Application in Fuel Cells

Sone et at. used a four-probe configuration to study the impact of temperature and RH on the ionic conductivity of a Nafion 117 membrane [50]. They quantified the conductivity change with RH and showed that the ionic conductivity of the membrane decreased with the heat-treatment temperature ranging from 80 to 120 °C. Heat treatment led to the microstructural change of the membrane and caused it to lose some of its ability to take up water. 

The two-probe configuration can also be used to measure the through-plane resistance of a membrane by sandwiching the membrane between two Pt sheets or foils  

EIS has also been used to measure the ionie resistanee of eatalyst layers. Poltarzewski et al. studied the ionie resistanees of eleetrodes with varying Nation loadings [36]. Guo et al. used porous eleetrode theory to derive a eompaet equation for the impedanee response of a eathode at the OCV [14]. They found an inerease in the ionie eonduetivity of eatalyst layers with an inerease in the Nation loading from 0.33 to 1.13 mg em .  

Ren et al. reported that the electroosmotic drag numbers increased with temperature for a fully hydrated Nafion 117 membrane [44]. The drag numbers were reported to be 2 and 5 H20/H at 15 and 130 °C, respectively. They obtained the drag numbers by balancing the water collected at the cathode in an operating DMFC, where water back-diffusion from the cathode to the anode is negligible. 

Wiezell et al. developed a steady-state model to explain the EIS of the HOR on porous electrodes [57]. The model predicted that the EIS of the HOR would have three to four loops. The high-frequency loop is due to the Volmer reaction and the medium-frequency loop is due to flic hydrogen adsorption. The low-frequency loops arise from the impact of flic water content on both the reaction kinetics and the membrane ionic conductance. Experimentally Wiezell et al. observed two semicircles at 10 and 0.01-0.1 Hz, and they attributed them to hydrogen adsorption and the impact of water, respectively [58]. They believed that the loop for the Volmer reaction required extremely high frequencies in order to be measured experimentally. 

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Chapter 3 discusses solid electrolytes and some of their early applications in fuel cells and catalysis. This material is quite familiar to the solid state ionics community but may be helpful to surface scientists, aqueous electrochemists and chemical reaction engineers. 

However, in the case of multimetallic catalysts, the problem of the stability of the surface layer is cmcial. Preferential dissolution of one metal is possible, leading to a modification of the nature and therefore the properties of the electrocatalyst. Changes in the size and crystal structure of nanoparticles are also possible, and should be checked. All these problems of ageing are crucial for applications in fuel cells. 

The oxide Ba2In205 is another well-studied phase that adopts the brownmillerite structure. This material disorders above 930°C to a perovskite-type structure containing oxygen vacancies. Both the Sr-Fe and Ba-In oxides are of interest for electrochemical applications in fuel cells and similar devices (Section 6.10). 

The discussion of Brouwer diagrams in this and the previous chapter make it clear that nonstoichiometric solids have an ionic and electronic component to the defect structure. In many solids one or the other of these dominates conductivity, so that materials can be loosely classified as insulators and ionic conductors or semiconductors with electronic conductivity. However, from a device point of view, especially for applications in fuel cells, batteries, electrochromic devices, and membranes for gas separation or hydrocarbon oxidation, there is considerable interest in materials in which the ionic and electronic contributions to the total conductivity are roughly equal. 

Which is the best catalyst for accelerating the reaction depends on the nature of the working materials. For the reaction of hydrogen or oxygen in potassium hydroxide solution, nickel or silver is suitable for carbonaceous fuels as well as for the reaction of oxygen in acid electrolytes platinum metals were up to the middle 60s, the only known catalysts. Precious metals are ruled out by price for wide application in fuel cells, and the search for cheaper catalysts has been actively pursued in many research laboratories. Many classes of inorganic substances (carbides, nitrides, oxides, sulfides, phosphides, etc.) have been investigated and, in particular, several chelates. 

The modified supported powder electrodes used in the experiments hitherto described on the anodic activity of CoTAA are out of the question for practical application in fuel cells, as they do not have sufficient mechanical stability and their ohmic resistance is very high (about 1—2 ohm). For these reasons, compact electrodes with CoTAA were prepared by pressing or rolling a mixture of CoTAA, activated carbon, polyethylene, and PTFE powders in a metal gauze. The electrodes prepared in this way show different activities depending on the composition and the sintering conditions. Electrodes prepared under optimal conditions can be loaded up to about 40 mA/cm2 at a potential of 350 mV at 70 °C in 3 M HCOOH, with relatively good catalyst utilization (about 5 A/g) and adequate stability. 

Considering their possible applications in fuel cells, hydrogen sensors, electro-chromic displays, and other industrial devices, there has been an intensive search for proton conducting crystals. In principle, this type of conduction may be achieved in two ways a) by substituting protons for other positively charged mobile structure elements of a particular crystal and b) by growing crystals which contain a sufficient amount of protons as regular structure elements. Diffusional motion (e.g., by a vacancy mechanism) then leads to proton conduction. Both sorts of proton conductors are known [P. Colomban (1992)]. 

A particular topical area in inclusion chemistry gas gas soption and separation. Gases of interest include H2 (for application in fuel cells), methane and C02. Recent developments in crystallography allow crystal structures to be obtained routinely under significant gas pressure. 

Nanotechnology Sees Applications in Fuel Cells and Solar Power—Micro Fuel Cells to Power Mobile Devices... 

Oxide ion conductors have been extensively investigated for their applications in fuel cells, oxygen sensors, oxygen pumps, and oxygen permeable membranes [81-108], The ion conduction effect was discovered more than a century ago by Nernst in zirconia products [83,84], To use zirconia, it... 

Lusardi, M., Bosio, B., Arato, E. (2004). An example of innovative application in fuel cell system development COj segregation using molten carbonate fuel cell. /. 

Hoogers, G. Portable applications. In Fuel Cell Technology Handbook Hoogers, G., Ed. CRC Press Boca Raton, 2003 9-1-9-14. 

Koryabkina, N. Ribeiro, F. Ruettinger, W. Determination of kinetic parameters for water-gas shift catalysts under realistic conditions for fuel cell applications. In Fuel Cell Technology Opportunities and Challenges, AICHE Spring National Meeting, March 10-14, New Orleans, 2002 92-97. 

When producing hydrogen as the final product, impurities such as CO, sulfur compounds, and other trace contaminants must be removed, particularly for application in fuel cells. Currently, pressure swing adsorption (PSA) is commonly used for the separation and purification of hydrogen from mixed gas streams. PSA systems are based on selective adsorbent beds. The gas mixture is introduced to the bed at an elevated pressure and the solid adsorbent selectively adsorbs certain components of the gas mixture, allowing the unadsorbed components, in this case hydrogen, to pass through the bed as purified gas. 

Finally, it should be noted that numerous perovskite-related materials with relatively low oxygen ionic conductivity at 700-1200 K have been excluded from consideration in this brief survey, but may have potential electrochemical applications in fuel cell anodes, current collectors, sensors, and catalytic reactors. Further information on these applications is available elsewhere 1-4, 11, 159, 217-219]. 

The perfluorosulfonic acid (Nafion) membrane found its application in fuel cells long before its introduction to the chlor-alkali industry (26-28). The Nafion membrane is used as the solid polymer electrolyte (separator/electrolyte) in fuel cells. Figure 2 shows the schematic of such an SPE fuel cell. 

Proton Conducting Electrolytes and Their Application in Fuel Cells... 

Nevertheless, classical heterogeneous catalysts like particulate noble metals may be immobilized on the nanotube surface as well. Nanoparticles of platinum or rhodium, for instance, can be deposited on cup-stacked carbon nanotubes by reductive precipitation (Figure 3.114b). The catalysts obtained this way suit an application in fuel cells run on methanol. Electrodes made from the nanotube material exhibit twice the efficiency as compared to the classical material XC-72-carbon. The particles of noble metal on the nanotube surface catalyze the direct conversion of methanol into CO2 (MeOH -1- H2O CO2 -1- 6 H -1- 6e ). A material to be employed in such fuel cells has to meet some essential requirements, including a maximal specific surface, a defined porosity and a high degree of crystalhnity. Carbon nanotubes are endowed with exactly these characteristics, which is why they are the most suitable material for electrodes. Their high price, however, is still prohibitive to an industrial scale application. 

Overall, a great deal of attention has been paid to inducing ionic conductivity in chitosan membranes for application in fuel cell membranes. In both cationic and anionic membranes, the fuel cell performance values are approaching that of the industry standard Nation membranes. 

PERSPECTIVE MATERIALS FOR APPLICATION IN FUEL-CELL TECHNOLOGIES... 

Perspective Materials For Application In Fuel-Cell Technologies... 

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