Molybdenum Dioxide (MoO₂) for Reforming Logistic Fuels and Direct JP-8 Solid Oxide Fuel Cell (SOFC)

Backgrounds:

Solid oxide fuel cells (SOFCs) that use high energy content of JP-8 military logistic fuels can allow more efficient electrical energy production to operate remote military bases, to build more energy efficient military vehicles and “more electrical airplanes and fleets” for U.S air force and navy with distributed power systems. However, the use of middle distillate heavy hydrocarbon fuels (e.g. kerosene based fuels, JP-8, Jet-A, diesel fuel) as feed stocks possesses a number of challenges. Primary among these challenges is the selection of appropriate catalytic materials and operating conditions to maximize the production of electrical power while avoiding carbon formation and sulfur poisoning. Hence, the key for the introduction of fuel cells into U.S. military applications will be the design and construction of logistic fuel cell designs based on novel catalysts that enhance catalyst durability and reduce costs.

Research Objectives:

The objective of this research is to develop a novel molybdenum based catalysts, with high coking-resistance, sulfur tolerance and sintering stability, for both JP-8 fuel reforming and direct JP-8 solid oxide fuel cell (SOFC) applications.

Catalytic Properties of Molybdenum Dioxide (MoO₂):

To directly oxidize JP-8 fuel in SOFCs, one needs an anode catalyst that possesses the following attributes:

High activity toward oxidation of JP-8 fuel	Resistance to coking
High oxygen anion and electronic conductivities	Tolerance to high sulfur concentrations

Our MoO₂ based anode will have all the attributes listed above and be able to directly process JP-8 fuel to generate electrical power. MoO₂ is a transition metal oxide that has been found to display metallic character, which is not a common characteristic of metal oxides. This is attributed to its relatively high density of states observed in the valence band energy region. The existence of these free electrons is considered to enhance the catalytic activity of Mo⁴⁺ in MoO₂, unlike Mo⁶⁺ in MoO₃, where all the valence electrons of the metal are bonded to neighboring oxygen atoms. MoO₂ has shown a high resistance to sintering at the typical operating temperatures of SOFC. Unlike previously used transition metal oxides (e.g., CeO₂) in SOFC applications, MoO₂ can conduct both electrons and ions, and has an ability to selectively transfer bulk lattice oxygen via Mars and van Krevelen reaction mechanism to the hydrocarbons. Due to this unique metallic and ion conducting properties of MoO₂, this oxide has been found to display a high oxidation activity for various long-chain hydrocarbons. By supplying a suitable amount of oxygen into a MoO₂ based anode, the redox cycles of Mo atoms become self-sustaining, leading to stable performances as well as a decrease in the amount of carbon accumulated on the catalyst surface. The MoO₂ also possesses a high sulfur tolerance where it maintains a high activity of processing the Jet-A fuel under its partial oxidation condition with the 2,000 ppmw of benzothiophene.

A direct outcome of our proposed work will be a total weight reduction of the SOFC system with a very simple control scheme by eliminating conventionally needed fuel reforming, water-gas-shift and desulfurization units. The result will be the development of robust, stable and highly efficient direct JP-8 SOFCs that can replace existing inefficient turbine-powered Auxiliary Power Units (APU) for the military applications and next generation of airplanes -- the “More Electric Aircraft” (MEA).

The broader impacts of this work will not only be a significant increase in fuel efficiency, (an efficient SOFC can save about 75% of fuel use), but a major reduction in pollutant emission. Replacement of the turbine-powered APU with a SOFC will lead to a reduction in CO₂ and other gaseous emissions and an elimination of ground noise.