Solid electrolytes catalytic operation

It is usual to operate an aqueous-medium fuel cell under pressure at temperatures well in excess of the normal boiling point, as this gives higher reactant activities and lower kinetic barriers (overpotential and reactant diffusion rates). An alternative to reliance on catalytic reduction of overpotential is use of molten salt or solid electrolytes that can operate at much higher temperatures than can be reached with aqueous cells. The ultimate limitations of any fuel cell are the thermal and electrochemical stabilities of the electrode materials. Metals tend to dissolve in the electrolyte or to form electrically insulating oxide layers on the anode. Platinum is a good choice for aqueous acidic media, but it is expensive and subject to poisoning. 

When the solid electrolytes are used as membranes, there are three different operation modes, as shown in Fig. 3. Id. Mode 1 is under open circuit operation, in which no net current passes through the membrane. The reactor in this mode often serves as a sensor or an in situ characterization technique for catalytic gas-solid reactions under work conditions, named solid electrolyte potentiometry (SEP)... 

Decreasing operation temperature of solid oxide fuel cells (SOFCs) and electrocatalytic reactors down to 800-1100 K requires developments of novel materials for electrodes and catalytic layers, applied onto the surface of solid electrolyte or mixed conducting membranes, with a high performance at reduced temperatures. Highly-dispersed active oxide powders can be prepared and deposited using various techniques, such as spray pyrolysis, sol-gel method, co-precipitation, electron beam deposition etc. However, most of these methods are relatively expensive or based on the use of complex equipment. This makes it necessary to search for alternative synthesis and porous-layer processing routes, enabling to decrease the costs of electrochemical cells. Recently, one synthesis technique based on the use... 

Munder, Rihko-Stmckmann, and Sundmacher (2007) investigated the performance of a bilayer-solid electrolyte membrane reactor in steady-state and forced-periodic operation modes. The obtained results showed that the solid electrolyte membrane reactor has a slightly higher yield under optimal operation conditions compared with the catalytic wall reactor, and forced-periodic operation of the bilayer-solid electrolyte... 

The basic phenomenology of EPOC when using -conducting supports is given in Fig. 1. The (usually porous) metal (Pt) catalyst-electrode, typically 40 nm to 4 mm thick, is deposited on an 8 mol % Y203-stabilized-Zr02 (YSZ) solid electrolyte. Under open-circuit operation (I = 0, no electrochemical rate), there is a catalytic rate, ro, of ethylene consumption for oxidation to CO2 (Fig. 1). 

As it was shown in Chaps. 2 and 6, solid electrolyte-based gas sensors include two electrodes, which should correspond to several requirements such as (1) electrodes should possess sufficiently high catalytic activity with respect to target gas (2) both electrodes must be stable at the operating temperature (3) they must have suitable porosity and pore size to improve the catalyst surface area and enhance the catalytic activity and (4) the catalyst should possess high electronic conductivity (Amar et al. 2011). 

The effect of impurities on fuel cells, often referred to as fuel cell contamination, has been identified as one of the most important issues in fuel cell operation and applications. Studies have shown that the component most affected by contamination is the MEA [3]. Three major effects of contamination on the MEA have been identified [3,4] (1) the kinetic effect, which involves poisoning of the catalysts or a decrease in catalytic activity (2) the conductivity effect, reflected in an increase in the solid electrolyte resistance and (3) the mass transfer effect, caused by changes in catalyst layer structure, interface properties, and hydrophobicity, hindering the mass transfer of hydrogen and/or oxygen. 

The anodes of these cells consist of a cermet (ceramic-metal composite) of nickel and the zirconia electrolyte. This material is made from a mixture of nickel oxide (NiO) and the YSZ electrolyte. The nickel oxide is reduced in situ to metallic nickel, forming highly disperse particles that serve as the catalyst for anodic fuel gas oxidation reactions. These particles are distributed uniformly in the solid electrolyte, and are prevented from agglomerating during fuel cell operation by this electrolyte, thus retaining their catalytic activity. The YSZ material present in the anode also improves the contact between the nickel catalyst and the fuel cell s electrolyte layer. 

The microscopic mechanisms responsible for the electrochemical behavior of metallic and oxide electrodes were exhaustively analyzed in the literature [17-22, 24, 60, 62-69] and are outside of the main scope of this chapter. One should only mention that the solid-electrolyte additions into the electrode composition make it possible to increase reversibility and to enlarge the domain of temperatures and chemical potentials where the electrode can be safely used, a result of the TPB expansion and microstructural stabilization. Although the mixed-conducting and catalytically active additives such as doped ceria might also be useful from the cell impedance point of view, their use for the reference electrodes is limited if oxygen nonstoichiometry changes may occur under the cell operation conditions. 

In addition to solid electrolyte potentiometry, the techniques of cyclic voltammetry" and linear potential sweep have also been used recently in solid electrolyte cells to investigate catalytic phenomena occurring on the gas-exposed electrode surfaces. The latter technique, in particular, is known in catalysis under the term potential-programmed reduction (PPR). With appropriate choice of the sweep rate and other operating parameters, both techniques can provide valuable kinetic" and thermodynamic information about catalytically active chemisorbed species and also about the NEMCA effect," as analyzed in detail in Section III. 

A solid oxide fuel cell (SOFC) consists of two electrodes anode and cathode, with a ceramic electrolyte between that transfers oxygen ions. A SOFC typically operates at a temperature between 700 and 1000 °C. at which temperature the ceramic electrolyte begins to exhibit sufficient ionic conductivity. This high operating temperature also accelerates electrochemical reactions therefore, a SOFC does not require precious metal catalysts to promote the reactions. More abundant materials such as nickel have sufficient catalytic activity to be used as SOFC electrodes. In addition, the SOFC is more fuel-flexible than other types of fuel cells, and reforming of hydrocarbon fuels can be performed inside the cell. This allows use of conventional hydrocarbon fuels in a SOFC without an external reformer. 

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