Fuel cell electrode can

To meet the requirements for electronic conductivity in both the SOFC anode and cathode, a metallic electronic conductor, usually nickel, is typically used in the anode, and a conductive perovskite, such as lanthanum strontium manganite (LSM), is typically used in the cathode. Because the electrochemical reactions in fuel cell electrodes can only occur at surfaces where electronic and ionically conductive phases and the gas phase are in contact with each other (Figure 6.1), it is common... 

Nevertheless, as discussed here, it was demonstrated that fuel-cell electrodes can be obtained with dispersion of plurimetallic particles in an electron-conducting polymer. Actually, the performances of these electrodes remain lower than the state... 

By varying the particle size, shapes, separation, and support on planar model electrocatalysts, the influence of these properties on the electrocatal3dic reactions, e.g., on fuel cell electrodes, can be evaluated systematically. Some new challenges arise, such as the adhesion of the catalyst particles on new types of support materials (e.g., glassy carbon). However, most of the procedures and concepts of preparation and characterization are the same as in heterogeneous catalysis and photocatalysis. 

Basically, a fuel cell electrode can, thus, be seen as a highly dispersed interface between Pt and electrolyte (ionoiner or water). Due to the random composition, complex spatial distributions of electrode potential, reaction rates, and concentrations of reactants and water evolve under PEMFC operation. A subtle electrode theory has to establish the links between these distributions. 

The catalytic layers (CLs) of fuel cell electrodes can be represented as an assembly of two interconnected porous systems (1) a microstructure of porous catalysts (together with their supports) flooded by electrolyte, and (2) a macroslructure in hydrophilic CLs of wide gas pores or, in hydrophobized CLs, agglomerates of hydrophobic particles with gas pores between the particles. 

Fuel cell electrodes can be examined for their electrocatalytic behavior by ex situ or in situ voltammetry tests. In the case of ex situ tests, also known as half-cell tests, the properties of the electrode are evaluated using a standard three-electrode cell where an aqueous solution (e.g., perchloric acid) simulates the proton-conducting electrolyte in a PEMFC. Half-cell tests are a convenient and relatively fast method of screening electrocatalysts however. 

Carbon-supported catalysts (e.g., those employed in fuel cell electrodes) can show potentially very different behavior for the ORR than that of the bulk metal. Yang et al. [37] showed that Pd/C catalysts in alkaline had high activity for ORR, and that the carbon support itself is active for the two-electron reduction of O2 to peroxide (which can then migrate to the Pd particles for subsequent two-electron reduction to water). It is shown that all carbon materials have some ORR activity in alkaline solution (but none in acid), normally for the two-electron reduction to peroxide, although some oxidized carbon surfaces can complete the serial reduction to water at higher overpotentials [24,38-40]. 

Example. The Pechini method for fuel cell electrode preparation. La, Ba, Mn niU ates - - CgHgO — citrate complex - - C2FI6O2 — gel. Metal nitrates are complexed with citric acid, and then heated with ethylene glycol to form a transparent gel. This is then heated to 600 K to decompose the organic content and then to temperatures between 1000 and 1300K to produce tire oxide powder. The oxide materials prepared from the liquid metal-organic procedures usually have a more uniform particle size, and under the best circumstances, this can be less than one micron. Hence these particles are much more easily sintered at lower temperatures than for the powders produced by tire other methods. 

Solid alkaline membrane fuel cells (SAMECs) can be a good alternative to PEMFCs. The activation of the oxidation of alcohols and reduction of oxygen occurring in fuel cells is easier in alkaline media than in acid media [Wang et al., 2003 Yang, 2004]. Therefore, less Pt or even non-noble metals can be used owing to the improved electrode kinetics. Eor example, Ag/C catalytic powder can be used as an efficient cathode material [Demarconnay et al., 2004 Lamy et al., 2006]. It has also... 

Oxides play many roles in modem electronic technology from insulators which can be used as capacitors, such as the perovskite BaTiOs, to the superconductors, of which the prototype was also a perovskite, Lao.sSro CutT A, where the value of x is a function of the temperature cycle and oxygen pressure which were used in the preparation of the material. Clearly the chemical difference between these two materials is that the capacitor production does not require oxygen partial pressure control as is the case in the superconductor. Intermediate between these extremes of electrical conduction are many semiconducting materials which are used as magnetic ferrites or fuel cell electrodes. The electrical properties of the semiconductors depend on the presence of transition metal ions which can be in two valence states, and the conduction mechanism involves the transfer of electrons or positive holes from one ion to another of the same species. The production problem associated with this behaviour arises from the fact that the relative concentration of each valence state depends on both the temperature and the oxygen partial pressure of the atmosphere. 

Another useful bimetallic for fuel cell electrodes is Pt/Ru. Ruthenium is readily oxidized to Ru02 by calcination after it is impregnated. The PZC of ruthenium oxide is unknown. Propose a comprehensive sequence of experiments with which the SEA method can be applied for the synthesis of a Pt/Ru bimetallic catalyst supported on carbon. The goal is to have intimate contact between the Pt and Ru phases in the final, reduced catalyst. 

Ohmic Polarization Ohmic losses occur because of resistance to the flow of ions in the electrolyte and resistance to flow of electrons through the electrode materials. The dominant ohmic losses, through the electrolyte, are reduced by decreasing the electrode separation and enhancing the ionic conductivity of the electrolyte. Because both the electrolyte and fuel cell electrodes obey Ohm s law, the ohmic losses can be expressed by the equation... 

As shown in Figure 3, the open-circuit potential represents the highest voltage obtainable for a single cell. This potential is derived from thermodynamics. The overall fuel-cell reaction can be broken down into the two global electrode reactions. If hydrogen is the primary fuel, it oxidizes at the anode according to the reaction... 

The importance of materials characterization in fuel cell modeling cannot be overemphasized, as model predictions can be only as accurate as their material property input. In general, the material and transport properties for a fuel cell model can be organized in five groups (1) transport properties of electrolytes, (2) electrokinetic data for catalyst layers or electrodes, (3) properties of diffusion layers or substrates, (4) properties of bipolar plates, and (5) thermodynamic and transport properties of chemical reactants and products. 

One of the most daunting issues is the catalyst. The reactions at the electrodes tend to go slowly, and making or coating the electrode with a catalyst is essential to speed things up. Platinum is one of the most effective catalysts because it binds the reactants and holds them in place so that the reaction can proceed. But the problem is cost—platinum is a rare metal and not at all cheap. An ounce (28.6 g) of platinum costs about 1,200 as of May 2009. Compare that to gold, a precious metal that costs about 970 per ounce (28.6 g) as of May 2009. Fuel cell electrodes would be cheaper if they were made of gold (but gold is not an effective catalyst) ... 

From the very beginning of fuel cell development, soot and other active carbons because of their high internal surface, amounting typically to 100 m2/g, had been the most important catalyst supports for fuel cell electrodes. Platinum can be utilized on soot to a higher extent than in the form of dispersed platinum as Pt black. Carbon-supported platinum is the fuel cell catalyst of choice for the cathode as well as for the anode (135, 136). 

Semiconductor fabrication techniques permit the feature size of Si-based devices to reach into the deep submicron regime [i]. Additionally, Si can be anodized electrochemically or chemically (e.g., in an HF-containing electrolyte) to produce a sponge-like porous layer of silicon, with pore dimensions that range from several microns in width to only a few nanometers [ii]. These properties of Si make it a useful substrate for fabricating sensor platforms, photonic devices and fuel cell electrodes [iii]. 

With respect to H2S contamination, it has been recognized for many years that the major impact is upon the fuel cell electrode kinetics [40], When H2S is added to humidified H2, EIS can be used to study the H2S contamination. Information gathered includes the effects of the H2S concentration, the exposure time, and the reversibility of the contamination. H2S can strongly adsorb on a Pt catalyst surface, blocking the active sites and then causing irreversible contamination. 

The external type of reference electrode is connected to the membrane via a liquid electrolyte bridge, such as a sulphuric acid solution, as shown in Figure 5.45. Compared with the internal reference electrode configuration, the external type is easier to use in a normal PEM fuel cell set-up because it needs minimal modifications. However, attention must also be paid to ensure that the liquid electrolyte has good contact with the membrane and does not flow into the cell. Furthermore, the use of a liquid electrolyte in an acid bridge can induce non-uniform hydration and a proton concentration gradient in the membrane, therefore interfering with the fuel cell electrodes. 

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