expensive and only partially able to prevent poisoning . Research indicates transition metals alone cannot achieve both requirements , emphasizing the need for synergistic design. To develop more effective DMFC system designs, I propose to model the methanol decomposition and O2 dissociation reactions in aqueous media over 1) supported gold and alloyed gold nanoparticles, which are more noble than Pt yet more active than bulk gold, and 2) metalloporphyrins (Figure 2), which are highly stable catalysts used for O2 dissociation in the body.
Biofuel Reforming Catalysts
The United States has the capacity to replace one third of its gasoline use with biomass derivatives . Biomass consists of large polymers with thousands of strongly bound structural units which are initially broken down into sugars or sugar alcohols and then catalytically processed into alkanes, for example ,
The catalysis occurs in aqueous media over supported transition metals, such as Pt supported on alumina. Alkane size is controlled by the relative rates of C-C bond forming reactions vs. C-O bond cleavage reactions, suggesting different designs for heavy alkane (diesel fuel), light alkane (gasoline), and methane production . For example, heavy alkane production catalysts should exhibit relatively fast C-C bond forming kinetics. Experimental evidence suggests C-C and C-H bond reactions occur on the metal, and C-O reactions occur on the supports . Theoretical analysis of the catalysis is lacking due to the large number of possible reactants, products, intermediates, and pathways. To motivate catalyst design, I propose to use small test molecules (ethane, methanol, acetic acid) to calculate C-C, C-H, and C-O bond reactions in aqueous media over supported transition metal catalysts in order to determine metal/support combinations most useful for heavy and light alkane production.
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