Porous carbon supports

Air electrode plays an important role in the operation of Li-air battery. A successful air cathode must satisfy many requirements such as good electrical conductivity, high surface area, fast oxygen diffusion, stable electrode integrity, and fast ionic conductivity. In nonaqueous Li-air batteries, the air cathode is a carbon-supported porous structure, which acts as a gas transport channel involving the formation and storage of discharge product. GDE helps to accomplish an air cathode to complete the cell such as that in PEM... 

Inorganic membranes (29,36) are generaUy more stable than their polymeric counterparts. Mechanical property data have not been definitive for good comparisons. IndustriaUy, tube bundle and honeycomb constmctions predominate with surface areas 20 to 200 m. Cross-flow is generaUy the preferred mode of operation. Packing densities are greater than 1000 /m. Porous ceramics, sintered metal, and metal oxides on porous carbon support... 

The electrochemical catalytic activity of various Pt-porous nanoparticles for the oxidation of methanol was shown in Figure 13. The peak mass current densities recorded after 100 scan cycles were 80mAmg Pt for those porous Pt nanoparticles. The peak mass current densities were 72, 62, 58 and 51mAmg Pt for those products formed for reaction time of 3, 5, 10, and 20min at the Pt(acac)2 HDD HDA ACA DPE molar ratio of 6 36 118 12 100, respectively. As a reference point, the catalytic activity of a commercial carbon-supported Pt... 

Note that for metal nanoparticles supported on porous carbon materials, it is even more difficult to establish the mechanism of the ORR. Indeed, for the above-described thin layer or porous RRDE (Section 15.3), H2O2 has very little chance to escape from the CL and be detected at the ring. H2O2 can readsorb either on Pt particles or on the carbon support, and undergo chemical decomposition or further electrochemical reduction, while diffusing out of the CL. This implies great difficulties in establishing the detailed ORR mechanism on nanometer-sized metal nanoparticles. 

The electrodes in the direct methanol fuel cell (DMFC) (i.e. the anode for oxidising the fuel and the cathode for the reduction of oxygen) are based on finely divided Pt dispersed onto a porous carbon support, and the electro-oxidation of methanol at a polycrystalline Pt electrode as a model for the DMFC has been the subject of numerous electrochemical studies dating back to the early years ot the 20th century. In this particular section, the discussion is restricted to the identity of the species that result from the chemisorption of methanol at Pt in acid electrolyte. This is principally because (i) the identity of the catalytic poison formed during the chemisorption of methanol has been a source of controversy for many years, and (ii) the advent of in situ IR culminated in this controversy being resolved. 

Two main types of catalyst layers are used in PEM fuel cells polyfefrafluo-roethylene (PTFE)-bound catalyst layers and thin-film catalyst layers [3]. The PTFE-bound CL is the earlier version, used mainly before 1990. If confains two components hydrophobic PTFE and Pt black catalyst or carbon-supported Pt catalyst. The PTFE acts as a binder holding the catalyst together to form a hydrophobic and structured porous matrix catalyst layer. This porous structure can simultaneously provide passages for reacfanf gas fransport to the catalyst surface and for wafer removal from fhe cafalysf layer. In fhe CL, the catalyst acts as both the reaction site and a medium for electron conduction. In the case of carbon-supported Pt catalysts, both carbon support and catalyst can act as electron conductors, but only Pt acts as the reaction site. 

The catalyst layer is composed of multiple components, primarily Nafion ion-omer and carbon-supported catalyst particles. The composition governs the macro- and mesostructures of the CL, which in turn have a significant influence on the effective properties of the CL and consequently the overall fuel cell performance. There is a trade-off between ionomer and catalyst loadings for optimum performance. For example, increased Nafion ionomer confenf can improve proton conduction, but the porous channels for reactanf gas fransfer and water removal are reduced. On the other hand, increased Pt loading can enhance the electrochemical reaction rate, and also increase the catalyst layer thickness. 

The porous carbon support is made by the pyrolysis of an 8 mm outer diameter tube with a wall thickness of 1 mm. Spending on the degree of pyrolysis (low temperature) the support can be weakly hydrophilic or more hydrophobic (high temperature). The latter material is weaker and more brittle. 

Glassy Carbon Immobilized porous Carbon Substrate Enzyme Support... 

A variety of industrial catalytic processes employ small metal-particle catalysts on porous inorganic supports. The particle sizes are increasingly in the nanometre size range which gives rise to nanocatalysts. As described in chapter 1, commonly used supports are ceramic oxides, like alumina and silica, or carbon. Metal (or metallic) catalysts in catalytic technologies contain a high dispersion of nanoscopic metal particles on ceramic oxide or carbon supports. This is to maximize the surface area with a minimum amount of metal for catalytic reactions. It is desirable to have all of the metal exposed to reactants. 

The SC-155 material calcined in oxidizing atmosphere at 1000 "C and until constant weight produced a self supported porous silica network (the carbon was burnt out) which was called S-155 this silica structure maintained the same shape and volume as the original composite. On the other hand, the SC-155 treated with 20 % HF aqueous solution until the elimination of Si02 produced a self supported carbon network called C-155, and this carbon structure maintained the same shape and volume as the original composite. 

The Au-catalyzed glycerol oxidation was influenced by the kind of support, the size of Au particles and the reaction conditions such as concentration of glycerol, p02 and molar ratio of NaOH to glycerol. As metal oxide supports showed inferior selectivity to glyceric acid compared to carbons, due to successive oxidation and C—C bond cleavage to form di-adds such as tartronic acid and glycolic acid, research has focused on Au NPs supported on carbon, as in the case of ethylene glycol oxidation [182]. Indeed, the catalytic activity was influenced by the kind of carbon support in terms of porous texture [183]. 

Polymer electrolyte fuel cell (PEFC) is considered as one of the most promising power sources for futurist s hydrogen economy. As shown in Fig. 1, operation of a Nation-based PEFC is dictated by transport processes and electrochemical reactions at cat-alyst/polymer electrolyte interfaces and transport processes in the polymer electrolyte membrane (PEM), in the catalyst layers consisting of precious metal (Pt or Ru) catalysts on porous carbon support and polymer electrolyte clusters, in gas diffusion layers (GDLs), and in flow channels. Specifically, oxidants, fuel, and reaction products flow in channels of millimeter scale and diffuse in GDL with a structure of micrometer scale. Nation, a sulfonic acid tetrafluorethy-lene copolymer and the most commonly used polymer electrolyte, consists of nanoscale hydrophobic domains and proton conducting hydrophilic domains with a scale of 2-5 nm. The diffusivities of the reactants (02, H2, and methanol) and reaction products (water and C02) in Nation and proton conductivity of Nation strongly depend on the nanostructures and their responses to the presence of water. Polymer electrolyte clusters in the catalyst layers also play a critical... 

C. Reverse selective surface diffusion Reverse selective skin layer, n (1-5 R > n Highly porous ceramic or 1 carbon support 7600 pm OD... 

The catalysts and electrode materials used in PAFCs are also similar to those in acidic H2/air fuel cells. Carbon-supported Pt is used as the catalyst at both anode and cathode, porous carbon paper serves as the electrode substrate, and graphite carbon forms the bipolar plates. Since a liquid electrolyte is used, an efficient water removal system is extremely important. Otherwise, the liquid electrolyte is easily lost with the removed water. An electrolyte matrix is needed to support the liquid phosphoric acid. In general, a Teflon -bonded silicon carbide is used as the matrix. 

A direct comparison between porous carbon and porous silica supports is difficult since the measurement conditions are different. In the case of silica the hydrogen release temperature is lower but a possible chemical reaction between the silica and the LiB H4 has to be considered. It is interesting that the porous carbon support seems to suppress the formation of diborane. 

Some of those developments at Oak Ridge were believed to spin off in some form at Union Carbide and some aspects of the efforts led to the commercialization of dyiuunically formed membranes primarily for ultrafiltration and hypeiTiltration (reverse osmosis) applications. In these dynamic membranes, a mixture of zirconium hydroxide and polyacrylic acid deposited on a porous support which provides the necessary mechanical strength. The support is mostly made of porous carbon although porous ceramic and stainless steel are also used. These non-sintered membranes, in great contrast to most of the membranes discussed in this book, are formed in situ and require periodic regeneration with new zirconium hydroxide and polyacrylic acid. 

The dynamic membranes originally developed by Union Carbide are protected by three core patents U.S, 3977967, 4078112, and 4412921 (Trulson and Litz, 1976 Bibeau, 1978 and Leung and Cacciola, 1983) and their foreign equivalents. Those patents cover a broad range of metal oxides such as zirconia, gamma alumina, magnesia>alumina spinel, tantalum oxide and silica as the membrane materials and carbon, alumina, aluminosilicates, sintered metals, fiberglass or paper as the potential porous support materials. However, their marketed product, trade named Ucarscp membranes, focused on dynamic membranes of hydrous zirconium oxide on porous carbon support. 

The year 1980 marked the entry of a new type of commercial ceramic membrane into the separation market. SPEC in France introduced a zirconia membrane on a porous carbon support called Carbosep. This was followed in 1984 by the introduction of alumina membranes on alumina supports, Membralox by Ceraver in France and Ceraflo by Norton in the U.S. With the advent of commercialization of these ceramic membranes in the eighties, the general interest level in inorganic membranes has been aroused to a historical high. Several companies involved in the gas diffusion processes were responsible for this upsurge of interest and applications. 

This is a topic of great practical interest because of water treatment and metal recovery applications. Its fundamental aspects are also important for the preparation of carbon-supported catalysts [22], where the catalyst precursor is typically dissolved in water prior to its loading onto the porous support. 

The support porous structure and the rate of solvent removal from the pores as well as the nature of solvent and metal compound dissolved can considerably influence both the distribution of the active component through the support grain and the catalyst dispersion [163,170-173]. As a rule, the resulting particles size of the active component will be smaller, the more liquid-phase ruptures caused by evaporation of the solvent from the support pores are attained before the solution saturation. Therefore, supports with an optimal porous structure are needed to prepare impregnated Me/C catalysts with the finest metal particles. As a result, carbon supports appropriate for synthesis of such catalysts are very limited in number. Besides, these catalysts will strongly suffer from the blocking effect (see Section 12.1.2) because some of the metal particles are localized in fine pores. 

Under certain approximations, using the concepts of percolation theory, the basic parameters can be related to the volume portions of the components of the layer. This offers a relationship between the structure of the porous composite catalyst layer and its performance. An optimum composition (in terms of volume fractions of electrolyte material, carbon and carbon-supported catalyst, and pore space) is a Holy Grail here. Albeit this goal can still be far away in view of the simplified character of the models used, these models give at least some rational scheme for... 

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