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§Fuel reformulation for fuel cell applications

§Catalytic phenomena involved in fuel cells

Fuel cells are electrochemical devices that convert chemical energy to electricity without combustion of the fuel. While fuel cell technologies have the promise of changing the energy landscape of the nation by providing clean and efficient energy for transportation as well as for residential, commercial and industrial uses, there are many scientific, technical and economical challenges that need to be overcome before they are widely used. Catalysis is at the heart of this effort.

Hydrocarbon steam reforming has been a process of major importance for production of hydrogen, synthesis gas and town gas for several decades. Since hydrogen is the most desirable fuel for fuel cells, there is a desperate need for efficient and cost-effective reforming technologies. Our research focus is on catalytically converting conventional and novel types of fuels (coal-derived liquids, gasoline/diesel, biofuels) to hydrogen.

The key challenges for our research efforts are derived from process economics and include:

Operation under minimized steam to carbon (S/C) ratios to avoid water storage, handling, and recirculation issues.

Carbonaceous deposit formation, particularly at lower S/C, reducing catalyst lifetime, requiring regeneration, and increasing pressure drop.

Loss of active surface area and metal-agglomeration causing short lifetimes under the conditions of reforming.

The sulfur tolerance of the catalysts where sulfur leads to deactivation of the catalyst.

A simplified fuel processing scheme showing the various operations needed to convert conventional fuels, such as gasoline or coal, to high-purity hydrogen for fuel cell applications.

Schematic showing the effect of ceria addition on coking/gasification mechanism during steam reforming.

The objective of this project is to examine the steam reforming of bio-ethanol over cobalt-based catalyst systems, enabling small-scale distributed hydrogen production from renewable sources. The study targets the development of a catalytic system active in the 350-550°C temperature range without relying on the use of precious metals. Fundamental mechanisms in the catalytic steam reforming of ethanol are investigated by employing various catalyst characterization techniques. Special emphasis will be placed on how these catalyst site requirements change for oxidative steam reforming. In particular, the conditions under which the autho-thermal reforming of ethanol prevails wil be characterized. The knowledge acquired through this study will bring industry closer to designing catalytic systems tailored for specific hydrogen production applications.

Flowchart showing the inter-related parameters responsible for ethanol reforming catalyst performance and stability.

The water-gas shift (WGS) reaction is an important step in the production of H₂, where CO, which is produced from steam reforming or coal gasification, is reacted with water to give H₂ and CO₂. There has been renewed interest in the WGS reaction in recent years because of its necessity in conjunction with PEM fuel cell power generation. The high-temperature shift (HTS) reaction is performed at 320-450 °C using Fe-Cr oxides catalysts, while the low-temperature shift (LTS) reaction is conducted at 200-250 °C. The LTS catalysts commonly used are Cu/ZnO/Al₂O₃ and precious metal-based catalysts. The WGS reactor currently represents the largest volume of any catalyst in a fuel processor due to the slow kinetics at temperatures where the equilibrium is favorable.

X-ray photoelectron spectra of Fe based WGS catalysts showing effect of promoters on metallic iron formation after reduction.

Existing WGS catalyst formulations are not yet commercially viable for use in coal-based or membrane fuel processors for fuel cell applications due to many disadvantages such as their sensitivity to air and low activity of the Fe-Cr catalysts, which leads to higher temperature of operation. Thus, the development of novel WGS catalysts with superior performance and low cost will have an impact on the widespread applications of fuel cell systems.

Proton exchange membrange (PEM) fuel cell anodes are easily poisoned by small concentrations of carbon monoxide in their fuel. Currently, hydrogen is produced mostly from hydrocarbon sources using steam or autothermal reforming followed by low and high temperature water gas shift reactors. The typical effluent from these reactors contains up to 2% carbon monoxide. To function properly, current PEM fuel cell anodes require less than 10 ppm carbon monoxide in their fuel. Preferential oxidation of carbon monoxide seems to be a simple and effective method to clean hydrogen prior to introduction to the fuel cell anode.

Our focus is on developing materials that can be both active and selective to the oxidation of carbon monoxide. We’ve developed a highly active and selective 10wt% Co/ZrO₂ catalyst.

These are the possible reactions that can occur when feeding hydrogen, oxygen and carbon monoxide into a catalytic reactor. The desired reaction is the oxidation of carbon monoxide while the major undesired reactions are methanation and H₂combustion.

This figure shows CO concentration versus temperature in a temperature programmed reaction experiment. At 175°C and below, CO oxidation dominates. Between 175 and 250°C, H₂ combustion becomes important. Above 250°C, methanation becomes important.

The high costs associated with the use of Pt catalysts coupled with the fact that the most significant potential loss in PEM fuel cells is due to the kinetics of the oxygen reduction reaction (ORR) in the cathode have driven the need to develop novel ORR catalysts. Based on the nature-inspired concept of using hemoglobin-type molecules to perform the oxygen reduction function, large organic macrocycles containing Fe or Co centers are reported to be active for ORR when supported on high-surface area carbon. Although these complex compounds are not stable for long periods of time in the fuel cell environment, when they are subjected to a heat treatment, the remaining nitrogen-containing carbon structures with metal centers are shown to be ORR active and stable. Our work is focused on the investigation of nanostructured heteroatom-containing carbon catalysts for oxygen reduction in PEM fuel cell cathodes.

Deconvoluted X-ray photoelectron spectra of Vulcan carbon samples after 2 hours of heat treatment at 900^oC in the presence of acetonitrile. (a) Fe- free sample (b) sample containing 2-wt% Fe showing increased pyridinic nitrogen formation. Pyridinic nitrogen has been hypothesized as the active site or a marker of the active site for ORR.

Solid oxide fuel cells (SOFCs) show great promise for generating clean power from a variety of fuels. A major roadblock against their implementation is a large cathodic resistance, which causes high operating temperatures, insufficient power densities and high fabrication costs. The large cathodic resistance is caused by slow oxygen activation kinetics and oxide ion transport of the current manganite-based cathode. Thus, the development of highly active and ionically conductive materials suitable for use as cathodes is needed to help SOFCs realize their wide-scale application. Our research strives to determine the factors that control the activity of the cathode for the oxygen reduction reaction, including oxygen adsorption, oxygen dissociation, and oxygen diffusion and the relation of these parameters to the nano-structure of the cathode material.

While many scientific studies focus on either the material preparation or electrochemical aspects of the ceramic components of the SOFC, our research focuses on the catalysis of the oxygen reduction process.

Reversible oxygen isotopic exchange over LSCF perovskite cathode material at 800^oC

Solid Oxide Fuel Cells are gaining popularity for use directly on hydrocarbon fuels and as on-board reformers in other energy systems. The current Ni-YSZ anode catalysts exhibit very good catalytic activity towards hydrocarbon reforming and good compatibility with the rest of the SOFC system but are however highly susceptible to poisoning due to sulfur in the fuel. Our research focuses on studying the mechanism of poisoning of the anode catalysts by sulfur in the fuel as well as developing materials that not only match the activity of the Ni-YSZ catalyst but also have tolerance to sulfur and coking.

X-ray Photoelectron Spectra showing presence of surface sulfur species on the Ni-YSZ catalysts on exposure to H₂S.

Schematic showing the effect of ceria addition on coking/gasification mechanism during steam reforming.

Flowchart showing the inter-related parameters responsible for ethanol reforming catalyst performance and stability.

X-ray photoelectron spectra of Fe based WGS catalysts showing effect of promoters on metallic iron formation after reduction.

These are the possible reactions that can occur when feeding hydrogen, oxygen and carbon monoxide into a catalytic reactor. The desired reaction is the oxidation of carbon monoxide while the major undesired reactions are methanation and H2 combustion.

This figure shows CO concentration versus temperature in a temperature programmed reaction experiment. At 175°C and below, CO oxidation dominates. Between 175 and 250°C, H2 combustion becomes important. Above 250°C, methanation becomes important.

These are the possible reactions that can occur when feeding hydrogen, oxygen and carbon monoxide into a catalytic reactor. The desired reaction is the oxidation of carbon monoxide while the major undesired reactions are methanation and H₂combustion.

This figure shows CO concentration versus temperature in a temperature programmed reaction experiment. At 175°C and below, CO oxidation dominates. Between 175 and 250°C, H₂ combustion becomes important. Above 250°C, methanation becomes important.