Fuel oxygen activation

A single-chamber solid oxide fuel cell (SC-SOFC), which operates using a mixture of fuel and oxidant gases, provides several advantages over the conventional double-chamber SOFC, such as simplified cell structure with no sealing required and direct use of hydrocarbon fuel [1, 2], The oxygen activity at the electrodes of the SC-SOFC is not fixed and one electrode (anode) has a higher electrocatalytic activity for the oxidation of the fuel than the other (cathode). Oxidation reactions of a hydrocarbon fuel can... 

A simple Langmuir-Hinshelwood model explains quantitatively the steady-state behavior (4) but it fails to explain the oscillatory phenomena that were observed. The origin of the limit cycles is not clear. Rate oscillations have not been reported previously for silver catalyzed oxidations. Oxidation of ethylene, propylene and ethylene oxide on the same silver surface and under the same temperature, space velocity and air-fuel ratio conditions did not give rise to oscillations. It thus appears that the oscillations are related specifically to the nature of chemisorbed propylene oxide. This is also supported by the lack of any correlation between the limits of oscillatory behavior and the surface oxygen activity as opposed to the isothermal oscillations of the platinum catalyzed ethylene oxidation where the SEP measurements showed that periodic phenomena occur only between specific values of the surface oxygen activity (6,9). 

As discussed in table 10.1, the mobile species within a fuel cell are ions, which necessitate the electrolyte being an ionic conductor and electronic insulator. If the oxygen ions are the only charge carriers, the electron motive force (EMF) of the cell is determined from the chemical potential of oxygen (i.e., oxygen activity), which is expressed by the Nernst equation as... 

Flame velocities of and temperatures in nitromethane flames supported by oxygen have been measured. The global activation energy is surprisingly low, viz. 10—16 kcal. mole", depending on the fuel/oxygen ratio [126]. 

Nonstoichiometry diminishes the already low thermal conductivity, lowers the melting point and strength, increases creep and fission product migration and release, and alters irradiation behavior. The increase in oxygen activity with burnup can be significant in leading rods in light-water reactors (5%burnup) and in fast breeder reactor fuels (10% burnup). 

Comparison of current densities, and the potenhal for onset and for maximum current, is hampered when attempts are made, as will be undertaken in the following sechons, to report on results from diverse sources, as there is as yet poor adherence to normalization of such data to standard or accepted conditions. The variability in experimental approaches includes differences, often unspecified, in specific surface areas of the elechodes used, the control of mass transport of fuel/ oxygen to the surface, the coverage and activity of the enzyme on the surface, the stabihty of the system, the operational (and ophmal) condihons (pH, ionic strength. 

E25.17 Electrocatalysts are compounds that are capable of reducing the kinetic barrier for electrochemical reactions (barrier known as overpotential). While platinum is the most efficient electrocatalyst for accelerating oxygen reduction at the fuel cell cathode, it is expensive (recall Section 25.18 Electrocatalysis). Current research is focused on the efficiency of a platinum monolayer by placing it on a stable metal or alloy clusters your book mentions the use of the alloy PtsN. An example would be a platinum monolayer fuel-cell anode electrocatalyst, which consists of ruthenium nanoparticles with a sub-monolayer of platinum. Other areas of research include using tethered metalloporphyrin complexes for oxygen activation and subsequent reduction. 

The electrocatalytic activity of Co(II) phthalocyanine is somewhat lower than that of silver for oxygen reduction in fuel cells. Activity varies with the central metal atom in the order Fe > Co > Cu for acid and neutral electrolytes (2S3). This result is in contrast to Jasinski s observation of lower activity for Fe than for Co ligands (282). The discrepancy can be explained by differences in the ligand structure and the supports used and possibly their conductivity. For instance, polymeric phthalocyanines show a fourfold increase in activity for oxygen reduction compared to monomeric ones, owing to a lO -lO increase in their conductivity (283). 

Adsorption can be classified as an accumulative process involving the transition of the contaminant from the water phase (or gas phase) to the sohd phase. Due to their log Kqc values, other hazardous substances (e.g. benzene) present at MTBE-contaminated sites show better adsorption on activated carbon. Alternatively, other adsorbents like zeolites have been widely investigated in laboratory studies as well as in one pilot-scale and one full-scale application, showing promising results. Nevertheless, it should be mentioned that the availability of data dealing with the adsorption behaviour of fuel oxygenates other than MTBE is limited. Further studies are needed to identify the most suitable adsorbent for each contaminant as well as for mixed con tarn-... 

Table 3 Freundlich parameters for the adsorption of other fuel oxygenates on activated carbon...

Complex FCC oxides of the fluorite type represent oxygen-conduction solid electrolytes (SOE s). They comprise a typical class of materials for the manufacture of sensors of oxygen activity in complex gas mixtures, oxygen pumps, electrolyzers and high-temperature fuel elements. These materials are based on doped oxides of cerium and thorium, zirconium and hafnium, and bismuth oxide. Materials based on zirconium oxide, for example, yttrium stabilized zirconia (YSZ) are the most known and studied among them. This fact is explained both by their processibility and a wide spectrum of practical applications and by the possibility to conduct studies on single crystals, which have the commercial name "fianites" and are used in jewelry. 

Since the electronic conductivity of nanocrystalline ScSZ becomes significant in reducing atmosphere (Fig.3), its contribution to the electrical transport should be considered. This can be discussed based on the dependence of the ionic transference number, t,- = cr,- / (oxygen activity. Such information is important for the development of ScSZ solid electrolyte for Solid Oxide Fuel Cells. Figure 4 presents the relationship between the ionic transference number and oxygen activity, which has been determined based on the presented conductivity measurements and the defect model [13]. 

In a ceramic electrochemical reactor oxygen activation is physically separate from substrate activation. Activation of dioxygen occurs at a cathodic surface, oxygen traverses the electrolyte as oxide ions, the ions react with a substrate on the anodic face, and the electrons return to complete the circuit. Two types of electrochemical membrane reactors are under study which differ in the route by which the electrons travel from anode to cathode these are illustrated schematically in Figure 1. In the shorted fuel cell reactor the electrons return through an external circuit, from which electrical power can be extracted. Electroceramic membranes allow the electrons to make their way from anode to cathode via electrically conductive phases within the membrane proper. 

Teske K, UUmann H, Rettig D (1983) Investigation of the oxygen activity of oxide fuels and fuel-fission product systems by solid electrolyte techniques. Part I qualification and limitations of the method. J Nucl Mater 116 260-266... 

Mass transport loss is defined as the loss in performance of the fuel cell due to limitations in mass transport processes. This performance loss is usually attributed to a reduction of the oxygen activity (associated to its partial pressure) at the electrode, in comparison to the oxygen partial pressure at the cell inlet. The accumulation of water in the transport pathways of the gaseous reactants can lead to increased mass transport losses and instability and can result in accelerated degradation. 

The converter function presupposes carefijl monitoring of the oxygen activity in the exhaust gases. This occurs with a k-sensor or a k-probe. The probe electronically orders the engine to keep the k-value (the air/fuel ratio) at an optimum, often near 1. For description of the k-probe see section 17.10.2.3 (Yttrium). The placing of the probe in the converter is shown in Figure 32.4. 

Oxygen Activation for Fuel Cell and Electrochemical Process Applications... 

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